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THE    WORKS 

OF 

FRANCIS    MAITLAND    BALFOUR, 


VOL.    II. 


<£iuttom 


Cambrfofle : 

PRINTED  BY  C.   J.   CLAY,  M.A.   AND  SON, 
AT  THE   UNIVERSITY   PRESS. 


<0&ftton. 


THE     WORKS 


OF 


FRANCIS    MAITLAND    BALFOUR, 

M.A.,    LL.D.,    F.R.S., 

FELLOW  OF  TRINITY   COLLEGE, 

AND   PROFESSOR  OF  ANIMAL  MORPHOLOGY  IN   THE   UNIVERSITY   OF 
CAMBRIDGE. 


EDITED    BY 
M.    FOSTER,    F.R.S., 

PROFESSOR   OF   PHYSIOLOGY   IN    THE   UNIVERSITY  OF  CAMBRIDGE; 
AND 

ADAM    SEDGWICK,    M.A., 

FELLOW  AND   LECTURER   OF  TRINITY  COLLEGE,   CAMBRIDGE. 


VOL.    II. 

A   TREATISE   ON    COMPARATIVE   EMBRYOLOGY. 
Vol.  I.    Inventebrata. 


Honiron : 

MACMILLAN    AND    CO. 
1885 


[The  /tight  of  Translation  is  reserved.] 


PREFACE. 

MY  aim  in  writing  this  work  has  been  to  give  such  an 
account  of  the  development  of  animal  forms  as  may  prove 
useful  both  to  students  and  to  those  engaged  in  embryo- 
logical  research.  The  present  volume,  save  in  the  intro- 
ductory chapters,  is  limited  to  a  description  of  the  develop- 
ment of  the  Invertebrata:  the  second  and  concluding 
volume  will  deal  with  the  Vertebrata,  and  with  the 
special  histories  of  the  several  organs. 

Since  the  work  is,  I  believe,  with  the  exception  of  a 
small  but  useful  volume  by  Packard,  the  first  attempt  to 
deal  in  a  complete  manner  with  the  whole  science  of 
Embryology  in  its  recent  aspects,  and  since  a  large 
portion  of  the  matter  contained  in  it  is  not  to  be  found  in 
the  ordinary  text  books,  it  appeared  desirable  to  give 
unusually  ample  references  to  original  sources.  I  have 
accordingly  placed  at  the  end  of  each  chapter,  or  in  some 
cases  of  each  section  of  a  chapter,  a  list  of  the  more 
important  papers  referring  to  the  subject  dealt  with.  The 
papers  in  each  list  are  numbered  continuously,  and  are 
referred  to  in  the  text  by  their  numbers.  These  lists  are 
reprinted  as  an  appendix  at  the  end  of  each  volume.  It 
will  of  course  be  understood  that  they  do  not  profess  to 
form  a  complete  bibliography  of  the  subject. 

B.  II.  B 


PREFACE. 


In  order  to  facilitate  the  use  of  the  work  by  students 
I  have  employed  two  types.  The  more  general  parts  of 
the  work  are  printed  in  large  type;  while  a  smaller  type 
is  used  for  much  of  the  theoretical  matter,  for  the  details 
of  various  special  modes  of  development,  for  the  histories 
of  the  less  important  forms,  and  for  controversial  matter 
generally.  The  student,  especially  when  commencing  his 
studies  in  Embryology,  may  advantageously  confine  his 
attention  to  the  matter  in  the  larger  type;  it  is  of  course 
assumed  that  he  already  possesses  a  competent  knowledge 
of  Comparative  Anatomy. 

Since  the  theory  of  evolution  became  accepted  as  an 
established  doctrine,  the  important  bearings  of  Embryo- 
l°&y  on  a^  morphological  views  have  been  universally 
recognised;  but  the  very  vigour  with  which  this  depart- 
ment of  science  has  been  pursued  during  the  last  few 
years  has  led  to  the  appearance  of  a  large  number  of 
incomplete  and  contradictory  observations  and  theories; 
and  to  arrange  these  into  anything  like  an  orderly  and 
systematic  exposition  has  been  no  easy  task.  Many 
Embryologists  will  indeed  probably  hold  that  any  attempt 
to  do  so  at  the  present  time  is  premature,  and  therefore 
doomed  to  failure.  I  must  leave  it  to  others  to  decide 
how  far  my  effort  has  been  justified.  That  what  I  have 
written  contains  errors  and  shortcomings  is  I  fear  only 
too  certain,  but  I  trust  that  those  who  are  most  capable 
of  detecting  them  will  also  be  most  charitable  in  excusing 
them. 

The  work  is  fully  illustrated,  and  most  of  the  figures 
have  been  especially  engraved  from  original  memoirs  or 
from  my  own  papers  or  drawings  by  Mr  Collings,  who 
has  spared  no  pains  to  r«  ndcr  the  woodcuts  as  clear  and 


PREFACE.  vii 


intelligible  as  possible.  I  trust  my  readers  will  not  be 
disappointed  with  the  results.  The  sources  from  which 
the  woodcuts  are  taken  have  been  in  all  cases  acknow- 
ledged, and  in  the. cases  where  no  source  is  given  the 
illustrations  are  my  own. 

I  take  this  opportunity  of  acknowledging  my  great 
obligations  to  Professors  Agassiz,  Huxley,  Gegenbaur, 
Lankester,  Turner,  Kolliker,  and  Claus,  to  Sir  John 
Lubbock,  Mr  Moseley,  and  Mr  P.  H.  Carpenter,  for  the 
use  of  electrotypes  of  woodcuts  from  their  works. 

I  am  also  under  great  obligations  to  numerous  friends 
who  have  helped  me  in  various  ways  in  the  course  of  my 
labour.  Professor  Kleinenberg,  of  Messina,  has  read 
through  the  whole  of  the  proofs,  and  has  made  numerous 
valuable  criticisms.  My  friend  and  former  pupil,  Mr 
Adam  Sedgwick,  has  been  of  the  greatest  assistance  to 
me  in  correcting  the  proofs.  I  have  had  the  benefit  of 
many  useful  suggestions  by  Professor  Lankester  es- 
pecially in  the  chapter  on  the  Mollusca,  and  Mr  P.  H. 
Carpenter  has  kindly  revised  the  chapter  on  the  Echino- 
dermata. 

I  am  also  much  indebted  to  Dr  Michael  Foster,  Mr 
Moseley,  and  Mr  Dew-Smith  for  aid  and  advice. 


CONTENTS  OF  VOLUME   I. 


INTRODUCTION.     Pp.  i — 16. 
CHAPTER  I.     THE  OVUM  AND  SPERMATOZOON. 

General  history  of  the  Ovum,  pp.  17 — 25.     Special  history  of  the  Ovum  in 
different  types,  pp.  -26 — 65.     The  Spermatozoon,  pp.  65 — 67. 

CHAPTER  II.    THE  MATURATION  AND  IMPREGNATION  OF  THE  OVUM. 

Maturation  of  the  Ovum,  and  formation  of  the  polar  bodies,  pp.  68 — 79, 
Impregnation  of  the  Ovum,  pp.  79—86.     Summary,  pp.  86. 

CHAPTER  III.     THE  SEGMENTATION  OF  THE  OVUM. 

Internal  phenomena  of  Segmentation,   pp.  88 — 92.      External  features  of 
Segmentation,  pp.  92 — 122. 

INTRODUCTION  TO  SYSTEMATIC  EMBRYOLOGY.  Pp.  125 — 130. 

CHAPTER  IV.     DICYEMID^E  AND  ORTHONECTID/E.  Pp.  131 — 137. 
CHAPTER  V.     PORIFERA.     Pp.  138 — 151. 
CHAPTER  VI.     CCELENTERATA. 

Hydrozoa,  pp.  152 — 167.     Actinozoa,  pp.  167 — 173.     Ctenophora,  pp.  173 
—  178.     Summary,  etc.,  pp.  178—182.     Alternations  of  generations,  pp.  182— 

187. 

CHAPTER  VII.     PLATYELMINTHES. 

Turbellaria,  pp.  189 — 196.    Nemertea,  pp.  196 — 204.    Trematoda,   pp.  205 
— 210.     Cestoda,  pp.  210 — 218. 

CHAPTER  VIII.     ROTIFERA.     Pp.  221 — 224. 


CONTENTS   OF   VOLUME   I. 


CHAPTER  IX.     MOLLUSCA. 

Formation  of  the  layers  and  larval  characters,  pp.  225  —  273.  Gasteropoda 
and  Pteropoda,  pp.  175—742.  Cephalopo<ia,  pp.  242—254.  Polyplacophora, 
pp.  am  —  357.  Scaphopoda,  pp.  257,  758.  Lamellibramhiata,  pp.  258  —  269 
General  review  of  Mollnscan  Larva,  pp.  270—273.  Development  of  organs, 
pp. 


CHAPTER  X.     POLYZOA. 

Entoprocta,  pp.  291—797.  Ectoprocta,  pp.  297—305.  Summary  and 
general  considerations,  pp.  305  —  308. 

CHAPTER  XI.     BRACHIOPODA. 

Development  of  the  layers,  pp.  31  1—313.  The  history  of  the  larva,  pp.  313— 
317.  Development  of  organs,  p.  317.  General  observations  on  the  affinities  of 
the  Brachiopoda,  pp.  317,  318. 

CHAPTER  XII.     CHJETOPODA. 

Formation  of  the  germinal  layers,  pp.  319  —  325.  The  larval  form,  pp.  325  — 
338.  Formation  of  organs,  pp.  338  —  342.  Alternations  of  generations,  pp. 
34».  343- 

CHAPTER  XIII.     DISCOPHORA. 

Formation  of  layers,  pp.  347—350.     History  of  larva,  pp.  351  —  354. 

CHAPTER  XIV.     GEPHYREA. 

Gephyrea  nuda,  pp.  355  —  361.  Gephyrea  tubicola,  pp.  361  —  364.  General 
considerations,  364. 

CHAPTER  XV. 

Chartognatha,  pp.  366—369.  Myzostomea,  pp.  369,  370.  Gastrotricha, 
P-  370- 

CHAPTER  XVI. 

Nematelminthes,  pp.  370—379.     Acanthocephala,  pp.  379—381. 

CHAPTER  XVII.    TRACHEATA. 

I'rototrachcata,  pp.  381—387.  Myriapoda,  pp.  387—395.  Insecta,  pp.  395  — 
429.  Embryonic  mcmbrants  and  the  formation  oj  '  the  layers,  pp.  400  —  406.  Forma- 
tion  of  the  organs,  pp.  406  —  417.  Spec  ial  types  of  larvcc,  pp.  417  —  419.  Mctamor- 
fikoiis  and  heterogamy,  pp.  420—419.  Arachnida,  pp.  431  —  455.  Formation 
of  tht  layen  and  general  development,  pp.  431  —  446.  Formation  of  the  organs, 
pp.  446—455'  Formation  of  the  layers  and  embryonic  envelopes  in  the  Tracheata, 
PP-  456—458. 


CONTENTS   OF  VOLUME   I.  xi 

CHAPTER  XVIII.     CRUSTACEA. 

History  of  larval  forms,  pp.  459 — 511.  Branchiopoda,  pp.  459 — 465.  Mala- 
costraca,  pp.  465 — 487.  Copepoda,  pp.  487 — 492.  Cirripedia,  pp.  492 — 500. 
Ostracoda,  pp.  500 — 502.  Phylogeny  of  the  Crustacea,  pp.  502 — 511.  The  forma- 
tion of  the  germinal  layers,  pp.  511 — 521.  Comparative  development  of  organs, 
pp.  521—529. 

CHAPTER  XIX. 

Pcecilopoda,  -pp.  534—538.  Pycnogonida,  pp.  538,  539.  Pentastomida, 
pp.  539 — 541.  Tardigrada,  p.  541.  Summary  of  Arthropodan  Development, 
pp.  541— 543. 

CHAPTER  XX.     ECHINODERMATA. 

Development  of  the  germinal  layers,  pp.  544 — 553.  Development  of  the 
larval  appendages  and  metamorphosis,  pp.  553 — 573.  Summary  and  general 
considerations,  pp.  573 — 576. 

CHAPTER  XXL     ENTEROPNEUSTA.     Pp.  579 — 583. 
INDEX.     Pp.  584 — 590. 

APPENDIX. 


EMBRYOLOGY. 


INTRODUCTION. 

EMBRYOLOGY  forms  a  large  and  important  department  of 
Biology.  Strictly  interpreted  according  to  the  meaning  of  the 
word,  it  ought  to  deal  with  the  growth  and  structure  of  organisms 
during  their  development  within  the  egg  membranes,  before  they 
are  capable  of  leading  an  independent  existence.  Modern  in- 
vestigations have  however  shewn  that  such  a  limitation  of  the 
science  would  have  a  purely  artificial  character,  and  the  term 
Embryology  is  now  employed  to  cover  the  anatomy  and  physi- 
ology of  the  organism  during  the  whole  period  included  between 
its  first  coming  into  being  and  its  attainment  of  the  adult  state. 

The  subject-matter  of  the  science  of  Embryology  admits  of  a 
twofold  classification.  It  may  be  placed  under  a  series  of  heads, 
each  dealing  either  with  a  special  group  of  organisms,  or  with  a 
special  department  of  the  whole  science.  If  classified  in  the 
first  of  these  ways  the  science  will  naturally  be  divided  into 
an  Embryology  of  Plants,  and  an  Embryology  of  Animals  ;  each 
of  which  admits  of  further  subdivision.  In  the  second  way 
the  subject  falls  under  two  primary  heads ;  viz.  Physiological 
Embryology  and  Anatomical  Embryology. 

The  present  treatise  deals  only  with  the  Embryology  of 
Animals,  and  is  further  confined  to  those  animals  known  as 
Metazoa.  The  science  is  moreover  treated  from  the  morpho- 
logical or  anatomical,  rather  than  from  the  physiological  side. 

B.  II.  I 


EMBRYOLOGY. 


The  marvellous  phenomenon  of  the  evolution  of  a  highly 
complicated  living  being  from  a  simple  undifferentiated  germ  in 
which  it  needs  the  aid  of  the  most  modern  microscopical  appli- 
ances to  detect  any  visible  signs  of  life,  has  not  unnaturally 
attracted  the  attention  of  biologists  from  the  very  earliest  periods. 
Before  the  establishment  of  the  cell  theory  the  origin  of  the 
organism  from  the  germ  was  not  known  to  be  an  occurrence 
of  the  same  nature  as  the  growth  of  the  fully  formed  individual, 
and  Embryological  investigations  were  mixed  up  with  irrelevant 
speculations  on  the  origin  of  life1. 

The  difficulties  of  understanding  the  formation  of  the  indivi- 
dual from  the  structureless  germ  led  anatomists  at  one  time  to 
accept  the  view  "according  to  which  the  embryo  preexisted, 
"even  though  invisible,  in  the  ovum,  and  the  changes  which 
"took  place  during  incubation  consisted  not  in  a  formation  of 
"  parts,  but  in  a  growth,  i.e.  in  an  expansion  with  concomitant 
"  changes  of  the  already  existing  germ." 

Great  as  is  the  interest  attaching  to  the  simple  and  isolated 
life  histories  of  individual  organisms,  this  interest  has  been 
increased  tenfold  by  the  generalizations  of  Mr  Charles  Darwin. 

It  has  long  been  recognized  that  the  embryos  and  larvae 
of  the  higher  forms  of  each  group  pass,  in  the  course  of  their 
development,  through  a  series  of  stages  in  which  they  more 
or  less  completely  resemble  the  lower  forms  of  the  group2. 
This  remarkable  phenomenon  receives  its  explanation  on  Mr 
Darwin's  theory  of  descent.  There  are,  according  to  this  theory, 
two  guiding,  and  in  a  certain  sense  antagonistic  principles  which 
have  rendered  possible  the  present  order  of  the  organic  world. 
These  are  known  as  the  laws  of  heredity  and  variation.  The 
first  of  these  laws  asserts  that  the  characters  of  an  organism 

1  To  this  general  statement  Wolff  forms  a  remarkable  exception,  for  though 
without  any  clear  knowledge  of  what  we  call  cells  he  had  very  distinct  notions  on  the 
relations  of  growth  and  development. 

*  Von  Boer  who  is  often  stated  to  have  established  the  above  generalization  really 
maintained  a  somewhat  different  view.  He  held  (Ueber  Entwickelungsgeschichte  d. 
Tkiere,  p.  314)  that  the  embryos  of  higher  forms  never  resembled  the  adult  stages  of 
lower  forms  but  merely  the  embryos  of  such  forms.  Von  Bacr  was  mistaken  in  thus 
absolutely  limiting  the  generalization,  but  his  statement  is  much  more  nearly  true  than 
a  definite  statement  of  the  exact  similarity  of  the  embryos  of  higher  forms  to  the 
adults  of  lower  ones. 


INTRODUCTION. 


at  all  stages  of  its  existence  are  reproduced  in  its  descendants  at 
corresponding  stages.  The  second  of  these  laws  asserts  that 
offspring  never  exactly  resemble  their  parents.  By  the  common 
action  of  these  two  principles  continuous  variation  from  a  parent 
type  becomes  a  possibility,  since  every  acquired  variation  has  a 
tendency  to  be  inherited. 

The  remarkable  law  of  development  enunciated  above,  which 
has  been  extended,  especially  by  the  researches  of  Huxley1  and 
Kowalevsky,  beyond  the  limits  of  the  more  or  less  artificial 
groups  created  by  naturalists,  to  the  whole  animal  kingdom,  is  a 
special  case  of  the  law  of  heredity.  This  law,  interpreted  in 
accordance  with  the  theory  of  descent,  asserts  that  each  organism 
in  the  course  of  its  individual  ontogeny  repeats  the  history  of  its 
ancestral  development.  It  may  be  stated  in  another  way  so  as 
to  bring  out  its  intimate  connection  with  the  laws  of  inheritance 
and  variation.  Each  organism  reproduces  the  variations  inherited 
from  all  its  ancestors  at  successive  stages  in  its  individual 
ontogeny  which  correspond  with  those  at  which  the  variations 
appeared  in  its  ancestors.  This  mode  of  stating  the  law  shews 
that  it  is  a  necessary  consequence  of  the  law  of  inheritance. 
The  above  considerations  clearly  bring  out  the  fact  that  Com- 
parative Embryology  has  important  bearings  on  Phylogeny,  or 
the  history  of  the  race  or  group,  which  constitutes  one  of  the 
most  important  branches  of  Zoology. 

Were  it  indeed  the  case  that  each  organism  contained  in  its 
development  a  full  record  of  its  origin,  the  problems  of  Phylogeny 
would  be  in  a  fair  way  towards  solution.  As  it  is,  however,  the 
law  above  enunciated  is,  like  all  physical  laws,  the  statement  of 
what  would  occur  without  interfering  conditions.  Such  a  state 
of  things  is  not  found  in  nature,  but  development  as  it  actually 
occurs  is  the  resultant  of  a  series  of  influences  of  which  that  of 
heredity  is  only  one.  As  a  consequence  of  this,  the  embryo- 
logical  record,  as  it  is  usually  presented  to  us,  is  both  imperfect 
and  misleading.  It  may  be  compared  to  an  ancient  manuscript 
with  many  of  the  sheets  lost,  others  displaced,  and  with  spurious 
passages  interpolated  by  a  later  hand.  The  embryological 

1  Huxley  was  the  first  to  shew  that  the  body  of  the  Coelenterata  was  formed  of 
two  layers,  and  to  identify  these  with  the  two  primary  germinal  layers  of  the  Verte- 
brata. 


EMBRYOLOGY. 


record  is  almost  always  abbreviated  in  accordance  with  the 
tendency  of  nature  (to  be  explained  on  the  principle  of  survival 
of  the  fittest)  to  attain  her  ends  by  the  easiest  means.  The  time 
and  sequence  of  the  development  of  parts  is  often  modified,  and 
finally,  secondary  structural  features  make  their  appearance 
to  fit  the  embryo  or  larva  for  special  conditions  of  existence. 
When  the  life  history  of  a  form  is  fully  known,  the  most  difficult 
part  of  his  task  is  still  before  the  scientific  embryologist.  Like 
the  scholar  with  his  manuscript,  the  embryologist  has  by  a 
process  of  careful  and  critical  examination  to  determine  where 
the  gaps  are  present,  to  detect  the  later  insertions,  and  to  place 
in  order  what  has  been  misplaced. 

The  aims  of  Comparative  Embryology  as  restricted  in  the 
present  work  are  two-fold:  (i)  to  form  a  basis  for  Phylogeny, 
and  (2)  to  form  a  basis  for  Organogeny  or  the  origin  and 
evolution  of  organs.  The  justification  for  employing  the  results 
of  Comparative  Embryology  in  the  solution  of  the  problems  in 
these  two  departments  of  science  is  to  be  found  in  the  law  above 
enunciated,  but  the  results  have  to  be  employed  with  the  quali- 
fications already  hinted  at ;  and  in  both  cases  a  knowledge  of 
Comparative  Anatomy  is  a  necessary  prelude  to  their  application. 

In  accordance  with  the  above  objects  Comparative  Embryo- 
logy may  be  divided  into  two  departments. 

The  scientific  method  employed  in  both  of  these  departments 
is  that  of  comparison,  and  is  in  fact  fundamentally  the  same  as 
the  method  of  Comparative  Anatomy.  By  this  method  it 
becomes  possible  with  greater  or  less  certainty  to  distinguish 
the  secondary  from  the  primary  or  ancestral  embryonic  characters, 
to  determine  the  relative  value  to  be  attached  to  the  results  of 
isolated  observations,  and  generally  to  construct  a  science  out  of 
the  rough  mass  of  collected  facts.  It  moreover  enables  each 
observer  to  know  to  what  points  it  is  important  to  direct  his 
attention,  and  so  prevents  that  simple  accumulation  of  dis- 
connected facts  which  is  too  apt  to  clog  and  hinder  the  advance 
of  the  science  it  is  intended  to  promote. 

In  the  department  of  Phylogeny  the  following  are  the  more 
important  points  aimed  at 

(i)  To  test  how  far  Comparative  Embryology  brings  to 
light  ancestral  forms  common  to  the  whole  of  the  Metazoa. 


INTRODUCTION.  5 


Examples  of  such  forms  have  been  identified  by  various  embryo- 
logists  in  the  ovum  itself,  supposed  to  represent  the  unicellular 
ancestral  form  of  the  Metazoa  :  in  the  ovum  at  the  close  of 
segmentation  regarded  as  the  polycellular  Protozoon  parent 
form  :  in  the  two-layered  gastrula,  etc.,  regarded  by  Haeckel  as 
the  ancestral  form  of  all  the  Metazoa1. 

(2)  How  far  some  special  embryonic  larval  form   is  con- 
stantly reproduced  in  the  ontogeny  of  the  members  of  one  or 
more  groups  of  the  animal  kingdom  ;   and  how  far  such  larval 
forms  may  be  interpreted  as  the  ancestral  type  for  those  groups. 

As  examples  of  such  forms  may  be  cited  the  six-limbed 
Nauplius  supposed  by  Fritz  Miiller  to  be  the  ancestral  form 
of  the  Crustacea  ;  the  trochosphere  larva  of  Lankester,  which  he 
considers  to  be  common  to  the  Mollusca,  Vermes,  and  Echino- 
dermata  :  the  planula  of  the  Ccelenterata,  etc. 

(3)  How  far  such  forms  agree  with  living  or  fossil  forms  in 
the  adult  state ;    such  an  agreement  being  held  to  imply  that 
the  living  or  fossil  form  in  question  is  closely  related  to  the 
parent  stock  of  the  group  in  which  the  larval  form  occurs.     It  is 
not  easy  to  cite  examples  of  a  very  close  agreement  of  this  kind 
between  the  larval  forms  of  one  group  and  the  existing  or  fossil 
forms  of  another.     The  larvae  of  some  of  the  Chaetopoda  with 
long]  provisional  setae  resemble  fossil  Chaetopods.     The  Rotifers 
have  many  points  of  resemblance  to  the  trochosphere,  especially 
to  that  form  of  trochosphere  characteristic  of  the  Mollusca.    The 
Turbellarians  have  some  features  in  common  with  the  Ccelente- 
rate  planula.      Some  of  the  Gephyrea   in    the   presence   of  a 
praeoral  lobe  resemble  certain  trochosphere  types.     The  larva 
of  the  Tunicata  has  the  characters  of  a  simple  type   of  the 
Chordata. 

Within  the  limits  of  a  single  group  agreements  of  this 
kind  are  fairly  numerous.  In  the  Craniata  the  tadpole  of 
the  Anura  has  its  living  representative  in  the  Pisces  and  perhaps 
especially  in  the  Myxinoids.  The  larval  forms  of  the  Insecta 
approach  Peripatus.  The  stalked  larva  of  Comatula  is  re- 
produced by  the  living  Pentacrinus  and  Rhizocrinus  etc. 

1  The  value  of  these  identifications  as  well  as  of  those  below  is  discussed  in  its 
appropriate  place  in  the  body  of  the  work.  Their  citation  here  is  not  to  be  regarded 
as  necessarily  implying  my  acceptance  of  them. 


EMBRYOLOGY. 


Numerous  examples  of  the  same  phenomenon  are  found  amongst 
the  Crustacea. 

(4)  How  far  organs  appear  in  the  embryo  or  larva  which 
cither  atrophy  or  become  functionless  in  the  adult  state,  and 
\vhich  persist  permanently  in  members  of  some  other  group  or 
in  lower  members  of  the  same  group.     Cases  of  this  kind  are  of 
the  most  constant  occurrence,  and  it  is  only  necessary  to  cite 
such  examples  as  the  gill-slits  and  Wolffian  body  in  the  embryos 
of  higher  Craniata  to  illustrate  the  kind  of  instance  alluded  to. 
The  same  conclusions  may  be  drawn  from  them  as  from  the 
cases  under  the  previous  heading. 

(5)  How  far  organs  pass  in  the  course  of  their  development 
through  a  condition  permanent  in  some  lower   form.     Phylo- 
genetic  conclusions  may  be  drawn  from  instances  of  this  cha- 
racter, though  they  have  a  more  important  bearing  on  Organology 
than  on  Phylogeny. 

The  considerations  which  were  used  to  shew  that  the  an- 
cestral history  is  reproduced  in  the  ontogeny  of  the  individual 
apply  with  equal  force  to  the  evolution  of  organs.  The  special 
questions  in  Organology,  on  which  Comparative  Embryology 
throws  light,  may  be  classified  under  the  following  heads. 

(1)  The  origin  and  homologies  of  what  are  known  as  the 
germinal  layers;  or  the  layers  into  which  the  embryo  becomes 
divided  immediately  after  the  segmentation. 

(2)  The  origin  of  primary  tissues,  epithelial,  nervous,  mus- 
cular, connective,  etc.,  and  their  relation  to  the  germinal  layers. 

(3)  The   origin  of  organs.      The   origin   of  the   primitive 
organs  is  intimately  connected  with  that  of  the  germinal  layers. 
The  first  differentiation  of  the  segmented  ovum  results  in  the 
cells  of  the  embryo  becoming  arranged  as  two  layers,  an  outer 
one  known  as  the  epiblast  and  an  inner  one  as  the  hypoblast. 
The  outer  of  these  forms  a  primitive  sensory  organ,  and  the 
inner  a  primitive  digestive  organ. 

(4)  The  gradual  evolution  of  the  more  complicated  organs 
and  systems  of  organs. 

This  part  of  the  subject,  even  more  than  that  dealing  with 
questions  of  Phylogeny,  is  intimately  bound  up  with  Com- 
parative Anatomy;  without  which  indeed  it  becomes  quite 
meaningless. 


INTRODUCTION. 


REPRODUCTION. 

A  study  of  reproduction  logically  precedes  that  of  Embry- 
ology. Reproduction  essentially  consists  in  the  separation  of  a 
portion  of  an  organism  which  has  the  capacity  of  developing  into 
a  form  similar  to  that  which  gave  it  origin.  The  simplest 
modes  of  reproduction  are  those  which  occur  amongst  the 
Protozoa. 

In  this  group,  reproduction  may  take  place  in  a  great  variety 
of  ways.  These  may  be  classified  in  three  groups:  (i)  fission, 
(2)  budding  or  gemmation,  (3)  spore  formation. 

Reproduction  in  all  these  ways  may  take  place  either  subse- 
quently to  and  apparently  in  consequence  of  a  very  important 
process  known  as  conjugation,  which  consists  in  the  temporary 
or  permanent  fusion  of  two  or  more  individuals,  or  spontane- 
ously, i.e.  independently  of  any  such  previous  conjugation. 

Reproduction  by  fission  consists  simply  in  the  division  of  the 
organism  into  two  similar  parts,  the  nucleus  when  present 
becoming  divided  simultaneously  with  the  cell  body.  This 
mode  of  reproduction  is  the  simplest  conceivable,  and  is  not 
followed  by  a  development,  since  the  two  organisms  produced 
are  exactly  similar,  except  in  size,  to  the  parent  form.  Besides 
single  fission,  a  process  of  multiple  fission  may  take  place,  as 
amongst  the  Flagellata,  where  Drysdale  and  Dallinger  have 
shewn  that  an  individual  enclosed  within  a  structureless  cyst 
may  divide  first  into  two,  then  into  four,  and  so  on. 

The  process  of  budding  differs  mainly  from  that  of  simple 
fission  in  the  fact  that  the  two  organisms  produced  are  dissimilar 
in  size,  and  also  that  the  separation  of  the  smaller  organism 
from  the  larger  is  preceded  by  a  process  of  growth  in  the  latter, 
so  that  in  the  separation  of  the  bud  no  essential  part  of  the 
parent  form  is  removed.  This  mode  of  reproduction  is  found 
amongst  the  Infusoria,  Acineta,  &c.  An  interesting  variation  in 
it  is  the  internal  gemmation  of  many  of  the  Acineta,  where  a 
portion  of  the  internal  protoplasm  with  part  of  the  nucleus  is 
separated  off  to  form  a  fresh  individual.  This  mode  of  gemma- 
tion is  connected  by  a  series  of  gradations  with  the  normal 


S  EMBRYOLOGY. 


external  gemmation.  The  organisms  produced  by  gemmation 
are  not  always  similar  at  birth  to  the  parent ;  e.g.  Acineta. 

Both  fission  and  gemmation  when  incomplete  lead  to  the 
formation  of  colonies. 

The  third  mode  of  reproduction,  by  spore  formation,  does 
not  essentially  differ  from  that  by  multiple  fission.  It  consists 
in  the  breaking  up  of  the  organisms  into  a  number  (usually  very 
considerable)  of  portions  ;  each  of  which  eventually  developes 
into  an  organism  like  the  parent  form.  All  gradations  between  a 
simultaneous  division  of  the  organism  into  such  spores  and  simple 
multiple  fission  are  to  be  found,  but  this  process  of  reproduction 
may  be  sometimes  distinguished  from  that  by  such  fission  by 
the  fact  that  the  two  processes  may  coexist  in  a  single  form, 
e.g.  the  biflagellate  monad  of  Drysdale  and  Dallinger.  In  the 
majority  of  cases  the  spores  produced  differ  at  first  from  the 
parent  organism  not  only  in  size  but  in  other  points,  such  as  the 
possession  of  a  flagellum,  etc.  They  may  even  be  without  a 
nucleus  when  the  parent  organism  is  nucleated,  as  in  the  Gre- 
garinidae. 

The  encystment,  which  in  many  cases  precedes  reproduction 
by  any  of  the  above  processes,  and  more  especially  by 
spores,  is  not  an  essential  condition  of  their  occurrence ;  and  is 
probably  in  the  first  instance  a  protective  arrangement  which 
has  become  secondarily  adapted  to  and  connected  with  re- 
production. 

As  has  been  already  stated,  all  the  above  modes  of  reproduc- 
tion take  place  in  some  of  the  Protozoa  without  any  anterior 
process  which  can  be  regarded  as  of  a  sexual  nature  ;  but  very 
often  they  are  preceded  by  the  temporary  or  permanent  fusion 
of  two  or  more  individuals,  such  fusion  being  known  as  con- 
jugation. 

In  most  cases  reproduction  by  spores  is  the  consequence  of 
conjugation,  but  in  the  Infusoria  etc.  where  the  fusion  at  conju- 
gation is  temporary  (except  Vorticella),  there  is  probably  merely 
a  renewed  activity — a  rejuvenescence — which  most  likely  results 
in  active  fission  or  budding.  In  the  Gregarinidse  reproduction 
by  spores  usually  follows  conjugation,  but  may  also  take  place 
without  it.  In  some  Flagellata  reproduction  by  spores  follows 
the  conjugation  of  two  individuals  in  a  different  stage  of  de- 


INTRODUCTION.  9 


velopment.  Thus  in  the  springing  Monad,  described  by 
Drysdale  and  Dallinger,  a  form  produced  by  the  fission  of  a 
monad  in  an  amoeboid  condition  fuses  with  an  ordinary  monad 
to  produce  an  individual,  which  then  breaks  up  into  spores. 
Another  instance  of  the  fusion  of  dissimilar  individuals  is 
afforded  by  Vorticella,  where  a  free-swimming  individual  conju- 
gates and  is  permanently  united  with  a  fixed  one  (Engelmann, 
Biitschli).  Conjugation  often  consists  in  the  fusion  of  more  than 
two  individuals.  In  conjugation  where  the  fusion  is  permanent, 
the  nuclei  of  the  conjugating  forms  usually  unite  before  the 
product  breaks  up  into  spores ;  and  where  temporary  fusion 
occurs  in  the  Infusoria  a  division  of  the  paranuclei  and  often  of 
the  nuclei  takes  place,  followed  by  the  ejection  of  parts  of  them, 
and  a  reproduction  of  new  paranuclei  and  nuclei  from  the 
remainder  of  the  original  structures. 

In  order  to  understand  the  meaning  of  conjugation  in  con- 
nection with  reproduction,  it  is  important  to  understand  how  the 
two  became  in  the  first  instance  related.  For  the  solution  of 
this  question  the  fact  that  many  Protozoa  have  the  capacity  of 
temporarily  or  permanently  fusing  together  without  an  imme- 
diate act  of  reproduction  is  of  great  importance.  A  good  example 
of  such  fusion  is  supplied  by  Actinophrys.  We  must  suppose  in 
fact  that  the  simple  coalescence  of  two  or  more  individuals  gives 
a  sufficient  amount  of  extra  vigour  to  their  product,  to  compen- 
sate the  race  for  the  loss  in  number  of  individuals  so  caused. 
This  extra  vigour  probably  first  exhibited  itself  especially  by 
increased  activity  in  reproduction,  till  finally  the  two  processes, 
viz.  that  of  conjugation  and  that  of  reproduction,  came  to  be 
inseparably  connected  together. 

The  reproduction  of  the  forms  above  the  Protozoa,  which  are 
known  as  the  Metazoa,  takes  place  by  two  methods,  viz.  a  sexual 
and  an  asexual  one.  The  sexual  process,  which  occurs  in  every 
known  Metazoon1,  consists  essentially,  as  is  shewn  in  the  second 
chapter  of  this  work,  in  the  fusion  of  two  cells  budded  off  from 
the  parent  organism,  viz.  the  female  cell  or  ovum,  and  the  male 
cell  or  spermatozoon,  and  of  the  subsequent  division  of  the 
compound  cell  so  produced  into  a  number  of  parts  which  build 

1  Dicyema,  if  it  is  a  true  Metazoon,  would  seem  to  form  an  exception  to  this  rule. 


10  EMBRYOLOGY. 


themselves  up  into  an  organism  resembling  one  of  the  parents. 
The  sexual  process  has  obviously  at  first  sight  a  very  close 
resemblance  to  the  process  of  conjugation.  Since  it  is  a  ques- 
tion of  fundamental  importance  to  determine  how  sexual  repro- 
duction originated,  it  becomes  necessary  to  examine  how  far 
this  apparent  resemblance  is  a  real  one,  and  how  far  sexual 
reproduction  can  be  derived  from  reproduction  following  upon 
conjugation. 

In  spite  of  the  general  similarity  between  the  two  processes 
there  is  an  obvious  difficulty  in  comparing  them,  in  that  the 
result  of  conjugation  is  usually  the  breaking  up  of  the  individual 
formed  by  the  fusion  of  two  other  individuals  into  a  number  of 
new  organisms,  while  the  result  of  the  fusion  which  takes  place 
in  sexual  reproduction  is  the  formation  of  a  single  new  organism. 
This  difference  between  the  two  processes,  great  as  it  is,  is  per- 
haps apparent  rather  than  real.  It  must  be  remembered  that  a 
single  individual  Metazoon  is  equivalent  to  a  number  of  Protozoa 
coalesced  to  form  a  single  organism  in  a  higher  state  of  aggre- 
gation. It  results  from  this  that  the  segmentation  of  the  ovum 
which  follows  the  sexual  act  may  be  compared  to  the  breaking 
up  of  the  product  of  conjugation  into  spores,  the  difference 
between  the  two  processes  consisting  in  the  fact  that  in  the  one 
case  the  spores  separate  each  to  form  an  independent  organism, 
while  in  the  other  they  remain  united  and  give  rise  to  a  single 
compound  organism. 

If  the  above  considerations  are  well  founded  it  seems  permis- 
sible to  accept  the  general  view  according  to  which  sexual 
reproduction  is  derived  from  conjugation.  It  is  necessary  to 
suppose  that,  in  a  colony  of  Protozoa  in  the  course  of  becoming 
a  Metazoon,  the  capacity  of  reproduction  by  spores  became 
localized  in  certain  definite  cells,  and  although  the  formation  of 
spores  from  these  cells  may  have  been  possible  without  previous 
conjugation,  yet  that  conjugation  gradually  became  established 
as  the  rule.  The  differentiation  of  primitively  similar  conjugating 
cells  into  male  and  female  cells  was  probably  a  very  early  occur- 
rence, since  indications  of  an  analogous  differentiation,  as  has 
already  been  mentioned,  are  found  in  certain  existing  Protozoa 
(Monads,  Vorticella,  etc.).  I  have  attempted  to  shew  in  the 
second  chapter  that  the  breaking  up  of  the  cell  into  spores 


INTRODUCTION.  1 1 


without  previous  conjugation  is  perhaps  provided  against  in  the 
extrusion  of  the  so-called  '  directive  body  '. 

With  the  differentiation  of  special  germinal  cells,  to  take  the 
place  of  the  whole  individual  in  the  act  of  conjugation,  the  pos- 
sibility of  each  act  of  conjugation  resulting  in  the  production  of 
only  a  single  organism  became  introduced.  Germinal  cells  can 
be  indefinitely  produced,  and  the  reproductive  capacity  of  a 
single  individual  is  therefore  unlimited ;  while  if  two  whole 
individuals  conjugated  and  only  produced  one  from  the  process, 
the  result  would  be  a  diminution  instead  of  an  increase  in  the 
race1. 

It  must  be  admitted  that,  in  the  present  state  of  our  know- 
ledge, the  passage  from  reproduction  by  spores  following  con- 
jugation, to  true  sexual  reproduction,  can  only  be  traced  in  a 
very  speculative  manner;  and  that  a  further  advance  in  our 
knowledge  may  prove  that  the  steps  which  I  have  attempted  to 
sketch  out  are  far  from  representing  the  true  origin  of  sexual 
differentiation.  The  peculiar  conjugation  and  fusion  of  two 
individuals  to  form  Diplozoon  paradoxum  may  be  alluded  to  in 
this  connection.  This  fusion  merely  results  in  the  attainment 
of  sexual  maturity  by  the  two  conjugating  individuals.  It  does 
not  appear  to  me  probable  that  this  conjugation  is  in  any  way 
connected  with  the  conjugation  of  the  Protozoa,  but  the  reverse 
must  be  borne  in  mind  as  a  possibility. 

It  is  not  easy  to  decide  whether  the  hermaphrodite  or  the 

1  In  the  vegetable  kingdom  there  are  numerous  types  of  Thallophytes,  which 
throw  a  considerable  amount  of  light  on  the  relation  between  sexual  reproduction  and 
conjugation.  Subjoined  are  a  few  of  the  more  striking  cases.  In  Pandorina  at  the 
time  of  sexual  reproduction  the  cells  which  constitute  a  colony  divide  each  into  sixteen, 
and  the  products  of  their  division  are  set  free.  Pairs  of  them  then  conjugate  and 
permanently  fuse.  After  a  resting  stage  the  protoplasm  is  set  free  from  its  envelope 
after  division  into  two  or  four  parts.  Each  of  these  then  divides  into  sixteen  coherent 
cells  and  constitutes  a  new  Pandorina  colony.  In  CEdogonium  the  fertilization  is 
effected  by  a  spermatozoon  fusing  with  an  oosphere  (ovum).  The  fertilized  oosphere 
(oospore)  then  undergoes  segmentation  like  the  ovum  of  an  animal;  but  the  segments, 
instead  of  uniting  to  form  a  single  organism,  separate  from  each  other,  and  each  of 
them  gives  rise  to  a  fresh  individual  (swarm-spore)  which  grows  into  a  perfect  CEdo- 
gonium. In  Coleochaete  the  impregnation  and  segmentation  take  place  nearly  as  in 
CEdogonium,  but  the  segments  remain  united  together,  acquire  definite  cell  walls,  and 
form  a  single  embryo.  There  is  in  fact  in  Coleochaete  a  true  sexual  reproduction  of 
the  ordinary  type.  (  Vide  S.  H.  Vines  "On  alternation  of  generation  in  the  Thallo- 
phytes." Journal  of  Botany,  Nov.,  1879.) 


12  EMBRYOLOGY. 


dioecious  state  is  the  primitive  one,  or  in  other  words  whether 
the  two  conjugating  cells,  from  which  I  have  supposed  the 
sexual  products  to  originate,  were  derived  in  the  first  instance 
from  one  or  from  two  colonies  of  Protozoa.  On  purely  d  priori 
grounds  it  seems  probable  that  they  were  originally  formed  in 
one  colony,  and  that  their  derivation  from  two  colonies  or 
individuals  was  inaugurated  when  the  spermatozoon  became 
motile.  There  can  be  no  doubt  that  the  dioecious  state  is  a 
very  early  one,  and  that  the  majority  of  existing  cases  of  herma- 
phroditism  are  secondary. 

The  above  considerations  with  reference  to  the  male  and 
female  cells  appear  to  indicate  that  they  were  primitively 
homodynamous ;  a  conclusion  which  is  on  the  whole  borne  out 
by  the  history  of  their  development. 

Although  the  modes  of  reproduction  amongst  the  Metazoa 
have  been  divided  into  the  classes  sexual  and  asexual,  there  is 
nevertheless  one  mode  of  asexual  reproduction  which  ought  to 
be  classified  with  the  sexual  rather  than  with  the  asexual 
modes.  I  mean  parthenogenesis,  which  consists  essentially  in 
the  development  of  the  ovum  into  a  fresh  individual  without 
previous  coalescence  with  the  male  element.  This  mode  of 
reproduction,  which  has  a  very  limited  range  in  the  animal 
kingdom,  being  confined  to  the  Arthropoda  and  Rotifera,  is 
undoubtedly  secondarily  derived  from  sexual  reproduction.  The 
conditions  of  its  occurrence  are  discussed  in  the  second  chapter. 

It  is  remarkable  that  in  certain  cases  the  absence  of  fertiliza- 
tion causes  the  production  of  males  (Bees,  a  Saw-fly,  Nematus 
ventricosus,  etc.);  more  usually  it  results  in  the  production  of 
females  only,  and  there  are  very  often  in  the  Arthropoda  a 
series  of  successive  generations  of  females  all  producing  ova 
which  develope  parthenogenetically  into  females;  eventually 
however,  usually  in  direct  or  indirect  connection  with  a  change 
of  food  or  temperature,  or  other  conditions,  ova  are  formed 
which  give  rise  without  fertilization  both  to  males  and  females. 

The  true  asexual  modes  of  reproduction  amongst  the  Metazoa 
consist  of  fission  and  gemmation.  Gemmation  is  by  far  the  most 
widely  disseminated  of  the  two.  Various  as  are  the  methods  in 
which  it  takes  place,  it  seems  nevertheless  that  cells  derived  from 
all  the  germinal  layers,  and  very  frequently  from  all  the  im- 


INTRODUCTION.  13 


portant  organs  of  the  adult,  assist  in  forming  the  bud.  Into  the 
details  of  the  process,  which  require  in  many  points  a  fuller 
elucidation,  it  is  not  my  purpose  to  enter. 

Gemmation  is  a  far  commoner  occurrence  amongst  the 
simpler  than  amongst  the  more  highly  organised  forms.  It 
appears  to  have  been  superadded  to  the  sexual  mode  of  repro- 
duction quite  independently  in  a  number  of  different  instances. 

While  there  is  no  difficulty  in  understanding  how  gemmation 
may  have  started  in  such  simple  types  as  the  Coelenterata,  the 
manner  in  which  it  first  originated  in  certain  highly  organised 
forms,  as  for  instance  the  Ascidians,  is  somewhat  obscure,  but  it 
seems  probable  that  it  began  with  the  division  of  the  developing 
germ  into  two  or  more  embryos,  at  a  very  early  stage  of  growth. 

Such  a  division  of  the  germ  is,  as  has  been  shewn  by 
Kleinenberg,  normal  in  Lumbricus  trapezoides1  and  Haeckel 
has  shewn  that  an  artificial  division  of  the  germ  in  the  Siphono- 
phora  leads  to  the  development  of  two  individuals.  It  has  been 
pointed  out  by  various  naturalists  that  the  production  of  double 
monsters  is  often  a  phenomenon  of  the  same  nature.  While  it 
is  next  to  impossible  to  understand  how  production  of  a  bud 
could  commence  for  the  first  time  in  the  adult  of  a  highly 
organised  form,  it  is  not  difficult  to  form  a  picture  of  the  steps 
by  which  the  fission  of  the  germ  might  eventually  lead  to  the 
formation  of  buds  in  the  adult  state. 

The  coexistence  of  sexual  reproduction  with  normal  asexual 
multiplication,  or  with  parthenogenesis,  has  led  to  a  remarkable 
phenomenon  in  the  animal  kingdom  known  as  alternations  of 
generations2. 

For  the  details  of  the  various  types  of  alternations  of 
generations,  and  their  origin,  the  reader  is  referred  to  the  body 
of  the  work ;  but  a  few  general  remarks  on  the  nature  and  origin 
of  the  process,  and  on  its  nomenclature,  may  conveniently  be 
introduced  in  this  place.  The  simplest  cases  are  those  in  which 

1  The  case  of  Pyrosoma,  which  might  be  cited  in  this  connection,  is  probably 
secondary. 

2  For  an  excellent  account  of  this  subject,  vide  Allen  Thompson's  article  Ovum  in 
Todd's  Cyclopaedia.     The  metamorphosis  of  the  Echinoderms  included  under  this 
head  in  Thompson's  article  is  now  known  not  to  be  a  proper  case  of  alternations  of 
generations. 


14  EMBRYOLOGY. 


an  individual  which  produces  by  sexual  means  gives  origin  to 
asexual  individuals  differently  organised  to  itself,  which  produce 
by  budding  the  original  sexual  form,  and  so  complete  a  cycle. 
Instances  of  this  kind  are  supplied  by  the  Hydrozoa,  Annelida 
and  Tunicata.  In  the  case  of  the  Tunicata  (Doliolum)  two 
different  asexual  generations  may  be  interpolated  between  the 
sexual  generations.  In  all  these  cases  the  origin  of  the  pheno- 
menon is  easily  understood.  It  appears,  as  is  most  clearly 
shewn  in  the  case  of  the  Annelida,  that  the  ancestors  of  the 
species  which  now  exhibit  alternations  of  generations  originally 
reproduced  themselves  at  the  same  time  both  sexually  and  by 
budding,  though  probably  the  two  modes  of  reproduction  did 
not  take  place  at  the  same  season.  Gradually  a  differentiation 
became  established,  by  which  sexual  reproduction  was  confined 
to  certain  individuals,  which  in  most  instances  did  not  also 
reproduce  asexually.  After  the  two  modes  of  reproduction 
became  confined  to  separate  individuals,  the  dissimilarity  in 
habits  of  life  necessitated  by  their  diverse  functions  caused  a 
difference  in  their  organization ;  and  thus  a  complete  alter- 
nation of  generations  became  established.  The  above  is  no 
merely  speculative  history,  since  all  gradations  between  com- 
plete alternations  of  generations  and  simple  budding  combined 
with  sexual  reproduction  can  be  traced  in  actually  existing  forms. 

The  alternation  of  generations  as  it  is  found  amongst  the 
Entoparasitic  Trematodes  and  most  Cestodes,  is  to  be  explained 
in  a  slightly  different  way. 

It  appears  that  in  these  parasitic  forms  a  complicated  meta- 
morphosis first  arose  from  the  parasite  having  to  accommodate 
itself  to  the  different  hosts  it  was  compelled  to  inhabit,  owing  to 
the  liability  of  its  primitive  and  subsequent  hosts  to  be  devoured1. 
A  capacity  for  asexual  multiplication — obviously  of  immense 
advantage  to  a  parasite — appears  to  have  been  acquired  in  some 
of  the  stages  of  this  metamorphosis,  and  an  alternation  of 
generations  thus  established. 

1  The  appearance  of  Vertebrata  on  the  globe  as  the  forms  which  most  frequently 
preyed  on  Invertebrate  forms,  and  were  themselves  not  so  liable  to  be  devoured,  has 
no  doubt  had  a  great  influence  on  the  metamorphosis  of  internal  parasites,  and  has 
amongst  other  things  resulted  in  these  parasites  usually  reaching  their  sexual  state  in 
a  vertebrate  host. 


INTRODUCTION.  1 5 


A  nearly  parallel  series  to  that  exhibiting  alternations  of 
sexual  generations  with  generations  which  produce  by  budding 
is  supplied  by  the  cases  where  sexual  generations  alternate  with 
parthenogenetic  ones,  or  in  some  instances  even  with  larvae 
which  reproduce  sexually  or  else  parthenogenetically. 

The  best  known  examples  of  this  form  of  alternations  of 
generations  are  found  amongst  the  Insecta1.  A  simple  case 
is  that  of  the  Aphides.  The  ova  deposited  by  impregnated 
females  give  rise  to  forms  differently  organised  to  the  parents 
but  provided  with  an  ovary2.  The  eggs  from  the  ovary  develope 
parthenogenetically  within  the  oviduct,  and  so  long  as  there 
is  plenty  of  food  and  warmth  the  generations  produced  are 
always  parthenogenetic  forms.  The  failure  of  warmth  and 
nutriment  causes  the  production  of  true  males  and  females,  and 
so  the  cycle  is  completed.  We  must  suppose  that  the  capacity 
possessed  by  so  many  female  insects  of  producing  eggs  capable 
of  developing  without  the  influence  of  the  male  element,  has 
been,  so  to  speak,  taken  hold  of  by  natural  selection,  and  has  led 
to  the  production  of  viviparous  parthenogenetic  forms,  by  which, 
so  long  as  food  is  abundant,  a  clear  economy  in  reproduction  is 
effected.  The  continuance  of  the  species  during  winter  is  secured 
by  the  production  of  males  and  females,  the  females  laying  eggs 
in  autumn  which  are  hatched  in  the  spring. 

In  Chermes  there  is  less  modification  of  the  primitive  condi- 
tion in  that  the  parthenogenetic  generations  lay  their  eggs  like 
the  impregnated  females.  In  the  gall-flies  (Cynipidae),  there  is 
frequently  an  alternation  of  generations  of  the  same  kind  as  in 
Chermes ;  there  being  no  viviparous  forms.  The  individuals  of 
the  different  generations  differ  from  each  other  to  some  extent 
in  all  these  cases. 

A  second  type  of  alternations  of  parthenogenetic  and  sexual 
generations  is  exemplified  by  the  cases  of  Chironomus  and 
Cecidomyia,  where  the  larva  which  develope  from  the  eggs  of 
the  fertilized  female  produce  parthenogenetically,  by  means  of 
true  ova,  forms  which  eventually  after  several  generations  (Ceci- 
domyia) of  larval  reproduction  give  rise  to  sexual  forms.  The 

1  For  details  vide  Chapter  on  Insecta. 

2  The  distinction  drawn  by  Huxley  between  ova  and  pseudova  does  not  appear  to 
me  a  convenient  one  in  practice. 


1 6  EMBRYOLOGY. 


explanation  is  here  practically  the  same  as  in  the  case  of  Aphis, 
and  is  paralleled  in  the  gemmiparous  series  by  the  production  of 
buds  in  the  larval  forms  of  Trematodes,  etc.  A  very  similar 
occurrence  takes  place  in  Ascaris  nigrovenosa  (vide  chapter  on 
Nematoidea),  except  that  larval  forms,  which  carry  on  reproduc- 
tion and  then  perish  without  developing  farther,  do  so  by  a  true 
sexual  process.  Thus  there  is  an  alternation  of  generations  of 
adult  and  larval  sexual  forms.  The  Axolotl  is  an  intermittent 
example  of  the  same  phenomenon. 

As  might  be  anticipated  from  the  mode  in  which  alternations 
of  generations  have  become  established,  incomplete  approxi- 
mations to  it  are  not  uncommon.  Such  approximations  are 
especially  found  in  the  Arthropoda,  where  alternations  of  sexual 
and  parthenogenetic  generations  frequently  take  place,  in  which 
the  individuals  of  different  generations  are  similarly  organised 
(Psychidae,  Apus,  &c.).  Another  approximation  is  afforded  by 
the  parthenogenetic  winter  eggs  of  Leptodora  amongst  the 
Phyllopods,  which  give  rise  to  Nauplius  larvae,  while  the  young 
hatched  from  the  summer  eggs  do  not  pass  through  a  meta- 
morphosis. Numerous  transitional  cases  are  also  found  amongst 
the  forms  in  which  there  is  an  alternation  of  sexual  and  gemmi- 
parous generations. 

The  whole  of  the  cases  to  which  allusion  has  been  made  in 
this  section  may  be  conveniently  classed  under  the  term  alterna- 
tions of  generations,  but  the  cases  of  alternation  of  two  sexual 
generations,  and  of  sexual  and  parthenogenetic  generations, 
are  classified  by  Leuckart,  Claus,  etc.  as  cases  of  heterogeny, 
which  they  oppose  to  the  other  form  of  alternation  of  genera- 
tions. If  special  terms  are  to  be  adopted  for  the  two  kinds  of 
alternation  of  generations,  it  would  be  perhaps  convenient  to 
classify  the  cases  of  alternations  of  sexual  and  gemmiparous 
generations  under  the  term  metagenesis,  and  to  employ  the 
term  heterogamy  for  the  cases  of  alternation  of  sexual  and 
parthenogenetic  generations. 

The  term  Nurse  (German  Amme),  employed  for  the  asexual 
generations  in  metagenesis,  may  advantageously  be  dropped 
altogether. 


CHAPTER    I. 


THE   OVUM   AND   SPERMATOZOON. 


THE  OVUM. 

THE  complete  developmental  history  of  any  being  constitutes 
a  cycle.  It  is  therefore  permissible  in  treating  of  this  history  to 
begin  at  any  point.  As  a  matter  of  convenience  the  ovum  ap- 
pears to  be  the  most  suitable  point  of  departure.  The  question 
as  to  the  germinal  layer  from  which  it  is  ultimately  derived  is 
dealt  with  in  a  subsequent  part  of  the  work ;  the  present  chapter 
deals  with  its  origin  and  growth. 


General  History  of  the  Ovum. 

Every  young  ovum  (fig.  i)  has  the  cha- 
racter of  a  simple  cell.  It  is  formed  of  a 
mass  of  naked  protoplasm  (a\  containing 
in  its  interior  a  nucleus  (b),  within  which 
there  is  a  nucleolus  (<;).  The  nucleus  and 
nucleolus  are  usually  known  as  the  ger- 
minal vesicle  and  germinal  spot. 

The  ovum  so  constituted  is  developed 
either  (i)  from  one  cell  out  of  an  aggrega- 
tion or  layer  of  cells  all  of  which  have  the 
capacity  of  becoming  ova ;  or  (2)  from  one 
out  of  a  number  of  cells  segmented  off 
from  a  polynuclear  mass  of  protoplasm,  not  divided  into  sepa- 
rate cells.  In  both  cases  the  cells  which  have  the  capacity  of 
becoming  ova  may  be  spoken  of  as  germinal  cells,  and  in  the 
case  where  the  ova  are  ultimately  developed  from  a  poly- 

B.  II.  2 


FIG.  i.  DIAGRAM  OF 
THE  OVUM.  (From  Ge- 
genbaur.) 

a.  Granular  proto- 
plasm, b.  Nucleus  (ger- 
minal vesicle),  c.  Nu- 
cleolus (germinal  spot). 


1 8  GENERAL  HISTORY. 


nuclear  mass  of  protoplasm  the  latter  structure  may  be  called  a 
germogen. 

In  some  cases  the  whole  of  the  germinal  cells  eventually 
become  ova,  but  as  a  rule  only  a  small  proportion  of  them  have 
this  fate,  the  remainder  undergoing  various  changes  to  be  spoken 
of  in  the  sequel. 

Extended  investigations  have  shewn  that  the  distinction 
between  germinal  cells  which  are  independent  cells  from  the 
first,  or  derived  from  a  germogen  in  which  the  nucleated  proto- 
plasm is  not  divided  into  cells,  is  an  unimportant  one ;  and 
closely  allied  forms  may  differ  in  this  respect.  It  is  moreover 
probable  that  a  germogen  of  nucleated  protoplasm  is  less  com- 
mon than  is  often  supposed  :  it  being  a  matter  of  great  difficulty 
to  determine  the  structure  of  the  organs  usually  so  described. 
A  germogen  is  stated  to  be  found  in  most  Platyelminthes, 
Nematoidea,  Discophora,  Insecta,  and  Crustacea. 

A  more  important  distinction  in  the  origin  of  the  germinal 
cells  is  that  afforded  by  their  position.  In  this  respect  three 
groups  may  be  distinguished,  (i)  The  germinal  cells  may  form 
the  lining  of  a  sack  or  tube,  having  the  form  of  a  syncytium  or 
of  an  epithelium  of  separate  cells  (Platyelminthes,  Mollusca,  Ro- 
tifera,  Echinodermata,  Nematoidea,  Arthropoda).  (2)  Or  they 
may  form  a  specialized  part  of  the  epithelium  lining  the  general 
body  cavity  (Chaetopoda,  Gephyrea,  Vertebrata).  (3)  Or  they 
may  form  a  mass  placed  between  the  two  elsewhere  contiguous 
primitive  germinal  layers  (Ccelenterata1). 

Types  of  transition  between  the  first  and  second  group  are 
not  uncommon.  Such  types,  properly  belonging  to  the  second 
group,  originate  by  a  special  membranous  sack  continuous  with 
the  oviduct  being  formed  round  the  primitively  free  patch  of 
germinal  cells.  Examples  of  this  are  afforded  by  the  Discophora, 
the  Teleostei,  etc.  It  is  very  probable  that  all  the  cases  which 
fall  under  the  first  heading  may  have  been  derived  from  types 
which  belonged  to  the  second  group. 

The  mode  of  conversion  of  the  germinal  cells  into  ova  is 
somewhat  diverse.  Before  the  change  takes  place  the  germinal 

1  In  nil  the  Metazoa  the  generative  organs  are  placed  between  the  primitive 
germinal  layers;  and  the  peculiarity  of  their  position  in  the  Ccelenterata  depends  on 
the  absence  of  a  body  cavity  and  of  a  distinct  mesoblast. 


THE  OVUM. 


FIG.  2.  OVUM  OF 
CARMARINA  (GERYO- 

NIA)  HASTATA.     (Copied 

from  Haeckel.) 

gd.  Body  of  ovum. 
gv.  Germinal  vesicle. 
gni.  Germinal  spot. 


cells  frequently  multiply  by  division.  The 
change  itself  usually  involves  a  considerable 
enlargement  of  the  germinal  cell,  and  gene- 
rally a  change  in  the  character  of  the  ger- 
minal vesicle,  which  in  most  young  ova 
(fig.  2)  is  very  large  as  compared  to  the 
body  of  the  ovum.  The  most  complicated 
history  of  this  kind  is  that  of  the  ovum  of 
the  Craniata.  (Vide  pp.  56,  57.) 

The  ovum  in  its  young  condition  is  obviously  nothing  but  a 
simple  cell ;  and  such  it  remains  till  the  period  when  it  attains 
maturity. 

Nevertheless  the  changes  which  it  undergoes  in  the  course  of 
its  growth  are  of  a  very  peculiar  kind,  and,  consisting  as  they  do 
in  many  instances  of  the  absorption  of  other  cells,  have  led 
various  biologists  to  hold  that  the  ovum  is  a  compound  struc- 
ture. It  becomes  therefore  necessary  to  consider  the  processes 
by  which  the  growth  and  nutrition  of  the  ovum  is  effected 
before  dealing  with  the  structure  of  the  ovum  at  all  periods  of 
its  history. 

The  ovum  is  of  course  nourished  like 
every  other  cell  by  the  nutritive  fluids  in 
which  it  is  surrounded,  and  special  provi- 
sions are  made  for  this,  in  that  the  ovary  is 
very  frequently  placed  in  contiguity  with 
vascular  channels.  But  in  addition  to  such 
nutrition  a  further  nutrition,  the  details  of 
which  are  given  in  the  special  part  of  this 
chapter,  is  provided  for  in  the  germinal 
cells  which  do  not  become  ova. 

In  the  simplest  case,  as  in  many  Hy- 
drozoa  (fig.  3),  the  germinal  cells  which  do 
not  become  ova  are  assimilated  by  the 
ovum  much  in  the  manner  of  an  Amoeba. 

In  other  cases  the  ovum  becomes  in- 
vested by  a  special  layer  of  cells,  which 
then  constitutes  what  is  known  as  a  fol- 
licle. The  cells  which  form  the  follicle  are 
often  germinal  cells,  e.g.  Holothuria,  Insecta  (fig.  17),  Vertebrata 

2 — 2 


FIG.     3.       FEMALE 

GONOPHORE  OF  TUBU- 
LARIA  MESEMBRYAN- 

THEMUM.  CONTAINING 

ONE  LARGE  OVUM  (ov) 
AND  A  NUMBER  OF  GER- 
MINAL CELLS  (g.C.). 

ep.  Epiblast  (Ecto- 
derm). Ay.  Hypoblast 
(Entoderm).  ov.  Ovum. 
g.c.  Germinal  cells. 


20  GENERAL   HISTORY. 


(fig-  !9)'  In  other  cases  they  seem  rather  to  be  adjoining  con- 
nective-tissue or  epithelioid  cells,  though  it  is  sometimes  difficult 
to  draw  the  line  between  such  cells  and  germinal  cells.  Ex- 
amples of  follicles  formed  of  ordinary  connective-tissue  cells, 
are  supplied  by  Asterias,  Bonellia  (fig.  16),  Cephalopoda  (fig. 
14),  etc. 

A  membrane  enclosing  the  ovum  without  a  lining  of  cells,  as  in  many 
Arachnida,  vide  p.  51,  has  no  true  analogy  with  a  follicle  and  does  not 
deserve  the  same  name. 

The  function  of  the  follicle  cells  appears  to  be,  to  elaborate 
nutriment  for  the  growth  of  the  ovum.  The  follicle  cells  are  not 
as  a  rule  directly  absorbed  into  the  body  of  the  ovum,  though 
in  some  instances,  as  in  Sepia  (vide  p.  40),  they  are  eventually 
assimilated  in  this  way. 

In  many  cases  some  of  the  germinal  cells  form  a  follicle, 
while  other  germinal  cells  form  a  mass  within  the  follicle 
destined  eventually  to  be  used  as  pabulum.  Insects  supply 
the  best  known  examples  of  this,  but  Piscicola,  Bonellia  (?)  may 
also  be  cited  as  examples  of  the  same  character.  In  the  Cra- 
niata  (pp.  56 — 58)  some  of  the  germinal  cells  which  advance  a 
certain  distance  on  the  road  towards  becoming  ova,  are  even- 
tually used  as  pabulum,  before  the  formation  of  the  follicle ; 
while  other  germinal  cells  form  at  a  later  period  the  follicular 
epithelium.  A  peculiar  case  is  that  of  the  Platyelminthes  (fig.  9), 
where  a  kind  of  follicle  is  constituted  by  the  cells  of  a  specially 
differentiated  part  of  the  ovary,  known  as  the  yolk-gland.  The 
cells  of  this  follicle  may  either  remain  distinct,  and  continue  to 
surround  the  ovum  after  its  development  has  commenced,  and 
so  be  used  as  food  by  the  embryo ;  or  they  may  secrete  yolk 
particles,  which  enter  directly  into  the  protoplasm  of  the  ovum. 

For  further  variations  in  the  mode  of  nutrition  the  reader  is 
referred  to  the  special  part  of  this  chapter.  Suffice  it  to  say 
that  none  of  the  known  modes  of  nutrition  indicate  that  the 
ovum  becomes  a  compound  body  any  more  than  the  fact  of  an 
Amoeba  feeding  on  another  Amoeba  would  imply  that  the  first 
Amoeba  ceased  thereby  to  be  a  unicellular  organism. 

The  constitution  of  the  ovum  may  be  considered  under  three 
heads  :— 


THE  OVUM. 


21 


(1)  The  body  of  the  ovum. 

(2)  The  nucleus  or  germinal  vesicle. 

(3)  The  investing  membranes. 

The  body  of  the  ovum.  The  essential  constituent  of  the 
body  of  the  ovum  is  an  active  living  protoplasm.  As  a  rule 
there  are  present  certain  extraneous  matters  in  addition,  which 
have  not  the  vital  properties  of  protoplasm.  The  most  impor- 
tant of  these  is  known  as  food-yolk,  which  appears  to  be 
generally  composed  of  an  albuminoid  matter. 

The  body  of  the  ovum  is  at  first  very  small  compared  with 
the  germinal  vesicle,  but  continually  increases  as  the  ovum 
approaches  towards  maturity.  It  is  at  first  comparatively  free 
from  food-yolk ;  but,  except  in  the  rare  instances  where  it  is 
almost  absent,  food-yolk  becomes  deposited  in  the  form  of 
granules,  or  highly  refracting  spheres,  by  the  inherent  activity 
of  the  protoplasm  during  the  later  stages  in  the  ripening  of 
the  ovum.  In  many  instances  the  protoplasm  of  the  ovum 
assumes  a  sponge-like  or  reticulate  arrangement,  a  fluid  yolk 
substance  being  placed  in  the  meshes  of  the  reticulum.  The 
character  of  the  food-yolk  varies  greatly.  Many  of  its  chief 
modifications  are  described  below.  There  is  not  unfrequently 
present  in  the  vitellus  a  peculiar  body  known  as  the  yolk 


FIG.  4.    A.  OVUM  OF  HYDRA  IN  THE  AMCEBOID  STATE,  WITH  YOLK  SPHERULES 

(PSEUDOCELLS)  AND  CHLOROPHYLL  GRANULES.     (After  Kleinenberg. ) 
gv.  Germinal  vesicle. 

B.  SINGLE  PSEUDOCELL  OF  HYDRA. 


22 


GENERAL   HISTORY. 


nucleus,  which  is  very  possibly  connected  with  the  formation  of 
the  food-yolk.  It  is  found  in  many  Arachnida,  Myriapoda, 
Amphibia,  etc.1 

More  important  for  the  subsequent  development  than  the 
variation  in  the  character  of  the  food-yolk  is  its  amount  and 
distribution.  In  a  large  number  of  forms  it  is  distributed  un- 
sym metrically,  the  yolk  being  especially  concentrated  at  one 
pole  of  the  ovum,  the  germinal  vesicle,  surrounded  by  a  special 
layer  of  protoplasm  comparatively  free  from  food-yolk,  being 
placed  at  the  opposite  pole.  In  the  Arthropoda  it  has  in  most 
instances  a  symmetrical  distribution.  Further  details  on  this 
subject  are  given  in  connection  with  the  segmentation ;  the 
character  of  which  is  greatly  influenced  by  the  distribution  of 
food-yolk. 

The  body  of  the  ovum  is  usually  spherical,  but  during  a 
period  in  its  development  it  not  unfrequently  exhibits  a  very 
irregular  amoeboid  form,  e.g.  Hydra  (fig.  4),  Halisarca. 

The  germinal  vesicle.  The 
germinal  vesicle  exhibits  all  the 
essential  characters  of  a  nucleus. 
It  has  a  more  or  less  spherical 
shape,  and  is  enveloped  by  a  distinct 
membrane  which  seems,  however, 
in  the  living  state  to  be  very  often 
of  a  viscous  semi-fluid  nature  and 
only  to  be  hardened  into  a  mem- 
brane by  the  action  of  reagents 
(Fol).  The  contents  of  the  germi- 
nal vesicle  are  for  the  most  part 
fluid,  but  may  be  more  or  less 
granular.  Their  most  characteris- 
tic components  are,  however,  a  protoplasmic  network  and  the 
germinal  spots8.  The  protoplasmic  network  stretches  from  the 
germinal  spots  to  the  investing  membrane,  but  is  especially 
concentrated  round  the  former.  (Fig.  5.)  The  germinal  spot 

1  For  details  on  the  yolk  nucleus  vide  Balbiani,  Lemons  s.  /.  Gtntration  d.  Vertchrcs. 
Paris,  1879.  In  this  work  the  author  maintains  very  peculiar  views  on  the  nature  and 
function  of  the  yolk  nucleus,  which  do  not  appear  to  me  well  founded. 

a  In  the  germinal  vesicles  of  very  young  ova  the  reticulum  is  often  absent. 


FIG.  5.  UNRIPE  OVUM  OK 
TOXOPNEUSTES  LiviDUS.  (Copied 
from  Hertwig.) 


THE   OVUM.  23 


forms  a  nearly  homogeneous  body,  with  frequently  one  or  more 
vacuoles.  It  often  occupies  an  eccentric  position  within  the 
germinal  vesicle,  and  is  usually  rendered  very  conspicuous  by  its 
high  refrangibility.  In  many  instances  it  has  been  shewn  to  be 
capable  of  amoeboid  movements  (Hertwig,  Eimer),  and  is  more- 
over more  solid  and  more  strongly  tinged  by  colouring  reagents 
than  the  remaining  constituents  of  the  germinal  vesicle. 

In  many  instances  there  is  only  one  germinal  spot,  or  else 
one  main  spot  and  two  or  three  accessory  smaller  spots.  In 
other  cases,  e.g.  Osseous  Fishes,  Echinaster  fallax,  Eucope  poly- 
styla,  there  are  a  large  number  of  nearly  equal  germinal  spots 
which  appear  to  result  from  the  division  or  endogenous  prolifera- 
tion of  the  original  spot  Sometimes  the  germinal  spots  are 
placed  immediately  within  the  membrane  of  the  germinal  vesicle 
(Elasmobranchii  and  Sagitta).  In  many  Lamellibranchiata,  in 
the  earth-worm,  and  in  many  Chsetopoda  the  components  of  the 
germinal  spot  become  separated  into  two  nearly  spherical 
masses  (fig.  12),  which  remain  in  contiguity  along  a  small  part 
of  their  circumference,  and  are  firmly  united  together.  The 
smaller  of  the  two  parts  is  more  highly  refractive  than  the 
larger.  Hertwig  has  shewn  that  the  germinal  spot  is  often 
composed  of  two  constituents  as  in  the  above  cases,  but  that  the 
more  highly  refractive  material  is  generally  completely  enclosed 
by  the  less  dense  substance.  By  Fol  the  germinal  spot  is  stated 
to  be  absent  in  a  species  of  Sagitta,  but  this  must  be  regarded 
as  doubtful.  In  young  ova  the  relative  size  of  the  germinal 
vesicle  is  very  considerable.  It  occupies  in  the  first  instance  a 
central  position  in  the  ovum,  but  at  maturity  is  almost  always 
found  in  close  proximity  to  the  surface.  Its  change  of  position 
in  a  large  number  of  instances  is  accomplished  during  the 
growth  of  the  ovum  in  the  ovary,  but  in  other  cases  does  not 
take  place  till  the  ovum  has  been  laid. 

As  the  ovum  attains  maturity,  important  changes  take  place 
in  the  constitution  of  the  germinal  vesicle,  which  are  described 
in  the  next  chapter. 

The  egg-membranes.  A  certain  number  of  ova  when 
ready  to  be  fertilized  are  naked  cells  devoid  of  any  form  of 
protecting  covering,  but  as  a  rule  the  ovum  is  invested  by  some 
form  of  membrane.  Such  coverings  present  great  variety  in 


GENERAL    HISTORY. 


FIG.   6.     OVUM   OF    Toxo- 

PNEUSTES  VARIEGATUS  WITH 
THE  PSEUDOPODIA-LIKE  PRO- 
CESSES OF  THE  PROTOPLASM 
PENETRATING  THE  ZONA  RADI- 

ATA  (zr).     (After  Selenka.) 


their  character  and  origin,  and  may  be  conveniently  (Ludwig, 
No.  4)  divided  into  two  great  groups,  viz.  (i)  those  derived  from 
the  protoplasm  of  the  ovum  itself  or  from  its  follicle,  which  may 
be  called  primary  egg-membranes;  and  (2)  those  formed  by 
the  wall  of  the  oviduct  or  otherwise,  such  as  the  egg-shell  of  a 
bird,  which  may  be  called  secondary 
egg-membranes. 

The  primary  egg-membranes  may 
again  be  divided  into  two  groups 
(Ed.  van  Beneden,  No.  1),  viz.,  (i) 
those  formed  by  the  protoplasm  of 
the  ovum,  to  which  the  name  vitel- 
line  membranes  will  be  applied  ; 
and  (2)  those  formed  by  the  cells  of 
the  follicle,  to  which  the  name 
chorion  will  be  applied. 

The  secondary  egg-membranes 
will  be  dealt  with  in  connection  with 
the  systematic  account  of  the  develop- 
ment of  the  various  groups.  They 
coexist  as  a  rule  with  primary  membranes,  though  in  some 
types  (Cephalophorous  Mollusca,  many  Platyelminthes,  etc.), 
they  constitute  the  only  protecting  coverings  of  the  ovum. 

The  vitelline  membranes  are  either  simple  structureless 
membranes  or  present  numerous  radial  pores.  Membranes 
with  the  latter  structure  are  very  widely  distributed,  Echino- 
dermata,  Gephyrea,  Vertebrata,  etc.  (  Vide  figs.  5  and  7.)  The 
function  of  the  pores  appears  to  be  a  nutritive  one.  They  either 
serve  for  the  emission  of  pseudopodia-like  processes  of  the  pro- 
toplasm of  the  ovum,  as  has  been  very  beautifully  shewn  in  the 
case  of  Toxopneustes  by  Selenka  (fig.  6),  or  they  admit  (?)  pro- 
cesses of  the  follicular  epithelial  cells  (Vertebrata),  Their 
presence  is  in  fact  probably  caused  by  the  existence  of  such 
processes,  which  prevent  the  continuous  deposition  of  the  mem- 
brane. The  term  zona  radiata  will  be  applied  to  perforated 
membranes  of  this  kind.  Two  vitelline  membranes,  one  per- 
forated and  the  other  homogeneous,  may  coexist  at  the  same 
time,  e.g.  Sipunculida,  Vertebrata.  (Fig.  7.) 

The  chorion  is  often  ornamented  with  various  processes,  etc. 


THE  OVUM. 


FIG.  7.  SECTION 
THROUGH  A  SMALL 
PART  OF  THE  SURFACE 
OF  AN  OVUM  OF  AN 


It  is  in  many  cases  doubtful  whether  a  par- 
ticular membrane  is  a  chorion  or  a  vitelline 
membrane. 

All  the  membranes  which  surround  the 
ovum  may  be  provided  with  a  special  aper- 
ture known  as  the  micropyle.  A  micropyle 
is  by  no  means  found  in  the  majority  of 
types,  and  there  is  no  homology  between 
the  various  apertures  so  named.  Micropyles 
have  two  functions,  either  (i)  to  assist  in  the 
nutrition  of  the  ovum  during  its  develop- 
ment, or  (2)  to  permit  the  entrance  of  the 
spermatozoa.  The  two  functions  may  in 
some  cases  coexist.  Micropyles  of  the  first 
class  are  developed  at  the  point  of  attach- 
ment of  the  ovum  to  the  wall  of  the  ovary  or  to  its  follicle. 
Good  examples  of  this  kind  of  micropyle  are  afforded  by  the 
Lamellibranchiata  (fig.  12),  Holothuria,  and  many  Annelida 
(Polynoe,  etc.).  The  micropyle  of  the  Lamellibranchiata  (p.  37) 
probably  serves  also  to  admit  the  spermatozoa.  The  second 
type  of  micropyle  is  found  in  many  Insecta,  Teleostei,  etc. 


fe.  Follicular  epi- 
thelium, vt.  Vitelline 
membrane.  Zn.  Zona 
radiata.  yk.  Yolk  with 
protoplasmic  network. 


GENERAL  BIBLIOGRAPHY  OF  THE  OVUM. 

(1)  Ed.  van  Ben ed en.     "  Recherches  sur  la  composition  et  la  signification  de 
1'ceuf,"  etc.     Mem.  cour.  d.  VAcad.  roy.  des  Sciences  de  Belgique,  Vol.  xxxiv.  1870. 

(2)  R.  Leuckart.     Artikel  '« Zeugung,"  R.  Wagner's  Handworterbuch  d.  Physio- 
logie,  Vol.  iv.  1853. 

(3)  Fr.  Leydig.     "  Die  Dotterfurchung  nach  ihrem  Vorkommen  in  d.  Thierwelt 
u.  n.  ihrer  Bedeutung."     Oken.  Isis,  1848. 

(4)  Ludwig.     "  Ueber  d.  Eibildung  im  Thierreiche."     Arbeiten  a.  d.  zooL-zoot. 
Instilut  Wiirzburg,  Vol.  i.  I8741. 

(5)  Allen  Thomson.     Article  "  Ovum"  in  Todd's  Cyclopaedia  of  Anatomy  and 
Physiology,  Vol.  v.  1859. 

(6)  W.  Waldeyer.    Eierstock  u.  Ei.    Leipzig,  1870. 


1  A  very  complete  and  critical  account  of  the  literature  is  contained  in  this  paper. 


26 


CCELENTERATA. 


Special  History  of  tJie  Ovum  tn  different  types. 
CCELENTERATA. 

(7)  Ed.  van  Beneden.     "  De  la  distinction  origiuelle  d.  testicule  et  fie  1'ovaire." 
Bull.  Accui.  roy.  Belgique,  3*  se'rie,  Vol.  xxxvn.  1874. 

(8)  R.  and  O.  Hertwig.     Der  Organismus  d.  Meduscn.     Jena,  1878. 

(9)  N.  Kleinenberg.     Hydra.     Leipzig,  1872. 

Amongst  the  Ccelenterata  the  ova  are  developed  in  imper- 
fectly specialized  organs,  which  are  situated  in  various  parts  of 
the  body,  for  the  most  part  in  the  space  between  the  epiblast 
and  the  hypoblast. 

In  Hydra  the  locality  where  the  ova  are  developed  only 
becomes  specialized  at  the  time  when  an  ovum  is  about  to  be 
formed.  At  one  or  more  points  the  interstitial  cells  of  the 
epiblast  increase  in  number  and  form  a  protuberance  of  germinal 
cells,  which  may  be  called  the  ovary.  In  this  ovary  a  single 
ovum  is  formed  by  the  special  growth  of  one  cell.  (Kleinenberg, 
No.  9.)  In  the  free  and  attached  gonophores  of  Hydrozoa,  the 
ova  appear  either  around  the  walls  of 
the  stomach,  or  the  radial  canals,  or 
around  other  parts  of  the  gastro-vas- 
cular  canals. 

Their  close  relations  to  the  gastro- 
vascular  canals  are  probably  determin- 
ed by  the  greater  nutritive  facilities 
thereby  afforded.  (Hertwig,  No.  8.) 

In  the  permanent  Medusa-forms 
the  ova  have  similar  relations  to  the 
gastro- vascular  system.  Amongst 
the  Actinozoa  the  ova  are  usually 
developed  between  the  epiblast  and 
the  hypoblast  in  the  walls  of  the 
gastric  mesenteries.  Amongst  the 
Ctenophora  the  ova  are  situated  in 
close  relation  with  the  peripheral 
canals  of  the  gastro-vascular  system, 
which  run  along  the  bases  of  the 
ciliated  bands.  There  are  many  ex- 


1'ic.    H.     RIPE   OVUM    OF 

EriBULIA    AURANTIACA.       THE 

GERMINAL  VESICLE  HAS  BE- 
COME INVISIBLE  WITHOUT  RE- 
AGENTS. 

Copied  from  Metschnikoff, 
"  Entwicklung  dcr  Siphonopho- 
ren."  Ztitschrift  f.  tciss.  Zool., 
Vol.  xxiv.  1874. 

p.d.  Peripheral  layer  of  den- 
ser protoplasm,  p.m.  Central 
area  consisting  of  a  protoplasmic 
mesh  work. 


THE   OVUM.  27 


amples  amongst  the  Ccelenterata  of  ova  which  retain  in  their 
mature  state  the  very  simple  constitution  which  has  been  de- 
scribed as  characteristic  of  all  young  ova ;  and  which  are,  when 
laid,  absolutely  without  any  trace  of  a  vitelline  membrane  or 
chorion.  In  many  other  cases  both  amongst  the  Medusae,  the 
Siphonophora,  and  the  Ctenophora,  the  ripe  egg  exhibits  a  dis- 
tinction into  two  parts.  The  outer  part  is  composed  of  a  dense 
protoplasm,  while  the  interior  is  composed  of  a  network  or  more 
properly  a  spongework  of  protoplasm  enclosing  in  its  meshes 
a  more  fluid  substance.  (Fig.  8.) 

In  some  cases  the  ovum  while  still  retaining  the  constitution  last 
described  becomes  invested  by  a  very  delicate  membrane.  Such  is  the 
constitution  of  the  ripe  ovum  of  Hippopodius  gleba  amongst  the  Siphono- 
phora1 and  of  the  eggs  of  Geryonia  amongst  the  permanent  Medusae2. 
The  ripe  eggs  of  the  Ctenophora  usually  present  a  similar  structure3. 
After  being  laid  they  are  found  to  be  invested  by  a  delicate  membrane 
separated  by  a  space  filled  with  fluid  from  the  body  of  the  ovum.  The 
latter  is  composed  of  two  layers,  an  outer  one  of  finely  granular  protoplasm 
and  an  inner  layer  consisting  of  a  protoplasmic  spongework  containing  in 
its  meshes  irregular  spheres.  These  latter  are  stated  by  Agassiz  to  be  of  a 
fatty  nature,  and  it  is  probable  that  in  most  cases  where  a  protoplasmic  net- 
work is  present,  this  alone  constitutes  the  active  protoplasm  ;  and  that  the 
substance  which  fills  up  its  meshes  is  to  be  looked  on  as  a  form  of  food -yolk 
or  deutoplasm,  though  it  appears  sometimes  to  have  the  power  of  assimila- 
ting the  firmer  yolk  particles. 

The  membrane  which  invests  the  ovum  of  many  of  the 
Ccelenterata  is  probably  a  vitelline  membrane. 

The  ova  of  the  Hydrozoa  take  their  origin,  in  most  groups 
at  any  rate4,  from  the  deeper  layer  of  the  epiblast  (interstitial 
layer  of  Kleinenberg).  The  interstitial  cells  in  the  ovarian 
region  form  primary  germinal  cells,  and  by  an  excess  of 
nutrition  certain  of  them  outstrip  their  fellows  and  become 
young  ova.  Such  ova  differ  from  the  full-grown  ova  already 

1  Metschnikoff.     Zeitschrift /.  wiss.  Zoologie,  Vol.  xxiv.  1874. 

2  Herman  Fol.    Jenaische  Zeitschrift,  Vol.  vil. 

3  Kowalevsky.     ' '  Entwicklungsgeschichte    d.    Rippenquallen."     Memoire   de 
FAcad.   Petersburg,    1866.     And  Alex.   Agassiz.     "Embryology  of  the  Cteno- 
phorEe."     Amer.  Acad.  of  Science  and  Arts,  Vol.  x.  No.  in. 

4  The  view  of  van  Beneden,  according  to  which  the  ova  have  an  endodermal 
(hypoblastic)  origin,  has  been  shewn  to  be  at  any  rate  confined  to  certain  groups. 
The  whole  question  of  the  origin  of  the  generative  products  from  the  germinal  layers 
in  the  Ccelenterata  is  still  involved  in  great  obscurity. 


28  CCELENTERATA. 


described,  mainly  in  the  fact  that  they  have  a  proportionately 
smaller  amount  of  protoplasm  round  the  germinal  vesicle. 
They  grow  to  a  considerable  extent  at  the  expense  of  germinal 
cells  which  do  not  become  converted  into  ova. 

The  ova  of  many  Ccelenterata  undergo  changes  of  a  more 
complicated  kind  before  attaining  their  full  development.  Of 
these  ova  that  of  Hydra  may  be  taken  as  the  type.  The  ovary 
of  Hydra  (Kleinenberg,  No.  9)  is  constituted  of  angular  flattish 
germinal  cells  of  which  no  single  one  can  be  at  first  dis- 
tinguished from  the  remainder.  As  growth  proceeds  one  of  the 
cells  occupying  a  central  position  becomes  distinguished  from 
the  remaining  cells  by  its  greater  size,  and  wedge-like  shape. 
It  constitutes  the  single  ovum  of  the  ovary.  After  it  has  become 
prominent  it  grows  rapidly  in  size,  and  throws  out  irregular 
processes.  The  germinal  vesicle,  which  for  a  considerable  time 
remains  unaltered,  also  at  length  begins  to  grow ;  and  the 
sharply  defined  germinal  spot  which  it  contains  after  reaching  a 
certain  size  completely  vanishes.  After  the  atrophy  of  the 
germinal  spot,  there  appears  in  the  middle  of  the  ovum  a 
number  of  roundish  yolk  granules. 

The  shape  of  the  ovum  becomes  more  irregular,  and  chloro- 
phyll granules,  in  addition  to  the  yolk  granules,  make  their 
appearance  in  it.  A  fresh  germinal  spot  of  circular  form  also 
arises  in  the  germinal  vesicle.  Protoplasmic  processes  are  next 
thrown  out  in  all  directions,  giving  to  the  ovum  a  marvellous 
amceboid  character.  (Fig.  4.)  The  amoeboid  form  of  the  ovum 
serves  no  doubt  to  give  it  a  larger  surface  for  nutrition.  Coin- 
cidently  with  the  assumption  of  an  amceboid  form  there  appear 
in  the  ovum  a  great  number  of  peculiar  bodies.  They  are 
vesicles  with  a  thick  wall  bearing  a  conical  projection  into  the 
interior  which  is  filled  with  fluid.  (Fig.  46.)  These  bodies 
are  formed  directly  from  the  protoplasm  of  the  ovum,  and  are 
to  be  compared  both  morphologically  and  physiologically  with 
the  yolk-spherules  of  such  an  ovum  as  that  of  the  Bird.  They 
are  called  pseudocells  by  Kleinenberg,  and  are  found  with 
slightly  varying  characters  in  many  ova  of  the  Hydrozoa. 

They  first  appear  as  small  highly  refracting  granules  ;  in  these  a  cavity 
is  formed  which  is  at  first  central  but  is  eventually  pushed  to  one  side  by  the 
formation  of  a  conical  projection  from  the  wall  of  the  vesicle. 


THE  OVUM.  29 


After  the  growth  of  the  ovum  is  completed  the  amoeboid 
processes  gradually  withdraw  themselves,  and  the  ovum  assumes 
a  spherical  form  ;  still  however  continuing  to  be  invested  by  the 
remaining  cells  of  the  ovary.  It  is  important  to  notice  that  the 
egg  of  Hydra  retains  throughout  its  whole  development  the 
characters  of  a  single  cell,  and  that  the  pseudocells  and  other 
structures  which  make  their  appearance  in  it  are  not  derived 
from  without,  and  supply  not  the  slightest  ground  for  regarding 
the  ovum  as  a  structure  compounded  of  more  than  one  cell. 

The  development  of  the  ova  of  the  Tubularidae,  which  has 
been  supposed  by  many  investigators  to  present  very  special 
peculiarities,  takes  place  on  essentially  the  same  type  as  that  of 
Hydra,  but  the  germinal  vesicle  remains  permanently  very 
small  and  difficult  to  observe.  The  mode  of  nutrition  of  the 
ovum  may  be  very  instructively  studied  in  this  type.  The 
process  is  one  of  actual  feeding,  much  as  an  Amoeba  might  feed 
on  other  organisms.  Adjoining  one  of  the  large  ova  of  the 
ovary  there  may  be  seen  a  number  of  small  germinal  cells. 
(Fig.  3.)  The  boundary  between  these  cells  and  the  ovum  is 
indistinct.  Just  beyond  the  edge  of  the  ovum  the  small  cells 
have  begun  to  undergo  retrogressive  changes ;  while  at  a  little 
distance  from  the  ovum  they  are  quite  normal  (g.c.y. 

PLATYELMINTHES. 

(10)  P.  Halle z.      Contributions  a  VHistoire  naturelle  des  Turbellaries.     Lille, 
1879. 

(11)  S.    Max   Schultze.     Beitrdge  z.  Naturgeschichte  d.  Turbellarien.     Greifs- 
wald,  1851. 

(12)  C.  Th.  von  Siebold.     "  Helminthologische  Beitrage."     Mullet's  Archiv, 
1836. 

(13)  C.  Th.  von  Siebold.     Lehrbuch  d.  vergleich.  Anat.  d.  wirbellosen  Thiere. 
Berlin,  1848. 

(14)  E.  Zeller.     "  Weitere  Beitrage  z.  Kenntniss  d.  Polystomen."    Zeit.f.  wiss. 
Zool.,  Bd.  xxvu.  1876. 

[Vide  also  Ed.  van  Beneden]  (No.  i). 

This  group,  under  which  I  include  the  Trematodes,  Cestodes, 

1  The  above  description  of  the  ova  of  the  Tubularidse  is  founded  on  sections  of 
the  gonophores  of  Tubularia  mesembryanthemum.  Dr  Kleinenberg  informs  me  how- 
ever that  the  absence  of  a  distinct  boundary  between  the  germinal  cells  and  the  ovum 
is  not  usual. 


30  PLATYELMINTHES. 


Turbellarians  and  Nemertines,  has  played  an  important  part  in 
all  controversies  relating  to  the  nature  and  composition  of  the 
ovum.  The  peculiarity  in  the  development  of  the  ovum  in 
most  members  of  this  group  consists  in  the  fact  that  two  organs 
assist  in  forming  what  is  usually  spoken  of  as  the  ovum.  One 
of  these  is  known  as  the  ovary  proper,  and  the  other  as  the 
vitellarium  or  yolk-gland.  In  the  sequel  the  term  ovum  will  be 
restricted  to  the  product  of  the  first  of  these  organs.  In  Trema- 
todes  the  ovary  forms  an  unpaired  organ  directly  continuous 
with  an  oviduct  into  which  there  open  the  ducts  from  paired 
yolk-glands. 

The  ovary  has  a  sack-like  form  and  contains  in  some 
instances  a  central  lumen  (Polystomum  integerrimum).  At  the 
blind  end  of  the  organ  is  placed  the  germinal  tissue.  This  part 
is,  according  to  the  accounts  of  the  majority  of  investigators, 
formed  of  a  polynuclear  mass  of  protoplasm  not  divided  into 
distinct  cells.  Whether  it  is  really  formed  of  undivided  proto- 
plasm or  not,  it  is  quite  certain  that  a  little  lower  down  in  the 
organ  distinct  cells  are  found,  which  have  been  segmented  off 
from  the  above  mass,  and  are  formed  of  a  large  nucleus  and 
nucleolus,  surrounded  by  a  delicate  layer  of  protoplasm.  These 
cells  are  the  young  ova.  They  usually  assume  a  more  or  less 
angular  form  from  mutual  pressure,  and,  in  the  cases  where  the 
ovary  has  a  lumen,  constitute  a  kind  of  epithelial  lining  for  the 
ovarian  tube.  They  become  successively  larger  in  passing 
down  the  ovary,  and,  though  in  most  cases  naked,  are  in  some 
instances  (Polystomum  integerrimum)  invested  by  a  delicate 
vitelline  membrane.  Eventually  the  ova  pass  into  the  oviduct 
and  become  free;  and  at  the  same  time  assume  a  spherical 
form. 

In  the  oviduct  the  ovum  receives  somewhat  remarkable 
investing  structures,  derived  from  the  organ  before  spoken  of  as 
the  yolk-gland.  The  yolk-gland  consists  of  a  number  of  small 
vesicles,  each  provided  with  a  special  duct,  connected  with  the 
main  duct  of  the  gland.  Each  vesicle  is  lined  by  an  epithelium 
of  cells  provided  with  doubly  contoured  membranes,  and  con- 
taining nuclei. 

As  the  yolk-cells  grow  older  refracting  spherules  become 
deposited  in  their  protoplasm,  which  either  completely  hide  the 


THE   OVUM. 


nucleus,  or  render  it  very  difficult  to  see.  In  the  majority  of 
cases  the  entire  cells  forming  the  lining  of  the  vesicles  constitute 
the  secretion  of  the  yolk-gland.  They  invest  the  ovum,  and 
around  them  is  formed  a  shell  or  membrane.  In  some  cases 
(e.g.  Polystomum  integerrimum)  the  yolk-cells  retain  their 
cellular  character  and  vitality  till  the  embryo  is  far  developed. 
In  other  cases  they  lose  their  membrane  and  nucleus  shortly 
after  the  formation  of  the  egg-shell,  and  break  up  into  a  fluid, 
holding  in  suspension  a  number  of  yolk-granules.  A  partial 
disorganisation  of  the  yolk-cells  can  also  take  place  before  they 
surround  the  ovum  ;  while  in  some  species  of  Distomum  they 
completely  break  up  before  leaving  the  yolk-gland. 

There  is  thus  a  complete  series  of  gra- 
dations between  the  investment  of  the 
ovum  by  a  number  of  distinct  cells,  and 
its  investment  by  a  layer  of  fluid  con- 
taining yolk-spherules  in  suspension.  In 
neither  the  one  case  nor  the  other  do  the 
investing  structures  take  any  share  in  the 
direct  formation  of  the  embryo  from  the 
ovum.  Physiologically  speaking  they  play 
the  same  part  as  the  white  in  the  fowl's 
egg- 

The  egg-shell,  which  is  usually  formed  by  a 
secretion  of  a  special  shell-gland  opening  into  the 
oviduct,  exhibits  one  or  two  peculiarities  in  the 
different  species  of  Trematodes.  In  Amphisto- 
mum  subclavatum  it  presents  at  one  extremity  a 
thickened  area,  which  is  pierced  by  a  narrow  mi- 
cropyle.  In  other  cases  one  extremity  of  the  egg- 
shell is  produced  into  a  long  process,  and  some- 
times even  both  extremities  are  armed  in  this  way. 
Opercula  and  other  types  of  armature  are  also 
found  in  different  forms. 

The  mode  of  development  of  the  ovum  in 
Cestodes  is  very  nearly  the  same  as  in  Trema- 
todes. 

The  ovum  becomes  enveloped  in  the  usual  secretion  of  the  yolk-gland  ; 
and  an  egg-shell  is  always  formed  by  the  secretion  of  a  special  shell-gland. 

Amongst  the  Turbellarians  and  Nemertines,  there  are  greater 
variations  in  the  arrangement  of  the  female  generative  glands, 


FIG.  9.  GENERATIVE 
SYSTEM  OF  VORTEX  VIRI- 
Dis.  (From  Gegenbaur, 
after  Max  Schultze.) 

t.  Testis.  v.d.  Vasa 
differentia,  v.s.  Seminal 
vesicle,  p.  Penis,  u.  Ute- 
rus, o.  Ovary,  v.  Vagina. 
g.v.  Yolk-glands,  r.s.  Re- 
ceptaculum  seminis. 


32  PLATYELMINTHES. 


than  in  the  preceding  types.  In  most  of  the  Rhabdocoela  and 
fresh-water  Dendrocoela  these  organs  resemble  in  their  funda- 
mental characters  those  of  the  Trematodes  and  Cestodes.  There 
are  present  a  paired  or  single  ovary  and  a  paired  yolk-gland. 
The  general  arrangement  of  the  organs  is  shewn  in  fig.  9. 

The  blind  end  of  the  ovaries  is  usually  (Ed.  van  Beneden,  etc.) 
stated  to  be  formed  of  a  polynuclear  protoplasmic  basis,  but 
Hallez  (No.  10)  has  recently  insisted  that,  even  at  the  extreme 
end  of  the  ovary,  the  germinal  cells  are  quite  distinct,  and  not 
confounded  together. 

With  one  or  two  exceptions  the  yolk-cells  secreted  by  the 
vitellarium  retain  their  vitality  till  they  are  swallowed  by  the 
embryo,  after  the  development  of  its  mouth.  The  few  not  so 
swallowed  become  disintegrated.  They  are  granular  nucleated 
cells,  and,  as  was  first  shewn  by  von  Siebold,  are  remarkable  for 
exhibiting  spontaneous  amoeboid  movements. 

Very  important  light  on  the  nature  of  the  vitellarium  is 
afforded  by  the  structure  of  the  generative  organs  in  Prorhyncus 
and  Macrostomum. 

In  Prorhyncus  there  is  no  separate  vitellarium,  but  the  lower 
part  of  the  ovarian  tube  functionally  and  morphologically 
replaces  it.  The  ovum  becomes  surrounded  by  yolk-cells, 
which  according  to  Hallez  (No.  10)  retain  their  vitality  for  a 
long  time.  According  to  Ed.  van  Beneden  yolk-spherules  are 
formed  in  the  protoplasm  of  the  ovum  itself,  in  addition  to  and 
independently  of  the  surrounding  yolk-cells.  In  Convoluta 
paradoxa  a  special  vitellarium  is  stated  to  be  absent ;  though  a 
deposit  of  yolk  is  formed  round  the  ovum  (Claparede). 

In  Macrostomum  again  the  yolk-glands  are  at  most  repre- 
sented by  a  lower  specialized  part  of  the  ovarian  tube.  The  ova  in 
passing  down  become  filled  with  yolk-spherules.  According  to 
Ed.  van  Beneden  these  spherules  are  formed  in  the  protoplasm 
of  the  ovum  itself;  but  this  is  explicitly  denied  by  Hallez,  who 
finds  that  they  are  formed  from  the  lining  cells  of  the  ovarian 
tube,  which,  instead  of  retaining  their  vitality  as  in  Prorhyncus, 
break  up  and  form  a  granular  mass  which  is  absorbed  by  the 
protoplasm  of  the  ovum. 

In  Prostomum  caledonicum  (Ed.  van  Beneden)  the  generative 
organs  are  formed  on  the  same  plan  as  in  other  Rhabdocoela,  but 


THE  OVUM.  33 


the  cells  which  form  the  yolk-gland  give  rise  to  yolk  particles 
which  enter  the  ovum,  instead  of  to  a  layer  of  yolk-cells  sur- 
rounding the  ovum. 

Amongst  the  marine  dendrocoelous  Turbellarians  the  ova  are  formed  in 
separate  sacks  widely  distributed  in  the  parenchyma  of  the  body  between 
the  alimentary  diverticula.  In  these  the  ova  undergo  their  complete  develop- 
ment, without  the  intervention  of  yolk-glands. 

The  ovaries  of  the  Nemertines  more  nearly  resemble  those  of  the 
marine  Dendrocoela  than  those  of  the  Rhabdoccela.  They  consist  of  a  series 
of  sacks  situated  on  the  two  sides  of  the  body  between  the  prolongations 
of  the  digestive  canal.  The  eggs  are  developed  in  these  sacks  in  a  perfectly 
normal  manner,  and  in  many  cases  become  filled  with  yolk-spherules  which 
arise  as  differentiations  of  the  protoplasm  of  the  ovum.  The  protecting 
membranes  of  the  ova  have  not  been  accurately  studied.  In  some  cases1 
two  membranes  are  present,  an  internal  and  an  external.  The  former, 
immediately  investing  the  vitellus,  is  very  delicate  :  the  external  one  is 
thicker  and  hyaline. 

The  constitution  of  the  female  generative  organs  of  the 
Trematodes  was  first  clearly  ascertained  by  von  Siebold  (No.  12). 
He  originally,  though  not  very  confidently,  propounded  the 
view  that  the  germinal  vesicles  alone  were  formed  in  the  ovary 
and  that  the  protoplasm  of  the  ovum  was  supplied  by  the 
yolk-gland.  This  view  has  long  been  abandoned,  and  von 
Siebold  (No.  13)  himself  was  the  first  to  recognize  that  true  ova 
with  a  protoplasmic  body  containing  a  germinal  vesicle  and 
germinal  spot  were  formed  in  the  ovary.  The  Trematodes  have 
however  not  ceased  to  play  an  important  part  in  forming  the 
current  views  upon  the  development  of  ova,  and  have  quite 
recently  served  Ed.  van  Beneden  as  his  type  in  exposing  his 
general  view  upon  this  subject. 

His  view  consists  fundamentally  in  regarding  the  secretion  of  the 
yolk-glands,  which  in  most  cases  merely  invests  the  ovum,  as  homologous 
with  the  yolk-spherules  which  fill  the  protoplasm  of  many  eggs  ;  and  he 
considers  the  part  of  the  ovary  where  in  most  forms  the  ova  receive  their 
supply  of  yolk  particles,  as  equivalent  to  the  vitellarium  of  the  Platy- 
elminthes.  He  further  appears  to  regard  the  primitive  state  as  that 
exemplified  in  Trematodes,  Cestodes,  etc.,  and  holds  that  the  ovarian 
types  characteristic  of  other  forms  are  secondarily  derived  from  this,  by 
the  coalescence  of  the  primitively  distinct  vitellarium  with  the  ovary  proper. 

1  Amphiporus  lactiflorius  and  Nemertes  gracilis.  Mclntosh.  Monograph  on 
British  Nemertines.  Ray  Society. 

B.  II.  3 


34  ECHINODERMATA. 


This  appears  to  me  a  case  of  putting  the  cart  before  the  horse.  To  my 
mind  the  vitellarium  is  to  be  regarded,  as  has  already  been  suggested  by 
Gegenbaur,  Hallez,  etc.  as  a  special  differentiation  of  the  primitively  simple 
ovarian  tube,  and  the  instances  of  Macrostomum  and  Prorhyncus  just  cited 
appear  to  me  to  indicate  some  of  the  steps  in  this  differentiation.  In 
Macrostomum  the  cells  of  the  lower  part  of  the  oviduct  simply  supply  a 
kind  of  nutriment  to  the  ovum  in  the  form  of  granular  yolk  particles, 
while  in  Prorhyncus  the  yolk-cells  of  the  lower  part  of  the  ovarian 
tube  form  a  complete  investment  of  independent  cells  for  the  ovum.  If 
this  lower  part  of  the  ovarian  tube  were  to  grow  out  as  a  special 
diverticulum  we  should  have  produced  a  normal  vitellarium.  But  even 
with  the  above  modification  the  theory  of  van  Beneden  appears  to  me  not 
completely  satisfactory.  The  view  that  the  yolk-spherules  are  of  the  same 
nature  as  the  yolk-cells  is  mainly  supported  by  the  case  of  Prostomum 
caledonicum,  where  the  vitellarium  produces  the  yolk  particles  which  fill 
the  ovum.  The  cases  of  Prorhyncus  and  Macrostomum  give  a  different 
complexion  to  that  of  Prostomum  caledonicum.  From  the  first  of  these 
especially  it  appears  that,  even  when  normal  yolk-cells  surround  the  ovum, 
yolk  particles  can  be  deposited  independently  in  the  protoplasm  of  the 
ovum. 

The  most  probable  view  of  the  nature  of  the  vitellarium  is 
that  of  Gegenbaur,  Hallez,  etc.,  according  to  which  it  is  to  be 
regarded  as  a  specially  modified  part  of  the  ovarian  tube.  On 
this  view  the  nature  and  function  of  the  yolk-cells  admit  of  a 
fairly  simple  explanation.  They  are  to  be  regarded  as  primary 
germinal  cells  like  those  in  the  ovaries  of  Hydra,  Tubularia,  etc., 
which  do  not  become  converted  into  ova.  Like  these  cells  they 
may  in  some  instances,  Macrostomum,  Prostomum,  etc.,  serve 
directly  in  the  nutrition  of  the  ovum.  In  other  cases  they  retain 
their  independence  and  serve  for  the  late  nutrition  of  the  embryo. 
In  both  instances  they  retain  the  faculty,  normally  possessed  by 
ova,  of  forming  yolk  particles  in  their  protoplasm. 


ECHINODERMATA. 

(15)  C.  K.  Hoffmann.     " Zur  Anatomic  d.  Echiniden  u.  Spatangen."     A't<;/<r- 
liindisch.  Archivf.  Zoologie,  Vol.  I.  1871. 

(16)  C.  K.  Hoffmann.     "  Zur  Anatomic  d.  Asteriden."    Niederldndisch.  Archiv 
/.  Zoologit,  Vol.  u.  1873. 

(17)  II.  Ludwig.     "Beitrage  zur  Anat.  d.  Crinoiden."     Zeit.  /.  wiss.  Zool., 
Vol.  xxvin.  1877. 

(18)  Job.  Miiller.     "  Ueber  d.  Canal  in  d.  Eiern  d.  Holothurien."    Muller's 
Archiv,  1854. 


THE   OVUM.  35 


(19)  C.  Semper.     Holothurien.     Leipzig,  1868. 

(20)  E.  Selenka.     Befruchtung d.  Eies  v.  Toxopneustes  variegatus,  1878. 

[  Vide  also  Ludwig  (No.  4),  etc.] 

The  eggs  of  the  Echinodermata  present  in  their  development 
certain  points  of  interest. 

The  ovaries  themselves  are  usually  surrounded  by  a  special 
vascular  dilatation.  In  the  Asteroidea,  the  Echinoidea,  and  the 
Holothuroidea  the  organs  have  the  form  of  sacks  ;  specially 
surrounded  in  the  two  former  groups,  and  probably  the  latter, 
by  a  vascular  sinus  formed  as  a  dilatation  of  one  of  the 
generative  vessels.  In  the  Crinoids  they  have  the  form  of  a 
hollow  rachis  completely  surrounded  by  a  blood-vessel.  (Fig. 
n,  &)  The  proximity  of  the  ovaries  (generative  organs)  to  the 
vascular  system  in  these  forms  has  clearly  the  same  physiological 
significance  as  the  proximity  of  the  ovaries  (generative  organs) 
to  the  radial  vessels  in  the  Ccelenterata. 

In  the  Asteroidea,  the  Echinoidea  and  the  Holothuroidea  the 
ovaries  have  the  form  of  sacks  lined  by  an  epithelium  of  germinal 
cells,  and  the  ova  are  formed  by  the  enlargement  of  these  cells, 
which,  when  they  have  reached  a  certain  size,  become  detached 
from  the  walls,  and  fall  into  the  cavity  of  the  ovarian  sack.  In 
Toxopneustes  (Selenka)  and  very  probably  in  other  forms  only 
a  few  of  the  epithelial  cells  undergo  conversion  into  ova  :  the 
remainder  undergo  repeated  division,  and,  as  in  so  many  other 
"  cases,  are  eventually  employed  in  the  nutrition  of  the  true  ova. 
In  the  nearly  ripe  ova  of  Asterias  Fol  has  described  a  flattened 
follicular  epithelium  the  origin  of  which  is  unknown. 

In  Holothuria  (Semper)  a  further  differentiation  of  the 
germinal  cells,  not  destined  to  become  ova,  takes  place.  They 
surround  the  enlarged  cell  which  forms  the  true  ovum,  for  which 
they  constitute  a  kind  of  follicular  capsule.  This  capsule  is 
attached  by  a  stalk  to  the  walls  of  the  ovary,  and  the  ovum  lies 
freely  in  it  except  for  an  area  nearly  opposite  its  (the  capsule's) 
point  of  attachment,  where  the  ovum  adheres  to  the  wall  of  the  cap- 
sule. Subsequently  the  follicle  cells  which  form  the  capsule  fuse 
together,  and  form  a  definite  membrane  in  which  only  the  nuclei 
remain  distinct.  Within  the  membranous  capsule  there  is  formed 
for  the  ovum  an  albuminous  zona  radiata.  At  the  point  where 
the  ovum  is  attached  to  its  capsule  this  membrane  cannot  be 

3—2 


ECHINODERMATA. 


FIG.    10.    OVUM  OF  Toxo- 

PNEUSTES  VARIEGATUS  WITH 
THE  PSEUDOPODIA-LIKE  PRO- 
JECTIONS OF  THE  PROTOPLASM 
PENETRATING  THE  ZONA  RADI- 


developed,  and  therefore  remains  in- 
complete. The  perforation  so  formed, 
becomes  the  micropyle  of  the  Holo- 
thurian  egg,  which  was  first  discover- 
ed by  Joh.  Miiller.  The  albuminous 
membrane  just  described  for  Holo- 
thurians  is  also  found  in  Asteroids 
(fig-  5)  an<3  Echinoids.  In  these 
groups  there  is  no  proper  micropyle, 
though  in  Ophiothrix  a  nutritive  pas- 
sage perforates  the  membrane  at  the 
attachment  of  the  ovum  before  the 

11,1  i  /*  riM^A 1  K.£\  J.  II^VJ       1111,      £*^i>i/V 

period  when  the  ovum  becomes  free     ATA  (zr).    (After  Selenka.) 
(Ludwig).    The  formation  of  the  zona 

radiata  has  been  studied  by  Selenka.  It  is  secreted  by  the 
protoplasm  of  the  ovum,  and  has  a  gelatinous  consistency,  and 
after  it  is  formed  the  peripheral  layer  of  the  protoplasm  of 
the  ovum  sends  out  through  it  pseudopodia-like  processes  to 
absorb  nutriment  from  without.  These 
processes  are  at  first  large  and  irregular, 
but  soon  become  finer  and  finer  (fig.  10), 
and  acquire  a  regular  radiating  arrange- 
ment. They  are  withdrawn  when  the  ovum 
is  ripe,  but  they  nevertheless  give  rise  to 
the  finely  radiated  appearance  of  the  mem- 
brane, the  radii  being  in  reality  delicate 
pores. 

In  the  Crinoids  the  generative  rachis 
consists  of  a  tube,  the  epithelium  of  which 
is  formed  of  the  primary  germinal  cells. 
(Fig.  n.)  While  some  of  these  cells  en- 
large and  become  ova,  the  remainder  supply 
the  elements  for  a  follicular  epithelium, 
which  is  established  round  the  ova,  exactly  MATURE  COMATULA. 

TT    ,     ,       .  (From    Gegenbaur,   after 

as  m  Holotnunans.  Ludwig.) 

p.  Tentacle,  g.  Lumen  of  genital  rachis.  w.  Water-vascular  vessel,  n.  Nerve 
cord.  b.  Blood-vessel  on  nerve  cord  and  round  genital  rachis.  eg.  Genital  canal. 
cd.  Dorsal  section  of  the  body  cavity,  cv.  Ventral  section  of  body  cavity. 


FIG.  n.  TRANSVERSE 


THE  OVUM. 


MOLLUSCA. 
Lamellibranchiata. 

(21)  H.    Lacaze-Duthiers.      "Organes   genitaux    des  Acephales   Lamelli- 
branches."     Ann.  Sci.  Nat.,  4me  serie,  Vol.  II.  1854. 

(22)  W.  Flemming.     "  Ueb.  d.  er.  Entwick.  am  Ei  d.  Teichmuschel. "   Archiv 
f.  mikr.  Anat.,  Vol.  x.  1874. 

(23)  W.   Flemming.     "  Studien   lib.  d.    Entwick.  d.  Najaden."    Sitz.  d.  k. 
Akad.  Wiss.  Wien,  Vol.  LXXI.  1875. 

(24)  Th.   von  Hessling.     "Einige  Bemerkungen,  etc."    Zeit.  f.  wiss.  Zool., 
Bd.  v.  1854. 

(25)  H.  von  Jhering.     "  Zur  Kenntniss  d.  Eibildung  bei  d.  Muscheln."     Zeit. 
f.  wiss.  ZooL>  Vol.  xxix.  1877. 

(26)  Keber.     De  Introitu  Spermatozoorum  in  ovuta,  etc.     Konigsberg,  1853. 

(27)  Fr.  Leydig.     "  Kleinere  Mittheilung  etc."    Mullet's  Archiv,  1854. 

Gasteropoda. 

(28)  C.  Semper.     "  Beitrage  z.  Anat.  u.  Physiol.  d.  Pulmonaten."     Zeit.  f. 
•wiss.  Zool.,  Vol.  vni.  1857. 

(29)  H.  Eisig.     "Beitrage  z.  Anat.  u.  Entwick.  d.  Pulmonaten."     Zeit. f.  wiss. 
Zool.,  Vol.  xix.  1869. 

(30)  Fr.  Leydig.    "  Ueb.  Paludina  vivipara."    Zeit.f.  wiss.  Zool.,  Vol.  II.  1850. 

Cephalopoda. 

(31)  Al.  Kolliker.     Entwicklungsgeschichte d,  Cephalopoden.     Zurich,  1844. 

(32)  E.  R.  Lankester.     "On  the  developmental  History  of  the  Mollusca." 
Phil.  Trans.,  1875. 


L  amellibranchiata. 

The  ova  of  the  Lamellibranchiata  present  several  points  of 
interest.  They  are  developed  in  pouches  of  the  ovary  which  are 
lined  by  a  flattened  germinal  epithelium,  or  sometimes  (?)  a 
syncytium.  Some  of  the  cells  of  this  epithelium  enlarge  and 
become  ova,  but  remain  attached  to  the  walls  of  their  pouches 
by  protoplasmic  stalks.  Round  the  ovum  there  appears  in  some 
forms  (Anodon,  Unio)  a  delicate  vitelline  membrane,  which  is 
incomplete  at  the  protoplasmic  stalk,  and  is  therefore  perforated 
by  an  aperture  which  forms  the  micropyle.  (Fig.  12.)  As  the 


38  MOLLUSCA. 


ovum  becomes  ripe  a  large  space  filled  with  albuminous  fluid 
becomes  established  between  the  ovum  and  its  membrane,  but 
the  ovum  remains  attached  to  the  membrane  at  the  micropyle. 
In  Scrobicularia  (von  Jhering,  No.  25)  the  membrane  round  the 
ovum  appears  from  the  first  as  an  albuminous  layer,  the  outer- 
most stratum  of  which  becomes  subsequently  hardened  as  the 
vitelline  membrane.  In  this  form  also  the  protoplasmic  stalk 
becomes,  in  pouches  largely  filled  with  ova,  extremely  long. 
The  ova  become  eventually  detached  by  the  stalk  rupturing, 
and  the  portion  of  it  which  remains  attached  to  the  vitelline 
membrane  falling  off.  The  function  of  the  stalk  and  of  the 
micropyle  during  the  development  of  the  ovum  is  undoubtedly  a 
nutritive  one. 

In  Anodon  and  Unio  yolk  granules 
similar  to  those  deposited  in  the  proto- 
plasm of  the  ovum  are  also  found  in  the 
epithelial  cells  of  the  ovarian  pouches 
(Flemming,  22),  and  there  can  be  but 
little  doubt  that  they  are  directly  trans- 
ported from  these  cells  into  the  ovum. 
These  cells  would  seem  therefore  to  play 
much  the  same  part  as  the  yolk-glands 

of  some  Turbellarians  (Prostomum  cale- 

v      T     ,-,       ,  .      ,  ,,  ,  FIG.  12.    MEDIUM-SIZED 

domcum).    In  Scrobicularia  yolk  granules     OVUM  OF  ANODONTA  COM- 

are  not  found  in  the  epithelium   of  the     PLAN  ATA.  (After  Flemming.) 

•.   •      j  •     ..u     j-1    4.   j  w/.  micropyle.     gs.  ger- 

pouches,  but  are  contained  in  the  dilated     minal  spot. 

disc  by  which  the  ovum  is  attached  to 

the  wall  of  its  pouch,  as  well  as  in  the  ovum  itself. 

On  the  ovum  becoming  detached  the  micropyle  still  remains 
as  an  aperture,  which  probably  has  the  function  of  admitting  the 
spermatozoa. 

The  shape  and  form  of  the  micropyle  vary  greatly.  In  Anodon  and 
Unio  it  is  a  projecting  trumpet-shaped  structure,  which  after  fertilization 
becomes  shortened  and  reduced  to  a  mere  aperture  which  is  finally 
stopped  up.  (Fig.  12.) 

In  other  forms  it  is  simply  a  perforation  in  the  vitelline  membrane 
which  is  sometimes  very  large.  In  a  species  of  Area,  which  I  had  an 
opportunity  of  observing  at  Valparaizo,  it  was  equal  to  nearly  the  circum- 
ference of  the  ovum. 


THE   OVUM.  39 


The  eggs  of  the  Lamellibranchiata  are  not  only  remarkable 
in  the  possession  of  a  micropyle,  but  in  certain  peculiarities  of  the 
yolk  and  of  the  germinal  vesicle. 

In  the  fresh-water  mussels  there  is  usally  found  in  young  and 
medium-sized  ova  a  peculiar  lens-shaped  body — Keber's  cor- 
puscle— which  is  placed  immediately  internal  to  the  micropyle. 
It  is  probably  in  some  way  connected  with  the  nutrition  of  the 
ovum,  though  the  fact  that  it  is  not  always  present  shews  that  it 
cannot  be  of  great  importance. 

A  dark  body  found  by  von  Jhering  in  the  neighbourhood  of 
the  germinal  vesicle  in  the  ripe  ovum  of  Scrobicularia  is  probably 
of  a  similar  nature  to  Keber's  corpuscle.  Both  bodies  may  be 
placed  in  the  same  category  as  the  so-called  yolk  nucleus  of  the 
spider's  and  frog's  ova. 

In  all  except  the  youngest  ova  of  Anodon  and  Unio  the 
germinal  spot  is  composed  of  two  nearly  complete  spheres  united 
together  for  a  small  part  of  their  circumference.  (Fig.  12,  gs.} 
The  smaller  of  these  has  a  higher  refractive  index  than  the 
larger,  and  often  contains  a  vacuole  :  the  two  parts  together 
appear  to  be  the  separated  components  (though  not  by  simple 
division)  of  the  primitive  nucleolus.  A  nucleolus  of  this  charac- 
ter is  not  universal  amongst  Lamellibranchiata,  but  a  similar 
separation  of  the  constituents  of  the  germinal  spot  has  been 
found  by  Flemming  in  Tichogonia,  in  which  however  the  more 
highly  refracting  body  envelopes  part  of  the  less  highly  re- 
fracting body  in  a  cap-like  fashion. 


Gasteropoda. 

The  ova  of  the  Gasteropoda  are  developed,  like  those  of  the 
Lamellibranchiata,  from  the  epithelial  cells  of  the  ovarian  acini 
or  pouches.  In  the  hermaphrodite  forms  both  ova  and  sperma- 
tozoa are  produced  in  the  same  pouches  (fig.  13),  some  of  the 
epithelial  cells  becoming  ova  and  others  spermatozoa.  The  ova 
are  usually  formed  in  the  wall  of  the  pouch,  and  the  sperma- 
tozoa internally  (Pulmonata)  (fig.  13  A),  or  a  further  differenti- 
ation of  parts  may  take  place  (fig.  13  B).  The  ova  of  Gastero- 
pods  are  exceptional  in  the  fact  that  a  vitelline  membrane  is 


MOLLUSCA. 


rarely  or  never  developed  around  them.  The  ovum  in  its  pas- 
sage to  the  exterior  becomes  enclosed  in  a  secretion  of  the 
albuminous  gland,  which  hardens  externally  to  form  a  special 
membrane. 


FIG.  13.    FOLLICLES  OF  THE  HERMAPHRODITE  GLANDS  OF  GASTEROPODA. 
(From  Gegenbaur.) 

A.  Of  Helix  hortensis.     The  ova  (aa)  are  developed  on  the  wall  of  the  follicle, 
and  the  seminal  masses  (b)  internally. 

B.  Of  Aeolidia.    The  seminal  portion  of  a  follicle  is  beset  peripherally  by  ovarian 
saccules  (a),     c.  Common  afferent  duct. 


Cephalopoda. 

Lankester  (No.  32)  has  brought  out  some  very  interesting 
points  with  reference  to  the  nutrition  of  the  eggs  of  Sepia  during 
their  growth.  The  eggs  develope  in  connective-tissue  pouches 
which  early  give  rise  to  a  double  pedunculated  capsule  of 
connective  tissue.  The  cells  of  the  inner  layer  of  this  capsule 
soon  assume  an  epithelial  character,  and  become  a  definite 
follicular  epithelium,  while  between  the  two  layers  there  pene- 
trates a  network  of  vascular  channels.  The  follicular  epithelium 
becomes  after  the  establishment  of  these  vascular  channels 
folded  in  a  most  remarkable  manner.  The  folds,  which  are 
shewn  in  section  in  fig.  14,  ic,  project  into  and  nearly  com- 
pletely fill  up  the  body  of  the  ovum.  An  enormous  increase  is 
thus  effected  in  the  nutritive  surface  exposed  by  the  epithelium. 
Each  fold  is  thoroughly  supplied  with  blood-vessels.  The 
plications  of  the  follicular  epithelium  give  rise  to  a  basket-work 
tracery  on  the  surface  of  the  ovum.  During  the  stage  when  the 
follicular  epithelium  has  the  above  structure,  its  cells  have  a 


Offcition 


THE    WORKS 


OF 


FRANCIS    MAITLAND    BALFOUR 


VOL.   II. 

A     TREATISE     ON     COMPARATIVE     EMBRYOLOGY. 
Vol.   I.     Invertebrata. 


Hmrtron : 

MACMILLAN    AND    CO. 
1885 


THE   OVUM. 


character  similar  to  that 
of  the  goblet-cells  of  a 
mucous  membrane,  and 
pour  out  their  metamor- 
phosed protoplasm  into 
the  body  of  the  ovum. 

After  the  above  mode 
of  nutrition  has  gone  on 
for  a  certain  time  a 
change  takes  place,  and 
the  ridges  gradually  dis- 
appear. This  is  caused 
by  the  epithelial  cells 
passing  off  from  the 
ridges  into  the  proto- 
plasm of  the  ovum ;  and 
becoming  assimilated, 
after  retaining  their  in- 
dividuality for  a  longer 
or  shorter  period.  When 
the  absorption  of  the 

ridges  is  completed  the  surface  of  the  ovum  assumes  a  perfectly 
regular  outline.  The  capsule  of  the  ovum  then  bursts  at  the 
opposite  pole  to  the  peduncle,  and  the  ovum  falls  into  the 
oviduct. 

The  ova  of  the  Cephalopoda,  like  those  of  the  Gasteropoda, 
are  quite  naked,  being  without  a  vitelline  membrane  or  chorion. 
The  egg-capsule  which  is  formed  for  them  in  their  passage  down 
the  oviduct  is  perforated  in  Sepia  by  a  micropylar  aperture. 


or: 


FIG.  14.    TRANSVERSE  SECTION  THROUGH  AN 
OVARIAN  EGG  OF  SEPIA.    (Copied  from  Lankester.) 

o.c.  outer  capsular  membrane,  i.e.  inner  cap- 
sular  membrane  with  follicular  epithelium,  b.v. 
blood-vessels  in  section  between  the  outer  and 
inner  capsular  membranes,  c.  vitellus. 

The  section  shews  the  folds  of  the  inner 
capsule  with  their  epithelium,  which  penetrate 
into  the  substance  of  the  ovum  for  the  purpose 
of  supplying  it  with  nourishment. 


CH^ETOPODA. 

(33)  Ed.    Claparede.     "  Les  Annelides   Chsetopodes   d.   Golfe  de   Naples." 
Mem.  d.  I.  Societ.  phys.  et  d'hist.  nat.  de  Geneve  1868 — 9  and  1870. 

(34)  E.  Ehlers.     Die  Borstenivurmer  nach  system,  und  anat.  Untersuchungen. 
Leipzig,  1864 — 68. 

(35)  E.    Selenka.      "Das   Gefass-System   d.    Aphrodite   aculeata."      Nieder- 
landisches  Archiv  f.  Zoo!.,  Vol.  II.  1873. 

The  ova  of  the  Chaetopoda  are  in  most  cases  developed  from 
the  special  tracts  of  the  epithelial  cells  lining  parts  of  the  body 


42  DISCOPHORA. 


cavity,  which  constitute  a  germinal  epithelium  (fig.  15).  Very 
frequently  (Aphrodite,  Arenicola),  as  is  so  common  in  other 
types,  these  tracts  of  germinal  cells  surround  the  blood-vessels. 


FIG.  15.     A  PARAPODIUM  OF  TOMOPTERIS.     (From  Gegenbaur.) 
o.  Collection  of  germinal  epithelial  cells  lining  the  body  cavity. 

In  some  cases  the  germinal  epithelium  thickens  to  form  a 
compact  organ,  for  which  the  outermost  cells  may  form  a 
more  or  less  definite  membranous  covering  (Oligochaeta,  etc.). 
The  ova  are  formed  by  the  enlargement,  accompanied  by  other 
changes,  of  these  germinal  cells.  During  their  early  development 
the  ova  are  frequently  surrounded  by  a  special  capsule,  which  is 
often  stalked,  and  provided  at  its  attachment  with  a  large  micro- 
pylar  aperture.  In  Aphrodite  and  Polynoe  this  arrangement, 
which  is  clearly  connected  with  the  nutrition  of  the  ovum,  is  very 
easily  seen.  The  ovum  is  dehisced  into  the  body  cavity  by  the 
bursting  of  its  capsule  or  the  rupture  of  the  stalk.  The  capsule 
is  always  eventually  thrown  off;  but  a  vitelline  membrane  is 
frequently  developed  after  the  detachment  of  the  ovum  into  the 
body  cavity.  The  vitelline  membrane  of  Spio  and  other  Poly- 
chaeta  is  provided  with  an  equatorial  ring  of  ampulliform 
vesicles. 

DISCOPHORA. 

(36)  H.  Dorner.     "  Ueber  d.  Gattung  Branchiobdella."     Zcit.  f.  tviss.  ZooL, 
Vol.  xv.  1865. 

(37)  R.  Leuckart.     Die  menschlicfun  Parasiten. 

(38)  Fr.  Leydig.     "Zur  Anatomic  v.  Piscicola  geometrica,  etc."    Zcit.f.wiss. 
Zool.,  Vol.  i.  1849. 

(39)  C.  O.  Whitman.     "  Embryology  of  Clepsine."     Quart.  J.  of  Micr.  Sci., 
Vol.  xvin.  1878. 

The  ovary  of  the  Discophora  is  formed  of  a  mass  of  cells  en- 
veloped  in   a  membranous  sack.     In   Branchiobdella    there    is 


THE   OVUM.  43 


placed  in  the  central  axis  of  these  cells  a  column  of  nucleated 
protoplasm  from  which  the  cells  themselves  are  budded  off.  The 
development  of  the  ovum  takes  place  by  the  enlargement,  etc.  of 
one  of  the  peripheral  cells,  which  eventually  bursts  the  wall  of 
the  sack  and  is  freely  dehisced  into  the  body  cavity. 

In  most  other  Leeches  (except  Piscicola  and  its  allies)  there 
is  found  a  more  specialized  arrangement  of  the  same  nature  as 
in  Branchiobdella.  There  are  one  or  more  coiled  egg-strings 
which  lie  freely  in  a  delicate  sack  continuous  with  the  oviduct. 
Each  egg-string  is  formed  of  a  central  rachis  and  of  a  peripheral 
layer  of  cells1.  The  ova  are  formed  by  the  enlargement  of  the 
peripheral  cells  accompanied  by  a  deposition  of  food-yolk. 
Food-yolk  appears  to  be  formed  in  the  rachis  even  more  ener- 
getically than  in  the  protoplasm  of  the  ova.  When  ripe  the  ova 
fall  into  the  ovarian  sack. 

In  Piscicola  the  development  of  the  ovum  is  somewhat  pecu- 
liar but  resembles  in  certain  respects  that  of  Bonellia  (p.  45). 
The  ova  are  developed  from  the  primitive  germinal  cells  which 
fill  up  the  ovarian  sack.  The  nuclei  in  these  cells  increase  in 
number,  and  a  nucleated  peripheral  layer  of  each  cell  becomes 
separated  from  the  central  part,  which  also  contains  nuclei. 
This  latter  part  next  divides  into  numerous  cells,  of  which  one 
eventually  forms  the  ovum,  and  the  remainder  constitute  a  mass 
of  cells  adjoining  it  as  in  Bonellia  (fig.  16).  This  mass  of  cells 
eventually  disappears,  and  is  probably  employed  in  the  nutrition 
of  the  ovum. 

The  ovaries  of  the  Leech  appear  to  belong  to  the  tubular 
type  in  that  the  ova  are  not  formed  from  part  of  the  epithelium 
lining  the  body  cavity;  but  if,  as  seems  probable,  the  true 
affinities  of  the  Leeches  are  with  the  Chaetopoda,  the  investment 
of  the  ovaries  must  be  of  a  secondary  nature.  It  should  be 
noted  that  the  ova  are  not,  as  in  the  ordinary  tubular  ovary, 
developed  from  the  epithelium  lining  the  ovarian  tube. 

1  The  rachis  is  stated  by  Whitman  (No.  39),  and  other  observers  to  be  formed  of 
nucleated  protoplasm,  but  further  investigations  on  this  point  are  still  required. 


44  GEPHYREA. 


GEPHYREA. 

(40)  Keferstein  u.  Ehlers.     Zoologische  Beitrage.     Leipzig,  1861. 

(41)  C.  Semper.    Holothurien,  1868,  p.  145. 

(42)  J.  W.  Spengel.     "  Beitrage  z.  Kenntniss  d.  Gephyreen."    Beitrage  a.  d. 
zool.  Station  «.  Neapel,  Vol.  i.  1879. 

(43)  J.  W.  Spengel.     "Anatomische  Mittheilungen  lib.  Gephyreen."     Tagebl. 
d.  Naturf.  Vers.     MUnchen,  1877. 

In  the  Gephyrea,  as  in  the  Chsetopoda,  the  ova  are  developed 
from  the  lining  cells  of  the  peritoneum  and  frequently  from  the 
cells  surrounding  parts  of  the  vascular  system  (Bonellia,  Thalas- 
sema).  In  many  cases  (Sipunculus,  Phascolosoma,  Echiurus) 
the  main  growth  of  the  ovum  takes  place  after  it  has  been 
dehisced  into  the  body  cavity. 

In  Sipunculus  the  ova  in  the  body  cavity  are  surrounded  by- 
a  follicle  which  is  thrown  off  before  they  become  ripe. 

Brandt  denies  the  existence  of  this  follicle  or  rather  its  cellular  nature 
Spengel's  (43)  observations  are  conclusive  in  favour  of  the  correctness  of 
the  original  interpretation  of  Keferstein  and  Ehlers.  The  follicles  would 
seem  to  be  formed  after  the  ova  have  become  free.  In  Phascolosoma  there 
is  no  follicle  (Semper,  Spengel). 

In  both  Phascolosoma  and  Sipunculus  a  vitelline  membrane 
with  radial  pores — zona  radiata — is  formed,  and  in  Phascolosoma 
the  external  part  of  this  is  separated  off  as  a  structureless 
vitelline  membrane.  The  formation  of  both  these  membranes 
from  the  protoplasm  of  the  ovum  is  rendered  certain  in  the 
latter  case  by  the  absence  of  a  follicular  epithelium. 

Some  interesting  observations  on  the  growth  and  origin  of 
the  ovum  in  Bonellia  have  been  made  by  Spengel. 

The  ova  originate  from  certain  cells  (germinal  cells)  in  the 
peritoneal  investment  of  the  ventral  vessel,  overlying  the  nervous 
cord.  These  cells,  which  are  well  marked  off  from  the  surround- 
ing flattened  peritoneal  elements,  increase  in  number  by  division, 
and  form  small  masses  surrounded  by  a  follicle  of  peritoneal 
cells,  and  attached  by  a  stalk  to  the  peritoneum.  The  central 
cell  of  each  mass  grows  larger  than  the  rest,  which  arrange 
themselves  in  a  columnar  fashion  round  it ;  it  is  not,  however, 
destined  to  become  the  ovum.  On  the  contrary  certain  of  the 
other  cells  adjoining  the  stalk  grow  larger,  and  finally  one  of 
these  becomes  distinguished  as  the  ovum  by  its  greater  size  and 


THE  OVUM.  45 


the  character  of  its  nucleus.     The  remainder  of  the  larger  cells 

become  of  the  same  size  as  their  neighbours.     The  ovum  now 

becomes  more  or  less  separate  from  the  mass  of  germinal  cells, 

rapidly  grows  in   size,  and  soon   forms   the  most  considerable 

constituent  of  the  follicle  (fig.  16,  ov).    The 

remaining  germinal  cells  are  quite  passive, 

and   though,   with   the   exception   of  the 

central  cell,  they  do  not  appear  to  atrophy, 

they   soon    constitute   a    relatively   small 

prominence  on  the  surface  of  the  ovum. 

By  the  rupture  of  the   stalk   the   whole 

follicle  becomes  eventually  detached,  and 

the  further  development  of  the  ovum  takes 

place    in    the    body    cavity.     A   vitelline     ^ONEL'LIA^A  MEDIUM 

membrane  is  formed,  and  eventually  the       STAGE  OF  DEVELOPMENT. 

y  (After  Spengel.) 

ovum  is  taken  into  the  oviduct  (segmental         ^  ovum  ftt  flattened 

organ).     At  this  time  or  slightly  before,     follicular  epithelium, 
the  follicle  cells  together  with  the  germinal  mass,  which  through- 
out exhibits  no  signs  of  atrophy,  become  thrown  off,  and  the 
ovum  is  left  invested  in  its  vitelline  membrane. 

NEMATODA. 

(44)  Ed.  Claparede.     De  la  formation  et  de  la  fecondation  des  ceufs  chez  les  Vers 
Nematodes.     Geneve,  1859. 

(45)  R.  Leuckart.     Die  menschlichen  Parasiten. 

(46)  H.  Munk.     "  Ueb.  Ei-  u.  Samenbildung  u.  Befruchtung  b.  d.  Nematoden." 
Zeit.f.  wiss.  ZooL,  Vol.  ix.  1858. 

(47)  H.  Nelson.     "On  the  reproduction  of  Ascaris  mystax,  etc."    Phil.  Trans. 
1852. 

(48)  A.Schneider.     Monographic  d.  Nematoden.     Berlin,  1866. 

The  female  organs  consist  as  a  rule  of  two  csecal  tubes  which 
unite  before  opening  to  the  exterior.  Each  of  these  is  divided 
into  a  vagina,  uterus,  oviduct,  and  ovary.  The  ovary  constitutes 
the  blind  end  of  the  tube,  and  is  formed  of  a  common  protoplas- 
mic column,  holding  a  number  of  nuclei  in  suspension.  The 
protoplasm  becomes  cleft  around  the  nuclei  in  the  uppermost 
part  of  the  tube ;  the  circumscription  of  the  ova  proceeds,  how- 
ever, very  gradually,  and  since  it  commences  at  the  periphery 
of  the  column  the  ova  remain  attached  by  stalks  to  a  central 
axis  with  one  end  free.  In  this  way  there  is  formed  a  rod-like 


46  INSECTA. 


structure  known  as  the  rachis,  which  consists  of  a  central  axis 
with  a  series  of  half  circumscribed  ova  radiately  arranged  round 
it.  In  the  lowest  part  of  the  ovary  the  ova  become  completely 
isolated  and  form  separate  cells. 

The  protoplasm  of  the  ova,  which  is  clear  in  the  terminal 
division  of  the  ovary,  becomes  in  most  forms  filled  lower  down 
with  yolk-spherules  secreted  in  the  body  of  the  ova.  These 
commence  to  appear  at  the  uppermost  extremity  of  the  rachis. 

In  some  instances,  e.g.  Cucullanus  elegans,  yolk-spherules  are  not 
formed.  In  the  Oxyuridae  the  ova  are  directly  segmented  off  from  the 
terminal  syncytium  of  protoplasm  without  the  intervention  of  a  rachis  ; 
and  are  therefore  formed  in  the  same  way  as  amongst  Trematodes,  etc. 

The  origin  of  the  membrane  around  the  ova  of  the  Nematoda  has  been 
much  disputed. 

At  the  time  when  the  ovum  is  detached  from  the  rachis  no  membrane 
is  present,  but  it  nevertheless  appears  from  Schneider's  observations  that  the 
region  at  which  it  is  detached  is  softer  than  other  parts,  so  that  a  kind  of 
micropyle  is  here  formed  which  disappears  after  impregnation.  A  delicate 
vitelline  membrane  then  appears,  around  which  there  is  subsequently 
established  an  egg-shell,  which  is  usually  stated  to  be  formed  as  a  secretion 
of  the  walls  of  the  uterus  ;  but  Schneider  and  Leuckart  have  given  strong 
grounds  for  believing  that  it  is  really  a  further  differentiation  of  the  vitel- 
line membrane  due  to  the  activity  of  the  protoplasm  of  the  ovum.  The 
originally  single  membrane  becomes  as  it  thickens  split  into  two  layers. 
The  outer  of  these  forms  the  true  egg-shell,  and  the  fertilization  of  the 
ovum  appears  to  be  a  necessary  prelude  to  its  production.  Round  the  egg- 
shell the  walls  of  the  uterus  often  secrete  a  special  albuminous  covering. 

The  egg-shell  exhibits  in  many  cases  peculiar  sculpturings  as  well 
as  terminal  prolongations. 

INSECTA. 

(49)  A.Brandt.     Ueber  das  Ei  u.  seine  Bildungsstdtte.     Leipzig,  1878. 

(50)  T.  H.  Huxley.     "  On  the  agamic  reproduction  and  morphology  of  Aphis." 
Linnean  Trans.,  Vol.  XXII.  1858.     Vide  also  Manual  of  Invertebrated  Animals,  1877. 

(51)  R.  Leuckart.     "Ueber  die  Micropyle  u.  den  feinern  Bau  d.  Schalcnliaut 
bei  den  Insecteneiern."    Mutter's  Archiv,  1855. 

(52)  Fr.  Ley  dig.    Der  Eierstock  u.  die  Samentasche  d.  Insecten.    Dresden,  1866. 

(53)  Lubbock.     ••  The  ova  and  pseudova  of  Insects."    Phil.  Trans.  1859. 

(54)  Stein.     Die  weiblichen  Geschlechtsorgane  d.  Kdfer.    Berlin,  1847. 

[Conf.  also  Glaus,  Landois,  Weismann,  Ludwig  (No.  4).] 

The  ovum  of  Insects  has  formed  the  subject  of  numerous 
investigations,  and  has  played  an  important  part  in  the  con- 
troversies on  the  nature  of  the  ovum. 


THE   OVUM. 


47 


The  ovaries  are  paired  organs,  rarely  directly  connected, 
each  consisting  of  more  or  fewer  ovarian  tubes  which  open  into 
a  common  oviduct.  The  oviducts  unite  into  a  vagina,  usually 
provided  with  a  spermatheca  and  accessory  glands,  which  need 
not  be  further  alluded  to.  Each  ovary  is  invested  by  a  peri- 
toneal covering,  which  assumes  various  characters,  and  either 
forms  a  loose  network  covering  the  whole  or  a  special  tunic 
round  each  egg-tube.  It  is  continuous  with  the  general  peri- 
toneal investment.  Each  ovarian  tube  (fig.  17)  consists  of  three 
sections:  (i)  a  terminal  thread, 
(2)  the  terminal  chamber  or  ger- 
mogen,  (3)  the  egg-tube  proper. 

The  whole  egg-tube  is  invested 
in  a  structureless  tunica  propria. 

The  terminal  threads  are  fine  prolon- 
gations of  the  ends  of  the  egg-tubes  usually 
continued  close  up  to  the  heart.  At  their 
extremities  they  frequently  anastomose, 
or  even  unite  into  a  common  thread.  In 
some  cases  they  are  absent.  They  form 
either  direct  continuations  of  the  ger- 
mogen  and  have  the  same  histological 
structure,  or  in  other  cases  are  simply 
prolongations  of  the  tunica  propria,  and 
serve  as  ligaments. 

The  germogen  usually  consists 
of  two  parts  :  an  upper,  filled  with 
nuclei  imbedded  in  protoplasm, 
and  a  lower,  in  which  distinct  cells 
have  become  differentiated. 

The  lower  part  of  the  egg-tubes 
is  filled  with  ova  which  advance  in 
development  towards  the  oviduct, 
and  lie  in  chambers  more  or  less 
distinctly  constricted  from  each 
other.  In  these  chambers  there 
are  in  most  forms  in  addition  to 
the  true  ova  a  certain  number  of 
nutritive  cells.  The  true  egg-tubes 
are  moreover  lined  by  an  epithe- 


FIG.  17.  A.  OVARIAN  TUBE  OF  THE 
FLEA,  PULEX  IRRITANS.  (From 
Gegenbaur,  after  Lubbock.) 

o.  ovum.    g.  germinal  vesicle. 
B.    OVARIAN  TUBE  OF  A  BEETLE, 
CARABUS  VIOLACEUS.  (After  Lub- 
bock.) 

o.    ovarian  segment,  formed  of  an 
ovum  a,  and  a  mass  of  yolk-cells,  b. 


48  INSECTA. 


Hal  layer  which  passes  in  and  forms  more  or  less  complete 
septa  between  the  successive  chambers.  The  points  which 
have  been  especially  controverted  are  (i)  the  relation  of  the 
ovum  to  the  germogen,  and  (2)  the  relation  of  the  nutritive 
or  yolk-cells  to  the  ovum.  To  the  controversies  on  these  points 
it  will  only  be  possible  to  give  a  passing  allusion. 

As  has  been  already  hinted  there  are  two  distinct  types 
of  ovaries,  viz.  those  without  the  so-called  nutritive  or  yolk- 
cells  and  those  with  them1. 

The  formation  of  the  ovum  is  most  simple  in  the  type 
without  yolk-cells,  which  will  for  that  reason  be  first  considered 
(fig.  17  A). 

The  germogen  is  constituted  of  a  number  of  nuclei  imbedded 
in  a  scanty  cementing  protoplasm.  In  the  lower  part  of  the 
germogen  the  nuclei  are  larger,  and  become  separated  off  from 
the  nucleated  protoplasm  above,  as  distinct  cells  with  a  thin 
layer  of  protoplasm  round  the  germinal  vesicle.  These  cells 
are  the  ova.  As  they  pass  down  the  egg-tube  their  protoplasm 
increases  in  bulk,  and  they  become  isolated  by  ingrowths  of  the 
epithelial  cells  the  origin  of  which  is  still  uncertain,  which  form 
round  each  ovum  a  special  follicle,  so  that  the  egg-tube  is  filled 
by  a  single  row  of  ova  each  in  an  epithelial  follicle  (fig.  17  A). 
The  larger  the  ova  the  more  columnar  is  the  epithelium  of  the 
follicle.  As  the  oviductal  extremity  of  the  egg-tube  is  ap- 
proached the  ova  increase  in  size,  and  their  protoplasm  is  more 
and  more  filled  with  yolk  particles. 

In  the  lower  part  of  the  egg-tube  the  epithelium  gives  rise  to 
a  chorion. 

The  epithelium  around  each  ovum  has  been  spoken  of  as  forming  a 
follicle,  and  it  is  implied  that  the  epithelium  round  each  ovum  travels  down 
the  egg-tube  with  the  ovum.  It  is  however  by  no  means  clear  from  the 
observations  of  the  majority  of  writers  that  this  is  the  case,  and  in  fact  the 
epithelium  is  generally  spoken  of  as  if  it  were  simply  the  epithelium  of  the 
egg-tube.  In  favour  of  the  view  here  adopted  the  following  considerations 
may  be  urged. 

Firstly,  there  is  considerable  evidence  that  the  superficial  layer  of 
the  germogen  gives  rise  to  the  epithelial  cells,  simultaneously  with  the 
formation  of  the  ova  from  the  deeper  layers. 

1  For  a  list  of  the  genera  with  and  without  nutritive  cells,  vide  Brandt,  pp.  47 
and  48. 


THE  OVUM.  49 


Secondly,  the  fact  that  the  epithelium  grows  in  between  the  separate 
ova  appears  to  render  it  almost  certain  that  this  part  of  the  epithelium 
must  travel  down  the  egg-tubes  with  the  ova. 

Thirdly,  the  epithelium  no  doubt  gives  rise  to  the  chorion,  and  considering 
the  peculiar  structure  of  the  chorion,  this  seems  possible  only  on  the  view 
that  the  epithelium  travels  down  the  egg-tube  with  the  ova. 

Fourthly,  when,  or  even  before,  the  egg  is  laid  the  epithelium  under- 
goes atrophy,  and  the  remains  of  it  have  been  compared  to  the  corpora 
lutea. 

If  the  view  about  the  epithelium  here  adopted  is  correct,  the  epithelium 
without  doubt  corresponds  to  the  follicular  epithelium  of  other  ova,  and  has 
the  same  origin  as  the  ova  themselves. 

The  ovaries  with  yolk-cells  differ  in  appearance  from  those 
without,  mainly  in  each  ovarian  chamber  of  an  egg-tube  con- 
taining two  elements,  usually  more  or  less  distinctly  separated. 
These  two  elements  are  (i)  at  the  lower  end  of  the  chamber,  the 
ovum,  and  (2)  at  the  upper,  large  cells  which  gradually  disappear 
as  the  ovum  grows  larger  (fig.  17  B). 

The  uppermost  part  of  the  egg-tube  is  formed,  as  in  the  pre- 
vious type,  by  a  mass  of  nucleated  protoplasm,  but  the  germinal 
cells  formed  from  it  do  not  all  become  ova.  The  germinal  cells 
leave  the  germogen  in  batches,  and  in  each  batch  one  of  the  cells 
may  usually  be  distinguished  from  the  very  first  as  the  ovum ; 
the  remainder  forming  the  nutritive  cells.  In  the  uppermost 
part  of  the  egg-tube  the  whole  mass  of  each  batch  is  very  small, 
and  the  successive  batches  are  very  imperfectly  constricted  from 
each  other.  Gradually  however  both  the  nutritive  cells  and  the 
ovum  grow  in  size,  and  then  as  a  rule,  the  Diptera  forming  a 
marked  exception,  the  chamber  containing  a  batch  becomes  con- 
stricted into  an  upper  section  with  the  nutritive  cells  and  a  lower 
one  with  the  ovum.  The  ovum  in  passing  down  the  tube  be- 
comes gradually  invested  by  a  layer  of  epithelial  cells,  which  in 
many  cases  pass  in  and  partially  separate  the  ovum  from  the 
nutritive  cells.  The  epithelium  appears  not  unfrequently  to  be 
continued  as  a  flat  layer  between  the  nutritive  cells  and  the  wall 
of  the  egg-tube. 

As  was  first  shewn  by  Huxley  and  Lubbock,  the  protoplasm  of  the 
ovum  is  often  continued  up  as  a  solid  cord,  which  terminates  freely  between 
the  nutritive  cells,  and  serves  to  bring  to  the  ovum  the  material  elaborated 
by  them.  It  is  present  in  its  most  primitive  form  in  the  somewhat 

B.  II.  4 


50  ARANEINA. 


aberrant  ovary  of  Coccus.  In  this  ovary  the  terminal  chamber  is  filled 
with  cells  which  are  united  to  a  central  rachis,  as  in  Nematodes,  and  the 
prolongation  from  the  ovum  is  continuous  with  this  rachis.  This  cord 
is  known  as  the  yolk-duct  (Dottergang)  by  German  writers.  Although  it 
is,  not  generally  present  in  a  distinct  form,  there  is  always  a  passage 
connecting  the  ovum  and  yolk-cells,  even  when  the  follicular  epithelium 
grows  in  and  nearly  separates  them. 

The  number  of  nutritive  cells  varies  from  two  (one  ?)  to 
several  dozen.  After  they  have  reached  a  maximum  they  gradu- 
ally atrophy,  and  are  finally  absorbed  without  apparently  fus- 
ing directly  with  the  ovum.  The  two  types  of  insect  ovaries 
appear  fundamentally  to  differ  in  this.  In  the  one  type  all  the 
germinal  cells  develop  into  ova ;  in  the  other  the  quantity  is,  so 
to  speak,  sacrificed  to  the  quality,  and  the  majority  of  germinal 
cells  are  modified  so  as  to  subserve  the  nutrition  of  the  few.  It 
is  still  undecided  whether  the  yolk-cells  absolutely  elaborate 
yolk  particles,  or  are  merely  conveyers  of  nutriment  to  the  ovum. 

The  egg-membranes  of  Insects  present  many  points  of  in- 
terest, which  are  however  for  the  most  part  beyond  the  scope  of 
this  work.  There  is  always  a  chorion  formed  as  a  cuticular 
deposit  of  the  follicle  cells,  which  is  frequently  sculptured,  finely 
perforated,  etc.,  and  is  in  many  instances  provided  with  a  micro- 
pyle,  developed,  according  to  Leydig,  at  the  upper  end  of  the 
ovum. 

Its  development  at  this  point  appears  to  be  due  to  the  fact 
that  the  follicle  is  here  incomplete ;  so  that  the  cuticular  mem- 
brane deposited  by  it  is  also  incomplete. 

A  true  vitelline  membrane  can  in  many  instances  be  demon- 
strated (Donacia,  etc.). 

ARANEINA. 

(65)     Victor  Carus.     "  Ueb.  d.  Entwick.  d.  Spinneneies.1'     Zcit.f.  wiss.  Zool., 
Vol.  ii.  1850. 

(56)    v.  Wittich.     "Die   Entstehung  d.    Arachnideneies   im    Eierstock,    etc." 
Mailer's  Archiv.  1849. 

[Conf.  Leydig,  Balbiani,  Ludwig  (No.  4),  etc.] 

The  ova  of  many  Araneina  are  remarkable  for  the  presence 
in  the  ovum  of  the  so-called  yolk-nucleus.  The  ova  develop 
from  the  epithelial  cells  lining  the  ovarian  sack.  Certain  of  these 
cells  grow  large  and  project  outwards,  invested  by  the  structure- 


THE  OVUM.  51 


less  membrane  of  the  ovarian  wall.  The  stalks  of  projections 
so  formed  are  turned  towards  the  lumen  of  the  ovary,  and  are 
plugged  with  the  epithelial  cells  which  line  the  ovarian  sack. 
When  ripe,  the  ova  pass  from  their  sacks  into  the  cavity  of  the 
ovary.  The  yolk-nucleus,  which  appears  very  early,  is  a  solid 
body  present  in  the  protoplasm  of  the  ovum.  It  is  not  found  in 
all  genera  of  Araneina.  At  its  full  development  it  exhibits  in 
the  fresh  condition  a  granular  structure,  but  very  soon  shews  an 
irregularly  concentric  stratification  which  becomes  more  marked 
on  the  addition  of  reagents.  According  to  Balbiani  this  strati- 
fication is  confined  to  the  superficial  layers,  while  internally  there 
is  a  body  with  all  the  characters  of  a  cell.  The  yolk-nucleus  is 
still  found  in  the  nearly  ripe  ovum,  though  it  always  disappears 
before  development  commences.  It  is  probably  connected  with 
the  nutrition  of  the  ovum,  though  nothing  is  certainly  known 
about  its  function. 

CRUSTACEA. 

(57)     Aug.  Weismann.     "  Ueb.    d.   Bildung  von  Wintereiern  bei  Leptodora 
hyalina."    Zeit.f.  wiss.  Zool.,  Vol.  xxvii.  1876. 

[For  general  literature  vide  Ludwig  No.  4  and  Ed.  van  Beneden,  No.  1.] 

Amongst  the  many  interesting  observations  on  the  Crustacean 
ova  I  will  only  allude  to  those  of  Weismann  on  the  ova  of  Lepto- 
dora, a  well-known  Cladoceran  form. 

The  phenomena  of  the  development  of  the  ova  in  this  form 
present  a  close  analogy  with  those  in  Insects. 

The  ovary  js  formed  of  (i)  a  germogen  containing  at  its  upper 
end  nucleated  protoplasm  and  lower  down  germinal  cells  in 
groups  of  four ;  (2)  of  a  portion  formed  of  successive  chambers 
in  each  of  which  there  is  a  row  of  four  germinal  cells.  Of  the 
four  cells  only  the  third  develops  into  an  ovum ;  the  remainder 
are  used  as  pabulum.  This  is  the  mode  of  development  in  the 
summer.  In  the  winter  the  sacrifice  of  a  larger  number  of  germi- 
nal cells  is  required  for  the  development  of  the  ova;  and  an 
ovum  is  produced  only  in  the  alternate  chambers.  In  the 
chambers  where  an  ovum  will  not  be  formed  an  epithelial  invest- 
ment becomes  first  established  round  the  four  germinal  cells. 
The  four  cells  then  coalesce,  and  form  a  spherical  ball  of  proto- 
plasm from  which  portions  are  budded  off  and  absorbed  by  the 

4—2 


52  CHORDATA. 


investing  epithelial  cells,  which  at  the  same  time  lose  their  nuclei. 
When  the  whole  of  the  central  ball  is  thus  absorbed  by  the 
epithelial  cells,  the  latter  become  used  by  the  winter  ovum  as 
food.  The  winter  ovum  at  its  full  development  is  formed  of  a 
central  mass  of  food-yolk  and  superficial  layer  of  protoplasm. 

CHORDATA. 
Urochorda.     (Tunicata.) 

(58)  A.-Kowalevsky.     "Weitere  Studien  ii.  d.    Entwicklung   d.   Ascidien." 
Archivf.  micr.  Anat.,  Vol.  VII.  1871. 

(59)  A.  Kowalevsky.     "  Ueber  Entwicklungsgeschichte  d.  Pyrosoma."     Arch, 
f.  micr.  Anat.,  Vol.  xr.  1875. 

(60)  Kupffer.     "  Stammverwandtschaft  zwischen  Ascidien  u.  Wirbelthieren." 
Arch.f.  micr.  Anat.,  Vol.  vi.  1870. 

(61)  Giard.     "  Etudes  critiques  des  travaux,  etc."    Archives  Zool.  experiment. , 
Vol.  i.  1872. 

(62)  C.    Semper.     "Ueber  die   Entstehung,   etc."    Arbeiten  a.  d.  zool.-zoot. 
Institut  Wurzburg,  Bd.  II.  1875. 

Cephalochorda. 

(63)  P.  Langerhans.     "Z.  Anatomic  d.  Amphioxus  lanceolatus,"  pp.  330 — 3. 
Archivf.  mikr.  Anat.,  Vol.  xii.  1876. 

'     Craniata. 

(64)  F.  M.  Balfour.     "On  the  structure  and  development  of  the  Vertebrate 
Ovary."     Quart.  J.  of  Micr.  Science,  Vol.  xvm.  1878. 

(65)  Th.  Eimer.     "  Untersuchungen  U.  d.  Eier  d.  Reptilien."     Archivf.  mikr. 
Anat.,  Vol.  vin.  1872. 

(66)  PflUger.     Die  Eierstocke  d.  Sdugethiere  u.  d.  Menschen.     Leipzig,  1863. 

(67)  J.  Foul  is.     "On  the  development  of  the  ova  and  structure  of  the  ovary  in 
Man  and  other  Mammalia."     Quart.},  of  Micr.  Science,  Vol.  xvi.  1876. 

(68)  J.  Foul  is.     "The  development  of  the  ova,  etc."    Journal  of  Anat.  and 
Phys.,  Vol.  xiii.  1878—9. 

(69)  C.  Gegenbaur.     "  Ueb.  d.  Bau  u.  d.  Entwicklung  d.  Wirbelthiereier  mit 
partieller  Dottertheilung."     Mailer's  Archtv,  1861. 

(70)  Alex.  Got te.    Entwicklungsgeschichte  d.  Unke.     Leipzig,  1875. 

(71)  W.  His.     Untersuchungen  iib.  d.  Ei  «.  d.  Eienhmcklung  bei  Knochenfischcn. 
Leipzig,  1873. 

(72)  A.    Kolliker.      Entwicklungsgeschichte  d.   Menschen  u.  hbherer   Thiere. 
Leipzig,  1878. 

(73)  J.  Mii Her.     "  Ueber  d.  zahlreichen  Porenkanale  in  d.  Eikapsel  d.  Fische." 
Mailer's  Archiv,  1854. 

(74)  W.  H.  Ransom.     "  On  the  impregnation  of  the  ovum  in  the  Stickleback." 
Pro.  R.  Society,  Vol.  vii.  1854. 

(75)  C.  Semper.     "Das  Urogenitalsystem  d.  Plagiostomen,  etc."    Arbeiten  a. 
d.  zool.-zoot.  Ins  tit.  Wurzburg,  Vol.  II.  1875. 

[Cf.  Ludwig,  No.  4,  Ed.  van  Beneden,  No.  1,  Waldeyer,  No.  6,  &c.] 


THE   OVUM.  53 


There  are  some  very  obscure  points  connected  with  the  growth 
of  the  ovum  of  the  Tunicata.  When  quite  young  the  ovum  is  a 
naked  cell  with  a  central  nucleus  containing  a  single  large 
nucleolus.  Around  it  is  a  flat  follicular  epithelium  enclosed  in 
a  membrana  propria  folliculi.  The  follicle  cells  soon  be- 
come larger  and  give  rise  to  an  envelope  round  the  egg  of  the 
nature  of  a  chorion.  At  the  same  time  they  frequently  become 
cubical  or  even  columnar,  and  filled  with  numerous  vacuoles. 

During  or  after  the  completion  of  the  above  changes  a  num- 
ber of  bodies  usually  spoken  of  as  test-cells  make  their  appear- 
ance in  the  superficial  protoplasm  of  the  egg,  which  by  the  time 
the  egg  is  ripe  arrange  themselves  in  many  species  as  a  definite 
layer  round  the  periphery  of  the  ovum.  These  bodies  have 
received  their  name  from  the  opinion,  now  known  to  be  erroneous 
(Hertwig  and  Semper),  that  they  eventually  migrated  into  the 
test  or  mantle  of  the  embryo  which  becomes  developed  round 
the  ovum.  By  Kowalevsky  (No.  58)  these  bodies  are  regarded 
as  true  cells,  and  are  believed  to  be  formed  by  some  of  the  cells 
of  the  original  follicular  epithelium  making  their  way  into  the 
vitellus  of  the  ovum  and  multiplying  there.  By  Kupffer  (No.  GO), 
and  Giard  (No.  61),  and  Fol,  they  are  also  regarded  as  true  cells 
but  are  believed  to  originate  spontaneously  in  the  vitellus. 
Finally  by  Semper  they  are  believed  not  to  be  cells,  but  to  be 
amoeboid  protoplasmic  bodies  which  are  pressed  out  from  the 
vitellus  under  the  stimulus  of  the  sea-water  or  otherwise. 

They  do  not  according  to  this  author  naturally  appear  till  the  ovum 
is  quite  ripe,  though  they  can  be  artificially  produced  at  an  earlier  period 
by  the  action  of  reagents  or  sea-water.  When  produced  in  the  natural 
course  of  things  the  vitellus  undergoes  a  contraction.  They  are  without 
any  apparent  function,  and  play  no  part  in  the  embiyonic  development. 
Semper's  results  are  very  peculiar,  but  owing  to  the  careful  study  which 
his  paper  displays  they  no  doubt  deserve  attention.  Further  investigations 
are  however  very  desirable.  Kowalevsky  from  his  researches  on  Pyrosoma 
(No.  59)  adheres  to  his  first  opinion,  though  he  abandons  the  view  that 
these  cells  are  connected  with  the  formation  of  the  test. 

In  the  passage  of  the  egg  through  the  oviduct  the  vacuolated 
follicle  cells  grow  out  into  very  peculiar  long  processes  or  villi. 
In  Ascidia  canina  these  processes  become  as  long  as  the  whole 
diameter  of  the  vitellus  (Kupffer,  No.  60). 


54  CHORDATA. 


In  Amphioxus  and  the  Craniata  the  ova  are  developed  as  in 
the  Chaetopoda,  Gephyrea,  etc.,  from  specialized  germinal  cells 
of  the  peritoneal  epithelium. 

In  Amphioxus  the  germinal  epithelium  which  constitutes  the 
essential  part  of  the  ovary  is  divided  into  a  number  of  distinct 
segments :  in  the  Craniata  no  such  division  is  observable. 

In  young  examples  of  Amphioxus  the  generative  organs  are 
in  an  indifferent  condition,  and  the  two  sexes  cannot  be  dis- 
tinguished. They  form  isolated  horse-shoe  shaped  masses  of 
cells,  which  occupy  a  position  at  the  base  of  the  myotomes,  in 
the  intervals  between  the  successive  segments  ;  and  extend  from 
the  hinder  end  of  the  respiratory  sack  to  the  abdominal  pore. 
They  are  situated  in  the  proper  body  cavity,  and  are  surrounded 
by  the  peritoneal  membrane.  Each  generative  mass  is  at  first 
solid,  and  is  formed  of  an  outer  layer  of  more  flattened  cells  and 
an  inner  mass  of  large  rounded  or  polygonal  cells.  In  its  interior 
there  appears  at  a  somewhat  later  period  a  central  cavity.  After 
the  cavity  has  appeared  the  sexes  can  be  distinguished  by  the 
different  behaviour  of  the  cells. 

In  all  the  Craniata,  the  ovary  forms  a  paired  ridge  (unless 
single  by  abortion  or  fusion)  attached  by  a  mesentery  to  the 
dorsal  wall  of  a  more  or  less  extended  region  of  the  abdominal 
cavity.  This  ridge  is  at  first  identical  in  the  two  sexes,  and 
arises  at  an  early  period  of  embryonic  life.  It  is  essentially 
formed  of  a  thickening  of  the  peritoneal  epithelium,  and  in 
Osseous  Fish,  Ganoids  (?)  and  Amphibia  the  ovary  remains 
during  embryonic  life  nearly  in  this  condition,  though  a  small 
prominence  of  the  adjacent  stroma  also  becomes  formed.  In 
other  Craniata  the  ridge,  though  at  first  in  this  condition,  very 
soon  becomes  much  more  prominent,  and  is  formed  of  a  central 
core  of  stroma  enclosed  in  the  germinal  epithelium  (fig.  1 8). 

The  thickened  germinal  epithelium  gives  rise  (in  the  case  of 
the  female)  to  the  ova  and  the  follicular  epithelium.  Whether 
the  genital  ridge  is  provided  with  a  core  of  stroma  or  no,  the 
germinal  epithelium  is  always  in  contact  on  one  side  with  the 
stroma,  from  which  it  is  at  first  separated  by  a  well-marked 
boundary  line ;  but  after  a  certain  time  there  appear  numerous 
vascular  ingrowths  from  the  stroma,  which  penetrate  through  all 
parts  of  the  germinal  epithelium,  and  break  it  up  into  a  sponge- 


THE   OVUM.  55 


like  structure  formed  of  trabeculae  of  germinal  epithelium  inter- 
penetrated by  vascular  strands  of  stroma.  The  trabeculae  of  the 
germinal  epithelium  form  the  egg-tubes  of  Pfliiger. 

With  reference  to  the  distribution  of  the  stroma  in  the  germi- 
nal epithelium,  it  may  be  said  in  a  general  way  that  there  is  a 
special  layer  close  to  the  surface  of  the  ovary,  which,  after  the 
formation  of  fresh  ova  has  nearly  ceased,  completely  isolates  a 
superficial  layer  of  the  germinal  epithelium  from  the  deeper  and 
major  part  of  it.  The  superficial  layer  is  frequently  (but  errone- 
ously) regarded  as  constituting  the  whole  of  the  germinal  epi- 
thelium. The  layer  of  stroma  below  the  superficial  epithelium 
forms  in  the  mammalian  ovary  the  tunica  albuginea.  As  the 
follicles  are  formed- in  the  trabeculae  of  germinal  epithelium  the 
stroma  grows  in  around  them,  and  forms  for  each  one  of  them  a 
special  tunic. 


FIG.  1 8.     TRANSVERSE  SECTION  THROUGH  THE  OVARY  OF  A  YOUNG  EMBRYO  OF 

SCYLLIIIM  CANICULA,  TO  SHEW  THE  PRIMITIVE  GERMINAL  CELLS  (po)  LYING  IN 
THE  GERMINAL  EPITHELIUM  ON  THE  OUTER  SIDE  OF  THE  OVARIAN  RIDGE. 

The  adult  ovaries  differ  in  a  corresponding  manner  to  the  em- 
bryonic genital  ridges  as  to  the  presence  of  a  core  of  stroma. 
The  ovaries  which  are  without  such  a  core  in  the  embryo,  are 
also  without  it  in  the  adult,  and  are  formed  of  a  double  layer  of 
tissue  entirely  derived  from  the  germinal  epithelium  with  its  in- 
growths of  stroma,  and  composed,  for  the  most  part,  of  ova  in 
all  stages  of  development.  In  the  case  of  the  other  ovaries  there 


56  CRANIATA. 


is  a  hilus  of  stroma — the  zona  vasculosa — internal  to  the  egg- 
bearing  region. 

In  Mammalia,  proportionately  to  the  ovary,  the  zona  vasculosa  is  at  a 
maximum,  and  in  Birds  and  Reptiles  it  is  relatively  far  less  developed.  In 
these  forms  the  germinal  epithelium  covers  the  whole  surface  of  the  ovary. 
In  Elasmobranchii  the  structure  of  the  ovary  is  somewhat  different,  owing 
to  the  presence  in  the  ovarian  ridge  of  a  large  quantity  of  a  peculiar 
lymphatic  tissue,  which  has  no  homologue  in  the  other  ovaries;  .and  still 
more  to  the  fact  that  the  true  germinal  epithelium  is  in  most  forms  entirely 
confined  to  the  outer  surface  of  the  ovary,  on  which  it  forms  a  layer 
of  thickened  epithelium  in  the  embryo  (fig.  17),  and  of  ovigerous  tissue  in 
the  adult. 

In  the  ovary  of  Mammalia  and  Reptilia  and  possibly  other  forms  there 
are  present  in  the  zona  vasculosa  during  embryonic  life  cords  of  epithelial 
tissue  derived  from  the  Malpighian  bodies;  these  cords  have  no  function 
in  the  female,  but  in  the  male  assist  in  forming  the  seminiferous  tubules. 

In  considering  the  development  of  the  ova  it  is  again  con- 
venient to  distinguish  between  Amphioxus  and  the  Craniata. 

In  Amphioxus  the  germinal  cells  destined  to  become  ova  are 
first  distinguished  by  the  larger  size  of  their  germinal  vesicles  and 
by  the  presence  of  certain  refracting  granules  in  their  protoplasm. 
They  subsequently  rapidly  enlarge  and  form  protuberances  on  the 
surface  of  the  ovary,  which  are  enveloped  for  three-quarters  of 
their  circumference  by  the  flattened  epithelioid  cells  of  the  peri- 
toneal membrane,  which  thus  form  a  kind  of  follicle.  As  the 
ova  become  ripe  yolk-granules  are  deposited  in  their  protoplasm, 
first  in  the  superficial  layer  and  subsequently  throughout.  The 
germinal  vesicle  also  passes  from  the  centre  to  the  surface.  A 
vitelline  membrane  is  formed  when  the  ova  are  mature. 

In  the  Craniata  the  ova  are  developed  from  the  cells  of  the 
germinal  epithelium.  In  the  types  with  larger  ova  (Teleostei, 
Elasmobranchii,  Amphibia,  Reptilia,  Aves),  at  a  very  early  period, 
sometimes  (Elasmobranchii)  even  before  the  formation  of  the 
genital  ridge,  certain  of  the  cells  which  are  destined  to  form  ova 
become  distinguished  by  their  greater  size,  and  by  the  possession 
of  an  abundant  clear  protoplasm  and  a  large  spherical  granular 
nucleus.  (Fig.  i8,/0.)  Such  special  cells  form  primitive  germi- 
nal cells,  and  are  common  to  both  sexes. 

For  a  considerable  period  after  their  first  formation  these  cells 
remain  stationary  in  their  development ;  but  their  number  in- 


THE   OVUM.  57 


creases,  partly,  it  appears,  by  an  addition  of  fresh  ones,  and  partly 
by  division.  Owing  to  the  latter  process  the  germinal  cells 
come  to  form  small  masses  or  nests.  The  following  description 
of  the  further  changes  of  these  cells  in  the  female  refers  in  the 
first  instance  to  Elasmobranchii,  but  holds  good  in  most  respects 
for  other  types  as  well. 

It  is  convenient  to  distinguish  two  modes  in  which  the  primi- 
tive germinal  cells  may  become  converted  into  permanent  ova, 
though  the  morphological  difference  between  the  two  modes  is 
of  no  great  importance. 

In  the  first  mode  the  protoplasm  of  all  the  cells  forming  a 
nest  unites  into  a  single  mass  containing  the  nuclei  of  the  pre- 
viously independent  ova  (fig.  19,  nn).  The  nuclei  in  the  nest  in- 
crease in  number,  probably  by  division,  and  at  the  same  time  the 
nest  itself  increases  in  size.  The  nuclei  while  increasing  in 


FIG.  19.    SECTION  THROUGH  PART  OF  THE  GERMINAL  EPITHELIUM  OF  THE  OVARY 

OF    SCYLLIUM    AT    THE    TIME   WHEN    THE    PRIMITIVE    GERMINAL    CELLS    ARE 
BECOMING  CONVERTED  INTO  OVA. 

nn.  Nests  formed  of  agglomerated  germinal  cells.  The  nuclei  of  these  cells  are 
imbedded  in  undivided  protoplasm,  do.  developing  ova.  o.  ovum  with  follicle. 
po.  primitive  germinal  cell.  dv.  blood-vessels. 

number  also  undergo  important  changes.  A  segregation  of  their 
contents  takes  place,  and  the  granular  part  (nuclear  substance) 
forms  a  mass  close  to  one  side  of  the  membrane  of  the  nucleus, 
while  the  remainder  of  the  nucleus  is  filled  with  a  clear  fluid. 
The  whole  nucleus  at  the  same  time  increases  somewhat  in  size. 
The  granular  mass  gradually  assumes  a  stellate  form,  and  finally 


58  CRANIATA. 


becomes  a  beautiful  reticulum,  of  the  character  so  well  known  in 
nuclei  (fig.  19,  do).  Two  or  three  special  nucleoli  are  present, 
and  form  the  nodal  points  of  the  reticulum,  while  its  meshes  are 
filled  up  with  the  clear  fluid  constituents  of  the  nucleus.  Not  all 
the  nuclei  undergo  the  above  changes ;  but  some  of  them  stop 
short  in  their  development,  undergo  atrophy,  and  appear  finally 
to  be  absorbed  as  pabulum  by  the  protoplasm  of  the  nest.  Such 
nuclei  in  a  state  of  degeneration  are  shewn  in  fig.  19.  Thus  only 
a  few  nuclei  out  of  a  nest  undergo  a  complete  development.  At 
first  the  protoplasm  of  the  nest  is  clear  and  transparent,  but  as 
the  nuclei  undergo  their  changes  the  protoplasm  becomes  more 
granular,  and  a  specially  large  quantity  of  granular  protoplasm 
is  generally  present  around  the  most  developed  nuclei,  and  these 
with  their  protoplasm  gradually  become  constricted  off  from  the 
nest,  and  constitute  the  permanent  ova  (fig.  19,  do).  The  rela- 
tive number  of  ova  which  may  develop  from  a  single  nest  is 
subject  to  great  variation.  The  object  of  the  whole  occurrence 
of  the  fusion  of  primitive  ova  and  the  subsequent  atrophy  of 
some  of  them  is  to  ensure  the  adequate  nutrition  of  a  certain 
number  of  them. 

In  the  second  and  rarer  mode  of  development  of  permanent 
ova  from  primitive  germinal  cells,  the  nuclei  and  protoplasm 
undergo  the  same  changes  as  in  the  first  mode,  but  the  cells  either 
remain  isolated,  and  never  form  part  of  a  nest,  or  form  part  of  a 
nest  in  which  no  fusion  of  protoplasm  takes  place,  and  in  which 
all  the  cells  develop  into  permanent  ova. 

The  isolated  ova  and  nests  are  situated,  during  the  whole  of 
the  above  changes,  amongst  the  general  undifferentiated  cells  of 
the  germinal  epithelium,  but  as  soon  as  a  permanent  ovum  be- 
comes formed  the  cells  adjoining  it  arrange  themselves  around  it 
as  a  special  layer,  and  so  give  rise  to  the  epithelium  of  the  follicle 
(fig.  19,  <?).  The  growths  of  stroma  into  the  germinal  epithelium 
appear  shortly  after  the  formation  of  the  earlier  follicles. 

Mammalia.  The  development  of  the  ovary  in  Mammalia 
differs  mainly  from  that  just  described  in  that  the  formation  of 
primitive  germinal  cells  from  the  indifferent  cells  of  the  germinal 
epithelium  takes  place  at  a  relatively  much  later  period. 

The  stroma  grows  into  the  germinal  epithelium  while  it  is  still 
formed  of  rounded  indifferent  cells,  and  divides  it  into  trabeculae 


THE   OVUM.  59 


as  described  above.  At  a  later  period  a  number  of  the  cells  in 
the  deeper  layer  of  the  epithelium,  as  well  as  certain  cells  in  the 
superficial  part,  become  primitive  germinal  cells,  while  the  re- 
mainder of  the  cells  become  smaller  and  are  destined  to  form  the 
follicle  cells. 

The  most  conspicuous  primitive  germinal  cells  are  situated 
in  the  superficial  layer  of  epithelium  ;  and  the  primitive  germinal 
cells  in  the  deeper  layers  of  the  germinal  epithelium  are  not 
nearly  so  marked  as  in  most  Craniata,  so  that  it  is  difficult  in  some 
cases  to  be  sure  of  their  destination  till  their  nucleus  commences 
to  undergo  its  characteristic  metamorphosis. 

The  change  of  the  primitive  ova  into  permanent  ova  takes 
place  in  the  same  manner  in  Mammals  as  in  Elasmobranchii, 
except  that  the  fusion  of  the  primitive  ova  into  polynuclear 
masses  is  much  rarer.  The  formation  of  the  at  first  quite  simple 
follicles  takes  place  while  the  ova  are  still  aggregated  in  large 
masses ;  and  the  first  follicles  are  formed  in  the  innermost  part 
of  the  germinal  epithelium.  Soon  after  their  formation  the  folli- 
cles become  isolated  by  connective-tissue  growths. 

Post-embryonic  development  of  the  ova. 

The  ova  of  the  Vertebrata  differ  greatly  in  size  and  structure. 
The  differences  in  size  depend  upon  the  quantity  of  the  food- 
yolk.  In  the  Amphioxus  and  Mammalia,  in  which  the  ova  are 
smallest,  the  comparatively  insignificant  amount  of  food-yolk  is 
distributed  uniformly  through  the  ovum.  A  larger  quantity  of 
it  is  present  in  the  ova  of  Amphibia,  Marsipobranchii  and  Teleostei, 
and  it  attains  an  immense  development  in  the  ova  of  Elasmo- 
branchii, Reptilia,  and  Aves. 

The  food-yolk  originates  from  a  differentiation  of  the  proto- 
plasm of  the  egg.  It  arises  as  a  number  of  small  highly  refract- 
ing particles  in  a  stratum  slightly  below  the  surface. 

In  the  Mammalian  ovum  these  particles  spread  through  the  protoplasm 
of  the  egg,  but  do  not  attain  any  considerable  development.  In  other 
forms  the  case  is  different.  In  Elasmobranch  Fishes  the  refracting  particles 
appear  to  develop  into  vesicles,  in  the  interior  of  which  there  arise 
solid  oval  or  even  rectangular  highly  refracting  bodies,  in  the  substance 
of  which  a  stratification  may  usually  be  observed,  which  gives  them 


60  CRANIATA. 


an  appearance  not  unlike  that  of  striated  muscle.  In  Teleostei  the 
yolk  assumes  very  different  characters  in  different  cases.  It  is  often 
formed  of  larger  or  smaller  vesicles  containing  in  their  interior  other 
bodies.  Stratified  plates  like  those  of  Elasmobranchii  are  also  not  un- 
common. In  the  ripe  ovum  of  Teleostei  the  food-yolk  usually  resolves 
itself  into  a  large  vitelline  sphere,  which  occupies  the  greater  part  of 
the  ovum,  and  is  formed  of  a  highly  refracting  fluid  material  which 
coagulates  on  the  addition  of  water.  It  contains  in  many  instances  one 
or  more  highly  refracting  bodies  known  as  oil  globules,  and  is  invested 
by  a  granular  protoplasmic  layer  continuous  with  the  germinal  disc,  in 
which  a  number  of  normal  yolk-spherules  are  frequently  present.  In  the 
ovum  of  the  Herring1  no  distinct  investing  protoplasmic  layer  or  germinal 
disc  is  present  till  after  impregnation,  but  the  ovum  is  formed  of  a  super- 
ficial layer  with  minute  yolk-spherules,  and  of  a  central  portion  with  larger 
yolk-spheres. 

In  Amphibia  the  yolk  very  often  appears  in  the  form  of  oval  or  quadri- 
lateral plates.  In  Reptilia  the  yolk-spherules  are  vesicles,  somewhat  similar 
to  the  white  yolk-spheres  of  Aves,  but  as  a  rule  without  the  highly  refracting 
spheres  in  their  interior.  The  peculiar  and  complicated  arrangement  and 
structure  of  the  white  and  yellow  yolk  in  Birds  is  fully  described  in  the 
"  Elements  of  Embryology,"  and  it  need  only  be  said  that  the  yolk  develops 
in  Birds  in  the  same  manner  as  in  other  types,  and  that  at  first  all  the  yolk- 
spherules  appear  in  the  form  of  white  yolk.  The  yellow  yolk-spheres  are  a 
peculiar  modification  of  white  yolk-spheres,  formed  comparatively  late  in  the 
development  of  the  egg  (fig.  20). 


FIG.  20.    YOLK  ELEMENTS  FROM  THE  EGG  OF  THE  FOWL. 
A.  Yellow  yolk.     B.  White  yolk. 

In  the  eggs  of  many  Amphibia  a  dark  granular  mass  known  as  the  yolk 
nucleus  makes  its  appearance ;  and  is  supposed,  without  any  very  clear  evi- 
dence, to  be  related  to  the  formation  of  the  yolk. 

A  body  in  the  form  of  a  shell  enclosing  a  dark  nucleus,  which 
is  perhaps  of  the  same  nature,  has  been  described  by  Eimer  in  the 
Reptilian  egg :  it  eventually  resolves  itself  into  a  number  of  angular 
fragments.  In  Elasmobranchii  a  similar  body  is  perhaps  present. 

The  food-yolk  just  described  is  imbedded  in  the  active  proto- 
plasmic portion  of  the  body  of  the  ovum.  In  the  case  of  the 

1  Kupffer,  Laichen  u.  Entwicklung  des  Ostsee-Harings.     Berlin,  1878. 


THE   OVUM.  6 1 


mammalian  ovum  the  food-yolk  is  fairly  uniformly  distributed, 
but  in  the  case  of  all  other  craniate  ova  the  protoplasm  of  the 
ovum  is  especially  concentrated  at  one  pole,  which  is  known  as 
the  upper  or  animal  pole,  and  the  food-yolk  is  more  especially 
concentrated  at  the  opposite  pole.  The  Herring's  ovum  forms 
an  apparent  exception  to  this  statement,  in  that  the  concentra- 
tion of  the  protoplasm  to  form  the  germinal  disc  does  not  take 
place  till  after  impregnation.  In  Amphibia  the  animal  pole  is 
mainly  marked  by  the  smaller  size  of  the  yolk-spherules,  but  in 
most  other  forms  a  small  portion  of  the  ovum  in  the  region  of 
the  germinal  vesicle  is  nearly  free  from  yolk-spherules,  and  then 
forms  a  more  or  less  specialized  part  known  as  the  germinal 
disc.  In  Aves,  Reptilia,  and  Elasmobranchii  the  germinal  disc 
shades  off  insensibly  into  the  yolk ;  but  in  Teleostei  it  is  more 
sharply  marked  off,  and  is  continued  more  or  less  completely 
round  the  periphery  of  the  ovum.  In  ova  with  true  germinal 
discs  it  is  the  germinal  disc  alone  which  undergoes  segmentation. 
The  protoplasm  of  vertebrate  ova  frequently  exhibits  a  reticulate 
or  sponge-like  structure  (fig.  21)  and  the  reticulum  in  many 
cases,  e.g.  Elasmobranchii  and  Reptilia,  serves  to  hold  the  yolk- 
spheres  together.  In  the  Tench  it  has  been  observed  by  Bam- 
beke  to  penetrate  into  the  vitelline  sphere. 

In  the  ova  of  the  Craniata  the  germinal  vesicle  is  generally 
polynucleolar.  In  Amphioxus  and  Petromyzon  there  is  how- 
ever but  a  single  nucleolus,  and  in  Mammalia  there  is  usually 
one  special  nucleolus  and  two  or  three  accessory  ones.  The 
opposite  extreme  is  reached  in  many  osseous  fish  where  the 
nucleoli  are  extremely  numerous.  The  protoplasmic  reticulum 
of  the  embryonic  germinal  vesicle  may  in  some  instances  be 
retained  till  the  ovum  is  nearly  ripe,  but  usually  assumes  a  very 
granular  form.  It  is  at  first  connected  with  the  nucleoli  which 
form  nodal  points  in  it,  but  this  relation  cannot  always  be 
detected  in  the  later  stages.  A  membrane,  which  in  the  case  of 
the  larger  ova  becomes  very  thick,  is  always  present  round  the 
germinal  vesicle.  It  is  said  to  be  perforated  in  some  Reptilian 
ova  (Eimer).  As  to  the  position  of  the  germinal  vesicle,  it  is  at 
first  situated  in  the  centre  of  the  ovum,  but  always  eventually 
travels  to  the  animal  pole,  and  as  the  egg  becomes  ripe  under- 
goes changes  which  will  be  more  especially  detailed  in  the  next 


62 


CHORD  ATA. 


chapter.    In  the  ova  with  a  large  amount  of  food-yolk  it  assumes 
an  eccentric  position  very  early. 

The  homologies  of  the  primary  egg-membranes  of  Craniata 
are  still  involved  in  some  obscurity.  There  seem  to  be  three 
membranes,  which  may  all  coexist,  and  of  which  one  or  more 
are  almost  always  present.  These  membranes  are — 

(1)  An    outermost    usually    homogeneous    non-perforated 
membrane,  which  is  by  most  authors  regarded  as  a  chorion, 
but  is  probably  a  vitelline  membrane — by  which  name  I  shall 
speak  of  it. 

(2)  A  radiately  striated  membrane  (internal  to  the  former 
when  the  two  coexist)  which  can  be  broken  up  into  a  series  of 
separate    columns.      These   give   to  the  membrane  its  radiate 
striation,  but  it  is  probable  that  between  the  columns  there  are 
pores  sufficiently  large  to  admit  of  the  passage  of  protoplasmic 
filaments.    This  membrane  will  be  spoken  of  as  the  zona  radiata. 
It  is  a  differentiation  of  the  outermost  layer  of  the  yolk. 

(3)  Within  the  zona  radiata  a  third  and  delicate  membrane 
is  occasionally  found,  especially  when  the  ovum  is  approaching 
maturity. 

In  Elasmobranchii  the  first  membrane  to 
be  formed  is  the  vitelline  membrane,  which 
appears  in  some  instances  before  the  forma- 
tion of  the  follicle — a  fact  which  appears  to 
shew  that  it  is  really  formed  as  a  differentia- 
tion of  the  protoplasm  of  the  egg.  In  most 
Elasmobranchii  this  membrane  attains  a 
very  considerable  development.  A  zona 
radiata  is  generally  (if  not  always)  present 
in  Elasmobranchii,  but  arises  at  a  later 
period  than  the  vitelline  membrane  (fig.  21, 
Zri).  The  zona  radiata  always  disappears 
long  before  the  ovum  is  ripe.  The  vitelline 
membrane  also  gradually  atrophies,  though 
it  lasts  much  longer  than  the  zona  radiata. 
When  the  egg  is  taken  up  by  the  oviduct  all  trace  of  both  mem- 
branes has  vanished.  In  Reptilia  precisely  the  same  arrange- 
ments of  the  membranes  are  found  as  in  Elasmobranchii,  except 
that  as  a  rule  the  zona  radiata  is  relatively  more  important. 


z.n 


FIG.  21.  SECTION 
THROUGH  A  SMALL 
PART  OF  THE  SURFACE 
OF  AN  OVUM  OF  AN  IM- 
MATURE FEMALE  OF 
SCYLLIUM  CANICULA. 

fe.  follicular  epithe- 
lium, vt.  vitelline 
membrane.  Zn.  zona 
radiata.  yk.  yolk  with 
protoplasmic  network. 


THE  OVUM.  63 


The  vitelline  membrane  is  thin  except  in  the  Crocodilia.  The 
third  innermost  membrane  is  found  according  to  Eimer  in  many 
Reptilia.  In  birds  both  vitelline  membrane  and  zona  radiata 
are  present,  but  the  latter  atrophies  early,  leaving  the  former  as 
the  sole  membrane  when  the  egg  is  ripe. 

In  osseous  .fish  the  vitelline  membrane  is  usually  either 
absent  or  may  perhaps  in  some  instances,  e.g.  the  Perch,  be 
imperfectly  represented.  In  the  ripe  ovum  of  the  Herring  there 
is  a  distinctly  developed  membrane  external  to  the  zona  radiata 
which  is  probably  the  vitelline  membrane.  The  zona  radiata 
attains  a  very  great  development,  and  is  generally  provided  with 
knobs  of  various  shapes  on  its  outer  surface.  A  delicate  mem- 
brane internal  to  this — my  third  membrane — has  often  been 
described,  but  there  is  still  some  doubt  about  its  existence.  In 
some  cases  an  external  less  granular  layer  of  the  ovum  itself  has 
been  described  as  a  special  membrane.  In  the  Perch  a  peculiar 
mucous  capsule,  penetrated  by  irregular  branched  prolongations 
of  the  follicle  cells,  is  present  in  addition  to  the  ordinary  mem- 
branes. In  Petromyzon  a  zona  radiata  appears  to  be  present, 
which  in  the  adult  is  divided  into  two  layers,  both  of  them 
radiately  striated  according  to  Calberla,  but  according  to  Kupffer 
and  Benecke  the  outer  one  is  not  perforated,  and  would  appear 
therefore  to  be  a  vitelline  membrane  as  defined  above.  A 
delicate  membrane  is  formed  at  a  comparatively  late  period 
around  the  ova  of  the  Amphibia,  and  is  stated  (Waldeyer,  No.  6, 
and  Kolessnikow)  to  have  a  delicate  radial  striation.  It  probably 
corresponds  with  the  zona  radiata. 

In  Mammalia  a  radiately  striated  membrane — the  zona 
radiata — is  generally  described  as  being  present,  and  internal  to 
it,  in  the  nearly  ripe  egg,  a  delicate  membrane  has  been  shewn 
by  E.  van  Beneden  to  exist.  Externally  to  the  zona  radiata 
there  may  be  observed  a  granular  membrane  irregular  on  its 
outer  surface  on  which  the  cells  of  the  discus  are  supported. 
This  membrane  is  more  or  less  distinctly  separated  from  the 
zona  radiata ;  and  by  tracing  back  its  development  it  appears 
very  probable  that  it  is  the  remnant  of  the  first-formed  membrane 
in  the  very  young  ovum,  and  therefore  the  vitelline  membrane. 

A  micropyle  (first  discovered  by  Ransom,  No.  74)  is  present 
in  a  large  number  of  osseous  fish  and  in  Petromyzon  (Calberla). 


64  CHORDATA. 


Doubts  have  been  thrown  on  its  existence  in  the  latter  form  by 
Kupffer  and  Benecke ;  and  at  any  rate  it  would  only  seem  to 
perforate  the  zona  radiata.  In  the  osseous  fish  in  which  it  has 
been  detected,  Salmonidae,  Percidae  (Gasterosteus),  Clupeidae, 
etc.,  it  forms  a  minute  perforation  of  the  zona  radiata  at  the 
animal  pole,  just  large  enough  to  admit  a  single  spermatozoon. 
Its  characters  differ  slightly  in  different  cases,  but  there  is  usually 
a  shallow  depression,  in  the  centre  of  which  it  is  situated. 

The  eggs  of  all  Craniata  (except  Petromyzon  (?))  appear  to 
be  enclosed  in  a  cellular  envelope  known  as  the  follicle.  The 
cells  which  form  this  are,  as  has  been  already  explained,  derived 
from  the  germinal  epithelium1,  and  frequently  arrange  themselves 
around  the  ovum  before  the  appearance  of  the  growths  of  stroma 
into  the  epithelium.  All  young  follicles  are  nearly  alike,  but  as 
they  grow  older  they  exhibit  various  modifications  in  the 
different  groups.  They  retain  their  simplest  condition  as  a  flat 
epithelial  layer  in  most  osseous  fish  and  Amphibia.  In  most 
other  forms  the  cells  become  at  some  period  columnar,  and  are 
generally  arranged  in  two  or  more  layers.  There  is  formed 
externally  to  the  epithelium  a  delicate  membrane — the  mem- 
brana  propria  folliculi — which  is  in  its  turn  enclosed  in  a 
vascular  connective-tissue  sheath. 

In  Elasmobranchii  and  many  Reptilia  (Lacertilia,  Ophidid) 
some  of  the  cells  become  much  larger  than  the  others,  and 
assume  a  funnel-shaped  form  with  the  narrow  end  in  contact 
with  the  egg-membrane.  These  large  cells,  which  have  a 
regular  arrangement  in  the  epithelium,  are  probably  in  some 
way  connected  with  the  nutrition.  They  have  only  been  noticed 
in  large-yolked  ova.  Many  observers  have  described  prolonga- 
tions of  the  follicle  cells  through  the  pores  of  the  zona  radiata  in 
Aves,  Reptilia  and  Teleostei. 

The  most  remarkable  modification  of  the  follicle  is  that 
which  is  found  in  Mammalia.  At  first  the  follicle  is  similar  to 
that  of  other  Vertebrata,  and  is  formed  of  flat  cells  derived  from 
the  germinal  cells  adjoining  the  ovum.  These  cells  next  become 
columnar  and  then  one  or  two  layers  deep.  Later  they  become 

1  For  the  different  views  maintained  by  Foulis,  Kolliker,  etc.  the  reader  is  referred 
to  the  writings  of  these  authors.  The  grounds  for  the  view  here  adopted  will  be  found 
in  my  paper  (No.  64). 


THE  OVUM.  65 


thicker  on  one  side  than  on  the  other,  and  there  appears  in  the 
thickened  mass  a  cavity,  which  gradually  becomes  more  dis- 
tended and  is  filled  with  an  albuminous  fluid.  As  the  cavity 
enlarges,  the  ovum  with  several  layers  of  cells  around  it  forms 
a  prominence  projecting  into  it.  The  whole  structure  with  its 
tunic  is  known  as  the  Graafian  follicle.  The  follicle  cells  are 
known  as  the  membrana  granulosa,  and  the  projection,  in 
which  the  ovum  lies,  as  the  discus  or  cumulus  proligerus. 
The  cells  of  the  discus  in  immediate  contiguity  to  the  ovum 
usually  form  a  more  or  less  specialized  layer  and  are  somewhat 
more  columnar  than  the  adjoining  cells. 

THE  SPERMATOZOON. 

Although  there  is  no  doubt  that  the  spermatozoon  in  most 
instances  plays  as  important  a  part  as  the  ovum  in  influencing 
the  characters  of  the  organism  which  is  evolved  from  the 
coalesced  product  of  the  ovum  and  spermatozoon,  yet  the 
actual  form  of  the  spermatozoon  has  not,  like  the  form  of  the 
ovum,  a  secondary  influence  on  the  early  phases  of  development. 
A  comparative  history  of  the  spermatozoon  is  therefore  of  less 
importance  for  my  purpose  than  that  of  the  ovum  ;  and  I  shall 
confine  myself  to  a  few  remarks  on  its  general  structure,  and 
mode  of  growth.  The  primary  origin  of  the  male  germinal  cells, 
and  their  relation  to  the  sperm-forming  cells,  is  dealt  with  in 
the  second  part  of  the  treatise. 

Although  the  minute  size  of  most  spermatozoa  places  great 
difficulties  in  the  way  of  a  satisfactory  investigation  of  them,  yet 
there  can  be  but  little  doubt  that  they  always  have  the  value  of 
cells.  In  the  vast  majority  of  instances  the  spermatic  cell  or 
spermatozoon  is  composed  of  (i)  a  spherical  or  oval  portion 
known  as  the  head,  formed  of  a  nucleus  enveloped  in  an 
extremely  delicate  layer  of  protoplasm,  and  (2)  of  a  motile 
protoplasmic  flagellum  known  as  the  tail;  which  together  with 
the  investing  layer  of  the  head  forms  the  body  of  the  cell. 

As  might  be  anticipated,  the  proportion,  size,  and  relations 

of  the  parts  of  the  spermatozoon  are  subject  to  great  variations. 

The  head  is  often  extremely  elongated  ;  and  it  is  in  many  cases 

rather  on  theoretical  grounds,  than  as  a  result  of  actual  observa- 

B.  II.  5 


66  THE  SPERMATOZOON. 

tion,  that  a  protoplasmic  layer  is  stated  to  be  continued  round 
the  nucleus  which  forms  the  main  constituent  of  the  head.  In 
some  of  the  elongated  forms  of  spermatozoa,  e.g.  in  Insecta, 
there  is  no  marked  distinction,  except  in  the  character  of  the 
protoplasm,  between  the  head  and  the  tail.  A  connecting 
element  is  frequently  interposed  between  the  head  and  tail, 
which  appears  however  to  be  constituted  of  the  same  material  as 
the  tail,  and  sometimes  forms  a  thickening  on  the  tail  close 
below  the  head  (Amphioxus).  A  very  remarkable  modification 
of  the  tail  is  found  in  many  Amphibia,  Reptilia  and  Mammalia. 
In  these  types  there  is  attached  to  what  appears  to  be  a  normal 
tail  a  delicate  membrane,  the  outer  edge  of  which  is  thickened 
to  form  a  kind  of  secondary  filament  In  the  living  spermato- 
zoon this  filament  is  in  a  state  of  constant  movement.  The 
membrane  winds  spirally  round  the  tail. 

In  the  majority  of  forms  the  tail  of  the  living  spermatozoon 
exhibits  sinuous  cilia-like  movements.  In  two  groups  the  move- 
ments are  however  of  an  amoeboid  character.  These  groups  are 
the  Nematoda  and  the  Crustacea ;  and  the  spermatozoa  in  both 
of  them  frequently  present  very  abnormal  forms.  In  Nematoda 
they  are  pear-shaped,  cylindrical,  spine-shaped,  etc.,  and  are 
mainly  formed  of  protoplasm  with  a  highly  refracting  nucleus. 
In  the  Crustacea  the  variations  of  form  are  still  greater.  In  the 
Malacostraca  they  are  sometimes  simply  spherical  (Squilla), 
while  in  Astacus  and  a  large  number  of  Decapoda  they  are 
composed  of  a  nucleated  body  with  stellate  rays.  In  Paludina 
amongst  the  Mollusca  there  are  two  forms  of  completely  deve- 
loped spermatozoa  existing  side  by  side  in  the  same  individual. 

The  spermatozoa  are  formed  by  the  breaking  up  of  the  male 
germinal  cells,  or  of  cells  secondarily  derived  from  them  by 
division.  The  cells  which  directly  give  rise  by  division  to  the 
spermatozoa  may  be  called  spermospores  and  are  equivalent 
to  the  ova  or  oospores. 

Amongst  the  Sponges  (Halisarca,  Schultze,  No.  141)  a 
germinal  cell,  similar  to  that  which  in  the  female  becomes  an 
ovum,  repeatedly  divides  and  eventually  gives  rise  to  a  ball  of 
cells  (a  spermosphere  or  sperm-morula),  each  constituent 
cell  of  which  becomes  converted  into  a  spermatozoon,  and  may 
be  designated  by  the  special  term  'spermoblast.' 


THE  SPERMATOZOON.  67 

In  most  Hydrozoa  the  subepithelial  epiblastic  cells  become 
converted  into  germinal  cells  (spermospores),  and  then  break  up 
to  form  spermoblasts,  each  of  which  becomes  a  spermatozoon. 

In  most  higher  Metazoa  the  spermospores  usually  form  the 
epithelium  of  an  ampulla  or  tube,  though  more  rarely  (many 
Chaetopoda,  Gephyrea,  etc.)  they  may  be  derived  from  cells  lining 
the  body-cavity,  as  in  the  case  of  ova.  The  spermatozoa  are 
formed  either  by  the  direct  division  of  the  spermospores  into  a 
number  of  cells,  spermoblasts,  each  of  which  grows  into  a  sper- 
matozoon ;  or  by  the  nucleus  of  the  spermospore  becoming 
subdivided  within  the  cell  body,  the  latter  differentiating  itself 
into  the  tails  of  the  spermatozoa  while  the  segments  of  the 
nucleus  give  rise  to  the  main  part  of  the  heads. 

In  many  instances  interstitial  cells  which  do  not  give  rise  to  spermatozoa, 
are  intermingled  with  the  spermospores. 

In  a  good  many  cases,  as  first  pointed  out  by  Blomfield1,  the  whole  of 
each  spermospore  does  not  become  converted  into  spermatozoa,  but  part, 
either  with  or  without  a  segment  of  the  original  nucleus,  remains  passive, 
and  carrying  as  it  does  the  off-budded  spermoblasts  may  be  called  the 
*  sperm-blastophor.'  This  passive  portion  of  protoplasm  is  not  employed 
in  the  regeneration  of  the  spermoblast.  This  very  singular  phenomenon 
has  been  observed  in  Elasmobranchii,  the  Frog,  the  Earthworm,  Helix,  etc.2, 
and  probably  has  a  much  wider  extension.  In  Elasmobranchii  (Semper) 
the  passive  portions  of  protoplasm  are  nucleated,  and  are  placed  on  the 
outer  side  of  the  columnar  spermospores  which  line  the  testicular  ampullae  ; 
they  are  not  distinctly  differentiated  till  the  nuclei,  segmented  from  the 
nucleus  of  the  primitive  spermospore  to  form  the  heads  of  the  spermatozoa, 
have  become  fairly  numerous.  In  the  Frog  the  passive  blastophor  also 
occurs  as  a  nucleated  mass  of  protoplasm  on  the  outer  side  of  the  spermo- 
spore. In  the  Earthworm  the  blastophor  forms  a  central  non-nucleated 
portion  of  the  spermospore ;  and  the  whole  periphery  of  each  spermospore 
becomes  converted  into  spermoblasts. 

It  has  been  already  stated  in  the  introduction  that  the  male 
and  female  generative  products  are  homodynamous,  but  the 
consideration  of  the  development  of  the  products  in  the  two 
sexes  shews  that  a  single  spermatozoon  is  not  equivalent  to  an 
ovum,  but  rather  that  the  whole  of  the  spermatozoa  derived  from 
a  spermospore  are  together  equivalent  to  one  ovum. 

1  Quart.  Journ.  of  Micro.  Science,  Vol.  XX.  1880. 

2  Blomfield,  loc.  cit.,  p.  83,  states  that  he  has  observed  this  fact  in  Lumbricus, 
Tubifer,  Hirudo,  Helix,  Arion,  Paludina,  Rana,  Salamandra,  and  Mus. 

5—2 


CHAPTER  II. 


THE  MATURATION  AND  IMPREGNATION  OF  THE  OVUM. 


Maturation  of  ttu  ovum  and  formation  of  tJie  polar  bodies. 

IN  the  preceding  chapter  the  changes  in  the  ovum  were  described 
nearly  up  to  the  period  when  it  became  ripe,  and  ready  to  be 
impregnated.  Preparatory  to  the  act  of  impregnation  there 
take  place  however  a  series  of  remarkable  changes,  which  more 
especially  concern  the  germinal  vesicle. 

The  attention  of  a  large  number  of  investigators  has  recently  been 
directed  to  these  changes  as  well 
as  to  the  phenomena  of  impregna- 
tion. The  results  of  their  investi- 
gations will  be  described  in  the 
present  chapter  ;  but  for  an  histori- 
cal account  of  these  investigations, 
as  well  as  for  a  determination  of 
the  delicate  questions  of  priority, 
the  reader  is  referred  to  Fol's 
memoir  (No.  87),  and  to  a  paper 
by  the  author  (No.  81). 

The  nature  of  the  changes 
which  take  place  in  the 
maturation  of  the  ovum  may 
perhaps  be  most  convenient- 
ly displayed  by  following  the 
history  of  a  single  ovum. 
For  this  purpose  the  eggs  of 
Asterias  glacialis,  which  have 
recently  formed  the  subject  of  a  series  of  beautiful  researches 
by  Fol  (87),  may  be  selected. 

The  ripe  ovum  (fig.  22),  when  detached  from  the  ovary   is 
formed  of  a  granular  vitellus  enveloped  in  a  mucilaginous  coat, 


FIG.  11.  RIPE  OVUM  OF  ASTERIAS  GLA- 
CIALIS ENVELOPED  IN  A  MUCILAGINOUS 
ENVELOPE,  AND  CONTAINING  AN  ECCEN- 
TRIC GERMINAL  VESICLE  AND  GERMINAL 

SPOT  (copied  from  Fol). 


MATURATION  OF  THE  OVUM. 


69 


the  zona  radiata.  It  contains  an  eccentrically-situated  germinal 
vesicle  and  a  germinal  spot.  In  the  former  is  -present  the  usual 
protoplasmic  reticulum.  As  soon  as  the  ovum  reaches  the  sea- 
water  the  germinal  vesicle  commences  to  undergo  a  peculiar 
metamorphosis.  It  exhibits  frequent  changes  of  form,  the  reti- 


FIG.  23.    Two  SUCCESSIVE  STAGES   IN  THE  GRADUAL  METAMORPHOSIS  OF  THE 

GERMINAL  VESICLE  AND  SPOT  OF  THE  OVUM  OF  ASTERIAS  GLACIALIS   IMMEDI- 
ATELY AFTER  IT  is  LAID  (copied  from  Fol). 

culum  vanishes,  its  membrane  becomes  gradually  absorbed,  its 
outline  indented  and  indistinct,  and  finally  its  contents  become 
to  a  certain  extent  confounded  with  the  vitellus  (fig.  23). 

The  germinal  spot  .at  the  same  time  loses  its  clearness  of  out- 
line and  gradually  disappears  from  view. 

At  this  stage,  and  between  it  and  the  stage  represented  in 
fig.  26,  the  action  of  reagents  brings  to  light  certain  appearances 
the  nature  of  which  is  not  yet  fully  cleared  up  for  Asterias,  which 
have  been  described  somewhat  differently  by  Fol  for  Ast.  glacialis 
and  Hertwig  for  Asteracanthion. 

Fol  finds  immediately  after  the  stage 
just  described  that  a  star  is  visible 
between  the  remains  of  the  germinal 
vesicle  and  the  surface  of  the  egg, 
which  is  connected  with  an  imperfectly- 
formed  nuclear  spindle  extending  to- 
wards the  germinal  vesicle1.  At  the 
end  of  the  nuclear  spindle  may  be  seen 
the  broken  up  fragments  of  the  germi- 
nal spot. 

At  a  slightly  later  stage,  in  the 
place  of  the  original  germinal  vesicle 
there  may  be  observed  in  the  fresh 

1  By  the  term  'nuclear  spindle'  I  refer  to  the  peculiar  form  of  a  double  striated  cone 
assumed  by  the  nucleus  just  before  division,  which  is  no  doubt  familiar  to  all  my 
readers.  I  use  the  term  star  for  the  peculiar  stellate  figure  usually  visible  at  the  poles 
of  the  nuclear  spindle.  For  a  further  description  of  these  parts  the  reader  is  referred 
to  Chapter  IV. 


FIG.  24.  OVUM  OF  ASTERIAS  GLA- 
CIALIS, SHEWING  THE  CLEAR  SPACES 
IN  THE  PLACE  OF  THE  GERMINAL 

VESICLE.  FRESH  PREPARATION  (copied 
from  Fol).     . 


GERMINAL  VESICLE. 


ovum  two  clear  spaces  (fig.  24),  one  ovoid  and  nearer  the  surface,  and  the 
second  more  irregular  in  form  and  situated  rather  deeper  in  the  vitellus.  In 
the  upper  space  parallel  stride  may  be  observed.  By  treatment  with  reagents 
the  first  clear  space  is  found  to  be  formed  of  a  horizontally-placed  spindle 
with  two  terminal  stars,  near  which  irregular  remains  of  the  germinal  spot 
may  be  seen.  Slightly  later  (fig.  25)  there  may  be  seen  on  the  lower  side  of 
the  spindle  a  somewhat  irregular  body,  which  may  possibly  be  part  of  the 
remains  of  the  germinal  spot,  though  Fol  holds  that  it  is  probably  part  of  the 
membrane  of  the  germinal  vesicle.  The  lower  clear  space  visible  in  the 
fresh  ovum  now  contains  a  round  body,  fig.  25.  Fol  concludes  that  the 
spindle  is  formed  out  of  part  of  the 
germinal  vesicle  and  not  from  the 
germinal  spot,  while  he  sees  in  the 
round  body  present  in  the  lower  of 
the  two  clear  spaces  the  metamor- 
phosed germinal  spot.  He  will  not, 
however,  assert  that  no  fragment  of 
the  germinal  spot  enters  into  the  for- 
mation of  the  spindle. 

The  following  is  Hertwig's  (No. 
92)  account  of  the  changes  in  the 
germinal  vesicle  in  Asteracanthion. 
Shortly  after  the  egg  is  laid  the  proto- 
plasm on  the  side  of  the  germinal 
vesicle  towards  the  surface  of  the  egg 
develops  a  prominence  which  presses 

inwards  the  wall  of  the  vesicle.  At  the  same  time  the  germinal  spot 
develops  a  large  vacuole,  in  the  interior  of  which  is  a  body  consisting  of 
nuclear  substance,  and  formed  of  a  firmer  and  more  refractive  material  than 
the  remainder  of  the  germinal  spot.  In  the  prominence  first  mentioned  as 
projecting  inwards  towards  the  germinal  vesicle  first  one  star,  formed  by 
radial  striae  of  protoplasm,  and  then  a  second  make  their  appearance  ;  while 
the  germinal  spot  appears  to  have  vanished,  the  outline  of  the  germinal 
vesicle  to  have  become  indistinct,  and  its  contents  to  have  mingled  with  the 
surrounding  protoplasm.  Treatment  with  reagents  demonstrates  that  in  the 
process  of  disappearance  of  the  germinal  spot  the  nuclear  mass  in  its  vacuole 
forms  a  rod-like  body,  the  free  end  of  which  is  situated  between  the  two  stars 
which  occupy  the  prominence  indenting  the  germinal  vesicle.  At  a  later 
period  granules  may  be  seen  at  the  end  of  the  rod  and  finally  the  rod  itself 
vanishes.  After  these -changes  by  the  aid  of  reagents  there  may  be  demon- 
strated a  spindle  between  the  two  stars,  which  Hertwig  believes  to  grow  in 
size  as  the  last  remnants  of  the  germinal  spot  gradually  vanish,  and  he 
maintains  that  the  spindle  is  formed  at  the  expense  of  the  germinal  spot. 
The  stage  with  this  spindle  corresponds  with  fig.  25. 

Several  of  Hertwig's  figures  closely  correspond  with  those  of  Fol,  and 
considering  how  conflicting  is  the  evidence  before  us,  it  seems  necessary 


FlG.  25.  OVUM  OF  ASTERIAS  GLA- 
CIALIS,  AT  THE  SAME  STAGE  AS  FIG.  24, 
TREATED  WITH  PICRIC  ACID  (copied 

from  Fol). 


MATURATION   OF   THE   OVUM. 


to  leave  open  for  Asterias  the  question  as  to  what  parts  of  the   germinal 
vesicle  are  concerned  in  forming  the  first  spindle. 

A  clearer  view  of  the  phenomena  which  take  place  at  this  stage 
has  been  obtained  by  Fol  in  the  case  of  Heteropods  (Pterotra- 
chaea).  In  the  ovum  a  few  minutes  after  it  has  been  laid  the 
germinal  vesicle  becomes  very  pale,  and  two  stars  make  their 
appearance  round  a  clear  substance  near  its  poles.  The  nucleus 
itself  is  somewhat  elongated,  and  commences  to  exhibit  at  its 
poles  longitudinal  striae,  which  gradually  extend  towards  the 
centre  at  the  expense  of  the  nuclear  reticulum,  from  a  metamor- 
phosis of  which  they  are  directly  derived.  When  the  striae  of  the 
two  sides  have  nearly  met,  thickenings  may  be  observed  in  the 
recticulum  between  them,  which  give  rise,  where  the  striae  of 
the  two  sides  unite,  to  the  central  thickenings  of  the  fibres 
(nuclear  plate).  In  this  way  a  complete  nuclear  spindle  is 
established1. 

The  important  result  of  Fol's  observations  on  Heteropods, 
which  tallies  also  with  what  is  found  in  Asterias,  is  that  a  spindle 
with  two  stars  at  its  poles  is  formed  from  the  metamorphosis  of 
the  germinal  vesicle  and  surrounding  protoplasm  (fig.  25). 

Polar  cells.  The  spindle  has  up  to 
this  time  been  situated  with  its  axis 
parallel  to  the  surface  of  the  egg,  but  in 
somewhat  older  specimens  a  vertical 
spindle  is  found,  with  one  end  projecting 
into  a  protoplasmic  prominence  which 
makes  its  appearance  on  the  surface  of 
the  egg  (fig.  26).  Hertwig  believes  that 
the  spindle  simply  travels  towards  the 
surface,  and  while  doing  so  changes  the 
direction  of  its  axis.  Fol  asserts,  how- 
ever, that  this  is  not  the  case,  but  that 
between  the  two  phases  of  the  spindle 
an  intermediate  one  is  found  in  which  a 
spindle  can  no  longer  be  seen  in  the  egg,  but  its  place  is  taken 
by  a  body  with  a  dentated  outline.  He  has  not  been  able  to 
arrive  at  a  conclusion  as  to  what  meaning  is  to  be  attached  to 


Fig.    26.    PORTION   OF 


OF  THE  DETACHMENT  OF 
THE  FIRST  POLAR  BODY  AND 
THE  WITHDRAWAL  OF  THE 
REMAINING  PART  OF  THE 
SPINDLE  WITHIN  THE  OVUM. 
PICRIC  ACID  PREPARATION 

(copied  from  Fol). 


For  the  further  details  on  the  nuclear  spindle  vide  the  next  Chapter. 


POLAR  CELLS. 


FIG.  27.  PORTION  OF  THE  OVUM 
OF  ASTERIAS  GLACIALIS,  WITH  THE 
FIRST  POLAR  CELL  AS  IT  APPEARS 
WHEN  LIVING  (copied  from  Fol). 


this  occurrence,  which  does  not  appear  to  take  place  in  Hetero- 
pods. 

In  any  case  the  spindle  which 
projects  into  the   prominence   on 
the  surface  of  the  egg  divides  into 
two  parts,  one  in  the  prominence 
and  one  in  the  egg  (fig.  26).     The 
prominence  itself  with  the  enclosed 
portion  of  the  spindle  becomes  con- 
stricted off  from  the  egg  to  form  a 
body,  well  known  to  embryologists 
as  the  polar  body  or  cell  (fig.  27).     Since  more  than  one  polar 
cell  is  formed,  that  which  is  the  earli- 
est to  appear  may  be  called  the  first 
polar  cell. 

The  part  of  the  spindle  which  re- 
mains in  the  egg  becomes  directly  con- 
verted into  a  second  spindle  by  the 
elongation  of  its  fibres,  without  pass- 
ing through  a  typical   nuclear   con- 
dition.    A  second  polar  cell  next  be- 
comes formed  in  the  same  manner  as 
the  first  (fig.  28),  and  the  portion  of 
the  spindle  remaining  in  the  egg  be- 
comes converted  into  two  or  three  clear  vesicles  (fig.  29),  which 
soon  unite  to  form  a  single  nucleus  (fig. 
30).     The  new  nucleus  which  is  clearly 
derived  from  part  of  the  original  germinal 
vesicle  is  called  the  female  pronucleus, 
for  reasons  which  will  appear  in  the  sequel. 

The  two  polar  cells  appear  to  be  situ- 
ated between  two  membranes,  the  outer 
of  which  is  very  delicate,  and  only  dis- 
tinct where  it  covers  the  polar  cells,  while 
the  inner  one  is  thicker  and  becomes, 
after  impregnation,  more  distinct,  and 
then  forms  what  Fol  speaks  of  as  the 
vitelline  membrane.  It  is  clear,  as  Hert- 
wig  has  pointed  out,  that  the  polar  bodies 


FIG.  28.  PORTION  OF  THE 
OVUM  OF  ASTERIAS  GLACIALIS 
IMMEDIATELY  AFTER  THE  FOR- 
MATION OF  THE  SECOND  POLAR 
CELL.  PICRIC  ACID  PREPARA- 


FIG.   29.     PORTION  OF 

THE  OVUM  OF  ASTERIAS 
GLACIALIS  AFTER  THK  FOR- 
MATION OF  THE  SF.COND 
POLAR  CELL,  SHEWING  THE 
PART  OF  THE  SPINDLE  RE- 
MAINING IN  THE  OVUM  BE- 
COMING CONVERTED  INTO 
TWO  CLEAR  VESICLES.  PlC- 
RIC  ACID  PREPARATION 

(copied  from  Fol). 


MATURATION   OF  THE  OVUM. 


73 


originate  by  a  regular  process  of  cell-division  and  have  the  value 
of  cells. 

A  peculiar  phenomenon  makes  its  appearance  in  the  eggs  of  Clepsine 
shortly  after  the  formation  of  the  polar  cells,  which  has  been  spoken  of  by 
Whitman  (No.  100)  as  the  formation  of  the  polar  rings.  The  following  is  his 
description  of  the  occurrence. 

"  Fifteen  minutes  after  the  elimination  of  the  polar  globules  (i.e.  cells)  a 
ring-like  depression  or  constriction  appears  in  the  yolk  around  the  oral  pole, 
and  in  this  depression  a  transparent  liquid  substance  (nuclear  ?)  is  collected 
forming  the  first  polar  ring.... The  same  phenomena  repeat  themselves 
later  at  the  aboral  pole.... The  rings  concentrate  to  form  two  discs.... Before 
the  first  cleavage  both  discs  plunge  deep  into  the  egg." 

The  nature  of  these  rings  is  at  present  quite  obscure. 

Considering  how  few  ova  have 
been  adequately  investigated  with 
reference  to  the  behaviour  of  the 
germinal  vesicle,  any  general  con- 
clusions which  may  at  present  be 
formed  are  to  be  regarded  as  pro- 
visional. 

There  is  however  abundant 
evidence  that  at  the  time  of  matu- 
ration of  the  egg  the  germinal 
vesicle  undergoes  peculiar  changes, 
which  are,  in  part  at  least,  of  a 
retrogressive  character.  These 
changes  may  begin  considerably 
before  the  egg  has  reached  the 
period  of  maturity,  or  may  not  take  place  till  after  it  has  been 
laid.  They  consist  in  an  appearance  of  irregularity  and  obscurity 
in  the  outline  of  the  germinal  vesicle,  the  absorption  of  its  mem- 
brane, the  partial  absorption  of  its  contents  in  the  yolk,  the  dis- 
appearance of  the  reticulum,  and  the  breaking  up  and  disappear- 
ance of  the  germinal  spot.  The  exact  fate  of  the  single  germinal 
spot,  or  the  numerous  spots  where  they  are  present,  is  still  obscure. 

The  retrogressive  metamorphosis  of  the  germinal  vesicle  is 
followed  in  a  large  number  of  instances  by  the  conversion  of  what 
remains  into  a  striated  spindle  similar  in  character  to  a  nucleus 
previous  to  division^  This  spindle  travels  to  the  surface  of  the 
ovum  and  undergoes  division  to  form  the  polar  cell  or  cells  in  the 


FIG.  30.    OVUM  OF  ASTERIAS 
GLACIALIS  WITH  THE  TWO  POLAR 

CELLS  AND  THE  FEMALE  PRONUCLEUS 
SURROUNDED  BY  RADIAL  STRIDE,  AS 
SEEN  IN  THE  LIVING  EGG  (copied  from 

Fol). 


74  POLAR  CELLS. 


manner  above  described.     The  part  which  remains  in  the  egg 
forms  eventually  the  female  pronucleus. 

The  germinal  vesicle  has  up  to  the  present  time  only  been 
observed  to  undergo  the  above  series  of  changes  in  a  certain 
number  of  instances,  which,  however,  include  examples  from 
several  divisions  of  the  Ccelenterata,  the  Echinodermata,  and  the 
Mollusca,  some  of  the  Vermes  [Turbellarians  (Leptoplana], 
Nematodes,  Hirudinea,  Alciope,  Sagitta],  Ascidians,  etc.  It  is 
very  possible,  not  to  say  probable,  that  such  changes  are  univer- 
sal in  the  animal  kingdom,  but  the  present  state  of  our  knowledge 
does  not  justify  us  in  saying  so. 

In  the  Craniata  especially  our  knowledge  of  the  formation  of  the  polar 
bodies  is  very  unsatisfactory.  In  Petromyzon  Kupffer  and  Benecke  have 
brought  forward  evidence  to  shew  that  one  polar  body  is  formed  prior  to 
the  impregnation,  and  a  second  in  connection  with  a  peculiar  prominence 
of  protoplasm  after  impregnation.  Part  of  the  germinal  vesicle  remains  in 
the  egg  as  the  female  pronucleus.  In  the  Sturgeon  the  germinal  vesicle 
atrophies  and  breaks  up  before  impregnation,  and  afterwards  part  is  found  as  a 
granular  mass  on  the  surface  of  the  egg,  while  part  forms  a  female  pronucleus. 

In  Amphibia  the  observations  of  Hertwig  (90)  and  Bambeke  (77)  tend  to 
shew  that  after  the  germinal  vesicle  has  assumed  a  superficial  situation  at 
the  pigmented  pole  of  the  ovum  its  contents  become  intermingled  with  the 
yolk,  and  are  in  part  extruded  from  the  ovum  as  a  granular  mass  after 
impregnation.  Part  of  them  remains  in  the  ovum  and  forms  a  female 
pronucleus.  Whether  there  is  a  proper  division  of  the  germinal  vesicle 
as  in  typical  cases  is  not  known. 

Oellacher  (95)  by  a  series  of  careful  observations  upon  the  egg  of  the  trout, 
and  subsequently  of  the  bird,  demonstrated  that  in  the  ovum  while  still  in 
the  ovary,  the  germinal  vesicle  underwent  a  kind  of  degeneration  and 
eventually  became  ejected,  in  part  at  any  rate.  My  own  observations  on 
Elasmobranchs,  which  require  enlargement  and  confirmation,  tend  to  shew 
that  this  part  may  be  the  membrane.  Ed.  van  Beneden  (78)  has  contributed 
some  important  observations  on  the  rabbit.  His  account  is  as  follows.  As 
the  ovum  approaches  maturity  the  germinal  vesicle  assumes  an  eccentric 
position,  and  fuses  with  the  peripheral  layer  of  the  egg  to  constitute  the 
cicatricular  lens.  The  germinal  spot  next  travels  to  the  surface  of  the 
cicatricular  lens  and  forms  the  nuclear  disc:  at  the  same  time  the  membrane 
of  the  germinal  vesicle  vanishes,  though  it  probably  unites  with  the  nuclear 
disc.  The  plasma  of  the  nucleus  then  collects  into  a  definite  mass  and  forms 
the  nucleoplasmic  body.  Finally  the  nuclear  disc  assumes  an  ellipsoidal 
form  and  becomes  the  nuclear  body.  Nothing  is  now  left  of  the  original 
germinal  vesicle  but  the  nuclear  body  and  the  nucleoplasmic  body,  both  still 
situated  within  the  ovum.  In  the  next  stage  no  trace  of  the  germinal 


MATURATION   OF   THE  OVUM.  75 

vesicle  can  be  detected  in  the  ovum,  but  outside  it,  close  to  the  point  where 
the  modified  remnants  of  the  vesicle  were  previously  situated,  there  is 
present  a  polar  body  which  is  composed  of  two  parts,  one  of  which  stains 
deeply  and  resembles  the  nuclear  body,  and  the  other  does  not  stain  but  is 
similar  to  the  nucleoplasmic  body.  Van  Beneden  concludes  that  the  parts  of 
the  polar  body  are  the  two  ejected  products  of  the  germinal  vesicle.  We  may 
be  perhaps  permitted  to  hold  that  further  observations  on  this  difficult  object 
will  demonstrate  that  part  of  the  germinal  vesicle  remains  in  the  ovum  to 
form  the  female  pronucleus. 

With  reference  to  invertebrate  forms  attention  may  be  called  to  the 
observations  of  Biitschli  (80).  Although  in  Cucullanus  a  normal  formation 
of  the  polar  bodies  takes  place,  yet  in  the  Nematodes  generally,  Biitschli  has 
been  unable  to  find  the  spindle  modification  of  the  germinal  vesicle,  but 
states  that  the  germinal  vesicle  undergoes  degeneration,  its  outline  becom- 
ing indistinct  and  the  germinal  spot  vanishing.  The  position  of  the 
germinal  vesicle  continues  to  be  marked  by  a  clear  space,  which  gradually 
approaches  the  surface  of  the  egg.  When  it  is  in  contact  with  the  surface 
a  small  spherical  body,  the  remnant  of  the  germinal  vesicle,  comes  into  view, 
and  eventually  becomes  ejected.  The  clear  space  subsequently  disappears. 

In  addition  to  the  types  just  quoted,  which  may  very  pro- 
bably turn  out  to  be  normal  in  the  mode  of  formation  of  the 
polar  bodies,  there  is  a  large  number  of  types,  including  the 
whole  of  the  Rotifera  and  Arthropoda  with  a  few  doubtful 
exceptions1,  in  which  the  polar  cells  cannot  as  yet  be  said  to 
have  been  satisfactorily  observed. 

The  more  important  of  the  doubtful  cases  amongst  the  Rotifera  and  Ar- 
thropoda are  the  following. 

Flemming  (83)  finds  that  in  the  summer  and  probably  parthenogenetic 
eggs  of  Lacinularia  socialis  the  germinal  vesicle  approaches  the  surface 
and  becomes  invisible,  and  that  subsequently  a  slight  indentation  in  the 
outline  of  the  egg  marks  the  point  of  its  disappearance.  In  the  hollow  of 
the  indentation  Flemming  believes  a  polar  cell  to  be  situated,  though  he 
has  not  definitely  seen  one. 

Hoek2  believes  that  he  has  found  a  polar  body  in  the  ovum  of  Balanus 
balanoides,  but  his  observations  are  not  perfectly  satisfactory. 

1  The  best  instance  of  what  appears  like  a  polar  cell  in  Arthropoda  is  a  body 
recently  found  by  Grobben  (" Entwicklungsgeschichte  d.  Moina  rectirostris."     Claus' 
Arbeiten,  Vol.  II.,  Wien,  1879)  near  the  surface  of  the  protoplasm  at  the  animal  pole 
of  the  summer  and  parthenogenetic  eggs  of  Moina  rectirostris,  one  of  the  Cladocera. 
The  body  stains  deeply  with  carmine,  but  differs  from  normal  polar  cells  in  not  being 
separated  from  the  ovum ;  and  its  identification  as  a  polar  cell  must  remain  doubtful 
till  it  has  been  shewn  to  originate  from  the  germinal  vesicle. 

2  "Zur  -Entwicklung  d.  Entomostraken."     Niederlandischer  Archiv.  f.  Zoologie, 
Vol.  in.  p.  62. 


76  FUNCTION  OF   POLAR  CELLS. 

Biitschli,  who  has  expressly  searched  for  the  polar  bodies  in  the  ova  of 
Rotifera,  was  unable  to  find  any  trace  of  them,  though  he  found  that  as  the 
egg  became  ripe  the  germinal  vesicle  became  half  its  original  size.  In  the 
parthenogenetic  eggs  of  Aphis  he  also  failed  to  find  a  trace  of  polar  bodies, 
though  the  germinal  vesicle,  after  the  germinal  spot  had  broken  up  into 
fragments,  approached  the  surface  and  disappeared. 

Whatever  may  be  the  eventual  result  of  more  extended 
investigation,  it  is  clear  that  the  formation  of  polar  cells 
according  to  the  type  described  above  is  a  very  constant 
occurrence.  Its  importance  is  increased  by  the  discovery  by 
Strasburger  of  the  existence  of  an  analogous  process  amongst 
plants.  Two  questions  about  it  obviously  present  themselves 
for  solution  :  (i)  What  are  the  conditions  of  its  occurrence  with 
reference  to  impregnation  ?  (2)  What  meaning  has  it  in  the 
development  of  the  ovum  or  the  embryo  ? 

The  answer  to  the  first  of  these  questions  is  not  difficult  to 
find.  The  formation  of  the  polar  bodies  is  independent  of 
impregnation,  and  is  the  final  act  of  the  normal  growth  of  the 
ovum.  In  a  few  types  the  polar  cells  are  formed  while  the 
ovum  is  still  in  the  ovary,  as,  for  instance,  in  some  species  of 
Echini,  Hydra,  etc.,  but,  according  to  our  present  knowledge,  far 
more  usually  after  the  ovum  has  been  laid.  In  some  instances 
the  budding-off  of  the  polar  cells  precedes,  and  in  other  instances 
follows  impregnation  ;  but  there  is  no  evidence  to  shew  that  in 
the  latter  cases  the  process  is  influenced  by  the  contact  with  the 
male  element.  In  Asterias,  as  has  been  shewn  by  O.  Hertwig 
and  Fol,  the  formation  of  the  polar  cells  may  indifferently  either 
precede  or  follow  impregnation — a  fact  which  affords  a  clear 
demonstration  of  the  independence  of  the  two  occurrences. 

To  the  second  of  the  two  questions  it  does  not  unfortunately 
seem  possible  at  present  to  give  an  answer  which  can  be  re- 
garded as  satisfactory. 

The  retrogressive  changes  in  the  membrane  of  the  germinal 
vesicle  which  usher  in  the  formation  of  the  polar  bodies  may 
very  probably  be  viewed  as  a  prelude  to  a  renewed  activity  of 
the  contents  of  the  vesicle  ;  and  are  perhaps  rendered  the  more 
necessary  from  the  thickness  of  the  membrane  which  results 
from  a  protracted  period  of  passive  growth.  This  suggestion 
does  not,  however,  help  us  to  explain  the  formation  of  polar 


MATURATION  OF  THE  OVUM. 


bodies  by  a  process  identical  with  cell-division.  The  ejection  of 
part  of  the  germinal  vesicle  in  the  formation  of  the  polar  cells 
may  probably  be  paralleled  by  the  ejection  of  part  or  the  whole 
of  the  original  nucleus  which,  if  we  may  trust  the  beautiful 
researches  of  Butschli,  takes  place  during  conjugation  in  In- 
fusoria as  a  preliminary  to  the  formation  of  a  fresh  nucleus. 
This  comparison  is  due  to  Butschli,  and  according  to  it  the 
formation  of  the  polar  bodies  would  have  to  be  regarded  as 
assisting,  in  some  as  yet  unknown  way,  the  process  of  regene- 
ration of  the  germinal  vesicle.  Views  analogous  to  this  are  held 
by  Strasburger  and  Hertwig,  who  regard  the  formation  of  the 
polar  bodies  in  the  light  of  a  process  of  excretion  or  removal  of 
useless  material.  Such  hypotheses  do  not,  unfortunately,  carry 
us  very  far. 

I  would  suggest  that  in  the  formation  of  the  polar  cells  part 
of  the  constituents  of  the  germinal  vesicle,  which  are  requisite 
for  its  functions  as  a  complete  and  independent  nucleus,  is 
removed,  to  make  room  for  the  supply  of  the  necessary  parts  to 
it  again  by  the  spermatic  nucleus. 

My  view  amounts  to  the  following,  viz.  that  after  the  forma- 
tion of  the  polar  cells  the  remainder  of  the  germinal  vesicle 
within  the  ovum  (the  female  pronucleus)  is  incapable  of  further 
development  without  the  addition  of  the  nuclear  part  of  the 
male  element  (spermatozoon),  and  that  if  polar  cells  were  not 
formed  parthenogenesis  might  normally  occur.  A  strong  sup- 
port for  this  hypothesis  would  be  afforded  were  it  to  be  definitely 
established  that  a  polar  body  is  not  formed  in  the  Arthropoda 
and  Rotifera  ;  since  the  normal  occurrence  of  parthenogenesis 
is  confined  to  these  two  groups.  It  is  certainly  a  remarkable 
coincidence  that  they  are  the  only  two  groups  in  which  polar 
bodies  have  not  so  far  been  satisfactorily  observed. 

It  is  perhaps  possible  that  the  part  removed  in  the  formation  of  the 
polar  cells  is  not  absolutely  essential  ;  and  this  seems  at  first  sight  to  follow 
from  the  fact  of  parthenogenesis  being  possible  in  instances  where  impreg- 
nation is  the  normal  occurrence.  The  genuineness  of  the  observations 
on  this  head  is  too  long  a  subject  to  enter  into  here1,  but  after  admitting, 

1  The  instances  quoted  by  Siebold,  Parthenogenesis  d.  Arthropoden,  are  not  quite 
satisfactory.  In  Hensen's  case,  p.  234,  impregnation  would  have  been  possible  if  we 
can  suppose  the  spermatozoa  to  be  capable  of  passing  into  the  body-cavity  through  the 


78  FUNCTION   OF.  POLAR  CELLS. 

as  we  probably  must,  that  there  are  genuine  cases  of  such  parthenogenesis, 
it  cannot  be  taken  for  granted  without  more  extended  observation  that  the 
occurrence  of  development  in  these  rare  instances  may  not  be  due  to  the 
polar  cells  not  having  been  formed  as  usual,  and  that  when  the  polar  cells 
are  formed  the  development  without  impregnation  is  impossible. 

Selenka  found  in  the  case  of  Purpura  lapillus  that  no  polar  body  was 
formed  in  the  eggs  which  did  not  develop,  but  in  the  case  of  Neritina, 
Biitschli  has  found  that  this  does  not  hold  good. 

The  remarkable  observations  of  Greeff  (No.  88)  on  the  parthenogenetic 
development  of  the  eggs  of  Asterias  rubens  tell,  however,  very  strongly 
against  the  above  hypothesis.  Greeff  has  found  that  under  normal 
circumstances  the  eggs  of  this  species  of  starfish  will  develop  without 
impregnation  in  simple  sea-water.  The  development  is  quite  regular  and 
normal,  though  much  slower  than  in  the  case  of  impregnated  eggs.  It  is 
not  definitely  stated  that  polar  cells  are  formed,  but  there  can  be  no  doubt 
that  this  is  implied.  GreefPs  account  is  so  precise  and  circumstantial  that 
it  is  not  easy  to  believe  that  any  error  can  have  crept  in  ;  but  neither 
Hertwig  nor  Fol  have  been  able  to  repeat  his  experiments,  and  we  may  be 
permitted  to  wait  for  further  confirmation  before  absolutely  accepting  them. 

To  the  suggestion  already  made  with  reference  to  the  function  of  the 
polar  cells,  I  will  venture  to  add  the  further  one,  that  the  function  of 
forming  polar  cells  has  been  acquired  by  the  ovum  for  the  express  purpose  of 
Preventing  parthenogenesis. 

The  explanation  given  by  Mr  Darwin  of  the  evil  effects  of  self-fertiliza- 
tion, viz.  the  want  of  sufficient  differentiation  in  the  sexual  elements1, 
would  apply  with  far  greater  force  to  cases  of  parthenogenesis. 

In  the  production  of  fresh  individuals,  two  circumstances  are  obviously 
favourable  to  the  species,  (i)  That  the  maximum  number  possible  of  fresh 
individuals  should  be  produced,  (2)  That  the  individuals  should  be  as 
vigorous  as  possible.  Sexual  differentiation  (even  in  hermaphrodites) 
is  clearly  very  inimical  to  the  production  of  the  maximum  number  of 
individuals.  There  can  be  little  doubt  that  the  ovum  is  potentially  capable 
of  developing  by  itself  into  a  fresh  individual,  and  therefore,  unless  the 
absence  of  sexual  differentiation  was  very  injurious  to  the  vigour  of  the 
progeny,  parthenogenesis  would  most  certainly  be  a  very  constant  occur- 
rence ;  and,  on  the  analogy  of  the  arrangements  in  plants  to  prevent  self- 
fertilization,  we  might  expect  to  find  some  contrivance  both  in  animals  and  in 

open  end  of  the  uninjured  oviduct ;  and  though  Oellacher's  instances  are  more  valuable, 
yet  sufficient  care  seems  hardly  to  have  been  taken,  especially  when  it  is  not  certain 
for  what  length  of  time  spermatozoa  may  be  able  to  live  in  the  oviduct.  For  Oellacher's 
precautions,  vide  Zeit.  fur  Wiss.  Zool.,  Bd.  xxii.,  p.  202.  A  better  instance  is  that 
of  a  sow  given  by  Bischoff,  Ann.  Sci.  Nat.,  series  3,  Vol.  n.,  1844.  The  unimpreg- 
nated  eggs  were  found  divided  into  segments,  but  the  segments  did  not  contain  the 
usual  nucleus,  and  were  perhaps  nothing  else  than  the  parts  of  an  ovum  in  a  state  of 
disruption. 

1  Darwin,  Cross-  and  Self- Fertilization  of  Plants,  p.  443. 


MATURATION  OF  THE  OVUM.  79 

plants  to  prevent  the  ovum  developing  by  itself  without  fertilization.  If 
my  view  about  the  polar  cells  is  correct,  the  formation  of  these  bodies 
functions  as  such  a  contrivance. 

Reproduction  by  budding  or  fission  has  probably  arisen  as  a  means  of 
increasing  the  number  of  individuals  produced,  so  that  the  co-existence  of 
asexual  with  sexual  reproduction  is  to  be  looked  on  as  a  kind  of  compromise 
for  the  loss  of  the  power  of  rapid  reproduction  due  to  the  absence  of 
parthenogenesis.  In  the  Arthropoda  and  Rotifera  the  place  of  budding  has 
been  taken  by  parthenogenesis,  which  may  be  a  frequent,  though  not  always 
a  necessary  occurrence,  as  in  various  Branchiopoda  (Apus,  Limnadia,  etc.) 
and  Lepidoptera  (Psyche  helix:,  etc.);  or  a  regular  occurrence  for  the  pro- 
duction of  one  sex,  as  in  Bees,  Wasps,  Nematus,  etc.  ;  or  an  occurrence 
confined  to  a  certain  stage  in  the  cycle  of  development  in  which  all  the 
individuals  reproduce  their  kind  parthenogenetically,  as  in  Aphis,  Ceci- 
domyia,  Gall  Insects  (Neuroterus,  etc.),  Daphnia1. 

On  my  hypothesis  the  possibility  of  parthenogenesis,  or  at  any  rate  its 
frequency,  in  Arthropoda  and  Rotifera  is  possibly  due  to  the  absence  of  polar 
cells.  In  the  case  of  all  animals,  so  far  as  is  known  to  me,  fertilization  of 
the  ovum  occasionally  occurs2,  but  there  are  instances  in  the  vegetable  king- 
dom where  so-called  parthenogenesis  appears  to  be  capable  of  recurring  for 
an  indefinite  period.  One  of  the  best  instances  appears  to  be  that  of 
Ccelebogyne,  an  introduced  exotic  Euphorbiaceous  plant  which  regularly 
produces  fertile  seeds  although  a  male  flower  never  appears.  The  recent 
researches  of  Strasburger  have  however  shewn  that  in  Ccelebogyne  and  other 
parthenogenetic  flowering  plants,  embryos  are  formed  by  the  budding  and 
subsequent  development  of  cells  belonging  to  the  ovule.  This  being  the 
case,  it  is  impossible  to  assert  of  these  plants  that  they  are  really  partheno- 
genetic, for  the  embryos  contained  in  the  seed  of  a  flower  which  has 
certainly  not  been  fertilized,  may  have  been  formed,  not  by  the  development 
of  the  ovum,  but  by  budding  from  the  surrounding  tissue  of  the  ovule. 

The  above  view  with  reference  to  the  nature  of  the  polar  bodies  is  not 
to  be  regarded  as  forming  more  than  an  hypothesis. 

Impregnation  of  the  Ovum. 

A  far  greater  amount  of  certainty  has  been  attained  as  to  the 
effects  of  impregnation  than  as  to  the  changes  of  the  germinal 
vesicle  which  precede  this,  and  there  appears,  moreover,  to  be  a 
greater  uniformity  in  the  series  of  resulting  phenomena. 

1  Mr  J.  A.  Osborne  has  recently  shewn  (Nature,  Sept.  4,  1879),  that  the  eggs  of  a 
Beetle  (Gastrophysa  raphani)  may  occasionally  develop,  up  to  a  certain  point  at  any 
rate,  without  the  male  influence. 

2  Dicyema,  which  is  an  apparent  exception,  has  not  yet  been  certainly  shewn  to 
develop  true  ova.      If  its  germs  are  true  ova  it  forms  an  exception  to  the  above 
rule. 


IMPREGNATION   OF  THE  OVUM. 


It  will  be  convenient  again  to  take  Asterias  glacialis  as  the 
type.  The  part  of  the  germinal  vesicle  which  remains  in  the 
egg,  after  the  formation  of  the  second  polar  cell,  becomes  con- 
verted into  a  number  of  small  vesicles  (fig.  29),  which  aggregate 


B. 

FIG.  31.  SMALL  PORTIONS  OF  THE  OVUM  OF  ASTERIAS  GLACIALIS.  THE  SPERMA- 
TOZOA ARE  SHEWN  ENVELOPED  IN  THE  MUCILAGINOUS  COAT.  IN  A.  A  PROMI- 
NENCE IS  RISING  FROM  THE  SURFACE  OF  THE  EGG  TOWARDS  THE  NEAREST 

SPERMATOZOON;  AND  IN  B.  THE  SPERMATOZOON  AND  PROMINENCE  HAVE  MET. 
(Copied  from  Fol.) 


themselves  into  a  single  clear  nucleus,  which 
toward  the  centre  of  the  egg  and  around 
which,  as  a  centre,  the  protoplasm  becomes 
radiately  striated  (fig.  30).  This  nucleus  is 
known  as  the  female  pronucleus.  By 
the  action  of  reagents  a  nucleolus  may  be 
shewn  in  it.  In  Asterias  glacialis  the  most 
favourable  period  for  fecundation  is  about  an 
hour  after  the  formation  of  the  female  pro- 
nucleus.  If  at  this  time  the  spermatozoa  are 
allowed  to  come  in  contact  with  the  egg, 
their  heads  soon  become  enveloped  in  the 
investing  mucilaginous  coat.  A  prominence, 
pointing  towards  the  nearest  spermatozoon, 
now  arises  from  the  superficial  layer  of  pro- 
toplasm of  the  egg,  and  grows  till  it  comes 
in  contact  with  the  spermatozoon  (fig.  31,  A 
and  B).  Under  normal  circumstances  the 
spermatozoon  which  meets  the  prominence  is 
the  only  one  concerned  in  the  fertilization, 


gradually  travels 


1''IG.  32.  1'OKTION 
OF  THE  OVUM  OF  AS- 
TERIAS  GLACIALIS  AF- 
TER THE  ENTRANCE 
OF  A  SPERMATOZOON 
INTO  THE  OVUM,  IT 
SHEWS  THE  PROMI- 
NENCE OF  THE  OVUM 
THROUGH  WHICH  THE 
SPERMATOZOON  HAS 
ENTERED.  A  VITEL- 
LINEMEMBRANEW1TH 
A  CRATER-LIKE  OPEN- 
ING HAS  BECOME  DIS- 
TINCTLY FORMED. 

(Copied  from  Fol.) 


IMPREGNATION   OF   THE  OVUM. 


8l 


and  it  makes  its  way  into  the  egg  by  passing  through  the  promi- 
nence. The  tail  of  the  spermatozoon,  no  longer  motile,  remains 
visible  for  some  time  after  the  head  has  bored  its  way  in,  but  its 
place  is  soon  taken  by  a  pale  conical  body,  which  is,  however, 
probably  in  part  a  product  of  the  metamorphosis  of  the  tail 
itself  (fig.  32).  It  eventually  becomes  absorbed  into  the  body  of 
the  ovum. 

At  the  moment  of  contact  between  the  spermatozoon  and 
the  egg  the  outermost  layer  of  the  protoplasm  of  the  latter 
raises  itself  as  a  distinct  membrane,  which  separates  from  the 
egg  and  prevents  the  entrance  of  other  spermatozoa.  At  the 
point  where  the  spermatozoon  entered  a  crater-like  opening  is 
left  in  the  membrane,  through  which  the  metamorphosed  tail  of 
the  spermatozoon  may  at  first  be  seen  projecting  (fig.  32). 

The  head  of  the  spermatozoon  when  in  the  egg  forms  a 
nucleus,  for  which  the  name  male 
pronucleus  may  be  conveniently 
adopted.  It  grows  in  size,  pro- 
bably by  assimilating  material 
from  the  ovum,  and  around  it  is 
formed  a  clear  space  free  from 
yolk-spherules.  Shortly  after  its 
formation  the  protoplasm  in  its 
neighbourhood  assumes  a  radiate 
arrangement  (fig.  33).  At  what- 
ever point  of  the  egg  the  sperma- 
tozoon may  have  entered,  it  grad- 
ually travels  towards  the  female  FlG.  33>  QVUM  OF  ASTERIAS 
pronucleus.  The  latter,  around  GLACIALIS,  WITH  MALE  AND  FEMALE 

PRONUCLEUS   AND   A    RADIAL    STRIA- 

which  the  protoplasm  no  longer     TION  OF  THE  PROTOPLASM  AROUND 
has    a    radiate    arrangement,    re-     THE  FORMER.    (Copied  from  FoL) 
mains  motionless  till  the  rays  of 

the  male  pronucleus  come  in  contact  with  it,  after  which  its 
condition  of  repose  is  exchanged  for  one  of  activity,  and  it 
rapidly  approaches  the  male  pronucleus,  apparently  by  means 
of  its  inherent  amoeboid  contractions,  and  eventually  fuses  with 
it  (figs.  34—36). 

As  the  male  pronucleus  approaches  the  female  the  latter, 
according  to  Selenka,  sends  out  protoplasmic  processes  which 
B.  n.  6 


82 


MALE  PRONUCLEUS. 


embrace  the  former.  The  actual  fusion  does  not  take  place  till 
after  the  pronuclei  have  been  in  contact  for  some  time.  While 
the  two  pronuclei  are  approaching  one  another  the  protoplasm 
of  the  egg  exhibits  amoeboid  movements. 

The  product  of  the  fusion  of  the  two  pronuclei  forms  the  first 
segmentation  nucleus  (fig.  37),  which  soon,  however,  divides  into 
the  two  nuclei  of  the  two  first  segmentation  spheres. 

The  phenomenon  which  has  just  been  described  consists 
essentially  in  the  fusion  of  the  male  cell  and  the  female  cell.  In 
this  act  the  protoplasm  of  the  two  cells  as  well  as  their  nuclei 
coalesce,  since  the  whole  spermatozoon  which  has  been  absorbed 
into  the  ovum  is  a  cell  of  which  the  head  is  the  nucleus. 

It  is  clear  that  the  ovum  after  fertilization  is  an  entirely 
different  body  to  the  ovum  prior  to  that  act,  and  unless  the  use 
of  the  same  term  for  the  two  conditions  of  the  ovum  had  become 
very  familiar,  a  special  term,  such  as  oosperm,  for  the  ovum 
after  its  fusion  with  the  spermatozoon,  would  be  very  convenient. 


FIGS.  34,  35,  AND  36.  THREE  SUCCESSIVE  STAGES  IN  THE  COALESCENCE  OF  THE 
MALE  AND  FEMALE  PRONUCLEI  IN  ASTERIAS  GLACIALIS.  FROM  THE  LIVING 
OVUM.  (Copied  from  Fol.) 

Of  the  earlier  observations  on  this  subject  there  need  perhaps  only  be 
cited  one  of  E.  van  Beneden,  on  the 
rabbit's  ovum,  shewing  the  presence  of 
two  nuclei  before  the  commencement  of 
segmentation.  Butschli  was  the  earliest 
to  state  from  observations  on  Rhabditis 
dolichura  that  the  first  segmentation 
nucleus  arose  from  the  fusion  of  two 
nuclei,  and  this  was  subsequently  shewn 
with  greater  detail  for  Ascaris  nigrove- 
nosa,  by  Auerbach  (76).  Neither  of  these 
authors  gave  at  the  first  the  correct  in- 
terpretation of  their  results.  At  a  later 
period  Butschli  (80)  arrived  at  the  con- 
clusion that  in  a  large  number  of  in-  FIG.  37  OVUM  OF  ASTERIAS 
stances  (Lymnaus,  Nephelis,  Cucullanus,  GLACIALIS,  AFTER  THE  COALESCENCE 

f  '      OF   THE   MALE  AND    FEMALE   PRONU- 

&c.),  the  nucleus  in  question  was  formed     CLEI.     (Copied  from  Fol.) 


IMPREGNATION  OF  THE  OVUM.  83 

by  the  fusion  of  two  or  more  nuclei,  and  Strasburger  at  first  made  a 
similar  statement  for  Phallusia,  though  he  has  since  withdrawn  it.  Though 
Biitschli's  statements  depend,  as  it  seems,  upon  a  false  interpretation  of 
appearances,  he  nevertheless  arrived  at  a  correct  view  with  reference  to 
what  occurs  in  impregnation.  Van  Beneden  (78)  described  in  the  rabbit 
the  formation  of  the  original  segmentation  nucleus  from  two  nuclei,  one 
peripheral  and  the  other  central,  and  deduced  from  his  observations  that 
the 'peripheral  nucleus  was  derived  from  the  spermatic  element.  It  was 
reserved  for  Oscar  Hertwig  (89)  to  describe  in  Echinus  lividus  the  en- 
trance of  a  spermatozoon  into  the  egg  and  the  formation  from  it  of  the 
male  pronucleus. 

The  general  fact  that  impregnation  consists  in  the  fusion 
of  the  spermatozoon  and  ovum  has  now  been  established  for 
some  forms  in  the  majority  of  invertebrate  groups  (Arthropoda 
and  Rotifera  excepted).  Amongst  Vertebrata  also  it  has  been 
shewn  by  E.  van  Beneden  that  the  first  segmentation  nucleus  is 
formed  by  the  coalescence  of  the  male  and  female  pronucleus. 
Calberla,  and  Kupffer  and  Benecke  have  demonstrated  that  a 
single  spermatozoon  enters  at  first  the  ovum  of  Petromyzon. 

The  contact  of  the  spermatozoon  with  the  egg-membrane  causes  in  Petro- 
myzon active  movements  of  the  protoplasm  of  the  ovum,  and  a  retreat 
of  the  protoplasm  from  the  membrane. 

In  Amphibia  the  appearance  of  a  peculiar  pigmented  streak 
extending  inwards  from  the  surface  of  the  pigmented  pole  of  the 
ovum,,  and  containing  in  a  clear  space  at  its  inner  extremity  a 
nucleus,  has  been  demonstrated  as  the  result  of  impregnation  by 
Bambeke  (77)  and  Hertwig  (90).  There  can  be  little  doubt  that 
this  nucleus  is  the  male  pronucleus,  and  that  the  pigmented 
streak  indicates  its  path  inwards.  Close  to  it  Hertwig  has 
shewn  that  another  nucleus  is  to  be  found,  the  female  pronucleus, 
and  that  eventually  the  two  join  together.  In  Amphibia  the 
phenomena  accompanying  impregnation  are  clearly  of  the  same 
nature  as  in  the  Invertebrata.  A  precisely  similar  series  of 
phenomena  to  those  in  Amphibia  has  been  shewn  by  Salensky 
to  take  place  in  the  Sturgeon. 

Although  there  is  a  general  agreement  between  the  most  recent  observers, 
Hertwig,  Fol,  Selenka,  Strasburger,  £c.,  as  to  the  main  facts  connected 
with  the  entrance  of  one  spermatozoon  into  the  egg,  the  formation  of  the 
male  pronucleus,  and  its  fusion  with  the  female  pronucleus,  there  still  exist 
differences  of  detail  in  the  different  descriptions,  which  partly,  no  doubt, 

6—2 


84  MALE  PRONUCLEUS. 


depend  upon  the  difficulties  of  observation,  but  partly  also  upon  the  observa- 
tions not  having  all  been  made  upon  the  same  species.  Hertwig  does  not 
enter  into  details  with  reference  to  the  actual  entrance  of  the  spermatozoon 
into  the  egg,  but  in  his  latest  paper  points  out  that  considerable  differences 
may  be  observed  in  the  occurrences  which  succeed  impregnation,  according 
to  the  relative  period  at  which  this  takes  place.  When,  in  Asterias,  the 
impregnation  is  effected  about  an  hour  after  the  egg  is  laid,  and  previously 
to  the  formation  of  the  polar  cells,  the  male  pronucleus  appears  at  first  to 
exert  but  little  influence  on  the  protoplasm,  but  after  the  formation  of  the 
second  polar  cell,  the  radial  striae  around  it  become  very  marked,  and  the 
pronucleus  rapidly  grows  in  size.  When  it  finally  unites  with  the  female 
pronucleus  it  is  equal  in  size  to  the  latter.  In  the  case  when  the  impregna- 
tion is  deferred  for  four  hours  the  male  pronucleus  never  becomes  so  large 
as  the  female  pronucleus.  With  reference  to  the  effect  of  the  time  at 
which  impregnation  takes  place,  Asterias  would  seem  to  serve  as  a  type. 
Thus  in  Hirudinea,  Mollusca,  and  Nematoidea  impregnation  normally  takes 
place  before  the  formation  of  the  polar  bodies  is  completed,  and  the  male 
pronucleus  is  accordingly  as  large  as  the  female.  In  Echinus,  on  the  other 
hand,  where  the  polar  bodies  are  formed  in  the  ovary,  the  male  pronucleus 
is  always  small. 

Selenka,  who  has  investigated  the  formation  of  the  male  pronucleus  in 
Toxopneustes  variegatus,  differs  in  certain  points  from  Fol.  He  finds  that 
usually,  though  not  always,  a  single  spermatozoon  enters  the  egg,  and  that 
though  the  entrance  may  be  effected  at  any  part  of  the  surface  it  generally 
occurs  at  the  point  marked  by  a  small  prominence  where  the  polar  cells 
are  formed.  The  spermatozoon  first  makes  its  way  through  the  mucous 
envelope  of  the  egg,  within  which  it  swims  about,  and  then  bores  with  its 
head  into  the  polar  prominence. 

One  important  point  has  been  so  far  only  indirectly  alluded 
to,  viz.  the  number  of  spermatozoa  required  to  effect  impregna- 
tion. 

The  concurrent  testimony  of  almost  all  observers  tends  to 
shew  that  one  only  is  required  for  this  purpose.  But  the 
number  of  cases  tested  is  too  small  to  admit  of  satisfactory 
generalization. 

Both  Hertwig  and  Fol  have  made  observations  on  the  result 
of  the  entrance  into  the  egg  of  several  spermatozoa.  Fol  finds 
that  when  the  impregnation  has  been  too  long  delayed  the 
vitelline  membrane  is  formed  with  comparative  slowness,  and 
several  spermatozoa  are  thus  enabled  to  penetrate.  Each  sper- 
matozoon forms  a  separate  pronucleus  with  a  surrounding  star ; 
and  several  male  pronuclei  usually  fuse  with  the  female  pro- 
nucleus.  Each  male  pronucleus  appears  to  exercise  a  repulsive 


IMPREGNATION   OF  THE  OVUM.  85 

influence  on  other  male  pronuclei,  but  to  be  attracted  by  the 
female  pronucleus.  When  there  are  several  male  pronuclei  the 
segmentation  is  irregular  and  the  resulting  larva  a  monstrosity. 
These  statements  of  Fol  and  Hertwig  are  up  to  a  certain  point 
in  contradiction  with  the  more  recent  results  of  Selenka.  In 
Toxopneustes  variegatus  Selenka  finds  that  though  impregnation 
is  usually  effected  by  a  single  spermatozoon  yet  several  may  be 
concerned  in  the  act.  The  development  continues,  however,  to 
be  normal  up  to  the  gastrula  stage,  at  any  rate,  if  three  or  even 
four  spermatozoa  enter  the  egg  almost  simultaneously.  Under 
such  circumstances  each  spermatozoon  forms  a  separate  pro- 
nucleus  and  star.  Selenka  is  of  opinion  (apparently  rather  on 
a  priori  grounds  than  as  a  result  of  direct  observation)  that 
normal  development  cannot  occur  when  more  than  one  male 
pronucleus  fuses  with  the  female  pronucleus ;  and  holds  that, 
where  he  has  observed  such  normal  development  after  the 
entrance  of  more  than  one  spermatozoon,  the  majority  of  male 
pronuclei  become  absorbed. 

It  may  be  noticed  that,  while  the  observations  of  Fol  and 
Hertwig  were  admittedly  made  upon  eggs  in  which  the  impreg- 
nation was  delayed  till  they  no  longer  displayed  their  pristine 
activity,  Selenka's  were  made  upon  quite  fresh  eggs ;  and  it 
seems  not  impossible  that  the  pathological  symptoms  in  the 
embryos  reared  by  the  two  former  authors  may  have  been  due 
to  the  imperfection  of  the  egg,  and  not  to  the  entrance  of  more 
than  one  spermatozoon.  This,  of  course,  is  merely  a  suggestion 
which  requires  to  be  tested  by  fresh  observations. 

Kupffer  and  Benecke  have  further  shewn  that  although  only 
one  spermatozoon  enters  the  ovum  directly  in  Petromyzon  yet 
other  spermatozoa  pass  through  the  vitelline  membrane,  and  are 
taken  into  a  peculiar  protoplasmic  protuberance  of  the  ovum 
which  appears  after  impregnation. 

The  act  of  impregnation  may  be  described  as  the  fusion  of 
the  ovum  and  spermatozoon,  and  the  most  important  feature  in 
this  act  appears  to  be  the  fusion  of  a  male  and  female  nucleus ; 
not  only  does  this  appear  in  the  actual  fusion  of  the  two  pro- 
nuclei,  but  it  is  brought  into  still  greater  prominence  by  the  fact 
that  the  female  pronucleus  is  a  product  of  the  nucleus  of  a 
primitive  ovum,  and  the  male  pronucleus  is  the  metamorphosed 


86  SUMMARY. 


head  of  the  spermatozoon  which,  as  stated  above,  contains  part 
of  the  nucleus  of  the  primitive  spermatic  cell.  The  spermatic 
cells  originate  from  cells  indistinguishable  from  the  primitive 
ova,  so  that  the  fusion  which  takes  place  is  the  fusion  of  morpho- 
logically similar  parts  in  the  two  sexes. 

These  conclusions  tally  very  satisfactorily  with  the  view 
adopted  in  the  Introduction,  that  impregnation  amongst  the 
Metazoa  was  derived  from  the  process  of  conjugation  amongst 
the  Protozoa. 

Summary. 

In  what  may  probably  be  regarded  as  a  normal  case  the 
following  series  of  events  accompanies  the  maturation  and  im- 
pregnation of  an  ovum  : — 

(1)  Transportation  of  the  germinal  vesicle  to  the  surface  of 
the  egg. 

(2)  Absorption  of  the  membrane  of  the  germinal  vesicle 
and  metamorphosis  of  the  germinal  spot  and  nuclear  reticulum. 

(3)  Assumption  of  a  spindle  character  by  the  remains  of 
the   germinal   vesicle,   these   remains    being   probably   in    part 
formed  from  the  germinal  spot. 

(4)  Entrance  of  one  end  of  the  spindle  into  a  protoplasmic 
prominence  at  the  surface  of  the  egg. 

(5)  Division  of  the  spindle  into  two  halves,  one  remaining 
in  the  egg,  the  other  in  the  prominence  ;  the  prominence  becom- 
ing at  the  same  time  nearly  constricted  off  from  the  egg  as  a 
polar  cell. 

(6)  Formation  of  a  second  polar  cell  in  the  same  manner  as 
the  first,  part  of  the  spindle  still  remaining  in  the  egg. 

(7)  Conversion  of  the  part  of  the  spindle  remaining  in  the 
egg  into  a  nucleus — the  female  pronucleus. 

(8)  Transportation  of  the  female  pronucleus  towards  the 
centre  of  the  egg. 

(9)  Entrance  of  one  spermatozoon  into  the  egg. 

(10)  Conversion  of  the  head  of  the  spermatozoon   into  a 
nucleus — the  male  pronucleus. 

(i  i)  Appearance  of  radial  striae  round  the  male  pronucleus, 
which  gradually  travels  towards  the  female  pronucleus. 


MATURATION   AND  IMPREGNATION  OF  THE  OVUM.  87 

(12)     Fusion  of  male  and  female  pronuclei  to  form  the  first 
segmentation  nucleus. 

(76)  Auerbach.     Organologische  Stiidien,  Heft  2.     Breslau,  1874. 

(77)  Bambeke.     "  Recherchess.  Embryologie  des  Batraciens."   Bull.detAcad. 
royale  de  Belgique,  sme  Ser.,  T.  LXI.,  1876. 

(78)  E.  van  Beneden.     "La  Maturation  de  1'OZuf des  Mammiferes."     Bull.de 
fAcad.  royale  de  Belgique,  2me  Sen,  T.  XL.  No.  12,  1875. 

(79)  Idem.    "  Contributions  a  1'Histoire  de  la  Vesicule  Germinative,  &c."   Bull. 
de  TAcad.  royale  de  Belgique,  sme  Se"r.,  T.  XLI.  No.  i,  1876. 

(80)  O.  Biitschli.    Eizelle,  Zelltheilung,  und Conjugation  der Infusorien.  Frank- 
furt, 1876. 

(81)  F.  M.  Balfour.     "On  the  Phenomena  accompanying  the  Maturation  and 
Impregnation  of  the  Ovum."     Quart.  J.  of  Micros.  Science,  Vol.  xvni.,  1878. 

(82)  Calberla.    " Befruchtungsvorgang  beim  Ei  von  Petromyzon  Planeri."    Zeit. 
f.  wiss.  Zool.,  Vol.  XXX. 

(83)  W.  Flemming.     "  Studien  in  d.  Entwickelungsgeschichte  der  Najaden." 
Sitz.  d.  k.  Akad.  Wien,  B.  LXXL,  1875. 

(84)  H.  Fol.     "Die  erste  Entwickelung  des  Geryonideneies."    Jenaische  Zeit- 
schrift,  Vol.  vii.,  1873. 

(85)  Idem.     "  Sur  le  Developpement  des   Pteropodes."     Archives  de  Zoologie 
Experimentale  et  Generale,  Vol.  IV.  and  V.,  1875 — 6. 

(86)  Idem.     "Sur  le  Commencement  de  1'Henogenie."     Archives  des  Sciences 
Physiques  et  Naturelles.     Geneve,  1877. 

(87)  Idem.     Recherches  s.  /.  Fecondation  et  I.  cornmen.  d.  FHenogenie.     Geneve, 
1879. 

(88)  R.  Greeff.     "  Ueb.  d.  Bau  u.  d.  Entwickelung  d.  Echinodermen. "  Sitzun. 
der  Gesellschaft  z.  Beforderung  d.  gesammten  Naturiviss.  z.  Marburg,  No.  5,  1876. 

(89)  Oscar  Hertwig.     "Beit.  z.  Kenntniss  d.  Bildung,  &c.,  d.  thier.  Eies." 
Morphologisches  Jahrbuch,  Vol.  I.,  1876. 

(90)  Idem.     Ibid.     Morphologisches  Jahrbuch,  Vol.  in.  Heft  i,  1877. 

(91)  Idem.  "  Weitere  Beitrage,  &c."    Morphologisches  Jahrbuch,  Vol.  ill.,  1877, 
Heft  3. 

(92)  Idem.    "Beit.  z.  Kenntniss,  &c."    Morphologisches  Jahrbuch,  Vol.  IV.  Heft 
i  and  2,  1878. 

(93)  N.  Kleinenberg.     Hydra.     Leipzig,  1872. 

(94)  C.  Kupffer  u.   B.  Benecke.     Der  Vorgang  d.  Befruchtung  am  Eie  d. 
Neunaugen.     Konigsberg,  1878. 

(95)  J.  Oellacher.     "Beitrage  zur  Geschichte  des  Keimblaschens  im  Wirbel- 
thiereie."     Archivf.  micr.  Anat.,  Bd.  viil.,  1872. 

(96)  W.  Salensky.     "  Befruchtung  u.  Furchung  d.  Sterlets-Eies."   Zoologischer 
Anzeiger,  No.  u,  1878. 

(97)  E.  Selenka.     Befruchtung  des  Eies  von  Toxopneustesvariegatus.    Leipzig, 

1878. 

(98)  Strasburger.      Ueber  Zellbildung u.  Zelltheilung.    Jena,  1876. 

(99)  Idem.     Ueber  Befruchtung  u.  Zelltheilung.    Jena,  1878. 

(100)  C.  O.  Whitman.     "The  Embryology  of  Clepsine."     Quart.  J.  of  Micr. 
Science,  Vol.  xvm.,  1878. 


CHAPTER   III. 


THE  SEGMENTATION    OF  THE   OVUM. 

THE  immediate  result  of  the  fusion  of  the  male  and  female  pro- 
nucleus  is  the  segmentation  or  division  of  the  ovum  usually  into 
two,  four,  eight,  etc.  successive  parts.  The  segmentation  may 
be  dealt  with  from  two  points  of  view,  viz.  (i)  the  nature  of  the 
vital  phenomena  which  take  place  in  the  ovum  during  its 
occurrence,  which  may  be  described  as  the  internal  phenomena 
of  segmentation.  (2)  The  external  characters  of  the  segmenta- 
tion. 

Internal  PJunomena  of  Segmentation. 

Numerous  descriptions  have  been  given  during  the  last  few 
years  of  the  internal  phenomena  of  segmentation.  The  most 
recent  contribution  on  this  head  is  that  of  Fol  (No.  87).  He 
appears  to  have  been  more  successful  than  other  observers  in 
obtaining  a  complete  history  of  the  changes  which  take  place, 
and  it  will  therefore  be  convenient  to  take  as  type  the  ovum  of 
ToxopneusUs  (Echinus]  lividus,  on  which  he  made  his  most 
complete  series  of  observations.  The  changes  which  take  place 
may  be  divided  into  a  series  of  stages.  The  ovum  immediately 
after  the  fusion  of  the  male  and  female  pronucleus  contains  a 
central  segmentation  nucleus. 

In  the  first  stage  a  clear  protoplasmic  layer  derived  from  the 
plasma  of  the  cell  is  formed  round  the  nucleus,  from  which  there 
start  outwards  a  number  of  radial  striae,  which  arc  rendered 
conspicuous  by  the  radial  arrangement  of  the  yolk-granules 


THE  SEGMENTATION   OF  THE  OVUM.  89 

between  them.  The  nucleus  during  this  process  remains  per- 
fectly passive. 

In  the  second  stage  the  nucleus  becomes  less  distinct  and 
somewhat  elongated,  and  around  it  the  protoplasmic  layer  of 
the  earlier  stage  is  arranged  in  the  form  of  a  disc-shaped  ring, 
compared  by  Fol  to  Saturn's  ring.  The  protoplasmic  rays  still 
take  their  origin  from  the  perinuclear  protoplasm.  This  stage 
has  a  considerable  duration  (20  minutes). 

In  the  third  stage  the  protoplasm  around  the  nucleus 
becomes  transported  to  the  two  nuclear  poles,  at  each  of  which 
it  forms  a  clear  mass  surrounded  by  a  star-shaped  figure 
formed  by  radial  striae.  The  nucleus  is  hardly  visible  in  the 
fresh  condition,  but  when  brought  into  view  by  reagents  is  found 
to  contain  many  highly  refractive  particles,  and  to  be  still 
enveloped  in  a  membrane. 

In  the  fourth  stage  the  nucleus  when  treated  by  reagents  has 
assumed  the  well-known  spindle  form.  The  striae  of  which  it  is 
composed  are  continuous  from  one  end  of  the  spindle  to  the 
other  and  are  thickened  at  the  centre.  The  central  thickenings 
constitute  the  so-called  nuclear  plate.  The  clear  protoplasmic 
masses  and  stars  are  present  as  before  at  the  apices  of  the 
nucleus,  and  the  rays  of  the  latter  converge  as  if  they  would 
meet  at  the  centre  of  the  clear  masses,  but  stop  short  at  their 
periphery.  There  is  no  trace  of  a  membrane  round  either  the 
nuclear  spindle  or  the  clear  masses ;  and  in  the  centre  of  the 
latter  is  a  collection  of  granules.  The  striae  of  the  polar  stars 
are  very  fine  but  distinct. 

Between  the  stage  with  a  completely  formed  spindle  and  the 
previous  one  the  intermediate  steps  have  not  been  made  out  for 
Toxopneustes  ;  but  for  Heteropods  Fol  has  been  able  to  demon- 
strate that  the  striae  of  the  spindle  and  their  central  thickenings 
are  formed,  as  in  the  case  of  the  spindle  derived  from  the 
germinal  vesicle,  from  the  metamorphosis  of  the  nuclear  reticulum. 
They  commence  to  be  formed  at  the  two  poles,  and  are  then  (in 
Heteropods)  in  immediate  contiguity  with  the  striae  of  the  stars. 
The  striae  gradually  grow  towards  the  centre  of  the  nucleus  and 
there  meet. 

In  the  fifth  stage  the  central  thickenings  of  the  spindle 
separate  into  two  sets,  which  travel  symmetrically  outwards 


QO  INTERNAL   PHENOMENA. 

towards  the  clear  masses,  growing  in  size  during  the  process. 
They  remain  however  united  for  a  short  time  by  delicate 
filaments — named  by  Fol  connective  filaments — which  very  soon 
disappear.  The  clear  masses  also  increase  in  size.  During  this 
stage  the  protoplasm  of  the  ovum  exhibits  active  amoeboid 
movements  preparatory  to  division. 

In  the  sixth  stage,  which  commences  when  the  central 
thickenings  of  the  spindle  have  reached  the  clear  polar  masses, 
the  division  of  the  ovum  into  two  parts  is  effected  by  an 
equatorial  constriction  at  right  angles  to  the  long  axis  of  the 
nucleus.  The  inner  vitelline  membrane  follows  the  furrow  for  a 
certain  distance,  but  does  not  divide  with  the  ovum.  All  con- 
nection between  the  two  parts  of  the  spindle  becomes  lost  during 
this  stage,  and  the  thickenings  of  the  fibres  of  the  spindle  give 
rise  to  a  number  of  spherical  vesicular  bodies,  which  pass  into 
the  clear  masses  and  become  intermingled  with  the  granules 
which  are  placed  there.  The  radii  of  the  stars  now  extend 
round  the  whole  circumference  of  each  of  the  clear  masses. 

In  the  seventh  stage  the  two  clear  masses  become  elongated 
and  travel  towards  the  outer  sides  of  their  segments  ;  while  the 
radii  connected  with  them  become  somewhat  bent,  as  if  a 
certain  amount  of  traction  had  been  exercised  on  them  in  the 
movement  of  the  clear  masses.  Shortly  afterwards  the  spherical 
vesicles,  each  of  which  appears  like  a  small  nucleus  and  contains 
a  central  nucleolus,  begin  to  unite  amongst  themselves,  and  to 
coalesce  with  the  neighbouring  granules.  Those  in  each  seg- 
ment finally  unite  to  form  a  nucleus  which  absorbs  the  substance 
of  the  clear  mass.  The  new  nucleus  is  therefore  partly  derived 
from  tfie  division  of  the  old  one  and  partly  from  the  plasma  of  the 
cell.  The  two  segments  formed  by  division  are  at  first  spherical, 
but  soon  become  flattened  against  each  other.  In  each  subse- 
quent division  of  these  cells  the  whole  of  the  above  changes  are 
repeated. 

The  phenomena  which  have  just  been  described  would 
appear  to  occur  in  the  segmentation  of  ova  with  remarkable 
constancy  and  without  any  very  considerable  variations. 

The  division  of  the  ovum  constitutes  a  special  case  of  cell  division,  and  it 
is  important  to  determine  to  what  extent  the  phenomena  of  ordinary  cell 
division  are  related  to  those  which  take  place  in  the  division  of  the  ovum. 


THE  SEGMENTATION   OF  THE  OVUM.  91 

Without  attempting  a  full  discussion  of  the  subject  I  will  confine  myself  to 
a  few  remarks  suggested  by  the  observations  of  Flemming,  Peremeschko  and 
Klein.  The  observations  of  these  authors  shew  that  in  the  course  of  the 
division  of  nuclei  in  the  salamander,  newt,  etc.  the  nuclear  reticulum  under- 
goes a  series  of  peculiar  changes  of  form,  and  after  the  membrane  of  the 
nucleus  has  vanished  divides  into  two  masses.  The  masses  form  the  basis 
for  the  new  nuclei,  and  become  reconverted  into  an  ordinary  nuclear  reticu- 
lum after  repeating,  in  the  reverse  order,  the  changes  of  form  undergone 
by  the  reticulum  previous  to  its  division. 

It  is  clear  without  further  explanation  that  the  conversion  of  the 
nuclear  reticulum  of  the  segmentation  nucleus  into  the  striae  of  the  spindle 
is  a  special  case  of  the  same  phenomenon  as  that  first  described  by  Flemming 
in  the  salamander.  There  are  however  some  considerable  differences.  In 
the  first  place  the  fibres  in  the  salamander  do  not,  according  to  Flemming, 
unite  in  the  middle  line,  though  they  appear  to  do  so  in  the  newt.  This  clearly 
cannot  be  regarded  as  a  fact  of  great  importance ;  nor  can  the  existence  of 
the  central  thickenings  of  the  striae  (nuclear  plate),  constant  as  it  is  for  the 
division  of  the  nucleus  of  the  ovum,  be  considered  as  constituting  a  funda- 
mental difference  between  the  two  cases.  More  important  is  the  fact  that 
the  striae  in  the  case  of  the  ovum  do  not  appear,  at  any  rate  have  not  been 
shewn,  to  form  themselves  again  into  a  nuclear  network. 

With  reference  to  the  last  point  it  is  however  to  be  borne  in  mind  (i)  that 
the  gradual  travelling  outwards  of  the  two  halves  of  the  nuclear  plate  is 
up  to  a  certain  point  a  repetition,  in  the  reverse  order,  of  the  mode  of 
formation  of  the  strise  of  the  spindle,  since  the  striae  first  appeared  at  the 
poles  and  gradually  grew  towards  the  middle  of  the  spindle :  (2)  that  there 
is  still  considerable  doubt  as  to  how  the  vesicular  bodies  formed  out  of  the 
nuclear  plate  reconstitute  themselves  into  a  nucleus. 

The  layer  of  clear  protoplasm  around  the  nucleus  during  its  division  has 
its  homologue  in  the  case  of  the  division  of  the  nuclei  of  the  salamander, 
and  the  rays  starting  from  this  are  also  found.  Klein  has  suggested  that  the 
extra-nuclear  rays  of  the  stars  around  the  poles  of  the  nucleus  are  derived 
from  a  metamorphosis  of  the  extra-nuclear  reticulum,  which  he  believes 
to  be  continuous  with  the  intra-nuclear  reticulum. 

The  delicate  connective  filaments  usually  visible  between  the  two  halves 
of  the  nuclear  plate  would  seem  from  Strasburger's  latest  observations 
(No.  104)  to  be  derived  from  the  nuclear  substance  between  the  striae  of  the 
spindle,  and  to  become  eventually  reabsorbed  into  the  newly-formed  nuclei. 

We  are  it  appears  to  me  still  in  complete  ignorance  as  to  the 
physical  causes  of  segmentation.  The  view  that  the  nucleus  is 
a  single  centre  of  attraction,  and  that  by  its  division  the  centre  of 
attraction  becomes  double  and  thereby  causes  division,  appears  to 
be  quite  untenable.  The  description  already  given  of  the  pheno- 
mena of  segmentation  is  in  itself  sufficient  to  refute  this  view. 


92  REGULAR   SEGMENTATION. 

Nor  is  it  in  the  least  proved  by  the  fact  (shewn  by  Hallez)  that 
the  plane  of  division  of  the  cell  always  bears  a  definite  relation 
to  the  direction  of  the  axis  of  the  nucleus. 

The  arguments  by  which  Kleinenberg  (93)  attempted  to  de- 
monstrate that  cell  division  was  a  phenomenon  caused  by  altera- 
tions in  the  molecular  cohesion  of  the  protoplasm  of  the  ovum 
still  in  my  opinion  hold  good,  but  recent  discoveries  as  to  the 
changes  which  take  place  in  the  nucleus  during  division  probably 
indicate  that  the  molecular  changes  which  take  place  in  the  co- 
hesion of  the  protoplasm  are  closely  related  to,  and  possibly 
caused  by,  those  in  the  nucleus.  These  alterations  of  cohesion 
are  produced  by  a  series  of  molecular  changes,  the  external  indi- 
cations of  which  are  to  be  found  in  the  visible  alterations  in  the 
constitution  of  the  body  of  the  cell  and  of  the  nucleus  prior  to 
division. 

BIBLIOGRAPHY. 

In  addition  to  the  papers  cited  in  the  last  Chapter,  vide 

(101)  W.  F lemming.     "  Beitrage  z.  Kenntniss  d.  Zelle  u.  ihrer  Lebenserschei- 
nungen."     Archiv f.  mikr.  Anat.,  Vol.  xvi.,  1878. 

(102)  E.   Klein.     "Observations  on  the  glandular  epithelium  and  division  of 
nuclei  in  the  skin  of  the  Newt."     Quart.  J.  of  Micr.  Science,  Vol.  xix.,  1879. 

(103)  Peremeschko.     "  Ueber  d.  Theilung  d.  thierischen  Zellen."     Archiv  f. 
mikr.  Anat.,  Vol.  xvi.,  1878. 

(104)  E.  Strasburger.     "  Ueber  ein  z.  Demonstration  geeignetes  Zelltheilungs- 
Object."     Site.  d.  Jenaischen  Gesell.f.  Med.  u.  Naturwiss.,  July  18,  1879. 


External  Features  of  Segmentation. 
In  the  simplest  known  type  of  segmentation  the  ovum  first 


,- 


FIG.  38.    VARIOUS  STAGES  IN  PROCESS  OF  SEGMENTATION.    (After  Gegenbaur.) 

of  all  divides  into  two,  then  four,  eight,  sixteen,  thirty-two,  sixty- 
four,  etc.  cells  (fig.  38).  These  cells  so  long  as  they  are  fairlylarge 
are  usually  known  as  segments  or  spheres.  At  the  close  of  such 


THE   SEGMENTATION   OF  THE   OVUM. 


93 


a  simple  segmentation  the  ovum  becomes  converted  into  a  sphere 
composed  of  segments  of  a  uniform  size.  These  segments  usu- 
ally form  a  wall  (fig.  39,  E),  one  row  of  cells  thick,  round  a  central 
cavity,  which  is  known  as  the  segmentation  cavity  or  cavity 
of  Von  Baer.  Such  a  sphere  is  known  as  a  blastosphere.  The 
central  cavity  usually  appears  very  early  in  the  segmentation,  in 
many  cases  when  only  four  segments  are  present  (fig.  39,  B). 

In  other  instances,  which  however  are  rarer  than  those  in 
which  a  segmentation  cavity  is  present,  there  is  no  trace  of  a 
central  cavity,  and  the  sphere  at  the  close  of  segmentation  is 
quite  solid.  In  such  instances  the  solid  sphere  is  known  as  a 
morula.  It  is  found  in  some  Sponges,  many  Coelenterata,  some 
Nemertines,  etc.,  and  in  Mammals  ;  in  which  group  the  segmen- 
tation is  not  however  quite  regular.  All  intermediate  conditions 
between  a  large  segmentation  cavity,  and  a  very  minute  central 
cavity  which  may  be  surrounded  by  more  than  a  single  row  of 
cells  have  been  described. 

The  segmentation  cavity  has  occasionally,  as  in  Sycandra,  the  Cteno- 
phora  and  Amphioxus,  the  form  of  an  axial  perforation  of  the  ovum  open  at 
both  extremities. 


FIG.  39.  THE  SEGMENTATION  OF  AMPHIOXUS.  (Copied  from  Kowalevsky.) 
sg.  segmentation  cavity.  A.  Stage  with  two  equal  segments.  B.  Stage  with  four 
equal  segments.  C.  Stage  after  the  four  segments  have  become  divided  by  an 
equatorial  furrow  into  eight  equal  segments.  D.  Stage  in  which  a  single  layer  of 
cells  encloses  a  central  segmentation  cavity.  E.  Somewhat  older  stage  in  optical 
section. 


94  REGULAR   SEGMENTATION. 

When  the  process  of  regular  segmentation  is  examined  some- 
what more  in  detail  it  is  found  to  follow  as  a  rule  a  rather  definite 
rhythm.  The  ovum  is  first  divided  in  a  plane  which  may  be 
called  vertical,  into  two  equal  parts  (fig.  39,  A).  This  division  is 
followed  by  a  second,  also  in  a  vertical  plane,  but  at  right  angles 
to  the  first  plane,  and  by  it  each  of  the  previous  segments  is 
halved  (fig.  39,  B.)  In  the  third  segmentation  the  plane  of  divi- 
sion is  horizontal  or  equatorial  and  divides  each  of  the  four  seg- 
ments into  two  halves,  making  eight  segments  in  all  (fig.  39,  C). 
In  the  fourth  period  the  segmentation  takes  place  in  two  vertical 
planes  each  at  an  angle  of  45°  with  one  of  the  previous  vertical 
planes.  All  the  segments  are  thus  again  divided  into  two  equal 
parts.  In  the  fifth  period  there  are  two  equatorial  planes  one  on 
each  side  of  the  original  equatorial  plane,  and  thirty-two  spheres 
are  present  at  the  close  of  this  period.  Sixty-four  segments  are 
formed  at  the  sixth  period,  but  beyond  the  fourth  and  fifth  periods 
the  original  regularity  is  not  usually  preserved. 

In  many  instances  the  type  of  segmentation  just  described  cannot  be 
distinctly  recognized.  All  that  can  be  noticed  is  that  at  each  fresh 
segmentation  every  segment  becomes  divided  into  two  equal  parts.  It  is 
not  absolutely  certain  that  there  is  not  always  some  slight  inequality  in 
the  segments  formed,  by  which,  what  are  known  as  the  animal  and  vegetative 
poles  of  the  ovum,  can  very  early  be  distinguished.  A  regular  segmen- 
tation is  found  in  species  in  most  groups  of  the  animal  kingdom.  It  is 
very  common  in  Sponges  and  Ccelenterates.  Though  less  common  so 
far  as  is  known  amongst  the  Vermes,  it  is  yet  found  in  many  of 
the  lower  types,  viz.  Nematoidea,  Gordiacea,  Trematoda,  Nemertea 
(apparently  as  a  rule),  Sagitta,  Chcetonotus,  some  Gephyrea  (Phoronis) ; 
though  not  usual  it  occurs  amongst  Cha?topoda,  e.g.  Serpula.  It  is  the 
usual  type  of  segmentation  amongst  the  Echinodermata.  Amongst  the 
Crustacea  it  appears  (for  the  earlier  phases  of  segmentation  at  any  rate) 
not  infrequently  amongst  the  lower  forms,  and  even  occurs  amongst  the 
Amphipoda  (Phronimd).  It  is  however  very  rare  amongst  the  Tracheata, 
Podura  affording  the  one  example  of  it  known  to  me.  It  is  almost  as  rare 
amongst  Mollusca  as  amongst  the  Tracheata,  but  occurs  in  Chiton  and  is 
nearly  approached  in  some  Nudibranchiata.  In  Vertebrata  it  is  most  nearly 
approached  in  Amphioxus^. 

Most  of  the  eggs  which  have  a  perfectly  regular  segmentation 
are  of  a  very  insignificant  size  and  rarely  contain  much  food- 

1  In  the  Rabbit  and  probably  other  Monodelphous  Mammalia  the  segmentation  is 
nearly  though  not  quite  regular. 


THE  SEGMENTATION   OF  THE  OVUM.  95 

yolk  :  in  the  vast  majority  of  eggs  there  is  present  however  a  con- 
siderable bulk  of  food  material  usually  in  the  form  of  highly  re- 
fracting yolk-spherules.  These  yolk-spherules  lie  embedded  in 
the  protoplasm  of  the  ovum,  but  are  in  most  instances  not  distri- 
buted uniformly,  being  less  closely  packed  and  smaller  at  one  pole 
of  the  ovum  than  elsewhere.  Where  the  yolk-spherules  are  few- 
est the  active  protoplasm  is  necessarily  most  concentrated,  and 
we  can  lay  down  as  a  general  law1  that  the  velocity  of  segmen- 
tation in  any  part  of  the  ovum  is  roughly  speaking  proportional 
to  the  concentration  of  the  protoplasm  there ;  and  that  the  size 
of  the  segments  is  inversely  proportional  to  the  concentration  of 
the  protoplasm.  Thus  the  segments  produced  from  that  part  of 
an  egg  where  the  yolk-spherules  are  most  bulky,  and  where 
therefore  the  protoplasm  is  least  concentrated,  are  larger  than 
the  remaining  segments,  and  their  formation  proceeds  more 
slowly. 

Though  where  much  food-yolk  is  present  it  is  generally  dis- 
tributed unequally,  yet  there  are  many  cases  in  which  it  is  not 
possible  to  notice  this  very  distinctly.  In  most  of  these  cases  the 
segmentation  is  all  the  same  unequal,  and  it  is  probable  that  they 
form  apparent  rather  than  real  exceptions  to  the  law  laid  down 
above.  Although  before  segmentation  the  protoplasm  may  be 
uniformly  distributed,  yet  in  many  instances,  e.g.  Mollusca,Vermes, 
etc.,  during  or  at  the  commencement  of  segmentation  the  proto- 
plasm becomes  aggregated  at  one  pole,  and  one  of  the  segments 
formed  consists  of  clear  protoplasm,  all  the  food-yolk  being  con- 
tained in  the  other  and  larger  segment. 

Unequal  Segmentation.  The  type  of  segmentation  I  now 
proceed  to  describe  has  been  called  by  Haeckel  (No.  105)  'un- 
equal segmentation',  a  term  which  may  conveniently  be 
adopted.  I  commence  by  describing  it  as  it  occurs  in  the  well- 
known  and  typical  instance  of  the  Frog2. 

The  ripe  ovum  of  the  common  Frog  and  of  most  other  tailless 
Amphibians  presents  the  following  structure.  One  half  appears 
black  and  the  other  white.  The  former  I  shall  call  the  upper 

1  Vide  F.  M.  Balfour,  "  Comparison  of  the  early  stages  of  development  in  Verte- 
brates."    Quart.  Jour,  of  Micr.  Science,  July,  1875. 

2  Vide  Remak,  Entwicklung  d.  Wirbelthiere;  and  Gotte,  Entwicklung  d.  Unke. 


96 


UNEQUAL  SEGMENTATION. 


pole,  the  latter  the  lower.     The  ovum  is  composed  of  protoplasm 
containing  in  suspension  numerous  yolk-spherules.     The  largest 


FIG.  40.    SEGMENTATION  OF  COMMON  FROG.     RANA  TEMPORARIA.     (Copied 
from  Ecker.) 

The  numbers  above  the  figures  refer  to  the  number  of  segments  at  the  stage  figured. 

of  these  are  situated  at  the  lower  pole,  the  smaller  ones  at  the 
upper  pole,  and  the  smallest  of  all  in  the  peripheral  layer  of  the 
upper  pole,  in  which  also  pigment  is  scattered  and  causes  the 
black  colour  visible  from  the  surface. 

The  first  formed  furrow  is  a  vertical  furrow.  It  commences 
in  the  upper  half  of  the  ovum,  through  which  it  extends  rapidly, 
and  then  more  slowly  through  the  lower.  As  soon  as  the  first 
furrow  has  extended  through  the  egg,  and  the  two  halves  have 
become  separated  from  each  other,  a  second  vertical  furrow 
appears  at  right  angles  to  the 
first  and  behaves  in  the  same 
way  (fig.  40,  4). 

The  next  furrow  is  equa- 
torial or  horizontal  (fig.  40,  8). 
It  does  not  arise  at  the  true 
equator  of  the  egg,  but  much 
nearer  to  its  upper  pole.  It 
extends  rapidly  round  the  egg 
and  divides  each  of  the  fourpre- 
vious  segments  into  two  parts, 
one  larger  and  one  smaller. 
Thus  at  the  end  of  this  stage 
there  are  present  four  small 
and  four  large  segments.  At 
the  meeting  point  of  these  a 


II 


FIG.  41.    SECTION  THROUGH  FROG'S 

OVUM  AT  THE  CLOSK  OK  SKGMKNTATION. 

sg.  segmentation  cavity.  //.  large  yolk-con- 
taining cells,  ep.  small  cells  at  formative 
pole  (epiblast). 


THE   SEGMENTATION   OF   THE  OVUM.  97 


small  cavity  appears,  which  is  the  segmentation  cavity,  already 
described  for  uniformly  segmenting  eggs.  It  increases  in  size  in 
subsequent  stages,  its  roof  being  formed  of  the  smaller  cells  and 
its  floor  of  the  larger.  The  appearance  of  the  equatorial  furrow 
is  followed  by  a  period  of  repose,  after  which  two  rapidly  suc- 
ceeding vertical  furrows  are  formed  in  the  upper  pole,  dividing 
each  of  the  four  segments  of  which  this  is  composed  into  two. 
After  a  short  period  these  furrows  extend  to  the  lower  pole, 
and  when  completed  16  segments  are  present — eight  larger  and 
eight  smaller  (fig.  40,  16).  A  pause  now  ensues,  after  which  the 
eight  upper  segments  become  divided  by  an  equatorial  furrow, 
and  somewhat  later  a  similar  furrow  divides  the  eight  lower  seg- 
ments. At  the  end  of  this  stage  there  are  therefore  present  16 
smaller  and  16  larger  segments  (fig.  40,  32).  After  64  segments 
have  been  formed  by  vertical  furrows  which  arise  symmetrically 
in  the  two  poles  (fig.  40,  64),  two  equatorial  furrows  appear  in  the 
upper  pole  before  a  fresh  furrow  arises  in  the  lower ;  so  that  there 
are  128  segments  in  the  upper  half,  and  only  32  in  the  lower. 
The  regularity  is  quite  lost  in  subsequent  stages,  but  the  upper 
pole  continues  to  undergo  a  more  rapid  segmentation  than  the 
lower.  While  the  segments  have  been  increasing  in  number  the 
segmentation  cavity  has  been  rapidly  growing  in  size  ;  and  at  the 
close  of  segmentation  the  egg  forms  a  sphere,  containing  an 
excentric  cavity,  and  composed  of  two  unequal  parts  (fig.  41). 
The  upper  part,  which  forms  the  roof  of  the  segmentation  cavity, 
is  formed  of  smaller  cells  :  the  lower  of  larger  yolk-containing 
cells. 

The  mode  of  segmentation  of  the  Frog's  ovum  is  typical  for 
unequally  segmenting  ova,  and  it  deserves  to  be  noticed  that  as 
regards  the  first  three  or  more  furrows  the  segmentation  occurs 
with  the  same  rhythm  in  the  unequally  segmenting  ova  as  in  those 
which  have  an  uniform  segmentation.  There  appear  two  verti- 
cal furrows  followed  by  an  equatorial  furrow.  The  general  laws 
which  were  stated  with  reference  to  the  velocity  of  segmentation 
and  the  size  of  the  resulting  segments  are  well  exemplified  in  the 
case  of  the  Frog's  ovum. 

The  majority  of  the  smaller  segments  in  the  segmented  Frog's 
ovum  are  destined  to  form  into  the  epiblast,  and  the  larger  seg- 
ments become  hypoblast  and  mesoblast. 

B.  II.  7 


98  UNEQUAL  SEGMENTATION. 

With  a  few  exceptions  (the  Rabbit,  Lymnaeus,  etc.)  the  majority  of  the 
smaller  segments  always  become  epi blast  and  of  the  larger  segments  hypo- 
blast. 

The  Frog's  ovum  serves  as  a  good  medium  type  for  unequally 
segmenting  ova.  There  are  many  cases  however  in  which  a 
regular  segmentation  is  far  more  closely  approached,  and  others 
in  which  it  is  less  so. 

One  familiar  instance  in  which  a  regular  segmentation  is 
nearly  approached  is  afforded  by  the  Rabbit's  ovum,  which  has 
indeed  usually  been  regarded  as  offering  an  example  of  a  regular 
segmentation. 

The  ovum  of  the  Rabbit1  becomes  first  divided  into  two  sub- 
equal  spheres.  The  larger  and  more  transparent  of  the  two  may, 
from  its  eventual  fate,  be  called  the  epiblastic  sphere,  and  the 
other  the  hypoblastic.  The  two  spheres  are  divided  into  four, 
and  then  by  an  equatorial  furrow  into  eight — four  epiblastic  and 
four  hypoblastic.  One  of  the  latter  assumes  a  central  position. 
The  four  epiblastic  spheres  now  divide  before  the  four  hypoblastic. 
There  is  thus  introduced  a  stage  with  twelve  spheres.  It  is 
followed  by  one  with  sixteen,  and  that  by  one  with  twenty-four. 
During  the  stages  with  sixteen  spheres  and  onwards  the  epiblastic 
spheres  gradually  envelop  the  hypoblastic,  which  remain  exposed 
on  the  surface  at  one  point  only.  There  is  no  segmentation 
cavity. 

In  Pedicellina,  one  of  the  entoproctous  Polyzoa,  there  is  a  sub- 
regular  segmentation,  where  however  the  two  primary  spheres 
can  be  distinguished  much  in  the  same  way  as  in  the  case  of  the 
Rabbit. 

A  very  characteristic  type  of  unequal  segmentation  is  that 
presented  by  the  majority  of  Gasteropods  and  Pteropods  and 
probably  also  of  some  Lamellibranchiata.  It  is  also  found  in 
some  Turbellarians,  in  Bonellia,  some  Annelids,  etc.  In  many 
instances  it  offers  a  good  example  of  the  type  where  in  the  course 
of  segmentation  the  protoplasm  becomes  aggregated  at  one  pole 
of  the  ovum,  or  of  its  segments,  to  become  separated  off  as  a  clear 
sphere. 

The  first  four  segments  formed  by  two  vertical  furrows  at 

1  Van  Beneden,  "  D^veloppement  embryonnaire  des  Mammiftres."  Bull,  de 
FAcad.  Belgique,  1874. 


THE   SEGMENTATION   OF   THE  OVUM.  99 

right  angles  are  equal,  but  from  these  there  are  budded  off  four 
smaller  segments,  which  in  subsequent  stages  divide  rapidly, 
receiving  however,  a  continual  accession  of  segments  budded  off 
from  the  larger  spheres.  The  four  larger  spheres  remain  conspi- 
cuous till  near  the  close  of  the  segmentation.  The  process  of 
budding,  by  which  the  smaller  spheres  become  separated  from 
the  larger,  consists  in  a  larger  sphere  throwing  out  a  prominence, 
which  then  becomes  constricted  off  from  it. 

In  the  extreme  forms  of  this  unequal  segmentation  we  find 
at  the  end  of  the  second  cleavage  two  larger  spheres  filled  with 
yolk  material  and  two  smaller  clear  spheres ;  and  in  the  later 
stages,  though  the  large  spheres  continue  to  bud  off  small 
spheres,  only  the  two  smaller  ones  undergo  a  regular  segmenta- 
tion, and  eventually  completely  envelop  the  former.  Such  a 
case  as  this  has  been  described  in  Aplysia  by  Lankester1. 

The  types  I  have  described  serve  to  exemplify  unequal  seg- 
mentation. The  Rabbit's  ovum  stands  at  one  end  of  the  series, 
that  of  Aplysia  at  the  other ;  and  the  Frog's  ovum  between  the 
two. 

Great  variations  are  presented  by  the  ova  with  unequal  seg- 
mentation as  to  the  presence  of  a  segmentation  cavity.  In  some 
instances,  e.g.  the  Frog,  such  a  cavity  is  well  developed.  In 
other  cases  it  is  small,  e.g.  most  Mollusca,  while  not  unfrequently 
it  is  altogether  absent. 

Before  leaving  this  important  type  of  segmentation,  it  will  be  well  to 
enter  with  slightly  greater  detail  into  some  of  the  more  typical  as  well  as 
some  of  the  special  forms  which  it  presents. 

As  an  example  of  the  typical  Molluscan  type  the  normal  Heteropod 
segmentation,  accurately  described  by  Fol2,  may  be  selected. 

The  ovum  divides  into  two  and  then  four  equal  segments  in  the  usual 
vertical  planes.  Each  segment  has  a  protoplasmic  and  a  vitelline  pole. 
The  protoplasmic  pole  is  turned  towards  the  polar  bodies.  In  the  third 
segmentation,  which  takes  place  along  an  equatorial  plane,  four  small 
protoplasmic  cells  or  segments  are  segmented  or  rather  budded  off  from  the 
four  large  segments,  so  that  there  are  four  small  segments  in  one  plane  and 
four  large  below  these.  In  the  fourth  segmentation  the  four  large  segments 
alone  are  active  and  give  rise  to  four  small  and  four  large  cells ;  so  that  there 
are  formed  in  all  eight  small  and  four  large  cells.  The  four  small  cells  of  the 


1  Phil.  Trans.  1875. 

2  Fol,  Archives  de  Zoologie  Experimenfale,  Vol.  iv.  1875. 


7—2 


100  UNEQUAL  SEGMENTATION. 

third  generation  next  divide,  forming  in  all  twelve  small  cells  and  four  large. 
The  small  cells  of  the  fourth  generation  then  divide,  and  subsequently  the 
four  large  cells  give  rise  to  four  new  small  ones,  so  that  there  are  twenty 
small  cells  and  four  large.  The  small  cells  form  a  cap  embracing  the  upper 
pole  of  the  large  segments.  It  may  be  noted  that  from  the  third  stage 
onwards  the  cells  increase  in  arithmetical  progression — a  characteristic 
feature  of  the  typical  gasteropod  segmentation. 

In  the  later  stages  of  segmentation  the  large  cells  cease  to  give  rise 
to  smaller  ones  in  the  same  manner  as  before.  One  of  them  divides 
first  into  two  unequal  parts,  of  which  the  smaller  becomes  pushed  in  to- 
wards the  centre  of  the  egg.  The  larger  cell  then  divides  again  into  two, 
arid  the  three  cells  so  formed  occupy  the  centre  of  a  shallow  depression. 
The  remaining  larger  cells  divide  in  the  same  way,  and  give  rise  to  smaller 
cells  which  line  a  pit  which  becomes  formed  on  one  side  of  the  ovum. 
The  original  smaller  cells  continue  in  the  meantime  to  divide  and  so  form 
a  layer  enclosing  the  larger,  leaving  exposed  however  the  opening  of  the 
pit  lined  by  the  latest  products  of  the  larger  cells. 


FIG.  47.  SEGMENTATION  OF  ANODON  PISCINALIS.  (Copied  rom  Flemming.) 
r.  polar  cells,  v.  vitelline  sphere,  i .  Commencing  division  into  two  segments ; 
one  mainly  formed  of  protoplasm,  the  other  of  yolk.  2.  Stage  with  four  segments. 
3.  Formation  of  blastosphere,  and  segmentation  cavity.  4.  Definite  segmentation 
of  the  yolk  sphere. 

The  eggs  of  Anodon  and  Unio  serve  as  excellent  examples  of  the  type 
in  which  the  ovum  has  a  uniform  structure  before  the  commencement  of 
segmentation,  but  in  which  a  separation  into  a  protoplasmic  and  a  nutritive 
portion  becomes  obvious  during  segmentation. 

In  Anodon1  the  egg  is  at  first  uniformly  granular,  but  after  impregnation 
it  throws  out  on  one  side  a  protuberance  nearly  free  from  granules  (fig. 
42,  1). 

In  the  case  of  this  clear  protuberance  and  of  the  similar  protuberances 
which  follow  it,  the  protoplasm  is  not  at  first  quite  free  from  food-yolk, 
but  only  becomes  so  on  being  separated  from  the  yolk-containing  part  of  the 
ovum.  We  must  therefore  suppose  that  the  production  of  the  clear 
segments  is  in  part  at  least  due  to  the  yolk  spherules  becoming  used  up  to 
form  protoplasm.  Such  a  formation  of  protoplasm  from  yolk  spherules  has 
been  clearly  shewn  to  occur  in  other  types  by  Bobretzky  and  Fol. 

1  Flemming,  "Entwick.  der  Najaden,"  Sitz.  d.  Akad.  Wiss.  Wien,  Bd.  4,  1875. 


THE  SEGMENTATION   OF  THE  OVUM. 


IOI 


The  protuberance  soon  becomes  separated  off  from  the  larger  part  of 
the  egg  as  a  small  segment  composed  of  clear  protoplasm.  From  the  larger 
segment  filled  with  food-yolk,  a  second  small  clear  segment  is  next  budded 
off,  and  simultaneously  (fig.  42,  2)  the  original  small  segment  divides  into 
two.  Thus  there  are  formed  four  segments,  one  large  and  three  small ;  the 
large  segment  as  before  being  filled  with  food-yolk.  The  continuation 
of  a  similar  process  of  budding  off  and  segmentation  eventually  results 
in  the  formation  of  a  considerable  number  of  small  and  of  one  large 
segment  (fig.  42,  3).  Between  this  large  and  the  small  segments  is  a  seg- 
mentation cavity. 

Eventually  the  large  yolk  segment,  which  has  hitherto  merely  budded 
off  a  series  of  small  segments  free  from  yolk,  itself  divides  into  two  similar 
parts.  This  process  is  then  repeated  (fig.  42,  4)  and  there  is  at  last  formed 
a  number  of  yolk  segments  filled  with  yolk  spheres,  which  occupy  the 
place  of  the  original  large  yolk  segment.  Between  these  yolk  segments 
and  the  small  segments  is  placed  the  segmentation  cavity. 

The  segmentation  of  the  ovum  of  Euaxes1  resembles  that  of  Unio  in  the 
budding  off  of  clear  segments  from  those  filled  with  yolk,  but  presents 
many  interesting  individualities. 

A  very  peculiar  modification  of  the  ordinary  Gasteropod  segmentation  is 
that  described  by  Bobretzky  for  Nassa  mutabilis2. 


FIG.  43.  SEGMENTATION  OF  NASSA  MUTABILIS  (from  Bobretzky).  A.  Upper  half 
divided  into  two  segments.  B.  One  of  these  has  fused  with  the  large  lower  seg- 
ment. C.  Four  small  and  one  large  segment,  one  of  the  former  fusing  with  the 
large  segment.  D.  Each  of  the  four  segments  has  given  rise  to  a  small  segment. 
E.  Small  segments  have  increased  to  thirty-six. 

1  Kowalevsky,  Mem.  Akad.  Petersburg,  Series  vn.  1871. 

2  Archiv.f.  mikr.  Anat.  Vol.  xni.  1877. 


102  UNEQUAL  SEGMENTATION. 

The  ovum  contains  a  large  amount  of  food-yolk,  and  the  protoplasm  is 
aggregated  at  the  formative  pole,  adjoining  which  are  placed  the  polar 
bodies.  An  equatorial  and  a  vertical  furrow  (fig.  43  A),  the  former  near 
the  upper  pole,  appear  simultaneously,  and  divide  the  ovum  into  three 
segments,  two  small,  each  with  a  protoplasmic  pole,  and  one  large  en- 
tirely formed  of  yolk  material.  One  of  the  two  small  segments  next  com- 
pletely fuses  with  the  large  segment  (fig.  43  B),  and  after  the  fusion  is  com- 
plete, a  triple  segmentation  of  the  large  segment  takes  place  as  at  the  first 
division,  and  at  the  same  time  the  single  small  segment  divides  into  two.  In 
this  way  four  partially  protoplasmic  segments  and  one  yolk  segment  are 
formed  (fig.  43  C).  One  of  the  small  segments  again  fuses  with  the  large 
segment,  so  that  the  number  of  segments  becomes  again  reduced  to  four, 
three  small  and  one  large.  The  protoplasmic  ends  of  these  segments  are  turned 
towards  each  other,  and  where  they  meet  four  very  small  cells  become  budded 
off,  one  from  each  segment  (fig.  43  D).  Four  small  cells  are  again  budded 
off  twice  in  succession,  while  the  original  small  cells  remain  passive,  so  that 
there  come  to  be  twelve  small  and  four  large  cells.  In  later  stages  the  four 
first-formed  small  cells  give  rise  to  still  smaller  cells  and  then  the  next- 
formed  do  the  same.  The  large  cells  continue  also  to  give  rise  to  small 
ones,  and  finally,  by  a  continuous  process  of  division,  and  fresh  budding  of 
small  cells  from  large  cells,  a  cap  of  small  cells  becomes  formed  covering 
the  four  large  cells  which  have  in  the  meantime  pressed  themselves  together 
(fig.  43  E).  A  segmentation  cavity  of  not  inconsiderable  dimensions  be- 
comes established  between  this  cap  of  small  cells  and  the  large  cells. 

Many  eggs,  such  as  those  of  the  Myriapods1,  present  an  irregular  seg- 
mentation ;  but  the  segmentation  is  hardly  unequal  in  the  sense  in  which  I 
have  been  using  the  term.  Such  cases  should  perhaps  be  placed  in  the  first 
rather  than  in  the  present  category. 

The  type  of  unequal  segmentation  is  on  the  whole  the  most  widely 
distributed  in  the  animal  kingdom.  There  is  hardly  a  group  without  ex- 
amples of  it. 

It  occurrs  in  Porifera,  Hydrozoa,  Actinozoa  and  Ctenophora.  Amongst 
the  Ctenophora  this  segmentation  is  of  the  most  typical  kind.  Four  equal 
segments  are  first  formed  in  the  two  first  periods.  In  the  third  period  a 
circumferential  furrow  separates  four  smaller  from  four  larger  segments. 

This  type  is  also  widely  distributed  amongst  the  unsegmented  (Gephyrea, 
Turbellaria),  as  well  as  the  segmented  Vermes,  and  is  typical  for  the 
Rotifera.  It  appears  to  be  very  rare  in  Echinoderms  (Echinaster  Sarsif). 
It  is  not  uncommon  in  early  stages  of  the  segmentation  of  the  lower 
Crustacea. 

For  Mollusca  (except  Cephalopoda)  it  is  typical.  Amongst  the  Ascidia 
it  occurs  in  several  forms  (Salpa,  Molgula]  and  amongst  the  Craniata  it 
is  typical  in  the  Cyclostomata,  Amphibia,  and  some  Ganoids,  e.g.  Acci- 
Penser. 

1  Metschnikoff,  Zeitschrift  f.  wiss.  '/.oohgie,  1X74. 


THE   SEGMENTATION   OF  THE   OVUM. 


103 


Partial  segmentation.  The  next  type  of  segmentation  we 
have  to  deal  with  has  long  been  recognized  as  partial  segmenta- 
tion. It  is  a  type  in  which  only  part  of  the  ovum,  called  the 
germinal  disc,  undergoes  segmentation,  the  remainder  usually 
forming  an  appendage  of  the  embryo  known  as  the  yolk-sack. 
Ova  belonging  to  the  two  groups  already  dealt  with  are  fre- 
quently classed  together  as  holoblastic  ova,  in  opposition  to  ova 
of  the  present  group  in  which  the  segmentation  is  only  partial, 
and  which  are  therefore  called  meroblastic.  For  embryological 


FIG.  44.     SURFACE  VIEWS  OF  THE  EARLY  STAGES  OF  THE  SEGMENTATION  IN  A 

FOWL'S  EGG.     (After  Coste.) 

a.  edge  of  germinal  disc.     b.  vertical  furrow,     c.  small  central  segment,     d.  larger 
peripheral  segment. 

purposes  this  is  in  many  ways  a  very  convenient  classification, 
but  ova  belonging  to  the  present  group  are  in  reality  separated 
by  no  sharp  line  from  those  belonging  to  the  group  just 
described. 

The  origin  and  nature  of  meroblastic  ova  will  best  be  under- 
stood by  taking  an  ovum  with  an  unequal  segmentation,  such  as 
that  of  the  frog,  and  considering  what  would  take  place  in 
accordance  with  the  laws  already  laid  down,  supposing  the 
amount  of  food-yolk  at  the  vitelline  pole  to  be  enormously 
increased.  What  would  happen  may  be  conveniently  illustrated 
by  fig.  44,  representing  the  segmentation  of  a  fowl's  egg.  There 
would  first  obviously  appear  a  vertical  furrow  at  the  formative 
or  protoplasmic  pole.  (Fig.  44  A,  b.}  This  would  gradually 
advance  round  the  ovum  and  commence  to  divide  it  into  two 
halves.  Before  the  furrow  had  however  proceeded  very  far  it 


PARTIAL  SEGMENTATION. 


would  come  to  the  vitelline  part  of  the  ovum ;  here,  according 
to  the  law  previously  enunciated,  it  would  travel  very  slowly, 
and  if  the  amount  of  the  food-yolk  was  practically  infinite  as 
compared  with  the  protoplasm,  it  would  absolutely  cease  to 
advance.  A  second  vertical  furrow  would  soon  be  formed, 
crossing  the  first  at  right  angles,  and  like  it  not  advancing 
beyond  the  edge  of  the  germinal  disc.  (Fig.  44  B.) 

The  next  furrow  should  be  an  equatorial  one  (as  a  matter  of 
fact  in  the  fowl's  ovum  an  equatorial  furrow  is  not  formed  till 
after  two  more  vertical  furrows  have  appeared).  The  equatorial 
furrow  would  however,  in  accordance  with  the  analogy  of  the 
frog,  not  be  formed  at  the  equator,  but  very  close  to  the  formative 
pole.  It  would  therefore  separate  off  as  a  distinct  segment  (fig. 
44  C,  c),  a  small  central,  i.e.  polar,  portion  of  each  of  the  imper- 
fect segments  formed 
by  the  previous  verti- 
cal furrows.  By  a 
continuation  of  the 
process  of  segmenta- 
tion, with  the  same 
alternation  of  vertical 
and  equatorial  furrows 
as  in  the  frog,  a  cap  or 
disc  of  small  segments 
would  obviously  be 
formed  at  the  proto- 
plasmic pole  of  the 
ovum,  outside  which 
would  be  a  number  of 
deep  radiating  grooves 
(fig-  45),  formed  by 
the  vertical  furrows, 
the  advance  of  which 
round  the  ovum  has  come  to  an  end  owing  to  the  too  great  pro- 
portion of  yolk  spheres  at  the  vitelline  pole. 

It  is  clear  from  the  above  that  an  immense  accumulation  of 
food -yolk  at  the  vitelline  pole  necessarily  causes  a  partial  seg- 
mentation. It  is  equally  clear  that  the  part  of  meroblastic  ova 
which  does  not  undergo  segmentation  is  not  a  new  addition 


FIG.  45.    SURFACE  VIEW  OF  THE  GERMINAL  DISC 

OF  FOWL'S  EGG  DURING  A  LATE  STAGE  OF  THE  SEG- 
MENTATION. 

c.  small  central  segmentation  spheres  ;  b.  larger 
segments  outside  these ;  a.  large,  imperfectly  cir- 
cumscribed, marginal  segments ;  e.  margin  of  ger- 
minal disc. 


THE  SEGMENTATION   OF  THE  OVUM.  IO5 

absent  in  other  cases.  It  is  on  the  contrary  to  be  regarded 
merely  as  a  part  of  the  ovum  in  which  the  yolk  spherules  have 
attained  to  a  very  great  bulk  as  compared  with  the  protoplasm  ; 
sometimes  even  to  the  complete  exclusion  of  the  protoplasm. 

An  ordinary  meroblastic  ovum  consists  then  of  a  small  disc 
at  the  formative  pole,  known  as  the  germinal  disc,  composed 
mainly  of  protoplasm  in  which  comparatively  little  food-yolk  is 
present  This  graduates  into  the  remainder  of  the  ovum,  being 
separated  from  it  by  a  more  or  less  sharp  line.  This  remainder 
of  the  ovum,  which  almost  always  forms  the  major  part,  usually 
consists  of  numerous  yolk  spherules,  embedded  in  a  very  scanty 
protoplasmic  matrix. 

In  some  cases,  e.g.  the  eggs  of  Elasmobranchii1,  the  protoplasm  is  pre- 
sent in  the  form  of  a  delicate  network ;  in  other  and  perhaps  the  majority  of 
cases,  too  little  protoplasm  is  present  to  be  detected,  or  the  protoplasm  may 
even  be  completely  absent.  In  some  Osseous  Fishes,  e.g.  Lota,  the  yolk 
forms  a  homogeneous  transparent  albuminoid  substance  containing  a  large 
globule  at  the  pole  furthest  removed  from  the  germinal  disc.  In  this  case 
the  germinal  disc  is  sharply  separated  from  the  yolk.  In  other  Osseous 
Fishes  the  separation  between  the  two  parts  is  not  so  sharp2.  In  these 
cases  we  find  adjoining  the  germinal  disc  a  finely  granular  material  con- 
taining a  large  proportion  of  protoplasm ;  this  graduates  into  a  material  with 
very  little  protoplasm  and  numerous  yolk  spherules,  which  is  in  its  turn 
continuous  with  an  homogeneous  albuminoid  yolk  substance.  In  Elasmo- 
branchii we  find  that  immediately  beneath  the  germinal  disc  there  is  present 
a  finely  granular  matter,  rich  in  protoplasm,  which  is  continuous  with  the 
normal  yolk. 

The  Elasmobranch  ovum  may  conveniently  serve  as  type  for  the  Verte- 
brata.  The  ovum  is  formed  of  a  spherical  vitellus  without  any  investing 
membrane.  The  germinal  disc  is  recognizable  on  this  as  a  small  yellow  spot 
about  i^  millimetres  in  diameter.  In  the  germinal  disc  a  furrow  appears 
bisecting  the  disc,  followed  by  a  second  furrow  at  right  angles  to  the  first. 
Thus  after  the  formation  of  the  second  furrow  the  disc  is  divided  into  four 
equal  areas.  Fresh  furrows  continue  to  rise,  and  eventually  a  circular 
furrow,  equivalent  to  the  equatorial  furrow  of  the  frog's  ovum,  makes  its 
appearance,  and  separates  off  a  number  of  smaller  central  segments  from 
peripheral  larger  segments.  In  the  later  stages  the  smaller  segments  at  first 
divide  more  rapidly  than  the  larger,  but  eventually  the  latter  also  divide 
rapidly,  and  the  germinal  disc  becomes  finally  formed  of  a  series  of  segments 

1  Vide  Schultze,  Archiv.f.  mikr.  Anat.  Vol.  XI.;  and  F.  M.  Balfour,  Monograph 
on  the  Development  of  Elasmobranch  Fishes. 

2  Vide  Klein,   Quart.  Joitrnal  of  Micr.  Science,  April,   1876.     Bambeke,  Mem. 
Cour.  Acad.  Belgique,  1875.     His,  Zeit.fiir  Anat.  u.  Entwicklung.  Vol.  I. 


106  NUCLEI  OF  THE  YOLK. 

of  a  fairly  uniform  size.  So  much  may  be  observed  in  surface  views  of  the 
segmenting  ovum,  and  it  may  be  noted  that  there  is  not  much  difference  to 
be  observed  between  the  segmentation  of  the  germinal  disc  of  the  Fowl's 
ovum  and  that  of  the  Elasmobranchii.  Indeed  the  figure  of  the  former  (fig. 
44)  would  serve  fairly  well  for  the  latter.  When  however  we  examine 
the  segmenting  germinal  discs  by  means  of  sections,  there  are  some  dif- 
ferences between  the  two  types,  and  several  interesting  features  which 
deserve  to  be  noticed  in  the  segmentation  of  the  Elasmobranchii.  In  the 
first  stages  the  furrows  visible  on  the  surface  are  merely  furrows,  which 
do  not  meet  so  as  to  isolate  distinct  segments  ;  they  merely,  in  fact,  form  a 
surface  pattern.  It  is  not  till  after  the  appearance  of  the  equatorial  furrow 
that  the  segments  begin  to  be  distinctly  isolated.  In  the  subsequent  stages 
not  only  do  the  segments  already  existing  in  the  germinal  disc  increase  by 
division,  but  fresh  segments  are  continually  being  formed  from  the  adjacent 
yolk,  and  added  to  those  already  present  in  the  germinal  disc.  (Fig.  46.) 


i  I  tffl 


FIG.  46.     SECTION  THROUGH  GERMINAL  DISC  OF  A  PRISTIURUS  EMBRYO  DURING 

THE  SEGMENTATION. 

n.  nucleus;  nx.  nucleus  modified  prior  to  division;  nx '.  modified  nucleus  of  the 
yolk ;  /.  furrow  appearing  in  the  yolk  adjacent  to  the  germinal  disc. 

This  fact  is  one  out  of  many  which  prove  that  the  germinal  disc  is  merely 
part  of  the  ovum  characterized  by  the  presence  of  more  protoplasm  than  the 
remainder  which  forms  the  so-called  food-yolk.  During  the  latest  stages  of 
segmentation  there  appear  in  the  yolk  around  the  blastoderm  a  number  of 
nuclei.  (Fig.  46,  nx'.}  These  are  connected  with  a  special  protoplasmic 
network  (already  described)  which  penetrates  through  the  yolk.  Towards 
the  end  of  segmentation,  and  during  the  early  periods  of  development  which 
succeed  the  segmentation,  these  nuclei  become  very  numerous.  (Fig.  47 
A,  «'.)  Around  many  of  them  a  protoplasmic  investment  is  established,  and 
cells  are  thus  formed  which  eventually  enter  the  blastoderm. 

The  result  of  segmentation  is  the  formation  of  a  lens-shaped  mass  of 
cells  lying  in  a  depression  on  the  yolk.  In  this  a  cavity  appears,  the 
homologue  of  the  segmentation  cavity  already  spoken  of.  It  lies  at  first  in 


THE  SEGMENTATION   OF  THE   OVUM.  107 

the  midst  of  the  cells  of  the  blastoderm,  but  very  soon  its  floor  of  cells 
vanishes,  and  it  lies  between  the  yolk  and  the  blastoderm.  (Fig.  47  A.)  Its 
subsequent  history  is  given  in  a  future  Chapter. 

Segmentation  proceeds  in  Osseous  Fishes  in  nearly  the  same  manner  as 
in  Elasmobranchii.  In  some  cases  the  germinal  disc  is  small  as  compared 
with  the  yolk,  in  other  cases  it  is  almost  as  large  as  the  yolk.  The  only 
points  which  deserve  special  notice  are  the  following  :  (i)  Nuclei,  precisely 
similar  to  those  in  the  Elasmobranch  yolk,  appear  in  the  protoplasmic 
matter  around  the  germinal  disc  ;  (2)  After  the  deposition  of  the  ova  there  is 
present  in  some  forms  a  network  of  protoplasm  extending  from  the  germinal 
disc  through  the  yolk1.  At  impregnation  this  withdraws  itself  from  the 
yolk.  It  is  to  be  compared  to  the  protoplasmic  network  of  the  Elasmo- 
branch ovum. 


FlG.  47.      TWO   LONGITUDINAL   SECTIONS   OF  THE   BLASTODERM   OF  A   PRISTIURUS 
EMBRYO  AT  STAGES  PRIOR  TO  THE  FORMATION  OF  THE  MEDULLARY  GROOVE. 

ep.  epiblast;  //.lower  layer  cells;  m.  mesoblast;  hy.  hypoblast;  sc.  segmentation 
cavity ;  es.  embryo  swelling ;  ri.  nuclei  of  yolk ;  er.  embryonic  rim. 

There  are  two  types  of  meroblastic  ova.  In  one  of  these 
(Aves,  Elasmobranchii)  the  germinal  disc  is  formed  in  the 
ovarian  ovum.  In  the  second  type  the  germinal  disc  is  formed 
after  impregnation  by  a  concentration  of  the  protoplasm  at  one 
pole.  This  concentration  is  analogous  to  what  has  already  been 
described  for  Anodon  and  other  Molluscan  ova  (p.  100). 

The  ova  of  some  Teleostei  are  intermediate  between  the  two 
types. 

The  ovum  of  the  wood-louse,  Oniscus  murarius2,  may  be  taken  as  an 
example  of  the  second  type  of  meroblastic  ovum.  In  this  egg  development 
commences  by  the  appearance  of  a  small  clear  mass  with  numerous 
transparent  vesicles.  This  mass  is  the  protoplasm  which  has  become 

1  Vide  Bambeke,  loc.  cit. 

2  Vide  Bobretzky,  Zeitschrift fur  wiss.  Zoologie,  Vol.  xxiv.,  1874. 


108  NUCLEI  OF  THE  YOLK. 

separated  from  the  yolk.  It  undergoes  segmentation  in  a  perfectly 
normal  fashion.  Examples  of  other  cases  of  this  kind  have  been  described 
by  Van  Beneden  and  Bessels1  in  Anchorella,  and  in  Hessia  by  Van 
Beneden2.  It  appears  from  their  researches  that  the  protoplasm  collects 
itself  together,  first  of  all  in  the  interior  of  the  egg,  and  then  travels  to  the 
surface.  It  arrives  at  the  surface  after  having  already  divided  into  two  or 
more  segments,  which  then  rapidly  divide  in  the  usual  manner  to  form  the 
blastoderm. 

There  are  some  grounds  for  thinking  that  the  cases  of  partial  segmen- 
tation in  the  Arthropoda  are  not  really  quite  comparable  with  those  in 
other  groups,  but  more  probably  fall  under  the  next  type  of  segmentation 
to  be  described.  The  grounds  for  this  view  are  mentioned  in  connection 
with  the  next  type. 

In  most  if  not  all  meroblastic  ova  there  appear  during  and 
after  segmentation  a  number  of  nuclei  in  the  yolk  adjoining  the 
blastoderm,  around  which  cells  become  differentiated.  (Figs.  46 
and  47.)  These  cells  join  the  part  of  the  blastoderm  formed  by 
the  normal  segmentation  of  the  germinal  disc.  Such  nuclei  are 
formed  in  all  craniate  meroblastic  ova3.  In  Cephalopods  they 
have  been  found  by  Lankester,  and  in  Oniscus  by  Bobretzky. 
They  have  been  by  some  authors  supposed  to  originate  from  the 
nuclei  of  the  blastoderm,  and  by  others  spontaneously  in  the 
yolk. 

Some  of  the  earliest  observations  on  these  nuclei  were  made  by  Lankes- 
ter4 in  the  Cephalopods.  He  found  that  they  appeared  first  in  a  ring- 
like  series  round  the  edge  of  the  blastoderm,  and  subsequently  all  over  the 
yolk  in  a  layer  a  little  below  the  surface.  He  observed  their  development 
in  the  living  ovum  and  found  that  they  "  commenced  as  minute  points,  gra- 
dually increasing  in  size  like  other  free-formed  nuclei."  A  cell  area  sub- 
sequently forms  around  them. 

By  E.  van  Beneden5  they  were  observed  in  a  Teleostean  ovum  to  appear 
nearly  simultaneously  in  considerable  numbers  in  the  granular  matter 
beneath  the  blastoderm.  Van  Beneden  concludes  from  the  simultaneous 
appearance  of  these  bodies  that  they  develop  autogenously.  Kupffer  at  an 
earlier  period  arrived  at  a  similar  conclusion.  My  own  observations  on  these 
nuclei  in  Elasmobranchii  on  the  whole  support  the  conclusions  to  be  derived 
from  Lankester's,  Kupffer's  and  Van  Beneden's  observations.  As  mentioned 
above,  the  nuclei  in  Elasmobranchii  do  not  appear  simultaneously,  but 

1  Loc.  cit.  2  Bulletins  de  FAcad.  Belgique,  Tom.  xxix.,  1870. 

*  Though  less  obvious  in  the  ovum  of  the  fowl  than  in  that  of  some  other  types, 
they  may  nevertheless  be  demonstrated  there  without  very  much  difficulty. 
4  Quart.  Journ.  of  Micr.  Science,  Vol.  xv.  pp.  39,  40. 
6  Quart.  Journ.  of  Micr.  Science,  Vol.  xvm.  p.  41. 


THE   SEGMENTATION   OF  THE  OVUM.  109 

increase  in  number  as  development  proceeds ;  and  it  is  possible  that  Van 
Beneden  may  be  mistaken  on  this  point.  No  evidence  came  before  me  01 
derivation  from  pre-existing  nuclei  in  the  blastoderm.  My  observations 
prove  however  that  the  nuclei  increase  by  division.  This  is  shewn  by  the 
fact  that  I  have  found  them  with  the  spindle  modification  (fig.  46,  nx'\  and 
that  in  most  cases  they  usually  exhibit  the  form  of  a  number  of  aggregated 
vesicles1,  which  is  a  character  of  nuclei  which  have  just  undergone  division. 
It  should  be  mentioned  however  that  I  failed  to  find  a  spindle  modification 
of  the  nuclei  in  the  later  stages.  Against  these  observations  must  be  set 
those  of  Bobretzky,  according  to  which  the  nuclei  in  Oniscus  are  really  the 
nuclei  of  cells  which  have  migrated  from  the  blastoderm.  Bobretzky's  obser- 
vations do  not  however  appear  to  be  very  conclusive. 

It  must  be  admitted  that  the  general  evidence  at  our  com- 
mand appears  to  indicate  that  the  nuclei  of  the  yolk  in  mero- 
blastic  ova  originate  spontaneously.  There  is  however  a  difficulty 
in  accepting  this  conclusion  in  the  fact  that  all  the  other  nuclei 
of  the  embryo  are  descendants  of  the  first  segmentation  nucleus ; 
and  for  this  reason  it  still  appears  to  me  possible  that  the  nuclei 
of  the  yolk  will  be  found  to  originate  from  the  continued 
division  of  one  primitive  nucleus,  itself  derived  from  the  segmen- 
tation nucleus. 

The  existence  of  these  nuclei  in  the  yolk  and  the  formation 
of  a  distinct  cell  body  around  them  is  a  strong  piece  of  evidence 
in  favour  of  the  view  above  maintained,  (which  is  not  universally 
accepted,)  that  the  part  of  the  ovum  of  meroblastic  ova  which 
does  not  segment  is  of  the  same  nature  as  that  which  does 
segment,  and  differs  only  in  being  relatively  deficient  in  active 
protoplasm. 

The  following  forms  have  meroblastic  ova  of  the  first  type :  the  Cephalo- 
poda, Pyrosoma,  Elasmobranchii,  Teleostei,  Reptilia,  Aves,  Ornithodelphia  (?). 
The  second  type  of  meroblastic  segmentation  occurs  in  many  Crustacea, 
(parasitic  Copepoda,  Isopoda  Mysis,  etc.).  It  is  also  stated  to  be  found  in 
Scorpio. 

The  ova  of  the  majority  of  groups  in  the  animal  kingdom 
segment  according  to  one  of  the  types  which  have  just  been 
described.  These  types  are  not  sharply  separated,  but  form  an 
unbroken  series,  commencing  with  the  ovum  which  segments 
uniformly,  and  ending  with  the  meroblastic  ovum. 

1  At  the  time  when  my  observations  on  Elasmobranchii  were  carried  out,  this 
peculiar  condition  of  the  nucleus  had  not  been  elucidated. 


I IO  CENTROLECITHAL  SEGMENTATION. 

It  is  convenient  to  distinguish  the  ova  which  segment 
uniformly  by  some  term  ;  and  I  should  propose  for  this  the 
term  alecithal1,  as  implying  that  they  are  without  food-yolk, 
or  that  what  little  food-yolk  there  is,  is  distributed  uniformly. 

The  ova  in  which  the  yolk  is  especially  concentrated  at  one 
pole  I  should  propose  to  call  telolecithal.  They  constitute 
together  a  group  with  an  unequal  or  partial  segmentation. 

The  telolecithal  ova  may  be  defined  in  the  following  way  : 
ova  in  which  the  food-yolk  is  not  distributed  uniformly,  but  is 
concentrated  at  one  pole  of  the  ovum.  When  only  a  moderate 
quantity  of  food-yolk  is  present  the  pole  at  which  it  is  concen- 
trated merely  segments  more  slowly  than  the  opposite  pole ;  but 
when  food-yolk  is  present  in  very  large  quantity  the  part  of  the 
ovum  in  which  it  is  located  is  incapable  of  segmentation,  and 
forms  a  special  appendage  known  as  the  yolk-sack. 

There  is  a  third  group  of  ova  including  a  series  of  types  of 
segmentation  nearly  parallel  to  the  telolecithal  group.  This 
group  takes  its  start  from  the  alecithal  ovum  as  do  the  teloleci- 
thal ova,  and  equally  with  these  includes  a  series  of  varieties 
of  segmentation  running  parallel  to  the  regular  and  unequal 
types  of  segmentation  which  directly  result  from  the  presence 
of  a  greater  or  smaller  quantity  of  food-yolk.  The  food-yolk  is 
however  placed,  not  at  one  pole,  but  at  the  centre  of  the  ovum. 
This  group  of  ova  I  propose  to  name  centrolecithal.  It  is 
especially  characteristic  of  the  Arthropoda,  if  not  entirely  con- 
fined to  that  group. 

Centrolecithal  ova.  As  might  be  anticipated  on  the  analogy 
of  the  types  of  segmentation  already  described,  the  concentration 
of  the  food-yolk  at  the  centre  of  the  ovum  does  not  always  take 
place  before  segmentation,  but  is  sometimes  deferred  till  even 
the  later  stages  of  this  process. 

Examples  of  a  regular  segmentation  in  centrolecithal  ova 
are  afforded  by  Palaemon  (Bobretzky)  and  Penaeus  (Haeckel). 
A  type  of  unequal  segmentation  like  that  of  the  Frog  occurs  in 
Gammarus  locusta  (Beneden  and  Bessels),  where  however  the 
formation  of  a  central  yolk  mass  does  not  appear  to  take  place 


1  For  this  term  as  well  as  for  the  terms  telolecithal  and  centrolecithal  I  am  indebted 
Mr  l.ankester. 


THE  SEGMENTATION   OF  THE  OVUM. 


Ill 


till  rather  late  in  the  segmentation.  More  irregular  examples  of 
unequal  segmentation  are  also  afforded  by  other  Crustaceans, 
e.g.  various  members  of  the  genus  Chondr acanthus  (Beneden  and 
Bessels)  and  by  Myriapods.  In  all  these  cases  segmentation 
ends  in  the  formation  of  a  layer  of  cells  enclosing  a  central  mass 
of  food-yolk. 

The  peculiarity  of  the  centrolecithal  ova  with  regular  or  un- 
equal segmentation  is  that  (owing  to  the  presence  of  the  yolk  in 
the  interior)  the  furrows  which  appear  on  the  surface  are  not 


FIG.  48.     SEGMENTATION  OF  A  CRUSTACEAN  OVUM  (PEN^EUS).    (After  Hseckel.) 

The  sections  illustrate  the  type  of  segmentation  in  which  the  yolk  is  aggregated  at 
the  centre  of  the  ovum. 

yk.  central  yolk  mass. 

i  and  2.  Surface  view  and  section  of  the  stage  with  four  segments.  In  2  it 
is  seen  that  the  furrows  visible  on  the  surface  do  not  penetrate  to  the  centre  of  the 
ovum. 

3  and  4.  Surface  view  and  section  of  ovum  near  the  end  of  segmentation.  The 
central  yolk  mass  is  very  clearly  seen  in  4. 

continued  to  the  centre  of  the  egg.  The  spheres  which  are  thus 
distinct  on  the  surface  are  really  united  internally.  Fig.  48, 
copied  from  Haeckel,  shews  this  in  a  diagrammatic  way. 

Many  ova,  which  in  the  later  stages  of  segmentation  exhibit 
the  characteristics  of  true  centrolecithal  ova,  in  the  early  stages 
actually  pass  through  nearly  the  same  phases  as  holoblastic  ova. 


112  CENTROLECITHAL  SEGMENTATION. 

Thus  in  Eupagurus  prideauxii*  (fig.  49),  and  probably  in  the 
majority  of  Decapods,  the  egg  is  divided  successively  into  two, 
four  and  eight  distinct  segments,  and  it  is  not  till  after  the  fourth 
phase  of  the  segmentation  that  the  spheres  fuse  in  the  centre  of 
the  egg.  Such  ova  belong  to  a  type  which  is  really  intermediate 


FIG.  49.   TRANSVERSE  SECTION  THROUGH  FOUR  STAGES  IN  THE  SEGMENTATION  OF 
EUPAGURUS  PRIDEAUXII.     (After  P.  Mayer.) 

between  the  ordinary  type  of  segmentation  and  that  with  a 
central  yolk  mass.  Eupagurus  presents  one  striking  peculiarity, 
viz.  that  the  nucleus  divides  into  two,  four  and  eight  nuclei,  each 
surrounded  by  a  delicate  layer  of  protoplasm  prolonged  into  a 
reticulum,  before  the  ovum  itself  commences  to  become  seg- 
mented. The  ovum  before  segmentation  is  therefore  in  the 
condition  of  a  syncytium. 

The  segmentation  of  Asellus  aquaticus2  is  very  similar  to  that  of  Eupagu- 
rus, etc.  but  the  ovum  at  the  very  first  divides  into  as  many  segments  (viz. 
eight)  as  there  are  nuclei. 

In  Gammarus  locusta  the  resemblance  to  ordinary  unequal  segmentation 
is  very  striking,  and  it  is  not  till  a  considerable  number  of  segments  have 
been  formed  that  a  central  yolk  mass  appears. 

1  Mayer,  Jtnaische  Zeitschrift,  Vol.  XI. 

3  Ed.  van  Beneden,  Butt,  d.  fAcad.  roy.  Bdgique,  2me  serie,  Tom.  Xxvm.  No.  7, 
1869,  p.  54. 


THE   SEGMENTATION   OF   THE  OVUM. 


In  all  the  above  types,  as  segmentation  proceeds,  the 
protoplasm  becomes  more  and  more  concentrated  at  the  surface, 
and  finally  a  superficial  layer  of  flat  blastoderm  cells  is  com- 
pletely segmented  off  from  the  yolk  below  (fig.  49  D). 

In  cases  like  those  of  Penaeus,  Eupagurus,  etc.,  the  yolk  in 
the  interior  is  at  first  nearly  homogeneous,  but  at  a  later  period 
it  generally  becomes  divided  up  partially  or  completely  into  a 
number  of  distinct  spheres,  which  may  have  nuclei  and  therefore 
have  the  value  of  cells.  In  many  cases  nuclei  have  however  not 
been  demonstrated  in  these  yolk  spheres,  though  probably 
present ;  yet,  till  they  have  been  demonstrated,  some  doubt 
must  remain  on  the  nature  of  these  yolk  spheres.  It  is  probable 
that  not  all  the  nuclei  which  result  from  the  division  of  the  first 
segmentation  nucleus  become  concerned  in  the  formation  of  the 
superficial  blastoderm,  but  that  some  remain  in  the  interior  of 
the  ovum  to  become  the  nuclei  of  the  yolk  spheres. 

In  Myriapods  (Chilognatha)  a  peculiar  form  of  segmentation  has  been 


FIG.  50.    SEGMENTATION  AND  FORMATION  OF  THE  BLASTODERM  IN  CHELIFER. 

(After  Metschnikoff.) 

In  A  the  ovum  is  divided  into  a  number  of  separate  segments.  In  B  a  number  of 
small  cells  have  appeared  (bl)  which  form  a  blastoderm  enveloping  the  large  yolk 
spheres.  In  C  the  blastoderm  has  become  divided  into  two  layers. 

B.  II.  8 


114  CENTROLECITHAL  SEGMENTATION. 

observed  by  Metschnikoff1.  The  ovum  commences  by  undergoing  a  per- 
fectly normal,  though  rather  irregular  total  segmentation.  But  after  the 
process  of  division  has  reached  a  certain  point,  scattered  masses  of  very 
small  cells  make  their  appearance  on  the  surface  of  the  large  spheres.  These 
small  cells  have  probably  arisen  in  a  manner  analogous  to  that  which 
characterizes  the  formation  of  the  superficial  cells  of  the  blastoderm  in  the 
types  of  centrolecithal  ova  already  described.  They  rapidly  increase  in 
number  and  eventually  form  a  continuous  blastoderm;  while  the  original 
large  segments  remain  in  the  centre  as  the  yolk  mass.  In  the  interesting 
Arachnid  CJulifer  segmentation  takes  place  in  nearly  the  same  manner  as  in 
Myriapods  (fig.  50). 

It  is  clear  that  it  is  not  possible  in  centrolecithal  ova  to  have 
any  type  of  segmentation  exactly  comparable  with  that  of 
meroblastic  ova.  There  are  however  some  types  which  fill  the 
place  of  the  meroblastic  ova  in  the  present  group,  in  as  much  as 
they  are  characterised  by  the  presence  of  a  large  bulk  of  food-yolk 
which  either  does  not  segment,  or  does  not  do  so  till  a  very  late 
stage  in  the  development.  The  essential  character  of  this  type  of 
segmentation  consists  in  the  division  of  the  germinal  vesicle  in 


FIG.  51.    FOUR  SUCCESSIVE  STAGES  IN  THE  SEGMENTATION  OF  THE  EGG  OF  TETRA- 
NYCHUS  TELARIUS.    (After  ClaparMe.) 

the  interior,  or  at  the  surface  of  the  ovum  into  two,  four,  etc. 
nuclei  (fig.  51).  These  nuclei  are  each  of  them  surrounded  by  a 
specially  concentrated  layer  of  protoplasm  (fig.  51)  which  is 

1  Zeitschrift  fur  wiss.  Zoo/.,  Vol.  xxiv.  1874. 


THE  SEGMENTATION   OF  THE  OVUM.  115 

continuous  with  a  general  protoplasmic  reticulum  passing 
through  the  ovum  [not  shewn  in  fig.  51].  The  yolk  is  contained 
in  the  meshes  of  this  reticulum  in  the  manner  already  described 
for  other  o.va. 

The  ovum,  like  that  of  Eupagurus  before  segmentation,  is 
now  a  syncytium.  Eventually  the  nuclei,  having  increased  by 
division  and  become  very  numerous,  travel,  unless  previously 
situated  there,  to  the  surface  of  the  ovum.  They  then  either 
simultaneously  or  in  succession  become,  together  with  protoplasm 
around  them,  segmented  off  from  the  yolk,  and  give  rise  to  a 
peripheral  blastoderm  enclosing  a  central  yolk  mass.  In  the 
latter  however  many  of  the  nuclei  usually  remain,  and  it  also 
very  often  undergoes  a  secondary  segmentation  into  a  number 
of  yolk  spheres. 

The  eggs  of  Insects  afford  numerous  examples  of  this  mode 
of  segmentation,  of  which  the  egg  of  Porthesia1  may  be  taken  as 
type.  After  impregnation  it  consists  of  a  central  mass  of  yolk 
which  passes  without  a  sharp  line  of  demarcation  into  a  peripheral 
layer  of  more  transparent  (protoplasmic)  material.  In  the 
earliest  stage  observed  by  Bobretzky  there  were  two  bodies  in 
the  interior  of  the  egg,  each  consisting  of  a  nucleus  enclosed  in  a 
thin  protoplasmic  layer  with  stellate  prolongations.  This  stage 
corresponds  with  the  division  into  two,  but  though  the  nucleus 
divides,  the  preponderating  amount  of  yolk  prevents  the  egg 
from  segmenting  at  the  same  time.  By  a  continuous  division 
of  the  nuclei  there  becomes  scattered  through  the  interior  of  the 
ovum  a  series  of  bodies,  each  formed  of  nucleus  and  a  thin  layer 
of  protoplasm  with  reticulate  processes.  After  a  certain  stage 
some  of  these  bodies  pass  to  the  surface,  simultaneously  (in 
Porthesia)  or  in  some  cases  successively.  At  the  surface  the 
protoplasm  round  each  nucleus  contracts  itself  into  a  rounded 
cell  body,  distinctly  cut  off  from  the  adjacent  yolk. 

The  cells  so  formed  give  rise  to  a  superficial  blastoderm  of  a 
single  layer  of  cells.  Many  of  the  nucleated  bodies  remain  in 
the  yolk,  and  after  a  certain  time,  which  varies  in  different  forms, 
the  yolk  becomes  segmented  up  into  a  number  of  rounded  or 
polygonal  bodies,  in  the  interior  of  each  of  which  one  of  the 


Bobretzky,  Zeit.f.  wiss.  Z00/.,-Bd.  xxxi.  1878. 

8—2 


CENTROLECITHAL  SEGMENTATION. 


above  nuclei  with  its  protoplasm  is  present.  This  process, 
known  as  the  secondary  segmentation  of  the  yolk,  is  really  part 
of  the  true  segmentation,  and  the  bodies  to  which  it  gives  rise 
are  true  cells. 

Other  examples  of  this  type  may  be  cited.  In  Aphis1  Metschnikoff 
shewed  that  the  first  segmentation  nucleus  divides  into  two,  each  of  which 
takes  up  a  position  in  the  clearer  peripheral  protoplasmic  layer  of  the  egg 
(fig.  52,  i  and  2).  Following  upon  further  division  the  nuclei  enveloped  in  a 
continuous  layer  of  protoplasm  arrange  themselves  in  a  regular  manner,  and 
form  a  syncytium,  which  becomes  segmented  into  definite  cells  (fig.  52,  3  and 
4).  The  existence  of  a  special  clear  superficial  layer  of  protoplasm  has  been 
questioned  by  Brandt. 


FIG.  57.     SEGMENTATION  OF  APHIS  ROSAE.    (Copied  from  Metschnikoff.) 
In  all  the  stages  there  is  seen  to  be  a  central  yolk  mass  surrounded  by  a  layer  of 

protoplasm. 

In  this  protoplasm  two  nuclei  have  appeared  in  i,  four  nuclei  in  2.    In  3  the  nuclei 

have  arranged  themselves  regularly,  and  in  4  the  protoplasm  has  become  divided  into 

a  number  of  columnar  cells  corresponding  to  the  nuclei. 

TV.  pole  of  the  blastoderm  which  has  no  share  in  forming  the  embryo. 

In  Tetranychus  telarius,  one  of  the  mites,  Claparede  found  on  the  surface 
of  the  ovum  a  nucleus  surrounded  by  granular  protoplasm  (fig.  51)  ;  which 
is  no  doubt  the  first  segmentation  nucleus.  By  a  series  of  divisions,  all 
on  the  surface,  a  layer  of  cells  becomes  formed  round  a  central  yolk  mass. 
The  result  here  is  the  same  as  in  Insects,  but  the  nucleus  with  its  granular 
protoplasm  is  from  the  first  superficial.  In  other  cases,  such  as  that  of  the 
common  fly2,  a  layer  of  protoplasm  is  stated  to  appear  investing  the  yolk ; 
and  in  this  there  arise  simultaneously  (?)  a  number  of  nuclei  at  regular  inter- 
vals, around  each  of  which  the  protoplasm  separates  itself  to  form  a  distinct 
cell.  Closely  allied  is  the  type  observed  by  Kowalevsky  in  Apis.  Develop- 
ment here  commences  by  the  appearance  of  a  number  of  protoplasmic 

1  Metschnikoff,  "  Embry.  Stud.  Insecten,"  Zcit.  fur  wiss.  Zoo!.,  Bd.  xvi.  1866. 
My  own  observations  on  this  form  accord  in  the  main  with  those  of  Metschnikoff. 

2  Vide  Weismann,   Entwicklung   d.    Dipteren;    and    Auerbach,    Organologische 
Studien. 


THE  SEGMENTATION   OF  THE  OVUM. 


117 


prominences,  each  forming  a  cell  provided  with  a  nucleus,  the  nuclei  having 
no  doubt  been  formed  by  previous  division  in  the  interior  of  the  ovum. 
They  appear  at  the  edge  of  the  yolk,  and  are  separated  from  one  another  by 
short  intervals.  Shortly  after  their  appearance  a  second  batch  of  similar 
bodies  appears,  filling  up  the  interspaces  between  the  first-formed  promi- 
nences. In  the  fresh-water  Gammarus  fluviatilis  the  protoplasm  is  stated 
first  of  all  to  collect  at  the  centre  of  the  ovum,  where  no  doubt  the  segmenta- 
tion nucleus  divides.  Subsequently  cells  appear  at  numerous  points  on  the 
surface,  and  by  repeated  division  constitute  an  uniform  blastoderm  investing 
the  central  yolk  mass.  This  mode  of  formation  of  the  blastoderm  is  closely 
allied  to  that  observed  by  Kowalevsky  in  Apis. 

Between  ova  with  a  segmentation  like  that  of  Insects,  and 
those  with  a  segmentation  like  that  of  Penaeus,  there  is  more 
than  one  intermediate  form.  The  Eupagurus  type,  with  the 
division  of  the  first  nucleus  into  eight  nuclei  before  the  division 


FIG.  53.    THREE  STAGES  IN  THE  SEGMENTATION  OF  PHILODROMUS  LIMBATUS. 
(After  Hub.  Ludwig.) 

of  the  ovum,  must  be  regarded  in  this  light ;  but  the  most 
instructive  example  of  such  a  transitional  type  of  segmentation 
is  that  afforded  by  Spiders1. 

The  first  phenomenon  which  can  be  observed  after  impreg- 
nation is  the  conglomeration  of  the  yolk  spheres  into  cylindrical 
columns,  which  finally  assume  a  radiating  form  diverging  from 
the  centre  of  the  egg.  In  the  centre  of  the  radiate  figure  is  a 
protoplasmic  mass,  probably  containing  a  nucleus,  which  sends 

i   Vide  Ludwig,  Zeit.f.  wiss.  Zool.,  1876. 


Il8  CENTROLECITHAL  SEGMENTATION. 

out  protoplasmic  filaments  through  the  columns  (fig.  53  A).  After 
a  certain  period  of  repose  the  figure  becomes  divided  into  two 
rosette-like  masses,  which  remain  united  for  some  time  by  a  proto- 
plasmic thread  :  this  thread  is  finally  ruptured  (fig.  53  B).  The 
whole  egg  does  not  in  this  process  divide  into  two  segments,  but 
merely  the  radiate  figure,  which  is  enclosed  in  a  finely  granular 
material.  The  two  rosettes  next  become  simultaneously  divided, 
giving  rise  to  four  rosettes  (fig.  53  C)  :  and  the  whole  process  is 
repeated  with  the  same  rhythm  as  in  a  regular  segmentation 
till  there  are  formed  thirty-two  rosettes  in  all  (fig.  54  A).  The 
rosettes  by  this  time  have  become  simple  columns,  which  by 
mutual  pressure  arrange  themselves  radiately  around  the  centre 
of  the  egg,  which  however  they  do  not  quite  reach. 

When  only  two  rosettes  are  present  the  protoplasm  with  its 
nucleus  occupies  a  central  position  in  each  rosette,  but  gradually, 
in  the  course  of  the  subsequent  subdivisions,  it  travels  towards 
the  periphery,  and  finally  occupies,  when  the  stage  with  thirty- 
two  rosettes  is  reached,  a  peripheral  position.  The  peripheral 
protoplasm  next  becomes  separated  off  as  a  nucleated  layer 


FIG.  54.    SURFACE  VIEW  AND  OPTICAL  SECTION  OF  A  LATE  STAGE  IN  THE 

SEGMENTATION  OF  PHILODROMUS  LIMBATUS  (Koch).      (After  Hub.  Ludwig.) 

bl.  blastoderm  ;  yk.  yolk  spheres. 

(fig-  54  B).  It  forms  the  proper  blastoderm,  and  in  it  the  nuclei 
rapidly  multiply  and  finally  around  each  an  hexagonal  or 
polygonal  area  of  protoplasm  is  marked  off;  and  a  blastoderm, 
formed  of  a  single  layer  of  flattened  cells,  is  thus  constituted. 
The  columns  within  the  blastoderm  now  form  (fig.  54  B)  more 
or  less  distinct  masses,  which  are  stated  by  Ludwig  to  be  with- 
out protoplasm. 


THE  SEGMENTATION   OF  THE  OVUM.  119 

From  observations  of  my  own  I  am  inclined  to  differ  from  Ludwig  as  to 
the  nature  of  the  parts  within  the  blastoderm.  My  observations  have  been 
made  on  Agelena  labyrinthica  and  commence  at  the  close  of  the  segmenta- 
tion. At  this  time  I  find  a  superficial  layer  of  flattened  cells,  and  within 
these  a  number  of  large  polyhedral  yolk  cells.  In  many,  and  I  believe  all, 
of  the  yolk  cells  there  is  a  nucleus  surrounded  by  protoplasm.  It  is  generally 
placed  at  one  side  and  not  in  the  centre  of  a  yolk  cell,  and  the  nuclei  are  so 
often  double  that  I  have  no  doubt  they  are  rapidly  undergoing  division.  It 
appears  to  me  probable  that,  at  the  time  when  the  superficial  layer  of  proto- 
plasm is  segmented  off  from  the  yolk  below,  the  nuclei  undergo  division,  and 
that  a  nucleus  with  surrounding  protoplasm  is  left  with  each  yolk  column. 
For  further  details  vide  Chapter  on  Arachnida. 

Although  by  the  close  of  the  segmentation  the  protoplasm 
has  travelled  to  a  superficial  position,  it  may  be  noted  that  at 
first  it  forms  a  small  mass  in  the  centre  of  the  egg,  and  only 
eventually  assumes  its  peripheral  situation.  It  is  moreover  clear 
that  in  the  Spider's  ovum  there  is,  so  to  speak,  an  attempt  at  a 
complete  segmentation,  which  however  only  results  in  an 
arrangement  of  the  constituents  of  the  ovum  in  masses  round 
each  nucleus,  and  not  in  a  true  division  of  the  ovum  into  distinct 
segments. 

It  seems  very  probable  that  Ludwig's  observations  on  the  segmentation 
of  Spiders  only  hold  good  for  species  with  comparatively  small  ova. 

In  connection  with  the  segmentation  of  the  Insects'  ovum  and  allied 
types  it  should  be  mentioned  that  Bobretzky,  to  whose  observations  we  are 
largely  indebted  for  our  knowledge  of  this  subject,  holds  somewhat  different 
views  from  those  adopted  in  the  text.  He  regards  the  nuclei  surrounded  by 
protoplasm,  which  are  produced  by  the  division  of  the  primitive  segmenta- 
tion nucleus,  as  so  many  distinct  cells.  These  cells  are  supposed  to  move 
about  freely  in  the  yolk,  which  acts  as  a  kind  of  intercellular  medium.  This 
view  does  not  commend  itself  to  me.  It  is  opposed  to  my  own  observations 
on  similar  nuclei  in  the  Spiders.  It  does  not  fit  in  with  our  knowledge  of  the 
nature  of  the  ovum,  and  it  cannot  be  reconciled  with  the  segmentation 
of  such  types  as  Spiders  or  even  Eupagurus,  with  which  the  segmentation  in 
Insects  is  undoubtedly  closely  related. 

The  majority  if  not  all  the  cases  in  which  a  central  yolk  mass  is  formed 
occur  in  the  Arthropoda,  in  which  group  centrolecithal  ova  are  undoubtedly 
in  a  majority.  In  Alcyonium  palmatum  the  segmentation  appears  however 
to  resemble  that  of  many  insects. 

One  or  two  peculiar  varieties  in  the  segmentation  of  ova  of  this  type 
may  be  spoken  of  here.  The  first  one  I  shall  mention  is  detailed  in  the 
important  paper  of  E.  Van  Beneden  and  Bessels  which  I  have  already  so 
often  had  occasion  to  quote :  it  is  characteristic  of  the  eggs  of  most  of  the 


120  SUMMARY. 


species  of  Chondracanthus,  a  genus  of  parasitic  Crustaceans.  The  ovum 
divides  in  the  usual  way  but  somewhat  irregularly  into  2,  4,  8  segments 
which  meet  in  a  central  yolk  mass ;  but  after  the  third  division  instead  of 
each  segment  dividing  into  two  equal  parts  it  divides  at  once  into  four,  and 
the  division  into  four  having  started,  reappears  at  every  successive  division. 
Thus  the  number  of  the  segments  at  successive  periods  is  2,  4,  8,  32,  128,  etc. 
In  another  peculiar  case,  an  instance  of  which1  is  afforded  by  Asellus  aqua- 
ticus,  after  each  of  the  earlier  segmentations  all  the  segments  fuse  and 
become  indistinguishable,  but  at  the  succeeding  segmentation  double  the 
number  of  segments  appears. 

Although,  as  has  been  already  stated,  it  does  not  seem  possible  to  have  a 
true  meroblastic  segmentation  in  centrolecithal  ova,  it  does  nevertheless 
appear  probable  that  the  apparent  cases  of  a  meroblastic  segmentation  in 
the  Arthropoda  are  derivatives  of  this  type  of  segmentation.  The  manner 
in  which  the  one  type  might  pass  into  the  other  may  perhaps  be  explained 
by  the  segmentation  in  Asellus  aquaticus^.  In  this  ovum  large  segments 
are  at  first  formed  around  a  central  yolk  mass,  in  the  peculiar  manner  men- 
tioned in  the  previous  paragraph,  but  at  the  close  of  the  first  period  of  seg- 
mentation minute  cells,  which  eventually  form  a  superficial  blastoderm,  are 
produced  from  the  yolk  cells.  They  do  not  however  appear  at  once  round 
the  whole  periphery  of  the  egg,  but  at  first  only  on  the  ventral  surface  and 
later  on  the  dorsal  surface.  If  the  amount  of  food-yolk  in  the  egg  were 
to  increase  so  as  to  render  the  formation  of  the  yolk  cells  impossible,  and  at 
the  same  time  the  formation  of  the  blastodermic  cells  were  to  take  place  at 
the  commencement,  instead  of  towards  the  close  of  the  segmentation,  a  mass 
of  protoplasm  with  a  nucleus  might  first  appear  at  the  surface  on  the  future 
ventral  side  of  the  egg,  then  divide  in  the  usual  way  for  meroblastic  ova,  and 
give  rise  to  a  layer  of  cells  gradually  extending  round  to  the  dorsal  surface. 
A  meroblastic  segmentation  might  perhaps  be  even  more  easily  derived  from 
the  type  found  in  Insects.  It  is  probable  that  the  cases  of  Scorpio,  Mysis, 
Oniscus,  the  parasitic  Isopoda,  and  some  parasitic  Copepoda  belong  to  this 
category ;  and  it  may  be  noticed  that  in  these  cases  the  blastopore  would  be 
situated  on  the  dorsal  and  not  on  the  ventral  side  of  the  ovum.  The  mor- 
phological importance  of  this  latter  fact  will  appear  in  the  sequel. 

The  results  arrived  at  in  the  present  section  may  be  shortly 
restated  in  the  following  way. 

(i)  A  comparatively  small  number  of  ova  contain  very 
little  or  no  food-yolk  embedded  in  their  protoplasm;  and  have 
what  food-yolk  may  be  present  distributed  uniformly.  In  such 
ova  the  segmentation  is  regular.  They  may  be  described  as 
alecithal  ova. 


1  Ed.  van  Beneden,  Bull.  Acad.  Belgique,  Vol.  xxvm.  1869. 


THE  SEGMENTATION   OF  THE  OVUM.  121 

(2)  The  distribution  of  food-yolk  in  the  protoplasm  of  the 
ovum  exercises  an  important  influence  on  the  segmentation. 

The  rapidity  with  which  any  part  of  an  ovum  segments  varies 
ceteris  paribus  with  the  relative  amount  of  protoplasm  it  contains; 
and  the  size  of  the  segments  formed  varies  inversely  to  the 
relative  amount  of  protoplasm.  When  the  proportion  of  pro- 
toplasm in  any  part  of  an  ovum  becomes  extremely  small, 
segmentation  does  not  occur  in  that  part. 

Ova  with  food-yolk  may  be  divided  into  two  great  groups 
according  to  the  eventual  arrangement  of  the  food-yolk  in  the 
protoplasm.  In  one  of  these,  the  food-yolk  when  present  is 
concentrated  at  the  vegetative  pole  of  the  ovum.  In  the  other 
group  it  is  concentrated  at  the  centre  of  the  ovum.  Ova  belong- 
ing to  the  former  group  are  known  as  telolecithal  ova,  those  to 
the  latter  as  centrolecithal. 

In  each  group  more  than  one  type  may  be  distinguished.  In 
the  first  group  these  types  are  (i)  unequal  segmentation,  (2) 
partial  segmentation.  The  features  of  these  three  types  have 
been  already  so  fully  explained  that  I  need  not  repeat  them  here. 

In  the  second  group  there  are  three  distinct  types,  (i)  equal 
segmentation,  (2)  unequal  segmentation.  These  two  being  ex- 
ternally similar  to  the  similarly  named  types  in  the  first  group. 
(3)  Superficial  segmentation.  This  is  unlike  anything  which  is 
present  in  the  first  group,  and  is  characterized  by  the  appearance 
of  a  superficial  layer  of  cells  round  a  central  yolk  mass.  These 
cells  may  either  appear  simultaneously  or  successively,  and  their 
nuclei  are  derived  from  the  segmentation  within  the  ovum  of  the 
first  segmentation  nucleus. 

The  types  of  ova  in  relation  to  the  characters  of  the  segmen- 
tation may  be  tabulated  in  the  following  way : 

Segmentation. 

(1)  alecithal       ) 

v  '  regular 

ova  j 

(2)  telolecithal  \  (a)  unequal 

ova  J  (b)  partial 

,  .  N  (a)  regular  (with  segments  united  in 

(3)  centre-          |  v  '      B  ' 

,     .,,    ,  central  yolk  mass) 

lecithal        >  / 

ova  W  une(lual    "  »  »       » 

(c)  superficial. 


122  SUMMARY. 


Although  the  various  types  of  segmentation  which  have  been 
described  present  very  different  aspects,  they  must  nevertheless 
be  looked  on  as  manifestations  of  the  same  inherited  tendency 
to  division,  which  differ  only  according  to  the  conditions  under 
which  the  tendency  displays  itself. 

This  tendency  is  probably  to  be  regarded  as  the  embryologi- 
cal  repetition  of  that  phase  in  the  evolution  of  the  Metazoa, 
which  constituted  the  transition  from  the  protozoon  to  the 
metazoon  condition. 

From  the  facts  narrated  in  this  chapter  the  reader  will  have 
gathered  that  similarity  or  dissimilarity  of  segmentation  is  no 
safe  guide  to  affinities.  In  many  cases,  it  is  true,  a  special  type 
of  segmentation  may  characterize  a  whole  group  ;  but  in  other 
cases  very  closely  allied  animals  present  the  greatest  differences 
with  respect  to  their  segmentation  ;  as  for  instance  the  different 
species  of  the  genus  Gammarus.  The  character  of  the  segmen- 
tation has  great  influence  on  the  early  phenomena  of  develop- 
ment, though  naturally  none  on  the  adult  form. 

EXTERNAL  FEATURES  OF  SEGMENTATION. 

(105)  E.    Haeckel.     "Die   Gastrula    u.    Eifurchung."    Jenaische    Zeitschrift, 
Vol.  IX.  1877. 

(106)  Fr.    Leydig.     "Die    Dotterfurchung    nach    ihrem   Vorkommen   in    d. 
Thierwelt  u.  n.  ihrer  Bedeutung."     Oken  his.  1848. 


PART  I. 

SYSTEM  A  TIC  EMBR  YOL  OG  Y. 


PART    I. 
SYSTEMATIC    EMBRYOLOGY. 

INTRODUCTION. 

IN  all  the  Metazoa  the  segmentation  is  followed  by  a  series 
of  changes  which  result  in  the  grouping  of  the  embryonic  cells 
into  definite  layers,  or  membranes,  known  as  the  germinal 
layers.  There  are  always  two  of  these  layers,  known  as  the 
epiblast  and  hypoblast;  and  in  the  majority  of  instances 
a  third  layer,  known  as  the  mesoblast,  becomes  interposed 
between  them.  It  is  by  the  further  differentiation  of  the  ger- 
minal layers  that  the  organs  of  the  adult  become  built  up. 
Owing  to  this  it  is  usual,  in  the  language  of  Embryology,  to 
speak  of  the  organs  as  derived  from  such  or  such  a  germinal 
layer. 

At  the  close  of  the  section  of  this  work  devoted  to  systematic 
embryology,  there  is  a  discussion  of  the  difficult  questions  which 
arise  as  to  the  complete  or  partial  homology  of  these  layers 
throughout  the  Metazoa,  and  as  to  the  meaning  to  be  attached 
to  the  various  processes  by  which  they  take  their  origin ;  but  a 
few  words  as  to  the  general  fate  of  the  layers,  and  the  general 
nature  of  the  processes  by  which  they  are  formed,  will  not  be 
out  of  place  here. 

Of  the  three  layers  the  epiblast  and  hypoblast  are  to  be 
regarded  as  the  primary.  The  epiblast  is  essentially  the  primi- 
tive integument,  and  constitutes  the  protective  and  sensory 
layer.  It  gives  rise  to  the  skin,  cuticle,  nervous  system,  and 
organs  of  special  sense.  The  hypoblast  is  essentially  the  diges- 
tive and  secretory  layer,  and  gives  rise  to  the  epithelium  lining 
the  alimentary  tract  and  the  glands  connected  with  it. 


126  INTRODUCTION. 


The  mesoblast  is  only  found  in  a  fully  developed  condition 
in  the  forms  more  highly  organized  than  the  Coelenterata.  It 
gives  origin  to  the  general  connective  tissue,  internal  skeleton, 
the  muscular  system,  the  lining  of  the  body-cavity,  the  vascular, 
and  excretory  systems.  It  probably  in  the  first  instance  origi- 
nated from  differentiations  of  the  two  primary  layers,  and  in  all 
groups  with  a  well-developed  body-cavity  it  is  divided  into  two 
strata.  One  of  them  forms  part  of  the  body-wall  and  is  known 
as  the  somatic  mesoblast,  the  other  forms 
part  of  the  wall  of  the  viscera  and  is  known 
as  the  splanchnic  mesoblast. 

A  very  large  number  not  to  say  the 
majority  of  organs  are  derived  from  parts  of 
two  of  the  germinal  layers.  Many  glands 
for  instance  have  a  lining  of  hypoblast  which 
is  coated  by  a  mesoblastic  layer. 

The  processes  by  which  the  germinal 
layers  take  their  origin  are  largely  influenced 
by  the  character  of  the  segmentation,  which,  FIG  DIAGRAM 
as  was  shewn  in  the  last  chapter,  is  mainly  OF  A  GASTRULA. 
dependent  on  the  distribution  of  the  food-  ™m  bl?stopore;  b. 

yolk.      When   the  segmentation    is   regular,     archenteron;  c.  hypo- 
'  blast ;  d.  epiblast. 

and  results  in  the  formation  of  a  blastosphere, 
the  epiblast  and  hypoblast  are  usually  differentiated  from  the 
uniform  cells  forming  the  wall  of  the  blastosphere  in  one  of  the 
two  following  ways. 

(1)  One-half  of  the  blastosphere  may  be  pushed  in  towards 
the  other  half.     A  two-layered  hemisphere  is  thus  established 
which  soon  elongates,  while  its  opening  narrows  to  a  small  pore 
(fig-   55)-     The  embryonic   form   produced   by   this   process  is 
known  as  a  gastrula.     The  process  by  which  it  originates  is 
known  as  embolic  invagination,  or  shortly  invagination.     Of 
the  two  layers  of  which  it  is  formed  the  inner  one  (c)  is  known  as 
the  hypoblast  and  the  outer  (d}  as  the  epiblast,  while  the  pore 
leading  into  its  cavity  lined  by  the  hypoblast  is  the  blastopore 
(a).     The  cavity  itself  is  the  archenteron  (b}. 

(2)  The  cells  of  the  blastosphere  may  divide  themselves  by 
a  process  of  concentric  splitting  into  two  layers  (fig.  56,  3).     The 
two   layers   are  as  before  the  epiblast  and  hypoblast,  and  the 


SYSTEMATIC  EMBRYOLOGY. 


127 


process  by  which  they  originate  is  known  as  delamination. 
The  central  cavity  or  archenteron  (F)  is  in  the  case  of  delamina- 
tion the  original  segmentation  cavity ;  and  not  an  entirely  new 
cavity  as  in  the  case  of  invagination.  By  the  perforation  of  the 
closed  two-walled  vesicle  resulting  from  delamination  an  embry- 


Fig.z 


FIG.  56.    DIAGRAM  SHEWING  THE  FORMATION  OF  A  GASTRULA  BY  DELAMINATION. 

(From  Lankester.) 
Fig.  i.     Ovum. 

Fig.  2.     Stage  in  segmentation. 

Fig.  3.     Commencement  of  delamination  after  the  appearance  of  a  central  cavity. 
Fig.  4.     Delamination  completed,  mouth  forming  at  M. 
In  fig.  i,  2  and  3  EC.  is  ectoplasm,  and  En.  is  entoplasm. 
In  fig.  4  EC.  is  epiblast  and  En.  hypoblast. 

onic  form  is  produced  which  cannot  be  distinguished  in  structure 
from  the  gastrula  produced  by  invagination  (fig.  56,  4).  The 
opening  (M)  in  this  case  is  not  however  known  as  the  blastopore 
but  as  the  mouth. 

When  segmentation  does  not  take  place  on  the  regular  type 
the  processes  above  described  are  as  a  rule  somewhat  modified. 
The  yolk  is  usually  concentrated  in  the  cells  which  would,  in 
the  case  of  a  simple  gastrula,  be  invaginated.  As  a  consequence 
of  this,  these  cells  become  (i)  distinctly  marked  off  from  the 
epiblast  cells  during  the  segmentation ;  and  (2)  very  much 
more  bulky  than  the  epiblast  cells.  The  bulkiness  of  the 


128 


INTRODUCTION. 


ms 


7,y 


hypoblast  cells  necessitates  a 
modification  of  the  normal  pro- 
cess of  embolic  invagination, 
and  causes  another  process  to 
be  substituted  for  it,  viz.  the 
growth  of  the  epiblast  cells  as 
a  thin  layer  over  the  hypoblast. 
This  process  (fig.  57)  is  known 
as  epibolic  invagination. 
The  point  where  the  complete 
enclosure  of  the  hypoblast  cells 
is  effected  is  known  as  the  blas- 
topore.  All  intermediate  con- 
ditions between  epibolic  and 
embolic  invagination  have  been  found. 

In  delamination,  when  the  segmentation  is  not  uniform,  or 
when  a  solid  morula  is  formed,  the  differentiation  of  the  epiblast 
and  hypoblast  is  effected  by  the  separation  of  the  central  solid 
mass  of  cells  from  the  peripheral  cells  (fig.  58  A). 


FIG.    57.       TRANSVERSE     SECTION 

THROUGH  THE  OVUM  OF  EUAXES 
DURING  AN  EARLY  STAGE  OF  DEVELOP- 
MENT. (After  Kowalevsky.) 

ep.   epiblast;    ms.  mesoblastic  band; 
hy.  hypoblast. 


FIG.  58.      TWO   STAGES  IN  THE  DEVELOPMENT  OF  STEPHANOMIA  PICTUM. 

(After  Metschnikoff.) 

A.  Stage  after  the  delamination.    ep.  epiblastic  invagination  to  form  pneuma- 
tocyst. 

B.  Later  stage  after  the  formation  of  the  gastric  cavity  in  the  solid  hypoblast, 
po.  polypite ;  t.  tentacle ;  pp.  pneumatophore ;   ep.  epiblastic  invagination   to   form 
pneumatocyst ;  hy.  hypoblast  surrounding  pneumatocyst. 


SYSTEMATIC  EMBRYOLOGY.  1 29 

In  the  case  of  epibolic  invagination  as  well  as  in  that  of  the 
type  of  delamination  just  spoken  of,  the  archenteric  cavity  is  in 
most  cases  secondarily  formed  in  the  solid  mass  of  hypoblast 
(fig.  58  B). 

In  ova  with  a  partial  segmentation  there  is  usually  some 
modification  of  the  epibolic  gastrula. 

Many  varieties  are  found  in  the  animal  kingdom  of  the  types 
of  invagination  and  delamination  just  characterized,  and  in  not 
a  few  forms  the  layers  originate  in  a  manner  which  cannot 
be  brought  into  connection  with  either  of  these  processes. 


FIG.    59.     EPIBOLIC  GASTRULA  OF  BONELLIA.     (After  Spengel.) 

A.  Stage  when  the  four  hypoblast  cells  are  nearly  enclosed. 

B.  Stage  after  the  formation  of  the  mesoblast  has  commenced  by  an  infolding  of 
the  lips  of  the  blastopore. 

ep.  epiblast ;  me.  mesoblast ;  bl.  blastopore. 

The  mesoblast  usually  originates  subsequently  to  the  two 
primary  layers.  It  then  springs  from  one  or  both  of  the  other 
layers,  but  its  modes  of  origin  are  so  various  that  it  would  be 
useless  to  attempt  to  classify  them  here.  In  cases  of  invagination 
it  often  arises  at  the  lips  of  the  blastopore  (fig.  57  and  59),  and 
in  other  cases  part  of  it  springs  as  paired  hollow  outgrowths  of 
the  walls  of  the  archenteron.  Such  outgrowths  are  shewn  in 
fig.  60,  B  and  C  at  pv.  The  cavity  of  the  outgrowths  forms  the 
body  cavity,  and  the  walls  of  the  outgrowths  the  somatic  and 
splanchnic  layers  of  mesoblast  (fig.  C.  sp.  and  so.).  The  archen- 
teron is  in  part  always  converted  into  a  section  of  the  permanent 
alimentary  tract;  and  the  section  of  the  alimentary  tract  so 
derived  is  known  as  the  mesenteron.  There  are  however 
usually  two  additional  parts  of  the  alimentary  tract,  known  as 
B.  II.  9 


130  INTRODUCTION. 


FIG.  60.  THREE  STAGES  IN  THE  DEVELOPMENT  OF  SAGITTA.  (A  and  C  after 
Butschli  and  B  after  Kowalevsky.)  The  three  embryos  are  represented  in  the  same 
positions. 

A.  Represents  the  gastrula  stage. 

B.  Represents  a  succeeding  stage  in  which  the  primitive  archenteron   is  com- 
mencing to  be  divided  into  three  parts,  the  two  lateral  of  which  are  destined  to  form 
the  mesoblast. 

C.  Represents  a  later  stage  in  which  the  mouth  involution  (»/)  has  become  con- 
tinuous with  alimentary  tract,  and  the  blastopore  has  become  closed. 

m.  mouth  ;  al.  alimentary  canal ;  ae.  archenteron  ;  bl.  p.  blastopore  ;  pv.  perivis- 
ceral  cavity ;  sp.  splanchnic  mesoblast ;  so.  somatic  mesoblast ;  ge.  generative  organs. 

the  stomodaeum  and  proctodaeum,  derived  from  epiblastic 
imaginations.  They  give  rise  respectively  to  the  oral  and  anal 
extremities  of  the  alimentary  tract. 

BIBLIOGRAPHY. 

(107)  K.  E.  von  Baer.     "  Ueb.  Entwicklungsgeschichte  d.  Thiere."     Konigs- 
berg,  1828—1837. 

(108)  C.  Claus.     Griindzilge  d.  Zoologie.     Marburg  und  Leipzig,  1879. 

(109)  C.  Gegenbaur.     Grundriss  d.  vergleichenden  Anatomic.     Leipzig,  1878. 
Vide  also  Translation.     Elements   of  Comparative  Anatomy.     Macmillan  and  Co., 
1878. 

(110)  E.  Haeckel.    Studien  z,  Gastraa-Theorie.    Jena,  1877,  and  dsojenaischc 
Zeitschrift,  Vols.  vin.  and  ix. 

(111)  E.  Haeckel.     Schbpfungsgeschichte.     Leipzig.      Vide  also  Translation. 
The  History  of  Creation,     King  and  Co.,  London,  1876. 

(112)  E.  Haeckel.     Anthropogenic.     Leipzig.     Vide  also  Translation.    Anthro- 
Pogeny  (Translation).     Kegan  Paul  and  Co.,  London,  1878. 

(113)  Th.    H.  Huxley.      The  Anatomy  of  Invcriebratcd  Animals.     Churchill, 
1877. 

(114)  E.  R.  Lankester.     "Notes  on  Embryology  and  Classification."     Quart. 
J.  of.  Micr.  Science,  Vol.  xvn.  1877. 

(115)  A.  S.  P.  Packard.     Life  Histories  of  Animals,  including  Man,  or  Outlines 
of  Comparative  Embryology.     Holt  and  Co.,  New  York,  1876. 

(116)  H.    Rathke.     Abhandlungen   2.    Bildung-    und    Enhvicklungsgesch.    d. 
Menschen  u.  d.  Thiere.     Leipzig,  1833. 


CHAPTER   IV. 

DICYEMID.E   AND   ORTHONECTID^. 

DlCYEMHXE. 

THE  structure  and  development  of  these  remarkable  para- 
sites in  the  renal  organs  of  the  Cephalopoda  have  recently  been 
greatly  elucidated  by  the  researches  of  E.  van  Beneden ;  and 
although  a  male  element  has  not  been  discovered,  yet  the 
embryos  originate  from  bodies  which  have  a  close  similarity  to 
ordinary  ova. 

Van  Beneden  has  shewn  that  Dicyema  consists  in  the  adult 
state  of  (i)  a  single  layer  of  ciliated  epiblast  cells,  somewhat 
modified  anteriorly  to  form  a  cephalic  enlargement;  and  of  (2) 
one  large  nucleated  hypoblast  cell  enclosed  within  the  epiblast. 
There  are  two  kinds  of  embryo,  both  developed  from  germs 
which  originate  in  the  hypoblast  cell.  The  two  kinds  of  embryo 
arise  in  individuals  of  somewhat  different  forms.  The  one  kind, 
called  by  Van  Beneden  the  vermiform  embryo,  arises  in  the 
more  elongated  and  thinner  examples  of  Dicyema  which  have 
been  named  Nematogens.  These  embryos  pass  directly  into 
the  parent  form  without  metamorphosis. 

The  second  kind  of  embryo,  called  infusoriform,  is  very 
different  from  the  parent,  and  has  a  free  existence.  Its  eventual 
history  is  not  known.  It  originates  in  the  shorter  and  thicker 
individuals  of  Dicyema;  which  have  been  called  Rhombogens. 

The  Vermiform  Embryos.  The  germs  or  cells  which  give 
rise  to  the  vermiform  embryos  originate  endogenously  in  the 
protoplasmic  reticulum  of  the  axial  hypoblast  cell.  They  appear 
as  small  but  well-defined  spheres,  with  a  minute  body  inj;he 

9—2 


132 


DICYEMID^E. 


centre.  In  these  spheres  a  cortical  layer  becomes  differentiated, 
which  gradually  increases  in  thickness  and  gives  rise  to  the  body 
of  a  cell,  the  nucleus  and  nucleolus  of  which  are  respectively 
formed  from  the  inner  part  of  the  original  sphere  and  the  minute 
central  body.  These  germs  can  originate  in  all  parts  of  the 
hypoblast  cell  and  are  frequently  very  numerous. 

The  germ  when  completely  formed  undergoes  a  segmentation 
very  similar  to  that  of  an  ordinary  ovum.  It  divides  first  into 
two  and  then  into  four  approximately  equal  segments.  Of  the 
four  segments  one,  however,  remains  passive  for  the  remainder 
of  the  development.  The  other  three  divide  and  arrange  them- 
selves so  as  partially  to  enclose  in  a  cup-like  fashion  the  passive 
cell  (fig.  6  1  A).  The  six  cells  resulting 
from  their  division  again  divide,  giving 
rise  to  twelve  cells,  which  nearly  enclose 
the  passive  cell,  leaving  only  a  small 
aperture  at  one  point.  The  whole  pro- 
cess by  which  the  central  cell  becomes 
enclosed  is,  as  E.  van  Beneden  points 

out,  identical  with  a  gastrula  formation 

,  FIG.  61.    A.  GASTRULA 

by   epibole,   and   the   space    where  the     STAGE  OF  DICYEMA  TYPUS. 


central  cell  is  left  uncovered  is  the  bias- 

topore.        The    central    cell     itself    gives      Gegenbaur,      after    E.     van 

origin    to    the    hypoblast    cell    of    the     Beneden-) 
adult,  and  the   peripheral    cells  to  the  epiblast. 

By  this  time  the  embryo  has  assumed  an  oval  form,  and  the 
blastopore  is  situated  at  the  pole  of  the  long  axis  of  the  oval 
where  the  cephalic  enlargement  is  eventually  formed. 

The  subsequent  development  consists  mainly  in  the  closure 
of  the  blastopore,  and  an  increase  in  the  number  of  the  epiblast 
cells.  Before  the  development  is  completed,  and  while  the 
embryo  is  still  in  the  body  of  the  parent,  two  germs,  destined 
themselves  to  give  rise  to  fresh  embryos,  appear  in  the  hypoblast 
cell,  one  on  each  side  of  the  nucleus  (fig.  61  B).  The  embryo 
continues  to  elongate,  while  the  anterior  cells  become  converted 
into  the  polar  cells.  Cilia  appear  simultaneously  over  the 
general  surface,  and  the  embryo  makes  its  way  out  of  the  body 
of  the  parent,  usually  at  the  cephalic  pole,  and  becomes  itself 
parasitic  in  the  renal  organ  of  the  host  in  which  it  finds  itself. 


INFUSORIFORM   EMBRYOS.  133 

At  the  time  of  birth  the  embryo  may  contain  a  number  of  germs 
and  sometimes  even  developing  embryos. 

Infusoriform  Embryos.  The  infusoriform  embryos  are 
capable  of  living  in  sea-water  and  almost  certainly  lead  a  free 
existence.  In  their  most  fully  developed  condition  so  far  known 
they  have  the  following  rather  complicated  structure  (fig.  62  D, 
E,  F,  G). 

The  body  is  somewhat  pyriform,  with  a  blunt  extremity 
which  is  directed  forwards  in  swimming,  and  a  more  pointed 
extremity  directed  backwards.  The  former  may  be  spoken  of  as 
the  anterior,  and  the  latter  as  the  posterior  extremity  or  tail. 
At  the  anterior  extremity  are  situated  a  pair  of  refractive  bodies 
(f)  which  lie  above  an  unpaired  organ  which  may  be  called  the 
urn. 

The  structure  of  the  urn,  the  refractive  bodies,  and  the  tail 
may  be  dealt  with  in  succession. 

The  urn  consists  of  three  parts:  (i)  a  wall  (#),  (2)  a  lid  (/), 
and  (3)  contents  (gr).  The  wall  of  the  urn  is  hemispherical  in 
form,  and  composed  of  two  halves  in  apposition  (fig.  F).  Its 
concavity  is  directed  forwards,  and  in  its  edge  are  imbedded  a 
number  of  rod-like  corpuscles  which  appear  as  a  ring  near  the 
surface  in  a  full-face  view  (fig.  D).  The  lid  has  the  form  of  a 
low  pyramid  with  its  apex  directed  outwards.  It  is  made  up  of 
four  segments  (fig.  D).  The  contents  of  the  urn,  which  com- 
pletely fill  up  its  cavity,  are  four  polynuclear  cells  arranged  in  the 
form  of  a  cross  which  appear  with  low  powers  as  granular  bodies 
(fig.  F).  They  are  frequently  ejected,  apparently  at  the  will  of 
the  embryo. 

The  refractive  bodies  (r),  two  in  number,  one  on  each  side  of 
the  middle  line,  are  composed  of  a  material  which  is  not  of  a 
fatty  nature,  and  which  is  passive  to  the  majority  of  reagents. 
Each  is  enveloped  in  a  special  capsule,  and  at  times  more  than 
one  refractive  body  is  present  in  each  capsule.  The  tail  is  a 
conical  structure  formed  of  ciliated  granular  cells. 

No  plausible  guess  has  been  made  as  to  the  function  either  of 
the  urn  or  of  the  refractive  bodies. 

The  infusoriform  embryos  originate  from  germs,  which  have 
however  a  different  origin  to  the  germs  of  the  vermiform 
embryos.  One  to  five  cells  appear  in  the  axial  hypoblast  cell,  in 


134 


a  way  not  clearly  made  out,  and  each  of  them  gives  rise  by  an 
endogenous  process  to  several  generations  of  cells,  all  of  which 
develop  into  infusoriform  embryos. 


FIG.  62.     INFUSORIFORM  EMBRYO  OF  DICYEMA. 

A.  B.  C.     Three  of  the  later  stages  in  the  development. 

D.  E.  F.  Three  different  views  of  the  full-grown  larva.  D.  from  the  front,  E. 
from  the  side,  and  F.  from  above. 

G.  side  view  of  urn. 

u.  wall  of  urn  ;  /.  lid  of  urn  ;  r.  refractive  bodies ;  gr.  granular  bodies  filling  the 
interior  of  the  urn. 

The  primitive  cell  is  called  by  Van  Beneden  a  Germogen. 
In  its  protoplasm  a  number  of  germs  first  appear  endogenously, 
but  the  nucleus  of  the  germogen  does  not  assist  in  their  forma- 
tion. They  eventually  become  detached  from  the  parent  cell, 
around  which  they  are  concentrically  arranged.  A  second  and 
then  a  third  generation  of  germs  are  formed  in  the  same  way,  till 
the  whole  of  the  protoplasm  of  the  primitive  cell  is  absorbed  in 
the  formation  of  these  germs,  and  nothing  of  it  remains  but  the 
nucleus.  The  germs  so  formed  are  arranged  in  about  three  con- 
centric layers,  of  which  the  innermost  is  the  youngest.  One  to 
five  masses  of  germs  may  be  present  in  a  single  Rhombogen. 
The  germs  undergo  a  division,  in  the  course  of  which  their  nuclei 
exhibit  very  beautifully  a  spindle  modification.  In  the  course  of 
the  segmentation  the  embryo  gradually  assumes  its  permanent 
form,  and  four  of  the  cells  composing  it  can  be  distinguished 
from  the  remainder  by  their  greater  size  (fig.  62  A,  ;/).  The  two 
largest  of  these  give  rise  to  the  wall  of  the  urn,  and  also  give 
origin  to  four  smaller  cells  (fig.  62  B,  gr)  which  eventually  be- 
come polynuclear  and  constitute  the  four  granular  cells  in  the 
urn.  The  two  other  cells  become  the  lid  of  the  urn.  The  parts 


INFUSORIFORM   EMBRYOS.  135 

of  the  urn  lie  at  first  side  by  side,  but  in  the  course  of  develop- 
ment the  cells  which  form  the  wall  of  the  urn  travel  inwards,  and 
the  four  granular  cells  are  carried  into  their  concavity.  At  the 
same  time  the  cells  which  form  the  lid  of  the  urn  alter  their 
position  so  as  to  overlie  the  wall  of  the  urn.  The  two  cells 
immediately  above  the  urn  give  rise  to  the  refractive  bodies 
(fig.  62  A,  B,  C,  r)  and  the  remainder  of  the  cells  of  the  embryo 
become  the  tail  (fig.  62  C).  The  embryo  becomes  ciliated,  and 
attains  its  nearly  full  development  before  leaving  the  parental 
tissues.  It  usually  passes  out  at  the  cephalic  extremity. 

As  has  already  been  stated,  it  is  probable  that  the  infusori- 
form  embryos  leave  the  renal  organs  of  their  host  and  lead  a  free 
existence.  What  becomes  of  them  afterwards  is  not  however 
known,  though  there  can  be  little  doubt  that  they  serve  to  carry 
the  species  to  new  hosts. 

Till  the  further  development  of  the  infusoriform  embryo  is 
known  it  is  not  possible  to  arrive  at  a  definite  conclusion  as  to 
the  affinities  of  this  strange  parasite.  Van  Beneden  is  anxious 
to  form  it,  on  account  of  its  simple  organization,  into  a  group 
between  the  Protozoa  and  the  Metazoa.  It  appears  however 
very  possible  that  the  simplicity  of  its  organization  is  the  result 
of  a  parasitic  existence  ;  a  view  which  receives  confirmation  from 
the  common  occurrence  of  the  process  of  endogenous  cell-forma- 
tion in  the  axial  hypoblast  cell.  It  has  been  clearly  shewn  by 
Strasburger  that  endogenous  cell-formation  is  secondarily 
derived  from  cell-division  ;  so  that  the  occurrence  of  this  pro- 
cess in  Dicyema  probably  indicates  that  the  hypoblast  was  primi- 
tively multicellular.  It  is  not  improbable  that  the  enigmatical 
infusoriform  embryo  may  develop  into  a  sexual  form,  the  pro- 
geny of  which  are  destined  to  complete  the  cycle  of  develop- 
ment by  becoming  again  parasitic  in  the  renal  organ  of  a 
Cephalopod. 

BIBLIOGRAPHY. 

(117)  E.  van  Beneden.     "  Recherches  sur  les  Dicyemides."     Bull.  d.  FAca- 
dtmie  roy.  de  Belgique,  i"  ser.  T.  XLI.  No.  6  and  T.  XLII.  No.  7,  1876.      Vide  this 
paper  for  a  full  account  of  the  literature. 

(118)  A.  Kolliker.     Ueber  Dicyema paradoxum  den  Schmarotzer  der  Venenan- 
hdnge  der  Cephalopoden. 

(119)  Aug.    Krohn.     "Ueb.    d.    Vorkommen  von   Entozoen,    etc."     Froriep 
Notizen,  vii.  1839. 


136  ORTHONECTIM:. 


ORTHONECTIM;. 

A  number  of  minute  parasites  infesting  various  Nemertines,  Turbella- 
rians,  and  Ophiuroids  have  recently  been  studied  by  Giard  and  Metschnikoff, 
the  former  of  whom  has  placed  them  in  a  special  group  which  he  calls  the 
Orthonectidae.  They  were  first  discovered  by  W.  C.  Mclntosh. 

In  the  adult  state  they  are1  (Metschnikoff)  somewhat  pear-shaped  bodies 
formed  of  a  kind  of  plasmodium  of  cells  with  irregular  lobate  processes. 
In  the  interior  of  this  body  are  eggs  in  all  stages  of  development. 
In  the  type  observed  by  Metschnikoff  (Intoshia  gigas)  the  ova  undergo 
a  regular  segmentation,  resulting  in  the  formation  of  a  blastosphere 
in  which  an  inner  layer  is  subsequently  formed  by  delamination.  A 
smaller  and  a  larger  kind  of  embryo  are  formed  ;  but  all  the  embryos 
in  each  female  belong  to  one  type.  The  larger  become  females  and  the 
smaller  males. 

The  female  embryos  are  ovoid.  The  outer  layer  of  cells  or  epiblast 
becomes  ciliated,  and  divided  into  nine  segments,  of  which  the  second 
is  marked  off  from  the  remainder  by  the  absence  of  cilia,  and  by  being 
provided  with  refractive  corpuscles.  The  inner  layer  which  surrounds 
a  central  cavity,  and  might  be  supposed  to  be  the  hypoblast,  becomes 
according  to  Metschnikoff  converted  into  ova. 

The  male  embryos  are  more  elongated  than  the  female,  from  which  they 
further  differ  in  only  having  six  segments.  The  cells  of  the  inner  layer 
eventually  divide  up  into  spermatozoa. 

The  larva}  probably  become  free,  and  while  in  the  free  state  impregna- 
tion would  appear  to  be  effected.  When  the  female  larvae  become  parasitic 
they  undergo  a  metamorphosis,  the  stages  of  which  have  not  been  observed ; 
but  in  the  course  of  which  the  epiblast  cells  probably  unite  into  a  plasmo- 
dium. 

The  observations  of  Giard  are  in  several  points  irreconcilable  with  those 
of  Metschnikoff,  but  from  the  statements  of  the  latter  it  appears  possible 
that  Giard  has  made  two  genera  from  the  males  and  females  of  one  species  ; 
and  that  Giard's  account  of  an  unequal  segmentation  followed  by  an 
epibolic  gastrula,  in  one  of  his  species,  has  arisen  from  two  segmenting  ova 
temporarily  fusing  together.  Giard  has  given  a  description  of  internal 
gemmiparous  reproduction,  upon  the  accuracy  of  which  doubts  have  been 
thrown  by  Metschnikoff.  The  affinities  of  the  Orthonectida:  are  as  obscure 
as  those  of  the  Dicyemida?  ;  though  there  can  be  but  little  doubt  that 
their  organization  has  been  much  simplified  in  correlation  with  their 
parasitic  habits.  The  origin  of  the  genital  products  in  the  axial  tissue  is 
a  feature  they  have  in  common  with  the  Dicyemidae. 

1  This  at  any  rate  holds  true  for  the  type  investigated  by  Metschnikoff.  The 
full  history  of  other  forms  is  not  yet  known. 


ORTHONECTID^E.  137 


BIBLIOGRAPHY. 

(120)  Alf.   Giard.     "  Les   Orthonectida  classe    nouv.    d.    Phylum   des  Vers." 
Journal  de  PAnat.  et  de  la  Physiol.,  Vol.  XV.  1879. 

(121)  El.    Metschnikoff.     "  Zur  Naturgeschichte  d.    Orthonectidae."     Zoolo- 
gischer  Anzeiger,  No.  40 — 43,  1879. 

[Ch.  Julin.  "  Rech.  sur  1'organization  et  le  devel.  d'Orthonectides."  Arch. 
BioL  Vol.  in.  1882. 

E.  Metschnikoff.  "  Untersuchungen  lib.  Orthonectidae."  Zeit.  f.  Wiss.  Zoo- 
logic,  Vol.  xxxv.  1881. 

For  general  account  of  Orthonectidse,  vide  Spengel.    Biolog.  Centralblatt,  No.  6.] 


CHAPTER  V. 


PORIFERA. 

ALTHOUGH  within  the  last  few  years  greater  advances  have 
probably  been  made  in  our  knowledge  of  the  development  of  the 
Porifera  than  of  any  other  group,  yet  there  is  much  that  is  still 
very  obscure,  and  it  is  not  possible  to  make  general  statements 
applying  to  the  whole  group. 

Calcispongiae.  The  form  which  has  so  far  been  most  com- 
pletely worked  out  is  Sycandra  raphanus,  one  of  the  Calcispon- 
giae  (Metschnikoff,  Nos.  132  and  134,  F.  E.  Schulze,  Nos.  139 
and  142),  and  I  shall  commence  my  account  with  the  life-history 
of  this  species. 

The  ovum  in  Sycandra  as  in  other  Spongida  has  the  form  of 
a  naked  amceboid  nucleated  mass  of  protoplasm.  From  the 
analogy  of  the  other  members  of  the  group,  there  is  no  doubt 
that  it  is  fertilized  by  a  male  spermatic  element,  though  this  has 
not  as  yet  been  shewn  to  be  the  case — and  the  changes  which 
accompany  fertilization  are  quite  unknown. 

The  segmentation  and  early  stages  of  development  take 
place  in  the  tissues  of  the  parent.  The  segmentation  is  some- 
what peculiar,  though  a  modification  of  a  regular  segmentation. 
The  ovum  divides  along  a  vertical  plane,  first  into  two,  and  then 
into  four  equal  segments.  But  even  when  two  segments  are 
formed,  each  of  them  has  one  end  pointed  and  the  other  broader. 
The  pointed  ends  give  rise  to  the  ciliated  cells  of  the  future 
larva,  and  the  broad  ends  to  the  granular  cells.  Instead  of  the 
next  division  taking  place,  as  is  usually  the  case,  in  a  horizontal 
(equatorial)  plane,  it  is  actually  effected  along  two  vertical  planes 


PORIFERA. 


'39 


intermediate  in  position  between  the  two  first  planes  of  segmen- 
tation. Eight  equal  segments  are  thus  formed,  each  of  which 
has  the  form  of  a  pyramid.  All  the  segments  are  situated  in  a 
single  tier,  and  are  so  arranged  as  to  give  to  the  whole  ovum  the 
form  of  a  flat  cone,  the  apex  of  which  is  formed  by  the  pointed 
extremities  of  the  constituent  segments  (fig.  63  B).  The  apices 
of  the  segments  do  not  however  quite  meet,  but  they  leave  a 
central  space,  which  is  an  actual  perforation  (fig.  63  A)  through 
the  axis  of  the  ovum,  open  at  both  ends.  The  first  indications 
of  this  perforation  appear  when  only  four  segments  are  present, 

A 

C 


FIG.  63.    SUCCESSIVE  STAGES  IN  THE  SEGMENTATION  OF  SYCANDRA  RAPHANUS. 
(Copied  from  F.  E.  Schulze.) 

A.  stage  with  eight  segments  still  arranged  in  pairs,  from  above. 

B.  side  view  of  stage  with  eight  segments. 

C.  side  view  of  stage  with  sixteen  segments. 

D.  side  view  of  stage  with  forty-eight  segments. 

E.  view  from  above  of  stage  with  forty-eight  segments. 

F.  side  view  of  embryo  in  the  blast osphere  stage,  eight  of  the  granular  cells  which 
give  rise  to  the  epiblast  of  the  adult  are  present  at  the  lower  pole. 

cs.  segmentation  cavity  ;  ec.  granular  cells  which  form  the  epiblast ;  en.  clear  cells 
which  form  the  hypoblast. 

and  it  is  to  be  regarded  as  the  homologue  of  the  segmentation 
cavity  of  other  ova.  The  next  plane  of  division  is  horizontal 
(equatorial),  and  the  apices  of  the  eight  cells  are  segmented  off 
as  a  tier  of  small  cells.  At  the  completion  of  this  division  (fig.  63 
C),  the  ovum  is  formed  of  sixteen  cells  arranged  in  two  superim- 
posed tiers.  The  ovum  now  assumes  somewhat  the  form  of  a 
biconvex  lens,  in  the  axis  of  which  the  central  perforation  is  still 


140  SYCANDRA. 


present.  At  the  close  of  the  next  stage,  forty-eight  cells  are  pre- 
sent arranged  in  four  tiers  (fig.  63  D  and  E),  the  two  outer  tiers 
containing  eight  cells  each,  and  the  two  inner  sixteen.  The  two 
inner  tiers  probably  arise  by  the  simultaneous  appearance  of  two 
equatorial  furrows  dividing  the  original  tiers  into  two,  and  by  the 
subsequent  simple  division  of  the  cells  of  the  two  inner  of  the 
tiers  so  formed.  At  the -close  of  the  stage  the  eight  basal  cells 
become  granular  (fig.  63  F).  At  the  same  time  the  central  part 
of  the  segmentation  cavity  becomes  enlarged,  while  its  terminal 
apertures  become  narrowed  and  finally,  shortly  after  the  end  of 
this  stage,  closed.  The  axial  perforation  thus  acquires  the 


KK;.  64.     LARVA  OF  SYCANDRA  RAPHANUS  AT  PSEUDCMJASTRUI.A  STACK,  IN  MM 

IN  THE  MATERNAL  TISSUES.     (Copied  from  F.  E.  Schulze.) 

me.  mesoblast  of  adult  ;  hy.  collared  cells  forming  hypoblast  of  the  adult  ;  en. 
clear  cells  of  larva  which  eventually  become  involuted  to  form  the  hypoblast  ;  >; . 
granular  cells  of  larva  which  give  rise  to  the  epiblast,  which  at  this  stage  are  partially 
involuted. 

character   of  a   closed  segmentation   cavity.     While  the  ovum 
itself  becomes  at  the  same  time  a  blastosphere. 

This  stage  nearly  completes  the  segmentation  :  in  the  next 
one,  the  cells  of  the  poles  of  the  blastosphere  increase  in  number, 


PORIFERA. 


and  the  cells  of  the  greater  part  of  the  blastosphere  become 
columnar  and  ciliated,  (fig.  64  en.)  while  the  granular  cells  (ec.} 
increase  to  about  thirty-two  in  number  and  appear  to  be  (parti- 
ally at  least)  involuted  into  the  segmentation  cavity,  reducing  this 
latter  to  a  mere  slit.  This  stage  forms  the  last  passed  by  the 
embryo  in  the  tissues  of  the  parent.  The  general  position  of  the 
embryo  while  still  in  this  situation  may  be  gathered  from  fig.  64, 
representing  the  embryo  in  situ.  The  embryo  is  always  placed 
close  to  one  of  the  radial  canals.  From  this  situation  it  makes 
its  way  through  the  lining  cells  into  a  canal  and  is  thence  trans- 
ported to  the  surrounding  water.  By  the  time  the  larva  has 
become  free,  the  semi-invaginated  granular  cells  have  increased 
in  bulk  and  become  everted  so  as  to  project  very  much  more 
prominently  than  in  the  encapsuled  state.  To  the  gastrula  stage, 
if  it  deserves  the  name,  passed  through  by  the  embryo  in  the 
tissues  of  the  parent,  no  importance  can  be  attached. 

The  larva,  after  it  has  left  the  parental  tissues,  has  an  oval 
form  and  is  transversely  divided  into  two  areas  (fig.  65  A).     One 


enl 


c .§. 


FlG.  65.   TWO  FREE  STAGES  IN  THE  DEVELOPMENT  OF  SYCANDRA  RAPHANUS. 

(Copied  from  Schulze.) 

A.  Amphiblastula  stage. 

B.  A  later  stage  after  the  ciliated  cells  have  commenced  to  become  invaginated. 
cs.  segmentation  cavity ;   ec.  granular  cells   which  will   form   the   epiblast ;    en. 

ciliated  cells  which  become  invaginated  to  form  the  hypoblast. 

of  these  areas  is  formed  of  the  elongated,  clear,  ciliated  cells, 
with  a  small  amount  of  pigment  near  their  inner  ends  (en.),  and 


1 42 


SYCANDRA. 


the  other  and  larger  area  of  the  thirty-two  granular  cells  already 
mentioned  (ec.).  Fifteen  or  sixteen  of  these  are  arranged  as  a 
special  ring  on  the  border  of  the  clear  cells.  In  the  centre  of  the 
embryo  is  a  segmentation  cavity  (c.s.)  which  lies  between  the 
granular  and  the  clear  cells,  but  is  mainly  bounded  by  the 
vaulted  inner  surface  of  the  latter.  This  stage  is  known  as  the 
amphiblastula  stage.  During  the  later  periods  of  the  amphi- 
blastula  stage  a  cavity  appears  in  the  granular  cells  dividing  them 
into  two  layers.  After  the  larva  has  for  some  time  enjoyed  a 
free  existence,  a  remarkable  series  of  changes  take  place,  which 
result  in  the  invagination  of  the  half  of  it  formed  of  the  clear 
cells,  and  form  a  prelude  to  the  permanent  attachment  of  the 
larva.  The  entire  process  of  invagination  is  completed  in  about 
half  an  hour.  The  whole  embryo  first  becomes  flattened,  but 
especially  the  ciliated  half,  which  gradually  becomes  less  promi- 
nent (fig.  65  B);  and  still  later  the  cells  composing  it  undergo  a 
true  process  of  invagination.  As  a  result  of  this  invagination 
the  segmentation  cavity  is  obliterated,  and  the  larva  assumes  a 
compressed  plano- 
convex form,  with 
a  central  gastrula 
cavity,  and  a  blasto- 
pore  in  the  middle 
of  the  flattened  sur- 
face. The  two  layers 
of  the  gastrula  may 
now  be  spoken  of  as 
epiblast  and  hypo- 
blast.  The  blasto- 
pore  becomes  gradu- 
ally narrowed  by  the 
growth  over  it  of  the 
outer  row  of  granu- 
lar cells.  When  it  has 
become  very  small 
the  attachment  of 


re 


FIG.  66.  FIXED  GASTRULA  STAGE  OF  SYCANDRA 
RAPHANUS.  (Copied  from  Schulze.) 

The  figure  shews  the  amoeboid  epiblast  cells  (ec.) 
derived  from  the  granular  cells  of  the  earlier  stage,  and 
the  columnar  hypoblast  cells,  lining  the  gastrula  cavity, 
derived  from  the  ciliated  cells  of  the  earlier  stage.  The 
larva  is  fixed  by  the  amoeboid  cells  on  the  side  on 
which  the  blastopore  is  situated. 


the  larva  takes  place  by  the  flat  surface  where  the  blastopore 
is  situated.  It  is  effected  by  protoplasmic  processes  of  the 
outer  ring  of  epiblast  cells,  which,  together  with  the  other 


PORIFERA.  143 


epiblast  cells,  now  become  amoeboid.  They  become  at  the 
same  time  clearer  and  permit  a  view  of  the  interior  of  the 
gastrula.  Between  the  epiblast  cells  and  the  hypoblast  cells  which 
line  the  gastrula  cavity  there  arises  a  hyaline  structureless  layer, 
which  is  more  closely  attached  to  the  epiblast  than  to  the  hypo- 
blast,  and  is  probably  derived  from  the  former.  A  view  of  the 
gastrula  stage  after  the  larva  has  become  fixed  is  given  in  fig.  66. 

There  would  seem  according  to  MetschnikofT's  observations 
(No.  134)  to  be  a  number  of  mesoblast  cells  interposed  between 
the  two  primary  layers,  which  he  derives  from  the  inner  part  of 
the  mass  of  granular  cells. 

After  invagination  the  cilia  of  the  hypoblast  cells  can  no 
longer  be  seen,  and  are  probably  absorbed  ;  and  their  disappear- 
ance is  nearly  coincident  with  the  complete  obliteration  of  the 
blastopore,  an  event  which  takes  place  shortly  after  the  attach- 
ment of  the  larva. 

Not  long  after  the  closure  of  the  blastopore,  calcareous 
spicules  make  their  appearance  in  the  larva  as  delicate  un- 
branched  rods  pointed  at  both  extremities.  They  appear  to  be 
formed  on  the  mesoblast  cells  situated  between  the  epiblast  and 
hypoblast1.  The  larva  when  once  fixed  rapidly  grows  in  length 
and  assumes  a  cylindrical  form  (fig.  67  A).  The  sides  of  the 
cylinder  are  beset  with  calcareous  spicules  which  project  beyond 
the  surface,  and,  in  addition  to  the  unbranched  forms,  spicules 
are  developed  with  three  and  four  rays  as  well  as  some  with  a 
blunt  extremity  and  serrated  edge.  The  extremity  of  the 
cylinder  opposite  the  attached  surface  is  flattened,  and,  though 
surrounded  by  a  ring  of  four-rayed  spicules,  is  itself  free  from 
them.  At  this  extremity  a  small  perforation  is  formed  leading 
into  the  gastric  cavity,  which  rapidly  increases  in  size  and  forms 
an  exhalent  osculum  (vs.).  A  series  of  inhalent  apertures  is  also 
formed  at  the  sides  of  the  cylinder.  The  relative  times  of 
appearance  of  the  single  osculum  and  the  smaller  apertures  are 
not  constant  for  the  different  larvae.  On  the  central  gastrula 
cavity  of  the  sponge  becoming  placed  in  communication  with 
the  external  water,  the  hypoblast  cells  lining  it  become  ciliated 

1  Metschnikoff  was  the  first  to  give  this  account  of  the  development  of  the  spicules 
in  Sycandra,  but  Prof.  Schulze  has  informed  me  by  letter  that  he  has  arrived  at  the 
same  result. 


144  SYCANDRA. 


afresh  (fig.  67  B,  rn.)  and  develop  the  peculiar  collar  characteris- 
tic of  the  hypoblast  cells  of  the  Spongida  (vide  fig.  64,  hy.). 
When  this  stage  of  development  is  reached  we  have  a  fully- 
formed  sponge  of  the  type  made  known  by  Haeckel  as 
Olynthus. 


FIG.  67.    THE  YOUNG  OF  SYCANDRA  RAPHANUS  SHORTLY  AFTER  THE  DEVELOP- 
MENT OF  THE  SPICULA.     (Copied  from  Schulze.) 

A.  View  from  the  side. 

B.  View  from  the  free  extremity. 

os.  osculum ;  cc.  epiblast ;  en.  hypoblast  composed  of  ciliated  cells.  The  terminal 
osculum  and  lateral  pores  are  represented  as  oval  white  spaces. 

When  young  examples  of  Sycandra  come  in  contact  shortly 
after  their  attachment  they  appear  to  fuse  together  temporarily 
or  else  permanently.  In  the  latter  case  colonies  are  produced 
by  their  fusion. 

Amongst  other  calcareous  sponges  the  larva  of  Ascandra  contorta 
(Haeckel  No.  126,  Barrois  No.  122)  presents  the  typical  amphiblastula  stage, 
and  so  probably  does  that  of  Ascandra  Lieberkiihnii  (Keller  No.  128).  In 
Leucandra  aspera  (Keller  No.  128,  Metschnikoflf  No.  134)  the  larva  passes 
through  an  amphiblastula  stage,  but  the  characters  of  the  cells  of  the  two 
halves  of  the  larva  do  not  differ  to  nearly  the  same  extent  as  in  Sycandra. 

Although  the  majority  of  calcareous  sponges  appear   to  agree  in  their 


PORIFERA. 


145 


mode  of  development  with  Sycandra,  nevertheless  the  concordant  researches 
of  O.  Schmidt  (No.  138)  and  Metschnikoff  (No.  134)  have  shewn  that  this 
is  not  true  for  the  genus  Ascetta  (As.  primordialis,  dathrus  and  bianco). 

The  larvae  of  these  forms  are  very  differently  constituted  to  those 
of  Sycandra.  They  have  an  oval  form  and  are  composed  of  a  single 
row  of  ciliated  columnar  cells :  their  two  extremities  only  differ  in  the  cells 
at  one  extremity  being  longer  than  those  at  the  other.  Especially  at  the 
pole  where  the  shorter  cells  are  situated  (Schmidt)  a  metamorphosis  of  the 
cells  takes  place.  One  after  the  other  they  lose  their  cilia,  become  granular, 
and  pass  into  the  interior  of  the  vesicle.  Here  they  become  differentiated 
into  two  classes  (Metschnikoff) ;  one  of  larger  and  more  granular  cells, 
and  the  other  of  smaller  cells  with  clearer  protoplasm.  Cells  of  the  former 
class  are  mainly  found  at  one  of  the  poles.  When  the  larva  becomes 
free  the  cells  in  the  interior  of  the  vesicle  increase  in  number  and  nearly 
fill  up  its  central  cavity.  After  a  short  free  existence  the  larva  becomes 
fixed,  and  the  epiblast  cells  lose  their  cilia  and  become  flattened.  At  a  later 
period  the  large  granular  cells  assume  a  radiate  arrangement  round  a  central 
cavity  and  become  clearly  marked  out  as  the  hypoblast  cells.  The  smaller 
cells  become  placed  between  the  epiblast  and  hypoblast  and  constitute  the 
mesoblast. 

Myxospongiae.  In  this  group  Halisarca  has  been  investi- 
gated by  Carter  (No.  123),  Barrois  (No.  122),  Schulze  (No.  141) 
and  Metschnikoff  (No.  134).  The  ova  develop  in  the  mesoblast, 
and  when  ripe  occupy  special  chambers  lined  by  a  layer  of 
epithelial  cells.  Schulze  has  found  the  spermatozoa  of  this 
genus  of  sponge  and  has  been  able  to  shew  that  the  sexes  may 
be  distinct,  though  many  species  of  Halisarca  are  hermaphrodite. 

The  segmentation  is,  roughly  speaking,  regular,  and  a  seg- 
mentation cavity  is  early  formed,  which  is  never,  as  in  Calci- 
spongise,  open  at  the  poles.  When  the  larva  leaves  the  parent  it 
is  an  oval  vesicle  formed  of  a  single  layer  of  columnar  ciliated 
cells.  Slight  differences  may  be  observed  between  the  two 
extremities  of  the  larvae  of  most  species.  One  of  these — the 
hinder  extremity — is  directed  backwards  in  swimming. 

The  further  history  of  the  larva  has  been  investigated  by 
Metschnikoff.  He  has  found  that  the  interior  of  the  vesicle 
becomes  gradually  filled  with  mesoblast  cells  of  a  peculiar  type, 
called  by  him  rosette-cells,  which  are  probably  derived  from  the 
walls  of  the  vesicle. 

When  the  metamorphosis  commences,  the  larva  assumes  a 
flattened  form,  and  cells  of  a  new  type,  viz.  normal  amoeboid 
B.  II.  10 


146  CERATOSPONGI^:. 


cells,  grow  in  amongst  the  rosette  cells.  The  new  cells  are  also 
derived  from  the  epiblast.  The  larvae  appear  to  fix  themselves 
by  the  hinder  extremity.  The  cilia  gradually  disappear,  and 
the  epiblast  cells  flatten  out  and  form  a  kind  of  cuticle.  For 
some  time  the  larva  remains  in  the  two-layered  condition,  but 
gradually  canals  (?  ciliated  chambers)  lined  by  hypoblast  cells 
become  formed.  They  appear  as  closed  spaces  with  walls 
of  ciliated  cells  derived  from  the  amoeboid  cells,  and  the 
different  parts  of  the  system  of  chambers  are  established  inde- 
pendently. In  H.  pontica  the  ciliated  chambers  are  formed 
before  the  attachment  of  the  larva.  The  development  was  not 
followed  up  to  the  formation  of  the  pores  placing  the  canal 
system  in  communication  with  the  exterior. 

The  young  sponges  at  a  somewhat  later  stage  have  been 
studied  by  Schulze  and  Barrois.  They  are  formed  of  an 
external  layer  of  flattened  cells,  not  clearly  ciliated  as  in  the 
adult,  within  which  are  a  normal  mesoblastic  tissue,  and  several 
spherical  chambers  lined  by  ciliated  cells  exactly  like  the  ciliated 
chambers  of  the  full-grown  sponge.  Irregular  invaginations  of 
the  epiblast  give  to  the  young  sponge  a  honeycombed  structure. 
The  ciliated  chambers  in  the  youngest  condition  of  the  sponge 
are  closed  ;  but  in  slightly  older  examples  they  come  into  com- 
munication with  the  passages  lined  by  hypoblast,  and  so  indirectly 
with  the  external  medium. 

CeratospongiSB.  Amongst  the  true  Ceratospongias  the  embryos  of 
two  of  the  Aplysinidae,  and  of  Spongelia  and  Euspongia  have  been  to  some 
extent  worked  out  by  Barrois  and  Schulze.  The  form  worked  out  by  Barrois 
is  called  by  him  Verongia  rosea.  The  segmentation  is  nearly  regular,  but 
from  the  first  the  segments  may  be  divided  according  to  their  constitution 
into  two  categories.  At  the  close  of  segmentation  the  embryo  is  oval  and 
covered  by  a  single  layer  of  columnar  ciliated  cells ;  these  cells  may  however 
be  divided  into  two  categories,  corresponding  with  those  observable  during 
the  segmentation.  A  certain  number  are  coloured  red  and  form  a  definite 
circular  mass  at  one  pole,  while  the  remainder,  which  constitute  the  major 
part  of  the  embryo,  have  a  pale  yellowish  colour.  Those  at  the  red  pole 
lose  their  cilia  in  the  free  larva,  but  around  the  area  formed  by  them  is  a 
special  ring  of  long  cilia.  The  chief  peculiarity  of  the  embryo  (made  known 
by  Schulze)  consists  in  the  fact  that  the  layer  of  cells  which  covers  the 
embryo  does  not,  as  in  other  sponge  embryos,  simply  enclose  a  space, 
but  the  interior  of  the  embryo  is  formed  of  a  mass  of  stellate  cells  like  the 
normal  mesoblast  of  full-grown  sponges. 


PORIFERA.  147 


This  feature  is  also  characteristic  of  the  embryos  of  Spongelia  and 
Euspongia. 

The  embryo  of  the  Gummineae  (Gummina  mimosa}  has  been  in- 
vestigated by  Barrois  (No.  122),  and  has  been  shewn  closely  to  resemble  the 
typical  larvae  of  calcareous  sponges  ;  one-half  being  formed  of  elongated 
ciliated  cells  and  the  other  of  rounded  granular  ones. 

Silicispongiae.  The  development  of  marine  silicious  sponges  is  but 
very  imperfectly  understood.  The  larvae  of  various  forms — Reniera  (Iso- 
dyctia),  Esperia  (Desmacidon),  Raspailia,  Halichondria,  Tethya — have  been 
described.  Barrois  has  shewn  that  the  egg  segments  regularly  and  that  in 
the  earlier  stages  a  segmentation  cavity  is  present.  In  the  later  stages  the 
embryo  appears  to  become  solid.  Externally  there  is  a  layer  of  ciliated  cells, 
and  within  a  mass  of  granular  matter  in  which  the  separate  cells  cannot  be 
made  out.  The  granular  matter  projects  at  one  pole,  and  forms  a  prominence 
possibly  equivalent  to  the  granular  cells  of  Sycandra.  In  some  forms,  e.g. 
Reniera,  the  edge  of  the  unciliated  granular  prominence  may  be  surrounded 
by  a  row  of  long  cilia.  In  later  stages  the  granular  material  may  project  at 
both  poles  or  even  at  other  points.  One  remarkable  feature  in  the 
development  of  the  Silicispongiae  is  the  appearance  of  spicula  between  the 
ciliated  cells  and  the  central  mass,  while  the  larva  is  still  free. 

Professor  Schulze  has  informed  me  that  these  spicula  are  developed 
in  mesoblast  cells  ;  while  the  horny  fibres  of  the  sponge  are  developed 
as  cuticular  products  of  special  mesoblast  cells  (spongioblasts). 

The  attachment  and  accompanying  metamorphosis  are  so  diversely 
described  that  no  satisfactory  account  can  be  given  of  them.  The  general 
statements  are  in  favour  of  the  attachment  taking  place  by  the  posterior 
extremity  where  the  granular  matter  projects. 

Carter  especially  gives  a  very  precise  account,  with  figures,  of  the 
attachment  of  the  larva  in  this  way.  He  also  figures  the  appearance  of  an 
osculum  at  the  opposite  pole1. 

A  very  elaborate  account  of  the  development  of  Spongilla  has  been 
published  in  Russian  by  Ganin,  of  which  a  German  abstract  has  also 
appeared  (No.  124). 

The  ovum  undergoes  a  regular  segmentation  and  becomes  a  solid  ova 
morula.  An  epiblast  of  smaller  cells  is  early  differentiated,  and  in  the 
interior  of  the  inner  cells  an  archenteron  becomes  subsequently  formed. 
The  inner  cells  next  become  divided  into  an  hypoblastic  layer  lining  the 

1  Keller  (No.  129)  has  recently  given  an  account  of  the  development  of  Halichon- 
dria (Chalinula)  fertilis.  He  finds  that  there  is  an  irregular  segmentation,  followed  by 
a  partial  epibolic  invagination,  the  inner  mass  of  cells  remaining  exposed  at  one  pole 
and  forming  there  a  prominence,  equivalent  to  the  granular  prominence  in  the  larvae 
of  other  Silicispongiae.  The  free  swimming  larva  resembles  the  larva  of  other  Sili- 
cispongiae in  the  possession  of  spicula,  etc.,  and  after  becoming  laterally  compressed 
attaches  itself  by  one  of  the  flattened  sides.  A  central  cavity  is  formed  in  the  interior 
with  ciliated  chambers  opening  into  it,  and  is  subsequently  placed  in  communication 
with  the  exterior  by  the  formation  of  an  aperture  which  constitutes  the  osculum. 

IO — 2 


148  SILICISPONGLE. 


archenteron,  and  a  mesoblastic  layer  between  this  and  the  now  ciliated 
epiblast.  At  the  narrow  hinder  end  of  the  embryo  the  mesoblast  becomes 
thickened,  and  largely  obliterates  the  archenteron.  In  this  part  of  the 
mesoblast  silicious  spicula  are  formed.  The  larva  becomes  attached  by 
its  hinder  extremity,  and  in  the  course  of  this  process  flattens  itself  out 
to  a  disc-like  form.  From  the  nearly  obliterated  archenteric  cavity  out- 
growths take  place  which  give  rise  to  the  ciliated  chambers.  These 
are  not  placed  directly  in  communication  with  the  exterior,  but  open,  if  I 
understand  Ganin  rightly,  into  a  space  in  the  mesoblast,  which  subsequently 
acquires  an  exterior  communication — the  primitive  osculum.  The  subse- 
quent pores  and  oscula  are  also  formed  as  openings  leading  into  the  meso- 
blastic cavity,  which  communicates  in  its  turn  with  the  ciliated  chambers. 

It  appears  that  in  the  present  unsatisfactory  state  of  our 
knowledge  the  larvae  of  the  Porifera  may  be  divided  into  two 
groups  :  viz.  (i)  those  which  have  the  form  of  a  blastosphere  or 
else  of  a  solid  morula ;  (?)  those  which  have  the  amphiblastula 
form. 

In  the  former  type  the  mesoblast  and  hypoblast  are  formed 
either  from  cells  budded  off  from  the  outer  cells  of  the  blasto- 
sphere or  from  the  solid  inner  mass  of  cells ;  while  the  outer 
ciliated  cells  become  the  epiblast.  This  type  of  larva,  which  is 
found  in  the  majority  of  sponges,  is  very  similar  in  its  general 
characters  and  development  to  many  Ccelenterate  planulae. 

The  second  type  of  larva  is  very  peculiar,  and  though  in  its 
fully  developed  form  it  is  confined  to  the  Calcispongiae,  where  it 
is  the  usual  form,  a  larval  type  with  the  same  characters  is 
perhaps  to  be  found  in  other  sponges,  e.g.  amongst  the  Gum- 
minese,  and  amongst  the  Silicispongiae  where  one-half  of  the 
embryo  is  without  cilia,  though  in  the  case  of  the  Silicispongise 
the  cells  of  the  ciliated  part  of  the  embryo  correspond  to  the 
granular  cells  of  the  larva  of  Sycandra. 

The  later  stages  in  the  development  of  the  larvae  of  the  Pori- 
fera are  not  similar  to  anything  we  know  of  in  other  groups. 

It  might  perhaps  be  possible  to  regard  sponges  as  degraded  descendants 
of  some  Actinozoon  type  such  as  Alcyonium,  with  branched  prolongations  of 
the  gastric  cavity,  but  there  does  not  appear  to  me  to  be  sufficient  evidence 
for  doing  so  at  present.  I  should  rather  prefer  to  regard  them  as  an 
independent  stock  of  the  Metazoa. 

In  this  connection  the  amphiblastula  larva  presents  some  points  of 
interest.  Does  this  larva  retain  the  characters  of  an  ancestral  type  of  the 
Spongida,  and  if  so,  what  does  its  form  mean  ?  It  is,  of  course,  possible  that 


PORIFERA.  149 


it  has  no  ancestral  meaning  but  has  been  secondarily  acquired  ;  but,  assuming 
that  this  is  not  the  case,  it  appears  to  me  that  the  characters  of  the  larva 
may  be  plausibly  explained  by  regarding  it  as  a  transitional  form  between 
the  Protozoa  and  Metazoa.  According  to  this  view  the  larva  is  to  be 
considered  as  a  colony  of  Protozoa,  one-half  of  the  individuals  of  which 
have  become  differentiated  into  nutritive  forms,  and  the  other  half  into 
locomotor  and  respiratory  forms.  The  granular  amoeboid  cells  represent 
the  nutritive  forms,  and  the  ciliated  cells  represent  the  locomotor  and 
respiratory  forms.  That  the  passage  from  the  Protozoa  to  the  Metazoa 
may  have  been  effected  by  such  a  differentiation  is  not  improbable  on 
a  priori  grounds. 

While  the  above  view  seems  fairly  satisfactory  for  the  free  swimming 
stage  of  the  larval  sponge,  there  arises  in  the  subsequent  development  a 
difficulty  which  appears  at  first  sight  fatal  to  it.  This  difficulty  is  the 
invagination  of  the  ciliated  cells  instead  of  the  granular  ones.  If  the 
granular  cells  represent  the  nutritive  individuals  of  the  colony,  they,  and 
not  the  ciliated  cells,  ought  most  certainly  to  give  rise  to  the  lining  of  the 
gastrula  cavity,  according  to  the  generally  accepted  views  of  the  morphology 
of  the  Spongida.  The  suggestion  which  I  would  venture  to  put  forward  in 
explanation  of  this  paradox  involves  a  completely  new  view  of  the  nature 
and  functions  of  the  germinal  layers  of  adult  Spongida. 

It  is  as  follows  : — When  the  free  swimming  ancestor  of  the  Spongida 
became  fixed,  the  ciliated  cells  by  which  its  movements  used  to  be  effected 
must  have  to  a  great  extent  become  functionless.  At  the  same  time  the 
amoeboid  nutritive  cells  would  need  to  expose  as  large  a  surface  as  possible. 
In  these  two  considerations  there  may,  perhaps,  be  found  a  sufficient  expla- 
nation of  the  invagination  of  the  ciliated  cells,  and  the  growth  of  the 
amoeboid  cells  over  them.  Though  respiration  was,  no  doubt,  mainly 
effected  by  the  ciliated  cells,  it  is  improbable  that  it  was  completely 
localized  in  them,  but  they  were  enabled  to  continue  performing  this 
function  through  the  formation  of  an  osculum  and  pores.  The  collared  cells 
which  line  the  ciliated  chambers,  or  in  some  cases  the  radial  tubes, 
are  undoubtedly  derived  from  the  invaginated  cells,  and,  if  there  is  any 
truth  in  the  above  suggestion,  the  collared  cells  in  the  adult  sponge  must 
be  mainly  respiratory  and  not  digestive  in  function,  while  the  epiblastic 
cells,  which  in  most  cases  line  the  inhalent  passages  through  its  substance1, 
ought  to  be  employed  to  absorb  nutriment.  The  recent  researches  of 
Metschnikoff  (No.  134)  on  this  head  shew  that  the  nutriment  is  largely 
carried  into  the  mesoblast  cells,  which  in  Sycandra  appear  to  be  derived 

1  That  the  greater  part  of  the  flat  cells  which  line  the  passages  of  most  Sponges 
are  really  derived  from  epiblastic  invaginations  appears  to  me  to  be  proved  by  Schulze's 
and  Barrois'  observations  on  the  young  fixed  stages  of  Halisarca.  Schulze's  (No.  140) 
observations  have  however  proved  that  the  flat  cells  lining  the  axial  gastric  chamber 
of  Sycandra  are  hypoblastic  in  origin,  and  the  observations  of  Keller  (No.  129)  and 
Ganin  (No.  124)  have  led  to  the  same  result  for  the  flat  epithelium  lining  part  of  the 
passages  of  the  Silicispongise. 


150  SUMMARY. 


from  the  granular  cells,  and  also  that  it  is  taken  up  by  the  cells  which  line 
the  passages,  though  not  by  the  superficial  epiblast  cells.  Whether  the 
collared  cells  generally  absorb  nutriment  is  not  clear  from  his  statements  : 
but  he  finds  that  they  do  not  do  so  in  Silicispongice. 

Professor  Schulze  has  informed  me  by  letter  that  he  finds  the  collared 
cells  to  be  respiratory  in  function,  while  the  cells  derived  from  the  granular 
cells  in  Sycandra  are  nutritive.  Carter1,  on  the  contrary,  from  his  obser- 
vations on  Spongilla,  has  fully  satisfied  himself  that  the  food  is  absorbed 
by  the  cells  lining  the  ciliated  chambers. 

If  it  is  eventually  proved  by  further  experiments  on  the  nutrition  of 
sponges,  that  digestion  is  mainly  carried  on  by  the  general  cells  lining  the 
passages  and  the  mesoblast  cells,  and  not  for  the  most  part  by  the  ciliated 
cells,  it  is  clear  that  the  epiblast,  mesoblast  and  hypoblast  of  sponges  will 
not  correspond  with  the  similarly  named  layers  in  the  Ccelenterata  and  other 
Metazoa.  The  invaginated  hypoblast  will  be  the  respiratory  layer  and 
the  epiblast  and  mesoblast  the  digestive  and  sensory  layers  ;  the  sensory 
function  being  probably  mainly  localized  in  the  epithelium  on  the  sur- 
face, and  the  digestive  one  in  the  epithelium  lining  the  passages  and  in 
the  mesoblast.  Such  a  fundamental  difference  in  the  primary  function  of 
the  germinal  layers  between  the  Spongida  and  the  other  Metazoa,  would 
necessarily  involve  the  creation  of  a  special  division  of  the  Metazoa  for  the 
reception  of  the  former  group. 

BIBLIOGRAPHY. 

(122)  C.  Barrois.     "Embryologie  de  quelques  eponges  de  la  Manche."     An- 
nales  des  Sc.  Nat.  ZooL,  vi.  ser.,  Vol.  ill.  1876. 

(123)  Carter.     "  Development  of  the  Marine  Sponges."     Annals  and  Mag.  of 
Nat.  Hist.,  4th  series,  Vol.  xiv.  1874. 

(124)  Can  in2.     "Zur  Entwicklung  d.  Spongilla  fluviatilis."     Zoologischer  An- 
zeiger.  Vol.  I.  No.  9,  1878. 

(125)  Robert  Grant.     "Observations  and  Experiments  on  the  Structure  and 
Functions  of  the  Sponge."     Edinburgh  Phil,  jf.,  Vol.  xm.  and  xiv.,  1825,  1826. 

(126)  E.  Haeckel.     Die  Kalkschwamme,  1872. 

(127)  E.  Haeckel.     Studien  zur  Gastraa-Theorie.    Jena,  1877. 

(128)  C.  Keller.     Untersuchungen  uber  Anatomic  und  EntwicklungsgeschichU 
einiger  Spongien.     Basel,  1876. 

(129)  C.    Keller.     "Studien   ub.   Organisation  u.    Entwick.    d.    Chalineen." 
Zeit.f.  wiss.  Zool.,  Bd.  xxvin.  1879. 

(130)  Lieberkiihn.     " Beitr.  z.  Entwick.  d.    Spongillen."     Mailer's  Archiv, 
1856. 

(131)  LieberkUhn.     "  Neue  Beitrage  zur  Anatomic  der  Spongien."     Miiller's 
Archiv,  1859. 

1  "On  the  Nutritive  and  Reproductive  Processes  of  Sponges."    Ann.  and  Mag. 
of  Nat.  Hist.,  Vol.  iv.  Ser.  v.  1879. 

2  There  is  a  Russian  paper  by  the  same  author,  containing  a  full  account,  with 
clear  illustrations,  of  his  observations. 


PORIFERA.  I  5  I 


(132)  El.  Metschnikoff.     " Zur  Entwicklungsgeschichte  der  Kalkschwamme." 
Zeit.f.  wiss.  Zool.,  Bd.  xxiv.  1874. 

(133)  El.  Metschnikoff.     "  Beitrage  zur  Morphologic  der  Spongien."     Zeit. 
f.  wiss.  Zool.,  Bd.  xxvu.  1876. 

(134)  El.  Metschnikoff.     *'  Spongeologische  Studien."     Zeit.  f.  wiss.  Zool., 
Bd.  xxxn.  1879. 

(135)  Miklucho  Maklay.    "  Beitrage  zur  Kenntniss  der  Spongien."    Jenaische 
Zeitschrift,  Bd.  iv.  1868. 

(136)  O.     Schmidt.     "  Zur  Orientirung  iiber  die  Entwicklung der  Schwamme." 
Zeit.f.  wiss.  Zool.,  Bd.  xxv.  1875. 

(137)  O.  Schmidt.     "  Nochmals  die  Gastrula  der   Kalkschwamme."     Archiv 
fur  mikrosk.  Anat.,  Bd.  xii.  1876. 

(138)  O.  Schmidt.     "Das  Larvenstadium  von  Ascetta  primordialis  und  Asc. 
clathrus."     Archiv  fur  mikrosk.  Anatomic,  Bd.  xiv.  1877. 

(139)  F.  E.  Schulze.     "  Ueber  den  Bau  und  die  Entwicklung  von  Sycandra 
raphanus."     Zeit.  f.  wiss.  Zool.,  Bd.  xxv.  1875. 

(140)  F.  E.  Schulze.     "  Zur  Entwicklungsgeschichte  von  Sycandra."     Zeit.f. 
wiss.  Zool.,  Bd.  xxvu.  1876. 

(141)  F.   E.  Schulze.     "  Untersuchung  lib.  d.  Bau,  etc.     Die  Gattung  Hali- 
sarca."     Zeit.f.  wiss.  Zool.,  Bd.  xxvni.  1877. 

(142)  F.  E.  Schulze.     "  Untersuchungen  iib.  d.  Bau,  etc.     Die  Metamorphose 
von  Sycandra  raphanus."     Zeit.f.  wiss.  Zool.,  Bd.  xxxi.  1878. 

(143)  F.  E.  Schulze.     "Untersuchungen  ii.  d.  Bau,  etc.     Die  Familie  Aply- 
sinidse."     Zeit.f.  wiss.  Zool.,  Bd.  xxx.  1878. 

(144)  F.  E.  Schulze.     "  Untersuchungen  ii.  d.  Bau,  etc.     Die  Gattung  Spon- 
gelia."     Zeit.f.  wiss.  Zool.,  Bd.  xxxn.  1878. 


CHAPTER   VI. 


C(ELENTERATA '. 

Hydroidea.  The  most  typical  mode  of  development  of  the 
Hydroidea  is  that  in  which  the  segmentation  leads  directly  to 
the  formation  of  a  free  ciliated  two-layered  larva,  known  since 
Dalyell's  observations  as  a  plan u la.  The  planula  is  characteris- 
tic of  almost  all  the  Hydromedusae  with  fixed  hydrosomes 
including  the  Hydrocoralla  (Stylasteridse  and  Millepora),  the 
most  important  exceptions  being  the  genus  Tubularia  and  one 
or  two  other  genera,  and  the  fresh-water  Hydra. 

In  a  typical  Sertularian  the  segmentation  is  approximately 
regular8  and  ends  according  to  the  usual  accounts  in  the  forma- 
tion of  a  solid  spherical  mass  of  cells.  A  process  of  delamina- 
tion  now  takes  place,  which  leads  to  the  formation  of  a  superficial 
layer  of  cubical  or  pyramidal  cells,  enclosing  a  central  solid 
mass  of  more  or  less  irregularly  arranged  cells. 

The  embryo,  in  the  cases  in  which  it  is  still  contained  within 
the  sporosack,  now  begins  to  exhibit  slight  changes  of  form,  and 

1  I.     HYDROZOA. 

\Hydroidca. 

1.  Hydromedusse.    \>/rachymedu^ 

2.  SiphODOphora.      \Cafycop**rM*. 

( Physophonda. 

3-    Acraspeda. 

II.  ACTINOZOA. 

1.  Alcyonaria.    (Octocoralla.) 

2.  Zoanthaiia.     (Hexacoralla.) 

III.  CTENOPHORA. 

9  For  a  detailed  description  of  the  development  of  a  single  species  the  reader 
referred  to  Allman's  description  of  Laomedia  flexuosa,  No.  149,  p.  85  /  seq. 


CtELENTERATA. 


153 


one  extremity  of  it  begins  to  elongate.  It  soon  becomes  free, 
and  rapidly  assumes  an  elongated  cylindrical  form,  while  a 
coating  of  cilia,  by  means  of  which  it  moves  sluggishly  about, 
appears  on  its  outer  surface.  A  central  cavity  appears  in  the 
interior,  and  the  inner  cells  form  themselves  into  a  definite 
hypoblast.  The  larva  has  now  become  a  planula,  and  consists  of 
a  closed  sack  with  double  walls.  It  continues  for  some  few  days 
to  move  about,  but  eventually  drops  its  cilia,  and  becomes 
dilated  at  one  extremity,  by  which  it  then  becomes  attached. 
The  base  of  attachment  becomes  gradually  enlarged  so  as  to 
form  a  disc,  which  spreads  out  and  is  frequently  divided  by 
fissures  into  radiating  lobes.  The  free  extremity  becomes  en- 
larged to  form  the  eventual  calyx. 

Over  the  whole  exterior  a  delicate  pellicle — the  future  peri- 
sarc — now  becomes  secreted.  Round  the  edge  of  the  anterior 
enlargement  a  row  of  tentacles  makes  its  appearance.  These,  in 
the  embryos  of  the  Tubularian  genera,  lie  some  little  way  behind 
the  apex  of  the  body.  After  a  certain  time  the  perisarc,  which 
has  hitherto  been  continuous,  becomes  ruptured  in  the  region  of 

C 


FIG.  68.    THREE  LARVA  STAGES  OF  EUCOPE  POLYSTYLA.    (After  Kowalevsky.) 

A.  Blastosphere  stage  with  hypoblast  spheres  becoming  budded   off  into  the 
central  cavity. 

B.  Planula  stage  with  solid  hypoblast. 

C.  Planula  stage  with  a  gastric  cavity. 

ep.  epiblast ;  hy.  hypoblast ;  al.  gastric  cavity. 

the  calyx,  and  the  tentacles  become  quite  free.     At  about  the 
same  period  a  mouth  is  formed  at  the  oral  apex. 


HYDROIDEA. 


The  development  of  Eucope  polystyla  (fig.  68),  one  of  the 
Campanularidae,  deviates  according  to  Kowalevsky  (No.  147)  in 
somewhat  important  points  from  the  usual  type.  The  whole 
development  takes  place  after  the  deposition  of  the  ovum.  The 
segmentation  results  in  the  formation  of  a  single-walled  blasto- 
sphere  with  a  large  central  cavity  (fig.  68  A).  This  cavity, 
somewhat  as  in  Ascetta,  becomes  filled  up  with  a  not  clearly  (?) 
cellular  material  derived  from  the  walls  of  the  blastosphere, 
which  must  be  regarded  as  the  hypoblast  (fig.  68  B).  The  larva 
elongates  and  becomes  ciliated,  and  the  epiblast  at  its  two 
extremities  becomes  thickened,  and  is  stated  by  Kowalevsky 
also  to  become  divided  into  two  layers.  The  alimentary  cavity 
appears  as  a  slit  in  the  middle  of  the  hypoblast  (fig.  68  C).  The 
cilia  after  a  time  disappear,  and  the  larva  then  becomes  fixed  by 
one  extremity.  It  flattens  itself  out  into  a  disc-like  form,  becomes 
divided  into  four  lobes,  and  covered  by  a  cuticle  (perisarc). 
From  the  disc  the  stalk  grows  out  which  dilates  at  its  free  ex- 
tremity into  the  calyx. 


In  both  the  groups  (Tubularia  and  Hydra) 
having  a  ciliated  planula  stage,  its  absence  may 
be  put  down  to  an  abbreviation  of  the  develop- 
ment, and  in  fact  a  two-layered  quiescent  stage, 
through  which  the  embryo  passes,  may  be 
regarded  as  representing  the  planula  stage. 

The  development  of  Tubularia,  which  has 
been  described  in  detail  by  Ciamician,  takes 
place  in  the  gonophore1.  The  segmentation 
is  irregular  and  leads  to  the  formation  of  an 
epibolic  gastrula,  four  large  central  cells  con- 
stituting the  hypoblast2.  The  larva  now  elon- 
gates, and  grows  out  laterally  into  two  pro- 
cesses which  constitute  the  first  pair  of 
tentacles.  At  this  stage  it  closely  resembles 
the  larvae  of  some  Medusas.  Additional  ten- 
tacles are  soon  formed ;  and  a  central  cavity 
appears  in  the  hypoblast,  the  cells  of  which 
have  in  the  meantime  become  more  numerous 
(fig.  69).  The  tentacles  are  directed  towards 


which  are  exceptional  in  not 


FIG.    69.    LONGITUDINAL 

SECTION  THROUGH  A  LARVA  OF 
TUBULARIAMESEMBRYANTHE- 
MUM  WHILE  STILL  IN  THE 

GONOPHORE.  The  lower  end 
is  the  oral  one. 

ep.  epiblast;   hy.  hypoblast 
of  tentacle ;  en.  enteric  cavity. 


1   Vide  Ciamician,  Zeit.f.  wiss.  Zool.,  Bd.  xxxn.  1879. 

3  In  examining  the  segmentation  by  means  of  sections  I  have  failed  to  detect  an 
epibolic  gastrula  or  such  irregularity  as  is  described  by  Ciamician.  Prof.  Kleinenberg 
informs  me  that  he  has  been  equally  unsuccessful. 


CCELENTERATA.  155 


the  aboral  side,  which  is  considerably  more  prominent  than  the  oral  one. 
They  contain  a  hypoblastic  axis.  The  aboral  end  continues  to  grow  and 
the  tentacles  gradually  assume  a  horizontal  position.  A  constriction  now 
appears,  dividing  the  larva  into  an  aboral  portion  which  will  eventually  form 
the  stalk,  and  an  oral  portion.  At  the  apex  of  the  latter  a  row  of  short 
tentacles — the  future  oral  tentacles — now  appears.  The  larva  has  at  this 
stage  the  form  known  as  Actinula.  In  this  condition  it  becomes  hatched, 
and  shortly  afterwards  it  becomes  fixed  by  the  aboral  end  and  grows  into 
a  colony. 

The  development  of  Myriothela  (Allman,  No.  150)  takes  place  on  the 
Tubularian  type.  The  ovum  invested  by  a  delicate  capsule  becomes  freed 
by  the  rupture  of  the  gonophore,  and  is  then  taken  up  by  the  remarkable 
claspers  characteristic  of  the  genus.  In  the  claspers  it  becomes  fecundated 
and  undergoes  its  further  development.  After  segmentation  a  gastric 
cavity  is  formed,  and  provisional  tentacles  arise  as  a  series  of  conical 
involutions  which  subsequently  become  evoluted.  Permanent  tentacles  are 
formed  as  conical  papillae  on  a  truncated  oral  process.  After  hatching  it 
has  a  few  days'  free  existence,  and  then  becomes  attached,  and  loses  its 
provisional  tentacles. 

Although  Hydra  itself  constitutes  the  simplest  type  of  Hydrozoon,  its 
development,  which  has  been  fully  investigated  by  Kleinenberg  (No.  161),  is 
in  some  respects  a  little  exceptional.  The  segmentation  is  regular,  but  a 
segmentation  cavity  is  not  formed.  The  peripheral  layer  of  cells  gradually 
becomes  converted  into  a  chitinous  membrane,  which  is  perhaps  homologous 
with  the  perisarc  of  marine  forms.  Between  the  membrane  and  the  germ  a 
second  pellicle  makes  its  appearance.  The  above  changes  require  about 
four  days  for  their  completion,  but  there  next  sets  in  a  period  of  relative 
quiescence  which  lasts  for  some  6 — 8  weeks.  During  this  period  the 
remaining  development  is  completed.  The  cells  of  the  germ  first  fuse 
together.  In  the  interior  of  the  protoplasm  a  clear  excentric  space  arises, 
which  gradually  extends  itself  and  forms  the  rudiment  of  the  gastric  cavity. 
The  outer  shell  in  the  meantime  becomes  less  firm,  and  is  finally  burst  and 
thrown  off,  owing  to  the  expansion  of  the  embryo  within. 

The  outermost  layer  of  the  protoplasm  becomes,  relatively  to  the  inner 
layer,  clear  and  transparent,  and  there  thus  arises  an  indication  of  a  division 
of  the  walls  of  the  archenteric  cavity  into  two  zones,  or  layers.  These  layers, 
which  form  the  epiblast  and  hypoblast,  are  definitely  established  on  the 
appearance  of  cells  with  contractile  tails1  in  the  clear  outer  zone,  between 
which  the  interstitial  epiblast  cells  subsequently  arise. 

The  embryo,  still  forming  a  closed  double-walled  sack,  elongates  itself, 
and  at  one  pole  its  wall  becomes  very  thin.  And  at  this  point  a  rupture 
takes  place  which  gives  rise  to  the  mouth.  Simultaneously  with  the  mouth 
the  tentacles  become  formed  as  hollow  processes,  according  to  Mereschkowsky 
two  being  formed  first  and  subsequently  the  others  in  pairs.  Very  shortly 

1  These  cells  are  the  so-called  nerve-muscle  cells.  Their  nature  is  discussed  in 
the  second  part  of  this  work. 


56  TRACHYMEDUS^E. 


afterwards  the  hitherto  uniform  hypoblast  becomes  divided  up  into  distinct 
cells.  The  thin  inner  pellicle  which  persists  after  the  rupture  of  the  outer 
membrane  becomes  in  the  meantime  absorbed.  With  these  changes  the 
embryo  practically  acquires  the  characters  of  the  adult. 

TrachymedusaB.  Amongst  the  Trachymedusae,  which  as 
has  now  been  satisfactorily  established  develop  directly  without 
alternations  of  generations,  the  embryology  of  species  both  of 
the  Geryonidae  and  the  ^Eginidae  has  been  studied. 

In  all  the  types  so  far  investigated  the  hypoblast  is  formed 
by  delamination,  and  there  is  a  more  or  less  well-marked  planula 
stage. 

The  development  of  Geryonia  (Carmarina)  hastata  has  been 
studied  by  Fol  (No.  155)  and  MetschnikofT  (No.  163)1.  The 
ovum,  when  laid,  is  invested  by  a  delicate  vitelline  membrane 
and  mucous  covering.  Its  protoplasm  is  formed  of  an  outer 
granular  and  dense  layer,  and  a  central  mass  of  a  more  spongy 
character.  The  segmentation  is  complete  and  regular,  and  up 
to  the  time  when  thirty-two  segments 
have  appeared  each  segment  is  composed 
of  both  constituents  of  the  protoplasm 
of  the  ovum.  A  segmentation  cavity 
appears  when  sixteen  segments  are 
formed,  and  becomes  somewhat  larger  at 
the  stage  with  thirty-two.  At  this  stage 
the  process  of  delamination  commences. 
Each  of  the  thirty-two  segments,  as 
shewn  in  the  accompanying  diagram 

(fig.  7O),  becomes  divided  into  two  unequal     DELAMINATION     OF    THE 
'     ,  r    ,  .     r  ,      OVUM       OF       GERYONIA. 

parts.     The  smaller  of  these  is  formed     (Copied  from  Fol.) 

almost    entirely   of    granular    material ;         «•  segmentation  cavity ; 

.         ,  .  .  r     .       .        a.  endoplasm ;  b.  ectoplasm. 

the  larger  contains  portions  of  both  The  dotted  lines  shew  the 
kinds  of  protoplasm.  In  the  next  seg-  C?u.r55e  of  the  next  planes  of 

division. 

mentation  the  thirty-two  large  cells  only 

are  concerned,  and  in  each  of  these  the  line  of  division  passes 
between  the  granular  and  the  transparent  protoplasm.  The 
sixty-four  lenticular  masses  of  granular  protoplasm  thus  formed 
constitute  an  outer  closed  epiblastic  vesicle,  within  which  the 

1  In  the  succeeding  account  I  have  followed  Fol,  who  differs  in  some  nvnor  points 
from  Metschnikoff. 


CCELENTERATA. 


157 


thirty-two  masses  of  transparent  protoplasm  form  an  hypoblastic 
vesicle.  The  embryo  at  this  stage  is  shewn  in  optical  section  in 
fig.  71. 

The  epiblastic  vesicle  now  grows  rapidly,  while  the  hypo- 
blastic vesicle  remains  nearly  passive  and  becomes  somewhat 
lens-shaped.  At  one  point  its  wall  comes  in  close  contact  with 
the  epiblast.  Elsewhere  a  wide  cavity  is  developed  between  the 
two  vesicles  which  becomes  filled  with  gelatinous  tissue.  At  this 
period  cilia  appear  on  the  surface,  and  the  larva  becomes  a  planula. 

The  succeeding  changes  lead  rapidly  to  the  formation  of  a 
typical  Medusa.  Where  the  epiblast  and 
hypoblast  are  in  contact  the  former  layer 
becomes  thickened  and  forms  a  disc-shaped 
structure.  The  centre  of  this  becomes 
somewhat  protuberant,  fuses  with  the  hy- 
poblast and  then  becomes  perforated  to 
form  the  mouth  (fig.  72  <?).  The  edge  of 
the  disc  forms  a  thickened  ridge,  the 
rudiment  of  the  velum  (v),  which  is  en- 
tirely formed  of  epiblast.  At  its  edge  six 
tentacles  (t]  arise,  into  which  are  con- 
tinued solid  prolongations  of  the  wall  of  the  now  somewhat 
hexagonal  gastric  chamber.  The  hypoblastic  axes  of  the  tenta- 
cles soon  lose  their  connection  with  the  gastric  wall. 

Up  to  this  time  the  larva  _==___=_^_ 

has  retained  a  more  or  less 
spherical  form,  and  the  cavity 
on  the  under  side  of  the 
umbrella  has  not  yet  become 
developed.  The  latter  now 
becomes  established  by  the 
whole  disc  assuming  a  vault- 
ed form  with  the  concavity 
directed  downwards.  The 
lining  of  the  cavity  so  formed 
is  derived  from  the  epiblast 
of  the  disc  already  spoken  of. 

The  exact  mode  of  formation  of  the  gastrovascular  canals  has  not  been 
worked  out.     It  has  however  been   established  by  the  researches  of  the 


FIG.  71.  EMBRYO  OF 
GERYONIA  AFTER  DELAMI- 
NATION.  (After  Fol.) 

ep.  epiblast;  hy.  hypo- 
blast. 


FIG.  72.    OPTICAL  SECTION  THROUGH 
THE  ORAL  POLE  OF  GERYONIA  AFTER  THE 

APPEARANCE   OF  THE  GELATINOUS    TISSUE 

OF  THE  DISC.     (After  Fol.) 

o.  mouth;  v.  velum;  /.  tentacle. 
The   shaded   part  represents  the  gelati- 
nous tissue. 


158  TRACHYMEDUS/E. 


Hertwigs  (No.  146)  and  Glaus  (No.  153)  that  the  radial  and  circular  vessels 
of  this  system  are  connected  together  in  adult  Medusae  by  an  hypoblastic 
lamella  ;  so  that  these  canals  would  seem  to  be  the  remnants  of  an  once- 
continuous  gastric  cavity.  This  mode  of  formation  is  established  in  the  case 
of  the  medusiform  buds ;  and  it  would  therefore  seem,  as  pointed  out  by  the 
Hertwigs,  a  fair  deduction  that  it  occurs  in  the  larva — a  conclusion  which  is 
confirmed  by  the  primitive  extension  of  the  gastric  cavity  to  the  edge  of  the 
disc  at  the  time  when  its  walls  give  rise  to  the  solid  axes  of  the  tentacles.  In 
the  course  of  the  subsequent  retirement  of  the  gastric  cavity  from  the  edge  of 
the  disc  the  gastrovascular  canals  probably  take  their  origin,  though  Fol  was 
unable  to  follow  the  changes  which  result  in  their  formation. 

On  the  completion  of  the  above  changes  the  larva  has  become 
a  fully  formed  Medusa,  but  it  undergoes  a  not  inconsiderable 
metamorphosis  before  the  attainment  of  the  adult  state. 

Two  species  of  ^Eginidae  have  been  studied  by  Metschnikoff 
(163),  viz.  Polyxenia  leucostyla  (^Egineta  flavescens},  and  ^Egi- 
nopsis  mediterranea.  In  both  of  these  forms  the  segmentation 


FIG.  73.    A  THREE-DAYS'  LARVA  OF  ^GINOPSIS  WITH  TWO  TENTACLES. 
(After  Metschnikoff.) 
m.  mouth;  /.  tentacle. 

results  in  the  formation  of  an  elongated  two-layered  ciliated 
planula,  without  a  central  cavity.  The  two  ends  of  this  grow 
out  into  two  long  processes — the  rudiments  of  a  pair  of  at  first 
aborally  directed  arms — which  contain  a  solid  hypoblastic  axis 
(fig.  73).  At  this  stage  the  larva  closely  resembles  the  larva  of 
Tubularia.  An  alimentary  cavity  is  hollowed  out  in  the  centre 
of  the  hypoblast  which  soon  opens  by  a  wide  oral  aperture  (m). 
A  second  pair  of  arms  becomes  formed,  which  are  at  first  much 
shorter  than  the  original  pair;  with  their  formation  a  radial 
symmetry  is  acquired.  Sense-organs  become  at  the  same  time 


OELENTERATA. 


159 


developed,  and  the  whole  embryo  assumes  a  medusiform  character. 
Fresh  tentacles  arise,  the  velum  and  cavity  of  the  umbrella  become 
established,  but  these  changes  do  not  involve  any  points  of  very 
special  interest. 

Siphonophora.  The  development  of  the  Siphonophora  has 
been  the  subject  of  careful  investigation  by  Haeckel  (158)  and 
Metschnikoff  (163).  The  ova  are  large  and  usually  (except 
Hippopodius)  without  a  membrane. 

They  are  formed  of  a  peripheral  denser  layer  of  protoplasm 
and  a  central  spongy  mass.  They  usually  undergo  their  entire 
development  in  the  water.  In  some  instances  they  have  been 
successfully  reared  by  artificial  impregnation. 

As  an  example  of  the  Calycophoridae  I  shall  take  Epibulia 
aurantiaca,  a  form  allied  to  Diphyes,  the  development  of  which 
has  been  studied  by  Metschnikoff1. 


FIG.  74.    THREE  LARVAL  STAGES  OF  EPIBULIA  AURANTIACA.    (After  Metschnikoff.) 

A.  Planula  stage. 

B.  Six-days'  larva  with  nectocalyx  (nc)  and  tentacle  (/"). 

C.  Somewhat  older  larva  with  gastric  cavity. 

ep.  epiblast;  hy.  hypoblast;  so.  somatocyst;  nc.  nectocalyx;  /.  tentacle;  c.  large 
yolk  cells ;  po.  polypite. 

1  In  my  description  of  the  development  of  the  Siphonophora  I  employ  Huxley's 
terminology. 


i6o 


SIPHONOPHORA. 


There  is  a  regular  segmentation,  unaccompanied  by  the 
formation  of  a  segmentation  cavity.  At  its  close  the  ovum 
becomes  a  spherical  ciliated  embryo.  This  embryo  soon  becomes 
elongated,  and  its  cells  differentiate  themselves  into  a  central 
and  a  peripheral  layer — the  epiblast  and  the  hypoblast  (fig.  74  A). 
At  this  stage  the  larva  has  the  typical  planula  form.  The  epiblast 
is  especially  thickened  at  a  pole,  which  may  be  called  the  oral 
pole,  and  towards  the  side  of  this,  which  will  be  spoken  of  as 
the  ventral  side.  Adjoining  this  thickened  layer  of  epiblast  a 
special  thin  layer  of  hypoblast  becomes  differentiated,  which  in 
opposition  to  the  main  mass  of  large  nutritive  cells  forms  the 
true  hypoblastic  epithelium 
(fig.  74  B,  hy\  On  this 
thickening  two  prominences 
make  their  appearance  (fig. 
74  B).  The  oral  of  these 
is  the  rudiment  of  a  ten- 
tacle (/),  and  the  aboral  of 
a  nectocalyx  (nc). 

The  former  of  these 
elongates  itself  in  succeed- 
ing stages  into  a  process  of 
both  epiblast  and  hypoblast. 
The  central  part  of  the 
nectocalyx  on  the  other  hand 
appears  to  originate  from  a 
thickening  of  the  epiblast 
in  which  the  cavity  of  the 
bell  becomes  subsequently 
hollowed  out.  Between 
this  part  and  the  external 
epiblast  which  gives  origin 
to  the  outermost  layer  of 
the  nectocalyx  a  layer  of 
hypoblast  is  interposed. 
When  the  nectocalyx  has 
become  to  a  certain  extent 
established  a  cavity — the 
commencement  of  the 


FlG.  75.  AN  ADVANCED  LARVA  OF  EPI- 
BULIA  AURANTIACA  WITH  ONE  LARGE  NECTO- 
CALYX. (After  Metschnikoff.) 

so.  somatocyst ;  nc.  second  imperfectly  de- 
veloped nectocalyx;  hph.  hydrophyllium;  po. 
polypite;  /.  tentacle. 


CCELENTERATA. 


161 


primitive  gastrovascular  cavity  of  the  adult — appears  in  the 
general  hypoblast  between  the  epithelial  and  nutritive  layers  in 
the  immediate  neighbourhood  of  its  attachment.  This  cavity 
becomes  prolonged  into  the  nectocalyx  to  form  the  four  gastro- 
vascular canals;  while  the  hypoblast  at  the  upper  end  of  the 
nectocalyx  forms  the  somatocyst  (fig.  74  C,  so).  The  primitive 
enteric  cavity  once  formed  rapidly  extends,  especially  in  an  oral 
direction  (fig.  74  C),  and  forms  a  widish  cavity  in  the  oral  part  of 
the  embryo.  At  the  pole  of  this  part  (fig.  74,  pd]  is  eventually 
formed  the  opening  of  the  mouth,  and  the  contained  cavity 
becomes  in  a  special  sense  the  gastric  cavity.  This  region  of  the 
embryo  may  be  spoken  of  as  the  polypite.  The  nectocalyx 
grows  with  great  rapidity  and  soon  forms  by  far  the  most 
prominent  part  of  the  larva  (fig.  75).  The  true  gastric  region  or 
polypite  (fig.  75, po)  continues  also  to  grow,  and  a  mouth  becomes 
formed  at  its  extremity.  The  aboral  end  of  the  original  body  of 
the  embryo  gradually  atrophies. 


FlG.  76.      TWO  STAGES  IN  THE  DEVELOPMENT  OF  STEPHANOMIA  PICTUM. 

(After  Metschnikoff.) 

A.  Stage  after  the  delamination.     ep.  epiblastic  invagination  to  form  pneuma- 
tocyst. 

B.  Later  stage  after  the  formation  of  the  gastric  cavity  in  the  solid  hypoblast, 
po.  polypite;    A  tentacle;  pp.  pneumatophore ;    ep.  epiblastic  invagination  to  form 
pneumatocyst ;  hy.  hypoblast  surrounding  pneumatocyst. 

At  the  junction  of  the  nectocalyx  and  polypite  the  ccenosarc 
becomes  formed,  and  rudiments  of  a  second  nectocalyx  (nc)  and 
B.  II.  Ir 


162  SIPHONOPHORA. 


second  polypite  early  become  visible ;  while  a  hydrophy Ilium  is 
formed  as  a  bud  which  covers  over  the  first  polypite  and  tentacle 
(JipJi).  With  the  development  of  the  hydrophyllium  the  first 
segment,  if  the  term  may  so  be  used,  is  complete.  The  second 
segment  of  which  a  rudiment  is  already  present  as  a  second 
polypite  is  intercalated  between  the  first  segment  and  the  necto- 
calyces. 

Amongst  the  Physophoridae  there  is  a  considerable  range  of 
variation  in  development ;  though  the  variations  concern  for  the 
most  part  not  very  important  points.  The  simplest  type  hitherto 
observed  is  that  of  StepJianomia  (Halistemma)  pictum.  The 
segmentation  and  formation  of  a  two-layered  planula  (fig.  76) 
take  place  in  the  usual  way.  Between  the  solid  central  mass  of 
nutritive  hypoblast  cells  and  the  epiblast  an  epithelial  hypoblastic 
layer  becomes  interposed  which  undergoes  a  special  thickening 
at  the  aboral  pole.  At  this  pole  a  solid  involution  of  epiblast 
next  becomes  formed,  to  which  a  layer  of  hypoblast  becomes 
applied.  The  structure  so  formed  is  the  rudiment  of  the  pneuma- 
tocyst  (<?/).  In  the  next  stage  the  air-cavity  of  the  pneumatocyst 
becomes  established  within  the  epiblast. 

The  gastrovascular  cavity  is  formed  in  the  midst  of  the 
nutritive  hypoblast  cells,  which  then  become  rapidly  absorbed 
leaving  the  gastrovascular  cavity  entirely  enclosed  by  the  epi- 
thelial layer  of  hypoblast  (fig.  76  B). 

By  the  above  changes  the  more  important  organs  of  the  larva 
have  become  established.  The  one  end  forms  the  pneumatophore, 
and  the  other,  the  oral  part,  the  polypite.  Between  the  two  there 
is  already  present  the  rudiment  of  a  tentacle,  and  a  second  tenta- 
cle soon  becomes  formed.  The  mouth  arises  as  a  perforation  at 
the  oral  end  of  the  larva. 

The  pneumatophore  contains  a  prolongation  of  the  gastro- 
vascular cavity,  the  fluid  in  which  bathes  the  outer  hypoblastic 
wall  of  the  pneumatocyst.  It  has  however  no  communication 
with  the  enclosed  cavity  of  the  pneumatocyst.  In  the  later 
developmental  stages  the  size  of  the  pneumatophore  becomes 
immensely  reduced  in  comparison  with  the  remainder  of  the 
larva. 

The  development  of  Physophora  agrees  closely  with  that  of  Stephanomia 
except  in   one   somewhat   important   point,  viz.    in   the  development   of  a 


CCELENTERATA.  163 


provisional  hydrophyllium.  This  arises  as  a  prominence  at  the  aboral  pole, 
containing  a  prolongation  of  the  gastrovascular  cavity.  Between  the  epiblast 
and  hypoblast  of  the  prominence  gelatinous  tissue  becomes  deposited,  and 
the  hydrophyllium  is  thus  converted  into  a  large  umbrella-like  organ  enclosing 
the  polypite.  The  two  together  have  a  close  resemblance  to  an  ordinary 
Medusa,  the  polypite  forming  the  manubrium,  and  the  hydrophyllium  the 
umbrella.  The  hydrophyllium  is  eventually  thrown  off. 

An  important  type  of  Physophorid  development  is  exemplified  in  Crystal- 
loides,  a  genus  closely  allied  to  Agalma.  In  this  type  the  greater  part  of  the 
original  ovum,  instead  of  directly  giving  rise  to  the  polypite,  becomes  a  kind 
of  yolk-sack,  from  which  the  polypite  is  secondarily  budded  (fig.  77,  yk). 
Agalma  sarsii  is  in  this  respect  intermediate  between  Crystalloides  and 
Physophora.  Both  these  types  are  remarkable  for  developing  a  series  of 
provisional  hydrophyllia  (fig.  77,  h.ph.}.  In  both  genera  the  first  of  these 
develops  as  in  Physophora,  and  for  a  long  time  is  the  only  one  functional. 

The  conclusions  to  be  drawn  from  the  above  description  may 
be  summed  up  as  follows.  In  all  the  Siphonophora,  so  far 
observed,  the  starting-point  for  further  development  is  a  typical 
ciliated  two-layered  planula.  The  inner  layer  or  hypoblast  is 
mainly  formed  of  large  nutritive  cells.  From  these  cells  an 
epithelial  hypoblastic  layer  becomes  secondarily  differentiated, 
the  exact  relations  of  which  differ  somewhat  in  the  various 
types.  The  nutritive  cells  themselves  do  not  appear  to  become 
directly  converted  into  the  permanent  hypoblastic  tissues.  The 
development  of  the  adult  from  the  planula  commences  by  the 
thickening  of  the  epiblastic  layer,  usually  at  one  pole  (the  future 
proximal  or  aboral  pole),  and  the  formation  at  this  pole  of  a 
series  of  bud-like  structures  (in  the  growth  of  which  both 
embryonic  layers  have  a  share),  which  become  converted  into 
the  hydrophyllia,  nectocalyces  etc.  The  main  oral  part  of  the 
planula  becomes  generally  converted  into  the  polypite,  though 
in  some  instances  (Crystalloides)  it  remains  as  a  yolk-sack,  and 
only  secondarily  gives  rise  to  a  polypite. 

Two  very  different  views  have  been  taken  as  to  the  nature  of 
the  various  component  parts  of  the  Siphonophora,  and  the 
embryological  evidence  has  been  appealed  to  by  both  sides  in 
confirmation  of  their  views.  By  Huxley  and  Metschnikoff  the 
various  parts — nectocalyces,  hydrophyllia,  hydrocysts,  polypites, 
generative  gonophores  etc.  are  regarded  as  simple  organs,  while 
by  Leuckart,  Haeckel,  Glaus  etc.  they  are  regarded  as  so  many 
different  individuals  forming  a  compound  stock.  The  difference 

II — 2 


164 


SIPHONOPHORA. 


between  these  two  views  is  not  merely  as  to  the  definition  of  an 

individual1.     The  question  really  is,  are  these  parts  originally 

derived  by  the  modification 

of  complete  zooids  like  the 

gonophores  and  trophosomes 

of  the  fixed  Hydrozoa  stocks, 

or  are  they  structures  derived 

from  the  modification  of  the 

tentacles  or  some  other  parts 

of  a  single  zooid  ? 

The  difficulty  of  deciding 
this  point  on  embryological 
evidence  depends  on  the  fact 
that  ontologically  a  tentacle 
and  a  true  bud  arise  in  the 
same  way,  viz.  as  papilliform 
outgrowths  containing  pro- 
longations of  both  the  primi- 
tive germinal  layers.  The 
balance  of  evidence  is  never- 
theless in  my  opinion  in 
favour  of  regarding  the  Si- 
phonophora  as  compound  stocks,  and  the  views  of  Claus  on  this 
subject  (Zoologie,  p.  271)  appear  to  me  the  most  satisfactory. 


FIG.  77.  LARVA  OF  CRYSTAI.LOJDES. 
(After  Haeckel.) 

h.ph.  hydrophyllium ;  h.  hydrocyst ;  /. 
tentacle  ;  pp.  pneumatophore  ;  po.  polypite  ; 
yk.  yolk-sack. 


The  most  primitive  condition  is  probably  that  like  Physophora  in  an 
early  stage  with  an  hydrophyllium  enclosing  a  polypite  (cf.  Haeckel  and 
Metschnikoff).  In  this  condition  the  whole  larva  may  be  compared  to  a 
single  Medusa  in  which  the  primitive  hydrophyllium  represents  the  umbrella 
of  the  Medusa,  and  the  polypite  the  manubrium.  The  tentacle  which 
appears  so  early  is  probably  not  to  be  regarded  as  a  modified  zooid,  but  as 
a  true  tentacle.  The  absence  of  a  ring  of  tentacles  is  correlated  with  the 
bilateral  symmetry  of  the  Siphonophora. 

The  primitive  zooid  of  a  Siphonophora  stock  is  thus  a  Medusa.  Like 
Sarsia  and  Wilsia  this  Medusa,  must  be  supposed  to  have  been  capable  of 
budding.  The  ordinary  nectocalyces  by  their  resemblance  to  the  umbrellas 
of  typical  Medusae  are  clearly  such  buds  of  the  medusiform  type.  The  same 
may  be  said  of  the  pneumatophore,  which,  as  pointed  out  by  Metschnikoff, 
is  identical  in  its  development  with  a  nectocalyx.  Both  are  formed  by  a 

1  From  the  expressions  used  by  Huxley,  Anatomy  of  Inverttb  rated  Animals, 
p.  149,  it  appears  to  me  possible  that  his  opposition  to  Leuckart's  view  is  mainly  as  to 
the  nature  of  the  individual. 


CCELENTERATA.  165 


solid  process  of  epiblast  in  which  a  cavity— the  cavity  of  the  nectocalyx  or 
pneumatocyst — is  eventually  hollowed  out.  Around  this  there  appears  a 
double  layer  of  hypoblast  containing  a  prolongation  of  the  gastrovascular 
cavity  ;  and  this  is  in  its  turn  enclosed  by  a  layer  of  epiblast  which  forms  the 
covering  of  the  convex  surface  of  the  nectocalyx  and  the  external  epiblast  of 
the  pneumatophore. 

The  generative  gonophores  are  clearly  also  zooids,  and  the  hydrophyllia 
are  probably  a  rudimentary  form  of  umbrella.  In  many  cases  (Epibulia, 
Stephanomia,  Halistemma  etc.)  the  hydrophyllium  of  the  primitive  polypite 
(manubrium)  is  absent.  In  such  instances  it  is  necessary  to  suppose  that 
the  umbrella  of  the  primitive  zooid  of  the  whole  colony  has  become  aborted. 
Leuckart  originally  took  a  somewhat  different  view  from  the  above  in  that 
he  regarded  the  starting-point  of  the  Siphonophora  to  be  a  compound  fixed 
Hydrozoon  stock,  which  became  detached  and  free-swimming. 

Acraspeda1.  The  embryonic  development  of  several  of  the 
forms  of  the  Acraspeda  has  been  investigated  by  Kowalevsky 
(No.  147)  and  Claus  (No.  153).  Their  observations  seem  to 
point  to  an  invaginate  gastrula  being  characteristic  of  this  group. 

Amongst  the  forms  with  alternations  of  generations  and  a 
fixed  larval  form  Chrysaora  and  Cassiopea  have  been  most  fully 
investigated.  The  ovum  of  the  former  undergoes  the  first  embry- 
onic phases  while  still  in  the  ovary.  In  the  latter  it  is  enclosed 
amongst  the  oral  processes.  A  complete  and  more  or  less 
regular  segmentation  leads  to  the  formation  of  a  single-walled 
blastosphere  with  a  small  segmentation  cavity.  The  wall  of  the 
blastosphere  next  becomes  invaginated,  giving  rise  to  an  arch- 
enteron  (fig.  78  A).  The  blastopore  soon  closes  up,  and  the 
archenteron  is  converted  into  a  closed  sack  completely  isolated 
from  the  epiblast  (fig.  78  B).  The  surface  of  the  larva  becomes 
in  the  meantime  covered  with  cilia.  The  free  larval  stage  thus 
reached  is  similar  to  the  ordinary  Hydrozoon  planula.  After 
the  closure  of  the  blastopore  the  larva  becomes  elongated,  and 
one  end  becomes  narrowed.  By  this  narrowed  extremity  the 
larva  soon  attaches  itself,  and  at  the  opposite  and  broader  end  a 
fresh  involution  of  the  epiblast  appears  (fig.  78  C) ;  this  gives 
rise  to  the  stomodseum,  which  is  placed  in  communication  with 
the  archenteron  on  the  absorption  of  the  septum  dividing  them. 
The  relation  of  the  stomodaeum  to  the  original  blastopore  has 
not  been  determined. 

!  I  use  this  term  for  the  group,  often  known  as  the  Discophora,  which  includes  the 
Pelagidte,  Rhizostomidae,  and  Lucernaridse. 


1 66  ACRASPEDA. 


At  the  point  of  attachment  there  is  developed  a  peculiar 
pedal  disc,  and  around  the  mouth  there  appears  a  fold  of  epiblast 
which  gives  rise  to  an  oral  disc  (fig.  78  D).  Two  tentacles  first 
make  their  appearance,  but  one  of  these  is  primarily  much  the 
largest,  though  eventually  the  second  overtakes  it  in  its  growth. 


FlG.   78.      FOUR  STAGES  IN  THE  DEVELOPMENT  OF  CHRYSAORA.      (After  Glaus.) 

A.  Gastrula  stage. 

B.  Stage  after  closure  of  blastopore. 

C.  Fixed  larva  with  commencing  stomodaeum. 

D.  Fixed  larva  with  mouth,  short  tentacles,  etc. 

ep.  epiblast;  hy.  hypoblast;  st.  stomodasum;  m.  mouth;  bl.  blastopore. 

A  second  pair  of  tentacles  next  becomes  formed,  giving  to  the 
larva  a  4-radial  symmetry.  Between  these  four  new  tentacles 
subsequently  sprout  out,  and  in  the  intermediate  planes  four 
ridge-like  thickenings  of  the  hypoblast,  projecting  into  the  cavity 
of  the  stomach,  make  their  appearance.  They  imperfectly  divide 
the  stomach  into  four  chambers,  to  each  of  which  one  of  the 
primary  tentacles  corresponds ;  they  may  be  regarded  as  homo- 
logous with  the  mesenteries  of  the  Actinozoa.  The  number  of 
tentacles  goes  on  increasing  somewhat  irregularly  up  to  sixteen. 
All  the  tentacles  contain  a  solid  hypoblastic  axis.  Muscular 
elements  are  developed  from  the  epiblast. 

With  the  above  changes  the  so-called  Hydra  tuba  or  Scyphi- 
stoma  form  is  reached  (vide  fig.  85).  The  peculiar  strobilization 
of  this  form  is  dealt  with  in  the  section  devoted  to  the  meta- 
morphosis. 


CCELENTERATA.  167 


Aurelia  is  stated  by  Kowalevsky  to  develop  in  the  same  way  as  Cassiopea; 
and  the  one  stage  of  Rhizostoma  observed  is  that  in  which  it  has  a  (probably 
invaginate)  gastrula  form. 

In  Pelagia  the  ovum  directly  gives  rise  to  a  form  like  the  parent.  The 
segmentation  and  the  invagination  take  place  nearly  as  in  Cassiopea,  but  the 
archenteric  cavity  is  relatively  much  smaller,  and  the  large  space  between  it 
and  the  epiblast  becomes  filled  with  the  gelatinous  tissue  which  forms  the 
umbrella.  The  blastopore  does  not  appear  to  close  but  to  become  directly 
converted  into  the  mouth.  As  in  Cassiopea  the  larva  takes  a  somewhat 
four-sided  pyramidal  form.  The  mouth  is  placed  at  the  base.  The  pyramid 
becomes  subsequently  flatter,  and  at  the  four  corners  four  tentacles  grow  out 
which  increase  to  eight  by  division.  The  flattening  continues  till  the  larva 
reaches  a  form  hardly  to  be  distinguished  from  the  Ephyra  resulting  from 
the  strobilization  of  the  fixed  Scyphistoma  form  of  other  Acraspeda. 

Alcyonidae.  In  the  Alcyonidae  the  segmentation  appears 
always  to  lead  to  the  formation  of  a  solid  morula,  which  becomes 
a  planula  by  delamination.  The  true  enteric  cavity  is  formed 
by  an  absorption  of  the  central  cells,  but  the  axial  portion  of  the 
gastric  cavity  and  mouth  are  formed  by  an  epiblastic  invagi- 
nation. 

The  development  of  these  types  has  been  mainly  studied  by  Kowalevsky 
(147),  and  my  knowledge  of  his  results  is  derived  from  German  abstracts  of 
the  original  Russian  memoirs. 

In  Alcyonium  palmatum  the  impregnation  is  external.  The  segmenta- 
tion is  very  exceptional  in  character.  It  commences  with  the  formation  of  a 
series  of  irregular  prominences  on  the  surface  of  the  ovum,  which  become 
segmented  off  to  form  a  superficial  layer  of  epiblast  cells.  The  inner  mass 
of  protoplasm  then  divides  up  into  polygonal  cells  to  form  the  hypoblast, 
which  would  thus  seem  to  be  formed  by  a  kind  of  delamination.  In  Clavu- 
laria  crassa  (No.  168)  there  is  a  complete  segmentation  followed  by  a 
delamination.  The  larva  of  AL  palmattim  elongates  and  becomes  ciliated, 
and  so  assumes  the  characters  of  a  typical  planula.  The  central  hypoblast 
is  formed  of  an  outer  granular  stratum  with  imperfectly  differentiated  cells 
— the  true  hypoblast — and  an  inner  homogeneous  mass  with  vacuoles. 

Some  of  the  larvae  become  fixed,  while  others  coalesce  together  and 
form  a  large  mass,  the  fate  of  which  has  not  been  further  studied.  An 
invagination  of  epiblast  takes  place  at  the  free  end  of  the  fixed  larva,  which 
gives  rise  to  the  so-called  gastric  cavity,  i.  e.  the  axial  portion  of  the  general 
enteric  cavity,  which  would  appear  to  be  in  reality  a  kind  of  stomodseum. 
Around  the  gastric  cavity  the  hypoblast  forms  eight  mesenteries,  the 
chambers  between  which  are  filled  with  the  homogeneous  material  which 
occupied  the  centre  of  the  ovum  in  the  previous  stage.  It  is  to  be 
presumed,  though  not  stated,  that  by  an  absorption  of  the  blind  end 
of  the  stomodaeal  invagination  the  gastric  chamber  is  placed  in  free 


168  ZOANTHARIA. 


communication  with  the  spaces  between  the  mesenteries1.  During  the  next 
stage  the  young  Alcyonium  also  acquires  eight  tentacles,  which  arise  as 
hollow  papillae  opening  into  the  eight  mesenteric  chambers.  By  this  stage 
also  the  matter  filling  up  the  mesenteric  chambers  is  nearly  absorbed. 

Between  the  epiblast  and  hypoblast  there  is  formed  an  homogeneous 
membrane,  which  penetrates  in  between  the  two  layers  of  hypoblast 
which  form  the  mesenteries.  On  the  outer  side  of  this  membrane,  and 
therefore  presumably  derived  from  the  epiblast,  is  a  layer  of  connective- 
tissue  cells,  which  eventually  gives  rise  to  the  abundant  gelatinous  tissue 
(ccenenchyma)  in  which  the  skeletal  elements  are  deposited.  In  Sympodimn 
coralloides  Kowalevsky  (No.  168)  has  shewn  still  more  completely  the  deriva- 
tion of  the  stellate  mesoblast  cells  from  the  epiblast.  He  finds  that  the 
calcareous  spicula  develop  in  these  cells  as  in  the  mesoblast  cells  of 
sponges.  The  branched  gastrovascular  canals  in  this  tissue  are  out- 
growths of  the  primitive  enteric  cavity.  A  layer  of  circular  muscles  is 
formed  at  a  late  period  from  the  epiblast,  but  the  longitudinal  muscles  of 
the  mesenteries  on  the  inner  side  of  the  homogeneous  membrane  are 
regarded  by  Kowalevsky  as  hypoblastic. 

A  ciliated  planula  with  delaminated  hypoblast  is  also  found  in  Gorgonia 
and  Corallium  rubrum.  In  the  former  genus  at  the  time  when  the  larva 
becomes  fixed,  the  hypoblast  is  formed  of  two  strata,  an  outer  one  of 
columnar  cells,  and  an  inner  one  of  round  ciliated  cells  lining  a  central 
enteric  cavity.  The  inner  layer  is  believed  by  Kowalevsky  to  become 
eventually  absorbed  and  to  be  homologous  with  the  inner  granular  mass  of 
Alcyonium. 

Zoantharia.  Amongst  the  Zoantharia  several  forms  have 
been  investigated  by  Kowalevsky  (147)  and  Lacaze  Duthiers 
(170),  of  which  some  are  stated  by  the  former  author  to  pass 
through  an  invaginate  gastrula  stage,  while  in  other  instances 
the  hypoblast  is  probably  formed  by  delamination. 

To  the  first  group  belongs  an  edible  form  of  Sea  Anemone 
found  near  Messina,  Cerianthus,  and  perhaps  also  Caryophyllium. 
In  the  first  of  these  segmentation  results  in  the  formation  of  a 
blastosphere.  A  normal  invagination  obliterating  the  segmenta- 
tion cavity  then  ensues,  and  the  blastopore  narrows  to  form  the 
mouth.  The  borders  of  the  mouth  bend  inwards  and  so  give 
rise  to  the  gastric  cavity  (stomoda^um)  which  as  in  the  Alcyonidae 
is  lined  by  epiblast.  Simultaneously  with  the  formation  of  the 
mouth  there  appear  the  two  first  mesenteries. 

In  Cerianthus  the  segmentation  is  unequal,  the  early  stages  are  the 
same  as  in  the  Actinia  just  described,  but  the  hypoblast  cells  give  rise 

1   The  German  abstract  is  very  obscure  as  to  the  formation  of  the  muulh. 


CCELENTERATA.  169 


to  a  mass  of  fatty  material  filling  up  the  enteric  cavity,  which   becomes 
eventually  absorbed. 

In  the  majority  of  the  Zoantharia  so  far  investigated,  includ- 
ing species  of  Actinia,  Sagartia,  Bunodes,  Astroides,  Astraea,  the 
segmentation,  which  is  often  unequal1  and  not  accompanied  by 
the  formation  of  a  segmentation  cavity,  results  in  a  solid  two- 
layered  ciliated  planula.  In  these  forms  the  impregnation  takes 
place  in  the  ovary,  and  the  early  stages  of  development  are 
passed  through  in  the  maternal  tissues. 

One  end  of  the  planula  becomes  somewhat  oval  and  develops 
a  special  bunch  of  cilia.  At  the  other  end  a  shallow  depression 
appears,  which  becomes  deeper  and  forms  an  involution  lined  by 
epiblast.  This  involution  is  the  stomodaeum,  and  becomes  the 
so-called  gastric  cavity.  The  true  enteric  cavity  lined  by  hypo- 
blast  is  for  some  time  filled  with  yolk  material.  The  larva 
always  swims  with  the  aboral  end  directed  forwards. 

Between  the  two  embryonic  layers  a  homogeneous  membrane 
is  formed,  similar  to  that  already  described  in  the  Alcyonidae. 

The  further  development  of  the  larvae  especially  concerns  the  formation 
of  mesenteries,  tentacles  and  calcareous  skeleton.  With  reference  to  this  sub- 
ject the  observations  of  Lacaze  Duthiers  are  especially  valuable  and  striking. 

In  the  adult  it  is  usually  possible  to  recognise  in  the  tentacles  a 
symmetry  of  six.  There  are  six  primary  tentacles,  six  secondary,  twelve 
tertiary,  twenty-four  quaternary,  etc.  In  the  hard  septa  of  the  skeleton 
the  same  law  is  followed  up  to  the  third  cycle,  but  beyond  that,  in  the 
cases  where  the  point  can  be  verified,  there  appear  to  be  only  twelve  septa 
in  each  additional  cycle.  The  observations  of  Lacaze  Duthiers  have  shewn 
that  this  symmetry  is  only  secondarily  acquired  and  does  not  in  the  least 
correspond  with  the  succession  of  the  parts  in  development. 

His  observations  were  conducted  on  three  species  of  Zoantharia  without 
a  skeleton,  viz.  Actinia  mesembryanthemum,  Sagartia,  and  Bunodes  gem- 
macea ;  while  Astroides  calycularis  served  as  the  type  for  his  investiga- 
tions on  the  corallum.  It  will  be  convenient  to  commence  with  his 
results  on  Actinia  mesembryanthemum  which  served  as  his  type. 

The  free  cylindrical  embryo,  with  the  aboral  end  directed  forwards  in 
swimming,  first  becomes  somewhat  flattened  and  the  mouth  elongated.  A 
bilateral  symmetry  is  thus  brought  about.  Two  mesenteries  now  make 
their  appearance  transversely  to  the  long  axis  of  the  mouth,  which  divide 
the  enteric  cavity  into  two  unequal  chambers.  The  mesenteries  consist  of 
a  fold  of  hypoblast  with  a  prolongation  of  the  epiblast  between  the  two 

1  I  have  this  on  the  authority  of  Kleinenberg.  The  existence  of  an  unequal 
segmentation  probably  indicates  an  epibolic  gastrula. 


ZOANTHARIA. 


limbs  of  the  fold.  The  larger  chamber  next  becomes  divided  by  two  fresh 
mesenteries  into  three,  and  a  similar  division  then  takes  place  in  the  smaller 
chamber.  The  stage  with  six  chambers  is  almost  immediately  succeeded 
by  one  with  eight,  owing  to  the  appearance  of  two  fresh  mesenteries  in  the 
second-formed  set  of  chambers.  At  the  stage  with  eight  chambers  there  is  a 
marked  period  of  repose.  The  number  of  chambers  is  increased  to  ten  by 
the  division  of  the  third-formed  set  of  chambers,  and  to  twelve  by  the 
division  of  the  fourth-formed  set.  It  will  be  observed  that  the  number  of 
the  chambers  increases  in  arithmetical  progression  by  the  continual  addition 
of  two,  alternately  cut  off  from  the  primitive  large  and  small  chambers. 
The  freshly  formed  chambers  are  always  formed  immediately  on  one  side  of 
the  primitive  mesenteries.  The  stages  with  six  and  ten  are  of  very  short 
duration.  The  two  primitive  chambers  are  necessarily  at  the  ends  of  the 
long  axis  of  the  mouth.  After  the  division  of  the  enteric  cavity  into  twelve 
chambers,  these  chambers  become  about  equal  in  size,  and  the  formation  of 
the  tentacles  commences.  The  law  regulating  the  appearance  of  the 
tentacles  is  nearly  the  same  as  that  for  the  mesenteries,  but  is  not  quite  so 
precise.  One  tentacle  makes  its  appearance  for  each  chamber.  The  most 
remarkable  feature  in  the  appearance  of  the  tentacles  is  due  to  the  fact  that 
the  tentacle  surmounting  the  primitive  largest  chamber  arises  before  any 
of  the  others,  and  long  retains  its  supremacy  (fig.  80  A).  This  fact,  coupled 
with  the  inequality  of  the  two  primitive  chambers,  supplies  some  grounds 
for  speculating  on  a  possible  descent  of  the  Ccelenterata  from  bilaterally 
symmetrical  forms  with  dis- 
tinctly differentiated  dorsal  and 
ventral  surfaces.  The  supre- 
macy of  the  first-formed  tentacle 
is  not  confined  to  the  Actinozoa, 
but  as  has  already  been  indi- 
cated, is  also  found  in  the  Scy- 
phistoma  (p.  166)  of  the  Acras- 
peda. 

After  the  twelve  tentacles 
have  become  established  they 
become  secondarily  divided  into 
two  cycles  of  six  respectively 
larger  and  smaller  tentacles, 
which  alternate  with  each  other. 
The  two  tentacles  pertaining  to 
the  two  original  chambers  be- 
long to  the  cycle  of  larger  ten- 
tacles. The  mesenteric  fila- 
ments appear  first  of  all  on  the 
primary  pair  of  septa.  The 
increase  in  the  number  of  ten- 
tacles and  chambers  from  12  to  24  has  been  found  to  take  place  in  a  very 


FlG.   So.       Two  STACKS    IN    Till',    HKVKI.OP- 

MENT  OF  ACTINIA  MESKMHRYANTHEMUM. 
(After  Laea/.e  Duthicrs.) 

In  the  younger  ciliated  embryo  A,  vkuol 
from  the  side,  only  one  tentacle  is  developed. 
m.  mouth. 

The  oMer  larva  B  is  viewed  from  the  face 
when  24  tentacles  have  just  become  establislu-d. 
The  letters  shew  the  true  order  of  succession  of 
the  tentacles;  but  c  and/are  transposed. 


CCELENTERATA.  171 


remarkable  and  unexpected  way.  The  law  is  expressed  by  Lacaze  Duthiers 
as  follows.  "The  appearance  of  the  new  chambers  is  not,  as  has  been 
believed,  a  consequence  of  the  production  of  a  single  chamber  between  each 
of  the  twelve  already  existing  chambers,  but  of  the  birth  of  two  new 
chambers  in  each  of  the  six  elements  (chambers)  of  the  smaller  cycle."  The 
result  of  this  law  is  that  a  pair  of  tentacles  of  the  third  cycle  is  placed  in 
every  alternate  space,  between  a  large  and  a  small  tentacle,  of  the  two 
already  existing  cycles,  which  may  conveniently  be  called  the  first  and 
second  cycles  (fig.  80  B). 

The  twenty-four  tentacles  formed  in  the  above  manner  are  obviously  at 
first  very  irregularly  arranged  (fig.  80  B),  but  they  soon  acquire  a  regular 
arrangement  in  three  graduated  cycles  of  6,  6  and  12.  The  first  cycle  of  the 
six  largest  tentacles  is  the  large  cycle  of  the  previous  stage,  but  the  two 
other  cycles  are  heterogeneous  in  their  origin,  each  of  them  being  composed 
partly  of  the  twelve  tentacles  last  formed,  and  partly  of  the  six  tentacles 
of  the  second  cycle  of  the  previous  stage. 

The  further  law  of  multiplication  has  been  thus  expressed  by  Lacaze 
Duthiers:  "The  number  of  chambers  and  still  later  that  of  the  corre- 
sponding tentacles  is  carried  from  24 — 48  and  from  48 — 96  by  the  birth  of 
a  pair  of  elements  in  each  of  the  12  or  24  chambers,  above  which  are  placed 
the  smallest  tentacles  which  together  constitute  the  fourth  or  fifth  cycle. 
Since,  after  the  formation  of  each  fresh  cycle,  the  arrangement  of  the 
tentacles  again  becomes  symmetrical,  it  is  obvious  that  all  the  equal  sized 
cycles  except  the  first  are  formed  of  tentacles  entirely  heterogeneous  as  to 
age." 

The  fixation  of  the  free  swimming  larva  takes  place  during  the  period 
when  the  tentacles  are  increasing  from  12  to  24. 

The  general  formation  of  the  chambers  in  Bunodes  and  Sagartia  is 
nearly  the  same  as  in  Actinia. 

In  the  two  types  of  Actinozoa  with  an  embolic  gastrula  stage  the  laws 
as  to  the  formation  of  the  tentacles  do  not  appear  to  be  the  same  as  those 
regulating  the  forms  observed  by  Lacaze  Duthiers. 

In  Cerianthus  four  tentacles  are  formed  simultaneously  at  the  period 
when  only  four  chambers  are  present.  In  Arachnitis  (Edwarsia)  the  suc- 
cession of  the  tentacles  is  stated  (A.  Agassiz,  166)  to  resemble  that  in 
Cerianthus.  There  are  originally  four  tentacles,  and  at  one  extremity  of  the 
long  axis  of  the  mouth  are  the  oldest  tentacles,  while  at  the  other  tentacles 
are  constantly  added  in  pairs.  An  odd  tentacle  is  always  found  at  the 
extremity  of  the  mouth  opposite  the  oldest  tentacles. 

In  the  other  species  with  an  embolic  gastrula  eight  tentacles  would  seem 
to  appear  simultaneously  at  the  period  when  eight  chambers  are  present ; 
though  on  this  point  Kowalevsky's  description  is  not  very  clear.  The 
presence  of  such  a  stage  would  seem  to  indicate  a  close  affinity  to  the 
Alcyonidas. 

Amongst  the  sclerodermatous  Actinozoa,  except  Caryophyllium,  the 
embryo  closely  resembles  that  of  the  delaminate  Malacodermata.  The  first 


ZOANTHARIA. 


stages  occur  in  the  ovary,  and  the  larva  is  dehisced  into  the  body  cavity  as 
a  two-layered  ciliated  planula. 

The  laws  affecting  the  formation  of  the  first  twelve  tentacles  and  septa 
appear  to  be  nearly  the  same  as  for  the  Malacodermata.  The  hard  parts 
begin  as  a  rule  to  be  formed  when  twelve  tentacles  have  appeared,  at  which 
period  also  the  fixation  of  the  larva  takes  place.  On  fixation  the  larva 
becomes  very  much  flattened. 

The  first  parts  of  the  corallum  to  appear  are  twelve  of  the  septa,  which 
arise  simultaneously  in  folds  of  the  enteric  wall  in  the  chambers  between 
the  mesenteries,  and  correspond  therefore  with  the  tentacles  and  not,  as 
might  be  supposed,  with  the  mesenteries.  Each  septum  is  formed  by  the 
coalescence  of  three  calcareous  plates  which  originate  in  separate  centres  of 
calcification.  The  concrescence  of  the  three  produces  a  Y-shaped  plate 
with  the  single  limb  directed  inwards  and  the  two  limbs  outwards  (fig.  81). 
The  theca  does  not  arise  till  after  the  septa  have  become  formed,  and  is  at 
first  a  somewhat  membranous  cup  quite  distinct  from  the  septa.  The 
columella  is  formed  still  later  by  the  coalescence  of  a  series  of  nodules  which 
are  formed  in  a  central  axis  enclosed  by  the  inner  ends  of  the  septa. 

After  the  formation  of  the  theca  the 
septa  become  divided  into  two  cycles  by 
the  predominant  growth  of  six  of  them. 
On  the  coalescence  of  the  septa  with  the 
theca  the  space  between  the  two  limbs  of 
the  Y  becomes  filled  up  with  calcareous 
tissue.  The  law  of  the  formation  of  the 
third  cycle  of  septa  (12 — 24)  has  not  been 
worked  out,  so  that  it  is  not  possible  to 
state  whether  it  follows  the  peculiar  prin- 
ciples regulating  the  growth  of  the  tentacles. 

The  whole  of  the  skeletal  parts  occupy 
a  position  between  the  epiblast  and  hypo- 
blast,  and  are  exactly  homologous  in  this 
respect  with  the  skeleton  of  the  Alcyonidae. 
By  Lacaze  Duthiers  they  are  however  be- 
lieved to  originate  in  the  hypoblast,  but 
from  the  observations  of  Kowalevsky  there 
can  be  little  doubt  that  they  arise  in  the 
connective  tissue  between  the  two  embry- 
onic layers  which  is  probably  epiblastic  in 
origin. 

A  peculiar  larva,  probably  belonging  to  the  Actinozoa,  has  been  described 
by  Semper1.  It  has  an  elongated  form  and  is  provided  with  a  longitudinal 
ridge  of  cilia.  There  is  a  mouth  at  one  end  of  the  body  and  an  anus  at 
the  opposite  extremity.  The  mouth  leads  into  an  oesophagus,  which  opens 


FIG.  81 .  LARVA  OF  ASTROIDES 
CALYCULARIS  SHORTLY  AFTER  IT 

HAS  BECOME  ATTACHED.      (After 

Lacaze  Duthiers.) 

The  figure  shews  the  develop- 
ment of  the  Y-shaped  septa  in  the 
intervals  between  the  mesenteries. 
The  position  of  the  latter  is  in- 
dicated by  the  faint  shading.  The 
theca  has  become  developed  ex- 
ternally. 


Ueb.  einige  tropische  Larven-formen."     Zeit.  f.  iviss.  ZooL,  vol.  xvn.  1867. 


CCELENTERATA.  173 


freely  into  a  stomach  with  six  mesenteries.  In  the  skin  are  numerous  thread- 
cells.  A  mesotrochal  worm-like  larva,  also  provided  with  thread-cells,  and 
found  at  the  same  time,  was  conjectured  by  Semper  to  be  a  younger  form  of 
this  larva. 

Cfcenophora.  The  ovum  of  the  Ctenophora  is  formed  of  an 
outer  granular  protoplasmic  layer  and  an  inner  spongy  mass  with 
fatty  spherules.  It  is  enveloped  in  a  delicate  vesicle,  the  diame- 
ter of  which  is  very  much  greater  than  that  of  the  contained 
ovum.  This  vesicle  appears  to  be  filled  with  sea-water,  in  which 
the  ovum  floats. 

Fertilized  ova  may  usually  be  easily  obtained  by  keeping  the 
captured  adults  in  water  from  12 — 24  hours.  The  two  main 
authorities  on  the  development  of  these  forms  (Kowalevsky,  No. 
147  and  178  and  Agassiz,  No.  172)  are  unfortunately  at  variance 
on  one  or  two  of  the  most  fundamental  points.  It  seems  how- 
ever that  the  embryonic  layers  are  formed  by  a  kind  of  epibolic 
gastrula ;  while  the  true  gastric  cavity,  as  distinct  from  the  gas- 
trovascular,  is  formed  by  an  invagination,  and  deserves  therefore 
to  be  regarded  as  a  form  of  stomodaeum. 

The  early  stages  are  very  closely  similar  in  all  the  types  so  far 
observed.  Segmentation  commences  by  the  outer  layer  of  the 
ovum,  which  throughout  behaves  as  the  active  layer,  forming  a 
protuberance  at  one  pole,  which  may  be  called  the  formative 
pole.  Close  below  this  protuberance  is  placed  the  nucleus.  In 
the  median  line  of  the  protuberance  a  furrow  appears  (fig.  82  A), 


FlG.  82.      FIVE  STAGES  IN  THE  DEVELOPMENT  OF  IDYIA  ROSEOLA.     (After  Agassiz. ) 

The  protoplasmic  layer  of  the  ovum  is  represented  in  black. 

which  gradually  deepens  till  it  divides  the  ovum  into  two.  The 
granular  layer  follows  the  furrow  so  that  each  of  the  fresh  seg- 
ments, like  the  original  ovum  is  completely  invested  by  a  layer 


1/4  CTENOPHORA. 


of  granular  protoplasm.  Each  segment  contains  a  nucleus.  A 
second  similar  division  at  right  angles  to  the  first  gives  rise  to 
four  segments  (fig.  82  B),  and  the  segments  so  formed  become 
again  divided  into  eight  (fig.  82  C).  In  the  division  into  eight, 
which  takes  place  in  a  vertical  plane,  the  segments  formed  are  of 
unequal  size,  four  of  them  being  much  smaller  than  the  others. 
The  eight  segments  are  arranged  in  the  form  of  a  slightly  curved 
disc  round  a  vertical  axis — the  future  long  axis  of  the  body  ; — 
and  there  is  a  cavity  in  this  axis  which,  like  the  segmentation 
cavity  of  Sycandra  raphanus^  is  open  at  both  extremities.  The 
disc  with  its  concavity  on  the  side  of  the  formative  pole  has  the 
shape  sometimes  of  an  ellipse  (fig.  82  C)  and  sometimes  of  a 
rectangle,  in  which  the  four  small  spheres  occupy  the  poles  of 
the  longer  axis.  A  bilateral  symmetry  is  thus  even  at  this  stage 
clearly  indicated. 

In  the  next  phase  of  segmentation  the  granular  layer  sur- 
rounding each  segment  again  forms  a  protuberance  at  the  forma- 
tive pole,  but,  instead  of  each  segment  becoming  divided  into 
two  equal  parts,  the  protoplasmic  protuberance  alone  is  divided 
off  from  the  main  segment.  In  this  way  sixteen  spheres  become 
formed,  of  which  eight  are  large  and  are  formed  mainly  of  the 
yolk  material  of  the  inner  part  of  the  ovum,  and  eight  are  small 
and  entirely  composed  of  the  granular  protoplasm.  The  eight 
small  spheres  form  a  ring  on  the  formative  surface  of  the  large 
spheres  (fig.  82  D). 

The  small  spheres  now  increase  very  rapidly  (fig.  82  E),  partly 
by  division  and  partly  by  the  formation  of  fresh  cells  from  the 
large  spheres ;  and  spread  over  the  large  spheres,  forming  in  this 
way  an  epibolic  gastrula.  They  constitute  a  layer  of  epiblast. 
(Fig.  83  A.)  The  large  cells  in  the  meantime  remain  relatively 
passive,  though  during  the  process  they  divide,  in  some  cases 
more  or  less  irregularly,  while  in  Eucharis  they  divide  into  six- 
teen. The  axial  segmentation  cavity  would  seem  during  the 
process  to  become  obliterated. 

There  is  an  important  discrepancy  between  the  statements  of  Kowalevsky 
and  Agassiz  as  to  the  course  of  the  growth  of  the  small  cells.  According 
to  Agassiz  the  small  cells  grow  most  rapidly  at  the  formative  pole  and  cover 
this  before  they  meet  at  the  opposite  pole.  The  reverse  statement  is  made 
by  Kowalevsky.  It  would  seem  that  the  above  discrepancy  is  due  to  an 


CCELENTERATA. 


175 


interchange  on  the  part  of  the  one  or  the  other  of  these  authors  of  the  two 
poles  of  the  embryo,  in  that  according  to  Agassiz  the  formation  of  the  mouth 
takes  place  at  the  formative  pole,  and  according  to  Kowalevsky  at  the  pole 
opposite  to  this. 

Without  attempting  to  decide  between  the  above  views,  we  shall  speak  of 
the  pole  at  which  the  mouth  is  formed  as  the  oral  pole. 

The  formation  of  the  alimentary  cavity  commences  shortly 
after  the  complete  investiture  of  the  embryo  by  the  epiblast 
cells.  At  the  oral  pole  an  invagination  of  epiblast  cells  takes 
place  (fig.  83  B),  which  makes  its  way  towards  the  opposite  pole. 
More  especially  from  the  figures  given  by  Agassiz,  and  from  the 
explanation  of  his  plates,  it  would  seem  that  a  large  chamber  is 
formed  in  the  hypoblast  at  the  end  of  the  invaginated  tube,  into 
which  this  tube  soon  opens  (fig.  83  C).  The  invaginated  tube 
would  seem  to  give  rise  to  the  so-called  stomach,  while  the 
chamber  at  its  aboral  extremity  is  no  doubt  the  infundibulum, 
which  as  may  be  gathered  from  Kowalevsky 's  statements,  is  lined 
by  a  flattened  epithelium.  At  a  later  period  the  gastrovascular 
canals  grow  out  from  the  infun- 
dibulum as  four  pouches,  which 
are  surrounded  by,  and  grow  at 
the  expense  of,  the  large  central 
cells,  which  have  in  the  mean- 
time arranged  themselves  in 
four  masses,  and  appear  to  serve 
as  a  kind  of  yolk.  The  nuclei 
of  these  large  cells  according  to 
Kowalevsky  disappear,  and  the 
cells  themselves  break  up  into 
continually  smaller  masses. 


FlG.  83.  FOUR  STAGES  IN  THE  DE- 
VELOPMENT OF  IDYIA  ROSEOLA.  (After 
Agassiz.) 

s.c,  sense  capsule;  st.  stomodreum. 


The  main  difficulty  in  the  above 
description  of  Agassiz  is  the  origin  of 
the  infundibulum.  In  the  absence  of 
definite  statements  on  this  head  it 
seems  reasonable  to  conclude  that  it  arises  as  a  space  hollowed  out  in  the 
central  cells,  and  that  its  walls  are  formed  of  elements  derived  from  the  yolk 
cells1.  On  this  interpretation  the  alimentary  canal  of  the  Ctenophora  would 


1  Chun  (No.  174)  gives  a  short  statement  of  his  observations,  which  accords  with 
the  interpretation  in  the  te*t. 


176  CTENOPHORA. 


consist,  as  in  the  Acraspcdote  Medusa:  and  Actinozoa,  of  two  sections:  (i) 
A  true  hypoblastic  section  consisting  of  the  infundibulum  and  the  gastro- 
vascular  canals  derived  from  it ;  and  (2)  an  epiblastic  section— the  stomo- 
d.i-um — forming  the  stomach. 

The  observations  of  Kowalevsky  on  the  alimentary  system  do  not  wholly 
tally  with  those  of  Agassiz.  He  finds  that  the  oral  side  of  the  embryo 
becomes  hollowed  out,  and  that  the  hollow,  lined  by  flattened  cells,  becomes 
constricted  off  as  the  infundibulum,  from  which  the  radial  canals  subse- 
quently grow  out.  To  the  infundibulum  there  leads  a  narrow  canal  lined  by 
a  columnar  epithelium  which  becomes  the  gastric  cavity. 

While  the  alimentary  canal  is  becoming  formed  a  series  of 
important  changes  takes  place  in  other  parts  of  the  embryo. 
The  rows  of  locomotive  paddles  first  appear  as  four  longitudinal 
equidistant  linear  thickenings  of  the  epiblast  near  the  aboral  pole 
(fig.  83  D).  On  the  projecting  surface  of  these  ridges  stiff  cilia 
appear  which  coalesce  together  to  form  the  paddles.  While  the 
embryo  is  still  within  the  egg  the  rows  of  paddles  are  quite  short 
and  also  double.  There  arc  in  Pleurobrachia  about  eight  or  nine 
pairs  of  paddles  in  each  row.  Each  double  row  eventually  sepa- 
rates into  two. 

In  all  the  forms  except  the  Eurostomata  (Beroe)  two  tentacles 
grow  out  as  thickenings  of  the  epiblast  (fig.  84  B,  /.).  They  are 
placed  at  the  opposite  poles  of  the  long  transverse  axis  of  the 
embryo. 

A  process  of  the  contractile  gelatinous  tissue  of  the  body,  the 
origin  of  which  is  described  below,  makes  its  way,  according  to 
Kowalevsky,  into  the  tentacles. 

The  central  apparatus  of  the  nervous  system  and  the  otoliths 
are  formed  at  the  aboral  pole  from  a  thickening  of  the  epiblast, 
but  the  full  details  of  their  formation  have  not  been  elucidated. 
It  may  be  well  to  preface  my  account  of  their  development  with 
a  short  statement  of  their  adult  structure. 

They  consist  in  the  adult  of  a  vesicle  with  a  ciliated  lining  situated  at 
the  bifurcation  of  the  two  anal  tubes,  and  of  certain  structures  connected 
with  this  vesicle.  From  the  floor  of  the  vesicle  is  suspended  a  mass  of 
otoliths  by  four  leaf-like  bodies  known  as  suspenders.  The  roof  is  very 
•  tc  ami  IMS  the  form  of  a  four-sided  pyramid.  Six  openings  lead  into 
the  vesicle.  Through  four  of  these,  placed  at  the  four  corners,  there  pass 
out  four  ciliated  grooves  continuous  with  the  suspenders.  These  grooves, 
after  leaving  the  otolithic  vesicle,  bifurcate  and  pass  to  the  eight  rows  of 
paddies.  At  the  two  sides  the  walls  of  the  vesicle  are  continuous  with  two 


CCELENTERATA.  177 


thickened  ciliated  plates  with  swollen  edges,  opposite  the  centres  of  which 
are  two  lateral  openings  into  the  vesicle,  completing  the  six  openings. 
Through  the  lateral  openings  the  sea  water  is  driven  by  the  action  of  the 
cilia  of  the  plates. 

The  development  of  these  parts  is  as  follows — In  the  aboral 
thickening  of  epiblast  a  cavity  makes  its  appearance,  the  walls 
of  which  constitute  the  rudiment  of  the  otolithic  vesicle  (fig.  83  B 
and  C,  s.c.).  The  roof  of  the  cavity  is  extremely  delicate.  On 
each  side  of  it  a  thickening  of  cells  becomes  established,  regarded 
by  Kowalevsky  as  the  rudiment  of  the  nervous  ganglia.  These 
thickenings  appear  to  give  origin  to  the  lateral  ciliated  plates. 
The  otoliths  arise  from  cells  at  four  separate  points  at  the  corners 
of  the  ciliated  plates  opposite  the  rows  of  paddles  (fig.  84  A,  ot.). 

In  Pleurobrachia  there  is  at  first  only  one  otolith  at  each 
corner.  The  otoliths  are  gradually  transported  towards  the 
centre  of  the  vesicle  (fig.  84  B,  ot)  and  are  there  attached,  though 
the  four  leaf-like  suspenders  do 
not  arise  till  very  late.  The  oto- 
liths go  on  increasing  in  number 
throughout  life. 

The  gelatinous  tissue  of  the 
Ctenophora  appears  as  a  homoge- 
neous layer  between  the  epiblast 
and  the  yolk-cells,  and  is  probably 
homologous  with  the  layer  formed  FlG.  ^  Two  STAGES  IN  THE 

in  the  same  situation  in  all  other     DEVELOPMENT  OF  PLEUROBRAcnrA 

,  r  TII  RHODODACTYLA.     (After  Agassiz.) 

ccelenterate  forms.     Into  the  layer 

*  of.  otolith  ;  /.  tentacle. 

a  number  of  anastomosing   cells, 

mainly  derived  from  the  epiblast,  though  according  to  Chun 
(No.  174)  also  in  part  from  the  hypoblast,  make  their  way. 
These  cells  would  appear  to  be  mainly,  if  not  entirely  (Chun), 
of  a  contractile  nature.  It  is  probable  that  the  great  mass  of  the 
gelatinous  tissue  of  the  adult  is  an  intercellular  substance  derived 
from  these  cells. 

The  whole  of  the  above  changes  are  completed  while  the 
embryo  is  still  enclosed  in  the  egg  capsule.  During  their 
accomplishment  the  oro-anal  axis,  which  was  originally  very 
short,  increases  greatly  in  length  (fig.  83),  so  that  the  embryo 
acquires  an  oval  form  similar  to  that  of  the  adult. 

B.  IT.  12 


I78 


SUMMARY. 


The  exact  period  of  leaving  the  egg  does  not  appear  to  be  very  constant 
but  the  hatching  never  takes  place  till  the  embryo  has  practically  acquired 
all  the  organs  of  the  adult. 

In  the  majority  of  types  the  differences  between  the  just  hatched  larva 
and  the  adult  are  inconsiderable,  and  in  all  cases  the  larva  has  a  somewhat 
oval  form.  In  the  case  of  the  Tasniatae  (Cestum,  etc.),  the  larva  has  the 
characteristic  oval  form,  and  the  subsequent  changes  amount  almost  to  a 
metamorphosis. 

The  larva  of  the  Lobatae,  such  as  Eucharis,  Bolina,  etc.,  can  hardly  be 
distinguished  from  Pleurobrachia,  and  undergoes  therefore  considerable 
changes  after  hatching. 

Eucharis  multicornis  while  still  in  the  larval  condition  is  stated  by  Chun 
to  become  sexually  mature. 

The  new  genus  Ctenaria  recently  described  by  Haeckel, 
which  is  intermediate  between  the  Ctenophora  and  the  Medusae 
clearly  proves  that  the  Ctenophora  are  more  closely  related  to 
the  Medusae  than  to  the  Actinozoa ;  but  their  development, 
especially  the  presence  of  a  stomodaeum,  shews  that  they  have 
affinities  (in  spite  of  the  rudimentary  velum  of  Ctenaria)  with  the 
Acraspedote  as  well  as  with  the  Craspedote  Medusae ;  and  it 
may  be  noted  that  the  Acraspeda  have  undoubted  affinities  with 
the  Actinozoa. 

Summary  and  general  considerations. 

Even  in  the  adult  condition  the  lower  forms  of  Ccelenterata 
do  not  rise  in  complexity  much  beyond  a  typical  gastrula. 
Ontogeny  nevertheless  brings  clearly  to  light  the  existence  of  a 
larval  form — the  planula — which  recurs  with  fair  constancy 
amongst  all  the  groups  except  the  Ctenophora. 

We  are  probably  justified  in  assuming  that  the  planula  is  a 
repetition  of  a  free  ancestral  form  of  the  Ccelenterata.  The  pla- 
nula, as  it  most  frequently  occurs,  is  a  two-layered  ciliated  nearly 
cylindrical  organism,  with  at  most  a  rudimentary  digestive  cavity 
hollowed  out  in  the  inner  layer,  and  as  a  rule  no  mouth.  In  the 
outer  layer  are  numerous  thread-cells. 

How  many  of  these  characters  did  the  ancestral  planula  possess  ?  I  think 
it  is  not  unreasonable  to  assume  that  the  only  two  characters  about  which 
there  can  be  much  doubt  are  the  rudimentary  condition  of  the  digestive 
cavity  and  the  absence  of  a  mouth.  Paradoxical  as  it  may  seem,  it  appears  to 
me  not  impossible  that  the  Ccelenterata  may  have  had  an  ancestor  in  which  a 
digestive  tract  was  physiologically  replaced  by  a  solid  mass  of  amoeboid  cells. 


COELENTERATA.  179 


This  ancestor  was  perhaps  common  to  the  Turbellarians  also.  The  constant 
presence  of  thread-cells  in  the  inner  layer  of  their  epiblast  fits  in  with  their 
derivation  from  a  form  similar  to  the  planula.  While  the  solid  parenchy- 
matous  digestive  canal  of  Convoluta  and  Schizoprora  and  other  forms 
amongst  the  Turbellarians,  though  very  probably  secondary,  may  perhaps 
be  explained  by  such  a  view  of  their  origin. 

The  planula  in  its  primitive  condition  is  not  bilaterally  symmetrical,  but 
frequently,  as  amongst  the  Actinozoa,  it  becomes  flattened  on  two  sides  before 
undergoing  its  conversion  into  the  adult  form.  Perhaps  the  bilateral  form 
of  planula  is  the  starting  point  both  for  the  Ccelenterata  and  the  Turbellaria. 
In  this  connection  the  peculiar  unilateral  development  of  a  tentacle  in 
Scyphistoma  and  Actinia  should  be  noted. 

The  planula  occurs  in  the  majority  of  sessile  forms  of  Hydro- 
zoa  except  the  Tubularidae  and  Hydra.  It  is  also  characteristic 
of  the  Trachy medusae  and  Siphonophora.  Amongst  the  Acras- 
peda  it  is  also  present,  but  has  an  exceptional  mode  of  ontogeny 
which  is  discussed  in  connection  with  the  germinal  layers. 

It  is  characteristic  both  of  the  Octocoralla  and  Hexaco- 
ralla,  but  is  not  found  in  the  Ctenophora. 

In  the  Tubularidae  and  in  Hydra  an  abbreviated  develop- 
ment leads  no  doubt  to  the  absence  of  a  free  planula  stage,  and 
the  absence  of  a  larval  form  amongst  the  Ctenophora  may,  as  has 
already  been  stated,  be  probably  explained  in  the  same  way. 

The  Ccelenterata  of  all  the  Metazoa  are  characterized  by  the 
greatest  simplicity  in  the  arrangement  of  their  germinal  layers  ; 
and  for  this  reason  very  considerable  interest  attaches  to  the 
mode  of  formation  of  the  layers  amongst  them.  Two  germinal 
layers  are  constantly  found,  which  correspond  in  a  general  way 
to  the  epiblast  and  hypoblast.  It  might  have  been  anticipated 
that  a  certain  amount  of  uniformity  would  have  existed  in  the 
mode  of  formation  of  the  layers.  This  however  is  not  the  case. 
In  perhaps  the  majority  of  forms  they  become  differentiated  by 
a  process  of  delamination,  but  in  a  not  inconsiderable  minority 
the  two  layers  owe  their  origin  to  an  invagination. 

Delamination  is  constant  (with  the  doubtful  exception  of 
some  Tubularidae)  amongst  the  Hydromedusae  and  Siphono- 
phora. It  is  perhaps  in  the  main  characteristic  of  the  Actino- 
zoa. 

Invagination  by  embole  takes  place,  so  far  as  is  known,  con- 
stantly amongst  the  Acraspeda  and  frequently  amongst  the 

12 — 2 


,gO  SUMMARY. 


Actinozoa ;  and  an  epibolic  invagination  is  characteristic  of  the 
Ctenophora. 

If  confidence  is  to  be  placed  in  the  recorded  observations  on 
which  this  summary  is  founded,  and  there  is  no  reason  why  in  a 
general  way  it  should  not  be  so  placed,  the  conclusion  is  inevit- 
able that  of  the  above  modes  of  development  the  one  must  be 
primitive  and  the  other  a  derivative  from  it,  for,  if  this  conclusion 
be  not  accepted,  the  absolutely  inadmissible  hypothesis  of  a 
double  origin  for  the  Coelenterata  would  have  to  be  adopted. 

Two  questions  arise  from  these  considerations  : — 

(1)  Which  is  the  primitive,  delamination  or  invagination  ? 

(2)  How  is  the  one  of  these  to  be  derived  from  the  other  ? 

There  is  a  great  deal  to  be  said  in  favour  of  both  delamin- 
ation and  invagination ;  but  it  will  be  convenient  to  defer  all 
discussion  of  the  question  to  the  general  chapter  on  the  forma- 
tion of  the  layers  throughout  the  animal  kingdom. 

The  hypoblast  cells  are  often  filled  with  yolk  material,  and 
secondary  modifications  are  thus  produced  in  the  development. 
The  most  important  examples  of  such  modifications  are  found  in 
the  Siphonophora  and  Ctenophora. 

In  the  simplest  forms  amongst  the  Hydrozoa  there  is  no  trace 
of  a  third  layer  or  mesoblast.  The  epiblast  is  typically  formed, 
as  was  first  shewn  by  Kleinenberg,  of  an  epithelial  layer  and  a 
subepithelial  interstitial  layer  of  cells.  The  cells  of  the  former 
are  frequently  produced  into  muscular  or  nervous  tails,  and  those 
of  the  latter  give  rise  to  the  thread  cells  and  generative  organs 
and  in  some  cases  to  muscles1.  In  many  cases,  amongst  all  the 
Ccelenterate  groups,  and  constantly  amongst  the  Ctenophora  the 
epiblast  is  simplified  and  reduced  to  a  single  layer.  The  hypo- 
blast  undergoes  in  most  cases  no  such  differentiation  but  simply 
forms  a  glandular  layer  lining  the  gastric  chamber  and  its  pro- 
longations into  the  tentacles  ;  but  in  the  Actinozoa  it  appears  to 
give  rise  to  muscles,  and  strong  evidence  has  been  brought  for- 
ward to  shew  that  in  some  groups  it  gives  rise  to  the  generative 
organs. 

Between  the  epiblast  and  hypoblast  a  structureless  lamella 
appears  always  to  be  interposed. 

1  The  questions  relating  to  the  generative  organs  of  the  Coelenterata  are  dealt  with 
in  the  second  part  of  this  work. 


CCELENTERATA.  l8l 


In  many  Coelenterata  further  differentiations  of  the  epiblast 
are  present.  In  many  forms  the  layer  gives  rise  to  a  hard  exter- 
nal skeleton.  This  is  most  widely  spread  amongst  the  Hydrozoa, 
where  in  the  majority  of  cases  it  takes  the  form  of  the  horny 
perisarc,  and  in  the  Hydrocoralla  (Millepora  and  Stylasteridse) 
of  a  hard  calcareous  skeleton.  The  skeleton  in  these  forms, 
though  closely  resembling  the  mesoblastic  skeleton  of  the  Actin- 
ozoa, has  been  shewn  by  Moseley  (164)  to  be  epiblastic. 

In  the  Actinozoa  an  epiblastic  skeleton  is  exceptional,  and 
according  to  most  authorities  absent.  Quite  recently  however 
Koch  (167)  has  found  that  the  axial  branched  skeleton  of  most 
of  the  Gorgonidae,  viz.  the  Gorgoninae  and  Isidinae,  is  separated 
from  the  ccenosarc  by  an  epithelium,  which  he  believes  to  be 
epiblastic,  and  to  which  no  doubt  the  axial  skeleton  owes  its 
origin.  A  similar  epithelium  surrounds  the  axis  of  the  Penna- 
tulidae. 

In  the  Medusae  the  epiblast  also  gives  rise  to  a  central 
nervous  system,  which  however  continues  to  form  a  constituent 
part  of  the  layer,  and  to  the  organs  of  special  sense1. 

A  special  differentiation  of  the  hypoblast  is  found  in  the 
solid  axis  of  the  tentacles.  This  axis  replaces  the  gastric 
prolongation  found  in  many  forms,  and  the  cells  composing 
it  differentiate  themselves  into  a  chorda-like  tissue,  which  has 
a  skeletal  function,  and  is  no  longer  connected  with  nutrition. 
This  axis  is  placed  by  many  morphologists  amongst  the  meso- 
blastic structures. 

In  all  the  higher  Ccelenterata  certain  tissues  become  inter- 
posed between  the  epiblast  and  hypoblast,  which  may  be  classi- 
fied together  as  the  mesoblast. 

The  most  important  of  these  are 

(1)  The  various  distinct  muscular  layers. 

(2)  The  gelatinous  tissue  of  the  Medusae  and  Ctenophora. 

(3)  The  skeletogenous  tissue  of  the  Actinozoa. 

In  most  cases  the  muscular  fibres  are  connected  with  epithe- 
lial cells,  but  in  certain  forms  amongst  the  Medusae  and  in  the 
majority  if  not  all  the  Actinozoa  they  constitute  a  distinct  layer, 
sometimes  separated  from  the  epiblast  by  a  structureless  mem- 

1  The  differentiation  of  the  nervous  and  muscular  systems  in  the  Hydrozoa  is 
treated  of  in  the  second  part  of  this  work. 


ALThK  NATIONS   OF   GENERATIONS. 

branc,  ALquorca  Mitrocoma.  Such  layers  when  on  the  outer 
of  the  membrane  separating  epiblast  and  hypoblast  are 
undoubtedly  epiblastic  in  origin,  but  in  some  cases  amongst  the 
Actinozoa  they  adjoin  the  hypoblast,  and  are  very  probably 
derived  from  this  layer. 

The  origin  of  the  gelatinous  tissue  is  still  involved  in  much 
obscurity. 

It  originates  as  a  homogeneous  layer  between  epiblast  and 
hypoblast,  which  in  the  Hydromedusae  never  becomes  cellular 
though  traversed  by  elastic  fibres. 

In  the  Acraspeda  it  contains  anastomosing  cells  in  the  main 
apparently  (Claus)  derived  from  the  hypoblast,  and  in  the  Cteno- 
phora  it  is  richly  supplied  with  muscular  stellate  cells  for  the 
most  part  of  epiblastic  origin,  though  some  are  stated  by  Chun 
to  come  from  the  hypoblast.  On  the  whole  it  seems  probable, 
that  the  gelatinous  tissue  may  be  regarded  as  a  product  of  both 
layers ;  and  there  are  some  grounds  for  thinking  that  it  is  an 
immense  development  of  the  membrane  always  interposed  be- 
tween the  two  primary  layers.  It  must  however  be  borne  in  mind 
that  a  membrane,  regarded  by  the  Hertwigs  as  the  equivalent  of 
the  ordinary  membrane  between  the  epiblast  and  hypoblast,  can 
be  usually  demonstrated  on  both  surfaces  of  the  gelatinous 
tissues  in  Medusae.  The  skeletogenous  layer  of  the  Actinozoa 
is  probably  the  morphological  homologue  of  the  gelatinous 
tissue ;  but  the  evidence  we  have  is  on  the  whole  in  favour  of  the 
connective-tissue  cells  it  contains  being  epiblastic  in  origin.  It 
gives  rise  to  the  skeleton  of  the  Hexacoralla,  to  the  spicular 
skeleton  of  Alcyonium,  the  axial  skeleton  of  Corallium,  and  the 
skeleton  of  the  Helioporidae  and  Tubiporidae. 


Alternations  of  generations. 

Alternation  of  generations  is  of  common  occurrence  amongst 
the  Hydrozoa,  and  something  analogous  to  it  has  been  found  to 
take  place  in  Fungia  amongst  the  Actinozoa.  It  is  not  known 
to  occur  in  the  Ctenophora. 

The  chief  interest  of  its  occurrence  amongst  the  Hydro- 
medusae  and  Siphonophora  is  the  fact  that  its  origin  can  be 


CCELENTERATA.  183 


traced  to  a  division  of  labour  in  the  colonial  systems  of  zooids 
so  characteristic  of  these  types. 

In  the  Hydromedusae  an  interesting  series  of  relations 
between  alternation  of  generations  and  the  division  of  the  zooids 
into  gonophores  and  trophosomes  can  be  made  out.  In  Hydra 
the  generative  and  nutritive  functions  are  united  in  the  same 
individual.  The  generative  swellings  in  these  forms  cannot,  as 
has  been  ably  argued  by  Kleinenberg,  be  regarded  as  rudimen- 
tary gonophores,  but  are  to  be  compared  to  the  generative  bands 
developed  in  the  Medusae  around  parts  of  the  gastro-vascular 
system.  A  condition  like  that  of  Hydra,  in  which  the  ovum 
directly  gives  rise  to  a  form  like  its  parent,  is  no  doubt  the 
primitive  one,  though  it  is  not  so  certain  that  Hydra  itself  is  a 
primitive  form.  The  relation  of  Hydra  to  the  Tubularidae  and 
Campanularidae  may  best  be  conceived  by  supposing  that  in 
Hydra  most  ordinary  buds  did  not  become  detached,  so  that  a 
compound  Hydra  became  formed  ;  but  that  at  certain  periods 
particular  buds  retained  their  primitive  capacity  of  becoming 
detached  and  subsequently  developed  generative  organs,  while 
the  ordinary  buds  lost  their  generative  function. 

It  would  obviously  be  advantageous  for  the  species  that  the 
detached  buds  with  generative  organs  should  be  locomotive,  so 
as  to  distribute  the  species  as  widely  as  possible,  and  such  buds 
in  connection  with  their  free  existence  would  naturally  acquire  a 
higher  organization  than  the  attached  trophosomes.  It  is  easy 
to  see  how,  by  a  series  of  steps  such  as  I  have  sketched  out,  a 
division  of  labour  might  take  place,  and  it  is  obvious  that  the 
embryos  produced  by  the  highly  organized  gonophores  would 
give  rise  to  a  fixed  form  from  which  the  fixed  colony  would  be 
budded.  Thus  an  alternation  of  generations  would  be  estab- 
lished as  a  necessary  sequel  to  such  a  division  of  labour.  To 
test  the  above  explanation  it  is  necessary  to  review  the  main 
facts  with  reference  to  alternations  of  generations  amongst  the 
Hydromedusae. 

Hydromedusae1.  In  many  instances  amongst  the  Tubula- 
ridae, Sertularidae  and  Campanularidae  medusiform  buds  are 
produced  which  become  detached  and  develop  sexual  organs. 

1  For  a  full  account  of  this  subject  the  reader  is  referred  to  the  beautiful  memoir 
of  Allman  (No.  149). 


IS4  ALTERNATIONS  OF  GENERATIONS. 

Such  Medusa  arc  divided  into  two  great  groups,  the  Ocellata  and 
Vesiculata,  according  to  the  characters  of  the  marginal  sense  organs. 
In  the  Ocellata  the  sense  organs  have  the  form  of  eyes,  and  in  the  Vesiculata 
of  auditory  vesicles.  The  latter  seem  to  be  usually  budded  off  from  the 
Campanularia  stocks,  and  the  generative  organs  extend  in  folded  bands  over 
the  radial  canals.  These  bands  have  been  regarded  by  Allman  as  composed 
of  rudimentary  gonophores,  and  he  called  the  Medusae  which  give  rise  to 
them  blastochemes.  He  regards  them  as  representing  a  more  complicated 
type  of  alternation  of  generations  with  three  instead  of  two  generations  in 
the  series.  The  Hertwigs  have  brought  what  appear  to  me  conclusive 
grounds  for  rejecting  this  view,  and  have  demonstrated  that  the  generative 
organs  of  these  types  resemble  those  of  ordinary  Medusas. 

In  many  forms  the  medusiform  buds  though  fully  developed 
do  not  become  detached ;  whether  detached  or  not  they  are 
known  as  phanerocodonic  gonophores.  In  other  forms 
again  buds  which  begin  as  if  they  were  going  to  form  Medusae 
never  reach  that  condition  but  remain  permanently  in  an  unde- 
veloped state.  They  have  been  called  by  Allman  adelocodonic 
gonophores. 

In  all  the  above  cases  two  generations  at  the  least  interpose 
between  the  successive  sexual  periods,  viz. : — 

(1)  A  trophosome  produced  directly  from  the  ovum. 

(2)  A  gonophore  budded  from  this. 

In  a  very  large  number  of  types  the  gonophores  do  not 
develop  directly  on  the  hydroid  stem,  but  arise  on  specially 
modified  zooids  resembling  rudimentary  trophosomes  which 
have  been  named  blastostyles  by  Allman.  On  the  sides  of 
each  blastostyle  a  series  of  gonophores  usually  becomes  de- 
veloped. The  blastostyles  either  remain  exposed  as  in  all  the 
Gymnoblastic  or  Tubularian  Hydroids,  or  as  in  all  the  Calypto- 
blastic  Hydroids  (Sertularidse  and  Campanularidae)  they  become 
invested  by  a  special  case — known  as  the  gonangium — which 
is  formed  of  perisarc  lined  by  epiblast.  In  the  forms  with 
blastostyles  three  generations  interpose  between  the  successive 
stages  of  sexual  reproduction,  (i)  the  trophosome  developed 
directly  from  the  ovum,  (2)  the  blastostyle  budded  from  this,  (3) 
the  gonophore  budded  from  the  blastostyle. 

Such  being  the  main  facts,  in  order  to  prove  that  the  existing  condition 
of  polymorphism  amongst  the  Hydromedusae  is  to  be  explained  as  hypo- 
t helically  suggested  above,  it  is  still  necessary  to  shew  that  (i)  the  free 


CCELENTERATA.  185 


medusiform  gonophores  are  really  only  modified  trophosomes,  or  rather  that 
the  trophosomes  and  gonophores  are  both  modifications  of  some  common 
type,  and  (2)  that  the  fixed  so-called  adelocodonic  gonophores  are  retrograde 
derivatives  of  the  free  medusiform  gonophores.  Unless  these  points  can  be 
established  it  might  be  maintained  that  the  Medusae  were  special  zooids, 
developed  de  novo  and  not  by  a  modification  of  trophosome  zooids.  To 
demonstrate  these  propositions  at  length  would  carry  me  too  far  into  the 
region  of  simple  Comparative  Anatomy,  and  I  content  myself  with  referring 
the  reader  to  a  discussion  of  the  Hertwigs  (No.  146,  p.  62)  where  the  first 
point  appears  to  me  fully  established.  With  reference  to  the  second  point  I 
will  only  say  that  the  structure  and  development  of  the  adelocodonic  gono- 
phores can  only  be  explained  on  the  assumption  that  they  are  retrograde 
forms  of  the  phanerocodonic  gonophores,  and  that  the  opposite  view,  that 
the  phanerocodonic  gonophores  are  derived  from  the  adelocodonic,  leads  to 
a  series  of  untenable  positions. 

The  Trachymedusae,  as  has  been  shewn  above,  develop  directly.  They 
are  probably  derived  from  gonophores  in  which  the  trophosome  has  dis- 
appeared from  the  developmental  cycle. 

To  sum  up,  three  types  of  development  are  found  amongst 
the  Hydromedusae. 

(1)  No   alternations    of   generations.     Permanent   form,    a 
sexual  trophosome.     Ex.  Hydra. 

(2)  Alternations  of  generations.     Trophosome  fixed,  gono- 
phore  free  or  attached.     Ex.  Gymnoblastic  and  Calyptoblastic 
Hydroids,  and  Hydrocoralla. 

(3)  No   alternations    of   generations.     Permanent   form,    a 
sexual  Medusa.     Ex.  Trachymedusse. 

Siphonophora.  In  the  Siphonophora  alternations  of  gener- 
ations take  place  in  the  same  way  as  in  the  Hydromedusae,  but 
the  starting  point  appears  to  be  a  Medusa.  The  gonophores 
may  remain  fixed  or  become  detached. 

Acraspeda.  With  the  exception  of  Pelagia  and  Lucernaria, 
in  which  the  development  involves  a  simple  metamorphosis,  all 
the  Acraspeda  undergo  a  form  of  alternations  of  generations. 
The  ovum,  as  already  described,  develops  into  a  fixed  form — the 
Scyphistoma — which  increases  asexually  by  normal  budding, 
and  can  even  form  a  permanent  colony. 

The  formation  of  the  sexual  Medusa  form  takes  place  by  a 
kind  of  strobilization  of  the  body  of  the  fixed  Scyphistoma. 
A  series  of  transverse  constrictions  becomes  formed  round 
the  body  below  the  mouth,  dividing  it  up  into  corresponding 


186 


ALTERNATIONS   OF   GENERATIONS. 


rings,  each  of  which  eventually  gives  rise  to  a  Medusa  known 
as  an  Ephyra  (fig.  85).  In  each 
of  these  rings  is  a  dilation  of  the 
stomach,  and  a  section  of  each  of 
the  four  rudimentary  mesenteries 
described  in  connection  with  the  de- 
velopment of  the  Scyphistoma.  As 
the  constrictions  become  deeper  the 
segments  of  the  body  between  them 
become  disc-like,  and  their  edges 
are  produced  into  eight  lobes  con- 
taining prolongations  of  the  gastric 
cavity  (fig.  85  C).  The  lower  sur- 
face of  each  disc,  which  forms  the 
future  aboral  surface  of  the  Medusa, 
becomes  convex,  in  part  owing  to 
the  development  of  gelatinous  tis- 
sue. On  the  opposite  surface  a 
muscular  layer  becomes  developed. 
the  body  of  the  Scyphistoma  gradually  grows  in  length  and 
continues  to  be  segmented,  so  that  a  series  of  Ephyrae  are 
uninterruptedly  formed,  of  which  those  near  the  base  are  the 
youngest.  The  original  terminal  ring  of  tentacles  of  the 
Scyphistoma  gradually  atrophies. 

In  the  further  development  of  the  Ephyrae  each  of  their  eight 
lobes  becomes  bifid  at  its  extremity. 

As  the  Ephyrae  successively  reach  this  condition  they  be- 
come detached,  and  by  a  series  of  remarkable  changes,  amount- 
ing almost  to  a  metamorphosis,  and  accompanied  by  an  enor- 
mous growth  in  size,  reach  the  adult  condition. 

The  alternation  of  generations  in  the  Acraspeda  cannot  be 
quite  so  simply  explained  as  in  the  Hydromedusae,  though  the 
principle  is  probably  the  same  in  the  two  cases. 

Actinozoa.  Amongst  the  Actinozoa  there  occurs  in  Fungia  a 
peculiar  process  which  is,  as  shewn  by  Semper  (171),  in  many 
ways  analogous  to  alternations  of  generations1.  From  the  larva 
a  nurse-stock  is  developed,  at  the  end  of  which  a  cup-like  coral 


B 


FIG.   85.    THREE    STAGES    IN 

THE    ALTERNATIONS    OF    GENERA- 
TIONS OF  AURELIA  AURITA.     (From 

Gegenbaur.) 

A.  Polype  stage. 

B.  Commencing  strobilization. 

C.  Completed  strobilization. 

During  the  above  process 


Vide  also  Moselcy.     Notes  by  a  Naturalist  of  the  Challenger •,  pp.  524  and  525. 


CCELENTERATA.  1 87 


resembling  the  adult  is  formed  as  a  bud.  The  bud  becomes 
detached  and  then  gives  rise  to  a  permanent  sexual  Fungia. 
From  the  nurse-stock  there  is  formed  however  a  fresh  bud  at 
the  centre  of  the  scar  left  on  the  detachment  of  the  old  one. 
The  fresh  bud  eventually  becomes  separated  from  the  nurse-stock 
leaving  a  small  portion  of  its  stem  behind  ;  each  succeeding  bud 
similarly  leaves  a  small  portion  of  its  stem,  so  that  the  nurse- 
stock  eventually  acquires  a  jointed  appearance.  In  the  above 
process  we  clearly  have,  as  in  the  Hydromedusae,  a  non-sexual 
form — the  nurse-stock — produced  directly  from  the  larva,  giving 
rise  by  budding  to  a  sexual  form ;  all  the  conditions  of  an  alter- 
nation of  generations  are  therefore  fulfilled.  It  seems  however 
possible  that  the  nurse-stock  itself  may  eventually  become  sexual. 

BIBLIOGRAPHY. 
Coelenterata.     General. 

(145)  Alex.   Agassi z.     Illustrated   Catalogue  of  the  Museum  of  Comparative 
Anatomy  at  Harvard  College,  No.  II.  American  Acalephse.     Cambridge,  U.  S.,  1865. 

(146)  O.  and  R.  Hertwig.     Der  Organismus  d.  Medusa  u.  seine  Stellung  z. 
Keimbldttertheorie.     Jena,  1878. 

(147)  A.    Kowalevsky.     " Untersuchungen   lib.  d.   Entwicklung   d.  Coelente- 
raten."     Nachrichten  d.   kaiser.  Gesell.  d.  Freunde  d.  Naturer  kenntniss  d.  Anthro- 
pologie  u.  Ethnographic.     Moskau,  1873.     (Russian.)     For  abstract  vide  Jahresberichte 
d.  Anat.  u.  Phys.  (Hoffman  u.  Schwalbe),  1873. 

Hydrozoa. 

(148)  L.  Agassiz.     Contributions  to  the  Natural  History  of  the  United  Slates  of 
America.     Boston,  1862.     Vol.  IV. 

(149)  G.  J.  Allman.    A  Monograph  of  the  Gymnoblastic  or  Tubularian  Hydroids. 
Ray  Society,  1871-2. 

(150)  G.  J.  Allman.     "On  the  structure  and  development  of  Myriothela."    Phil. 
Trans.,  Vol.  CLXV.  p.  2. 

(151)  P.  J.  van  Beneden.     "Mem.  sur  les  Campanulaires  de  la  Cote  d'Ostende 
considered  sous  le  rapport  physiologique,   embryogenique,  et  zoologique."     Now. 
Mem.  de  ?  Acad.  de  Srux.,  Tom.  xvn.  1844. 

(152)  P.  J.  van  Beneden.     "  Recherches  sur  PEmbryog&ne  des  Tubulaires  et 
1'histoire  naturelle  des  differents  genres  de  cette  famille  qui  habitent  la  Cote  d'Ostende." 
Nouv.  Mem.  de  F  Acad.  de  Brux.,  Tom.  xvn.  1844. 

(153)  C.  Claus.     " Polypen  u.  Quallen  d.  Adria."     Denk.  d.  math.-naturwiss. 
Classed,  k.  k.  Akad.  d.  Wiss.  Wien,  Vol.  xxxvui.  1877. 

(154)  J.    G.  Daly  ell.     Rare  and  Remarkable  Animals  of  Scotland.     London, 
1847. 

(155)  H.  Fol.     "Die  erste  Entwicklung  d.  Geryonideneies. "     Jenaische  Zeit- 
schrift,  Vol.  VII.  1873. 

(156)  Carl   Gegenbaur.     Zur  Lehre  vom    Generationswechsel  und  der  Fort- 
pflanzung  bei  Medusen  und  Polypen.     WUrzburg,  1854. 


1 88  BIBLIOGRAPHY. 


(157)  Thomas  Hincks.  "On  the  development  of  the  Hydroid  Polypes,  Clava- 
lella  and  Stauridia;  with  remarks  on  the  relation  between  the  Polype  and  the  Medu- 
soid,  and  between  the  Polype  and  the  Medusa."  Brit.  Assoc.  Rep.,  1861. 

(168)  E.  Haeckel.  Zur  Entwicklungsgeschichte  d.  Siphonophoren.  Utrecht, 
1869. 

(159)  Th.  H.Huxley.     Oceanic  Hydrozoa.     Ray  Society,  1858. 

(160)  Geo.   Johnston.     A    History  of  British  Zoophytes.     Edin.  1838.     2nd 
Edition,  1847. 

(161)  N.    Kleinenberg.     Hydra,    eine   anatomisch-entwicklungsgeschichtliche 
Untersuchung.     Leipzig,  1872. 

(162)  El.   Metschnikoff.      "Ueber  die  Entwicklung  einiger  Ccelenteraten. " 
Bull,  tie  CAcad.  dt  St  Petersbourg,  XV.  1870. 

(163)  El.  Metschnikoff.     "Studien  iiber  Entwicklungsgeschichte  d.  Medusen 
u.  Siphonophoren.     Zeit.f.  wiss.  Zool.,  Bd.  xxiv.  1874. 

(164)  H.  N.  Moseley.     "On  the  structure  of  the  Stylasteridae."     Phil.  Trans. 
1878. 

(165)  F.  E.  Schulze.     Ueber  den  Bau  und  die  Entwicklung  von  Cordylophora 
lacustris.     Leipzig,  1871. 

Actinozoa. 

(166)  Al.  Agassiz.     "Arachnitis  (Edwarsia)  brachiolata."    Proc.  Boston  Nat. 
Hist.  Society,  1860. 

(167)  Koch.     "Das  Skelet  d.  Alcyonarien."    Morpholog.  Jahrbuch,  Bd.  iv. 
1878. 

(168)  A.  Kowalevsky.     "Z.  Entwicklung  d.  Alcyoniden,  Sympodium  coral- 
loides  und  Clavularia  crassa."     Zoologischtr  Anzeiger,  No.  38,  1879. 

(169)  H.  LacazeDuthiers.     Histoire  nat.  du  Corail.     Paris,  1864. 

(170)  H.  Lacaze  Duthiers.     "  Developpement  des Coralliaires. "    Archives de 
Zoologie  experimentale  et  generate,  Vol.  i.  1872  and  Vol.  II.  1873. 

(171)  C.  Semper.     "  Ueber  Generationswechsel  bei  Steinkorallen  etc."    Zeit. 
/.  wiss.  Zool.,  Bd.  xxn.  1872. 

Ctenophora. 

(172)  Alex.  Agassiz.     " Embryology  of  the  Ctenophorse. "     Mem.ofthe  Amer. 
Acad.  of  Arts  and  Sciences,  Vol.  x.  No.  in.  1874. 

(173)  G.  J.  Allman.     "Contributions  to  our  knowledge  of  the  structure  and 
development  of  the  Beroidae."    Proc.  Roy.  Soc.  Edinburgh,  Vol.  iv.  1862. 

(174)  C.   Chun.     Das   Nervensystem   u.   die    Musculatur  d.    Rippenquallcn." 
Abkand.  d.  Senkenberg.  Gesellseh.,  B.  XI.  1879. 

(175)  C.  Claus.     "Bemerkungen  u.  Ctenophoren  u.  Medusen."    Zeit.  f.  wiss. 
Zool.,  xiv.  1864. 

(176)  H.  Fol.     Ein  Beitrag  z.  A  nat.  u.  Entwickl.  einiger  Rippenquallen.  1869. 

(177)  C.   Gegenbaur.     "Studien  ii.  Organis.   u.    System    d.    Ctenophoren." 
Archivf.  Natnrgesch.,  xxil.  1856. 

(178)  A.  Kowalevsky.     "Entwicklungsgeschichte  d.  Rippenquallen."     Mem. 
Acad.  St  Petersbourg,  VII.  serie,  Tom.  x.  No.  4.  1866. 

(179)  J.    Price.     "Embryology  of  Ciliogrades."    Proceed,  of  British  Assoc., 
1846. 

(180)  C.  Semper.     "Entwicklung  d.   Eucharis   multicornis."    Zeit.  f.  wiss. 
Zool.,  Vol.  tx.  1858. 


CHAPTER  VII. 

PLATYELMINTHES '. 
TURBELLARIA. 

ALTHOUGH  there  is  perhaps  no  group  in  the  animal  kingdom 
the  ontogeny  of  which  would  better  repay  a  thorough  investiga- 
gation  than  the  Turbellarians,  yet  the  difficulties  to  be  overcome 
have  hitherto  proved  too  great. 

The  fresh-water  Rhabdocoela  and  Dendroccela  do  not  under- 
go any  metamorphosis,  and  leave  the  ovum  in  a  condition  in 
which  they  cannot  easily  be  distinguished  in  their  general  appear- 
ance from  Infusoria.  Many  marine  Dendroccela  also  develop 
directly,  while,  as  was  first  shewn  by  Joh.  Miiller,  other  marine 
Dendroccela  undergo  a  more  or  less  complicated  metamorphosis. 

Marine  Dendroccela.  Of  the  marine  Dendroccela  which  do 
not  undergo  a  metamorphosis  the  form  most  fully  worked  out  is 
Leptoplana  tremellaris — (vide  Keferstein,  No.  187,  and  Hallez, 
No.  185). 

The  ova  are  surrounded  by  large  albuminous  capsules 
secreted  by  a  special  gland.  They  are  laid  a  great  number  at  a 

1  I.    Turbellaria. 

1.  Dendrocoela. 

2.  Rhabdocoela. 

II.    Nemertea. 

1.  Anopla. 

2.  Enopla. 

Hi.    Trematoda. 

1.  Distomese. 

2.  Polystomeae. 

iv.    Cestoda, 


TURBELLARIA. 


time,  and  adhere  together  so  as  to  form  masses  not  unlike  the 
spawn  of  nudibranchiate  Molluscs. 

Within  the  egg-capsule  the  ovum  floats  freely  and  undergoes 
a  segmentation  similar  in  many  respects  to  the  characteristic 
molluscan  type.  The  ovum  divides  into  two,  and  then  into  four 
parts,  from  each  of  which  a  small  segment  is  then  separated  off. 
The  four  small  segments,  which  appear  to  give  rise  to  the  epi- 
blast,  increase  in  number  by  division  and  gradually  envelop  the 
large  segments1;  so  that  an  epibolic  invagination  clearly  takes 
place.  Between  the  small  and  the  large  cells  is  a  small  segmen- 
tation cavity,  fig.  86  A  and  B.  At  the  time  when  twelve  epiblast 
cells  are  present,  each  of  the  four  large  cells  divides  into  two  un- 
equal parts  (Hallez),  fig.  86  A.  In  this  way  four  large  (Jiy)  and 
four  small  cells  (;«)  are  formed.  The  latter  are  placed  at  the 
opposite  pole  of  the  ovum  to  the  epiblast  cells,  and  give  rise  to 
the  mesoblast,  while  the  four  large  cells  remain  as  the  hypoblast. 

In  the  course  of  the  enclosure  of  the  hypoblast  cells  by  the 


FIG.  86.    SECTIONS  THROUGH  THE  OVUM  OF  LEPTOPLANA  TREMELLARIS  IN  THREE 
STAGES  OF  DEVELOPMENT.     (After  Hallez.) 

ep.  epiblast;  m.  mesoblast;  hy.  yolk  cells  (hypoblast);  bl.  blastopore. 

epiblast,  the  mesoblast  cells  gradually  travel  towards  the  forma- 
tive pole  (fig.  86  B).  In  the  process  they  become  first  of  all 
divided  so  as  to  form  four  linear  streaks,  and  finally  unite  into  a 
continuous  layer  between  the  epiblast  and  hypoblast,  which 
obliterates  the  segmentation  cavity  (fig.  86  C,  m). 

Before  the  completion  of  the  epibole  a  closely  packed  layer 
of  fine  cilia  appears,  which  causes  a  rotation  of  the  embryo  within 
the  egg-capsule.  During  the  above  changes  a  fifth  hypoblast 
cell  is  formed  by  the  division  of  one  of  those  already  present ; 
and  at  a  later  period  four  of  the  hypoblast  cells  give  rise  within 

1   It  is  probable,  though  it  has  not  been  observed,  that  the  growth  of  the  layer  of 
small  rclls  is  a^isted  by  the  formation  of  fresh  cells  from  the  hypoblast  spheres. 


PLATYELMINTHES.  191 


the  nearly  closed  blastoporic  area  to  four  small  cells.  In  con- 
nection with  these  cells  a  complete  hyploblastic  wall  becomes 
subsequently  established,  which  encloses  the  original  large  hypo- 
blast  cells.  The  latter  then  become  resolved  into  a  vitelline 
mass. 

From  a  comparison  with  other  types  it  may  be  regarded  as 
probable  that  the  enteric  wall  originates  by  a  process  of  continu- 
ous budding  off  of  small  cells  from  the  large  cells,  which  com- 
mences with  the  formation  of  the  four  cells  above  mentioned. 

The  blastopore  becomes  nearly  obliterated,  but  whether  it 
gives  rise  to  the  mouth,  which  is  formed  in  the  same  place,  has 
not  been  determined.  In  front  of  the  mouth  a  small  and  very 
transitory  rudiment  of  an  upper  lip  makes  its  appearance.  The 
protrusible  pharynx  is  stated  by  Hallez  to  arise  as  an  hypoblastic 
bud,  while  its  sheath  has  an  epiblastic  origin.  Two  pairs  of 
eyes  and  the  supra-cesophageal  ganglia  also  become  early 
developed. 

The  peripheral  ciliated  layer  of  small  cells  becomes  divided 
into  two  strata,  of  which  the  outer  remains  ciliated  and  forms 
the  true  epiblast :  the  inner  probably  forms  the  cutis.  In  it  are 
developed  rod-like  bodies,  which  seem  to  be  homologous  with 
the  thread  cells  of  the  Ccelenterata,  so  that  if  the  views  put 
forward  in  the  previous  chapter  as  to  the  similarity  of  the  turbel- 
larian  and  ccelenterate  larvae  are  correct,  the  cutis  corresponds 
with  the  deeper  layer  of  the  ccelenterate  epiblast.  The  meso- 
blast,  like  the  epiblast,  becomes  divided  into  two  strata.  The 
outer  one  is  stated  to  form  the  circular  and  longitudinal  muscles; 
the  inner  one  to  give  rise  to  a  muscular  reticulum,  the  spaces 
within  which  constitute  the  parenchymatous  body  cavity. 

The  later  changes  are  not  of  great  importance.  At  a  period  slightly 
after  the  formation  of  the  mouth  and  ganglia  two  pairs  of  stiff  hairs  become 
formed  at  the  sides  of  the  body.  The  embryo  has  by  this  time  grown  so  as 
to  fill  up  its  capsule,  in  which  however  it  continues  rapidly  to  rotate,  and  also 
commences  to  exhibit  active  contractions.  It  next  becomes  hatched,  and 
passes  from  a  spherical  to  a  flattened  elongated  form.  The  ventral  oral 
opening  is  at  first  central,  but  soon,  by  a  process  of  unequal  growth,  becomes 
carried  towards  the  posterior  end  of  the  body.  The  pairs  of  stiff  hairs  in  the 
meantime  considerably  increase  in  number.  The  remains  of  the  yolk  cells 
now  disappear,  and  the  enteric  walls  become  more  distinct.  The  alimentary 
canal,  which  is  at  first  simple  in  outline  like  that  of  a  rhabdoccelous  Turbel- 


192 


TURBELLARIA. 


larian,  soon  assumes  a  dendritic  form.  The  young  animal  after  these  changes 
resembles  its  parent,  except  in  the  possession  of  only  two  pairs  of  eyes  and 
in  the  absence  of  generative  organs. 

Of  the  types  with  a  complete  metamorphosis  the  free  larvae 
of  various  species  of  Thysanozoon  have  been  observed  by  Joh. 
Miiller  (190)  and  Moseley  (189), 
and  the  complete  development  of 
Eurylepta  auriculata  has  been 
studied  by  Hallez. 

The  stages  within  the  egg  of 
this  latter  type  agree  precisely 
with  those  already  described  in 
Leptoplana.  After  the  formation 
of  the  mouth  the  body  elongates, 
remaining  however  cylindrical.  A 
fold  forms  on  the  anterior  side  of 
the  mouth,  giving  rise  to  a  large 
upper  lip.  Two  posterior  processes  FIG.  87.  LARVA  OF  EURYLEPTA 

AURICULATA  IMMEDIATELY  AFTER 

are   next   formed,  and   other  pro-     HATCHING. 

cesses  soon  arise,  constituting  the     SID^  j£J 

whole  of  those  found  in  the  free 

larva.     The   embryo  next  shakes  off  its  egg  membranes  by  a 

series  of  vigorous  contractions.    When  free  it  has  the  form  repre- 

sented in  the  annexed  figure  (fig.  87). 

It  is  so  similar  to  Miiller's  (fig.  88)  and  Moseley's  larvae  that 
all  three  may  be  dealt  with  together. 

The  body  is  somewhat  oval,  and  slightly  pointed  behind. 
At  the  anterior  end  are  placed  the  eyes,  two  in  the  youngest 
larva  of  Miiller,  and  twelve  in  the  older  larva  (fig.  88),  and  in 
the  middle  of  the  ventral  surface  is  the  mouth.  It  is  surrounded 
by  a  strong  fold,  and  leads  into  an  alimentary  canal,  which  is  at 
first  simple,  but  in  the  older  larvae  is  much  branched.  A  bilobed 
ganglion  connected  with  two  nerve  cords  is  placed  anteriorly. 
The  superficial  epithelium  is  ciliated,  and  below  it  is  a  layer  of 
cells  (cutis)  derived  from  the  primitive  epiblast,  in  which  are 
formed  the  usual  rods  (Hallez).  The  chief  peculiarity  of  the 
larva  consists  in  the  presence  of  elongated  processes  covered 
with  long  cilia,  and  so  connected  together  by  a  ciliated  band 
that  the  whole  together  forms,  in  Miiller's  larva  at  any  rate,  a 


VIEWED   FROM   THE 
Hallez.) 


PLATYELMINTHES. 


193 


lobed  prceoral  ciliated  band  (fig.  88).  This  band  is  not  quite  so 
clear  in  Hallez'  figures.  Miiller's  youngest  larva  was  provided 
with  eight  very  long  lobes;  three  were  dorsal,  viz.  a  median 
anterior,  and  two  lateral  placed  far  back  ;  three  ventral,  viz.  a 
median  in  the  front  of  the  mouth  forming  a  large  upper  lip,  and 
two  processes  at  the  sides  of  the  mouth.  The  number  was  com- 
pleted by  two  lateral  processes  of  the 
body.  All  the  processes  except  the 
dorsal  median  one  are  shewn  in  fig.  88. 
In  Hallez'  larva,  fig.  87,  the  six  posterior 
processes  form  a  rather  definite  ring, 
while  one  flagellum  projects  from  the 
front  end  of  the  body  immediately  below 
the  eyes,  and  a  second  flagellum  behind. 
In  Moseley's  youngest  larva  six,  pro- 
cesses only  were  present,  though  subse- 
quently eight  became  formed  as  in  Miiller's 
larvae. 

The  metamorphosis  consists  in  the 
whole  animal  growing  longer  and  flatter, 
and  in  the  arms  becoming  gradually 
shorter  and  shorter  till  they  finally  dis- 
appear altogether,  and  the  larva  acquires 
the  ordinary  adult  form. 

The  lobed  larval  form  of  the  Turbellaria  has  some  points  of 
resemblance  to  the  Pilidium  form  of  nemertine  larva  described 
below,  yet  its  resemblance  to  this  interesting  larva  is  less  close 
than  would  appear  to  be  the  case  with  certain  turbellarian  larval 
forms  recently  described  by  Gotte  and  Metschnikoff,  which  are 
in  some  respects  intermediate  in  character  between  the  larva  of 
Leptoplana  and  those  just  described. 

The  observations  of  Gotte  (No.  184)  were  made  on  Planaria  Neapolitana 
and  Thysanozoon  Diesingi,  and  those  of  Metschnikoff  (No.  188)  on  Stylo- 
chopsis  ponticus.  The  larvae  of  all  these  forms  undergo  more  or  less  of  a 
metamorphosis,  but  the  accounts  of  their  development  are  not  easily 
reconciled1.  The  early  stages  of  Planaria  are  like  those  of  Leptoplana,  as 

1  The  account  of  Metschnikoff  s  observations  on  Stylochopsis  ponticus  given  in 
the  German  abstract  is  too  obscure  to  be  placed  in  the  text,  but  the  following  are  the 
more  important  points  which  can  be  gleaned  from  it. 

The  ovum  becomes  first  divided  into  eight  segments.  By  further  division  along 
the  equatorial  zone,  a  ring  of  small  cells  is  formed  which  becomes  the  epiblast.  The 

B.  II.  13 


FIG.  88.  MULLER'S  TUR- 
BELLARIAN LARVA  (PRO- 
BABLY THYSANOZOON). 
VIEWED  FROM  THE  VEN- 
TRAL SURFACE.  (After 
Miiller.) 

The  ciliated  band  is  re- 
presented by  the  black  line. 

m.  mouth ;  «./.  upper 
lip. 


194 


TURBELLARIA. 


FIG.  89.  PLANA- 
RIAN   LARVA   (PRO- 

BABLY        PLAN  ARIA 

ANGULATA).  (From 
Agassi  z.) 


described  by  Keferstein.  Four  large  hypoblast  cells  become  surrounded  by 
small  epiblast  cells,  which  commence  to  be  formed  on  the  dorsal  side.  The 
hypoblast  cells  divide  and  arrange  themselves  in  two  bilaterally-symmetrical 
rows.  A  small  blastopore  is  left  by  the  small  cells  on  the  ventral  surface, 
which  communicates  with  an  otherwise  closed  and  ciliated  cavity  which  is 
formed  between  the  two  rows  of  hypoblast  cells.  The  blastopore  would 
seem  to  remain  permanently  open,  and  to  be  placed  at  the  base  of  a  deep 
pit,  lined  by  epiblast  cells,  which  constitutes  the  stomodaeum. 

The  embryo  now  becomes  dorsally  convex,  while  the  ventral  surface 
becomes  marked  with  a  median  furrow  and  grows  out  laterally  into  two 
lobes,  and  anteriorly  into  a  ventrally-directed  upper  lip.  The  whole  surface 
becomes  ciliated,  and  the  cilia  are  especially  prominent  on  the  ventral 
processes  and  the  summit  of  the  dorsal  dome.  A  bunch 
of  strong  cilia  becomes  formed  in  front  of  the  dome, 
and  a  less  marked  bunch  behind.  The  larva  is  now 
stated  by  Gotte  closely  to  resemble  a  Pilidium.  It  soon, 
however,  extends  itself,  and  the  two  bunches  of  cilia 
become  situated  at  the  anterior  and  posterior  extremities 
of  the  body.  The  ventral  processes  become  incon- 
spicuous prominences  of  the  side  of  the  body.  Gotte 
believes  that  the  larva  undergoes  no  further  metamor- 
phosis. 

A  type  of  Planarian  larva  (figs.  89  and  90)— possibly  Plan,  angulata, 
observed  by  Alex.  Agassiz  (No.  181), — is  very 
different  from  any  other  so  far  described,  and 
is  remarkable  for  being  divided  into  a  series  of 
segments  corresponding  in  number  with  the 
diverticula  of  the  digestive  cavity.  In  the 
youngest  specimen  (fig.  89)  the  body  was  nearly 
cylindrical,  and  divided  into  eleven  rings,  cor- 
responding with  as  many  digestive  diverticula. 
Two  eye-spots  were  present.  In  a  later  stage 
two  poles  are  at  this  time  formed  of  large  cells.  At  one  pole  four  small  cells  appear, 
which  are  compared  by  Metschnikoff  to  the  pole  cells  of  the  Diptera  (vide  Chapter 
on  the  development  of  the  Insecta).  At  the  opposite  pole  a  blastopore  is  formed 
leading  into  a  small  segmentation  cavity.  The  epiblast  also  now  gradually  grows 
over  the  large  cells.  At  the  blastopore  pole  the  large  cells  give  rise  to  the  hypoblast 
and  the  small  cells  at  the  opposite  pole  assist  in  forming  the  epiblast.  The  blastopore 
dtappeUB,  and  with  it  the  segmentation  cavity,  while  the  hypoblast,  forming  a  solid 
mass,  becomes  divided  into  two  halves  (Cf.  Planaria  Neapolitana).  The  embryo  be- 
comes ciliated  and  begins  to  rotate;  and  the  eyes,  and  somewhat  later  (?)  the  nervous 
ganglion  make  their  appearance. 

In  the  interior  a  wide  cavity  develops  between  the  hypoblast  cells,  which  becomes 
ciliated  and  is  placed  in  communication  with  the  exterior  by  an  invaginated  stoma- 
dseum  which  forms  the  pharynx. 

The  larva  now,  as  in  Planaria  Neapolitana,  takes  on  a  Pilidium-like  form.  Lateral 
I- >!•(.•>  and  an  anterior  lip  grow  out  from  the  under  surface,  and  become  covered  with 
long  cilia,  while  at  the  upper  pole  a  long  flagellum  makes  its  appearance. 


FIG.  90.  PLANARIAN  LARVA 
(PROBABLY  PLANARIA  ANGU- 
LATA). (From  Agassiz.) 


PLATYELMINTHES.  195 


(fig.  90)  the  body  was  considerably  flattened  and  had  approached  more  to 
the  planarian  form. 

If  Agassiz'  interesting  observations  can  be  trusted  we  have  in  this  larva 
indications  of  a  distinct  segmentation,  which  are  of  some  morphological 
importance,  especially  when  taken  in  connection  with  the  traces  of  segmen- 
tation found  amongst  the  Nemertines. 

A  further  type,  with  an  incomplete  metamorphosis,  has  been  observed  by 
Girard  (183).  It  is  remarkable  for  having  an  uniform.segmentation,  and  for 
presenting  a  quiescent  stage  after  passing  through  a  free  larval  condition 
with  a  large  upper  lip. 

Fresh-water  Dendroccela.  The  development  of  the  fresh- 
water Dendrocoela  has  been  especially  investigated  by  Knappert 
(No.  186)  and  Metschnikoff  (No.  188). 

The  ova  are  very  delicate  minute  naked  cells,  which  to  the 
number  of  4 — 6  or  more  become  enveloped  in  a  capsule  or 
cocoon  together  with  a  large  mass  of  yolk  cells  derived  from  the 
vitellarium.  The  yolk  cells  exhibit  peristaltic  movements  and 
send  out  amoeboid  processes.  Each  ovum  when  laid  becomes 
surrounded  by  an  extremely  delicate  membrane,  which  dis- 
appears during  the  course  of  development.  The  capsules  consist 
of  a  spherical  case  and  a  stalk.  The  latter  is  first  emitted  from 
the  female  opening  as  a  thread-like  body.  Its  free  end  becomes 
attached,  and  then  the  remainder  of  the  capsule  is  ejected. 

Impregnation  takes  place  before  the  formation  of  the  capsule.  The 
segmentation  is  complete.  The  ovum  first  divides  into  two  segments.  One 
of  these  next  divides,  forming  three  segments.  There  are  subsequently 
stages  with  four,  eight,  sixteen,  and  thirty-two  segments. 

Metschnikoff's  results  on  the  stages  subsequent  to  the  segmentation  are 
not  in  complete  harmony  with  those  of  Knappert ;  but  no  doubt  represent 
an  advance  in  our  knowledge,  and  I  shall  follow  them  here.  His  observa- 
tions were  made  on  Planaria  polychroa. 

In  the  earliest  stage  observed  by  him  the  segmentation  was  already  far 
advanced,  but  no  membrane  was  present  round  the  ovum.  At  a  later  stage 
the  ovum  becomes  more  or  less  bell-shaped  or  hemispherical,  and  encloses 
within  its  concavity  a  mass  of  yolk  elements.  It  is  now  formed  of  three 
concentric  layers.  An  outer  layer  of  flattened  cells — the  epiblast,  a  middle 
layer  of  fused  cells — the  mesoblast,  and  an  inner  solid  mass  of  yolk  cells — 
the  hypoblast. 

At  the  upper  pole  is  formed  the  protrusible  pharynx  (cf.  Knappert),  which 
is  provided  with  a  provisional  musculature  and  a  lumen.  By  its  contractions 
it  takes  up  the  yolk  elements  which  surround  the  embryo,  and  the  rapid 
growth  of  the  embryo  no  doubt  takes  place  at  their  expense.  The  embryo 

13—2 


196  NEMERTEA. 


gradually  loses  its  hemispherical  form,  and  assumes  an  elongated  and 
flattened  shape.  It  acquires  a  coating  of  cilia  by  means  of  which  it  rotates. 
On  the  fifth  day  it  is  hatched. 

The  alimentary  tract  long  remains  solid,  even  after  it  has  acquired  its 
branched  form.  The  pharynx  becomes  withdrawn  as  soon  as  the  larva  is 
hatched.  It  loses  its  provisional  muscles,  and  subsequently  acquires  a 
permanent  musculature.  The  young  after  hatching  attach  themselves  to  the 
body  of  their  parent,  on  which  they  feed  (?). 

Rhabdoccela.  The  development  of  some  of  the  Rhabdoccela 
has  recently  been  studied  by  Hallez.  The  ova  are  mostly  laid 
in  capsules,  one  in  each  capsule.  Sometimes  the  development 
commences  before  the  capsules  are  laid,  at  other  times  not  till 
afterwards.  In  certain  forms  (Mesostomum)  there  are  summer 
eggs  with  thin  capsules  which  develop  within  the  parent,  while 
hard  capsules,  forming  what  are  known  as  winter  eggs,  are  laid 
in  the  autumn,  and  the  embryo  hatched  in  the  spring. 

The  ova  of  the  Rhabdoccela  like  those  of  the  fresh-water  Dendroccela 
are  enveloped  in  yolk  elements  derived  from  the  vitellarium. 

The  segmentation  probably  takes  place  in  the  same  way  as  in  Lepto- 
plana.  A  stage  with  four  equal  cells  has  been  observed  by  Hallez,  and 
there  is  subsequently  an  epibolic  gastrula.  The  embryo  becomes  ciliated 
while  still  within  the  capsule  and,  according  to  Hallez,  the  pharynx  arises 
as  a  bud  of  the  hypoblast.  The  proboscis  in  Prostomum  originates  as  an 
epiblastic  invagination. 

NEMERTEA. 

Some  Nemertea  develop  without  and  some  with  a  meta- 
morphosis. 

The  most  remarkable  type  of  Nemertine  development  with  a 
metamorphosis  is  that  in  which  the  ovum  develops  into  a 
peculiar  larval  form  known  as  Pi  lid  ium,  within  which  the  perfect 
worm  is  subsequently  evolved.  Closely  allied  to  this  type  is  one 
in  which  the  sexual  worm  is  developed  within  a  larval  form  as  in 
1'ilidium,  but  in  which  the  larva  has  no  free  swimming  stage,  and 
is  therefore  without  the  characteristic  appendages  of  the  Pilidium. 
This  is  known  as  the  type  of  Desor  and  is  confined  (?)  to  the 
genus  Lineus.  The  Pilidium  and  the  Desor  type  may  be  first 
considered  (vide  Barrois,  No.  192). 

The  type  of  Desor.  The  segmentation  is  regular  and  leads 
to  the  formation  of  a  blastosphere  with  a  large  segmentation 


PLATYELMINTHES.  197 


cavity.     The  blastosphere  is  converted  by  invagination  into  a 
gastrula  (fig.  91  A).     The  blastopore  is  soon  carried  relatively 


FIG.  91.     THREE  STAGES  IN  THE  DEVELOPMENT  OF  LINEUS.    (After  Barrois.) 

A  is  a  side  view  in  optical  section. 

B  and  C  are  two  later  stages  from  the  ventral  (oral)  surface. 

ae.  archenteron ;  sc.  segmentation  cavity ;  hy.  hypoblast ;  me.  mesoblast ;  ep.  epi- 
blast ;  m.  mouth ;  st.  stomach ;  pr.  d.  prostomial  disc  ;  po.  d.  metastomial  disc ;  pr. 
proboscis. 

forwards  by  the  elongation  backwards  of  the  archenteron,  and, 
according  to  Barrois,  actually  forms  the  mouth.  Owing  to  the 
elongation  of  the  archenteric  cavity  the  embryo  assumes  a  bila- 
teral form  (fig.  92  A)  in  which  the  dorsal  and  ventral  surfaces 
can  be  distinguished,  the  mouth  (m.)  being  situated  on  the 
ventral  surface. 

Immediately  after  the  completion  of  the  gastrula  a  remarkable 
series  of  phenomena  takes  place.  The  embryo  when  viewed 
from  the  ventral  surface  assumes  a  pentagonal  form  (fig.  91  B), 
and  four  invaginations  of  the  epiblast  make  their  appearance  on 
the  ventral  surface  (fig.  92  A),  two  in  front  of  {pr.  d.)  and  two 
behind  {po.  d.)  the  mouth ;  they  result  in  the  formation  of  four 
thickened  discs.  These  discs  soon  become  separated  from  the 
external  skin,  which  closes  in  forming  an  unbroken  layer  over 
them  (fig.  91  C).  The  discs  grow  rapidly,  and  first  the  prosto- 
mial pair  and  subsequently  the  metastomial  fuse  together,  and 
finally  the  whole  four  unite  into  a  continuous  ventral  plate ; 
analogous  it  would  seem  to  the  ventral  plate  of  chsetopodan  and 


198 


NEMERTEA. 


arthropodan  embryos.  The  plate  so  formed  gradually  extends 
itself  so  as  to  close  over  the  dorsal  surface,  and  to  form  a 
complete  skin  within  the  primitive  larval  skin,  which  at  this 
period  is  richly  ciliated,  though  the  embryo  is  not  yet  hatched 


FIG.  91.    THREE  STAGES  IN  THE  DEVELOPMENT  OF  LINEUS.    (After  Barrois.) 

A.     Side  view  of  an  embryo  at  a  very  early  stage  as  an  opaque  object. 

B  and  C.     Two  late  stages,  seen  as  transparent  objects  from  the  ventral  surface. 

at.  archenteron;  m.  mouth;  pr, d.  prostomial  disc;  po.d.  metastomial  disc; 
cs.  lateral  pit  developing  in  B  as  a  diverticulum  from  the  oesophagus;  pr.  proboscis  ; 
ms.  muscular  layer  (?);  Is.  larval  skin  about  to  be  thrown  off;  me.  mesoblast; 
st.  stomach. 

(fig.  91  C).  While  these  changes  are  taking  place,  there  are 
budded  off  from  the  invaginated  discs  a  number  of  fatty  cells, 
which  fill  up  the  space  between  the  discs  and  the  archenteron, 
and  eventually  form  the  mesoblastic  reticulum.  During  this 
stage  the  rudiment  of  the  proboscis  also  makes  its  appearance  as 
a  solid  process  of  epiblast,  which  grows  backwards  from  the 
point  of  fusion  of  the  two  prostomial  discs  at  the  front  end  of 
the  embryo  (fig.  91  C,  pr.).  A  lumen  is  excavated  in  it  at  a 
later  period.  The  lateral  organs  or  cephalic  pits  arise  in  a 
somewhat  unexpected  fashion  as  a  pair  of  diverticula  from  the 


PLATYELMINTHES.  199 


oesophagus  (fig.  92  B,  cs.)1,  which  soon  fuse  with  the  walls  of  the 
body  at  the  junction  of  the  prostomial  and  metastomial  plates 
(fig.  92  C,  cs.),  although  they  remain  for  some  time  attached  to 
the  oesophagus  by  a  solid  cord. 

During  these  changes  the  original  larval  skin  separates  itself 
from  the  subjacent  layer  formed  by  the  discs  (fig.  92,  B  and  C), 
and  is  soon  thrown  off  completely,  leaving  the  already  ciliated 
(fig.  92  C)  external  layer  of  the  invaginated  discs  as  the  external 
skin  of  the  young  Nemertine.  During,  and  subsequently  to,  the 
casting  off  of  the  embryonic  skin,  important  changes  take  place 
in  the  constitution  of  the  various  layers  of  the  body,  resulting  in 
the  formation  of  the  vascular  system  and  other  mesoblastic 
organs,  the  nervous  system,  and  the  permanent  alimentary  tract. 
These  changes  appear  to  me  to  stand  in  need  of  further  elucida- 
tion ;  and  the  account  below  must  be  received  with  a  certain 
amount  of  caution. 

It  has  been  already  stated  that  the  two  discs  give  rise  to  fatty  cells, 
which  occupy  the  space  between  the  walls  of  the  body  and  the  archenteron. 
At  the  period  of  the  casting  off  of  the  embryonic  skin  fresh  changes  take 
place.  The  discs  become  very  much  thickened,  and  then  divide  into  two 
layers,  which  become  the  epidermis  and  subjacent  muscular  layers.  The 
muscular  layers  arise  in  two  masses,  separated  by  the  two  cephalic  sacks. 
The  anterior  mass  is  formed  as  an  unpaired  anterior  thickening,  followed  by 
two  lateral  thickenings.  The  posterior  mass  is  much  thinner,  in  correspond- 
ence with  the  rapid  elongation  of  the  metastomial  portion  of  the  embryo. 

The  cells  originally  split  off  from  the  discs  undergo  considerable  changes, 
some  of  them  arrange  themselves  around  the  proboscis  as  a  definite  mem- 
brane, which  becomes  the  proboscidean  sheath,  some  also  form  a  true 
splanchnic  layer  of  mesoblast,  and  the  remainder,  which  are  especially  con- 
centrated during  early  embryonic  life  in  the  anterior  parts  of  the  body,  form 
the  general  interstitial  connective  tissue.  The  cephalic  ganglia  are  stated  to 
become  gradually  differentiated  in  the  prostomial  mesoblast,  and  the  two 
cords  connected  with  them  in  the  metastomial  mesoblast. 

At  the  time  when  the  larval  skin  is  cast  off  the  original  mouth  becomes 
closed,  and  it  is  not  till  some  time  afterwards  that  a  permanent  mouth  is 
formed  in  the  same  situation.  During  the  early  part  of  embryonic  life  the 
intestine  is  lined  with  columnar  cells,  but,  before  the  loss  of  the  larval  skin, 
the  walls  of  the  intestine  undergo  a  peculiar  metamorphosis.  Their  cells 
either  fuse  or  become  indistinguishable,  and  their  protoplasm  appears  to 
become  converted  into  yolk-spherules,  which  fill  up  the  whole  space  within 

1  Biitschli  for  Pilidium  regards  these  pits  as  formed  by  imaginations  of  the  epiblast, 
but  Metschnikoff  s  statements  are  in  accordance  with  those  in  the  text. 


2OO  NEMERTEA. 


the  walls  of  the  body,  and  are  only  prevented  from  extending  forwards  by 
a  membrane  of  connective  tissue.  This  mass  gradually  forms  itself  into  a 
distinct  canal,  lined  by  columnar  cells. 

Pilidium.  In  the  case  of  the  true  Piltdium  type,  the  larva  is 
hatched  very  early  and  leads  the  usual  existence  of  surface 
larvae.  A  regular  segmentation  is  followed  by  an  invagination 
which  does  not  however  cause  the  complete  obliteration  of  the 
segmentation  cavity  (fig.  93  A,  a.e.). 

The  primitive  alimentary  tract  so  formed  becomes  divided 
into  cesophageal  and  gastric  regions  (fig.  93  B,  oe.  and  .$•/.).  Even 
while  the  invagination  of  the  archenteron  is  proceeding,  the 
larva  becomes  ciliated  throughout,  and  assumes  a  somewhat 
conical  form,  the  apex  of  the  cone  being  opposite  the  flat  ventral 
surface  on  which  the  mouth  is  situated  (fig.  93,  A  and  B).  From 


FIG.  93.    Two  STAGES  IN  THE  DEVELOPMENT  OF  PILIDIUM.    (After  Metschnikoff.) 
of.  archenteron  ;  <r.  oesophagus ;  st.  stomach  ;  am.  amnion  ;  pr.d.  prostomial  disc ; 
PO. d.  metastomial  disc;  c.s.  cephalic  sack. 

the  apex  a  flagellum  projects  in  many  forms,  giving  the  larva  a 
helmet-like  appearance.  In  other  forms  a  bunch  of  long  cilia 
takes  the  place  of  the  flagellum  (fig.  94),  and  in  others  again  the 
flagellum  is  not  represented.  After  the  completion  of  the  inva- 
gination a  lobe  grows  out  on  each  side  of  the  mouth,  and  less 
well  developed  lobes  may  appear  anteriorly  and  posteriorly. 
Round  the  edge  of  the  ventral  surface  a  ciliated  band  makes  its 
appearance. 


PLATYELMINTHES. 


201 


Two  pairs  of  imaginations  of  the  skin,  just  as  in  the  type  of 
Desor,  now  make  their  appearance,  one  pair  in  front  of  and  the 
other  behind  the  mouth  (fig.  93  B,  pr.d.  and  po.d.},  and  each  of 
them  by  the  closure 
of  the  opening  of 
invagination  forms 
a  sack,  the  outer 
wall  of  which  be- 
comes very  thin 
and  the  inner 
wall  (correspond- 
ing with  the  whole 
invagination  of  the 
type  of  Desor)  very 
thick.  The  inner 
walls  of  the  four 
thickenings,  which 
I  may  speak  of  as 
discs,  now  fuse  to- 
gether, each  disc 
first  uniting  with 
its  fellow,  and 
finally  the  two 
pairs  uniting. 

A  ventral  ger- 
minal plate  is  thus 
established,  which 
gradually  grows 
round  the  intestine 
of  the  Pilidium  to 
form  the  skin  of 
the  future  Nemer- 
tine.  The  outer 
thin  layer  of  each  of  the  discs  grows  part  passu  with  the  inner 
layer,  and  furnishes  an  amnion-like  covering  for  the  embryo 
which  is  forming  within  the  Pilidium  (fig.  94,  an}. 

In  connection  with  the  young  vermiform  Nemertine  there  is 
formed  on  each  side  an  outgrowth  from  the  oesophagus  (fig.  94) 
which  is  eventually  placed  in  communication  with  the  exterior 


FIG.  94. 

A.  PILIDIUM   WITH    AN    ADVANCED    NEMERTINE 
WORM. 

B.  RIPE     EMBRYO     OF     THE     NfiMERTEA     IN    THE 
POSITION     IT     OCCUPIES      IN      PlLIDIUM.        (Both     after 

Butschli.) 

&.  oesophagus ;  st.   stomach ;   i.  intestine ;  pr.   pro- 
boscis ;  Ip.  lateral  pit ;  an.  amnion  ;  n.  nervous  system. 


202  NEMERTEA. 


by  a  ciliated  canal1.  The  proboscis  arises  as  an  hollow  invagi- 
nation  at  the  point  where  the  two  anterior  discs  fuse  in  front. 

When  the  young  Nemertine  has  become  pretty  well  formed 
within  the  Pilidium  it  becomes  ciliated,  begins  to  move,  and 
eventually  frees  itself  and  leads  an  independent  existence, 
leaving  its  amnion  in  the  Pilidium  which  continues  to  live  for 
some  time. 

The  central  nervous  system  (fig.  94)  is  developed  either 
before  or  after  the  detachment  of  the  young  Nemertine,  accord- 
ing to  Metschnikoff  as  a  thickening  of  the  epiblast.  The  young 
Nemertine  is  at  first  without  an  anus. 

The  development  of  the  Nemertine  within  the  Pilidium  is 
clearly  identical  with  that  of  the  Lineus  embryo  within  the 
larval  skin ;  the  formation  of  an  amnion  in  the  Pilidium  consti- 
tuting the  only  important  difference  which  can  be  pointed  out 
between  the  modes  of  origin  of  the  young  Nemertine  in  the  two 
types. 

So  far  as  is  known  the  forms  which  develop  in  a  Pilidium,  or 
according  to  the  type  of  Desor,  all  belong  to  the  division  of  the 
Nemertines  without  stylets  in  the  proboscis,  known  as  the 
Anopla. 

Development  without  Metamorphosis.  The  majority  of 
the  Nemertea,  including  the  whole  (?)  of  the  Enopla,  develop 
without  a  metamorphosis.  The  observations  which  have  been 
made  on  this  type  are  not  very  satisfactory,  but  appear  to 
indicate  that  the  formation  of  the  hypoblast  may  take  place 
cither  by  invagination  or  by  delamination. 

Invaginate  types  have  been  observed  by  Barrois  (No.  192),  Dieck  (No. 
196)  and  Hubrecht. 

Barrois'  fullest  observations  were  made  on  Amphipoi~us  lactifloreus  (one 
of  the  Enopla),  and  those  of  Dieck  on  Cephalothrix  galathece  (one  of  the 
Anopla). 

A  regular  segmentation  is  followed  by  a  blastosphere  stage  with  a  small 
segmentation  cavity.  In  Barrois'  type  the  inner  ends  of  the  cells  of  the 
blastosphere  are  stated  to  fuse  into  a  kind  of  syncytium.  A  small  invagina- 
tion takes  place,  and  the  cells  which  take  part  in  it  separate  from  the 

1  This  is  the  view  of  both  Metschnikoff  (No.  202)  and  Leuckart  and  Pagcnstecher 
(No.  201),  and  is  further  confirmed  by  Barrois,  but  Biitschli  (No.  193),  though  he  has 
not  observed  the  earliest  stages  of  their  outgrowth,  believes  them  to  be  invaginations  of 
the  Nemertine  skin. 


PLATYELMINTHES.  203 


epiblast,  and  then  fuse  with  the  syncytium  within  the  blastosphere.  Dieck 
finds  that  in  Cephalothrix  the  invaginated  mass  simply  vanishes. 

Barrois'  statements  about  the  fusion  of  the  syncytium  derived  from  the 
epiblast  cells  with  the  invaginated  cells  must  be  regarded  as  very  doubtful. 
The  formation  of  the  germinal  layers  takes  place,  according  to  Barrois, 
by  the  separation  of  the  internal  mass  of  cells  into  mesoblast  and  hypoblast. 
The  proboscis  is  formed,  according  to  this  author,  from  the  mesoblastic 
tissues.  Dieck,  on  the  other  hand,  with  greater  probability,  states  that  the 
proboscis  is  formed  by  an  invagination.  In  Cephalothrix  a  further  point 
deserves  notice,  in  that  the  whole  of  the  primitive  epiblast  becomes  shed. 
In  this  fact  there  may  perhaps  be  recognised  the  last  trace  of  a  metamor- 
phosis like  that  in  the  type  of  Desor. 

Delaminate  types  have  been  studied  by  Barrois  (No.  192)  and  Hoffman 
(No.  198),  both  of  whom  give  circumstantial  accounts  of  their  develop- 
ment. 

Hoffman's  account  is  especially  deserving  of  attention,  since  his  observa- 
tions were,  to  a  great  extent,  made  by  means  of  artificial  sections.  The 
following  account  is  taken  from  him.  His  observations  were  made  on 
Tetrastemma  varicolor,  and  Tetrastemma  appears  to  be  the  genus  in  which 
this  type  of  development  has  been  most  completely  made  out.  After  a 
regular  segmentation  the  embryo  forms  a  solid  mass  of  cells,  the  outermost 
of  which  soon  become  distinguished  as  a  separate  epiblastic  layer.  At  the 
same  time  the  larva  leaves  the  egg,  and  the  epiblast  cells  become  coated  by 
an  uniform  covering  of  cilia.  At  the  anterior  extremity  of  the  body  is  a 
bunch  of  long  cilia ;  and  at  the  hinder  end  two  stiff  bristles  are  formed,  but 
soon  disappear. 

The  internal  mass  of  cells  is  still  quite  uniform,  but  as  the  larva  grows  in 
length  the  outermost  of  them  arrange  themselves  as  a  columnar  layer, 
constituting  the  mesoblast.  Of  the  cells  internal  to  the  mesoblast  the  outer 
become  columnar,  and  are  converted  into  the  walls  of  the  alimentary  tract, 
while  the  inner  ones  undergo  fatty  degeneration,  and  form  a  kind  of  food- 
yolk.  In  the  later  development  the  characters  of  the  adult  are  gradually 
acquired  without  metamorphosis,  and  the  larval  skin  passes  directly  into 
that  of  the  adult.  Both  mouth  and  anus  are  formed  nearly  simultaneously 
by  a  rupture  of  the  enteric  wall  from  within.  The  nervous  system  arises  as 
a  thickening  of  the  epiblast,  which  Hoffman  states  he  has  been  able  to  see 
in  sections.  Hoffman  also  states  that  the  epithelium  of  the  proboscis  is 
formed  as  a  diverticulum  of  the  alimentary  tract,  and  that  its  sheath  is 
formed  by  a  special  mesoblastic  growth. 

Barrois  is  less  precise  than  Hoffman,  from  whom  he  differs  in  certain 
particulars.  Hoffman's  statements  about  the  proboscis  are  important  if 
accurate,  but  require  further  confirmation. 

Malacobdella.  The  early  stages  in  development  of  the  peculiar  ecto- 
parasitic  Nemertine  Malacobdella  have  been  worked  by  Hoffman  (No.  199) 
by  means  of  sections,  and  there  appears  to  be  a  close  agreement  between 
the  development  of  Malacobdella  and  that  of  Tetrastemma. 

The  segmentation  is  uniform,  and  there  is  no  trace  of  a  segmentation 


204  NEMERTEA. 


cavity.  On  the  third  day  after  impregnation  the  outermost  cells  of  the 
embryo  become  flattened  and  ciliated,  and  distinguished  from  the  remain- 
ing spherical  cells  of  the  embryo  as  the  epiblast.  With  the  appearance 
of  cilia  a  rotation  of  the  embryo  commences.  On  the  fourth  day  the 
embryo  becomes  oval,  and  at  one  of  the  poles — the  future  anal  pole — a 
separation  takes  place  between  the  epiblast  and  the  inner  cells,  giving  rise 
to  the  body  cavity.  In  it  are  a  number  of  loose  oval  cells,  which  soon 
become  stellate,  and  form  a  mesoblastic  reticulum  connecting  the  body-wall 
nnd  central  cells  of  the  embryo,  which  may  now  be  spoken  of  as  hypoblast. 
The  body-cavity  increases  in  size,  leaving  at  last  the  hypoblast  and  epiblast 
united  only  at  one  point— the  oral  pole— at  which,  on  the  fifth  day,  a  crown 
of  long  cilia  appears.  The  solid  mass  of  hypoblast  in  the  interior  becomes 
differentiated  into  an  outer  layer  of  cells — the  true  glandular  epithelium  of 
the  alimentary  tract — and  an  inner  core,  the  cells  of  which  soon  undergo 
fatty  degeneration,  and  serve  as  food-yolk. 

The  later  stages  of  development,  and  the  formation  of  the  proboscis, 
etc.,  have  not  been  worked  out. 

General  considerations.  Of  the  types  of  larvae  hitherto 
found  amongst  the  Nemertea,  those  with  a  metamorphosis,  viz. 
the  Pilidium  type  and  that  of  Desor,  are  to  be  regarded  as  the 
primitive.  But  even  in  Pilidium  there  are  evidences  of  a  great 
abbreviation  in  development.  Pilidium  itself  is  probably  a  more 
or  less  modified  ancestral  form,  while  the  peculiar  development  of 
the  Nemertine  within  it  is  to  be  explained  as  a  very  much  short- 
ened record  of  a  long  series  of  changes  by  which  the  Pilidium  be- 
came gradually  converted  into  a  Nemertine.  The  formation  of 
the  body  wall  of  the  Nemertine  by  four  epiblastic  invaginations 
is  a  remarkable  cmbryological  phenomenon,  for  which  it  is  not 
easy  to  assign  a  satisfactory  meaning  ;  and  it  is  probable  that  it  is 
merely  a  secondary  process  of  growth  similar  to  the  formation  of 
imaginal  discs  in  the  larvae  of  Diptera  (vide  Chapter  on  Trache- 
ata),  which  has  had  its  origin  in  the  abbreviation  of  the  develop- 
ment just  alluded  to.  The  development  on  the  type  of  Desor  is 
clearly  a  simplification  of  the  Pilidium  type,  and  its  peculiarities 
are  to  be  explained  by  the  fact  that  the  first  larval  form  has  no 
free  existence.  The  types  without  metamorphosis  have  no  doubt 
a  development  of  a  still  more  simplified  character ;  they  are  re- 
markable however  in  presenting  us,  if  the  existing  descriptions 
are  to  be  trusted,  with  examples  of  delamination  and  invagination 
coexisting  in  closely  allied  forms. 


PLATYELMINTHES.  20$ 


TREMATODA. 

The  eggs  of  the  Trematoda  consist  of  a  germ  or  true  ovum 
enclosed  in  a  mass  of  yolk  cells,  which  undergo  disintegration 
and  subsequent  absorption  at  varying  periods  of  the  develop- 
ment. From  the  observations  of  E.  van  Beneden  (No.  218)» 
Zeller  (No.  217),  etc.  it  is  known  that  the  segmentation  is 
usually  complete,  but  generally  somewhat  irregular. 

Unfortunately  we  are  still  completely  in  the  dark  as  to  the 
mode  of  formation  of  the  germinal  layers.  The  embryos  of  the 
entoparasitic  forms  or  Distomeae  become  free  in  a  very  imperfect 
condition,  and  the  ova  are  small ;  while  in  the  Polystomeae  the 
development  is  as  a  rule  nearly  completed  before  hatching,  and 
the  ova  are  large.  It  will  be  convenient  to  treat  separately  the 
development  of  the  two  groups. 

Distomeae.  The  embryos  of  the  Distomeae  are  hatched 
either  in  some  moist  place  or  more  usually  in  water.  In  the 
majority  of  genera  the  larvae  pass  through  a  complicated  meta- 
morphosis, accompanied  by  alternations  of  generations.  But  for 
some  genera,  e.g.  Holostomum,  etc.,  the  life  history  has  not  yet 
been  made  out.  The  whole  life  history  of  comparatively  few 
forms  has  been  followed,  but  sufficient  fragments  are  known 
to  justify  us  in  making  certain  general  statements,  which  no 
doubt  hold  true  for  a  large  proportion  of  the  Distomeae. 

The  larvae  are  usually  ciliated  (fig.  95  A),  but  sometimes 
naked. 

The  ciliated  forms  are  generally  completely  covered  with  cilia,  but  in 
Distommn  lanceolatum  the  cilia  are  confined  to  an  area  at  the  front  end  of 
the  body,  in  the  centre  of  which  a  median  spine  is  placed.  An  x  shaped 
pigment  spot,  sometimes  provided  with  a  rudimentary  lens  (Monostomum 
mutabile\  is  also  generally  situated  on  the  dorsal  surface. 

In  some  intances  a  more  or  less  completely  developed  alimentary  tract  is 
present  (Monostomum  capitellum,  Amphistomum  subclavatum\  but  usually 
there  can  only  be  distinguished  in  the  interior  of  the  larva  a  transparent 
mass  of  cells  bounded  by  a  more  or  less  distinctly  marked  body  wall  with 
ciliated  excretory  channels. 

Ed.  van  Beneden  has  shewn  that  the  ciliated  covering  is  developed 
while  the  embryo  is  still  in  the  egg,  and  long  before  the  yolk  cells  are  com- 
pletely absorbed.  It  would  seem  that  even  before  hatching  this  ciliated 
covering  is  to  a  great  extent  independent  of  the  mass  within.  In  the 


206  TREMATODA. 


larva  of  Monostomum  mutabile  (fig.  95  A),  which  offers  an  example  of 
an  extreme  case  of  the  kind,  there  is  present  within  the  ciliated  epidermis 
a  fully-developed  independent  worm. 

The  non-ciliated  larvae  are  less  highly  organized  than  the  ciliated  forms, 
and  are  covered  by  a  cuticle  :  their  anterior  extremity  is  sometimes  provided 
with  a  circular  plate  armed  with  radiate  ridges  and  spines. 

The  free-swimming  or  creeping  embryos  make  their  way  into 
or  on  to  the  body  of  some  invertebrate  (occasionally  vertebrate) 
form,  usually  a  Mollusc,  to  undergo  the  first  stage  in  their 
metamorphosis.  They  may  either  do  this  on  the  gills  of  their 
host,  or  very  frequently  they  bore  their  way  into  the  interior  of 
the  body.  Soon  after  the  larvae  have  reached  a  satisfactory 
position  the  epidermis  becomes  stripped  off,  and  there  emerges  a 
second  larval  form  developed  in  the  interior  of  the  first  larva, 
much  as  a  Nemertine  is  developed  within  the  larva  of  Desor. 
In  the  case  of  Monostomum  mutabile  the  new  worm  is,  as 
stated  above,  fully  formed  within  the  ciliated  larva  at  the  time 
of  hatching. 

The  worm  which  proceeds  from  the  above  metamorphosis 
has  different  characters  corresponding  with  those  of  the  larva 
from  which  it  proceeded.  If  the  original  larva  had  an  alimen- 
tary canal  it  has  one  also,  and  then  grows  into  the  form  known 
as  a  Redia  (Fig.  95,  B  and  C). 

The  Redia  has  anteriorly  a  mouth  leading  into  a  muscular 
pharynx  and  thence  into  a  caecal  stomach.  Posteriorly  the  body 
is  prolonged  into  a  kind  of  blunt  caudal  process,  at  the  com- 
mencement of  which  are  a  pair  of  lateral  papillae.  There  is  a 
perivisceral  cavity,  and  the  body  walls  are  traversed  by  excretory 
tubes. 

If  the  original  larva  is  without  an  alimentary  tract,  the 
second  form  becomes  what  is  known  as  a  Sporocyst.  The 
Sporocyst  is  a  simple  elongated  sack  with  a  central  body  cavity  ; 
when  derived  from  the  metamorphosis  of  a  ciliated  embryo  its 
walls  are  provided  with  excretory  tubes,  but  such  tubes  are 
absent  in  Sporocysts  developed  from  non-ciliated  larvae.  Some 
Sporocysts  send  out  numerous  branches  amongst  the  viscera  of 
their  hosts. 

The  Rediiu  and  Sporocysts  rapidly  grow  in  size  and  some- 
times increase  by  transverse  division.  In  the  course  of  their 


PLATYELMINTHES. 


2O7 


further  development  one  of  two  things  may  happen.  They  may 
either  (i)  develop  fresh  Rediae  or  Sporocysts  by  a  process  of 
internal  budding  (fig.  95  C) ;  or  else  (2)  there  may  be  formed  in 
them,  by  an  analogous  pro- 
cess, larvae  with  long  tails 
known  as  Cercariae  (fig.  95 
D.)  The  direct  develop- 
ment of  Cercariae  is  the 
usual  course,  though  in 
Distomum  globiparum  the 
reverse  is  true  ;  but  where 
this  does  not  take  place  the 
Rediae  or  Sporocysts  of  the 
second  generation  give  rise 
to  Cercariae. 

The  Cercarias  are  deve- 
loped from  spherical  masses 
of  cells  found  in  the  body 
cavity  of  the  Sporocyst  or 
Redia.  The  exact  origin  of 
these  masses  is  still  some- 
what obscure,  but  they  are 
stated  by  Wagener  (No.  212) 
to  be  derived  from  the  body  wall, 
regarded  as  internal  buds. 

The  spherical  bodies  grow  rapidly  in  size,  their  posterior 
extremity  is  prolonged  into  a  process  which  forms  the  tail,  while 
the  anterior  part  forms  the  trunk.  When  fully  formed  (fig.  95  E), 
the  trunk  has  very  much  the  organization  of  an  adult  Distomum. 
There  is  an  anterior  and  a  ventral  sucker,  the  former  of  which 
contains  the  opening  of  the  mouth,  and  is  often  provided  with  a 
special  chitinous  armature.  The  mouth  leads  into  a  muscular 
pharynx,  and  this  into  a  bilobed  caecal  alimentary  tract.  An 
excretory  system  of  the  ordinary  type  is  present,  consisting  of 
longitudinal  contractile  trunks  continuous  anteriorly  with  branch- 
ed ciliated  canals,  which,  as  has  recently  been  shewn  by  Biitschli, 
may  be  provided  with  funnel-shaped  ciliated  internal  openings1. 


FIG.  95.  VARIOUS  STAGES  IN  THE  META- 
MORPHOSIS OF  THE  DISTOME^;  (from  Huxley.) 

A.  Ciliated  larva  of  Monostomum  muta- 
bile.  a.  larval  skin.  b.  Redia  developed 
within  it.  B.  Redia  of  Monostomum  muta- 
bile.  C.  Redia  of  Distomum  pacificum,  with 
germs  of  a  second  brood  of  Rediae.  D.  Redia 
containing  Cercariae.  E.  Cercaria.  F.  Full- 
grown  Distomum. 


They  are  probably  to  be 


1  O.  Biitschli,   "Bemerkungen  iib.  d.  excretorischen  Gefassapparat  d.  Trematoden." 
Zoologischer  Anzeiger,  1879,  No.  42. 


208  TREMATODA. 


The  contractile  trunks  unite  posteriorly,  but  instead  of  opening 
directly  to  the  exterior  are  prolonged  into  a  vessel  which 
traverses  the  substance  of  the  tail,  and  after  a  longer  or  shorter 
course  bifurcates  into  two  branches  which  open  laterally. 

The  tail  is  provided  with  an  axial  rod  of  hyaline  connective 
tissue,  like  the  notochord  of  the  tail  of  a  larval  Ascidian,  and  is 
frequently  provided  with  membranous  expansions.  It  is  used  as 
a  swimming  organ.  Beneath  the  epidermis  are  layers  of  circular 
and  longitudinal  muscular  fibres,  the  latter  arranged  in  the  tail 
as  two  bands. 

The  Cercariae  when  fully  developed  leave  the  Sporocyst  or 
Redia,  and  then  their  host,  and  become  free.  In  most  Rediae 
there  is  a  special  opening,  not  far  from  the  mouth,  by  which  they 
pass  out.  There  is  no  such  opening  in  Sporocysts,  but  the 
Cercariae  bore  their  way  through  the  walls. 

After  leaving  their  parent  the  Cercariae  pass  into  the  external 
medium,  and  for  a  short  period  have  a  free  existence.  They 
soon  however  enter  a  new  host,  making  their  w;  y  into  its  body 
by  a  process  of  boring,  which  is  effected  by  the  head  (especially 
when  armed  with  chitinous  processes)  assisted  by  movements  of 
the  tail. 

The  second  host  is  usually  some  Invertebrate  (Mollusc, 
Worm,  Crustacean,  Insect  larva,  &c.),  but  occasionally  a  Fish  or 
Amphibian  or  even  a  vegetable.  The  tail  is  very  often  lost  as 
the  Cercaria  bores  its  way  into  its  host,  but  whether  it  is  so  or 
not,  the  Cercaria,  after  it  has  once  reached  a  suitable  post  in  its 
new  host,  assumes  a  quiescent  condition,  and  surrounds  itself 
with  a  many-layered  capsule.  The  cephalic  armature  and  tail 
(if  still  present)  are  then  exuviated,  and  the  generative  organs 
gradually  become  apparent  though  very  small.  In  other  respects 
the  organization  is  not  much  altered. 

Though  an  encysted  Cercaria  may  remain  some  months 
without  further  change,  it  eventually  dies  unless  it  be  introduced 
into  its  permanent  vertebrate  host,  an  act  which  is  usually 
effected  by  the  host  in  which  it  is  encysted  being  devoured. 
It  then  becomes  freed  from  its  capsule  as  a  fully  formed  Trema- 
tode,  in  which  the  generative  organs  rapidly  complete  their 
development. 

In  some  cases  the  Rediae  or  Sporocysts  do  not  give  rise  to 


PLATYELMINTHES.  2OQ 


tailed  Cercariae,  but  to  tailless  forms.  In  such  cases,  as  a  rule, 
the  encystment  takes  place  in  the  host  of  the  Redia  or  Sporocyst, 
but  the  tailless  larvae  sometimes  pass  through  a  free  stage  like 
the  Cercariae.  In  the  case  of  Distomum  cygnoides,  parasitic  in 
the  bladder  of  the  Frog,  the  Cercaria  passes  directly  into  the 
adult  host  without  the  intervention  of  an  intermediate  host. 

The  life  history  of  a  typical  entoparasitic  Trematode  is  shortly 
as  follows  : 

1 i )  It  leaves  the  egg  as  a  ciliated  or  non-  ciliated  free  larva. 

(2)  This  larva  makes  its  way  on  to  the  gills  or  into  the 
body  of  some  Mollusc  or  other  host,  throws  off  its  epidermis  and 
becomes  a  Redia  or  Sporocyst. 

(3)  In  the  body  cavity  of  the  Redia  or  Sporocyst  nume- 
rous tailed  larvae,  known  as  Cercariae,  are  developed  by  a  process 
of  internal  gemmation. 

(4)  The  Cercariae  pass  out  of  the  body  of  their  parent, 
and  out  of  their  host,  and  become  for  a  short  time  free.     They 
then  pass  into  a  second,  usually  invertebrate  host,  and  encyst. 

(5)  If  their  second  host  is  swallowed  by  the  vertebrate 
host  of  the  adult  of  the  species,  the  encysted  forms  become  free, 
and  attain  to  sexual  maturity. 

The  majority  of  these  stages  are  simply  parts  of  a  complicated 
metamorphosis,  but  in  the  coexistence  of  larval  budding  (giving 
rise  to  Cercariae  or  fresh  Rediae)  with  true  sexual  reproduction 
there  is  in  addition  a  true  alternation  of  generations. 

Polystomeae.  The  ova  of  the  Polystomeae  are  usually  large 
and  not  very  numerous,  and  they  are  in  most  cases  provided 
with  some  process  for  attachment.  Some  species  of  Polystomeae, 
e.g.  Gyrodactylus,  are  however  viviparous.  The  young  leave  the 
egg  in  a  nearly  perfect  state,  and  at  the  utmost  undergo  a  slight 
metamorphosis  and  no  alternations  of  generations.  Some  how- 
ever (Polystomum,  Diplozoon)  are  provided  with  temporary  cilia, 
but  the  number  investigated  is  too  small  to  determine  whether 
ciliation  is  the  rule  or  the  exception.  The  ciliated  larvae  have  a 
short  free  existence.  The  cilia  are  developed  on  special  cells 
which  may  be  arranged  in  transverse  bands  in  the  same  way  as 
in  the  larvae  of  many  Chaetopods,  but  are  not,  in  the  larvae  at 
present  known,  distributed  uniformly.  When  the  free  larvae 
become  parasitic  the  cells  with  cilia  shrink  up. 

B.  II.  H 


2IO  '  1'^TODA. 


In  Polystotninn  inlc^rrimum,  which  lives  in  the  urinary  bladder  of  Rana 
temporaria,  the  eggs  when  laid  in  the  spring  pass  out  into  the  water.  The 
segmentation  is  complete,  and  the  embryo  when  hatched  is  provided  with 
most  of  the  adult  organs,  but  presents  certain  striking  larval  characters. 
It  has  five  rings  of  ciliated  cells.  Three  of  these  are  placed  anteriorly,  and 
are  especially  developed  on  the  ventral  surface,  the  posterior  one  being 
incomplete  dorsally ;  two  are  placed  posteriorly,  and  are  especially  devel- 
oped on  the  dorsal  surface.  Anteriorly  there  is  a  tuft  of  cilia. 

The  larva  itself  resembles  somewhat  an  adult  Gyrodactylus,  and  is  pro- 
vided (i)  with  a  large  posterior  disc  armed  with  hooks,  and  (2)  with  two 
pairs  of  eyes  which  persist  in  the  adult  state.  After  a  certain  period  of  free 
existence  the  larva  attaches  itself  to  the  gills  of  a  tadpole.  The  rings  of  cili- 
ated cells  shrink  up,  and  some  of  the  six  pairs  of  suckers  found  in  the  adult 
commence  to  be  formed  on  the  posterior  disc.  When  the  bladder  of  the  tad- 
pole is  developed,  the  young  Polystomum  passes  down  the  alimentary  tract  to 
the  cloaca,  and  thence  to  the  urinary  bladder,  where  it  slowly  attains  to  sexual 
maturity.  When  the  larva  becomes  attached  to  the  gills  of  a  very  young 
tadpole,  its  development  is  somewhat  more  rapid  in  consequence  of  better 
nutrition  from  the  more  delicate  gills.  It  then  reaches  its  full  development 
in  the  gill  cavity,  and.  though  smaller  and  provided  with  differently 
organised  generative  organs  to  the  normal  form,  produces  generative  pro- 
ducts and  dies  without  being  transported  to  the  bladder  (vide  Zeller, 
Nos.  216  and  217). 

The  ova  of  Diplozoon,  a  form  parasitic  on  the  gills  of  freshwater  fish 
(Phoxinus,  etc.),  are  provided  with  a  long  spiral  filament  (Zeller,  No.  215). 
The  embryo  has  five  ciliated  areas,  four  lateral  and  one  posterior.  The 
young  form  is  known  as  Diporpa.  Sexual  maturity  is  not  attained  till  two 
individuals  unite  permanently  together.  They  unite  by  the  ventral  sucker 
of  each  of  them  becoming  attached  to  the  dorsal  papilla  of  the  other.  Sub- 
sequently these  parts  coalesce,  and  the  ventral  suckers  disappear  in  the 
process.  Gyrodactylus,  parasitic,  like  Diplozoon,  on  the  gills  of  freshwater 
fishes  (Gasterosteus,  etc.),  is  remarkable  for  its  mode  of  reproduction.  It  is 
viviparous,  producing  a  single  young  one  at  a  time,  and,  what  is  still  more 
remarkable,  the  young  while  still  within  its  parent  produces  a  young  one, 
and  this  again  a  young  one,  so  that  three  generations  may  be  present  within 
the  parent.  It  seems  probable  that  the  second  and  third  generations  are 
produced  asexually,  the  generative  organs  not  being  developed  ;  while  the 
young  Gyrodactylus  of  the  first  generation  springs  from  a  fertilized  ovum 
(Wagener,  No.  214). 

CESTODA. 

On  anatomical  grounds  the  affinity  of  the  Cestoda  to  the 
Trematoda  has  been  insisted  on  by  the  majority  of  anatomists. 
The  existence  of  such  intermediate  forms  as  Amphilina  tends  to 


PLATYELMINTHES.  211 


strengthen  this  view ;  and  the  striking  resemblances  between 
the  two  groups  in  the  structure  of  the  egg  and  characters  of 
the  metamorphosis  appear  to  me  to  remove  all  doubt  about  the 
matter. 

The  ripe  egg  is  formed  of  a  minute  germ  enveloped  in  yolk 
cells,  the  whole  being  surrounded  by  a  membrane,  which  is  very 
delicate  in  most  forms,  but  in  certain  types  has  a  firmer  consist- 
ency, and  is  provided  with  an  aperture,  covered  by  an  operculum, 
by  which  the  larva  escapes. 

The  early  development,  up  to  the  formation  of  a  six-hooked 
larva,  generally  takes  place  in  the  uterus,  but  in  the  types  with  a 
firmer  egg-shell  it  takes  place  after  the  egg  has  been  deposited 
in  water. 

The  segmentation  (E.  van  Beneden,  No.  218,  Metschnikofif, 
No.  228)  is  complete,  and  during  its  occurrence  the  yolk  cells 
surrounding  the  germ  are  gradually  absorbed,  so  that  the  mass 
of  segmentation  spheres  grows  in  size,  till  at  the  close  of  segmen- 
tation it  fills  up  nearly  the  whole  egg-shell. 

As  was  first  shewn  by  Kolliker  for  Bothriocephalus  salmonis, 
the  embryonic  cells  separate  themselves  at  the  close  of  segmen- 
tation into  a  superficial  layer  and  a  central  mass. 

The  further  development  takes  place  on  two  types.  In  the 
cases  where  the  egg-shell  is  strong,  and  the  egg  is  laid  prior  to 
the  formation  of  the  embryo,  a  ciliated  larva  is  developed  (Bo- 
thriocephalus latus,  ditremus,  Schistocephalus  dimorphus,  Ligula 
simplicissima,  etc.1). 

Of  these  forms  Bothriocephalus  latus  may  be  taken  as  type. 

The  development  of  the  embryo  requires  many  months  for 
its  completion.  The  outer  layer  becomes  ciliated  while  the 
central  mass  has  already  become  developed  into  a  six-hooked 
embryo.  The  embryo  leaves  its  shell  by  the  opercular  aperture, 
and  for  some  time  swims  rapidly  about  by  means  of  its  long  cilia. 
The  ciliated  coating  is  eventually  stripped  off,  and  the  six-hooked 
larva  emerges. 

In  the  second  type  of  embryo  the  external  cellular  layer  does 
not  become  ciliated.  This  is  the  most  usual  arrangement,  and  is 
even  found  in  many  species  of  Bothriocephalus. 

1  Vide  for  list  of  such  forms  at  present  known  Willemoes  Suhm,  No.  231. 

14—2 


212  CESTODA. 

The  central  mass  of  cells  becomes  developed,  as  in  the  other 
type,  into  a  six-hooked  (rarely  four-hooked)  embryo  (fig.  96  G), 
but  the  superficial  layer  separates  from  the  central,  and  either 
disappears  or  becomes  (Bothriocephalus  proboscideus]  a  cuticular 
layer.  Between  the  six-hooked  embryo  and  the  outer  layer  of 
cells  one  or  more  thick  membranes  become  deposited  (E.  van 
Beneden).  The  eggs  are  carried  out  of  the  alimentary  canal  in 
the  proglottis  and  transported  to  various  situations  on  land  or 
in  water.  They  usually  remain  within  the  proglottis,  invested 
by  their  thick  shell,  till  taken  up  into  the  alimentary  canal  of 
a  suitable  host,  or  they  may  be  swallowed  after  the  death  and 
decay  of  the  proglottis.  They  are  subsequently  hatched  after 
their  shell  has  become  softened  by  the  action  of  the  digestive 
fluids. 

Before  proceeding  to  describe  their  further  history,  the  close 
resemblance  between  the  first  developmental  stages  of  Cestoda, 
especially  in  the  case  of  the  ciliated  larvae,  and  those  of  Trema- 
toda,  may  be  pointed  out. 

In  both  there  is  a  ciliated  larva,  and  in  both  there  is  developed 


FK;.  96.  DIAGRAMS  OF  VARIOUS  STAGES  IN  TMK  DKVKI.OPMENT  OF  THE 
••DA.  (From  Huxley.) 

A.  Cysticercus.  H.  and  C.  Cysticerci  in  the  everted  (B)  and  inverted  (C)  con- 
dition. I),  (''"'minis.  K.  and  F.  I  )iat;rams  of  Kchinococcus.  It  is  most  probable  that 
T;unia  heads  are  not  developed  directly  from  the  wall  of  the  cyst  as  represented  in 
the  diagram,  (i.  Six-hooked  embryo. 

within  the  ciliated  skin  a  second  larva,  which  becomes  freed  by 
the  stripping  off  of  the  ciliated  skin. 

The  type  of  development  has  moreover  many  analogies  with 
that  of  the  Nemcrtine  larva  of  Desor,  p.  163  (cf.  Mctschnikoff), 
and  is  probably  like  that  an  abbreviated  record  of  a  long  history. 

The  suitable  host  for  the  six-hooked   embryo  to  enter   is 


PLATYELMINTHES.  213 


rarely  the  same  as  the  host  for  the  sexual  form.  The  embryos 
having  become  transported  into  the  alimentary  canal  of  such  a 
host,  and  become  free,  if  previously  invested  by  the  egg-shell, 
soon  make  their  way,  apparently  by  the  help  of  their  hooks, 
through  the  wall  of  the  alimentary  tract,  and  are  transported  in 
the  blood  or  otherwise  into  some  suitable  place  for  them  to 
undergo  their  next  transformation.  This  place  may  be  the  liver, 
lungs,  muscles,  connective  tissue,  or  even  the  brain  (e.g.  Ccenurus 
cerebralis  in  the  brain  of  sheep). 

Here  they  become  enclosed  in  a  granular  deposit  from  the 
surrounding  tissues,  which  becomes  in  its  turn  enclosed  in  a  con- 
nective-tissue coat.  Within  lies  the  solid  embryo,  the  hooks  of 
which  in  many  cases  disappear  or  become  impossible  to  make 
out.  In  other  forms,  e.g.  Cysticercus  limacis,  they  remain  visible, 
and  then  mark  the  anterior  pole  of  the  worm  (fig.  98,  c.}.  The 
central  part  of  the  body  next  becomes  transformed  into  a  material 
composed  of  clear  non-nucleated  vesicles.  Accompanying  these 
changes  the  embryo  grows  rapidly  in  size ;  a  cuticle  is  deposited 
by  its  outer  layer,  in  which  also  an  external  layer  of  circular 
muscular  fibres  and  an  internal  layer  of  longitudinal  fibres  become 
differentiated,  and  internal  to  both  there  is  formed  a  layer  of 
granular  cells. 

With  the  rapid  growth  of  the  body  a  central  cavity  is  formed, 
which  becomes  filled  with  fluid,  and  the  embryo  assumes  the  form 
of  a  vesicle.  At  the  same  time  a  system  of  excretory  vessels, 
sometimes  opening  by  a  posterior  pore,  becomes  visible  in  the 
wall  of  the  vesicle. 

The  embryo  has  now  reached  a  condition  in  which  it  is  known 
as  a  cystic-  or  bladder-worm,  and  may  be  compared  in  almost 
every  respect  with  the  sporocyst  of  a  Trematode  (Huxley). 

The  next  important  change  consists  in  the  development  of  a 
head,  which  becomes  the  head  of  the  adult  Tamia.  This  is 
formed  in  an  involution  of  the  outer  wall  of  the  anterior  ex- 
tremity of  the  cystic  worm.  This  involution  forms  a  papilliform 
projection  on  the  inner  surface  of  the  wall  of  the  cystic  worm, 
with  an  axial  cavity  opening  by  a  pore  on  the  outer  surface. 
The  layer  of  cells  forming  the  papilla  soon  becomes  divided 
into  two  laminae,  of  which  the  outer  forms  a  kind  of  investing 
membrane  for  the  papilla.  The  papilla  itself  now  becomes 


2I4 


CESTODA. 


Kic.  97.     CYS- 

TICERCUS      CELLU- 
LOSE.    (From  Ge- 
fenbaur,  after  von 
iebold.) 

a.  Caudal  ve- 
sicle, c.  Anterior 
part  of  body,  d, 
head. 


moulded  into  a  Cestode  head,  which  however  is  developed  in 
an  inverted  position.  The  suckers  and  hooks 
(when  present)  of  the  head  are  developed  on  a 
surface  bounding  the  axial  lumen  of  the  papilla, 
which  is  the  true  morphological  outer  surface, 
while  the  apparent  outer  surface  of  the  papilla 
is  that  which  eventually  forms  the  interior  of 
the  (at  first)  hollow  head.  Before  the  external 
armature  of  the  head  has  become  established, 
four  longitudinal  excretory  vessels,  continuous 
with  those  in  the  body  of  the  cystic  worm,  make 
their  appearance.  They  are  united  by  a  circular 
vessel  at  the  apex  of  the  head.  The  develop- 
ment is  by  no  means  completed  with  the  simple 
growth  of  the  head,  but  the  whole  inverted  papilla  continues  to 
grow  in  length,  and  gives  rise  to  what  afterwards  becomes  part 
of  the  trunk.  The  whole  papilla  eventually  becomes  everted, 
and  then  the  cystic  worm  takes  the  form  (fig.  97)  of  a  head  and 
unsegmented  trunk  with  a  vesicle — the  body  of  the  cystic  worm 
— attached  behind.  The  whole  larva  is  known  as  a  Cysticercus. 
The  term  scolex,  which  is  also  sometimes  employed,  may  be 
conveniently  retained  for  the  head  and  trunk  only.  The  head 
differs  mainly  from  that  of  the  adult  in  being  hollow. 

There  are  great  variations  in  the  relative  size  of  the  head  and  the 
vesicle  of  Cysticerci.  In  some  forms  the 
vesicle  is  very  small  (fig.  98),  e.g.  Cysticercus 
limacis ;  it  is  medium-sized  in  Cysticercus 
cellulosce  (fig.  97),  and  in  some  forms  is  much 
larger.  The  embryonic  hooks,  when  they 
persist,  are  found  at  the  junction  of  the  trunk 
and  the  vesicle  (fig.  98  A,  c}.  Though  the 
majority  of  cystic  worms  only  develope  one 
head,  this  is  not  invariably  the  case.  There 
is  a  cystic  worm  found  in  the  brain  of  the 
sheep  known  as  Ccenurus  cerebralis — the  larva 
of  Tcenia  caenurus,  parasitic  in  the  intestine 
of  the  dog — which  forms  an  exception  to  this 
rule.  There  appears,  to  start  with,  a  tuft  of 
three  or  four  heads,  and  finally  many  hun- 
dred heads  are  developed  (fig.  96  D).  They 


FK;.  98.    CYSTICERCUS 

\\Y\\\  SMALL  CAUl'Al    VESICLE, 

A.  Head  involuted.  B. 
Head  everted. 

a.  Scolex.  />.  caudal  vesicle. 
c.  (in  A)  six  embryonic  hooks. 


are  arranged  in  groups  at  one  (the  anterior?)  pole  of  the  cystic  worm. 


PLATYELMINTHES. 


A  still  more  complicated  form  of  cystic  worm  is  that  known  as  Echino- 
coccus,  parasitic  in  the  liver,  lungs,  etc.  of  man  and  various  domestic  Un- 
gulata.  In  the  adult  state  it  is  known  as  Tcenia  echinococcus  and  infests 
the  intestine  of  the  dog.  The  cystic  worm  developed  from  the  six-hooked 
embryo  has  usually  a  spherical  form,  and  is  invested  in  a  very  thick  cuticle 
(fig.  96  E  and  F,  and  fig.  99).  It  does  not  itself  directly  give  rise  to  Taenia 
heads,  but  after  it  reaches  a  certain  size  there  are  formed  on  the  inner 
side  of  its  walls  small  protuberances,  which  soon  grow  out  into  vesicles 
connected  with  the  walls  of  the  cyst  by  narrow  stalks  (figs.  96  F  and  99  C). 
In  the  interior  of  these  vesicles  a  cuticle  is  developed.  It  is  in  these 
secondary  vesicles  that  the  heads  originate.  According  to  Leuckart,  they 
either  arise  as  outgrowths  of  the  wall  of  the  vesicle  on  the  inner  face 
of  which  the  armature  is  developed,  which  subsequently  become  involuted 
and  remain  attached  to  the  wall  of  the  vesicle  by  a  narrow  stalk,  or  they 
arise  from  the  first  as  papilliform  projections  into  the  lumen  of  the  vesicle, 
on  the  outer  side  of  which  the  armature  is  formed.  Recent  observers  only 
admit  the  second  of  these  modes  of  development.  The  Echinococcus  larva, 
in  addition  to  giving  rise  to  the  above  head-producing  vesicles,  also  gives 
rise  by  budding  to  fresh  cysts,  which  resemble  in  all  respects  the  parent 
cyst.  These  cysts  may  either  be  detached  in  the  interior  (fig.  96  F)  of 
the  parent  or  externally.  They  appear  to  spring  in  most  cases  from  the 
walls  of  the  parent  cyst,  but  there  are  some  discrepancies  between  the 
various  accounts  of  the  process.  In  the  cysts  of  the  second  generation 
vesicles  are  produced  in  which  new  heads  are  formed.  As  the  primitive 
cyst  grows,  it  naturally  becomes  more  and  more  complicated,  and  the  num- 
ber of  heads  to  which  one  larva  may  give  rise  becomes  in  this  way  almost 
unlimited. 

Cysticerci  may  remain  a  long  time  without  further  develop- 
ment, and  human  beings  have  been  known  to  be  infested  with 
an  Echinococcus  cyst  for  over  thirty  years.  When  however  the 
Cysticercus  with  its  head  is  fully  developed,  it  is  in  a  condition 
to  be  carried  into  its  final  host.  This  takes  place  by  the  part  of 
one  animal  infested  with  cysticerci  becoming  eaten  by  the  host 
in  question.  In  the  alimentary  canal  of  the  final  host  the  con- 
nective-tissue capsule  is  digested,  and  then  the  vesicular  caudal 
appendage  undergoes  the  same  fate,  while  the  head,  with  its 
suckers  and  hooks,  attaches  itself  to  the  walls  of  the  intestine. 
The  head  and  rudimentary  trunk,  which  have  been  up  to  this 
time  hollow,  now  become  solid  by  the  deposition  of  an  axial 
tissue;  and  the  trunk  very  soon  becomes  divided  into  segments, 
known  as  proglottides  (fig.  99  A).  These  segments  are  not 
formed  in  the  same  succession  as  those  of  Chaetopods ;  the 


216 


CKSTODA. 


youngest  of  them  is  that  nearest  to  the  head,  and  the  oldest 
that  furthest  removed  from  it.     Each  segment  appears  in  fact 
to   be  a  sexual  individual,  and  is 
capable  of  becoming  detached  and 
leading  for  some  time  an  indepen- 
dent  existence.      In    some   cases, 
e.g.  Cysticcrcus  fasciolaris,  the  seg- 
mentation of  the  trunk  may  take 
place  while  the  larva  is  still  in  its 
intermediate  host. 

The  stages  in  the  evolution  of 
the  Cestoda  are  shortly  as  follows : 

1.  Stage  with  embryonic  epi- 
dermis  either   ciliated  (Bothrioce- 
phalus,  etc.)  or  still  enclosed  in  the 
egg-shell.     This  stage  corresponds 
to  the  ciliated  larval  stage  of  the 
Trematoda. 

2.  Six-hooked  embryonic  stage 
after  the  embryonic  epidermis  has 
been  thrown  off.     During  this  stage 
the  embryo  is  transported  into  the 
alimentary  tract  of  its  intermediate 
host,  and  boring  its  way  into  the 
tissues,  becomes  encapsuled. 

3.  It  develops  during  the  en- 
capsuled  state  into  a  cystic  worm, 
equivalent     to    the    sporocyst    of 
Trematoda. 

4.  The  cystic  worm  while  still  encapsuled  develops  a  head 
with  suckers  and  hooks,  becoming  a  Cysticercus.    In  some  forms 
(Ccenurus,  Echinococcus)  reproduction  by  budding  takes  place  at 
this  stage.     The  head  and  trunk  are  known  as  the  scolex. 

5.  The    Cysticercus    is    transported    into   the   second    and 
permanent  host  by  the  infested  tissue  being  eaten.    The  bladder- 
like  remains  of  the  cystic  worm  are  then  digested,  and  by  a 
process  of  successive  budding  a  chain  of  sexual  proglottides  are 
formed  from  the  head,  which  remains  asexual. 

The   above   development "  is   to   be  regarded   as  a  case   of 


FIG.  99.    ECHINOCOCCUS  VETE- 
RINORUM.     (From  Huxley.) 

A.  Tsenia  head  or  scolex.     a. 
hooks,      b.    suckers,      c.     cilia     in 
water  vessel,     d.    refracting    parti- 
cles in  body  wall. 

B.  single  hooks. 

C.  portion  of  cyst.     a.  cuticle. 
b.    membranous    wall    of    primary 
cyst.     c.  and   e.  scolex   heads,      d. 
secondary  cyst. 


PLATYELMINTHES. 


217 


FIG.  99  A.    TETRARHYNCUS.     (From 
Gegenbaur  ;  after  Van  Beneden.) 

A.  Asexual  state. 

B.  Sexual  stage  with  ripe  proglottides. 


complicated  metamorphosis  secondarily  produced  by  the  neces- 
sities of  a  parasitic  condition,  to  which  an  alternation  of  sexual 
and  gemmiparous  generations 
has  been  added.  The  alter-  A- 
nation  of  generations  only 
occurs  at  the  last  stage  of  the 
development,  when  the  so-  B| 
called  head,  without  generative 
organs,  produces  by  budding 
a  chain  of  sexual  forms,  the 
embryos  of  which,  after  pass- 
ing through  a  complicated 
metamorphosis,  again  become 
Cestode  heads. 

In  the  case  of  Ccenurus  and  Echinococcus  two  or  more 
asexual  generations  are  interpolated  between  the  sexual  ones. 
It  is  not  quite  clear  whether  the  production  of  the  Taenia  head 
from  the  cystic  worm  may  not  be  regarded  as  a  case  of  budding. 
There  are  some  grounds  for  comparing  the  scolex  to  the  Cercaria 
of  Trematodes,  cf.  Archigetes. 

As  might  be  anticipated  from  the  character  of  the  Cestode  metamor- 
phosis, the  two  hosts  required  for  the  development  are  usually  forms  so 
related  that  the  final  host  feeds  upon  the  intermediate  host.  As  familiar 
examples  of  this  may  be  cited  the  pig,  the  muscles  of  which  may  be 
infested  by  Cysticercus  cellulosce,  which  becomes  the  Tcenia  solium  of  man. 
Similarly  a  Cysticercus  infesting  the  muscles  of  the  ox  becomes  the  TcBnia 
mediocanellata  of  man.  The  Cysticercus  piscifonnis  of  the  rabbit  becomes 
the  Tcenia  serrata  of  the  dog.  The  Coenurus  cerebralis  of  the  sheep's 
brain  becomes  the  Tcenia  ccenurus  of  the  dog.  The  Echinococcus  of  man 
and  the  domestic  herbivores  becomes  the  Tcenia  echinococcus  of  the  dog. 

Cystic  worms  infest  not  only  Mammalian  forms,  but  lower  Vertebrates, 
various  fishes  which  form  the  food  of  other  fishes,  and  Invertebrates  liable 
to  be  preyed  on  by  vertebrate  hosts.  So  far  the  Cestodes  (except  Archi- 
getes) are  only  known  to  attain  sexual  maturity  in  the  alimentary  tracts  of 
Vertebrata. 

The  rule  that  the  intermediate  host  is  not  the  same  as  the  final  host  does 
not  appear  to  be  without  exception.  Redon1  has  shewn  by  experiments 
on  himself  that  a  Cysticercus  (celluloses)  taken  from  a  human  subject 
developes  into  Tcenia  solium  in  the  intestines  of  a  man.  Redon  took 
four  cysts  of  a  Cysticercus  from  a  human  subject,  and  after  three  months 
passed  some  proglottides,  and  subsequently  the  head  of  Tania  solium. 

1  Annal.  d.  Scien.  Nat.,  6th  Series,  Vol.  vi.  1877. 


2l8  CESTODA. 


Some  important  variations  of  the  typical  development  are  known. 

The  so-called  head  or  scolex  may  be  formed  without  the  intervention 
of  a  cystic  stage.  In  Archigetes  (Leuckart,  No.  227),  which  infests,  in 
the  Cysticercus  condition,  the  body-cavity  of  various  invertebrate  forms 
(Tubifex,  etc.),  the  six-hooked  embryo  becomes  elongated  and  divided  into 
two  sections,  one  forming  the  head,  while  the  other,  with  the  six  embryonic 
hooks,  forms  an  appendage,  homologous  with  the  caudal  vesicle  of  other 
Cysticerci. 

The  embryo  of  Tcenia  elliptica  similarly  gives  rise  to  a  Cysticercus 
infesting  the  dog-louse  (Trichodectes  cants],  without  passing  through  a 
vesicular  condition ;  but  the  caudal  vesicle  disappears,  so  that  it  forms 
simply  a  scolex.  These  cases  may,  it  appears  to  me,  be  probably  regarded 
as  more  primitive  than  the  ordinary  ones,  where  the  cystic  condition  has 
become  exaggerated  as  an  effect  of  a  parasitic  life. 

In  some  cases  the  larva  of  a  Taenia  has  a  free  existence  in  the  scolex 
condition.  Such  a  form,  the  larva  of  Phyllobothrium,  has  been  observed  by 
Claparede1.  It  was  not  ciliated,  and  was  without  a  caudal  vesicle;  and 
was  no  doubt  actively  migrating  from  an  intermediate  host  to  its  permanent 
host. 

Scolex  forms,  without  a  caudal  vesicle,  are  found  in  the  mantle  cavity 
of  Cephalopoda,  and  appear  to  be  occupying  an  intermediate  host  in  their 
passage  from  the  host  of  the  cystic  worm  to  that  of  the  sexual  form. 

Archigetes,  already  mentioned,  has  been  shewn  by  Leuckart  (No.  227) 
to  become  sexually  mature  in  the  Cysticercus  state,  and  thus  affords  an 
interesting  example  of  paedogenesis.  It  is  not  known  for  certain  whether 
under  normal  circumstances  it  reaches  the  mature  state  in  another  host. 

Amphilina.  The  early  stages  of  this  interesting  form  have  been 
investigated  by  Salensky  (No.  229),  and  exhibit  clear  affinities  to  those  of 
the  true  Cestoda.  An  embryonic  provisional  skin  is  formed  as  in  Cestodes  ; 
and  pole-cells  also  appear.  Within  the  provisional  skin  is  formed  an 
embryo  with  ten  hooks.  After  hatching  the  provisional  skin  is  at  once 
thrown  off,  and  the  larva,  which  is  then  covered  by  a  layer  of  very 
fine  cilia,  becomes  free.  The  further  metamorphosis  is  not  known. 

BIBLIOGRAPHY. 
Turbellaria. 

(181)  Alex.  Agassiz.     "On  the  young  stages  of  a  few  Annelids"  (Planaria 
angulatd).     Annals  Lyceum  Nat.  Hist,  of  Nav  York,  Vol.  vili.  1866. 

(182)  Dalyell.     "  Powers  of  the  Creator. " 

(183)  C.  Girard.     "  Embryonic  development  of  Planocera  elliptica."    Jour,  of 
Acad.  of  Nat.  Set.  Philadelphia.     New  Series,  Vol.  1 1.  1854. 

(184)  Alex.   Gotte.     "Zur  Entwicklungsgeschichte  d.  Seeplanarien. "     Zoolo- 
gischtr  Anzeiger,  No.  4,  1878. 

1  Beobachtungen  iib.  Anat.  H.  lint-wick.  Wirbell.  Thiere.     Leipzig,  1863. 


PLATYELMINTHES.  219 


(185)  P.  Halle  z.     Contributions  a  Chistoire  natitrelle  des  Ttirbellaries.     Thesis  a 
la  faculte  des  Sciences  p.  le  grade  d.  Docteur  es-sciences  naturelles,  Lille,  1879. 

(186)  Knappert.     "  Bijdragen  tot   de   Ontwikkelings-Geschiedenis  der   Zoet- 
water-Planarien."     Provinciaal  Utrechtsch  Genootschap  van  Kunsten  en   Wetenschap- 
pen.     Utrecht,  1865. 

(187)  W.  Keferstein.     "  Beitrage  z.  Anat.  u.  Entwick.  ein.  Seeplanarien  von 
St.  Malo."     Abh.  d.  konig.  GeselL  d.  Wiss.  zu  Gottingen.     Bd.  xiv.  1868. 

(188)  El.  Metschnikoff.    "  Untersuchungen lib.  d.  Entwicklungd.  Planarien." 
Notizen  d.  neurussischen  Gesellschaft  d.  Naturforscher.     Odessa,  Bd.  V.   1877.     Vide 
Hoffman  and  Schwalbe's  Bericht  for  1878. 

(189)  H.  N.  Moseley.     "On  Stylochus  pelagicus  and  a  new  species  of  pelagic 
Planarian,  with  notes  on  other  pelagic  species,  on  the  larval  forms  of  Thysanozoon, 
etc."     Quart.  Journ.  of  Micr.  Science.     Vol.  xvn.  1877. 

(190)  J.  Miiller.     "  Ueber  eine  eigenthtimliche  Wurmlarva  a.  d.  Classe  d.  Tur- 
bellarien,  etc."     Miiller's  Archiv  f.  Anat.  u.  Phys.  1850. 

(191)     "Ueber  verschiedene  Formen  von  Seethieren."     Miiller's  Archiv  f. 

Anat.  und  Phys.  1854. 

Nemertea. 

(192)  J.  Barrois.     "  L'Embryologie  des  Nemertes."     An.  Sd.  Nat.  Vol.  VI. 
1877. 

(193)  O.  Biitschli.     Archiv  f.  Naturgeschichte,  1873. 

(194)  A.  Krohn.     "  Ueb.  Pilidium  u.  Actinotrocha."     Miiller's  Archiv,  1858. 

(195)  E.  Des  or.     "  Embryology  of  Nemertes."     Proceedings  of  the  Boston  Nat. 
History  Society,  Vol.  vi.  1848. 

(196)  G.  Dieck.     " Entwicklungsgeschichte  d.  Nemertinen."     Jenaische  Zeit- 
schrift,  Vol.  viii.  1874. 

(197)  C.  Gegenbaur.     " Bemerkungen  iib.  Pilidium  gyrans,  etc."     Zeitschrift 
fur  wiss.  Zool.,  Bd.  v.  1854. 

(198)  C.  K.  Hoffmann.    " Entwicklungsgeschichte  von  Tetrastemma  tricolor." 
Niederldndisches  Archiv,  Vol.  ill.  1876,  1877. 

(199)     "Zur  Anatomic  und  Ontogenie  von  Malacobdella."    Niederldndisches 

Archiv,  Vol.  IV.  1877. 

(200)  W.  C.  Mc  In  tosh.     British  Annelids.      The  Nemerteans.     Ray  Society, 

1873-4- 

(201)  Leuckart  u.  Pagenstecher.    "  Untersuchungen  iib.  niedere  Seethiere." 
Miiller's  Archiv,  1858. 

(202)  E.  Metschnikoff.     "  Studien  iib.  die  Entwicklung  d.  Echinodermen  u. 
Nemertinen."     Mem.  Acad.  imp.  Petersburg,  vn.  Ser.  Tom.  xiv.  No.  8,  1869. 

Trematoda. 

(203)  T.  S.  Cobbold.     Entozoa.     Groombridge  and  Son,  1864. 

(204)  Parasites  ;  a  Treatise  on  the  Entozoa,  etc.    Churchill,  1879. 

(205)  Filippi.     Mem.  p.  servir  a  Fhistoire  genetiqite  dts   Tremalodes.     Ann. 
Scien.  Nat.  4th  Series,  Vol.  u.  1854,  and  Mem.  Acad.  Torino,  1855 — 1859. 

(206)  R.  Leuckart.     Die  menschlichen  Parasiten,  Vol.  I.  1863,  p.  485,  et  seq. 

(207)  H.  A.  Pagenstecher.     Trematoden  u.  Trematodenlarven.     Heidelberg, 
1857. 


220  mm  i'  H.KAPHY. 


(208)  C.  Th.  von  Siebold.     Lehrbttch  d.  vergleich.  Anat.  wirhclloscr  Thicrc. 
Merlin,  1848. 

(209)  J.  J.  S.  Steenstrup.      Gcnerationswcchsel.      1842. 

(210)  R.  v.  Willemoes-Suhm.     "  Zur  Naturgeschichte  d.    Polystomum  inte- 
gerrimum,  etc."     Zeit.  f.  wiss.  Zool.  Vol.  xxn.  1872. 

(211)  -    -  "Helminthologische  Notizen  III."     Zeit.  /.  wiss.  Zool.  Vol.  xxm. 
1873.     Vide  this  paper  for  a  summary  of  known  observations  and  literature. 

(212)  G.  R.  Wagener.    Bt'ilriige  zur  Entwicklnngsgeschichte  d.  Eingeweidewiir- 
mtr.     Haarlem,  1855. 

(21H)     G.  R.  Wagener.    "  Helminthologische  Bemerkungen,  etc."    Zeit.  f.  wiss. 
Zool.  Vol.  ix.  1850. 

(214)  G.  R.  Wagener.     "  Ueb.  Gyrodactylus  elegans."     Archiv  f.  Anat.  u. 
Phys.  1860. 

(215)  E.  Zeller.     "  Untersuchungen  lib.  d.   Entwicklung  d.    Uiplozoon   para- 
doxum."     Zeit.f.  wiss.  Zool.  Vol.  xxn.  1872. 

(216)  E.  Zeller.     "  Untersuchungen  ii.  d.  Entwick.  u.  Ban  d.  Polystomum  inte- 
gerrimum."     Zeit.f.  wiss.  Zool.  Vol.  xxn.  1872. 

(217)  E.  Zeller.     "Weitere  Beitrage  z.  Kenntniss  d.   Polystomen."    Zeit.  f. 
wiss.  Zool.  Vol.  xxvn.  1876. 

Cestoda. 

(218)  Ed.  van  Beneden.     "  Recherches  sur  la  composition  et  la  signification  d. 
Toeuf."     Mem.  cour.  Acad.  roy.  Belgique.     Vol.  xxxiv.  1868. 

(219)  P.  J.   van   Beneden.     "  Les  vers  Cestoides  considered  sous  le  rapport 
physiologique  embryogenique,  etc."     Bui.  Acad.  Scien.  Bruxelles.     Vol.  xvn.  1850. 

(220)  T.  S.  Cob  bo  Id.     Entozoa.     Groombridge  and  Son,  1864. 

(221)     Parasites ;  a  treatise  on  the  Entozoa,  etc.     Churchill,  1879. 

(222)  Th.  H.  Huxley.     "On  the  Anatomy  and  Development  of  Echinococcus 
veterinorum."    Proc.  Zool.  Soc.  Vol.  xx.  1852. 

(223)  J.  Knoch.     "Die  Naturgesch.  d.  breiten  Bandwurmer."     Mini.    Acad. 
Imp.  Pctersbourg,  Vol.  V.  Ser.  7,  1863. 

(224)  F.  Kiichenmeister.     "  Ueber  d.  Umwandlung  d.  Finnen  Cysticerci  in 
Bandwiirmer  (Taenien)."     Prag  Vierteljahrsschr.  1852. 

(226)    "  Experimente  Ub.  d.  Entstehung  d.  Cestoden.     2°   Stufe  zunachst 

d.  Ccenurus  cerebralis."     Gunsburg,  Zeitsch.  klin.  Med.  IV.  1853. 

(226)  R.  Leuckart.     Die  Menschlichen  Parasiten,  Vol.  I.  Leipzig,  1863.    Vide 
also  additions  at  the  end  of  the  ist  and  2nd  volume. 

(227)  R.  Leuckart.     "Archigetes  Sieboldii,  eine  geschlechtsreife  Cestodenam- 
me."     Zeit.f.  wiss.  Zool.,  Vol.  XXX.  Supplement,  1878. 

(228)  El.  Metschnikoff.     "Observations  sur  le  developpement  de  quelques 
animaux  ( Bothriocephalus  proboscideus)."     Bull.  Acad.  Imp.  St  Petersbourg,    Vol. 

XIII.   1869. 

(229)  W.  Salensky.     "Ueb.  d.  Bau  u.  d.  Entwicklungsgeschichte  d.  Amphi- 
lina."     Zeit.  f.  wiss.  Zool.,  Vol.  xxiv.  1874. 

(230)  Von  Siebold.     Burdach's  Physiologie. 

(231)  R.    von   Willemoes-Suhm.     "Helminthologische  Notizen."     Zeit.  f. 
wiss.  Zool.,  Vol.  xix.  xx.  xxn.  1869,  70  and  73. 


CHAPTER   VIII. 

ROTIFERA. 

FOR  many  reasons  a  complete  knowledge  of  the  ontogeny  of 
the  Rotifera  is  desirable.  They  constitute  a  group  which  retain 
in  the  trochal  disc  an  organ  common  to  the  embryos  of  many 
other  groups,  but  which  in  most  other  instances  is  lost  in  the 
adult  state.  In  the  character  of  the  excretory  organs  they 
exhibit  affinities  with  the  Platyelminthes,  while  in  other  respects 
they  possibly  approach  the  Arthropoda  (e.g.  Pedalion  ?).  The 
interesting  Trochosphcera  cequatorialis  of  Semper  closely  re- 
sembles a  monotrochal  polychaetous  larva. 

Up  to  the  present  time  our  embryological  knowledge  is 
mainly  confined  to  a  series  of  observations  by  Salensky  on 
Brachionus  urceolaris,  and  to  scattered  statements  on  other  larval 
forms  by  Huxley,  etc. 

In  many  cases  Rotifers  lay  summer  and  winter  eggs  of  a 
different  character.  The  former  are  always  provided  with  a  thin 
membrane,  and  frequently  undergo  development  within  the 
oviduct.  They  are  hatched  in  the  autumn.  The  winter  eggs 
are  always  provided  with  a  thick  shell. 

The  summer  eggs  are  of  two  kinds,  viz.  smaller  eggs  which 
become  males,  and  larger,  females.  On  the  authority  of  Cohn 
(No.  232)  they  are  believed  to  develope  parthenogenetically. 
Males  are  not  found  in  summer,  and  only  seem  to  be  produced 
from  the  summer  eggs.  Cohn's  observations,  especially  on 
Conochilus  volvox,  are  however  not  quite  satisfactory.  Huxley 
(No.  234)  came  to  the  conclusion  that  the  winter  eggs  of  Lacinu- 
laria  developed  without  previous  fertilization. 

The  following  are  the  more  important  results  of  Salensky's 
observations  (No.  236)  on  Brachionus  urceolaris. 

The  ovum  is  attached  by  a  short  stalk  to  the  hind  end  of  the 
body  of  the  female,  in  which  position  it  undergoes  its  develop- 
ment. It  will  be  convenient  to  treat  separately  the  development 
of  the  female  and  male,  and  to  commence  with  the  former.  The 


HRACHIONUS. 


female  ovum  divides  into  two  unequal  spheres,  of  which  the 
smaller  in  the  subsequent  stages  segments  more  rapidly  than  the 
larger.  The  segmentation  ends  with  the  formation  of  an  epibolic 
gastrula.  The  solid  inner  mass  of  cells  derived  from  the  larger 
sphere  constitutes  the  hypoblast,  and  is  more  granular  than  the 
epiblast  The  evolution  of  the  embryo  commences  with  the 
formation  of  a  depression  on  the  ventral  surface,  at  the  bottom 
of  which  the  stomodaeum  is  formed  by  an  invagination.  At  the 
hinder  part  of  the  depression  there  rises  up  a  rounded  protuber- 
ance which  eventually  becomes  the  caudal  appendage  or  foot. 
Immediately  behind  the  mouth  is  formed  an  underlip. 

On  the  sides  of  the  ventral  depression  are  two  ridges  which 
form  the  lateral  boundaries  of  the  trochal  disc.  They  appear  to 
unite  with  the  under  lip. 

In  a  later  stage  the  anterior  part  of  the  body  becomes  marked 
off  from  the  posterior  as  a  praeoral  lobe,  and  the  hypoblast  is  at 
the  same  time  confined  to  the  posterior  part.  The  supra-oeso- 
phageal  ganglion  is  early  formed  as  an  epiblastic  thickening  on 
the  dorsal  side  of  the  praeoral  lobe. 

The  first  cilia  to  appear  arise  at  the  apex  of  the  praeoral  lobe. 
At  a  later  period  the  lateral 
ridges  of  the  trochal  disc  meet 
dorsally  and  so  enclose  the  prae- 
oral lobe.  They  then  become 
coated  by  a  ring  of  cilia,  to  which 
a  second  ring,  completing  the 
double  ring  of  the  adult,  is  added 
later. 

In  the  trunk  an  indication  of 
a  division  into  two  segments 
makes  its  appearance  shortly 
after  the  development  of  the 
praeoral  lobe.  Before  this  period 
the  proctodaeum  is  established  as 
a  shallow  pit  immediately  behind 
the  insertion  of  the  foot.  The 
latter  structure  soon  becomes 
pointed  and  forked  (fig.  100,  /). 

The   complete   establishment 


FIG.  100.  EMBRYO  OF  BRACHIO- 
NUS  URCEOLARIS  SHORTLY  BEFORE  IT 
is  HATCHED.  (After  Salensky.) 

m.  mouth  ;  ms.  masticatory  appa- 
ratus ;  me.  mesenteron  ;  an.  anus  ;  Id. 
lateral  gland  ;  ov.  ovary  ;  /.  tail,  ;'.  e. 
foot ;  tr.  trochal  disc  ;  sg.  supra-ceso- 
phageal  ganglion. 


ROTIFERA.  223 


of  the  alimentary  canal  occurs  late.  The  stomodaeum  (fig.  100) 
gives  rise  to  the  mouth  (m),  oesophagus  and  masticatory  appara- 
tus (ins).  The  mesenteron  is  formed  from  the  median  part  of 
the  hypoblast ;  the  lateral  parts  of  which  appear  to  give  rise  to 
the  great  lateral  glandular  structures  (Id)  which  open  into  the 
stomach,  and  to  the  ovaries  (?)  (ov)  etc.  The  proctodaeum 
becomes  the  cloaca  and  anus  (an).  The  origin  of  the  mesoblast 
is  not  certainly  known.  The  shell  is  formed  before  the  larva  is 
hatched — an  occurrence  which  does  not  take  place  till  the  larva 
closely  resembles  the  adult. 

The  early  developmental  stages  of  the  male  are  closely 
similar  to  those  of  the  female ;  and  the  chief  difference  between 
the  two  appears  to  consist  in  the  development  of  the  male  being 
arrested  at  a  certain  point. 

The  larvae  of  Lacinularia  (Huxley,  No.  234)  are  provided 
with  a  praeoral  circlet  of  cilia  containing  two  eye-spots1,  and  a 
perianal  patch  of  cilia.  They  closely  resemble  some  telotrochal 
polychaetous  larvae. 

Salensky  has  compared  the  larva  of  Brachionus  to  that  of  a 
cephalophorous  Mollusc,  more  especially  to  the  larva  of  Calyp- 
traea  on  which  he  has  made  important  observations.  The 
praeoral  lobe,  with  the  ciliated  band,  no  doubt  admits  of  a 
comparison  with  the  velum  of  the  larva  of  Molluscs ;  but  it  does 
so  equally,  as  was  first  pointed  out  by  Huxley,  with  the  ciliated 
praeoral  lobe  of  the  larvae  of  many  Vermes.  It  further  deserves 
to  be  noted  that  the  trochal  disc  of  a  Rotifer  differs  from  the 
velum  of  a  Mollusc  in  that  the  eyes  and  ganglia  are  placed 
dorsally  to  it,  and  not,  as  in  the  velum  of  a  Mollusc,  within  it. 
The  larva  of  Lacinularia  appears  to  be  an  exception  to  this, 
since  two  eye-spots  are  stated  to  lie  within  the  circlet  of  cilia. 
More  important  in  the  comparison  is  the  so-called  foot  (tail), 
which  arises  in  the  embryo  as  a  prominence  between  the  mouth 
and  anus,  and  in  this  respect  exactly  corresponds  with  the 
Molluscan  foot. 

If  Salensky 's  comparison  is  correct,  and  there  is  something 
to  be  said  for  it,  the  foot  or  tail  of  Rotifers  is  not  a  post-anal 
portion  of  the  trunk,  but  a  ventral  appendage,  and  the  segmen- 

1  In  Leydig's  figure  of  the  larva,  Zeit,  f.  iviss.  Zool.  Vol.  ill.  1851,  the  eye-spots 
lie  just  outside  the  ciliated  ring. 


224  BRACHIONUS. 


tation  which  it  frequently  exhibits  is  not  to  be  compared  with  a 
true  segmentation  of  the  trunk.  If  the  Rotifers,  as  seems  not 
impossible,  exhibit  crustacean  affinities,  the  '  foot '  may  perhaps 
be  best  compared  with  the  peculiar  ventral  spine  of  the  Nauplius 
larva  of  Lepas  fascicularis  (vide  Chapter  on  Crustacea)  which  in 
the  arrangement  of  its  spines  and  other  points  also  exhibits  a 
kind  of  segmentation. 

BIBLIOGRAPHY. 

(232)  F.  Cohn.     "Ueb.  d.  Fortpflanzung  von  Raderthiere."     Zeit.f.  wiss.  Zool. 
Vol.  vii.  1856. 

(233)  F.  Cohn.     "  Bemerkungen  ii.  Raderthiere."     Zeit.  f.  wiss.  Zool.    Vol.  ix. 
1858,  and  Vol.  xn.  1862. 

(234)  T.  H.   Huxley.     "  Lacinularia   socialis."     Trans,   of  the  Microscopical 
Society,  1853. 

(235)  Fr.  Leydig.     "Ueb.  d.    Bau  u.    d.   systematische   Stellung  d.   Rader- 
thiere."    Zeit.f.  wiss.  Zool.  Vol.  vi.  1854. 

(236)  W.  Salensky.     "  Beit.  z.  Entwick.  von  Brachionus  urceolaris."     Zeit.f. 
wiss.  Zool.  Vol.  xxii.  1872. 

(237)  C.  Semper.     "  Zoologische  Aphorismen.     Trochosphaera  sequatorialis." 
Zeit.f.  wiss.  Zool.  Vol.  xxn.  1877. 


CHAPTER  IX. 


MOLLUSCA1. 

ALTHOUGH  the  majority  of  important  developmental  features 
are  common  to  the  whole  of  the  Mollusca,  yet  at  the  same  time 
many  of  the  subdivisions  have  well-marked  larval  types  of  their 
own.  It  will  for  this  reason  be  convenient  in  considering  the 
larval  characters  to  deal  successively  with  the  different  sub- 
divisions, but  to  take  the  whole  group  at  once  in  considering  the 
development  of  the  organs. 

Formation  of  the  layers  and  larval  characters. 
ODONTOPHORA. 

Gasteropoda  and  Pteropoda.  There  is  a  very  close  agree- 
ment amongst  the  Gasteropoda  and  Pteropoda  in  the  general 
characters  of  the  larva ;  but  owing  to  the  fact  that  the  eggs  of 

1  The  classification  of  the  Mollusca  adopted  in  the  present  chapter  is  shewn  in  the 
subjoined  table : 

I.     ODONTOPHORA.  II.     LAMELLIBRANCHIATA. 

1.  Gasteropoda.  «.    Dimya. 

a.  Prosobranchiata.  b.     Monomya. 

b.  Opisthobranchiata. 

c.  Pulmonata. 

d.  Heteropoda. 

2.  Pteropoda. 

a.  Gymnosomata. 

b.  Thecosomata. 

3.  Cephalopoda. 

a.  Tetrabranchiata. 

b.  Dibranchiata. 

4-  Polyplacophora. 

5-  Scaphopoda. 

B,  II.  15 


226  GASTEROPODA  AND   PTEROPODA. 

the  various  species  differ  immensely  as  to  the  amount  of  food- 
yolk,  considerable  differences  obtain  in  the  mode  of  formation  of 
the  layers  and  of  the  alimentary  tract. 

The  spheres  at  a  very  early  stage  of  segmentation1  become 
divided  into  two  categories,  one  of  them  destined  to  give  rise 
mainly  to  the  hypoblast,  the  other  mainly  to  the  epiblast.  Ac- 
cording as  there  is  much  or  little  food-yolk  the  hypoblast  spheres 
are  either  very  bulky  or  the  reverse.  In  all  cases  the  epiblast 
cells  lie  at  one  pole,  which  may  be  called  the  formative  pole, 
and  the  hypoblast  cells  at  the  opposite  pole.  When  the  bulk  of 
the  food-yolk  is  very  great,  the  number  of  hypoblast  spheres  is 
small.  Thus  in  Aplysia  there  are  only  two  such  spheres.  In  other 
cases,  where  there  is  but  little  food-yolk,  they  may  be  nearly 
as  numerous  as  the  epiblast  cells.  In  all  these  cases,  however, 
as  was  first  shewn  by  Lankester  and  Selenka,  a  gastrula  becomes 
formed  either  by  normal  invagination  as  in  the  case  of  Paludina 
(fig.  107),  or  by  epibole  as  in  Nassa  mutabilis  (fig.  105).  In  both 
cases  the  hypoblast  becomes  completely  enclosed  by  the  epiblast. 
T/ie  blastopore  is  always  situated  opposite  the  original  formative 
pole.  In  the  large  majority  of  cases  (i.e.  Marine  Gasteropoda, 
Heteropoda,  and  Pteropoda)  the  blastopore  becomes  gradually 
narrowed  to  a  circular  opening  which  eventually  occupies  the 
position  of  the  mouth.  It  either  closes  or  remains  permanently 
open  at  this  point.  In  some  cases  the  blastopore  remains  per- 
manently open  and  becomes  the  anus.  The  best  authenticated 
instance  of  this  is  Paludina  vivipara,  as  was  first  shewn  by 
Lankester  (No.  263). 

In  some  instances  the  blastopore  assumes  before  closing  a 
very  narrow  slit-like  form,  and  would  seem  to  extend  along  the 
future  ventral  region  of  the  body  from  the  mouth  to  the  anus. 
This  appears,  according  to  Lankester  (No.  262),  to  be  the  con- 
dition in  Lymnaeus,  but  while  Lankester  believes  that  the  closure 
proceeds  from  the  oral  towards  the  anal  extremity,  other  inves- 
tigators hold  that  it  does  so  in  the  reverse  direction.  Fol  (No. 
*2 4!);  has  also  described  a  similar  type  of  blastopore.  In  an  un- 
determined marine  Gasteropod,  with  an  embolic  gastrula,  observed 
by  myself  at  Valparaiso,  the  blastopore  had  the  same  elongated 

1  The  reader  is  referred  for  the  segmentation  to  pp.  98 — 101,  and  to  the  special 
description  of  separate  types. 


MOLLUSCA.  227 


form  as  in  Lymnaeus,  but  the  whole  of  it  soon  became  closed 
except  the  oral  extremity  ;  but  whether  this  finally  closed  could 
not  be  determined.  It  is  probable  that  the  typical  form  of  the 
blastopore  is  the  elongated  form  observed  by  Lankester  and  my- 
self, in  which  an  unclosed  portion  can  indifferently  remain  at 
either  extremity;  and  that  from  this  primitive  condition  the 
various  modifications  above  described  have  been  derived1. 

Before  the  blastopore  closes  or  becomes  converted  into  the 
oral  or  anal  aperture,  a  number  of  very  important  embryonic 
organs  make  their  appearance ;  but  before  describing  these  it 
will  be  convenient  to  state  what  is  known  with  reference  to  the 
third'  embryonic  layer  or  mesoblast. 

This  layer  generally  originates  in  a  number  of  cells  at  the  lips 
of  the  blastopore,  which  then  gradually  make  their  way  dorsal- 
wards  and  forwards,  and  form  a  complete  layer  between  the  epi- 
blast  and  hypoblast.  The  above  general  mode  of  formation  of 
the  mesoblast  may  be  seen  in  fig.  107,  representing  three  stages 
in  the  development  of  Paludina. 

In  some  cases  the  mesoblast  arises  from  certain  of  the  seg- 
mentation spheres  intermediate  in  size  between  the  epiblast  and 
hypoblast  spheres.  This  is  the  case  in  Nassa  mutabilis,  where 
the  mesoblast  appears  when  the  epiblast  only  forms  a  very  small 
cap  at  the  formative  pole  of  the  ovum  ;  and  in  this  case  the  meso- 
blast cells  accompany  the  epiblast  cells  in  their  growth  over  the 
hypoblast  (fig.  105). 

In  other  cases  the  exact  derivation  of  the  mesoblast  cells  is 
quite  uncertain.  The  evidence  is  perhaps  in  favour  of  their 
originating  from  the  hypoblast.  It  is  also  uncertain  whether  the 
mesoblast  is  bilaterally  symmetrical  at  the  time  of  its  origin.  It 
is  stated  by  Rabl  to  be  so  in  Lymnaeus2. 

In  the  case  of  Paludina  the  mesoblast  becomes  two  layers 

1  Rabl  (No.  268)  describes  a  blastopore  of  this  form  in  Planorbis  which  closes  at 
the  mouth. 

2  Rabl  (No.  268)  has  quite  recently  given  a  more  detailed  account  than  previous 
observers  of  the  origin  of  the  mesoblast  in  Planorbis.     He  finds  that  it   originates 
from  the  posterior  one  of  the  four  large  cells  which  remain  distinct  throughout  the 
segmentation.     By  the  division  of  this  cell  two  '  mesoblasts '  are  formed,  one  on  each 
side  of  the  middle  line  at  the  hinder  end  of  the  embryo.     Each  of  these  again  divides 
into  two,  an  anterior  and  a  posterior.     By  the  division  of  the  mesoblasts  there  arise 
two   linear   rows   of  mesoblastic   cells — the   mesoblastic  bands — which  are  directed 

15—2 


228 


GASTEROPODA   AND   PTEROPODA. 


thick,  and  tlun  splits  into  a  splanchnic  and  somatic  layer,  of 
which  the  former  attaches  itself  to  the  hypoblast,  and  gives  rise 
to  the  muscular  and  connective-tissue  wall  of  the  alimentary  tract, 
and  the  latter  attaches  itself  to  the  epiblast,  and  forms  the  mus- 
cular and  connective-tissue  wall  of  the  body  and  other  structures. 
The  two  layers  remain  connected  by  protoplasmic  strands,  and 
the  space  between  them  forms  the  body  cavity  (fig.  107).  In 
most  instances  there  would  appear  to  be  at  first  no  such  definite 
splitting  of  the  mesoblast,  but  the  layer  has  the  form  of  a  scattered 
network  of  cells  between  the  epiblast  and  the  hypoblast.  Finally 
certain  of  the  cells  form  a  definite  layer  over  the  walls  of  the 
alimentary  canal,  and  constitute  the  splanchnic  mesoblast,  and 
the  remaining  cells  constitute  the  somatic  mesoblast. 

We  must  now  return  to  the  embryo  at  the  time  when  the 
blastopore  is  becoming  narrowed.  First  of  all  it  will  be  necessary 
to  define  the  terms  to  be  applied  to  the  various  regions  of  the 
body — and  these  will 
best  be  understood  by 
taking  a  fully  formed 
larva  such  as  that  re- 
presented in  fig.  101. 
The  ventral  surface  I 
consider  to  be  that 
comprised  between  the 
mouth  (m)  and  the 
anus,  which  is  very 
nearly  in  the  position 
(i)  in  the  figure.  As  a 
great  protuberance  on 
the  ventral  surface  is 
placed  the  foot/  The 
long  axis  of  the  body, 
at  this  period  though  not  necessarily  in  the  adult,  is  that  passing 


FIG.  101.     DIAGRAM  OF  AN  EMBRYO  OF  I'I.KI- 
ROBRANCHIDIUM.     (From  Lankester.) 

f.  foot;  ot.  otocyst ;  m.  mouth;  v.  velum; 
ng.  nerve  ganglion ;  ry.  residual  yolk  spheres ;  s/is. 
shell-gland ;  i.  intestine. 


forwards  and  divided  transversely  into  two  parts,  an  anterior  continued  from  the  front 
mesoblast,  and  a  posterior  from  the  hinder  mesoblast. 

If  Rabl's  account  is  correct,  there  is  a  striking  similarity  between  the  origin  of  the 
mesoblast  in  Mollusca  and  in  Chaetopoda.  It  appears  to  me  very  probable  that  the 
mesoblastic  bands  are  formed  (as  in  Lumbricus)  not  only  from  the  products  of  the 
division  of  the  mesoblasts,  but  also  from  cells  budded  off  from  one  or  both  of  the 
primary  germinal  layers. 


MOLLUSCA.  229 


through  the  mouth  and  the  shell-gland  (shs.) :  while  the  dorsal 
surface  is  that  opposite  the  ventral  as  already  defined. 

Before  the  blastopore  has  attained  its  final  condition  three 
organs  make  their  appearance,  which  are  eminently  characteristic 
of  the  typical  molluscan  larva.  These  organs  are  (i)  the  velum, 
(2)  the  shell-gland,  (3)  the  foot. 

The  velum  is  a  provisional  larval  organ,  which  has  the  form 
of  a  praeoral  ring  of  cilia,  supported  by  a  ridge  of  cells,  often  in 
the  form  of  a  double  row,  the  ventral  end  of  which  lies  immedi- 
ately dorsal  to  the  mouth.  Its  typical  position  is  shewn  in  fig. 
101,  v.  There  are  considerable  variations  in  its  mode  and  extent 
of  development  etc.,  but  there  is  no  reason  to  think  that  it  is 
entirely  absent  in  any  group  of  Gasteropoda  or  Pteropoda.  In  a 
few  individual  instances,  especially  amongst  viviparous  forms 
and  land  Pulmonata,  it  has  been  stated  to  be  absent.  Semper 
(No.  274)  failed  to  find  it  in  Vitrina,  Bulimus  citrinus,  Vaginulus 
luzonicus,  and  Paludina  costata.  It  is  very  probably  absent  in 
Helix,  etc. 

In  some  cases,  e.g.  Limax  (Gegenbaur),  Neritina  (Claparede), 
Pterotrachaea  (Gegenbaur),  the  larva  is  stated  to  be  coated  by  an 
uniform  covering  of  cilia  before  the  formation  of  the  velum,  but 
the  researches  of  Fol  have  thrown  very  considerable  doubt  on 
these  statements.  In  some  cases  amongst  the  Nudibranchiata 
(Haddon)  and  Pteropoda  there  are  one  or  two  long  cilia  in  the 
middle  of  the  velar  area.  In  many  Nudibranchiata  (Haddon) 
there  is  present  a  more  or  less  complete  post-oral  ring  of  small 
cilia,  which  belongs  to  the  velum. 

The  cilia  on  the  velum  cause  a  rotation  of  the  larva  within 
the  egg  capsule.  Cilia  are  in  most  cases  (Paludina,  etc.)  developed 
on  the  foot  and  on  a  small  anal  area. 

The  shell-gland  arises  as  an  epiblastic  thickening  on  the  pos- 
terior and  dorsal  side.  In  this  thickening  a  deep  invagination 
(fig.  101,  shs.)  is  soon  formed,  in  which  a  chitinous  plug  may 
become  developed  (Paludina,  Cymbulia  ?  etc.),  and  in  abnormal 
larvae  such  a  chitinous  plug  is  generally  formed. 

The  foot  is  a  simple  prominence  of  epiblast  on  the  ventral 
surface,  in  the  cavity  of  which  there  are  usually  a  number  of  meso- 
blast  cells  (fig.  ioi,y).  The  larval  form  just  described  has  been 
named  by  Lankester  the  trochosphere  larva. 


230          GASTEROPODA  AND  PTEROPODA. 

Before  considering  the  further  external  changes  which  the 
larva  undergoes,  it  will  be  well  to  complete  the  history  of  the 
invaginated  hypoblast. 

The  hypoblast  has  after  its  invagination  either  the  form  of  a 
sack  (fig.  102)  or  of  a  solid  mass  (fig.  101).  Whether  the  mouth 
be  the  blastopore  or  no,  the  permanent 
oesophagus  is  formed  of  epiblast  cells,  so 
that  the  oesophagus  and  buccal  cavity 
are  always  lined  by  epiblast.  When  the 
blastopore  remains  permanently  open  the 
outer  part  of  the  oesophagus  grows  as  a 
prominent  ridge  round  the  opening. 

The  mesenteric  sack  itself  becomes 

IMG.    102.     EMBRYO   OF 
differentiated   into  a  stomach   adjoining     A  HETEROPOD.    (Fom  Ge- 

the  oesophagus,  a  liver  opening  immedi-     genbaur ;  after  Foh) 

.  0.   mouth;   v.   velum;  g. 

ately  behind  this,  and  an  intestine.     The     archenteron ;  p.  foot ;  c.  body 
cells  forming  the  hepatic  diverticula  and     cavity ;  s'  shell-Sland- 
sometimes   also   those   of  the  stomach  may  during  larval  life 
secrete  in  their  interior  peculiar  albuminous  products,  similar  to 
ordinary  food-yolk. 

The  proctodaeum,  except  when  it  is  the  blastopore,  arises 
later  than  the  mouth.  It  is  frequently  developed  from  a  pair  of 
projecting  epiblast  cells  symmetrically  placed  in  the  median 
ventral  line  behind  the  foot.  It  eventually  forms  a  very  shallow 
invagination  meeting  the  intestine.  Its  opening  is  the  anus. 
The  anus,  though  at  first  always  symmetrical  and  ventral,  subse- 
quently, on  the  formation  of  the  pallial  cavity,  opens  into  this 
usually  on  the  right  and  dorsal  side. 

In  the  cases  where  the  hypoblast  is  not  invaginated  in  the 
form  of  a  sack  the  formation  of  the  mesenteron  is  somewhat 
complicated,  and  is  described  in  the  sequel. 

From  the  trochosphere  stage  the  larva  passes  into  what  has 
been  called  by  Lankester  the  veliger  stage  (fig.  103),  which  is 
especially  characteristic  of  Gasteropod  and  Pteropod  Mollusca. 

The  shell-gland  (with  a  few  exceptions  to  be  spoken  of  subse- 
quently) of  the  previous  stage  flattens  out,  forming  a  disc-like 
area,  on  the  surface  of  which  a  delicate  shell  becomes  developed, 
while  the  epiblast  of  the  edges  of  the  disc  becomes  thickened. 
The  disc-like  area  is  the  mantle.  The  edge  of  the  area  and  with 


MOLLUSCA. 


231 


it  the  shell  now  rapidly  extend,  especially  in  a  dorsal  direction. 
Up  to  this  time  the  embryo  has  been  symmetrical,  but  in  most 
Gasteropods  the  shell  and  mantle  extend  very  much  more  to- 
wards the  left  than  towards  the  right  side,  and  a  commencement 
of  the  permanent  spiral  shell  is  thus  produced. 

The  edge  of  the  mantle  forms  a  projecting  lip  separating  the 
dorsal  visceral  sack  from  the  head  and  foot.  An  invagination 
appears,  usually  on  the  right  in  Gasteropods,  and  eventually 
extends  to  the  dorsal  side  (fig.  103  B).  It  gives  rise  to  the 


FIG.  103.    LARVAE  OF  CEPHALOPHOROUS  MOLLUSCA  IN  THE  VELIGER  STAGE. 
(From  Gegenbaur.) 

A.  and  B.     Earlier  and  later  stage  of  Gasteropod.     C.  Pteropod  (Cymbulia). 
v,  velum  ;  c.  shell ;  p.  foot ;  op.  operculum  ;  /.  tentacle. 

pallial  or  branchial  cavity,  and  receives  also  the  openings  of  the 
digestive,  generative  and  urinary  organs.  In  most  Pteropods  it 
is  also  formed  to  the  right,  and  usually  eventually  extends  after- 
wards towards  the  ventral  surface  (fig.  103  C).  In  the  pallial 
cavity  the  gills  are  formed,  in  those  groups  in  which  they  are 
present,  as  solid  processes  frequently  ciliated.  They  are  coated 
by  epiblast  and  contain  a  core  of  mesoblast.  They  soon  become 
hollow  and  contractile. 

The  velum  in  the  more  typical  forms  loses  its  simple  circular 
form,  and  becomes  a  projecting  bilobed  organ,  which  serves  the 
larva  after  it  is  hatched  as  the  organ  of  locomotion  (fig.  103  B 
and  C).  The  extent  of  the  development  of  the  velum  varies 
greatly.  In  the  Heteropods  especially  it  becomes  very  large,  and 
in  Atlanta  it  becomes  six-lobed,  each  lateral  half  presenting 
three  subdivisions.  It  is  usually  armed  on  its  projecting  edge 
with  several  rows  of  long  cilia,  and  below  this  with  short  cilia 


232  GASTEROPODA    AND    PTEROPODA. 

which  bring  food  to  the  mouth.  It  persists  in  many  forms  for  a 
very  long  period.  Within  the  area  of  the  velum  there  appear  the 
tentacles  and  eyes  (fig.  103  B).  The  latter  are  usually  formed  at 
the  base  of  the  tentacles. 

The  foot  grows  in  most  forms  to  a  very  considerable  size. 
On  its  hinder  and  dorsal  surface  is  formed  the  operculum  as  a 
chitinos  plate  which  originates  in  a  depression  lined  by  thick- 
ened epiblast,  much  in  the  same  way  as  the  shell  (fig.  103  B  and 
C,  op}.  In  the  typical  larval  forms  it  is  only  possible  to  distin- 
guish the  anterior  flattened  surface  of  the  foot  for  locomotion  and 
the  posterior  opercular  region,  but  special  modifications  of  the 
foot  are  found  in  the  Pteropods  and  Heteropods,  which  are 
described  with  those  groups.  The  foot  very  often  becomes  richly 
ciliated,  and  otic  vesicles  are  early  developed  in  it  (fig.  101,  of). 

All  the  Gasteropods  and  Pteropods  have  a  shell-bearing 
larval  form  like  that  first  described,  with  the  exception  of  a  few 
forms,  such  as  Limax  and  perhaps  some  other  Pulmonata,  in 
which  the  "shell-gland  closes  up  and  gives  rise  to  an  internal 
shell. 

The  subsequent  metamorphosis  in  the  different  groups  is  very 
various,  but  in  all  cases  it  is  accompanied  by  the  disappearance 
of  the  velum,  though  in  some  cases  remnants  of  the  velum  may 
persist  as  the  subtentacular  lobes  (Lymnaeus,  Lankester)  or  the 
lip  tentacles  (Tergipes,  Nordmann).  In  prosobranchiate  Gaste- 
ropods the  larval  shell  is  gradually  added  to,  and  frequently 
replaced  by,  a  permanent  shell,  though  the  free-swimming  velig- 
crous  larva  may  have  a  long  existence.  In  many  of  the  Opis- 
thobranchiata  the  larval  shell  is  lost  in  the  adult  and  in  others 
reduced.  Lankester,  who  has  especially  worked  at  the  early 
stages  of  this  group,  has  shewn  that  the  larvae  are  in  almost 
every  respect  identical  with  those  of  prosobranchiate  Gastero- 
pods. They  are  all  provided  with  a  subnautiloid  shell,  an  oper- 
culated  foot,  etc.  The  metamorphosis  has  unfortunately  been 
satisfactorily  observed  in  but  few  instances.  In  Heteropods  and 
Pteropods  the  embryonic  shell  is  in  many  cases  lost  in  the  adult. 

The  following  sections  contain  a  special  account  of  the  develop- 
ment in  the  various  groups  of  Gasteropoda  and  Pteropoda 
which  will  complete  the  necessarily  sketchy  account  of  the  pre- 
ceding pages. 


MOLLUSCA. 


233 


Gasteropoda.  To  illustrate  the  development  of  the  Gasteropoda  I 
have  given  a  detailed  description  of  two  types,  viz.  Nassa  mutabilis  and 
Paludina  vivipara. 

Nassa  mutabilis.  This  form,  the  development  of  which  has  been 
very  thoroughly  worked  out  by  Bobretzky  (No.  242),  will  serve  as  an 
example  of  a  marine  Gasteropod  with  a  large  food-yolk.  The  segmentation 


FIG.  104.    SEGMENTATION  OF  NASSA  MUTABILIS.    (From  Bobretzky.) 
A.  Upper  half  divided  into  two  segments.     B.  One  of  these  has  fused  with  the 
large  lower  segment.     C.  Four  small  and  one  large  segment,  one  of  the  former  fusing 
with  the  large  segment.     D.   Each  of  the  four  segments  has  given  rise  to  a  fresh 
small  segment.     E.  Small  segments  have  increased  to  thirty-six. 

has  already  been  described,  p.  102.  It  will  be  convenient  to  take  up  the  de- 
velopment at  a  late  stage  of  the  segmentation.  The  embryo  is  then  formed 
of  a  cap  of  small  cells  which  may  be  spoken  of  as  the  blastoderm  resting 
upon  four  large  yolk-cells  of  which  one  is  considerably  larger  than  the 
others  (fig,  104  A).  The  small  and  the  large  cells  are  separated  by  a 
segmentation  cavity.  The  general  features  at  this  stage  are  shewn  in 
fig.  105  A,  representing  a  longitudinal  section  through  the  largest  yolk- 
cell  and  a  smaller  yolk- cell  opposite  to  it.  The  blastoderm  is  for  the  most 
part  one  cell  thick,  but  it  will  be  noticed  that,  at  the  edge  of  the  blastoderm 
adjoining  the  largest  yolk-cell,  there  are  placed  two  cells  underneath 
the  edge  of  the  blastoderm  (me).  These  cells  are  the  commencement  of 
the  mesoblast.  In  the  later  stages  of  development  the  blastoderm  con- 
tinues to  grow  over  the  yolk-cells,  and  as  it  grows  the  three  smaller  yolk- 
cells  travel  round  the  side  of  the  largest  yolk-cell  with  it.  As  they  do 
so  they  give  rise  to  a  layer  of  protoplasmic  cells  (fig.  105,  hy)  which  form 
a  thickened  layer  at  the  edge  of  the  blastoderm  and  therefore  round  the 


234 


GASTEROPODA. 


lips  of  the  blastopore.  These  cells  form  the  hypoblast.  The  whole  of  the 
protoplasmic  matter  of  the  yolk-cells  is  employed  in  the  formation  of  the 
hypoblast.  The  rest  of  them  remains  as  a  mass  of  yolk.  A  longitudinal 
section  of  the  embryo  at  a  slightly  later  stage,  when  the  blastopore  has 
become  quite  narrowed,  is  represented  in  fig.  105  C.  The  greater  part  of 
the  dorsal  surface  is  not  represented. 

Two  definite  organs  have  already  become  established.     One  of  these  is  a 
pit  lined  by  thickened  epiblast  on  the  posterior  and  dorsal  side  (sg).     This 


FIG.  105.     LONGITUDINAL  SECTION  THROUGH  THE  EMBRYO  OF  NASSA  MUTA- 
BILIS.     (After  Bobretzky.) 

A.     Stage  when  the  mesoblast  is  commencing  to  be  formed. 

I'..     Stage  when  the  yolk  is  half  enclosed.    The  hypoblast  is  seen  at  the  lips  of  the 
blastopore. 

C.  Stage  when  the  blastopore  (bp}  is  nearly  obliterated. 

D.  The  blastopore  is  closed. 

ep.   epiblast ;    me.    mesoblast ;    hy.  hypoblast ;    bp.   blastopore ;    in.   intestine ; 
st.  stomach  ;  /.  foot ;  sg.  shell-gland  ;  m.  mouth. 

is  the  shell-gland.  The  other  is  the  foot  (/)  which  arises  as  a  ventral 
prominence  of  thickened  epiblast  immediately  behind  the  blastopore.  The 
hypoblast  forms  a  ring  of  columnar  cells  round  the  blastopore.  On  the 


MOLLUSCA. 


235 


posterior  side  its  cells  have  bent  over  so  as  to  form  a  narrow  tube  (*«),  the 
rudiment  of  the  intestine. 

In  the  next  stage  (fig.  105  D)  the  blastopore  completely  closes,  but  its 
position  is  marked  by  a  shallow  pit  (;;z)  where  the  stomodaeum  is  eventually 
formed.  The  foot  (/)  is  more  prominent,  and  on  its  hinder  border  is 
formed  the  operculum.  The  shell-gland  (not  shewn  in  the  figure)  has 
flattened  out,  and  its  thickened  borders  commence  to  extend  especially  over 
the  dorsal  side  of  the  embryo.  A  delicate  shell  has  become  formed.  In 
front  of  and  dorsal  to  the  mouth,  a  ciliated  ring-shaped  ridge  of  cells,  which 
is  however  incomplete  dorsally,  gives  rise  to  the  velum.  On  each  side  of  the 
foot  there  appears  a  protuberance  of  epiblast  cells,  which  forms  a  provisional 
renal  organ.  The  hypoblast  now  forms  a  complete  layer  ventrally,  bound- 
ing a  cavity  which  may  be  conveniently  spoken  of  as  the  stomach  (.$•/), 
which  is  open  to  the  yolk  above.  Posteriorly  however  a  completely  closed 
intestine  is  present,  which  ends  blindly  behind  (in). 

The  shell  and  with  it  the  mantle  grow  rapidly,  and  the  primitive 
symmetry  is  early  interfered  with  by  the  shell  extending  much  more 
towards  the  left  than  the  right.  The  anus  soon  becomes  formed  and  places 
the  intestine  in  communication  with  the  exterior. 

With  the  growth  of  the  shell  and  mantle  the  foot  and  the  head  become 
sharply  separated  from  the  visceral 
sack  (fig.  1 06).    The  oesophagus  (m] 
becomes  elongated.     The  eyes  and 
auditory  sacks  become  formed. 

With  further  growth  the  asym- 
metry of  the  embryo  becomes  more 
marked.  The  intestine  takes  a  trans- 
verse direction  to  the  right  side  of 
the  body,  and  the  anus  opens  on  the 
right  side  and  close  to  the  foot  in  the 
mantle  cavity  which  is  formed  by  an 
epiblastic  invagination  in  this  region. 
The  cavity  of  the  stomach  (fig.  106, 
st}  increases  enormously  and  passes 


ce.v 


FIG.    106.     LONGITUDINAL  SECTION 

THROUGH     AN     ADVANCED     EMBRYO     OF 

NASSA  MUTABILIS.   (After  Bobretzky.) 


f.     foot ;    m.    mouth ; 
vesicle  ;  sf.  stomach. 


cephalic 


to  the  left  side  of  the  body,  pushing 
the  food-yolk  at  the  same  time  to 
the  right  side,  and  the  point  where 
it  communicates  with  the  intestine  becomes  carried  towards  the  posterior 
dorsal  end  of  the  visceral  sack.  The  walls  of  the  stomach  gradually  extend 
so  as  to  narrow  the  opening  to  the  yolk.  The  part  of  it  adjoining  the 
oesophagus  becomes  the  true  stomach,  the  remainder  the  liver  ;  its  interior  is 
filled  with  coagulable  fluid. 

Paludina.  Paludina—  Lankester  (No.  263)  and  Butschli  (No  244)— is 
a  viviparous  form  characterised  by  the  small  amount  of  food-yolk.  The 
hypoblast  and  epiblast  cells  are  distinguished  very  early,  but  soon  become  of 
nearly  the  same  size. 


236 


GASTEROPODA. 


In  the  later  stages  of  segmentation  the  epiblast  cells  differ  from  the 
hypoblast  cells  in  the  absence  of  pigment.  The  segmentation  cavity,  if 
developed,  is  small.  A  perfectly  regular  gastrula  is  formed  (fig.  107  A  and 
B),  which  is  preceded  by  the  embryo  assuming  a  flattened  form.  The 
blastopore  is  at  first  wide,  but  gradually  narrows,  and  finally  assumes  a 
slightly  excentric  position.  //  becomes  not  the  mouth,  but  the  anus. 

When  the  blastopore  has  become  fairly  narrow,  mesoblast  cells  (B,  me.} 
appear  around  it,  between  the  epiblast  and  hypoblast.  Whether  they  are 


FlG.  107.       FOUR  STAGES  IN  THE  DEVELOPMENT  OF  PALUDINA  VIVIPARA. 

(Copied  from  Biitschli.) 

ep.  epiblast  ;  hy.  hypoblast ;  me.  mesoblast ;  bl.  blastopore ;  an.  anus  ;  st.  stomo- 
dceum  ;  sh.  shell-gland  ;  V.  velum;  x.  primitive  excretory  organ. 

bilaterally   arranged   or   no   is   not   clear;    and   though   coloured   like   the 
hypoblast,  their  actual  development  from  this  layer  has  not  been  followed. 

The  velum  appears  about  the  same  time  as  the  mesoblast,  in  the  form  of 
a  double  ring  of  ciliated  cells  at  about  the  middle  of  the  body  (B  and  C,  V\ 
The  mesoblast  rapidly  extends  so  as  to  occupy  the  whole  space  between 
the  epiblast  and  hypoblast,  and  at  the  same  time  becomes  divided  into  two 
layers  (C).  Shortly  afterwards  a  space— the  body  cavity— appears  be- 
tween the  two  layers  (D)  which  then  attach  themselves  respectively  to  the 
epiblast  and  hypoblast,  and  constitute  the  somatic  and  splanchnic  layers  of 
mesoblast.  The  two  layers  remain  connected  by  transverse  strands. 


MOLLUSCA.  237 


By  a  change  in  the  relations  of  the  various  parts  and  especially  by 
the  growth  of  the  posterior  region  of  the  body,  the  velum  now  occupies  a 
position  at  the  end  of  the  body  opposite  the  blast opore.  Immediately 
behind  it  there  appear  two  organs,  one  on  the  dorsal  and  one  on  the 
ventral  side.  That  on  the  dorsal  side  (sh)  is  a  deep  pit — the  shell-gland — 
which  is  continuous  with  a  layer  of  columnar  epiblast  which  ends  near  the 
anus.  The  other  organ  (j/),  situated  on  the  ventral  side,  is  a  simple  de- 
pression, and  is  the  rudiment  of  the  stomodaeum.  Between  it  and  the 
dorsally  placed  anus  is  a  slight  prominence—  the  rudiment  of  the  foot.  On 
the  two  sides  of  the  body,  between  the  epiblast  and  hypoblast  on  a  level 
with  the  shell-gland  are  placed  two  masses  of  excretory  cells,  the  pro- 
visional kidneys  (D,  x).  These  are  probably  not  homologous  with  the 
provisional  renal  organ  of  Nassa  and  other  marine  Prosobranchiata.  At 
a  later  period  a  ciliated  cavity  appears  in  them,  which  probably  communi- 
cates with  the  exterior  at  the  side  of  the  throat. 

In  the  later  stages  the  foot  grows  rapidly,  and  forms  a  very  prominent 
mass  between  the  mouth  and  the  anus.  An  operculum  is  developed  some- 
what late  in  a  shallow  groove  lined  by  thickened  epiblast. 

A  provisional  chitinous  plug  is  formed  in  the  shell-gland  which  soon 
becomes  everted.  The  shell  is  formed  in  the  usual  way  on  the  everted 
surface  of  the  shell-gland.  The  thickened  edge  of  this  part  becomes  the 
edge  of  the  mantle,  and  soon  projects  in  the  neighbourhood  of  the  anus  as 
a  marked  fold. 

With  the  rapid  growth  of  the  larva  the  invaginated  mesenteron  becomes 
relatively  reduced  in  size.  In  its  central  part  yolk  spherules  become 
deposited,  while  the  part  adjoining  the  blastopore  (anus)  becomes  elongated 
to  give  rise  to  the  intestine.  The  stomodaeum  grows  greatly  in  length  and 
joins  the  dorsal  part  of  the  archenteron  which  then  becomes  the  stomach. 
The  part  of  the  mesenteron  with  yolk  spherules  forms  the  liver.  With 
the  development  of  the  visceral  sack  the  anus  shifts  its  position.  It  first 
passes  somewhat  to  the  left,  and  is  then  carried  completely  to  the  right. 

The  development  of  Entoconcha  mirabilis  (Joh.  Miiller,  No.  265),  a 
remarkable  Prosobranchiate  parasitic  in  the  body  cavity  of  Synapta,  which 
in  the  adult  state  is  reduced  to  little  more  than  an  hermaphrodite  generative 
sack,  deserves  a  short  description.  It  is  viviparous,  and  the  ovum  gives 
rise  to  a  larva  which  from  the  hardly  sufficient  characters  of  the  foot  and 
shell  is  supposed  to  be  related  to  Natica. 

There  is  nothing  very  striking  in  the  development.  The  food  yolk  is 
scanty.  The  velum,  as  might  be  anticipated  from  the  viviparous  develop- 
ment, is  small.  The  tentacles  are  placed  not  within,  but  behind  the  velar 
area.  There  is  a  natica-like  shell,  a  large  mantle-cavity,  and  a  large  two- 
lobed  foot. 

In  Buccinum,  and  Neritina  only  one  out  of  the  many  ova  included 
in  each  egg-capsule  develops.  The  rest  atrophy  and  are  used  as  food 
by  the  one  which  develops. 

Opisthobranchiata.      It  will  be  convenient  to  take  a  species  of 


238  GASTEROPODA. 


Pleurobranchidium  (Aplysia),  observed  by  Lankester  (No.  239),  as  a  type  of 
Nudibranchiate  development.  The  ovum  first  divides  into  two  segments, 
and  from  these  small  segments  are  budded  off,  which  gradually  grow 
round  and  enclose  the  two  large  segments.  The  small  segments  now  form 
the  epiblast. 

At  the  aboral  pole  the  epiblast  becomes  thickened  and  invaginated  to 
form  the  shell-gland,  and  shortly  afterwards  the  velum  and  foot  are  formed 
in  the  normal  way,  and  a  stomodaeum  appears  close  to  the  ventral  edge  of 
the  velum  (fig.  101).  The  two  yolk  cells  (ry)  still  remain  distinct,  but  a 
true  hypoblastic  layer  (probably  derived  from  them,  though  this  has  not 
been  made  out)  soon  becomes  established.  Prominent  cells  early  make 
their  appearance  at  the  base  of  the  foot,  which  become  at  a  later  period 
invaginated  to  form  the  anus.  Otolithic  sacks  (of)  become  formed  in  the 
foot,  and  the  supraoesophageal  ganglia  from  a  differentiation  of  the  epiblast 

(ng\ 

At  a  later  period  the  shell-gland  becomes  everted,  and  a  nautiloid  shell 
developed.  The  alimentary  tract  becomes  completed,  though  the  two  yolk 
cells  long  retain  their  original  distinctness.  The  shell-muscle  is  developed, 
and  peculiar  pigmented  bodies  are  formed  below  the  velum.  The  foot 
becomes  prominent  and  acquires  an  operculum. 

The  metamorphosis  of  Tergipes  has  been  more  or  less  completely  worked 
out  by  Nordmann  and  by  Schultze  (No.  271). 

In  Tergipes  Edwardsii  worked  out  by  the  former  author,  the  larva  when 
hatched  is  provided  with  a  large  velum,  eyes,  tentacles,  an  elongated 
operculated  foot,  and  mantle.  In  the  next  stage  both  shell  and  operculum 
are  thrown  off,  and  the  body  becomes  elongated  and  pointed  behind.  Still 
later  a  pair  of  gill-processes  with  hepatic  diverticula  becomes  formed. 

The  velum  next  becomes  reduced,  and  two  small  processes,  which  give 
rise  to  the  lip  tentacles  and  a  second  pair  of  gills,  sprout  out.  An  ecdysis 
now  takes  place,  and  leads  to  further  changes  which  soon  result  in  the 
attainment  of  the  adult  form. 

In  Tergipes  lacinulatus,  observed  by  Schultze,  the  velum  atrophies  before 
the  shell  and  operculum  are  thrown  off. 

Pulmonata.  The  development  of  the  fresh-water  Pulmonata  appears 
from  Lankester's  observations  on  the  pond-snail  (Lymnaeus)  to  be  very 
similar  in  all  important  particulars  to  that  of  marine  Branchiogasteropoda. 
The  velum  is  however  less  developed  than  in  most  marine  forms.  The 
shell-gland,  etc.  have  the  normal  development.  In  Lymnaeus  the  blasto- 
pore  has  an  elongated  form  and  it  is  still  a  matter  of  dispute  whether  it 
closes  at  the  mouth  or  anus. 

In  the  Helicidae  there  is  a  gastrula  by  epibole.  The  shell-gland,  as 
may  be  gathered  from  Von  Jhering's  figures,  has  the  usual  form,  and  an 
external  shell  of  the  usual  larval  type  is  developed.  There  is  a  ciliated 
process  above  the  mouth,  which  extends  into  the  lumen  of  the  mouth.  This 
process  is  often  regarded  as  a  rudimentary  velum,  but  probably  has  not  this 
value.  There  is  no  other  organ  which  can  be  homologous  with  the  velum. 


MOLLUSCA.  239 


The  development  of  Limax  presents  some  peculiarities.  The  yolk- 
spheres  (hypoblast)  form  a  large  mass  enclosed  by  the  epiblast  cells.  A 
shell-gland  is  formed  in  the  usual  situation,  which  however,  instead  of  being 
everted,  as  in  ordinary  forms,  becomes  closed,  and  in  its  interior  are 
deposited  calcareous  plates  which  give  rise  to  the  permanently  internal 
shell.  The  foot  grows  out  posteriorly,  and  contains  a  large  provisional 
contractile  vesicle,  traversed  by  muscular  strands  which  contract  rhyth- 
mically. 

Although  an  external  shell  is  present  in  Clausilia  in  the  adult,  the 
shell-gland  becomes  closed  in  the  embryo  as  in  Limax,  and  an  internal 
plate-like  shell  is  developed.  The  shell  is  at  first  covered  by  a  complete 
epithelium,  which  eventually  gives  way  in  the  centre,  leaving  covered  only 
the  edges  of  the  shell.  It  thus  comes  about  that  the  original  internal  shell 
becomes  an  external  one.  It  is  very  difficult  to  bring  this  mode  of  develop- 
ment of  the  external  shell  into  relation  with  that  of  other  forms.  Clausilia 
like  Limax  develops  a  large  pedal  sinus. 

In  both  Limax  and  Clausilia  cilia  are  early  developed  and  cause  a 
rotation  of  the  embryo,  but  how  far  they  give  rise  to  a  distinct  velum  is 
not  clear. 

Heteropoda.  The  Heteropod  embryos  present  in  their  early  develop- 
ment the  closest  resemblance  to  those  of  other  Gasteropods.  The  seg- 
mentation takes  place  according  to  the  most  usual  Gasteropod  type ;  (vide 
p.  99)  and  after  the  yolk  cells  have  ceased  to  give  origin  to  epiblast  cells 
they  divide  towards  the  nutritive  pole,  become  invaginated,  and  line  a 
spacious  archenteron.  The  epiblast  cells  at  the  formative  pole  gradually 
envelop  the  yolk  (hypoblast)  cells,  and  the  blastopore  very  early  narrows 
and  becomes  the  permanent  mouth. 

Simultaneously  with  the  narrowing  of  the  blastopore,  the  shell-gland  is 
formed  at  the  aboral  pole,  and  the  foot  on  the  ventral  side.  The  velum 
appears  as  a  patch  of  cilia  on  the  dorsal  side,  which  then  gradually  extends 
ventrally  so  as  to  form  a  complete  circle  just  dorsal  to  the  mouth. 

The  larva,  after  these  changes  have  been  completed,  is  represented  in 
fig.  102. 

In  later  stages  the  shell-gland  becomes  everted,  and  a  shell  is  developed 
in  all  the  forms  both  with  and  without  shells  in  the  adult.  The  foot  grows 
very  rapidly,  and  an  operculum  is  in  all  cases  formed  behind.  A  bilobed 
invagination  in  front  gives  rise  to  the  mucous  gland.  The  velum  enlarges 
and  becomes  bilobed. 

Though  the  blastopore  remains  permanently  open  as  the  mouth,  the 
oesophagus  is  formed  as  an  epiblastic  ingrowth.  The  rudiment  of  the 
proctodaeum  appears  as  two  epiblastic  cells  symmetrically  placed  behind  the 
foot,  which  subsequently  pass  to  the  right  side,  and  give  rise  to  a  shallow 
invagination  which  meets  the  meseriteric  sack.  In  the  latter  structure  the 
cells  of  part  of  the  wall  develop  a  peculiar  nutritive  material,  and  form  a 
nutritive  sack  which  eventually  becomes  the  liver.  The  part  of  the  sack 
connected  with  the  epiblastic  oesophagus  becomes  constricted  off  as  the 


240 


HETEROPODA. 


stomach.     The  remainder,  which   unites  with   the   proctodaeum,  forms  the 
intestine. 

The  structural  peculiarities  of  the  adult  are  formed  by  a  post-larval 
metamorphosis.  The  caudal  appendage  of  Pterotrachea  and  Firoloidea  is 
formed  as  an  outgrowth  of  the  upper  border  of  the  hind  end  of  the  foot. 
The  so-called  fin  arises  as  a  cylindrical  process  in  front  of  the  base  of  the 
foot,  which  is  eventually  flattened  laterally.  In  the  Atlantidae  it  is  in  some 
cases  at  first  vermiform,  and  in  other  cases  attains  directly  its  adult  struc- 
ture. The  embryonic  foot  itself  gives  rise  in  Pterotrachea,  Firoloidea  and 
Carinaria  to  the  tail,  on  the  dorsal  and  posterior  side  of  which  the  operculum 
may  still  be  seen  in  young  specimens.  In  Atlanta  it  forms  the  posterior  part 
of  the  foot  on  which  the  operculum  persists  through  life. 

The  embryonic  shell  is  completely  lost  in  Pterotrachea  and  Firoloidea, 
and  the  shell  is  rudimentary  in  Carinaria.  With  its  atrophy  the  mantle 
region  also  becomes  much  reduced. 

The  velum  is  enormously  developed  in  many  Heteropods.  In  Atlanta  it 
is  six-lobed,  each  of  the  two  primitive  lateral  lobes  being  prolonged  into 
three  processes,  two  in  front,  and  one  behind.  As  in  all  other  cases,  it 
atrophies  in  the  course  of  the  post-larval  metamorphosis. 

Pteropoda.  The  early  larval 
form  of  the  Pteropods  is  closely 
similar  to  that  of  marine  Gastero- 
pods.  There  are  usually  only  three 
hypoblastic  spheres  at  the  close  of 
the  segmentation  in  the  Thecoso- 
mata,  and  a  somewhat  larger  num- 
ber in  the  Gymnosomata.  The  blas- 
topore  closes  at  the  oral  region,  on 
the  nutritive  side  of  the  ovum,  and 
the  shell-gland  is  placed  at  the 
original  formative  pole.  The  velum, 
shell-gland  and  foot  have  the  usual 
relations.  Although  many  of  the 
adult  forms  are  symmetrical,  there  is 
very  early  an  asymmetry  visible  in 
the  larva,  shewing  that  the  Pteropods 
are  descended  from  asymmetrical 
ancestors.  In  the  Gymnosomata 
there  is  a  second  larval  stage  after 
the  loss  of  the  shell  when  the  larva 
is  provided  with  three  rings  of  cilia 
(fig.  109).  In  most  forms  of  Ptero- 
pods the  dorsal  part  of  the  body, 
covered  by  the  mantle,  is  produced 
into  a  visceral  sack  like  that  of  the 


of 


-mr- 


Cephalopoda  (fig.  108). 


FIG.  108.  EMBRYO  OF  CAVOLINIA 
(HYALEA)  TRIDENTATA.  (After  Fol.) 

M.  mouth  ;  a.  anus ;  s.  stomach  ;  /'. 
intestine ;  <r.  nutritive  sack  ;  tub.  mantle  ; 
me.  mantle  cavity ;  fCn.  contractile  sinus  ; 
ft.  heart ;  r.  renal  sack  :  f.  foot ;  pn.  epi- 
podia  ;  q.  shell ;  of.  otolithic  sack, 


MOLLUSCA. 


24I 


The  velum  varies  considerably  in  its  development  in  different  forms.  In 
the  Hyaleidee  it  is  comparatively  small  and  atrophies  early;  while  in 
Cymbulia  (fig.  103)  and  the  Gymnosomata  it  is  large  and  bilobed,  and 
persists  till  after  the  foot  has  attained  its  full  development. 

The  free  edge  of  the  velum  is  provided  with  long  motor  cilia,  and  its 
lower  border  with  small  cilia  which  bring  the  food  to  the  mouth.  In 
Cleodora  there  is  a  median  bunch  of  cilia  in  the  centre  of  the  velum  like  that 
in  the  Lamellibranchiata,  Nudibranchiata,  etc. 

The  shell-gland  forms  a  pit  at  the  aboral  end  of  the  body,  and  in 
Cymbulia  a  chitinous  plug  appears  to  be  normally  formed  in  this  pit.  The 
pit  afterwards  everts  itself.  The  edge  of  the  everted  area  becomes  thickened 
and  gradually  travels  towards  the  anterior  end  of  the  body.  On  this  everted 
area  a  small  plate  is  developed,  which  forms  the  commencement  of  the 
embryonic  shell  with  which  the  larvae  of  all  Pteropods  are  provided. 

The  remainder  of  the  embryonic  shell  is  secreted  in  successive  rings  by 
the  thickened  edge  of  the  mantle,  and  grows  with  this  till  it  reaches  the 
neck  (fig.  108).  The  permanent  shell  is  added  subsequently,  usually  on  a 
very  different  model  to  the  larval  shell.  The  fate  of  the  embryonic  shell  is 
very  various  in  different  forms.  In  the  Hyaleidae  the  animal  withdraws  itself 
from  the  larval  shell,  which  becomes  shut  off  from  the  permanent  shell  by  a 
diaphragm.  The  larval  shell  then  becomes  detached. 

In  the  Styliolidae  the  per- 
manent shell  becomes  twice 
the  size  of  the  embryonic 
shell  while  the  animal  is  still 
in  an  embryonic  condition, 
but  the  larval  shell  persists 
for  life.  In  the  Cymbulidae 
there  is  an  embryonic  and 
secondary  shell,  which  per- 
sist together  during  larval 
life.  They  are  eventually 
cast  off  at  the  same  time 
and  replaced  by  a  perma- 
nent shell. 

In  the  Gymnosomata  an 
embryonic  shell  is  develop- 
ed, and  a  secondary  shell 
added  to  it  during  embryo- 
nic life.  Both  are  cast  off 
before  the  adult  condition 
is  attained.  After  the  shell 
has  been  cast  off  three  cili- 
ated rings  are  developed  (fig.  109).  The  anterior  of  these  is  placed  between 
the  velum  and  the  foot,  and  the  two  hinder  ones  on  the  elongated  posterior 
part  of  the  body. 

B.  II.  16 


FIG.  109.  FREE  SWIMMING  PNEUMODERMON 
LARVAE.  (After  Gegenbaur,  copied  from  Bronn.) 

The  velum  has  atrophied  in  both  larvae. 

In  A  three  ciliated  bands  are  present,  and  the 
auditory  vesicles  are  visible. 

In  B  the  tentacles  with  suckers  and  the  epi- 
podia  have  become  developed. 

an.    anus. 


242  CEPHALOPODA. 


The  ciliated  rings  give  to  these  larvae  a  resemblance  to  Chcetopod  larvae  ; 
but  there  can  be  no  doubt  that  this  resemblance  is  a  purely  superficial  one- 
The  anterior  ring  atrophies  early  (fig.  109  B),  and  the  second  one  soon 
follows  suit.  It  is  probable  that  the  hindermost  one  does  not  persist 
through  life,  although  it  has  been  observed  in  forms  with  fully  developed 
sexual  organs.  Most  of  these  larvae  have  not  been  traced  to  their  adult 
forms.  They  have  been  referred  to  Pneumodermon,  Clio,  etc. 

The  most  characteristic  organ  of  the  Pteropods  is  the  foot,  which  is 
prolonged  into  two  enormous  lateral  wings,  the  epipodia.  These  develope 
at  different  periods  in  different  larvae,  but  are  always  distinct  lateral  out- 
growths of  the  foot. 

In  the  Hyaleidic  the  foot  is  early  conspicuous,  and  soon  sends  out  two 
lateral  prolongations  (fig.  108  pn.}  which  develop  with  enormous  rapidity 
as  compared  with  the  medium  portion,  and  give  rise  to  the  epipodia.  The 
whole  of  the  foot  becomes  ciliated. 

In  the  Cymbulidae,  though  not  in  other  forms,  an  operculum  is  developed 
on  the  hinder  surface  of  the  foot  (fig.  103  C).  The  epipodia  are  late  in 
appearing. 

In  the  Gymnosomata  the  foot  is  developed  very  early,  but  remains  small. 
The  epipodia  do  not  appear  till  very  late  in  larval  life  (fig.  109  B). 

In  Pneumodermon  and  some  other  Gymnosomata  there  appear  on  the 
hinder  part  of  the  head  peculiar  tentacles  with  suckers  like  those  of  the 
Cephalopoda  (fig.  109  B).  It  is  not  certain  that  these  tentacles  are  geneti- 
cally related  to  the  arms  of  the  Cephalopoda. 

Cephalopoda.  The  eggs  of  the  Cephalopoda  are  usually 
laid  in  special  capsules  formed  in  the  oviduct,  which  differ 
considerably  in  the  different  members  of  the  group. 

In  the  case  of  Argonauta  each  egg  is  enveloped  in  an  elongated  capsule 
provided  with  a  stalk.  By  means  of  the  stalk  the  eggs  are  attached  together 
in  bunches,  and  these  again  are  connected  together  and  form  transparent 
masses,  which  are  placed  in  the  back  of  the  shell.  In  octopus  the  eggs  are 
small  and  transparent :  each  of  them  is  enclosed  in  a  stalked  capsule.  In 
Loligo  the  eggs  are  enveloped  in  elongated  sack-like  gelatinous  cords, 
each  containing  about  thirty  or  forty  eggs.  The  cords  are  attached  in 
bunches  to  submarine  objects.  In  Sepia  each  egg  is  independently  en- 
veloped in  a  spindle-shaped  black  capsule,  which  is  attached  to  a  stone 
or  other  object. 

In  a  decapod  form  with  pelagic  larvae,  described  by  Grenacher  (No.  280), 
the  eggs  were  enclosed  in  a  somewhat  cylindrical  gelatinous  mass.  In  each 
mass  there  were  an  immense  number  of  eggs  arranged  in  spirals.  Each 
ovum  was  enclosed  in  a  structureless  membrane,  within  which  it  floated  in  a 
colourless  albumen. 

The  ovum  itself  within  the  capsule  is  a  nearly  homogeneous 
granular  mass,  without  a  distinct  envelope.     Development  com- 


MOLLUSCA. 


243 


mences  by  the  segregation,  at  the  narrow  pole  of  the  ovum  oppo- 
site the  egg-stalk,  of  the  greater  part  of  the  protoplasmic  forma- 
tive material1.  This  material  forms  a  disc  equivalent  to  the 
germinal  disc  of  meroblastic  vertebrate  ova.  The  germinal  disc 
in  Sepia  and  Loligo  does  not,  however,  undergo  a  quite  symme- 
trical segmentation  (Bobretzky,  No.  279).  When  eight  segments 
are  present,  two  of  them  close  together  are  much  smaller  and 
narrower  than  the  remainder ;  and  when,  in  the  succeeding 
stages  small  segments  are  formed  from  the  inner  ends  of  the 
large  ones,  those  derived  from  the  two  smaller  segments  continue 
to  be  smaller  than  the  remainder:  so  that  throughout  the  seg- 
mentation one  pole  of  the  blastoderm  is  formed  of  smaller 
segments,  and  the  blastoderm  exhibits  a  bilateral  symmetry2. 
The  partial  segmentation  results  in  the  formation  of  a  blastoderm 
covering  one  pole  of  the  egg,  but,  unlike  the  vertebrate  blasto- 
derm, formed  of  a  single  row  of  cells.  This  blastoderm  very 
soon  becomes  two  or  three  cells  deep  at  its  edge,  and  the  cells 
below  the  surface  constitute  the  layer  from  which  the  mesoblast 
and  hypoblast  originate  (fig.  1 10  ms).  The  origin  of  the  meso- 
blast at  the  edge  of  the  blastoderm  is  a  phenomenon  equivalent 
to  its  origin  at  the  lips  of  the  blastopore  in  so  many  other  types. 
The  external  layer  forms  the  epiblast. 

The  whole  blastoderm  does  not  take  its  origin  from  the  seg- 
mentation spheres,  but,  as  was  discovered  by  Lankester  (282),  a 
number  of  nuclei  arise  spontaneously  in  the  yolk  outside  the 
blastoderm,  around  which  cell-bodies  become  subsequently 
formed.  They  make  their  appearance  near  to,  but  not  at  the 
surface,  extending  first  in  a  ring-like  series  in  advance  of  the 
margin  of  the  blastoderm,  but  subsequently  appearing  indiscrim- 
inately over  all  parts  of  the  egg.  They  take  no  share  in  form- 
ing the  epiblast,  but  would  seem,  according  to  Lankester,  to 
assist  in  giving  rise  to  the  lower  layer  cells,  and  also  to  a 
layer  of  flattened  cells  which  eventually  completely  encloses  the 
yolk,  and  may  be  called  the  yolk  membrane.  The  cells  of  the 
yolk  membrane  first  of  all  appear  at  the  thickened  edge  of  the 

1  In  Octopus  and  Argonauta  (Lankester)  as  soon  as  the  blastoderm  is  completed 
the  egg  reverses  its  position  in  the  egg-shell ;  the  cleavage  pole  taking  up  a  position 
nearest  the  stalk. 

2  I  do  not  know  the  relation  of  this  axis  of  symmetry  to  the  future  embryo. 

1 6—  2 


244  CEPHALOPODA. 


blastoderm.  From  this  point  they  spread  inwards  under  the  centre 

of  the  blastoderm  (fig. 

115  tti),  and,  together 

with  the  epiblast  cells, 

outwards  over  theyolk 

generally ;      so     that 

before   long   (on   the 

tenth  day  in   Loligo)  FIG.  no.     SECTION  THROUGH  THE  BLASTODERM 

jay  "5VJ        op    A    LoLIGO    OVUM    AT    THE    BEGINNING    OF    THE 

the  yolk  becomes  com-      FOURTH  DAY.     (After  Bobretzky.) 

pletely  invested  by  a  ms.  mesoblast ;  d.  cell  at  the  edge  of  the  blasto- 

i  r       11  derm;  c.  one  of  the  segmentation  cells. 

In  the  non-germinal  region  the  blastoderm  is  formed  of  two 
layers,  (i)  a  flattened  epiblast,  and  (2)  the  yolk  membrane.  In 
the  region  of  the  original  germinal  disc  the  epiblast  cells  become 
columnar,  and  below  them  is  placed  a  ring  of  lower  layer  cells, 
which  gradually  extends  towards  the  centre  so  as  finally  to  form 
a  complete  layer.  Below  this  again  comes  the  yolk  membrane 
just  spoken  of. 

Before  describing  the  further  fate  of  the  separate  layers  it  is 
necessary  to  say  a  few  words  as  to  the  external  features  of  the 
embryo.  In  the  adult  Cephalopod  it  is  convenient,  for  the  sake 
of  comparison  with  other  Mollusca,  to  speak  of  the  narrow  space 
enclosed  in  the  arms,  which  contains  the  mouth,  as  the  ventral 
surface ;  the  aboral  apex  as  the  dorsal  surface ;  and  what  is 
usually  called  the  upper  surface  as  the  anterior  and  the  lower 
one  as  the  posterior. 

Employing  this  terminology  the  centre  of  the  original  blasto- 
derm is  the  dorsal  apex  of  the  embryo.  In  the  typical  forms 
with  a  large  yolk-sack  the  whole  embryo  is  formed  out  of  the 
original  germinal  disc ;  the  part  of  the  blastoderm  which  is 
continued  as  a  thin  layer  over  the  remainder  of  the  egg  forms  a 
large  ventral  yolk-sack  appended  to  the  head  of  the  embryo. 
The  following  description  applies  especially  to  two  types,  which 
form  the  extremes  of  the  series  in  reference  to  the  development 
of  the  yolk-sack.  The  first  of  these  with  a  large  yolk-sack  is 
Sepia,  of  which  Kolliker  in  his  classical  memoir  (No.  281)  has 
published  a  series  of  beautiful  figures.  The  second,  with  a  small 
yolk-sack,  is  the  pelagic  larva  of  an  unknown  adult  described  by 
Grenadier  (No.  280). 


MOLLUSCA.  245 


In  a  young  blastoderm  of  Sepia  viewed  from  the  dorsal 
surface,  a  series  of  structures  appear  which  are  represented  in 
fig.  in  A.  In  the  middle  is  a  somewhat  rhomboid  prominence 
which  forms  the  rudiment  of  the  mantle  (mt).  In  its  centre  is  a 
pit  which  forms  the  shell-gland.  On  each  side  of  the  mantle  is 
a  somewhat  curved  fold  (/).  These  folds  eventually  coalesce  to 
form  the  funnel.  They  are  divided  into  two  parts  by  a  small 
body  which  forms  the  cartilage  of  the  funnel.  The  smaller  part 
of  the  fold  behind  this  body  gives  rise  to  the  true  funnel,  the  part 
in  front  becomes  (Kolliker)  the  strong  muscle  connecting  the 
funnel  with  the  neck-cartilage.  In  front  and  to  the  sides  are 
two  kidney-.shaped  bodies  (oc)  the  optic  pits.  Behind  the  mantle 
are  two  buds  (br}}  the  rudiments  of  the  gills. 


FlG.  in.      TWO  SURFACE  VIEWS  OF  THE  GERMINAL  DISC  OF  SEPIA. 

(After  Kolliker.) 

mt.  mantle ;  oc.  eye ;  /.  folds  of  funnel ;  br.  branchiae ;  an.  posterior  portion  of 
alimentary  tract ;  m.  mouth,     i,  2,3,  4,  5,  arms  ;  /.  cephalic  lobe. 

In  the  somewhat  later  stage  rudiments  of  the  two  posterior 
pairs  of  arms  make  their  appearance  outside  and  behind  the 
rudiments  of  the  funnel.  The  head  is  indicated  by  a  pair  of 
lateral  swellings  on  each  side,  the  outer  of  which  carries  the  eyes. 
The  whole  embryo  now  becomes  ciliated,  though  the  ciliation 
does  not  cause  the  usual  rotation.  At  a  slightly  later  stage  the 
second,  third,  and  fourth  pairs  of  arms  make  their  appearance 
slightly  in  front  of  those  already  present.  The  posterior  parts  of 
the  funnel  rudiments  approach  each  other,  and  the  anterior  meet 
the  rudiments  of  the  neck-cartilage.  The  gills  begin  to  be 
covered  by  the  mantle-edge,  which  now  projects  as  a  marked 
fold.  At  a  slightly  later  period  two  fresh  rudiments  may  be 


246 


CEPHALOPODA. 


noted,  viz.  the  oral  (fig.  1 1 1  B,  m)  and  anal  invaginations,  the 
latter  of  which  is  extremely  shallow  and  appears  at  the  apex  of 
a  small  papilla  which  may  be  spoken  of  as  the  anal  papilla. 
These  invaginations  appear  at  the  two  opposite  poles  (anterior 
and  posterior)  of  the  blastoderm.  Shortly  after  this  the  rudi- 
ment of  the  first  pair  of  arms  arises  considerably  in  front  of  the 
other  rudiments,  at  the  sides  of  the  outer  pair  of  cephalic  swell- 
ings (fig.  1 1 1  B,  i). 

Fig.  1 1 1  B  represents  a  view  from  the  dorsal  surface  of  an 
embryo  at  this  stage.  In  the  centre  is  the  mantle  with  the  shell- 
gland  which  is  now  very  considerably  raised  beyond  the  general 
surface.  Concentric  with  the  edge  of  the  mantle  are  the  two 


FIG.  112.    SIDE  VIEWS  OF  THREE  LATE  STAGES   IN  THE  DEVELOPMENT  OF 

SEPIA.     (After  Kolliker.) 
/;/.  mouth  ;  yk.  yolk-sack  ;  oc.  eye  ;  tut.  mantle. 

halves  of  the  funnel,  the  anterior  half  meeting  the  dorsal  or  neck- 
cartilage  and  the  posterior  halves  approaching  each  other.  The 
oral  invagination  is  shewn  at  ;;/  and  the  anal  immediately  in 
front  of  an.  The  gills,  nearly  covered  by  the  mantle,  are  seen  at 
br.  At /are  the  cephalic  swellings,  and  the  eye  is  seen  at  oc. 


MOLLUSCA.  247 


The  arms  I — 5  form  a  ring  outside  these  parts.  The  whole  of 
the  embryo,  with  the  exception  of  the  gills,  the  funnel,  and  the 
outer  border  of  the  blastoderm,  is  richly  ciliated. 

The  embryo  up  to  this  time  has  had  the  form  of  a  disc  or 
saucer  on  the  surface  of  the  yolk.  After  this  stage  it  rapidly 
assumes  its  permanent  dome-like  form,  and  becomes  at  the  same 
time  folded  off  from  the  yolk.  The  blastoderm  is  very  slow  in 
enveloping  the  yolk,  and  the  whole  yolk  is  not  completely  in- 
vested till  a  considerably  later  stage  than  that  represented  in  fig. 
1 1 1  B.  As  soon  as  the  blastoderm  covers  the  yolk-sack  cilia 
appear  upon  it.  The  mantle  grows  very  rapidly,  and  its  free 
border  soon  projects  over  the  funnel  and  gills.  After  the  two 
halves  of  the  funnel  have  coalesced  into  a  tube,  it  comes  to  pro- 
ject again  beyond  the  edge  of  the  mantle. 

On  the  completion  of  the  above  changes  the  resemblance  of 
the  embryo  to  a  Cuttle-fish  becomes  quite  obvious.  Three  of 
the  stages  in  the  accomplishment  of  these  changes  are  represent- 
ed in  fig.  112. 

To  the  ventral  side  of  the  embryo  is  attached  the  enormous 
external  yolk-sack  (yk}y  which  is  continuous  with  an  internal 
section  situated  within  the  body  of  the  embryo.  The  general 
relations  of  the  embryo  to  the  yolk  will  best  be  understood  by 
reference  to  the  longitudinal  section  of  Loligo,  fig.  1 27. 

The  arms  gradually  increase  in  length,  and  the  second  pair 
passes  in  front  of  the  first  so  as  eventually  to  lie  completely  in 
front  of  the  mouth.  The  arms  thus  come  to  form  a  complete 
ring  surrounding  the  mouth,  of  which  the  original  second  pair, 
and  not,  as  might  be  anticipated,  the  first,  completes  the  circle 
in  front.  The  second  pair  develops  into  the  long  arms  of  the 
adult. 

After  the  embryo  has  attained  more  or  less  completely  its 
definite  form  (fig.  112  C)  it  grows  rapidly  in  size  as  compared 
with  the  yolk-sack.  The  latter  structure  is  at  first  four  or  five 
times  as  big  as  the  embryo,  but,  by  the  time  of  hatching,  the  em- 
bryo is  two  to  three  times  as  big  as  the  yolk-sack. 

Loligo  mainly  differs  from  Sepia  in  the  early  enclosure  of  the  yolk  by  the 
blastoderm,  and  in  the  embryo  exhibiting  the  phenomena  of  rotation  within 
the  egg-capsule  so  characteristic  of  other  Mollusca. 

In  Argonauta  the  yolk-sack  is  still  smaller  than  in  Loligo,  and  the  yolk  is 


248  CEPHALOPODA. 


early  completely  enclosed  by  the  blastoderm.  A  well  developed  outer  yolk- 
sack  is  present  during  early  embryonic  life,  but  is  completely  absorbed 
within  the  body  before  its  close.  Cilia  appear  on  the  blastoderm  very  early, 
but  vanish  again  when  the  yolk  is  about  two-thirds  enclosed.  There  is, 
during  embryonic  life,  no  trace  of  a  shell,  but  the  mantle  and  other  parts  of 
the  body  become  covered  by  peculiar  bunches  of  fine  setae.  The  shell-gland 
develops  normally  in  both  Octopus  and  Argonauta,  but  disappears  again 
without  closing  up  to  form  a  sack  (Lankester). 

The  pelagic  Decapod  larva  described  by  Grenacher,  which 
forms  my  second  type,  must  be  placed  with  reference  to  the  de- 
velopment of  the  yolk-sack  at  the  opposite  pole  to  Sepia.  Seg- 
mentation, as  in  other  Cephalopods,  is  partial,  but  the  blastoderm 
almost  completely  envelops  the  yolk  before  any  organs  are  de- 
veloped ;  and  no  external  yolk-sack  is  present.  At  a  stage 
slightly  before  the  closure  of  the  yolk-blastopore  the  mantle  is 
formed  as  a  slight  prominence  at  the  blastodermic  pole  of  the 
egg,  and  even  at  this  early  stage  is  marked  by  the  presence  of 
chromatophores.  The  edge  of  the  blastoderm  is  ciliated.  At  a 
slightly  later  stage  the  embryo  becomes  more  cylindrical,  the 
edge  of  the  mantle  becomes  marked  by  a  fold,  which  divides  the 
embryo  transversely  into  two  unequal  parts,  a  smaller  region 
covered  by  the  mantle,  and  a  larger  region  beyond  this.  The 
yolk  is  still  exposed,  but  rudiments  of  the  optic  pit  and  of  two 
pairs  of  arms  have  appeared.  The  first-formed  arms  are  appa- 
rently the  anterior,  and  not,  as  in  Sepia,  the  posterior. 

At  a  still  later  stage,  represented  in  lateral  and  posterior 
views  in  fig.  113  A  and  B,  considerable  changes  are  effected. 
The  yolk-blastopore  is  nearly  though  not  quite  closed.  The 
mantle  fold  (int)  is  much  more  prominent,  and  on  the  posterior 
side  on  a  level  with  its  edge  may  be  seen  the  rudiments  of  the 
gills  (br).  The  funnel  is  formed  as  two  independent  folds  on 
each  side  (in/1  and  mf*),  which  apparently  correspond  with  the 
two  divisions  of  the  funnel  rudiments  in  Sepia.  The  eye  has 
undergone  considerable  changes.  Close  to  each  rudiment  of  the 
funnel  may  be  seen  a  fresh  sense-organ — the  auditory  sack  (ac). 
The  ventral  (upper  in  the  figure)  end  of  the  body  now  forms  a 
marked  protuberance,  probably  equivalent  to  the  foot  of  other 
Mollusca  (vide  p.  225),  at  the  sides  of  which  are  seen  the  rudi- 
ments of  the  arms  (i,  2,  3).  To  the  two  previously  present  a 
third  one,  on  the  posterior  side,  has  been  added.  The  blastopore 


MOLLUSCA. 


249 


is  placed  on  the  anterior  side  of  the  ventral  protuberance,  and 
immediately  dorsal  to  this  is  an  invagination  (os)  which  gives 
rise  to  the  stomodaeum.  The  ciliation  at  the  edge  of  the  blas- 
topore  still  persists,  but  does  not  lead  to  the  rotation  of  the 
embryo. 

In  later  stages  (fig.  1130  the  blastopore  becomes  closed, 
and  the  mantle  region  increases  in  length  as  compared  with  the 
remainder  of  the  body.  The  ventral  halves  of  the  funnel,  each 
in  the  form  of  a  half  tube,  coalesce  together  to  form  a  single 


FIG.  113.  THREE  EMBRYOS  OF  A  CEPHALOPOD  WITH  A  VERY  SMALL  YOLK-SACK. 
(After  Grenacher.) 

a.  blastopore  ;  br.  branchiae ;  inf.1  and  mf.2  posterior  and  anterior  folds  of  the 
funnel ;  g.  op.  optic  ganglion  (?) ;  oc.  eye ;  wk.  white  body ;  ac.  auditory  pit ;  os. 
stomodaeum;  an.  anus;  mt.  mantle;  i,  2,  3.  ist,  2nd,  and  3rd  pairs  of  arms. 

tube  (inf)  in  the  same  manner  as  in  Sepia.  A  shallow  procto- 
daeum  (an}  is  formed  between  the  two  branchiae.  The  eyes  (oc) 
stand  out  as  lateral  projections,  and  the  arms  become  much 
longer. 

Still  later  a  fourth  pair  of  arms  is  added  as  a  bud  from  each 
of  the  posterior  pair,  and  with  the  growth  in  length  of  the  arms 
the  suckers  make  their  appearance.  The  mouth  is  gradually 
carried  up  so  as  to  be  surrounded  by  the  arms.  The  ciliation  of 
the  surface  becomes  more  extensive. 

During  the  whole  of  the  above  development  the  interior  of 
the  embryo  is  filled  with  yolk,  although  no  external  yolk-sack  is 


250  CEPHALOPODA. 


present.  The  internal  yolk-sack  falls  into  three  sections ;  a 
cephalic  section,  a  section  in  the  neck,  and  an  abdominal  section. 
Of  these,  that  in  the  neck  is  the  first  to  be  absorbed.  The 
cephalic  portion  fills  out  the  ventral  protuberance  already  spoken 
of.  The  hinder  section  becomes  occupied  by  the  liver  which 
exactly  fits  itself  into  this  space  as  it  absorbs  the  material  pre- 
viously there. 

It  will  be  convenient  at  this  point  to  complete  the  account  of 
the  Cephalopoda  by  a  short  history  of  their  germinal  layers,  and 
by  a  fuller  description  of  the  mantle,  shell,  and  funnel  than  that 
given  in  the  preceding  pages. 

It  has  already  been  shewn  that  in  the  region  of  the  germinal 
disc  a  thick  layer  of  cells  becomes  interposed  between  the  epi- 
blast  and  the  yolk  membrane.  This  layer  (fig.  115  m)  is  mainly 
mesoblastic,  but  also  contains  the  elements  which  form  the 

c7is 


FIG.  114.    LONGITUDINAL  VERTICAL  SECTION  THROUGH  A  LOLIGO  OVUM 

WHEN   THE   MESENTERIC   CAVITY   IS  JUST   COMMENCING  TO   BE   FORMED. 

(After  Bobretzky. ) 

gls.  salivary  gland  ;  brd.  sheath  of  radula ;  oe.  oesophagus;  ds.  yolk  -sack  ;  c/is. 
shell-gland  ;  int.  mantle  ;  pdh.  mesenteron ;  x.  epiblastic  thickening  between  the 
folds  of  the  funnel. 

lining  of  the  alimentary  tract.  Its  cells  first  become  differenti- 
ated into  mesoblast  and  hypoblast  after  the  shell-gland  has 
become  a  fairly  deep  pit.  The  mode  of  differentiation  is  shewn 
in  fig.  1 14.  On  the  posterior  side  of  the  mantle,  at  the  point 
marked  in  fig.  1 1 1  B,  an,  a  cavity  is  formed  between  the  yolk 
membrane  and  the  mesoblast  cells  (fig.  n^pd/i).  This  cavity 
is  the  commencement  of  the  anal  extremity  of  the  mesenteron, 
and  the  columnar  cells  lining  it  constitute  the  hypoblast.  The 


MOLLUSCA.  251 


remainder  of  the  lower  layer  cells  are  the  mesoblast.  The 
mesenteron  gradually  extends  itself  till  it  meets  the  stomodaeum 
(fig.  127).  The  proctodaeum  is  formed  as  a  shallow  pit  close  to 
the  first  formed  part  of  the  mesenteron. 

The  mesoblast  gives  rise  not  only  to  the  organs  usually 
formed  in  this  layer,  but  also  to  the  nervous  centres,  etc. 

The  mantle  and  shell.  The  mantle  first  arises  as  a  thick- 
ening of  the  epiblast  on  the  dorsal  surface  of  the  embryo.  The 
thickened  integument,  with  the  subjacent  mesoblast,  soon  forms 
a  definite  projection,  in  the  centre  of  which  appears  a  circular 
pit  (figs.  1 14  chs  and  115  shs).  This  pit,  which  has  already  been 
spoken  of  as  the  shell-gland,  resembles  very  closely  the  shell- 
gland  of  other  Mollusca.  The  fold  around  the  edge  of  the  shell- 


FIG.  115.    DIAGRAM  OF  A  VERTICAL  SECTION  THROUGH  THE  MANTLE  REGION  OK 

AN  EMBRYO  LOLIGO.     (From  Lankester.) 

[This  figure  is  turned  the  reverse  way  up  to  fig.  114.] 

ep.  epiblast ;  y.  food-yolk ;  m.  mesoblast ;  m'.  cellular  yolk  membrane  ;  shs.  shell- 
gland. 

gland  grows  inwards  so  as  gradually  to  circumscribe  its  opening, 
which  before  long  becomes  completely  obliterated ;  and  the 
gland  forms  a  closed  sack  lined  by  epiblast  which  grows  in  an 
anterior  direction  (figs.  114  and  127  cctt). 

The  edges  of  the  mantle  now  begin  to  project,  especially  on 
the  posterior  side  (fig.  127),  and  within  the  cavity  formed  by  this 
projecting  lip  there  are  placed  the  anus  (an],  gills,  etc.  The  pro- 
jecting lip  of  the  mantle  is  formed  both  of  epiblast  and  meso- 
blast. The  whole  of  the  anterior  side  of  the  mantle  is  filled  by 
the  elongated  shell-sack  (cck),  within  which  the  shell  or  pen  soon 
becomes  secreted. 


252  CEPHALOPODA. 


There  are  certain  difficulties  in  comparing  the  shell-gland  of  the  Cephalo- 
poda with  that  of  other  Mollusca  which  will  best  be  rendered  clear  by  the 
following  quotation  from  Lankester1: 

"The  position  and  mode  of  development  of  the  shell-gland  of  the  Cepha- 
lopoda exactly  agree  with  that  of  the  shell-gland  as  seen  in  the  other  Mol- 
luscan  embryos  figured  in  this  paper.  We  are  therefore  fairly  entitled  to 
conclude  from  the  embryological  evidence  that  the  pen-sack  of  Cephalopoda 
is  identical  with  the  shell  gland  of  other  Mollusca. 

"  But  here — forming  an  interesting  example  of  the  interaction  of  the 
various  sources  of  evidence  in  genealogical  biology — palaeontology  crosses 
the  path  of  embryology.  I  think  it  is  certain  that  if  we  possessed  no  fossil 
remains  of  Cephalopoda  the  conclusion  that  the  pen-sack  is  a  special  develop- 
ment of  the  shell-gland  would  have  to  be  accepted. 

"  But  the  consideration  of  the  nature  of  the  shell  of  the  Belemnites  and 
its  relation  to  the  pen  of  living  Cuttle-fish  brings  a  new  light  to  bear  on  the 
matter.  Reserving  anything  like  a  decided  opinion  as  to  the  question  in 
hand,  I  may  briefly  state  the  hypothesis  suggested  by  the  facts  ascertained  as 
to  the  Belemnitidae.  The  complete  shell  of  a  Belemnite  is  essentially  a 
straightened  nautilus-shell  (therefore  an  external  shell  inherited  from  a 
nautilus-like  ancestor),  which,  like  the  nautiloid  shell  of  Spirula,  has  become 
enclosed  by  growths  of  the  mantle,  and  unlike  the  shell  of  Spirula,  has 
received  large  additions  of  calcareous  matter  from  those  enclosing  over- 
growths. On  the  lower  surface  of  the  enclosed  nautilus-shell  of  the  Belemnite 
— the  phragmacone — a  series  of  layers  of  calcareous  matter  have  been 
thrown  down  forming  the  guard  ;  above,  the  shell  has  been  continued  into 
the  extensive  chamber  formed  by  the  folds  of  the  mantle,  so  as  to  form  the 
flattened  pen-like  pro-ostracum  of  Huxley. 

"  Whether  in  the  Belemnites  the  folds  of  the  mantle  which  thus  covered 
in  and  added  to  the  original  chambered  shell,  were  completely  closed  so  as 
to  form  a  sack  or  remained  partially  open  with  contiguous  flaps  must  be 
doubtful. 

"  In  Spirula  we  have  an  originally  external  shell  enclosed  but  not  added 
to  by  the  enclosing  mantle  sack. 

"  In  Spirulirostra,  a  tertiary  fossil,  we  have  a  shell  very  similar  to  that 
of  Spirula,  with  a  small  guard  of  laminated  structure  developed  as  in  the 
Belemnite  (see  the  figures  in  Bronn  Classen  u.  Ordnungen  des  Thierreichs}. 

"  In  the  Belemnites  the  original  nautiloid  shell  is  small  as  compared  with 
Spirulirostra.  It  appears  to  be  largest  in  Huxley's  genus  Xiphoteuthis. 
Hence  in  the  series  Spirula,  Spirulirostra,  Xiphoteuthis,  Belemnites,  we 
have  evidence  of  the  enclosure  of  an  external  shell  by  growths  from  the 
mantle  (as  in  Aplysia),  of  the  addition  to  that  shell  of  calcareous  matter  from 
the  walls  of  its  enclosing  sack,  and  of  the  gradual  change  of  the  relative 
proportions  of  the  original  nucleus  (the  nautiloid  phragmacone)  and  its 

1  "Development    of   Pond    Snail."     Quart.   J.  of  Micro.    Science,    1874,   pp. 


MOLLUSCA.  253 


superadded  pro-ostracal  and  rostral  elements  tending  to  the  disappearance 
of  the  nucleus  (the  original  external  shell).  If  this  view  be  correct  as  to  the 
nature  of  these  shells,  it  is  clear  that  the  shell-gland  and  its  plug  has  nothing 
to  do  with  them.  The  shell-gland  must  have  preceded  the  original  nautiloid 
shell,  and  must  be  looked  for  in  such  a  relation  whenever  the  embryology  of  the 
pearly  Nautilus  can  be  studied.  Now,  everything  points  to  the  close  agreement 
of  the  Belemnitidie  with  the  living  Dibranchiata.  The  hooklets  on  the  arms, 
the  ink-bag,  the  horny  jaws,  and  general  form  of  the  body,  leave  no  room  for 
doubt  on  that  point  ;  it  is  more  than  probable  that  the  living  Dibranchiata 
are  modified  descendants  of  the  mesozoic  Belemnitidae.  If  this  be  so,  the 
pens  of  Loligo  and  Sepia  must  be  traced  to  the  more  complex  shell  of  the 
Belemnite.  This  is  not  difficult  if  we  suppose  the  originally  external  shell 
the  phragmacone,  around  which  as  a  nucleus  the  guard  and  pro-ostracum 
were  developed,  to  have  finally  disappeared.  The  enclosing  folds  of  the 
mantle  remain  as  a  sack  and  perform  their  part,  producing  the  chitino- 
calcareous  pen  of  the  living  Dibranch,  in  which  parts  can  be  recognised  as 
corresponding  to  the  pro-ostracum,  and  probably  also  to  the  guard  of  the 
Belemnite.  If  this  be  the  case,  if  the  pen  of  Sepia  and  Loligo  correspond  to 
the  entire  Belemnite  shell  minus  the  phragmacone-nucleus,  it  is  clear  that 
the  sack  which  develops  so  early  in  Loligo  and  which  appears  to  correspond 
to  the  shell-gland  of  the  other  Molluscs  cannot  be  held  to  do  so.  The  sack 
thus  formed  in  Loligo  must  be  held  to  represent  the  sack  formed  by  the 
primaeval  up-growth  of  mantle-folds  over  the  young  nautiloid  shell  of  its 
Belemnitoid  ancestors,  and  has  accordingly  no  general  significance  for  the 
whole  Molluscan  group,  but  is  a  special  organ  belonging  only  to  the  Dibran- 
chiate  stem,  similar  to— but  not  necessarily  genetically  connected  with — the 
mantle-fold  in  which  the  shell  of  the  adult  Aplysia  and  its  congeners  is  con- 
cealed. The  pen,  then,  of  Cephalopods  would  not  represent  the  plug  of  the 
shell-gland.  In  regard  to  this  view  of  the  case,  it  may  be  remarked  that  I 
have  found  no  trace  in  the  embryonic  history  of  the  living  Dibranchiata  of  a 
structure  representing  the  phragmacone  ;  and  further,  it  is  possible,  though 
little  importance  can  be  attached  to  this  suggestion,  that  the  Dibranchiate 
pen-sack,  as  seen  in  its  earliest  stage  in  the  embryo  Loligo,  etc.,  is  fused 
with  the  surviving  remnants  of  an  embryonic  shell-gland.  When  the 
embryology  of  Nautilus  pompilius  is  worked  out,  we  shall  probably  know 
with  some  certainty  the  fate  of  the  Molluscan  shell-gland  in  the  group  of  the 
Cephalopoda." 

The  funnel.  The  general  development  of  the  funnel  has 
already  been  sufficiently  indicated.  The  folds  of  which  it  is 
formed  are  composed  both  of  epiblast  and  mesoblast.  The 
mesoblast  of  the  anterior  part  of  each  half  of  the  funnel  would 
appear  to  give  rise  to  a  muscle  passing  from  the  cartilage  of  the 
neck  to  the  funnel  proper.  The  posterior  parts  gradually 
approximate,  but  meet  in  the  first  instance  ventrally.  The  two 


254 


POLYPLACOPHORA. 


folds  at  first  merely  form  the  side  of  a  groove  or  imperfect  tube 
(fig.  1 13  C  and  124  ff.),  but  soon  the  free  edges  unite  and  so  give 
rise  to  a  perfect  tube,  the  primitive  origin  of  which  by  the  coal- 
escence of  two  halves  would  not  be  suspected.  In  Nautilus  the 
two  halves  remain  permanently  separate  but  overlap  each  other, 
so  as  to  form  a  functional  tube. 

Polyplacophora.  The  external  characters  of  the  embryo  of 
Chiton  have  long  been  known  through  the  classical  observations 
of  Loven  (No.  285),  while  the  formation  of  the  layers  and  the  in- 
ternal phenomena  of  development  have  recently  been  elucidated 
by  Kowalevsky  (No.  284).  The  eggs  are  laid  in  April,  May, 
and  June,  and  are  enclosed  in  a  kind  of  chorion  with  calcareous 


FIG.  1 1 6. 

I.  CHITON  WOSSNESSENSKII.     (After  Middendorf.) 

II.  CHITON  DISSECTED  to  shew  o.  the  mouth ;  g.  the  nervous  ring ;  ao.  the 
aorta;  c.  the  ventricle;  ^.  an  auricle;  br.  the  left  branchiae;  od.  oviducts.     (After 
Cuvier.) 

III.,  IV.,  V.    STAGES  OF  DEVELOPMENT  OF  CHITON  CINEREUS.    (After  Loven.) 
The  figure  is  taken  from  Huxley. 

protuberances.     The   segmentation   remains   regular   till   sixty- 
four  segments  are  formed.     The  cells  composing  the  formative 


MOLLUSCA.  255 


half  of  the  ovum  then  divide  more  rapidly  than  the  remainder ; 
there  is  in  this  way  formed  an  elongated  sphere,  half  of  which  is 
composed  of  small  cells  and  half  of  larger  cells.  In  the  interior 
is  a  small  segmentation  cavity.  From  its  eventual  fate  the 
hemisphere  of  the  smaller  cells  may  be  called  the  anterior  pole, 
and  that  of  the  larger  cells  the  posterior.  An  involution  of  the 
cells  at  the  apex  of  the  posterior  pole  (though  not  of  the  whole 
hemisphere  of  larger  cells)  now  takes  place,  and  gives  rise  to  the 
archenteron.  At  the  same  time  an  equatorial  double  ring  of 
large  cells  appears  on  the  surface  between  the  two  poles,  which 
becomes  ciliated  and  forms  the  velum.  At  the  apex  of  the  an- 
terior pole  a  tuft  of  cilia,  or  at  first  a  single  flagellum,  is  estab- 
lished (fig.  116  in.  and  IV.). 

In  the  succeeding  developmental  period  the  blastopore,  which 
has  so  far  had  the  form  of  a  circular  pore  at  the  posterior  ex- 
tremity of  the  body,  undergoes  a  series  of  very  remarkable 
changes.  In  conjunction  with  a  gradual  elongation  of  the  larva 
it  travels  to  the  ventral  side,  and  is  prolonged  forwards  to  the 
velum  as  a  groove.  The  middle  part  of  the  groove  is  next  con- 
verted into  a  tube,  which  opens  externally  in  front,  and  post- 
eriorly communicates  with  the  archenteron.  The  walls  of  this 
tube  subsequently  fuse  together,  obliterating  the  lumen,  and 
necessarily  causing  at  the  same  time  the  closure  of  the  blasto- 
pore. The  tube  itself  becomes  thereby  converted  into  a  plate 
of  cells  on  the  ventral  surface  between  the  epiblast  and  the 
hypoblast1. 

While  the  above  changes  have  been  taking  place  the  meso- 
blast  has  become  established.  It  is  derived  from  the  lateral  and 
ventral  cells  of  the  hypoblast. 

After  the  establishment  of  the  germinal  layers  the  further 
evolution  of  the  larva  makes  rapid  progress.  A  transverse 
groove  is  formed  immediately  behind  the  velum,  which  is 
especially  deep  on  the  ventral  surface  ;  and  the  stomodaeum  is 
formed  as  an  invagination  of  the  anterior  wall  of  the  deeper 
section  of  the  groove.  Behind  the  stomodaeum  the  remainder 
of  the  ventral  surface  grows  out  as  a  flattened  foot. 

1  There  is  a  striking  similarity  between  the  changes  of  the  blastopore  in  Chiton 
and  the  formation  of  the  neurenteric  canal  of  Chordata ;  especially  if  Kowalevsky  is 
correct  in  stating  that  the  pedal  nerves  are  developed  from  the  ventral  plate. 


256  POLYPLACOPHORA. 


The  dorsal  surface  behind  the  velum  constitutes  the  mantle, 
and  becomes  divided  by  six  or  seven  transverse  grooves  into 
segment-like  areas,  which  may  be  called  mantle  plates  (fig.  116 
IV.).  These  areas  would  seem  (?)  to  correspond  to  so  many 
flattened-out  shell-glands.  Immediately  behind  the  velum  the 
eyes  appear  as  two  black  spots  (fig.  1 16  IV.). 

While  the  above  external  changes  take  place  the  archenteron 
undergoes  considerable  modifications.  Its  anterior  section  gives 
rise,  according  to  Kowalevsky,  to  a  dorsal  (?)  sack  in  which  the 
radula  is  formed  ;  while  the  liver  arises  from  it  as  two  lateral 
diverticula. 

From  the  above  statements  it  would  appear  that  Kowalevsky  holds  that 
the  oesophagus  and  radula  sack  are  both  derived  from  the  walls  of  the 
archenteron  and  not  from  the  stomodaeum.  Such  an  origin  for  these  organs 
is  without  parallel  amongst  Mollusca. 

The  larva  becomes  about  this  time  hatched,  and  after  swim- 
ming about  for  some  time  attaches  itself  by  the  foot,  throws  off 
its  larval  organs,  cilia,  etc.,  and  develops  the  shell. 

The  shell  appears  first  of  all  during  larval  life  in  the  form  of  spicula  on 
the  middle  and  sides  of  the  head,  and  later  on  the  middle  and  sides  of  the 
post-oral  mantle  plates  (fig.  116  v.).  The  permanent  shell  arises  somewhat 
later  as  a  series  of  median  and  lateral  calcareous  plates,  first  of  all  on  the 
posterior  part  of  the  velar  area,  and  subsequently  on  the  mantle  plates  behind. 
The  three  calcareous  patches  of  each  plate  fuse  together  and  give  rise  to  the 
permanent  shell  plates.  The  original  spicula  are  displaced  to  the  sides, 
where  they  partly  remain,  and  are  partly  replaced  by  new  spicula. 

The  nervous  system  is  formed  during  larval  life  as  four  longitudinal 
cords  : — two  lateral — the  branchial  cords,  and  two  ventral — the  pedal. 
Paired  anterior  thickenings  of  the  pedal  cords  meet  in  front  of  the  mouth  to 
form  the  cesophageal  ring.  The  pedal  cords  and  their  derivatives  are 
believed  by  Kowalevsky  to  be  developed  from  the  lateral  parts  of  the  plate 
formed  by  the  metamorphosis  of  the  blastopore.  The  median  part  of  the 
plate  is  still  visible  after  the  formation  of  these  parts. 

The  chief  peculiarity  of  the  larva  of  Chiton  (apart  from  the 
peculiar  ventral  plate)  consists  in  the  elongation  and  dorsal 
segmentation  of  the  posterior  part  of  the  body.  The  velum  has 
the  normal  situation  and  relation  to  its  mouth.  The  position  of 
the  eyes  behind  it  is  however  abnormal. 

The  elongation  and  segmentation  of  the  posterior  part  of  the 
trunk  is  probably  to  be  regarded  as  indicating  that  Chiton  has 


MOLLUSCA.  257 


early  branched  off  from  the  main  group  of  the  Odontophora 
along  a  special  line  of  its  own,  and  not  that  the  remaining  Odon- 
tophora are  descended  from  Chiton-like  ancestral  forms.  The 
shell  of  Mollusca  on  this  view  is  not  to  be  derived  from  one  of 
the  plates  of  Chiton,  but  the  plates  of  Chiton  are  to  be  derived 
from  the  segmentation  of  a  primitive  simple  shell.  The  segmen- 
tation exhibited  is  of  a  kind  which  all  the  trochosphere  larval 
forms  seem  to  have  been  capable  of  acquiring.  The  bilateral 
symmetry  of  Chiton,  which  is  quite  as  well  marked  as  that  of 
the  Lamellibranchiata,  indicates  that  it  is  a  primitive  phylum  of 
the  Odontophora. 

Scaphopoda.  The  external  characters  of  the  peculiar  larva 
of  this  interesting  group  have  been  fully  worked  out  by  Lacaze 
Duthiers  (No.  286). 

The  segmentation  is  unequal  and  conforms  to  the  usual 
molluscan  type.  At  its  close  the  embryo  becomes  somewhat 
elongated,  and  there  appears  on  its  surface  a  series  of  transverse 
ciliated  rings.  As  soon  as  these  become  formed  the  larva  is 
hatched,  and  swims  about  by  means  of  its  cilia.  Six  ciliated 
bands  are  formed  in  all,  and  in  addition  a  tuft  of  cilia  is  formed 
in  a  depression  at  the  anterior  extremity. 

The  larva  thus  constituted  is  very  different  in  appearance  to 
the  larvae  already  described,  and  its  parts  very  difficult  to 
identify ;  the  next  stages  in  the  development  shew  however  that 
the  whole  region  of  the  body  taken  up  by  the  ciliated  rings  is 
part  of  the  velar  area,  while  the  small  papilliform  region  behind 
is  the  post-velar  part  of  the  embryo.  This  latter  part  grows 
rapidly,  and  at  the  same  time  the  ciliated  rings  become  reduced 
to  four ;  which  gradually  approach  each  other,  while  the  region 
on  which  they  are  placed  grows  in  diameter.  The  rings  finally 
unite,  and  form  a  single  ring  on  a  projecting  velar  ridge.  In 
the  centre  of  this  ring  is  placed  the  terminal  tuft  of  cilia  on  a 
much  reduced  prominence. 

By  the  time  that  these  changes  have  been  effected  in  the 
velum,  the  post-velar  part  of  the  embryo  has  become  by  far  the 
largest  section  of  the  embryo,  so  that  the  velum  forms  a  project- 
ing disc  at  the  front  end  of  an  elongated  body.  The  mantle  is 
formed  as  two  lateral  outgrowths  near  the  hinder  extremity  of 
the  body  which  leave  between  them  a  ventral  groove  lined  by 
B.  II.  1 7 


258  LAMELLIBRANCHIATA. 

cilia ;  on  their  dorsal  side  is  formed  a  delicate  shell.  The 
mantle  lobes  continue  to  grow,  and  by  the  time  the  above 
changes  in  the  velum  are  effected  they  meet  and  unite  in  the 
ventral  line  and  convert  the  groove  between  them  into  a  com- 
plete tube  open  in  front  and  behind.  A  stream  of  water  is 
driven  through  this  tube  by  the  action  of  the  cilia.  The  shell, 
which  is  at  first  disc-shaped  like  the  shell  of  other  molluscan 
larvae,  moulds  itself  upon  the  mantle  and  is  so  converted  into  a 
tube.  At  the  front  end  of  the  mantle  tube,  which  does  not  at 
first  cover  the  velum,  there  is  formed  the  foot.  It  arises  as  a 
protuberance  of  the  ventral  wall  of  the  body,  which  rapidly 
grows  forwards,  becomes  trilobed  as  in  the  adult,  and  ciliated. 

On  the  completion  of  these  changes  the  larva  mainly  differs 
in  appearance  from  the  adult  by  the  projection  of  the  velum 
beyond  the  edge  of  the  shell.  The  velum  soon  however  begins 
to  atrophy  ;  and  the  larva  sinks  to  the  bottom.  The  mantle  tube 
and  shell  grow  forward  and  completely  envelop  the  velum, 
which  shortly  afterwards  disappears.  The  mouth  is  formed  on 
the  ventral  side  of  the  velum  at  the  base  of  the  foot ;  at  its  sides 
arise  the  peculiar  tentacles  so  characteristic  of  the  adult  Denta- 
lium. 


LAMELLIBRANCHIATA. 

The  larvae  of  Lamellibranchiata  have  in  a  general  way  the 
same  characters  as  those  of  Gasteropods  and  Pteropods.  A 
trochosphere  stage  with  a  velum  but  without  a  shell  is  succeeded 
by  a  veliger  stage  with  a  still  more  developed  velum,  a  dorsal 
shell,  and  a  ventral  foot. 

The  segmentation  is  unequal,  and  in  a  general  way  like  that 
of  Gasteropoda,  but  the  specially  characteristic  Gasteropodan 
type  with  four  large  yolk  spheres  is  only  known  to  occur  in 
Pisidium,  and  a  type  of  segmentation  similar  to  that  of  Anodon 
(p.  100)  appears  to  be  the  most  frequent. 

There  is  an  epibolic  or  embolic  gastrula,  but  the  further 
history  of  the  formation  of  the  germinal  layers  has  been  worked 
out  so  imperfectly,  and  for  so  few  types,  that  it  is  not  possible  to 
make  general  statements  about  it.  What  is  known  on  this  head 


MOLLUSCA. 


259 


is  mentioned  in  connection  with  the  description  of  the  develop- 
ment of  special  types. 

The  blastopore  in  some  cases  closes  at  the  point  where  the 
anus  (Pisidium),  and  probably  in  other  cases  where  the  mouth,  is 
eventually  formed.  In  Anodon  it  is  stated  to  close  at  a  point 
corresponding  neither  with  the  mouth  nor  the  anus,  but  on  the 
dorsal  surface ! 

The  embryo  assumes  a  somewhat  oval  form,  and  in  the  free 
marine  forms  there  appears  very  early  in  front  of  the  mouth  a 
well-developed  velum.  This  is  formed  according  to  Love"n  from 
two  papillae,  and  takes  the  form  of  a  circular  ridge  armed  with 
long  cilia.  In  the  centre  of  the  velar  area  there  is  usually 


FIG.  117.    THREE  STAGES  IN  THE  DEVELOPMENT  OF  CARDIUM.     (After  Loven.) 
hy.  hypoblast ;  b.  foot ;  m.  mouth ;  an.  anus ;   V.  velum ;  cm.  anterior  adductor 
muscle. 

present  a  single  long  flagellum  (fig.  117  B  and  C).  The  velum 
never  becomes  bilobed. 

In  the  later  stages,  after  the  development  of  the  shell,  the 
velum  becomes  highly  retractile  and  can  be  nearly  completely 
withdrawn  within  the  mantle  by  special  muscles.  It  forms  the 
chief  organ  of  locomotion  of  the  free  larva. 

In  some  fresh-water  forms,  which  have  no  free  larval  exist- 
ence, the  velum  is  very  much  reduced  (Anodon,  Unio,  Cyclas)  or 
even  aborted  (Pisidium).  In  these  forms  as  well  as  in  Teredo 
and  probably  other  marine  forms  (e.g.  Ostrea)  the  central  flagel- 
lum is  absent.  It  has  been  suggested  by  Loven,  though  without 
any  direct  evidence,  that  the  labial  tentacles  of  adult  Lamelli- 
branchiata  are  the  remains  of  the  velum.  The  velar  area  is  in 
any  case  the  only  representative  of  the  head.  In  some  marine 
forms  a  general  covering  of  cilia  arises  before  the  formation  of 

17-2 


260 


LAMELLIBRANCHIATA. 


the  velum ;  and  in  Montacuta  and  other  types  there  is  developed, 
as  in  many  Gasteropoda,  a  circum-anal  patch  of  cilia. 

A  shell-gland  appears  at  a  very  early  period  on  the  dorsal 
surface  in  Pisidium,  Cyclas  and  Ostrea,  and  probably  in  most 
marine  forms  (fig.  118,  s/is).  It  is  somewhat  saddle-shaped,  and 
formed  of  elongated  non-ciliated  cells  bounding  a  groove.  It 
flattens  out  and  on  its  surface  is  formed  the  shell,  which  appears 
usually  to  have  the  form  of  an  unpaired  saddle-shaped  cuticle,  on 
the  two  sides  of  which  the  valves  are  subsequently  formed  by  a 
deposit  of  calcareous  salts.  In  Pisidium  the  two  valves  are 
stated  by  Lankester  to  be  at  first  quite  independent  and  widely 
separated,  and  it  has  been  suggested  by  Lankester,  though  not 
proved,  that  the  ligament  of  the  shell  is  developed  in  the  median 
part  of  the  groove  of  the  shell-gland. 

The  mantle  lobes  are  developed  as  lateral  outgrowths  of  the 
body  :  they  usually  have  a  considerable  extension  before  they 
are  covered  by  the  shell.  In  Anodon  and  Unio  the  larval 
mantle  lobes  are,  however,  formed  in  a  somewhat  exceptional 
way,  and  are  from  the  first  completely  covered  by  the  valves  of 
the  larval  shell.  The  larval  mantle  lobes  and  shell  in  Anodon 
and  Unio  are  subsequently  replaced  by  the  permanent  structures. 

The  adductor  muscles  are  formed  soon  after  the  appearance 
of  the  shell.  The 
posterior  sometimes 
appears  first,  e.g. 
Mytilus,andat  other 
times  the  anterior, 
e.g.  Cardium. 

The  foot  arises 
in  the  usual  way  as 
a  prominence  be- 
tween the  mouth 
and  anus.  In  com- 
parison with  Gaster- 
opoda it  is  late  in 
appearing,  and  in 
many  cases  does  not 
become  prominent 

till  the  shell  has   at-      /'-~intestine  J  *hs-  shell-gland 


P'lG.    Il8.       AN    EMBRYO   OF   PlSIDIUM    PUSILLUM. 

(From  Lankester.) 

/.  foot ;  m.  mouth  ;  ph.  pharynx ;  gs.  bilobecl  stomach ; 
ell-j 


MOLLUSCA.  261 


tained  a  considerable  size.  In  its  hinder  part  a  provisional 
paired  byssus-gland  is  developed  from  the  epidermis  in  Cyclas 
and  other  forms.  In  other  cases,  e.g.  Mytilus,  the  byssus-gland 
is  permanent.  The  byssus-gland  occupies  very  much  the  position 
of  the  Gasteropod  operculum,  and  would  appear  very  probably 
to  correspond  with  this  organ.  The  anterior  part  of  the  foot  is 
usually  ciliated. 

The  gills  appear  rather  late  in  larval  development  along  the 
base  of  the  foot  on  either  side,  between  the  mantle  and  the  foot 
(fig.  1 20,  br).  They  arise  as  a  linear  row  of  separate  ciliated 
somewhat  knobbed  papillae.  A  second  row  appears  later.  The 
two  rows  give  rise  respectively  to  the  two  gill  lamellae  of  each 
side. 

The  further  history  of  the  development  of  the  gills  has  been  studied  by 
Lacaze  Duthiers  (No.  297)  in  Mytilus.  The  first  row  of  gill  papillae  formed 
becomes  the  innermost  of  the  two  lamellae  of  the  adult.  The  number  of 
papillae  goes  on  increasing  from  before  backwards.  When  about  eleven 
have  been  formed,  their  somewhat  swollen  free  extremities  unite  together, 
the  basal  portions  being  separated  by  slits. 

The  free  limb  is  formed  by  the  free  end  of  the  gill  lamella  bending  upon 
itself  towards  the  inner  side  and  growing  towards  the  line  of  attachment  of 
the  lamella.  The  free  limb  is  at  first  not  composed  of  separate  bars,  but  of 
a  continuous  membrane.  Before  this  membrane  has  grown  very  wide, 
perforations  are  formed  in  it  corresponding  to  the  spaces  between  the  bars  of 
the  attached  limb. 

The  outer  gill  lamella  develops  in  precisely  the  same  way  as,  but  some- 
what later  than,  the  inner.  The  rudiments  of  it  appear  when  about  twenty 
papillae  of  the  inner  lamella  are  formed.  Its  first  papillae  are  formed  near 
the  hind  border  of  the  inner  lamella,  and  new  papillae  are  added  both  in 
front  and  behind.  Its  free  limb  is  on  the  outer  side. 

In  Mytilus  the  two  limbs  (free  and  attached)  of  each  bar  of  the  gill  are 
joined  at  wide  intervals  by  extensile  processes,  the  '  inter-lamellar  junctions,' 
and  the  successive  bars  are  attached  together  by  ciliated  junctions.  In 
many  other  types  the  concrescences  between  the  various  parts  of  the  gills 
are  carried  much  further ;  the  maximum  of  concrescence  being  perhaps 
attained  in  Anodon  and  Unio1. 

Large  paired  auditory  sacks  seem  always  to  be  developed  in 
the  foot;  and  clearly  correspond  with  the  auditory  sacks  in 
Gasteropoda. 

1  R.  H.  Peck,  "Gills  of  Lamellibranch  Mollusca."  Quart.  J.  of  M.  Science, 
Vol.  xvn.  1877. 


262  LAMELL1BRANCHIATA. 

Eyes  are  frequently  present  in  the  larva,  though  they  dis- 
appear in  the  adult.  In  Montacuta  and  other  types  a  pair  of 
these  organs  is  formed  at  the  base  of  the  velum  on  each  side  of 
the  oesophagus,  not  far  from  the  auditory  sacks.  They  are 
provided  with  a  lens. 

A  row  of  similar  organs  is  present  in  the  larva  of  Teredo  in 
front  of  the  foot. 

Cardium.  As  an  example  of  a  marine  Lamellibranchiate  I  may  take 
Cardium  pygmaeum,  the  development  of  which  has  been  studied  by  Lovdn 
(No.  291).  The  ova,  surrounded  by  a  thickish  capsule,  are  impregnated  in 
the  cloaca.  The  segmentation  takes  place  much  as  in  Nassa  (vide  p.  101), 
and  the  small  segments  gradually  envelop  the  large  hypoblast  spheres  ;  so 
that  there  would  seem  to  be  a  gastrula  by  epibole.  After  the  hypoblast  has 
become  enveloped  by  the  epiblast,  one  side  of  the  embryo  is  somewhat  flat- 
tened and  marked  by  a  deepish  depression  (fig.  117  A).  From  LoveVs 
description  it  appears  to  me  probable  that  the  depression  on  the  flattened 
side  occupies  the  position  of  the  blastopore,  and  that  the  depression  itself  is 
the  stomodaeum.  At  this  stage  the  embryo  becomes  covered  with  short  cilia 
which  cause  it  to  rotate  within  the  egg-capsule. 

Close  above  the  mouth  there  appear  two  small  papillae.  These  gradually 
separate  and  give  rise  to  a  circular  ridge  covered  with  long  cilia,  which 
encircles  the  embryo  anteriorly  to  the  ventrally-placed  mouth.  This 
structure  is  the  velum.  In  its  centre  is  a  single  long  flagellum  (fig.  117  B). 
Shortly  after  this  the  shell  appears  as  a  saddle-shaped  structure  on  the 
hinder  part  of  the  dorsal  surface  of  the  embryo.  It  is  formed  at  first  of 
two  halves  which  meet  behind  without  the  trace  of  a  hinge  (fig.  117  C). 
The  two  valves  rapidly  grow  and  partially  cover  over  the  velum,  and  below 
them  the  mantle-folds  soon  sprout  out  as  lateral  flaps. 

The  alimentary  tract  has  by  this  tirhe  become  differentiated  (fig.  117  C). 
It  consists  of  a  mouth  (;;/)  and  ciliated  oesophagus  probably  derived  from 
the  stomodaeum,  a  stomach  and  intestine  derived  from  the  true  hypoblast, 
and  an  hepatic  organ  consisting  of  two  separate  lobes  opening  into  the 
stomach.  The  anus  (an]  appears  not  far  behind  the  mouth,  and  between 
the  two  is  a  very  slightly  developed  rudiment  of  the  foot  (V).  The  anterior 
adductor  muscle  (cm]  appears  at  this  stage,  though  the  posterior  is  not  yet 
differentiated. 

The  larva  is  now  ready  to  be  hatched,  but  the  further  stages  of  its 
development  were  not  followed. 

Ostrea.  The  larvae  of  Ostrea,  figured  by  Salensky  (No.  293),  shew  a 
close  resemblance  to  those  of  Cardium.  The  velum  is  however  a  simple 
ring  of  cilia  without  a  central  flagellum.  The  proctodacum  would  appear  to 
be  formed  later  than  the  stomodaeum,  and  the  earliest  stage  figured  is  too 
far  advanced  to  throw  light  on  the  position  of  the  blastopore. 

Pisidium.     The  development  of  Pisidium  has  been  investigated  by 


MOLLUSCA. 


263 


Lankester  (No.  239).  The  ovum  is  invested  by  a  vitelline  membrane  and 
undergoes  development  in  a  brood-pouch  at  the  base  of  the  inner  gill  lamella. 

The  segmentation  commences  by  a  division  into  four  equal  spheres,  each 
of  which,  as  in  so  many  other  Mollusca,  then  gives  rise  by  budding  to  a 
small  sphere.  The  later  stages  of  segmentation  have  not  been  followed  in 
detail,  but  the  result  of  segmentation  is  a  blastosphere.  An  invagination, 
presumably  at  the  lower  pole,  now  takes  place,  and  gives  rise  to  an 
archenteric  sack. 

The  embryo  now  rapidly  grows  in  size.  The  blastopore  becomes  closed 
and  the  archenteric  sack  forms  a  small  mass  attached  at  one  point  to  the 
walls  of  the  embryonic  vesicle  (fig.  119,  hy).  In  the  space  between  the  walls 
of  the  archenteron  and  those  of  the  embryonic  vesicle  stellate  mesoblast  cells 


FIG.   119.    THREE  VIEWS  OF  AN  EMBRYO  OF  PISIDIUM  IMMEDIATELY  AFTER 
THE  CLOSURE  OF  THE  BLASTOPORE.     (After  Lankester.) 

A.  View  from  the  surface. 

B.  Optical  section  through  the  median  plane. 

C.  Optical  section  through  a  plane  a  little  below  the  surface. 

ep.  epiblast ;  me.  mesoblast ;  hy.  hypoblast ;  p.  cells  apparently  budding  from  the 
hypoblast  to  form  mesoblastic  elements. 


264 


LAMELLIBRANCHIATA. 


make  their  appearance,  derived  in  the  main  from  the  epiblast,  though 
probably  in  part  also  from  the  hypoblastic  vesicle  (vide  fig.  119  C,  p}.  The 
cavity  between  the  hypoblast  and  epiblast,  which  contains  these  cells,  is  the 
body  cavity.  Fig.  1 19  represents  three  views  of  the  embryo  at  this  stage. 
A  is  a  surface  view  shewing  the  epiblast ;  B  is  an  optical  section  through 
the  median  plane  shewing  the  hypoblast  and  some  of  the  mesoblast  cells  ; 
and  C  is  an  optical  section  shewing  the  mesoblast  cells.  A  prominence  on 
one  side  of  the  embryo  now  develops  which  forms  the  commencement  of  the 
foot,  and  the  archenteric  sack  grows  out  at  its  free  extremity  into  two  lobes, 
but  remains  attached  to  the  epiblast  by  an  imperforate  pedicle.  The  next 
organ  to  appear  is  the  stomodasum.  It  arises  as  a  ciliated  epiblastic  in- 
growth which  meets  the  free  end  of  the  archenteric  sack,  fuses  with  it,  and 
shortly  afterwards  opens  into  it  (fig.  118,  ph).  Between  the  mouth  and  the 
attachment  of  the  enteric  pedicle  is  placed  the  foot  (/),  which  becomes 
ciliated.  On  the  dorsal  side  of  the  enteric  pedicle  there  appears  a  saddle- 
shaped  patch  of  epiblast  cells  bounding  the  sides  of  a  groove  (shs).  This  is 
the  rudiment  of  the  shell-gland. 

The  enteric  pedicle,  or  intestine  as  it  may  now  be  called,  soon  acquires  a 
lumen,  though  still  imperforate  at  its  termination  where  the  anus  is 
eventually  formed.  Ventral  to  the  intestine  is  placed  a  mass  of  cells — the 
rudiment  of  the  organ  of  Bojanus.  It  is  stated  to  be  developed  as  an 
ingrowth  of  the  epiblast. 

In  a  slightly  later  stage  the  shell-gland  rapidly  increases  in  size  and 
flattens  out,  and  on  the  two  sides  of  it  there  appear  the  rudiments  of  the  two 
valves,  which  are  at  first  quite  distinct,  and  separated  by  a  considerable 
interval  (fig.  120).  Before  the  appearance  of  the  valves  of  the  shell,  the 
mantle  folds  have  already  grown  out  from  the  sides  of  the  body. 

At  a  somewhat  later  stage  the  gills 
appear  as  a  linear  series  of  small  inde- 
pendent buds  within  the  folds  of  the  mantle 
behind  the  foot  (fig.  120,  br).  The  ante- 
rior adductor  also  becomes  differentiated. 

The  alimentary  tract  in  the  meantime 
has  undergone  considerable  changes.  The 
primitive  lateral  lobes  dilate  enormously 
and  become  ciliated.  At  a  still  later  stage 
their  walls  undergo  peculiar  changes,  the 
nature  of  which  is  somewhat  obscure,  but 
they  appear  to  me  to  be  of  the  same  charac- 
ter as  those  in  many  Pteropods  and  Gas- 
teropods,  where  the  cells  of  the  hepatic 
diverticula,  to  which  the  lobes  of  Pisidium 
apparently  correspond,  become  filled  with 
an  albuminous  material. 

The  later  stages  in  Pisidium  have  not 
been  followed. 


FIG.  120.  DIAGRAMMATIC  VIEW 
OF  ADVANCED  LARVA  OF  PiSIDIUM. 
(Copied  from  Lankesler.) 

tti.  mouth  ;  a.  anus  ;  B.  organ 
of  Bojanus ;  mn.  mantle  ;  f.  foot. 


MOLLUSCA.  265 


It  is  remarkable  that  in  Pisidium  a  veliger  stage  does  not  occur.  This  is 
probably  due  to  the  development  taking  place  within  the  brood-pouch.  The 
late  development  of  the  otocysts  is  also  remarkable.  A  byssus-gland  was 
not  formed  up  to  the  stage  observed.  In  Cyclas  calyculata  (Schmidt), 
a  byssus-gland  also  appears  to  be  absent. 

Cyclas.  The  development  of  Cyclas  as  described  by  Von  Jhering  is 
very  unlike  that  of  Pisidium,  and  the  differences  would  seem  to  be  too  great 
to  be  accounted  for  except  by  errors  of  observation. 

The  segmentation  of  Cyclas  is  similar  to  that  of  Anodon  (vide  p.  82),  and 
a  mass  of  large  cells  enclosed  by  the  smaller  cells  gives  rise  to  the  hypoblast. 
In  the  interior  of  this  mass  there  appears  a  lumen,  and  a  process  from  it 
grows  towards  and  meets  the  epiblast,  and  gives  rise  to  the  oesophagus  and 
mouth, — a  mode  of  development  of  these  parts  without  parallel  amongst 
Mollusca.  A  very  rudimentary  velum  would  appear,  according  to  Leydig 
(No.  290),  to  be  developed  at  the  cephalic  extremity.  A  ,  shell-gland  is 
formed  of  the  same  character  as  in  Gasteropods.  According  to  Leydig  the 
shell  appears  as  a  single  saddle-like  structure  on  the  dorsal  surface ;  the 
lateral  parts  of  this  become  calcified,  and  give  rise  to  the  two  valves,  but  are 
united  in  the  middle  by  the  membranous  median  portion.  At  the  two  sides 
of  the  body  the  mantle  lobes  are  formed,  as  in  Pisidium. 

Very  shortly  after  the  formation  of  the  shell  the  byssus-gland  appears  as 
a  pair  of  small  follicles  in  the  hinder  part  of  the  foot.  It  rapidly  grows 
larger  and  becomes  a  paired  pyriform  gland,  in  which  are  secreted  the  byssus 
threads  which  serve  to  attach  all  the  embryos  at  a  common  point  to  the 
walls  of  the  brood-pouch. 

The  foot  is  large,  and  ciliated  anteriorly.  Otolithic  sacks  and  peda 
ganglia  are  developed  in  it  very  early. 

Unio.  The  ovum  of  Anodonta  and  Unio  is  enveloped  in  a  vitelline 
membrane,  the  surface  of  which  is  raised  into  a  projecting  trumpet-like  tube 
perforated  at  its  extremity  (fig.  12).  This  structure  is  the  micropyle.  The 
micropyle  disappears  in  Anodonta  piscinalis  when  the  egg  is  ripe,  but  in 
Unio  persists  during  the  whole  development.  The  ova  are  transported,  in  a 
manner  not  certainly  made  out,  into  the  space  between  the  two  limbs  of  the 
outer  gills  of  the  mother,  and  there  undergo  their  early  development.  The 
animal  or  upper  pole  of  the  egg  is  placed  at  the  pole  opposite  to  the 
micropyle. 

The  segmentation  is  unequal  (vide  p.  100)  and  results  in  the  formation  of 
a  blastosphere  with  a  large  segmentation  cavity.  The  greater  part  of  the 
circumference  of  the  egg  is  formed  of  small  uniform  spheres,  but  the  lower 
(with  reference  to  the  segmentation)  pole  is  taken  up  by  a  single  large  cell. 
The  small  spheres  become  the  epiblast,  and  the  large  cell  gives  rise  to 
hypoblast  and  mesoblast1. 

1  The  account  of  the  remainder  of  the  development  till  the  larva  becomes  hatched 
is  taken  from  Rabl,  No.  292. 


266  LAMELLIBRANCHIATA. 

The  single  large  cell  next  divides  into  two,  and  then  four,  and  finally  into 
about  ten  to  fifteen  cells.  These  cells  form  an  especial  area  of  more  granular 
cells  than  the  other  cells  of  the  blastosphere.  Most  of  them  are  nearly  of 
the  same  size,  but  two  of  them  (according  to  Rabl),  in  contact  with  each 
other,  but  placed  on  the  future  right  and  left  sides  of  the  embryo,  are  con- 
siderably larger  than  the  remainder.  These  two  cells  soon  pass  into  the 
cavity  of  the  blastosphere,  while  at  the  same  time  the  area  of  granular  cells 
becomes  flattened  out,  and  then  becomes  involuted  as  a  small  sack  with  a 
transversely  elongated  opening,  which  does  not  nearly  fill  up  the  cavity  of 
the  blastosphere.  This  involuted  sack  is  the  archenteron. 

The  two  large  cells,  which  lie  in  immediate  contact  with  what,  following 
Rabl,  I  shall  call  the  anterior  lip  of  the  blastopore,  next  bud  off  small  cells, 
which  first  form  a  layer  covering  the  walls  of  the  archenteron,  but  sub- 
sequently develop  into  a  network  filling  up  the  whole  cavity  of  the  primitive 
blastosphere.  The  space  between  these  cells  is  the  primitive  body  cavity. 
For  a  long  time  the  two  primitive  mesoblast  cells  retain  their  preponderating 
size1.  At  the  hinder  end  of  the  body,  and  at  the  end  opposite  therefore 
to  the  two  mesoblast  cells,  are  placed  three  especially  large  epiblast  cells. 

In  Anodonta  and  Unio  tumidus  there  appears  at  this  period  a  patch  of 
long  cilia  at  the  anterior  end  of  the  body.  These  cilia  cause  a  rotation  of 
the  embryo  and  would  appear  to  be  the  velum.  In  Unio  pictorum  they 
do  not  appear  till  much  later. 

Immediately  following  this  stage  the  changes  in  the  embryo  take  place 
with  great  rapidity.  In  the  first  place  a  special  mass  of  mesoblast  cells 
appears  at  the  hinder  end  of  the  archenteric  sack  ;  and  becoming  elongated 
transversely  gives  rise  to  the  single  adductor  muscle.  On  the  subsequent 
formation  of  the  shell  the  muscle  becomes  inserted  in  its  two  valves. 
The  blastopore  next  becomes  closed,  and  the  small  archenteron  grows  for- 
wards till  it  meets  the  epiblast  anteriorly,  and  at  the  same  time  detaches 
itself  from  the  epiblast  in  the  region  where  the  blastopore  was  placed. 
Where  it  comes  in  contact  with  the  wall  of  the  body  in  front  a  small 
epiblastic  invagination  arises,  which  meets  and  opens  into  the  archenteric 
sack  and  forms  the  permanent  mouth. 

While  these  changes  have  been  taking  place  the  shell  is  formed  as  a 
continuous  saddle-shaped  plate  on  the  dorsal  surface.  From  this  plate  the 
two  valves  are  subsequently  differentiated.  On  the  dorsal  surface  they 
meet  with  a  straight  hinge-line.  Each  valve  is  at  first  rounded,  but  subse- 
quently becomes  triangular  with  the  hinge-line  as  base.  The  valves  are  not 
quite  equi-sided,  but  the  anterior  side  is  less  convex  than  the  posterior.  At 
a  later  period  a  beak-shaped  organ  is  formed  at  the  apex  of  each  valve  in  the 
same  manner  as  the  remainder  of  the  shell.  This  organ  is  placed  at  about 
a  right  angle  with  the  main  portion  of  the  valve.  It  is  pointed  at  its  ex- 

1  In  this  description  I  follow  Rabl's  nomenclature.  According  to  his  statements 
the  ventral  part  of  the  body  is  the  original  animal  pole— the  dorsal  the  lower  pole ; 
the  anterior  end  the  mesoblastic  side  of  the  opening  of  invagination. 


MOLLUSCA.  267 


tremity  and  bears  numerous  sharp  spines  on  its  outer  side,  which  are 
especially  large  in  the  median  line  (vide  fig.  121  A).  It  is  employed  in  fixing 
the  larva,  after  it  is  hatched,  on  to  the  fish  on  which  it  is  for  some  time 
parasitic.  The  shell  is  perforated  by  numerous  pores. 

After  the  shell  has  become  formed  a  new  structure  makes  its  appearance 
which  is  known  as  the  byssus-gland.  It  is  developed  as  an  invagination  of 
the  epiblast  at  the  hinder  end  of  the  body  :  Rabl  was  unable  to  determine 
whether  it  was  formed  from  the  three  large  epiblastic  cells  present  there  or 
no,  It  subsequently  forms  an  elongated  gland  with  three  coils  or  so  round 
the  adductor  muscle  on  the  left  side  of  the  body,  but  opening  in  the  median 
ventral  line.  It  secretes  an  elongated  cord  by  which  the  larva  becomes 
suspended  after  hatching. 

For  some  time  the  ventral  portion  of  the  body  projects  behind  the  ends 
of  the  valves  of  the  shell,  but  before  these  are  completely  formed  a  median 
invagination  of  the  body  wall  takes  place,  which  obliterates  to  a  large  extent 
the  body  cavity,  and  gives  rise  to  two  great  lateral  lobes,  one  for  each  valve. 
These  lobes  are  the  mantle  lobes. 

Before  the  mantle  lobes  are  fully  formed  peculiar  sense-organs,  usually 
four  in  number,  make  their  appearance  on  each  lobe.  Each  of  them  consists 
of  a  columnar  cell,  bearing  at  its  free  end  a  cuticle  from  which  numerous 
fine  bristles  proceed.  Covering  the  cell  and  the  parts  adjoining  it  is  a 
delicate  membrane  perforated  for  the  passage  of  the  bristles.  The  largest 
and  first  formed  of  these  organs  is  placed  near  the  anterior  and  dorsal  part 
of  the  mantle.  The  three  others  are  placed  near  the  free  end  of  the  mantle 
(vide  fig.  121  A).  These  organs  probably  have  the  function  of  enabling  the 
larva  to  detect  the  passage  of  a  fish  in  its  vicinity,  and  to  assist  it  therefore 
in  attaching  itself.  When  the  embryo  is  nearly  ripe  there  appears  im- 
mediately ventral  to  and  behind  the  velum  a  shallow  pit  on  each  side  of  the 
middle  line,  and  the  two  pits  appear  to  be  connected  by  a  median  transverse 
bridge.  These  structures  have  been  the  cause  of  great  perplexity  to  different 
investigators,  and  their  meaning  is  not  yet  clear.  According  to  Rabl  the 
median  structure  is  the  somewhat  bilobed  archenteron,  and  according  to 
his  view  it  is  not  really  connected  with  the  laterally  placed  pits.  The  cilia 
of  the  velum  overlie  these  latter  structures  and  make  them  appear  as  if  their 
edges  were  ciliated.  They  are  regarded  by  Rabl  as  the  rudiments  of  the 
nervous  system. 

With  the  development  of  the  shell,  the  mantle,  and  the  sense-organs,  the 
young  mussel  reaches  its  full  larval  development,  and  is  now  known  as  a 
Glochidium  (fig.  121  A). 

If  the  parent,  with  Glochidia  in  its  gills,  is  placed  in  a  tank  with  fish,  it 
very  soon  (as  I  have  found  from  numerous  experiments)  ejects  the  larvae 
from  its  gills,  and  as  soon  as  this  occurs  the  larvae  become  free  from  the  egg- 
membrane,  attach  themselves  by  the  byssus-cord,  and  when  suspended  in 
this  position  continually  close  and  open  their  shells  by  the  contraction  of  the 
adductor  muscle.  If  the  mussels  are  not  placed  in  a  tank  with  fish  the  larvae 
may  remain  for  a  long  time  in  the  gills. 


268  LAMELLIBRANCHIATA. 


•p.  ad 


B. 
FIG.  121. 

A.    GLOCHIDIUM  IMMEDIATELY  AFTER  IT  is  HATCHED. 
ad.  adductor ;  s/t.  shell ;  by.  byssus  cord  ;  s.  sense  organs. 

B.     GLOCHIDIUM  AFTER  IT  HAS  BEEN  ON  THE  FISH  FOR  SOME  WEEKS. 
br.  branchiae ;  au.  v.  auditory  sack  ;  f.  foot ;  a.  ad.  and  p.  ad.  anterior  and  posterior 
adductors ;  al.  mesenteron ;  mt.  mantle. 

Before  passing  on  to  state  what  is  known  with  reference  to  the  larval 
metamorphosis,  it  may  be  well  to  call  attention  to  certain,  and  to  my  mind 
not  inconsiderable,  difficulties  in  the  way  of  accepting  in  all  particulars 
Rabl's  account  of  the  development. 

In  all  Gasteropod  Molluscs  the  lower  or  vegetative  pole  of  the  ovum  is 
ventral,  not  dorsal  as  Rabl  would  make  it  in  Unio.  The  blastopore  in  other 
Molluscs  always  coincides  either  with  the  mouth  or  anus,  or  extends  between 
the  two.  The  surface  on  which  the  foot  is  formed  is  the  ventral  surface. 
On  the  dorsal  surface  are  placed,  (i)  the  velum  near  the  mouth,  (2)  the  shell- 
gland  near  the  anus.  In  Anodon  the  velum  is  placed  just  dorsal  to  the 
mouth,  then  according  to  Rabl  follows  the  blastopore,  and  in  the  region  of 
the  blastopore  is  formed  the  shell.  The  blastopore  is  therefore  dorsal  in 
position.  It  occupies  in  fact  the  ordinary  place  of  the  shell-gland,  and  looks 
very  much  like  this  organ  (which  is  not  otherwise  present  in  Anodon  and 
Unio).  Without  necessarily  considering  Rabl's  interpretations  false,  I  think 
that  the  above  difficulties  should  have  been  at  any  rate  discussed  in  his  paper. 
More  especially  is  this  the  case  when  there  is  no  doubt  that  Rabl  has 
made  in  his  paper  on  Lymnaeus  a  confusion  between  the  mouth  and  the 
shell-gland. 

Investigations  on  the  post-embryonic  metamorphosis  of  Glochidium  have 
been  made  by  Braun  (No.  287),  and  several  years  ago  I  made  a  series  of 
observations  on  this  subject,  the  results  of  which  agree  in  most  points  with 
those  of  Braun.  I  was  however  unsuccessful  in  carrying  on  my  observations 
till  the  young  mussel  left  its  host. 

The  free  Glochidia  very  soon  attach  themselves  to  the  gills,  fins,  or  other 
parts  of  fish  which  are  placed  in  the  tank  containing  them;  after  attachment 
they  become  covered  by  a  growth  of  the  epidermic  cells  of  their  host,  and 
undergo  their  metamorphosis. 


MOLLUSCA.  269 


The  first  change  that  takes  place  is  the  disappearance  of  the  byssus  and 
the  byssus  organ.  This  occurs  very  soon  ;  shortly  afterwards  all  traces  of 
the  velum  and  sense  organs  also  become  lost. 

At  the  time  of  the  disappearance  of  these  bodies,  at  the  point  of  the 
projection  from  which  the  byssus  cord  arose,  and  very  possibly  from  this 
very  projection,  the  foot  arises  as  a  rounded  process  which  rapidly  grows 
and  soon  becomes  ciliated  (fig.  121  B,/). 

The  single  adductor  muscle  begins  to  atrophy  very  early,  but  before  its 
entire  disappearance  rudiments  are  formed  at  the  two  ends  of  the  body, 
which  at  a  later  period  can  be  distinctly  recognised  as  the  anterior  and 
posterior  adductor  muscles  (fig.  121  B,  a.ad  a.n&  p.ad). 

After  the  formation  of  these  parts  the  gills  arise  as  solid  and  at  first 
somewhat  knobbed  papillae  covered  with  a  ciliated  epidermis,  on  each  side 
of,  but  somewhat  in  front  of  (!)  the  foot  (fig.  121  B,  br).  In  the  foot  there 
soon  appear  the  auditory  sacks  (au.v\  and  the  foot  itself  becomes  a  long 
tongue-like  ciliated  organ  projecting  backwards1. 

The  mantle  lobes  undergo  great  changes,  and  indeed  by  Braun  the 
mantle  lobes  are  stated  to  be  formed  almost  entirely  de  novo.  The  perma- 
nent shell  is  (Braun)  formed  on  the  dorsal  surface  of  the  still  parasitic  larva 
in  the  form  of  two  small  independent  plates.  I  have  not  followed  the  changes 
of  the  alimentary  canal,  etc.,  but  at  an  early  stage  there  is  visible,  dorsal  to 
the  foot,  a  simple  enteric  sack. 

By  the  time  the  larva  leaves  its  host  all  the  organs  of  the  adult,  except 
the  generative  organs,  have  become  established. 

The  post-embryonic  development  of  the  organs  of  Glochidium  is  similar 
in  the  main  to  that  of  other  Lamellibranchiata.  This  fact  is  of  some 
importance  on  account  of  the  peculiarities  of  the  earlier  developmental 
stages. 

The  byssus  organ,  the  toothed  processes  of  the  shell,  and  the  sense  organs 
of  the  Glochidium  can  hardly  be  ancestral  rudiments,  but  must  be  organs 
which  have  been  specially  developed  for  the  peculiar  mode  of  life  of  the 
Glochidium.  Whether  the  single  muscle  is  to  be  counted  amongst  such 
provisional  organs  is  perhaps  a  more  doubtful  point,  but  I  am  inclined  to 
think  that  it  ought  to  be  so. 

If  however  the  single  muscle  is  an  ancestral  organ,  it  is  important  to 
observe  that  it  entirely  disappears  as  development  goes  on  and  the  two 
adductor  muscles  in  the  adult  are  developed  independently  of  it. 

1  The  position  of  the  foot  and  gills  in  the  larva  represented  in  Fig.  119  B  would  be 
more  normal  if  the  convex  and  not  the  natter  side  of  the  shell  were  the  anterior.  I 
have  followed  Rabl  and  Flemming  in  the  determinations  of  the  anterior  and  posterior 
end  of  the  embryo,  but  failed  to  rear  my  larvae  up  to  a  stage  at  which  the  presence  of 
the  heart  or  some  other  organ  would  definitely  confirm  their  interpretation.  I  ori- 
ginally adopted  myself  the  other  view,  and  in  case  they  are  mistaken,  the  so-called 
velum  would  be  a  circum-anal  patch  of  cilia,  while  the  position  of  the  primitive  meso- 
blast  cells  as  well  as  of  the  byssus  would  better  suit  my  view  than  that  adopted  in  the 
text  on  the  authority  of  the  above  observers. 


2/0  SUMMARY. 


General  review  of  the  characters  of  the  Molhiscan  larvae. 

The  typical  larva  of  a  Mollusc,  as  has  been  more  especially 
pointed  out  by  Lankester,  is  essentially  similar  to  the  larva  of  a 
number  of  invertebrate  types,  and  especially  the  Chaetopoda, 
with  the  addition  of  certain  special  organs  characteristic  of  the 
Mollusca. 

It  has  a  bent  alimentary  tract,  with  a  mouth  on  the  ventral 
surface  and  a  terminal  or  ventral  anus.  The  alimentary  tract  is 
divided  into  three  regions  :  oesophagus,  stomach,  and  intestine. 
There  is  a  variously  developed  praeoral  lobe  with  a  ring  of  cilia 
— the  velum,  and  a  perianal  lobe,  often  with  a  patch  of  cilia 
(Paludina,  etc.).  In  all  these  characters  it  is  essentially  similar 
to  a  Chaetopod  larva.  The  two  characteristic  molluscan  organs 
are  (i)  a  foot  between  the  mouth  and  anus,  and  (2)  an  in- 
vagination  of  the  epiblast  on  the  dorsal  side  at  the  hinder 
end  of  the  body,  which  is  connected  with  the  formation  of  the 
shell. 

The  larvae  of  most  Gasteropoda,  Pteropoda,  and  Lamelli- 
branchiata  present  no  features  which  call  for  special  remark  ; 
but  the  larvae  of  the  Gymnosomata  amongst  the  Pteropoda,  and 
of  the  Scaphopoda,  Polyplacophora  and  Cephalopoda  present 
interesting  peculiarities. 

The  larvae  of  the  Gymnosomata  are  peculiar  in  the  presence 
of  three  transverse  ciliated  rings,  situated  behind  the  velum  (Fig. 
109).  These  rings  might  be  regarded  as  indications  of  a  rudi- 
mentary segmentation ;  but,  as  already  indicated,  this  view 
is  not  satisfactory.  There  is  every  reason  for  thinking  that 
these  rings  have  been  specially  acquired  by  these  larvae. 

At  first  sight  the  larvae  of  the  Gymnosomata  might  be 
supposed  to  resemble  those  of  the  Scaphopoda,  which  are  also 
provided  with  transverse  ciliated  rings  ;  but,  as  shewn  above,  the 
rings  of  the  Scaphopoda  are  merely  parts  of  the  extended  velar 
ring. 

Thus,  the  ciliated  rings  of  the  two  larvae — so  similar  in 
appearance — are  in  reality  structures  of  entirely  different  values, 
being  in  the  one  case  parts  of  the  velum,  and  in  the  other  special 
developments  of  cilia  behind  the  velum. 


MOLLUSCA.  271 


The  great  peculiarity  of  the  early  larva  of  the  Scaphopoda 
is  the  enormous  development  of  the  praeoral  lobe,  which  gives 
room  for  the  development  of  the  ciliated  rings.  In  the  pre- 
sence of  a  central  tuft  of  cilia,  at  the  anterior  extremity,  the 
larva  of  the  Scaphopoda  resembles  that  of  the  Lamellibranchi- 
ata,  etc. 

The  larva  of  the  Polyplacophora  resembles  that  of  Lamelli- 
branchiata  in  its  anterior  flagellum,  and  that  of  the  Scaphopoda 
in  the  large  development  of  the  praeoral  lobe ;  but  is  quite  pecu- 
liar amongst  Mollusca  in  the  transverse  segmentation  of  the 
mantle  area. 

The  embryo  of  the  Cephalopoda  agrees  very  closely  with 
that  of  normal  Odontophora  in  the  formation  of  the  mantle  and 
(?)  of  the  shell-gland,  but  is  quite  exceptional  (i)  in  the  almost 
invariable  presence  of  a  more  or  less  developed  external  yolk- 
sack,  (2)  in  the  absence  of  a  velum,  (3)  in  the  absence  of  a 
median  foot,  and  in  the  presence  of  the  arms. 

The  presence  of  a  yolk-sack  may  most  conveniently  be  spoken 
of  in  connection  with  the  foot,  and  we  may  therefore  pass  on  to 
the  question  of  the  velum. 

The  velum  is  one  of  the  most  characteristic  embryonic 
appendages  of  the  Mollusca,  and  its  absence  in  the  Cephalopoda 
is  certainly  very  striking.  By  some  investigators  the  arms  have 
been  regarded  as  representing  the  velum,  but  considering  that 
they  are  primitively  placed  on  the  posterior  and  ventral  side  of 
the  mouth,  and  that  the  velum  is  essentially  an  organ  on  the 
dorsal  side  of  the  mouth,  this  view  cannot,  in  my  opinion,  be 
maintained  with  any  plausibility. 

Various  views  have  been  put  forward  with  reference  to  the 
Cephalopod  foot.  Huxley's  view,  which  is  the  one  most  gene- 
rally adopted,  is  given  in  the  following  quotation1. 

"  But  that  which  particularly  distinguishes  the  Cephalopoda 
"  is  the  form  and  disposition  of  the  foot.  The  margins  of  this 
"  organ  are,  in  fact,  produced  into  eight  or  more  processes  termed 
"arms,  or  brachia ;  and  its  antero-lateral  portions  have  grown 
"  over  and  united  in  front  of  the  mouth,  which  thus  comes, 
"  apparently,  to  be  placed  in  the  centre  of  the  pedal  disk.  More- 

1   The  Anatomy  of  Invertebrated  Animals,  p.  519. 


2/2  SUMMARY. 


"  over,  two  muscular  lobes  which  correspond  with  the  epipodia  of 
"the  Pteropods  and  Branchiogasteropods,  developed  from  the 
"  sides  of  the  foot,  unite  posteriorly,  and,  folding  over,  give  rise  to 
"  a  more  or  less  completely  tubular  organ  —  the  funnel  or  infun- 


Grenacher,  from  his  observations  on  the  development  of 
Cephalopoda,  argues  strongly  against  this  view,  and  maintains 
that  no  median  structure  comparable  with  the  foot  is  present  in 
this  group  :  and  that  the  arms  cannot  be  regarded  as  taking  the 
place  of  the  foot,  but  are  more  probably  representatives  of  the 
velum. 

The  difficulty  of  arriving  at  a  decision  on  this  subject  is 
mainly  due  to  the  presence  of  the  yolk-sack,  which,  amongst  the 
Cephalopoda  as  amongst  the  Vertebrata,  is  the  cause  of  consider- 
able modifications  in  the  course  of  the  development.  The  foot  is 
essentially  a  protuberance  on  the  ventral  surface,  between  the 
mouth  and  the  anus.  In  Gasteropods  it  is  usually  not  filled  with 
yolk,  but  contains  a  cavity,  traversed  by  contractile  mesoblastic 
cells.  In  this  group  the  blastopore  is  a  slit-like  opening  (vide 
p.  187)  extending  over  the  region  of  the  foot,  from  the  mouth  to 
the  anus,  the  final  point  of  the  closure  of  which  is  usually  at  the 
oral  but  sometimes  at  the  anal  extremity.  In  Cephalopods  the 
position  of  the  Gasteropod  foot  is  occupied  by  the  external  yolk- 
sack.  In  normal  forms  the  blastopore  closes  at  the  apex  of  the 
yolk-sack,  and  at  the  two  sides  of  the  yolk-sack  the  arms  grow 
out.  These  considerations  seem  to  point  to  the  conclusion  that 
the  normal  Gasteropod  foot  is  represented  in  the  Cephalopod 
embryo  by  the  yolk-sack,  which  has,  owing  to  the  immense  bulk 
of  food-yolk  present  in  the  ovum,  become  filled  with  food-yolk 
and  enormously  dilated.  The  closure  of  the  blastopore  at  the 
apex  of  the  yolk-sack,  and  not  at  its  oral  or  anal  side,  is  what 
might  naturally  be  anticipated  from  the  great  extension  of  this 
part. 

Grenacher's  type  of  larva,  where  the  external  yolk-sack  is 
practically  absent,  appears  to  me  to  lend  confirmation  to  this 
view.  If  the  reader  will  turn  to  fig.  1  13,  he  will  observe  a  promi- 
nence between  the  mouth  and  anus,  which  exactly  resembles  the 
ordinary  Gasteropod  foot.  At  the  sides  of  this  prominence  are 
placed  the  rudiments  of  the  arms.  This  prominence  is  filled 


MOLLUSCA.  273 


with  yolk,  and  represents  the  rudiment  of  the  external  yolk-sack 
of  the  typical  Cephalopod  embryo.  The  blastopore,  owing  to 
the  smaller  bulk  of  the  food-yolk,  reverts  more  nearly  to  its 
normal  position  on  the  oral  side  of  this  prominence. 

If  the  above  considerations  have  the  weight  which  I  attribute 
to  them,  the  unpaired  part  of  the  Cephalopod  foot  has  been  over- 
looked in  the  embryo  on  account  of  the  enormous  dilatation  it 
has  undergone  from  being  filled  with  food-yolk  ;  and  also  owing 
to  the  fact  that  in  the  adult  the  median  part  of  the  foot  is  unre- 
presented. The  arms  are  clearly,  as  Huxley  states,  processes  of 
the  margin  of  the  foot. 

Both  Grenacher  and  Huxley  agree  in  regarding  the  funnel  as 
representing  the  coalesced  epipodia;  but  Grenacher  points  out 
that  the  anterior  folds  which  assist  in  forming  the  funnel  (vide 
p.  253)  represent  the  great  lateral  epipodia  of  the  Pteropod  foot, 
and  the  posterior  folds  the  so-called  horse-shoe  shaped  portion  of 
the  Pteropod  foot. 

Development  of  Organs. 

The  epiblast.  With  reference  to  the  general  structure  of 
the  epiblast  there  is  nothing  very  specially  deserving  of  notice. 
It  gives  rise  to  the  whole  of  the  general  epidermis  and  to  the 
epithelium  of  the  organs  of  sense.  The  most  remarkable  feature 
about  it  is  a  negative  one,  viz.  that  it  does  not,  in  all  cases  at  any 
rate,  give  rise  to  the  nervous  system. 

The  epiblast  of  the  mantle  has  the  special  capacity  of  secret- 
ing a  shell,  and  the  integument  of  the  foot  has  also  a  more  or 
less  similar  property  in  that  it  forms  the  operculum,  and  a 
byssus  in  some  Lamellibranchiata,  other  parts  of  the  integument 
form  the  radula,  setae  in  Chiton,  and  other  similar  structures. 

Nervous  system.  The  origin  of  the  nervous  system  in 
Mollusca  is  still  involved  in  some  obscurity.  It  is  the  general 
opinion  amongst  the  majority  of  investigators  that  the  nervous 
ganglia  in  Gasteropods  and  Pteropods  are  formed  from  detached 
thickenings  of  the  epiblast.  Both  Lankester  (No.  239)  and  Fol 
(No.  249 — 251)  have  arrived  at  this  conclusion,  and  Rabl  has 
shewn  by  sections  that  in  Planorbis  there  are  two  lateral  thick- 
enings of  the  epiblast  in  the  velar  area ;  from  which  the  supra- 
B.  II.  1 8 


2/4  NERVOUS   SYSTEM. 


cesophageal  ganglia  become  subsequently  separated  off.  The 
observations  on  the  pedal  ganglia  are  less  precise :  they  very 
probably  arise  as  thickenings  of  the  epiblast  of  the  side  of  the 
foot. 

According  to  Fol,  the  nervous  system  in  the  Hyaleacea  amongst  the 
Pteropoda  originates  in  a  somewhat  different  way.  A  disc-like  area  appears 
in  the  centre  of  the  velum,  which  soon  becomes  nearly  divided  into  two 
halves.  From  each  of  these  there  is  formed  by  invagination  a  small  sack. 
The  axes  of  invagination  of  the  two  sacks  meet  at  an  angle  on  the  surface. 
The  cavities  of  the  sacks  become  obliterated  ;  the  sacks  themselves  become 
detached  from  the  surface,  fuse  in  the  middle  line,  and  come  to  lie  astride  of 
the  oesophagus.  Fol  has  detected  a  similar  process  in  Limax.  The  exact 
origin  of  the  pedal  ganglia  was  not  observed,  but  Fol  is  inclined  to  believe 
that  they  develop  from  the  mesoblast  of  the  foot. 

A  very  different  view  is  held  by  Bobretzky  (No.  242),  whose  observations 
were  made  by  means  of  sections. 

The  supra- cesophageal  and  pedal  ganglia  are  formed  according  to  this 
author  as  independent  and  ill-defined  local  thickenings  of  cells  which  are 
apparently  mesoblastic.  The  two  sets  of  ganglia  appear  nearly  simultane- 
ously, and  later  than  the  rudiments  of  the  auditory  and  optic  organs. 

In  the  Cephalopoda  there  seems  to  be  but  little  doubt,  as 
first  pointed  out  by  Lankester,  that  the  various  ganglia  originate 
in  what  is  apparently  mesoblastic  tissue. 

There  is  still  very  much  requiring  to  be  made  out  with 
reference  to  their  origin,  unless  details  on  this  subject  are  given 
in  Bobretzky's  Russian  memoir.  It  would  seem  however  that 
each  ganglion  develops  as  an  independent  differentiation  of  the 
mesoblast  (unless  the  optic  and  cerebral  ganglia  are  from  the 
first  continuous)1.  The  corresponding  ganglia  of  the  two  sides 
become  subsequently  united  and  the  various  ganglia  become 
connected  by  their  proper  commissural  cords.  The  ganglia  are 
shewn  in  figures  124,  126,  and  127. 

In  Lamellibranchiata  the  development  of  the  nervous  system 
has  not  been  worked  out. 

The  two  points  which  are  most  striking  in  the  development  of  the 
nervous  system  of  Mollusca  are  (i)  the  fact  that  in  the  Cephalopoda  at  any 
rate  it  is  developed  from  tissue  apparently  mesoblastic  ;  and  (2)  the  fact  that 
the  several  ganglia  frequently  originate  quite  independently,  and  subse- 
quently become  connected. 

1  Ussow  states  that  they  are  independent. 


MOLLUSCA. 


2/5 


With  reference  to  the  first  of  these  points  it  should  be  noticed  that  the 
supra-cesophageal  and  pedal  ganglia  are  at  first  respectively  connected  with 
the  optic  and  auditory  organs,  and  that  these  sense  organs  are  in  some  cases 
at  any  rate  developed  anteriorly  in  point  of  time  to  the  ganglia.  It  seems 
perhaps  not  impossible  that  primitively  the  ganglia  may  have  been  simply 
differentiations  of  the  walls  of  the  sense  organ,  and  perhaps  their  apparent 
derivation  from  the  mesoblast  is  really  a  derivation  from  cells  which 
primitively  belonged  to  the  walls  of  these  sense  organs.  Bobretzky's 
observations  on  Fusus  fit  in  well  with  this  view. 

In  the  Hyaleacea  and  in  other  Pteropods,  where  the  eyes  are  absent  in 
the  adult,  Fol  finds  the  supra-cesophageal  ganglia  resulting  from  a  pair  of 
epiblastic  invaginations.  May  not  these  invaginations  be  really  rudiments 
of  the  eyes  as  well  as  of  the  ganglia  ?  Fol  also,  it  is  true,  describes  a  similar 
mode  of  origin  for  these  ganglia  in  Limax.  It  would  be  interesting  to  have 
further  observations  on  this  subject.  The  independent  origin  of  the  pedal 
and  supra-cesophageal  ganglia  finds  its  parallel  amongst  the  Chaetopoda. 


Pal, 


N.op 


FIG.  122.    THREE  DIAGRAMMATIC  SECTIONS  OF  THE  EYES  OF  MOLLUSCA. 

(After  Grenacher.) 

A.  Nautilus.     B.  Gasteropod  (Limax  or  Helix).     C.  Dibranchiate  Cephalopod. 
Pal.  eyelid;  Co.  cornea;  Co.ep.  epithelium  of  ciliary  body  ;  Ir.  iris;  Int.  Int1... 
Znt4.  different  parts  of  the  integument ;  /.  lens  ;  I1,  outer  segment  of  lens  ;  R.  retina; 
N.op.  optic  nerve;    G.op.  optic  ganglion;   x.  inner  layer  of  retina;    N.S.  nervous 
stratum  of  retina. 

The  supra-cesophageal  ganglia  appear  always  to  develop  within  the 
region  of  the  velar  area.  This  area  corresponds  with  the  prae-oral  lobe  of 
the  Chaetopod  larva,  at  the  apex  of  which  is  developed  the  supra-cesophageal 
ganglion.  Embryology  thus  confirms  the  results  of  Comparative  Anatomy 
in  reference  to  the  homology  of  these  ganglia  in  the  two  groups. 

Optic  organs1.     An  eye  is  present  in  most  Gasteropods  and 

1  For  a  fuller  account  of  this  subject  the  reader  is  referred  to  the  chapter  on  '  The 
Development  of  the  Eye.' 

1 8— 2 


276 


OPTIC  ORGANS. 


in  many  larval  Pteropods.  Although  its  development  has  not 
been  fully  worked  out,  yet  it  has  clearly  been  shewn  by 
Bobretzky  and  other  investigators  that  it  originates  as  an  involu- 
tion of  the  epidermis,  which  first  forms  a  cup  and  eventually  a 
closed  vesicle.  The  posterior  wall  of  the  vesicle  gives  rise  to  the 
retina,  the  anterior  to  the  inner  epithelium  of  the  cornea.  The 
external  epidermis  becomes  continued  over  the  outer  surface  of 
the  vesicle. 

The  lens  is  formed  in  the  interior  of  the  vesicle,  probably  as 
a  cuticular  deposit,  which  increases  by  the  addition  of  concentric 
layers.  Pigment  becomes  deposited  between  the  cells  of  the 
retina.  Fig.  122  B  is  a  diagrammatic  representation  of  the  adult 
eye  of  a  Gasteropod. 

The  Cephalopod  eye  is  formed,  as  first  shewn  by  Lankester, 
as  a  pit  in  the  epiblast  round  which  a  fold  arises  (fig.  123  A)  and 
gradually  grows  over  the  mouth  of  the  pit  so  as  to  shut  it  off 
from  communication  with  the  exterior  (fig.  123  B). 

The  epiblast  lining  the  posterior  region  of  the  vesicle  gives 
rise  to  the  retina,  that 
lining  the  anterior  region 
to  the  ciliary  body  and 
processes.  It  is  impor- 
tant to  notice  that  the 
condition  of  the  eye  just 
before  the  above  pit  be- 
comes closed  is  exactly 
that  which  is  permanent 
in  Nautilus  (vide  fig.  122 
A).  After  the  pit  has 
become  closed  a  meso- 
blastic  layer  grows  in 
between  its  wall  and  the 
external  epiblast. 

The  lens  becomes  formed  in  two  independent  segments. 
The  inner  and  larger  of  these  arises  as  a  rod- like  process  (fig. 
124)  projecting  from  the  front  wall  of  the  optic  vesicle  into  the 
cavity  of  the  vesicle.  It  is  a  cuticular  structure  and  therefore 
without  cells.  By  the  deposition  of  a  series  of  concentric  layers 
it  soon  assumes  a  spherical  form  (fig.  125,  hi}.  The  condition 


FlG.  123.  TWO  SECTIONS  THROUGH  THE 
DEVELOPING  EYE  OF  A  CEPHALOPOD  TO  SHEW 
THE  FORMATION  OF  THE  OPTIC  CUP.  (After 

Lankester.) 


MOLLUSCA. 


277 


of  the  eye,  with  a  closed  optic  vesicle  and  the  lens  projecting 
into  it,  is  that  which  is  permanent  in  the  majority  of  Gasteropods 
(vide  fig.  122  B).  At  about  the  time  when  the  lens  first  becomes 
formed  a  fold  composed  of  epiblast  and  mesoblast  appears  round 
the  edge  of  the  optic  cup  (fig.  124,^),  and  gives  rise  to  a  structure 

*d     ffc 

adk 


FIG.  124.    TRANSVERSE  SECTION  THROUGH  THE  HEAD  OF  AN  ADVANCED  EMBRYO 

OF  LOLIGO.     (After  Bobretzky.) 

vd.  oesophagus ;  gls.  salivary  gland  ;  g.us.  visceral  ganglion  ;  gc.  cerebral  ganglion; 
g.op.  optic  ganglion ;  adk.  optic  cartilage ;  ak.  and  y.  lateral  cartilage  or  (?)  white 
body ;  rt.  retina ;  gm.  limiting  membrane ;  vk.  ciliary  region  of  eye  ;  cc.  iris ;  ac. 
auditory  sack  (the  epithelium  lining  the  auditory  sacks  is  not  represented) ;  vc.  vena 
cava  ;  ff.  folds  of  funnel. 

known  in  the  adult  as  the  iris.  Shortly  afterwards  this  becomes 
more  prominent  (fig.  125,  if),  and  at  the  same  time  the  layers  of 
cells  of  the  ciliary  region  in  front  of  the  inner  segment  of  the 
lens  become  reduced  to  the  condition  of  mere  membranes  (fig. 
125  B);  and  in  front  of  them  the  anterior  or  outer  segment  of 
the  lens  becomes  formed  as  a  cuticular  deposit  (fig.  125  B,  vl). 
At  a  still  later  period  a  fresh  fold  of  epiblast  and  mesoblast 
appears  round  the  eye  and  gradually  constitutes  the  anterior 
optic  chamber  (vide  fig.  122  C,  Co).  In  most  forms  this  chamber 
communicates  with  the  exterior  by  a  small  aperture,  but  in 
some  it  is  completely  closed.  The  fold  itself  gives  rise  to  the 
cornea  in  front  and  to  the  sclerotic  at  the  sides.  At  a  later 


278  AUDITORY  ORGANS. 


period  another  fold  may  appear  forming  the  eyelids  (fig.  122  C, 
Pal). 

Auditory  organs.  A  pair  of  auditory  sacks  is  found  in  the 
larvae  of  almost  all  Gasteropods  and  Pteropods,  and  usually 
originates  very  early.  They  are  placed  in  the  front  part  of  the 
foot,  and  on  the  formation  of  the  pedal  ganglia  come  into  close 
connection  with  it,  though  they  receive  their  nervous  supply  in 
the  adult  from  the  supra-cesophageal  ganglia. 

In  a  very  considerable  number  of  cases  amongst  Gasteropods 
and  Pteropods  the  auditory  organs  have  been  observed  to  develop 
as  invaginations  of  the  epiblast,  which  give  rise  to  closed  vesicles 
lying  in  the  foot,  e.g.  Paludina,  Nassa,  Heteropods,  Limax,  some 
Pteropods  (Clio). 

This  is  no  doubt  the  primitive  mode  of  origin,  but  in  other 
cases,  which  perhaps  require  confirmation,  the  sacks  are  stated 
to  originate  from  a  differentiation  of  solid  thickenings  of  the  epi- 
dermis or  of  the  tissues  subjacent  to  it. 

The  auditory  sacks  are  provided  with  an  otolith,  which 
according  to  Fol's  observations  is  first  formed  in  the  wall  of  the 
sack. 

In  Cephalopods  the  auditory  organs  are  formed  as  epiblastic 
pits  on  the  posterior  surface  of  the  embryo,  and  are  at  first 
widely  separated  (fig.  113,  ac).  The  openings  of  the  pits  become 
narrowed,  and  finally  the  original  pits  form  small  sacks  lined 
by  an  epithelium,  and  communicating  with  the  exterior  by 
narrow  ducts,  equivalent  to  the  recessus  vestibuli  of  Vertebrates, 
and  named,  after  their  discoverer,  Kolliker's  ducts.  The  ex- 
ternal openings  of  these  ducts  become  completely  closed  at 
about  the  same  time  as  the  shell-gland,  and  the  ducts  remain  as 
ciliated  diverticula  of  the  auditory  pits.  The  widely  separated 
auditory  sacks  gradually  approach  in  the  middle  ventral  line, 
and  are  immediately  invested  by  the  visceral  ganglia  (fig.  124, 
ac).  They  finally  come  to  lie  in  contact  on  the  inner  side  of 
the  funnel. 

On  the  side  opposite  Kolliker's  duct,  an  epithelial  ridge  is 
formed — the  crista  acustica — the  cells  of  which  give  rise  to  an 
otolith  connected  with  the  crista  by  a  granular  material.  At  a 
later  period  of  development  three  regions  of  the  epithelium  of 
the  sack  become  especially  differentiated.  Each  of  these  regions 


MOLLUSCA. 


279 


is  provided  with  two  rows 
of  cells,  bearing  on  their  free 
edges  numerous  very  short 
auditory  hairs.  The  cells  of 
each  row  are  placed  nearly 
at  right  angles  to  those  of 
the  adjoining  row. 

Muscular  system.  The 
muscular  system  in  all 
groups  of  Molluscs  is  de- 
rived entirely  from  the 
mesoblast. 

The  greater  part  of  the 
system  takes  its  origin  from 
the  somatic  mesoblast.  In 
almost  all  Gasteropod  and 
Pteropod  larvae  there  is  pre- 
sent a  well-developed  spin- 
dle muscle  attaching  the 
embryo  to  the  shell.  This 
muscle  appears  to  be  absent 
in  the  Cephalopoda. 

Body  cavity  and  vas- 
cular system.  The  body 
cavity  in  Gasteropods  and 
Pteropods  originates  either 
by  a  definite  splitting  of 
the  mesoblast,  or  by  the  ap- 


B 


FIG.  125.  SECTIONS  THROUGH  THE  DE- 
VELOPING EYE  OF  LOLIGO  AT  TWO  STAGES. 
(After  Bobretzky.) 

hi.  inner  segment  of  lens ;  vl.  outer  segment 
of  lens;  a  and  a  .  epithelium  lining  the  anterior 
optic  chamber;  gz.  large  epiblast  cells  of 
ciliary  body ;  cc.  small  epiblast  cells  of  ciliary 
body;  ms.  layer  of  mesoblast  between  the  two 
epiblastic  layers  of  the  ciliary  body;  af.  and 
if.  fold  of  iris;  rt.  retina;  rt" .  inner  layer 

pearance    of    intercellular     of  retina;  "' rods ;  aq'  equatorial  cartilase" 
spaces.     It  becomes  divided  into  numerous  sinuses  which  freely 
communicate  with  the  vascular  system. 

Very  different  accounts  have  been  given  by  different  investi- 
gators of  the  development  of  the  heart  in  the  Gasteropoda  and 
Pteropoda. 

It  would  seem  however  in  most  cases  to  arise  as  a  solid  mass 
of  mesoblast  cells  at  the  hind  end  of  the  pallial  cavity,  which 
subsequently  becomes  hollowed  out  and  divided  into  an  auricle 
and  ventricle.  Bobretzky's  careful  observations  have  fully  estab- 
lished this  mode  of  development  for  Nassa. 


280  RENAL   ORGANS. 


In  Pteropods  the  heart  is  formed  (Fol)  close  to  the  anus,  but  slightly 
dorsal  to  it  (fig.  108,  h).  The  pericardium  is  formed  from  the  mesoblast  at 
a  considerably  later  period  than  the  heart. 

A  very  different  account  of  the  formation  of  the  heart  is  given  by 
Hiitschli  for  Paludina.  He  states  that  there  appears  an  immense  contrac- 
tile sack  on  the  left  side  of  the  body.  This  becomes  subsequently  reduced 
in  size,  and  in  the  middle  of  it  appears  the  heart,  probably  from  a  fold 
of  its  wall.  The  original  sack  would  appear  to  give  rise  to  the  pericardium. 

In  connection  with  the  vascular  system  mention  may  be 
made  of  certain  contractile  sinuses  frequently  found  in  the  larvae 
of  Gasteropoda  and  Pteropoda.  One  of  these  is  placed  at  the 
base  of  the  foot,  and  the  other  on  the  dorsal  surface  within  the 
mantle  cavity  immediately  below  the  velum1.  The  completeness 
of  the  differentiation  of  these  sinuses  varies  considerably;  in 
some  forms  they  are  true  sacks  with  definite  walls,  in  other  cases 
mere  spaces  traversed  by  muscular  strands.  They  are  found  in 
the  majority  of  marine  Gasteropods,  Heteropods  and  Pteropods. 
In  Limax  a  large  posteriorly  placed  pedal  sinus  is  well  developed, 
and  there  is  also  a  sinus  in  the  visceral  sack.  The  rhythmical 
contraction  of  the  yolk-sack  of  Cephalopods  appears  to  be  a 
phenomenon  of  the  same  nature  as  the  contraction  of  the  foot 
sinus  of  Limax. 

In  Calyptraea  (Salensky)  there  is  an  enormous  provisional 
cephalic  dilatation  within  the  velum  which  does  not  appear  to  be 
contractile.  Similar  though  less  marked  cephalic  vesicles  are 
found  in  Fusus,  Buccinum  and  most  marine  Gasteropods. 

In  Cephalopods  the  vascular  system  is  formed  by  a  series  of 
independent  (?)  spaces  originating  in  the  mesoblast,  the  cells 
around  which  give  rise  to  the  walls  of  the  vessels.  The  branchial 
hearts  are  formed  at  about  the  time  at  which  the  shell-gland 
becomes  closed.  The  aortic  heart  (fig.  127,  c)  is  formed  of  two 
independent  halves  which  subsequently  coalesce  (Bobretzky). 

The  true  body  cavity  arises  as  a  space  in  the  mesoblast  sub- 
sequently to  the  formation  of  the  main  vascular  trunks. 

Renal  organs.  Amongst  the  Gasteropods  and  Pteropods 
there  are  present  provisional  renal  organs,  which  may  be  of  two 
kinds,  and  a  permanent  renal  organ. 

1  Rabl  holds  that  there  is  no  contractile  dorsal  sinus,  but  that  the  appearance  of 
contraction  there  is  due  to  the  contractions  of  the  foot. 


MOLLUSCA.  28 1 


The  provisional  organs  consist  of  either  (i)  an  external 
paired  mass  of  excretory  cells  or  (2)  an  internal  organ  provided 
with  a  duct,  which  is  not  in  all  cases  certainly  known  to  open 
externally.  The  former  structure  is  found  especially  in  the 
marine  Prosobranchiates  (Nassa,  etc.)  where  it  has  been  fully 
studied  by  Bobretzky.  It  consists  of  a  mass  of  cells  on  each 
side  of  the  body,  close  to  the  base  of  the  foot,  and  not  far 
behind  the  velum.  This  mass  grows  very  large,  and  below  it 
may  be  seen  a  continuous  layer  of  epiblast.  The  cells  forming 
it  fuse  together,  their  nuclei  disappear,  and  numerous  vacuoles 
containing  concretions  arise  in  them.  At  a  later  stage  all  the 
vacuoles  unite  together  and  form  a  cavity  filled  with  a  brown 
granular  mass. 

The  provisional  internal  renal  organ  is  found  in  many  pulmo- 
nate  Gasteropods — Lymnaeus,  Planorbis,  etc.  It  consists  of  a 
paired  V-shaped  ciliated  tube  with  a  pedal  and  cephalic  limb. 
The  former  has  an  external  opening,  but  the  termination  of  the 
latter  is  still  in  doubt. 

It  consists,  according  to  Biitschli's  description  (No.  244),  in  the  fresh- 
water Pulmonata  (Lymnasus,  Planorbis)  of  a  round  sack,  close  to  the  head, 
opening  by  an  elongated  and  richly  ciliated  tube  in  the  neighbourhood  of 
the  eye.  From  the  sack  a  second  shorter  tube  passes  off  towards  the  foot, 
which  seems  however  to  end  blindly.  The  cells  lining  the  sack  contain 
concretions,  and  there  is  one  especially  large  cell  in  the  lumen  of  the  sack 
attached  on  the  side  turned  towards  the  eye.  It  coexists  in  Lymnasus  with 
provisional  renal  organs  of  the  type  of  those  in  marine  Prosobranchiata. 

A  somewhat  different  description  of  the  structure  and  development  of 
this  organ  in  Planorbis  has  recently  been  given  by  Rabl  (No.  268).  It 
consists  of  a  V-shaped  tube  on  each  side  with  both  extremities  opening  into 
the  body  cavity.  The  one  limb  is  directed  towards  the  velar  area,  the  other 
towards  the  foot.  It  is  developed  from  the  mesoblast  cells  of  the  anterior 
part  of  the  mesoblastic  band.  The  large  mesoblast  (p.  227)  of  each  side 
grows  into  two  processes,  the  two  limbs  of  the  future  organ.  A  lumen  in 
the  cell  is  continued  into  each  limb,  while  continuations  of  the  two  limbs  of 
the  V  are  formed  from  the  hollowing  out  of  the  central  parts  of  the  adjoining 
mesoblast  cells. 

In  Limax  embryos  Gegenbaur  found  a  pair  of  elongated 
provisional  branched  renal  sacks,  the  walls  of  which  contained 
concretions.  These  sacks  are  provided  with  anteriorly  directed 
ducts  opening  on  the  dorsal  side  of  the  mouth.  This  organ  is 


282  GENERATIVE  GLANDS. 

probably  of  the  same  nature  as  the  provisional  renal  organ  in 
other  Pulmonata. 

Permanent  renal  organ.  According  to  the  most  recent  ob- 
server (Rabl,  No.  268),  whose  statements  are  supported  by  the 
sections  figured,  the  permanent  renal  organ  in  Gasteropods  is 
developed  from  a  mass  of  mesoblast  cells  close  to  the  end  of  the 
intestine.  This  is  first  carried  somewhat  to  the  left  side,  and 
then  becomes  elongated  and  hollow,  and  attaches  itself  to  the 
epiblast  on  the  left  side  of  the  anus  (fig.  108,  r).  After  the 
formation  of  the  heart  the  inner  end  opens  into  the  pericardium 
and  becomes  ciliated,  the  median  part  becomes  glandular  and 
concrements  appear  in  its  lining  cells,  and  the  terminal  part 
forms  the  duct. 

Previous  observers  have  usually  derived  this  organ  from  the  epiblast; 
according  to  Rabl  this  is  owing  to  their  having  studied  too  late  a  stage  in  the 
development. 

In  Cephalopoda  the  excretory  sacks  or  organ  of  Bojanus  are 
apparently  differentiations  of  the  mesoblast1.  At  an  early  stage 
part  of  their  walls  envelops  the  branchial  veins.  From  this 
part  of  the  wall  the  true  glandular  section  of  the  organ  would 
seem  to  be  formed.  The  epithelium  forming  the  inner  wall  of 
each  sack  is  at  an  early  age  very  columnar. 

The  development  of  the  organ  of  Bojanus  in  Lamellibranchiata 
has  been  studied  by  Lankester.  He  finds  that  it  develops  as  a 
paired  invagination  of  the  epiblast  immediately  ventral  to  the 
anus. 

Generative  glands.  The  generative  glands  in  Mollusca 
would  appear  to  be  usually  developed  in  the  post-larval  period, 
but  our  knowledge  on  this  subject  is  extremely  scanty. 

In  Pteropods  Fol  believes  that  he  has  proved  that  the  hermaphrodite 
gland  originates  from  two  independent  formations,  one  (the  testicular) 
epiblastic  in  origin,  and  the  other  (the  ovarian)  hypoblastic. 

These  views  of  Fol  do  not  appear  to  me  nearly  sufficiently  substantiated 
to  be  at  present  accepted. 

The  generative  glands  in  Cephalopoda  appear  to  be  simple 
differentiations  of  the  mesoblast.  They  are  at  first  very  closely 

1  I  conclude  this  from  Bobretzky's  figures. 


MOLLUSCA.  283 


connected  with  the  aortic  heart  (fig.  127,  &/),  but  soon  become 
completely  separated  from  it. 

Alimentary  tract.  The  formation  of  the  archenteron,  and 
the  relation  of  its  opening  to  the  permanent  mouth  and  anus,  has 
already  been  described  and  needs  no  further  elucidation.  It  will 
be  convenient  to  treat  the  subject  of  this  section  under  three 
headings  for  each  group — viz.  (i)  the  mesenteron,  (2)  the  sto- 
modaeum,  and  (3)  the  proctodaeum. 

The  mesenteron.  In  the  Gasteropoda  and  Pteropoda  the 
mesenteron,  as  has  already  been  mentioned,  forms  a  simple  sack, 
which  may  however,  owing  to  the  presence  of  food-yolk,  be  at 
first  without  a  lumen.  Of  this  sack  an  anterior  portion  gives 
rise  to  the  stomach  and  liver,  and  a  posterior  to  the  intestine. 
This  latter  portion  is  the  first  to  be  distinctly  differentiated  as 
such,  and  forms  a  narrowish  tube  connecting  the  anterior  dila- 
tation with  the  anus.  In  the  meantime  the  cells  of  a  great 
part  of  the  anterior  portion  of  the  mesenteron  undergo  peculiar 
changes.  They  enlarge,  and  in  each  of  them  a  deposit  of  food 
material  appears,  which  is  often  at  any  rate  derived  from  the 
absorption  of  the  albumen  in  which  the  embryo  floats.  The  cells 
on  the  dorsal  side,  adjoining  the  cesophageal  invagination,  and 
the  whole  of  the  cells  on  the  ventral  side  do  not  however  undergo 
these  changes.  There  thus  arises  an  anterior  and  ventral  region 
adjoining  the  oesophagus,  which  becomes  completely  enclosed  by 
small  cells  and  forms  the  true  stomach.  The  part  behind  and 
dorsal  to  the  stomach  is  lined  by  the  large  nutritive  cells  and 
forms  the  liver.  It  opens  into  the  stomach  at  the  junction  of  the 
latter  with  the  intestine,  which  in  the  later  stages  becomes  bent 
somewhat  forwards  and  to  the  right.  Still  later  the  hepatic 
region  becomes  branched,  the  albuminous  contents  of  its  cells 
are  replaced  by  a  coloured  secretion,  and  it  becomes  bodily 
converted  into  the  liver.  The  stomach  is  usually  richly  ciliated. 

The  various  modifications  of  the  above  type  of  development  of  the 
alimentary  tract  are  to  be  regarded  as  due  to  the  disturbing  influence  of 
food-yolk.  Where  primitively  the  hypoblast  cells  are  very  bulky,  though 
invaginated  in  a  normal  way,  the  wall  of  the  hepatic  region  becomes 
immensely  swollen  with  food-yolk,  e.g.  Natica.  In  other  cases  amongst 
certain  Pteropods  (Fol,  No.  249)  where  the  hypoblast  is  still  more  bulky, 
part  of  the  archenteric  walls  becomes  converted  into  a  bilobed  sack  opening 


284  ALIMENTARY   TRACT. 

into  the  pyloric  region,  in  the  walls  of  which  a  large  deposit  of  food  material 
is  stored,  which  gradually  passes  into  the  remainder  of  the  alimentary  tract 
and  is  there  digested.  The  bilobed  nutritive  sack,  as  it  is  called  by  Fol,  is 
eventually  completely  absorbed,  though  the  liver  in  some,  if  not  all  cases, 
grows  out  as  a  fresh  sack  from  its  duct. 

The  formation  of  the  permanent  alimentary  tract,  when  the  hypoblast  is 
so  bulky  that  there  is  no  true  archenteric  cavity,  has  been  especially  investi- 
gated by  Bobretzky  (No.  242). 

In  the  case  of  a  species  of  Fusus  the  hypoblast,  when  enclosed  by  the 
epiblast,  is  composed  of  four  cells  only.  The  blastopore  remains  perma- 
nently open  at  the  oral  region,  and  around  it  the  oesophagus  grows  in  a 
wall-like  fashion.  The  protoplasmic  portions  of  the  four  hypoblast  cells  are 
turned  towards  the  cesophageal  opening,  and  from  them  are  budded  off 
small  cells  which  are  continuous  at  the  blastopore  with  the  epiblast  of  the 
oesophagus.  These  cells  give  rise  posteriorly  to  the  intestine  and  anteriorly 
to  the  sack,  which  becomes  the  stomach  and  liver.  This  sack  always 
remains  open  towards  the  four  primitive  yolk  cells.  The  cells  of  the 
posterior  part  of  it  become  larger  and  larger  and  form  the  hepatic  sack, 
which  fills  up  the  left  and  posterior  part  of  the  visceral  sack,  pushing  the 
yolk  cells  to  the  right.  The  cells  lining  the  hepatic  sack  become  pyramidal 
in  shape,  and  each  of  them  is  filled  with  a  peculiar  mass  of  albuminous 
material.  The  cells  adjoining  the  opening  of  the  oesophagus  remain  small, 
become  ciliated,  and  form  the  stomach.  They  are  not  sharply  separated  off 
from  the  cells  of  the  hepatic  sack.  The  yolk  cells  remain  distinct  on  the 
right  side  of  the  body  during  larval  life,  and  their  food  material  is  gradually 
absorbed  for  the  nutrition  of  the  embryo. 

A  modification  of  the  above  mode  of  development,  where  the  food 
material  is  still  more  bulky  and  the  blastopore  closed,  is  found  in  Nassa, 
and  has  already  been  described  (vide  p.  233). 

The  stomodceum.  The  stomodaeum  in  most  cases  is  formed 
as  a  simple  epiblastic  imagination  which  meets  and  opens  into 
the  mesenteron.  When  the  blastopore  remains  permanently 
open  at  the  oral  region  the  stomodaeum  is  formed  as  an  epiblastic 
wall  round  its  opening.  In  all  cases  the  stomodaeum  gives  rise 
to  the  mouth  and  oesophagus.  At  a  subsequent  period  there  are 
developed  in  the  oral  region  of  the  stomodaeum  the  radula  in  a 
special  ventral  pit,  and  the  salivary  glands — the  latter  as  simple 
outgrowths. 

The  oesophagus  is  usually  ciliated. 

The  proctod&um.  Except  where  the  blastopore  remains  as 
the  permanent  anus  (Paludina)  the  proctodseum  is  always  formed 
subsequently  to  the  mouth.  Its  formation  is  usually  preluded 
by  the  appearance  of  two  projecting  epiblast  cells,  but  it  is 


MOLLUSCA.  285 


always  developed  as  a  very  shallow  epiblastic  invagination,  which 
does  not  give  rise  to  any  part  of  the  true  intestine. 

In  the  Cephalopods  the  alimentary  tract  is  formed,  as  in 
other  cephalophorous  Mollusca,  of  three  sections,  (i)  A  stomo- 
daeum,  formed  by  an  epiblastic  invagination,  which  gives  rise  to 
the  mouth,  oesophagus  and  salivary  glands.  (2)  A  proctodseum, 
which  is  an  extremely  small  epiblastic  invagination.  (3)  A 
mesenteron,  lined  by  true  hypoblast,  which  forms  the  main 

cTts 

•mf 


brd 


c,h 


FIG.  126.    LONGITUDINAL  VERTICAL  SECTION  THROUGH  A  LOLIGO  OVUM  WHEN 

THE  MESENTERIC  CAVITY  IS  JUST  COMMENCING  TO  BE  FORMED.     (After  Bobretzky.) 

gls.  salivary  gland  ;  brd.  sheath  of  radula ;  oe.  oesophagus ;  ds.  yolk-sack ;  chs. 
shell-gland  ;  mt.  mantle ;  pdh.  mesenteron  ;  x.  epiblastic  thickening  between  the  folds 
of  the  funnel. 

section  of  the  alimentary  tract,  viz.  the  stomach,  intestine,  the 
liver,  and  ink  sack1. 

The  mesenteron.  The  mesenteron  is  first  visible  from  the 
surface  as  a  small  tubercle  on  the  posterior  side  of  the  mantle 
between  the  rudiments  of  the  two  gills  (fig.  1 1 1  B,  an).  Within 
this,  as  was  first  shewn  by  Lankester,  a  cavity  appears. 

This  cavity  is  as  in  Gasteropods  open  to  the  yolk-sack,  and 
only  separated  from  the  yolk  itself  by  the  yolk  membrane 
already  spoken  of.  It  is  at  first  lined  by  indifferent  cells  of  the 
lower  layer  of  the  blastoderm,  which  however  soon  become 
columnar  and  form  a  definite  hypoblastic  layer  (fig.  126,  pdk). 
Between  the  hypoblast  and  epiblast  there  is  a  very  well  marked 
layer  of  mesoblast.  As  the  mesenteric  cavity  extends,  its  walls 

1  The  following  description  applies  specially  to  Loligo. 


286 


ALIMENTARY  TRACT. 


meet  the  epiblast,  and  at  the  point  of  contact  of  the  two  layers 
the  epiblast  becomes  slightly  pitted  in.  At  this  point  the  anus 
is  formed  at  a  considerably  later  period  (fig.  127,  an}. 

On  the  ventral  side  of  the  primitive  mesenteron  an  outgrowth 
appears  very  early,  which  becomes  the  ink  sack  (fig.  127,  bi). 

The  mesenteric  cavity,  still  open  to  the  yolk,  gradually  ex- 
tends itself  in  a  dorsal  di- 
rection over  the  yolk-sack, 
but  remains  for  some  time 
completely  open  to  it  ven- 
trally,  and  only  separated 
from  the  actual  yolk  by 
theyolk  membrane.  There 
early  grow  out  from  the 
walls  of  the  mesenteron 
a  pair  of  hepatic  diver- 
ticula. 

As  the  mesenteric 
cavity  extends  it  dilates 
at  its  distal  extremity 
into  a  chamber  destined 
to  form  the  stomach  (fig. 
127,  mg).  At  about  this 
time  the  anus  becomes 
perforated.  Shortly  af- 
terwards the  mesenteron 
meets  and  opens  into 
the  oesophagus  at  the 
dorsal  extremity  of  the 
yolk  sack,  but  at  the  time 
when  this  takes  place  the 
hypoblast  has  extended 
round  the  entire  cavity, 
and  has  shut  it  off  from 


—tr 


ypd 


the  yolk.  The  yolk  mem- 
brane throughout  the 
whole  of  this  period  is 
quite  passive,  and  has  no 
share  in  forming  the  walls  of  the  alimentary  tract. 


FIG.  127.   LONGITUDINAL  SECTION  THROUGH 

AN  ADVANCED  EMBRYO  OF  LOLIGO.      (After   Bo- 

bretzky.) 

os.  mouth ;  gls.  salivary  gland  ;  brd.  sheath  of 
radula  ;  ao.  anterior  aorta;  ao1.  posterior  aorta; 
ra.  branch  of  posterior  aorta  to  shell  sack  ;  ma. 
branch  of  posterior  aorta  to  mantle ;  c.  aortic 
heart  ;  oe.  oesophagus  ;  mg.  stomach ;  an.  anus  ; 
bi.  ink  sack  ;  kd.  germinal  tissue  ;  eih.  shell  sack ; 
•vc.  vena  cava  ;  g.vs.  visceral  ganglion  ;  g-pd.  pedal 
ganglion  ;  ac.  auditory  sack  ;  tr.  funnel. 


MOLLUSCA.  287 


The  stomodaum.  The  stomodaeum  appears  as  an  epiblastic 
imagination  at  the  anterior  side  of  the  blastoderm,  before  any 
trace  of  the  mesenteron  is  present.  It  rapidly  grows  deeper, 
and,  shortly  after  the  mesenteric  cavity  becomes  formed,  an 
outgrowth  arises  from  its  wall  adjoining  the  yolk-sack,  which 
gives  rise  to  the  salivary  glands  (figs.  126  and  127,  gls).  Im- 
mediately behind  the  opening  of  the  salivary  glands  there 
appears  on  its  floor  a  swelling  which  becomes  the  odontophore, 
and  behind  this  a  pocket  of  the  stomodaeal  wall  forms  the 
sheath  of  the  radula  (figs.  126  and  127,  brd}.  Behind  this  again 
the  oesophagus  is  continued  dorsalwards  as  a  very  narrow  tube, 
which  eventually  opens  into  the  stomach  (fig.  127). 

The  terminal  portion  of  the  rudiment  of  the  salivary  gland 
divides  into  two  parts,  each  of  which  sends  out  numerous  diver- 
ticula  which  constitute  the  permanent  glands.  The  greater  part 
of  the  original  outgrowth  remains  as  the  unpaired  duct  of  the 
two  glands1. 

In  the  larva  observed  by  Grenacher  the  anterior  pair  of 
salivary  glands  originated  from  independent  lateral  outgrowths 
of  the  floor  of  the  mouth,  close  to  the  opening  of  the  posterior 
salivary  glands. 

The  yolk-sack  of  the  Cephalopoda.  The  yolk,  as  has  already  been  stated, 
becomes  at  an  early  period  completely  enclosed  in  a  membrane  formed  of 
flattened  cells,  which  constitutes  a  definite  yolk-sack.  It  is,  in  the  more 
typical  forms  of  Cephalopoda,  divided  into  an  external  and  an  internal 
section,  of  which  the  former  is  probably  a  special  differentiation  of  the 
median  part  of  the  foot  of  other  cephalophorous  Mollusca  (vide  p.  272).  At 
no  period  does  the  yolk-sack  communicate  with  the  alimentary  tract.  The 
two  sections  of  the  yolk-sack  are  at  first  not  separated  by  a  constriction.  In 
the  second  half  of  embryonic  life  the  condition  of  the  yolk-sack  undergoes 
considerable  changes.  The  internal  part  grows  greatly  in  size  at  the  expense 
of  the  external,  and  the  latter  diminishes  very  rapidly  and  becomes  con- 
stricted off  from  the  internal  part  of  the  sack,  with  which  it  remains  con- 
nected by  a  narrow  vitelline  duct. 

The  internal  yolk-sack  becomes  divided  into  three  sections  :  a  dilated 
section  in  the  head,  a  narrow  section  in  the  neck,  and  an  enormously 
developed  portion  in  the  mantle  region.  It  is  the  latter  part  which  mainly 
grows  at  the  expense  of  the  external  yolk-sack.  It  gives  off  at  its  dorsal 
end  two  lobes,  which  pass  round  and  embrace  the  lower  part  of  the  cesopha- 

1  In  Loligo  only  a  single  pair  of  salivary  glands  is  present. 


288  ALIMENTARY  CANAL. 

gus.    The  passage  of  the  yolk  from  the  external  to  the  internal  yolk-sack  is 
probably  largely  due  to  the  contractions  of  the  former. 

The  external  yolk-sack  is  not  vascular,  and  probably  the  absorption  of 
the  yolk  for  the  nutrition  of  the  embryo  can  only  take  place  in  the  internal 
yolk-sack.  The  most  remarkable  feature  of  the  Cephalopod  yolk-sack  is  the 
fact  that  it  lies  on  the  opposite  side  of  the  alimentary  tract  to  the  yolk  cells, 
which  form  a  rudimentary  yolk-sack  in  such  Gasteropoda  as  Nassa  and 
Fusus.  In  these  forms,  the  yolk-sack  is  at  first  dorsal,  but  subsequently  is 
carried  by  the  growth  of  the  liver  to  the  right  side.  In  Cephalopoda  on  the 
contrary,  the  yolk-sack  is  placed  on  the  ventral  side  of  the  body. 

What  is  known  of  the  development  of  the  alimentary  tract  in 
the  Polyplacophora  has  already  been  mentioned. 

In  the  Lamellibranchiata  (Lankester,  No.  239),  the  mesen- 
teron  early  grows  out  into  two  lateral  lobes  which  form  the  liver, 
while  the  part  between  them  forms  the  stomach. 

In  Pisidium  the  intestine  is  formed  from  the  original  pedicle 
of  invagination,  which  remains  permanently  attached  to  the 
epiblast.  The  stomodaeum  is  formed  by  the  usual  epiblastic 
invagination,  and  becomes  the  mouth  and  cesophagus.  The 
development  of  the  crystalline  rod  and  its  sack  do  not  appear  to 
be  known.  In  the  adult  the  sack  of  the  crystalline  rod  opens 
into  a  part  of  the  alimentary  tract  which  appears  to  belong  to 
the  mesenteron.  Were  however  the  development  to  shew  them 
to  be  really  derived  from  the  stomodaeum  they  might  be  inter- 
preted as  rudiments  of  the  organ  which  constitutes  the  odonto- 
phore  and  its  sack  in  cephalophorous  Mollusca — an  interpretation 
which  would  be  of  considerable  phylogenetic  interest. 

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(239)  E.  R.  Lankester.     "On  the  developmental  history  of  the  Mollusca." 
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(240)  H.  G.  Bronn  and  W.  Keferstein.    Die  Klassen  u.  Oninungen  d.  Thier- 
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(241)  J.  Alder  and  A.  Hancock.     "Devel.  of  Nudibr."     Ann.  and  Magaz. 
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(262)  E.  R.  Lankester.    "  On  the  development  of  the  Pond-Snail."    Quart.  J. 
of  Micr.  Scie.,  Vol.  xiv.     1874. 

(263)  E.  R.  Lankester.     "On  the  coincidence  of  the  blastopore  and  anus  in 
Paludina  vivipara."     Quart.  J.  of  Micr.  Scie.,  Vol.  xvi.  1876. 

(264)  F.  Leydig.     "  Ueber  Paludina  vivipara."     Zeitschr.  f.  w.  Zool.,  Vol.  n. 
1850. 

(265)  J.  Muller.     Ueber  Synapta  dig.  u.  iib.  d.  Erzeug.  v.  Schnecken  in  Holoth., 
1852. 

B.  II.  19 


290  BIBLIOGRAPHY. 


(266)  J.  Muller.     "Bemerk.  aus  d.  Entwickl.  der  Pteropoden."     Monatsber. 
Berl.  Akad.,  1857. 

(267)  C.  Rabl.     "  Die  Ontogenie  d.  Susswasser-Pulmonaten. "    Jenaische  Zeit- 
schrift,  Vol.  IX.     1875. 

(268)  C.   Rabl.     "  Ueb.  d.  Entwick.  d.  Tellerschnecke  (Planorbis)."    Morph. 
Jahrbuch,  Vol.  v.     1879. 

(269)  W.  Salensky.     "  Beitr.  zur  Entwickl.  d.  Prosobr."     Zeitschr.  f.  w.  Zool., 
Vol.  xxn.     1872. 

(270)  O.  Schmidt.     "Ueb.  Entwick.  von  Limax  agrestis."     Muller's  Archiv, 
1851. 

(271)  Max  S.  Schultze.    "Ueber  d.  Entwick.  des Tergipes lacinulatus."  Arch, 
f.  Naturg.,  Jahrg.  xv.     1849. 

(272)  E.  Selenka.     "  Entwick.  von  Tergipes  claviger."  Niederl.  Arch.f.  Zool., 
Vol.  I.     1871. 

(273)  E.  Selenka.     "Die  Anlage  d.  Keimbl.  bei  Purpura  lapillus."    NiederL 
Arch.f.  Zool.,  Vol.  I.     1872. 

(274)  C.  Semper.     "Entwickl.  der  Ampullaria  polita,  etc."    Natuurk.  Ver- 
handl.  Utrechts  Genootsch.,  1862. 

(275)  An.  Sleeker.     "  Furchung  u.  Keimblatterbildung  bei  Calyptrsea."    Mor- 
phol.  Jahrbuch,  Vol.  II.     1876. 

(276)  A.Stuart.     "  Ueb.  d.  Entwickl.  einiger  Opisthobr."   Zeitschr.  f.  10.  Zool. , 
Vol.  xv.     1865. 

(277)  N.  A.  Warneck.     "Ueber  d.  Bild.  u.  Entwick.  d.  Embryos  bei  Gaste- 
rop."    Bullet.  Soc.  natural,  de  Moscou,  T.  xxm.     1850. 

Cephalopoda. 

(278)  P.  J.   van  Beneden.     "  Recherches  sur  1'Embryogenie  des  Sepioles." 
Nouv.  Mini.  Acad.  Roy.  de  Bruxelles,  Vol.  xiv.     1841. 

(279)  N.  Bobretzky.    Observation  on  the  development  of  the  Cephalopoda 
(Russian).    Nachrichten  d.  kaiserlichen  Gesell.  d.  Freunde  der  Naturwiss.  Anthropolog. 
Ethnogr.  bei  d.  Universitdt  Moskau. 

(280)  H.  Grenacher.    "  Zur  Entwicklungsgeschichte  d.  Cephalopoden."    Zeit. 
/.  wiss.  Zool.,  Bd.  xxiv.     1874. 

(281)  A.  Kolliker.     Entwicklungsgeschichte  d.  Cephalopoden.     Zurich,  1844. 

(282)  E.   R.   Lank  ester.     "Observations  on  the  development  of  the  Cepha- 
lopoda."    Quart.  J.  of  Micr.  Science,  Vol.  xv.     1875. 

(283)  E.  Metschnikoff.     *•  Le  developpement  des  Sepioles."    Archiv  d.  Sc. 
phys,  et  nat.y  Vol.  XXX.     Genfcve,  1867. 


Polyplacophora. 

(284)  A.   Kowalevsky.      "Ueb.  d.  Entwick.  d.  Chitonen."     Zoologischer  An- 
zeiger,  No.  37.     1879. 

(285)  S.  L.  Loven.     "  Om  utvecklingcn  hos  slagtet  Chiton."    Stockholm  bfver- 
sigt,    xn.   1855.     [Vide  also  Ann.  and  Mag.  of  Nat.  Hist.,  Vol.  xvn.    1856,   and 
Archiv  f.  Naturgeschichte,  1856.] 


BIBLIOGRAPHY.  29 1 


Scaphopoda. 

H.  Lacaze-Duthiers.     "  Developpement  du  Dentale."    Ann.  d.  Sci. 
Nat.,  Series  iv.  Vol.  VH.     1857. 


L  amellibranchiata. 

(287)  M.  Braun.     "  Postembryonale   Entwicklung   d.    Siisswasser-Muscheln." 
Zoologischer  Garten. 

(288)  C.  G.  C  arus.    "  Neue  Untersuch.  lib.  d.  Entwickl.  unserer  Flussmuschel." 
Verh.  Leop.-Car.  Akad.,  Vol.  xvi.     1832. 

(289)  W.  Flemming.     "  Studien  in  d.  Entwicklungsgeschichte  der  Najaden." 
Sitz.  d.  k.  Akad.  Wiss.  Wien,  Vol.  LXXI.     1875. 

(290)  F.  Leydig.     "  Ueber  Cyclas  Cornea."     Muller's  Archiv,  1855. 

(291)  S.   L.   Loven.      "  Bidrag  til  Kanned.  om  Utweckl.  af  Moll.  Acephala 
Lamellibr."     Vetensk.  Akad.  Hand!.,  1848.     [Vide  also  Arch.  f.  Naturg.,  1849.] 

(292)  C.  Rabl.     «'  Ueber  d.  Entwicklungsgeschichte  d.  Malermuschel."    Jena- 
ische  Zeitschrift,  Vol.  x.     1876. 

(293)  W.  Salensky.    "  Bemerkungen  iiber  Haeckels Gastrsea-Theorie  (Ostrea)." 
Arch.  f.  Naturg.,  1874. 

(294)  O.   Schmidt.     "Ueb.   d.  Entwick.   von  Cyclas  calyculata."     Muller's 
Arch.,  1854. 

(295)  O.Schmidt.     "  Zur  Entwickl.  der  Najaden."    Wien,Sitzungsber.math.- 
nat.  CL,  Vol.  xix.     1856. 

(296)  P.  Stepanoff.     "Ueber  die  Geschlechtsorgane  u.  die  Entwicklung  von 
Cyclas."     Archiv  f.  Naturgeschichte,  1865. 

(297)  H.  Lacaze-Duthiers.     "Developpement  d.  branchies  d.  Mollusques 
Acephales."    An.  Sc.  Nat.,  Ser.  iv.  Vol.  v.     1856. 


I9—2 


CHAPTER   X. 
POLYZOA1. 

ENTOPROCTA. 

THE  development  of  the  larvae  of  Pedicellina  is  known  from 
the  researches  of  Hatschek  (No.  299)  far  more  completely  than 
that  of  Loxosoma,  though  it  does  not  apparently  differ  from  it 
except  in  certain  details.  In  both  the  known  Entoproctous 
genera  the  segmentation  is  regular  or  nearly  so,  though  Hatschek 
believes  that  he  has  detected  in  Pedicellina  a  slight  difference 
between  the  two  first  segmentation  spheres,  and  regards  them  as 
constituting  the  animal  and  vegetative  poles  of  the  embryo.  The 
segmentation  in  Pedicellina,  to  which  genus  alone  the  remainder 
of  the  description  applies,  results  in  the  formation  of  a  single- 
layered  blastosphere,  with  a  small  segmentation  cavity,  in  which 
the  animal  and  vegetative  poles  can  readily  be  distinguished 
owing  to  the  smaller  size  of  the  cells  at  the  animal  pole. 

The  hypoblast  cells  and  the  vegetative2  pole  become  invagi- 


1  The  classification  of  the  Polyzoa  adopted  in  this  chapter  is  shewn  in  the  sub- 
joined table : 

I.    Entoprocta. 
II.    Ectoprocta. 

fa.     Chilostomata. 

1.  GYMNOUEMATA  \b.    Ctenostomata. 

v.     Cyclostomata. 

2.  PHYLACTOL^EMATA. 

3.  PODOSTOMATA  (Rhabdopleuro). 

2  The  succeeding  statements  about   the  gastrula   are   derived   from    Hatschek. 
According  to  Salensky  a  segmentation  cavity  is  not  present,  and  the  hypoblast  would 
seem  to  be  formed  by  delamination  or  epibole.     Barrois  finds  a  gastrula  in  both 
Loxosoma  and  Pedicellina,  but  gives  no  details.     Uljanin  finds  a  segmentation  cavity 
in  Pedicellina,  and   Schmidt  would   appear  to  have   observed  a  gastrula  stage   in 
Loxosoma.     None  of  the  accounts  we  have  can  be  compared  in  fulness  of  detail  to 
that  of  Hatschek. 


.    POLYZOA.  293 


nated  in  the  normal  manner  (fig.  128  A),  the  blastopore  becomes 
narrowed  to  a  slit  with  an  antero-posterior  direction,  i.e.  parallel 
to  the  line  connecting  the  mouth  and  anus  in  the  adult.  At 
the  hinder  extremity  of  the  blastopore  there  are  present  two 
conspicuously  large  cells  (fig.  128  B,  me),  one  on  each  side  of 
the  middle  line.  These  cells  give  rise  to  the  fnesoblast.  On 
the  completion  of  the  invagination  the  mesoblasts  become 
covered  by  the  epiblast  (fig.  128  C,  me).  The  blastopore  then 
closes,  but  in  the  position  it  occupied  the  epiblast  becomes 
thickened  to  form  the  rudiment  of  the  vestibule,  which  at  this 
stage  constitutes  a  disc  marked  off  by  a  shallow  groove  from  the 
remainder  of  the  body. 


FIG.  128.  THREE  STAGES  IN  THE  DEVELOPMENT  OF  PEDICELLINA  ECHINATA. 
(After  Hatschek.) 

s.c.  segmentation  cavity ;  a.e.  archenteron ;  ep.  epiblast ;  me.  mesoblast ;  hy. 
hypoblast. 

A  is  the  commencing  gastrula  stage  from  the  side  in  optical  section. 

B  is  a  slightly  later  stage  from  above  in  optical  section.  It  shews  the  two  primi- 
tive mesoblast  cells. 

C  is  a  later  stage  after  the  closure  of  the  blastopore,  viewed  from  the  side  in 
optical  section. 

At  the  anterior  extremity  of  this  disc  an  invagination  arises 
to  form  the  oesophagus  (fig.  129  A,  oe)  ;  and  not  long  afterwards 
a  posterior  invagination  to  form  the  rectum  (fig.  129  B,  an.i). 
The  oral  disc  and  the  oesophagus  are  richly  ciliated.  The 
cesophagus  first,  and  afterwards  the  rectum  unite  with  the 
archenteron  (fig.  130),  the  walls  of  which  soon  become  differ- 
entiated into  a  stomach  and  intestine,  and  on  the  upper  wall 
of  the  former  the  hepatic  cells  become  especially  conspicuous 
(fig.  130). 


294  ENTOPROCTA. 


During  the  completion  of  the  alimentary  canal  a  number  of 
important  structures  is  formed.  The  disc  in  which  the  oral  and 
anal  apertures  are  situated  becomes  converted  into  a  true  vesti- 


FlG.     129.       TWO    STAGES    IN    THE    DEVELOPMENT    OF    PEDICELLINA.       (After 

Hatschek.) 

oe.  oesophagus ;  ae.   archenteron  ;  an.i.   anal  invagination  ;  f.   fold  of   epiblast ; 
f.g.  ciliated  disc ;  x.  problematical  body  derived  from  hypoblast  (probably  a  bud). 

bule.  On  its  floor,  between  the  mouth  and  the  anus,  there  arises 
a  marked  prominence  with  a  tuft  of  cilia  (fig.  130  B),  which 
persists  in  the  adult. 

This  prominence  is  perhaps  equivalent  to  the  epistome  of  the 
Phylactolsemata  and  the  disc-like  organ  of  Rhabdopleura,  which 
Lankester  has  compared  to  the  molluscan  foot1. 

Very  shortly  after  the  first  formation  of  the  vestibule  there 
appears  at  the  opposite  end  of  the  larva  a  thickening  of  the 
epiblast,  which  soon  becomes  invaginated,  and  forms  an  eversible 
pit  (fig.  129  A  and  B, /.£-.).  Round  its  mouth  there  is  formed  a 
ring  of  stiff  cilia  (fig.  130,  f.g.).  This  organ  is  very  possibly 
equivalent  to  the  cement  gland  described  by  Kowalevsky  in  the 
adult  Loxosoma.  I  shall  speak  of  it  as  the  ciliated  disc. 

The  epiblast  cells  early  secrete  a  cuticle. 

The  two  mesoblast  cells  soon  increase  by  division,  and 
occupy  the  space  between  the  alimentary  canal  and  the  body 
wall.  They  do  not  become  divided  into  a  splanchnic  and 
somatic  layer ;  but  give  rise  to  the  interstitial  connective  tissue 

1  Lankester.  "Remarks  on  the  Affinities  of  Rhabdopleura."  Quart.  J.  of 
Micro.  Science,  Vol.  XIV.  1874. 


POLYZOA.  295 


and  muscles.  From  the  mesoblast  there  is  also  formed,  ac- 
cording to  Hatschek,  a  pair  of  ciliated  excretory  canals,  in  the 
space  between  the  mouth  and  anus  (fig.  130  B,  npk.).  The 
development  of  the  nervous  system  has  not  been  observed. 

At  a  comparatively  late  stage  in  the  development  there  is 
formed  round  the  edge  of  the  vestibule  a  ring  of  long  cilia  (fig. 
1306,  m.). 

A  remarkable  organ  makes  its  appearance  on  the  dorsal  side 
of  the  oesophagus  (the  side  opposite  the  adult  ganglion)  formed 
of  an  oval  mass  of  cells  attached  to  the  epiblast  at  the  apex  of  a 
small  ciliated  papilla  (fig.  130  A  and  B,  *.).  This  organ  will  be 


FlG.     130.       TWO     STAGES     IN    THE    DEVELOPMENT    OF    PEDICELLINA.      (After 

Hatschek.) 

v.  vestibule ;  m.  mouth  ;  /.  liver  ;  kg.  hind-gut ;  a.  anus  ;  and.  anal  invagination ; 
nph.  duct  of  kidney  \  fg.  ciliated  disc  ;  x.  dorsal  organ  (probably  bud). 

spoken  of  as  the  dorsal  organ.  According  to  Hatschek  it 
develops  as  a  solid  outgrowth  of  the  hypoblastic  walls  of  the 
mesenteron  shortly  before  the  mesenteron  joins  the  oesophagus 
(fig.  129  B,  jr.).  The  cells  composing  it  arrange  themselves  as  a 
sack,  which  acquires  an  external  opening  on  the  dorsal  surface 
(fig.  130  A,  jr.).  In  a  later  stage  the  lumen  of  the  sack  dis- 
appears, but  at  the  junction  of  the  organ  with  the  epiblast  a  pit 


296  ENTOPROCTA. 


is  formed,  lined  with  ciliated  cells,  which  is  capable  of  being 
protruded  as  a  papilla.  The  organ  itself  becomes  invested  by  a 
lining  of  cells,  which  Hatschek  regards  as  mesoblastic.  A  nearly 
similar  organ  to  this  is  found  in  the  embryo  of  Loxosoma  [Vogt 
(No.  302)  and  Barrois  (No.  298)].  Here  however  it  is  double, 
and  forms  a  kind  of  disc  connected  with  two  eye  spots. 

Hatschek  has  made  with  reference  to  the  dorsal  organ  the 
extremely  plausible  suggestion  that  it  is  a  rudimentary  bud,  and 
that  the  hypoblastic  sack  it  contains  gives  rise  to  the  hypoblast 
of  the  young  polype  developed  from  the  bud.  Although,  owing 
to  the  deficiency  of  our  observations  on  the  attachment  of  the 
larva,  this  suggestion  has  not  received  direct  confirmation,  yet 
the  relations  of  dorsal  organs  in  Pedicellina  and  Loxosoma 
respectively  strongly  confirm  Hatschek's  view  of  their  nature. 
Both  of  these  forms  increase  in  the  adult  state  by  budding :  in 
Pedicellina  there  is  a  single  row  of  buds  formed  successively 
on  the  dorsal  side  of  the  stem,  corresponding  with  the  single 
dorsal  organ  of  the  embryo ;  while  in  Loxosoma  a  double  row 
of  buds,  right  and  left,  is  formed,  in  correspondence  with  the 
double  nature  of  the  dorsal  organ. 

As  to  the  mode  of  attachment  of  the  embryo  next  to  nothing  is  known, 
the  few  observations  we  have  being  due  to  Barrois.  From  these  observa- 
tions it  would  appear  probable  that  the  larva,  as  is  usual  amongst  Polyzoa, 
does  not  become  directly  converted  into  the  permanent  form,  but  that, 
on  becoming  fixed,  it  undergoes  a  metamorphosis  in  the  course  of  which  its 
organs  atrophy.  I  would  venture  to  suggest  that  the  whole  free-swimming 
larva  atrophies,  while  the  dorsal  organ  alone  develops  into  the  fixed  form 1. 

Although  the  changes  which  take  place  during  budding  do  not  fall  within 
the  province  of  this  work,  it  may  be  well  to  state  that  Hatschek  has 
observed  during  this  process  the  development  of  the  nervous  system  and 
the  generative  organs.  The  nervous  system  arises  as  an  unpaired  thickening 
of  the  epiblastic  floor  of  the  vestibule,  between  the  mouth  and  the  anus. 
On  becoming  constricted  off  from  the  epiblast  the  nerve  ganglion  contains  a 
central  cavity  which  afterwards  vanishes. 

The  generative  organs  originate  as  a  pair  of  specially  large  mesoblast 
cells  in  the  space  between  the  stomach  and  the  floor  of  the  vestibule.  These 
two  cells,  surrounded  by  an  investment  of  flattened  mesoblast  cells,  sub- 

1  My  view  of  the  metamorphosis  which  takes  place  during  the  fixation  of  the 
larva  involves  the  supposition  that  in  Loxosoma,  about  the  attachment  of  which  we 
know  absolutely  nothing,  two  buds  are  directly  formed  in  accordance  with  the  double 
nature  of  the  dorsal  organ. 


POLYZOA.  297 


sequently  divide  and  form  two  masses.  At  a  still  later  period  each  mass 
divides  into  an  anterior  and  a  posterior  part ;  the  former  giving  rise  to  the 
ovary,  the  latter  to  the  testis.  The  similarity  of  this  mode  of  development 
of  the  generative  organs  to  that  observed  by  Butschli  in  Sagitta,  which 
is  described  in  the  sequel,  is  very  striking. 

ECTOPROCTA. 

Although  the  embryology  of  the  Ectoprocta  has  been  in- 
vestigated by  a  very  considerable  number  of  the  distinguished 
naturalists  of  the  century,  many  points  connected  with  it  still 
stand  in  great  need  of  further  elucidation.  The  original  nature 
of  the  embryo  was  rightly  interpreted  by  Grant,  Dalyell  and 
other  naturalists,  but  it  was  not  till  Huxley  demonstrated  the 
presence  of  both  the  ovary  and  testis  that  the  true  sexual  origin 
of  the  embryo  in  the  ovicells  became  an  established  fact  in 
science.  The  recent  memoir  of  Barrois  (No.  298),  though  it 
contains  the  record  of  a  vast  amount  of  research,  and  marks  a 
great  advance  in  our  knowledge,  still  leaves  a  great  number  of 
points,  both  with  reference  to  the  early  development  and  to  the 
larval  metamorphosis  in  a  very  unsatisfactory  condition. 

Four  larval  forms  can  be  distinguished,  viz. 

(1)  A  larval  form  which  with  slight  modifications  is  common 
to  all  the  genera  of  the  Chilostomata  (except  Membranipora  and 
Flustrella)  and  of  the  Ctenostomata. 

(2)  A  bivalved  larva  of  Membranipora  known  as  Cyphon- 
autesy  the  true  nature  of  which  was  first  recognized  by  Schneider 
(No.  322),  and  the  closely  allied  larva  of  Flustrella. 

(3)  The   typical    Cyclostomatous   larva,   for   the   first   full 
description  of  which  we  are  indebted  to  Barrois  (No.  298). 

(4)  The  larva  of  the  Phylactolaemata. 

Chilostomata  and  Ctenostomata.  As  an  example  of  the 
first  type  of  larvae,  Alcyonidium  my  till,  one  of  the  Ctenostomata, 
may  be  conveniently  selected  for  description,  as  having  been 
more  completely  worked  out  by  Barrois  than  perhaps  any  other 
form.  The  segmentation  commences  in  the  normal  manner  by 
the  appearance  of  two  vertical  furrows  followed  by  an  equatorial 
furrow,  which  divide  the  ovum  into  eight  equal  spheres.  The 
stage  with  eight  spheres  is  followed,  according  to  Barrois,  by  one 
with  sixteen,  formed  in  a  remarkable  manner  by  the  simultaneous 


298  ECTOPROCTA. 

appearance  of  two  vertical  furrows,  both  parallel  to  one  of  the 
original  vertical  furrows,  so  that  the  segmentation  spheres  at  this 
stage  are  arranged  in  two  layers  of  eight  each.  In  the  next 
stage  segmentation  takes  place  along  two  fresh  vertical  planes, 
similar  to  those  of  the  last  stage,  but  at  right  angles  to  them,  and 
therefore  parallel  to  the  second  of  the  two  primitive  vertical 
furrows.  At  the  close  of  this  stage  there  are  thirty-two  cells 
arranged  in  two  layers  of  sixteen  eachj  and  when  viewed  from 
the  surface  each  of  these  layers  presents  a  regularly  symmetrical 
pattern.  Up  to  the  stage  with  sixteen  cells  the  two  poles  of  the 
egg,  separated  by  the  primitive  equatorial  plane  of  segmentation, 
remain  equal,  but  during  the  stage  with  thirty-two  cells  a 
peculiar  change  takes  place  in  the  character  of  the  cells  at  the 
two  poles.  At  the  one  pole,  which  will  be  spoken  of  as  the  oral 
pole,  the  four  central  cells  become  much  larger  than  the  twelve 
peripheral  cells. 

The  stages  immediately  following  are  still  involved  in  much 
obscurity,  and  have  been  described  very  differently  by  Barrois  in 
his  original  memoir  (No.  298),  and  in  a  subsequent  note  (No. 
307)1.  In  the  latter  he  states  that  the  four  large  cells  of  the 
oral  face  become  enclosed  by  the  division  and  growth  of  the 
twelve  peripheral  cells.  They  are  thus  carried  into  the  interior 
of  the  ovum  ;  and  there  divide  into  a  central  vitelline  mass — the 
hypoblast — and  a  peripheral  mesoblastic  layer. 

The  eight  peripheral  cells  of  the  aboral  pole  divide  vertically, 
and,  owing  to  the  eight  central  cells  at  the  aboral  pole  dividing 
transversely  so  as  to  form  a  protuberance  on  the  aboral  surface, 
they  constitute  a  transverse  ring  of  large  cells  round  the  ovum, 
which  become  ciliated  and  constitute  the  main  ciliated  band  of 
the  embryo,  corresponding  to  the  ciliated  band  at  the  edge  of 
the  vestibule  of  the  entoproctous  larvae.  They  divide  the  embryo 
into  an  aboral  and  an  oral  region.  The  central  part  of  the 
aboral  projection  forms  a  structure  which  I  shall  speak  of  as  the 
ciliated  disc.  It  probably  corresponds  with  the  ciliated  disc  in 
the  Entoprocta.  An  invagination  is  next  formed  on  the  oral 

1  The  note  (No.  307)  refers  in  the  first  instance  to  the  changes  in  the  larvae  of  the 
Chilostomata,  but  the  similarity  of  the  larvoe  of  the  Ctenostomata  to  those  of  the 
Chilostomata  renders  it  practically  certain  that  the  corrections,  in  so  far  as  they  apply 
to  the  one  group,  apply  also  to  the  other. 


POLYZOA.  299 


surface,  which  gives  rise  to  a  sack  opening  to  the  exterior 
(fig.  131,  st.).  This  was  originally  held  by  Barrois  to  be  the 
stomach  ;  but  Barrois  now  prefers  to  call  it  '  the  internal  sack.' 
To  my  mind  it  is  probably  the  stomodaeum.  The  embryo  has 
become  in  the  meantime  laterally  compressed,  and,  at  what 
I  shall  call  the  anterior  end  of  the  oral  disc,  a  structure  makes 
its  appearance  (fig.  131,  m),  which  is  probably  homologous  with 
the  dorsal  organ  of  the  larva  of  Pedicellina  and  may  go  by  the 
same  name.  It  was  originally  interpreted  by  Barrois  as  the 
pharynx1. 

The  larva,  having  now  acquired  all  the  important  structures 
it  is  destined  to  possess,  becomes  free.  It  is  shewn  in  fig.  131  ; 
the  oral  face  being  turned  upwards.  There  are  two  rings  of 


FIG.  131.    FREE-SWIMMING  LARVA  OF  ALCYONIDIUM  MYTILI.    (After  Barrois.) 
m  (?)  dorsal  organ  ;  st.  stomodseum  (?) ;  s.  ciliated  disc. 

cilia,  one  round  the  edge  of  the  ciliated  disc,  and  a  second  with 
larger  cilia  on  the  ring  of  large  cells  described  above.  This 
ring  projects  somewhat ;  its  projecting  edge  being  directed 
towards  the  ciliated  disc.  The  dorsal  organ  (m?)  is  placed  on 
the  oral  face  at  the  bottom  of  an  elongated  groove,  in  front 
of  which  is  a  bunch  of  long  cilia  or  flagella.  Two  long  flagella 
are  also  developed  at  the  posterior  extremity  of  the  oral  face, 
and  two  pairs  (an  anterior  and  a  posterior)  of  eye-spots  also 
appear.  Towards  the  posterior  extremity  of  the  oral  face  is 
seen  a  body  marked  st,  which  forms  the  internal  sack.  If  I  am 

1  The  interpretation  of  the  larvae  given  in  the  text  must  be  regarded  as  somewhat 
tentative.  The  opacity  of  the  free  larvae  is  very  great,  and  almost  every  one  of  the 
numerous  authors  who  have  worked  on  these  larvae  have  arrived  at  different  conclu- 
sions, as  to  the  physiological  significance  of  the  various  parts. 


300  ECTOPROCTA. 


right  in  regarding  this  as  the  stomodaeum,  it  is  probable  that  it 
never  unites  with  the  invaginated  hypoblast,  and  that  the 
alimentary  tract  of  the  larva  remains  therefore  permanently  in 
an  imperfect  condition. 

Careful  observations  have  been  made  by  Repiachoff  (No.  318)  on  the 
early  development  of  Tendra,  which  accord  in  some  respects  with  the 
results  arrived  at  by  Barrois  in  his  second  memoir.  The  observations  are 
not,  unfortunately,  carried  down  to  the  complete  development  of  the  larva. 

The  ovum  divides  in  the  normal  way  into  two  and  then  four  uniform 
segments.  These  four  next  become  divided  by  an  equatorial  furrow  into 
four  dorsal  and  four  ventral  segments,  the  former  constituting  the  aboral 
pole  and  forming  the  epiblast,  and  the  latter  the  oral  pole.  The  stages  with 
sixteen  and  thirty-two  cells  appear  to  be  formed  in  the  same  manner  as  in 
Alcyonidium — but  between  the  two  layers  of  cells  forming  the  oral  and 
aboral  poles  a  well-marked  segmentation  cavity  arises  at  the  stage  with 
sixteen  segments.  At  the  stage  with  thirty-two  cells  the  four  middle  cells  of 
the  oral  side,  which  are  larger  than  the  others,  become  divided  into  two 
tiers,  in  such  a  manner  as  to  form  a  prominence  projecting  into  the 
segmentation  cavity.  By  the  appearance  of  a  lumen  in  this  prominence 
it  becomes  converted  into  an  archenteron,  which  communicates  with  the 
exterior  by  a  blastopore  in  the  middle  of  the  oral  surface.  The  blastopore 
becomes  eventually  closed. 

The  archenteric  sack  of  Repiachoff  is  clearly  the  same  structure  as 
Barrois'  four  invaginated  cells  of  the  oral  face,  their  further  history  has 
unfortunately  not  been  followed  out  by  Repiachoff. 

The  free  larva  swims  about  for  some  time,  and  then  fixes 
itself  and  undergoes  a  metamorphosis;  but  the  exact  course  of 
this  metamorphosis  is  still  very  imperfectly  known. 

According  to  the  latest  statements  of  Barrois  the  attachment 
takes  place  by  the  oral  face1.  The  ciliated  disc,  which  in  the 
free  larva  forms  a  kind  of  cup  directed  towards  the  aboral  end, 
turns  in  upon  itself  towards  the  oral  face.  It  subsequently 
undergoes  degeneration  and  forms  a  nutritive  or  yolk-mass. 
The  skin  of  the  larva  after  these  changes  gives  rise  to  the 
ectocyst  or  cell  of  the  future  polype.  The  future  polype  itself 
appears  to  originate,  in  part  at  any  rate,  from  the  so-called 
dorsal  organ*. 

1  Barrois  himself  held  the  opposite  view  in  his  earlier  memoir,  and  other  observers 
have  done  the  same. 

a  The  statements  on  this  head  are  so  unsatisfactory  and  contradictory  that  it  does 
not  appear  to  me  worth  while  quoting  them  here ;  even  the  latest  accounts  of  Barrois, 
which  entirely  contradict  his  early  statements,  can  hardly  be  regarded  as  satisfactory. 


POLYZOA.  301 


The  first  distinct  rudiment  of  the  polype  appears  as  a  white 
body,  which  gradually  develops  into  the  alimentary  canal  and 
lophophore.  While  this  is  developing  the  ectocyst  grows  rapidly 
larger,  and  the  yolk  in  its  interior  separates  from  the  walls  and 
occupies  a  position  in  the  body  cavity  of  the  future  polype, 
usually  behind  the  developing  alimentary  canal.  According 
to  Nitsche  (No.  316)  it  is  attached  to  a  protoplasmic  cord 
(funiculus)  which  connects  the  fundus  of  the  stomach  with  the 
wall  of  the  cell.  It  is  probably  (Nitsche,  etc.)  simply  employed 
as  nutritive  material,  but,  according  to  Barrois,  becomes  con- 
verted into  the  muscles,  especially  the  retractor  muscles. 

Adopting  the  hypothesis  already  suggested  in  the  case  of  the 
Entoprocta  the  metamorphosis  just  described  would  seem  to  be 
a  case  of  budding  accompanied  by  the  destruction  of  the  original 
larva. 

This  view  of  the  nature  of  the  post-embryonic  metamorphosis  is  appa- 
rently that  of  Claparede  and  Salensky,  and  is  supported  by  Claparede's 
statement  that  the  formation  of  the  first  polype  '  resembles  to  a  hair '  that  of 
the  subsequent  buds.  The  mode  of  budding  would,  however,  appear  to 
present  certain  peculiarities,  in  that  the  whole  larval  skin  passes  directly  into 
the  bud,  while  from  the  rudimentary  bud  of  the  larva  the  lophophore  and 
alimentary  tract  only  of  the  fixed  polype  are  formed. 

Flustrella  and  Cyphonautes.  The  next  group  of  larval 
forms  is  that  of  which  Cyphonautes  is  the  best  known  type. 
The  larvae  composing  it  at  first  sight  appear  to  have  but  little  in 
common  with  the  larvae  hitherto  described.  The  researches 
of  Barrois  (No.  298)  and  Metschnikoff  (No.  314),  (but  especially 
those  of  the  former  on  the  early  stages  of  Flustrella  hispida,  the 
larva  of  which  is  very  similar  in  form  to  Cyphonautes,  though 
without  so  great  a  complexity  of  organisation),  have  given  a 
satisfactory  basis  for  a  general  comparison  of  Cyphonautes  with 
other  ectoproctous  larvae. 

The  segmentation  and  early  stages  of  the  embryo  of  Flus- 
trella resemble  closely  those  of  Alcyonidium.  A  projecting  ring 
of  large  cells  is  formed,  dividing  the  larva  into  oral  and  aboral 
parts.  The  oral  part  soon  however  becomes  very  small  as  com- 
pared with  the  aboral,  and  becomes  vertically  flattened  so  as  to  be 
nearly  on  a  level  with  the  ring  of  large  cells.  In  the  next  stage 
the  flattening  becomes  completed  ;  and  the  ring  of  large  cells 


302  ECTOPROCTA. 


surrounds,  like  the  vestibule  of  the  Entoprocta,  a  flat  oral  disc. 
The  aboral  side  is  dome-shaped,  and  forms  the  greater  part 
of  the  embryo. 


FIG.  132.     ADVANCED  LARVA  OF  FLUSTRELLA  HISPIDA.    (After  Barrois.) 
m  (?)  groove  above  dorsal  organ  ;  Ph.  dorsal  organ ;  st.  stomodoeum  (?) ;  s.  ciliated 
disc  at  aboral  end  of  body. 

In  the  next  stage  a  small  disc — the  ciliated  disc — is  formed 
in  the  middle  of  the  aboral  dome.  The  larva  becomes  laterally 
compressed.  The  ring  of  large  cells  which  now  constitute  the 
edge  of  the  vestibule  is  covered,  as  in  the  larva  of  Pedicellina,  by 
cilia,  which  are  specially  long  in  front  of  the  dorsal  organ. 

In  the  next  stage  the  ciliated  disc  (fig.  132,  s.)  becomes 
reduced  in  size,  but  surmounted  by  a  ring  of  cilia  round  the 
edge,  and  a  tuft  of  cilia  in  the  centre.  The  chief  difference 
between  this  larva  and  that  of  Alcyonidium  depends  on  the 
small  size  of  the  ciliated  disc,  and  the  oral  position  of  the  ciliated 
ring  in  the  former.  There  are  intermediate  types  between  these 
forms  of  larvae. 

This  stage  immediately  precedes  the  liberation  of  the  larva. 
The  free  larva  differs  from  that  in  the  ovicell  mainly  in  the 
possession  of  a  shell  formed  as  a  cuticular  structure,  composed 
of  two  valves  placed  on  the  two  sides  of  the  embryo.  The 
aboral  ciliated  disc,  still  more  reduced  in  size,  loses  its  cilia,  and 
becomes  enclosed  between  the  two  valves  of  the  shell. 

The  post-embryonic  metamorphosis  follows,  so  far  as  is  known, 
the  course  already  described  for  the  larva  of  Alcyonidium. 


POLYZOA. 


303 


Cyphonautes  (fig.  133)  forms  at  certain  seasons  of  the  year 
one  of  the  commonest  captures  in  the  surface  net.  It  was  origi- 
nally described  by  Ehrenberg,  but  the  important  discovery  of  its 
true  nature  as  the  larva  of  Membranipora  (the  common  species  C. 
compressus  is  the  larva  of  Mem.  pilosa),  a  genus  of  the  chilosto- 
matous  Polyzoa,  was  made  by  Schneider  (No.  322).  The  younger 
stages  of  the  larva  have  not  been  worked  out,  but  from  a 
comparison  with  the  last  described  larva  it  is  easy  to  make  out 
the  general  relationship  of  the  parts.  The  larva  has  a  triangular 
form  with  an  aboral  apex,  corresponding  with  the  summit  of  the 
dome  of  the  Flustrella  larva,  and  an  oral  base.  It  is  enclosed  in 
a  bivalve  shell,  the  two  valves  of  which  meet  along  the  two  sides, 
but  are  separate  along  the  base.  At  the  apex  an  opening  is  left 
between  the  two  valves,  through  which  a  ciliated  disc  (/.  g)  of 
the  same  character  and  nature  as  that  of  previous  larvae  can  be 
protruded. 

The  oral  side  or  base  is  girthed  by  a  somewhat  sinuous 
ciliated  edge,  which  is  continued  round  the  anterior  and  posterior 
extremities  of  the  oral  disc.  It  is  no  doubt  equivalent  to  the 
ciliated  ring  of  other  larvae.  Two  openings  are  present  on  the 
oral  face,  both  enclosed  in  a  special  lobe  of  the  ciliated  ring. 
The  larger  of  these  leads 
into  a  depression,  which 
may  be  called  the  ves- 
tibule; and  is  situated 
on  the  posterior  side  of 
the  oral  surface.  The 
smaller  of  the  two,  on 
the  anterior  side,  leads 
into  a  cavity  which  is 
apparently  (Hatschek) 
equivalent  to  the  rudi- 
mentary bud  or  dorsal 
organ  of  other  larvae. 
The  deeper  part  of  the 
vestibule  leads  into  the 
mouth  (m)  and  oesopha- 
gus ;  the  latter  is  con- 
tinued till  close  to  the 


CYPHONAUTES  (LARVA  OF  MEM- 
( After  Hatschek.) 


FIG.   133 
BRANIPORA). 

m.    mouth;    a.   anus;    f.g.   ciliated   disc;    x. 
problematical  body  (probably  a  bud). 

apex   of  the   larva,   there   bends  upon 


304  ECTOPROCTA. 


itself,  dilates  into  a  stomach,  and  is  continued  parallel  to  the 
oesophagus  as  the  rectum  which  opens  by  an  anus  (a1)  at  the 
posterior  end  of  the  vestibule.  A  peculiar  paired  organ  is 
situated  on  each  side  nearly  above  the  stomach.  Its  nature 
is  somewhat  doubtful.  It  was  regarded  as  muscular  by  Clapa- 
rede  (No.  309),  though  this,  as  shewn  by  Schneider,  is  no  doubt 
a  mistake.  Allman  (No.  305)  regards  it  as  hepatic,  and  Hat- 
schek  as  a  thickening  of  the  epidermis.  Close  to  each  of  these 
organs  is  a  small  body  regarded  by  Claparede  as  an  accessory 
muscle.  It  is  placed  in  the  normal  position  for  a  Polyzoon 
ganglion,  and  may  perhaps  be  therefore  regarded  as  nervous  in 
nature.  Allman  points  out  its  similarity  to  a  bilobed  ganglion, 
but  is  not  inclined  to  take  this  view  of  it.  The  constitution 
of  the  parts  contained  in  the  anterior  cavity  (x)  is  somewhat 
obscure.  The  most  elaborate  descriptions  of  them  are  given  by 
Schneider  and  Allman.  Lining  the  cavity  is  apparently  a  mass 
of  spherical  bodies,  connected  with  which  is  a  tongue-like  process 
provided  with  long  cilia,  which  can  be  protruded  from  the  orifice. 
Internal  to  this  is  a  striated  body.  A  good  figure  of  the  whole 
structure  is  given  by  Schneider. 

The  general  similarity  of  Cyphonautes  to  the  other  larvae  is 
quite  obvious  from  the  above  description  and  figure.  In  the 
presence  of  an  anus,  a  vestibule,  and  possibly  a  nervous 
system,  it  clearly  exhibits  a  far  more  complicated  organisa- 
tion than  any  other  Polyzoon  larvae  except  those  of  the 
Entoprocta. 

The  post-embryonic  metamorphosis  of  Cyphonautes,  ad- 
mirably investigated  by  Schneider,  takes  place  in  the  same 
manner  as  that  of  other  larvae,  and  is  accompanied  by  the  de- 
generation of  the  larval  organs,  and  the  formation  of  a  clear 
body,  which  gives  rise  to  the  alimentary  cavity  and  lophophore 
of  the  fixed  polype.  The  larval  shell  takes  part  in  the  formation 
of  the  ectocyst  of  the  polype. 

Cyclostomata.  We  owe  to  Barrois  by  far  the  fullest  account  of  the 
development  of  the  Cyclostomata,  but  how  far  his  interpretations  are  to  be 
trusted  is  very  doubtful.  The  larvae  differ  very  considerably  from  the 
normal  larvae  of  the  Chilostomata  and  Ctenostomata ;  the  difference  being 
mainly  due  to  the  enormous  development  of  the  ciliated  disc.  Barrois  has 
investigated  the  larvae  of  three  genera,  Phalangella,  Crisia,  and  Diastopora, 


POLYZOA.  305 


and  states  that  they  very  closely  resemble  each  other.     The  ovum  is  ex- 
tremely minute. 

The  segmentation,  so  far  as  it  has  been  made  out,  is  regular.  During 
the  segmentation  growth  is  very  rapid,  and  eventually  there  is  formed  a 
blastosphere  many  times  larger  than  the  original  ovum.  The  blastosphere 
becomes  flattened,  and  is  converted  into  a  gastrula  by  bending  up  into 
a  cup-like  form.  The  gastrula  opening  is  stated  to  remain  as  the  permanent 
mouth,  which  has  a  terminal  and  central  position.  A  transverse  ring-like 
thickening  is  formed  round  the  larva,  which  probably  corresponds  with  the 
ciliated  ring  of  previous  larvae  ;  and  the  body  of  the  larva  in  front  of  this 
ring  becomes  ciliated.  The  aboral  end  of  the  larva  becomes  thickened,  and 
grows  out  into  an  elongated  prominence,  which  probably  corresponds  to  the 
ciliated  disc.  The  ring  before  mentioned  becomes  at  the  same  time  more 
prominent,  and  forms  a  cylindrical  sheath  for  the  ciliated  disc.  At  the 
time  when  the  larva  becomes  liberated  from  the  maternal  cell  it  has  the 
form  of  a  barrel  with  a  slight  constriction  in  the  middle  separating  the  oral 
from  the  aboral  end.  At  the  centre  of  the  oral  face  is  situated  the  mouth, 
leading  into  a  wide  stomach,  while  the  aboral  end  is  formed  of  the  ciliated 
disc  enclosed  in  its  sheath.  The  whole  surface  is  now  ciliated.  No 
structure  equivalent  to  the  dorsal  organ  or  bud  is  described  by  Barrois,  but 
in  other  respects,  if  the  ciliated  disc  is  really  equivalent  in  the  two  forms,  a 
general  comparison  on  the  line  indicated  above  between  this  larva  and  the 
normal  larvae  of  the  Ctenostomata  and  Chilostomata  seems  quite  possible. 
The  fixation  and  subsequent  development  of  the  larva  take  place  in  the 
normal  manner. 

Phylactolaemata.  The  development  of  the  phylactolaema- 
tous  Polyzoa  has  been  studied  by  Metschnikoff  (No.  315),  who 
describes  the  eggs  as  undergoing  a  complete  segmentation  within 
a  peculiar  brood-pouch  developed  from  the  walls  of  the  body  of 
the  parent.  After  segmentation  the  cells  of  the  embryo  arrange 
themselves  in  two  layers  round  a  central  cavity.  The  embryo 
then  forms  the  well-known  cyst,  from  which  a  colony  is  formed 
by  a  process  of  budding. 

General  considerations  on  the  Larva  of  the  Polyzoa. 

The  different  forms  of  embryo  amongst  the  Polyzoa  are 
represented  in  figs.  130  B,  131,  132,  and  133  in  what  I  regard  as 
identical  positions,  and  fig.  133  A  is  a  figure  of  what  may  be 
regarded  as  an  idealized  larval  Polyzoon.  In  all  the  larvae  there 
is  present  a  ciliated  ring,  which  separates  an  oral  from  an  aboral 
face,  and  is  apparently  homologous  throughout  the  series.  In 
the  adult  it  is  probably  represented  by  the  lophophore.  On  the 
B.  IT  20 


306  SUMMARY. 


oral  face  is  situated  in  all  cases  the  mouth,  and  in  the  entoproc- 
tous  larvae  and  Cyphonautes  also  the  anus.  It  thus  appears  that 
Cyphonautes,  though  the  larva  of  an  ectoproctous  form,  is  itself 
entoproctous — a  fact  which  tends  to  shew  that  the  Entoprocta 
are  the  more  primitive  forms.  In  all  the  larvae,  except  possibly 
those  of  the  Cyclostomata,  there  is  present  on  the  anterior  side 
of  the  mouth,  in  the  Ectoprocta  on  the  oral,  and  in  the  Ento- 
procta on  the  aboral  side  of  the  ciliated  ring,  an  organ,  to  which 
is  attached  externally  a  plume  of  long  cilia.  This  organ  has 
been  identified  throughout  the  series 
in  accordance  with  Hatschek's  view 
as  the  dorsal  organ  or  rudimentary 
bud ;  but  it  is  well  to  bear  in  mind 
that  this  identification  is  of  a  purely 
hypothetical  character. 

On  the  aboral  side  of  the  ciliated 
ring  there  is  present  in  all  the  larv* 
an  organ,  which  has  been  called  the         ,„.  m0uth;  an.  anus;^.  sto- 
ciliated  disc,  which  is  probably  homo-     mach'  *-  ciliated  disc- 
logous  throughout  the  series.    It  perhaps  remains  in  the  adult  of 
Loxosoma  as  the  cement  gland,  but  not  in  other  forms. 

The  Polyzoa  present  a  simple  and  almost  certainly  degraded 
organisation  in  the  adult  state  ;  it  is  therefore  more  than  usually 
necessary  to  turn  to  their  larvae  for  the  elucidation  of  their 
affinities,  and  various  plausible  suggestions  have  been  made  as 
to  the  interpretation  of  the  characters  of  the  larvae. 

Lankester1  has  suggested  that  the  larvae  are  essentially 
similar  to  those  of  Molluscs.  He  compares  the  main  ciliated 
ring  to  the  velum,  but  has  ingeniously  suggested  that  it  repre- 
sents not  the  simple  velar  ring  of  most  molluscan  larvae,  but 
a  more  extended  longitudinal  ring,  of  which  the  gills  of  Lamel- 
libranchiata  are  supposed  by  him  to  be  remnants,  and  to  which 
the  Echinoderm  larvae  with  one  continuous  ciliated  band  furnish 
a  parallel. 

The  foot  he  finds  in  the  epistome  of  the  Phylactolaemata, 
and  the  disc  of  Rhabdopleura — both  situated  between  the 
mouth  and  anus,  and  therefore  in  the  situation  of  the  molluscan 

1  Lankester.  "Remarks  on  the  affinities  of  Rhabdopleura."  Quart.  J.  of 
Mitro.  Science,  Vol.  XIV.  1874. 


POLYZOA.  307 


foot.  The  peculiar  prominence  between  the  mouth  and  the 
anus  in  Pedicellina  (vide  fig.  130  B)  and  Loxosoma  is  probably 
the  same  structure. 

Finally  he  identifies  my  ciliated  disc,  which  as  mentioned 
above  is  perhaps  equivalent  to  the  cement  gland  in  the  adult 
Loxosoma,  as  the  molluscan  shell-gland.  Lankester's  interpre- 
tations are  very  plausible,  but  at  the  same  time  they  appear  to 
me  to  involve  considerable  difficulties. 

There  is  absolutely  no  evidence  amongst  the  Mollusca  of  the 
existence  of  a  primitive  longitudinal  ciliated  ring,  such  as  he 
supposes  to  have  existed,  and  Lankester  is  debarred  from 
regarding  the  ciliated  ring  of  the  Polyzoa  as  equivalent  to  the 
simple  velar  ring  of  the  Mollusca,  because  his  shell-gland  lies  in 
the  centre  and  not  as  it  should  do  on  the  posterior  side  of  the 
ciliated  ring. 

Another  difficulty  which  I  find  is  the  invariable  ciliation  of 
Lankester's  shell-gland — a  ciliation  which  never  occurs  amongst 
Mollusca. 

It  appears  to  me  that  a  more  satisfactory  comparison  of  the 
larvae  of  the  Polyzoa  with  those  of  the  Mollusca  is  obtained  by 
dropping  the  view  that  the  ciliated  disc  is  the  shell-gland,  and 
by  regarding  the  ciliated  ring  as  equivalent  to  the  velum.  This 
mode  of  comparison  has  been  adopted  by  Hatschek. 

The  larva  ceases  however  on  this  view  to  have  any  special 
molluscan  characters  (except  possibly  the  organ  which  Lankester 
has  identified  as  the  foot),  and  only  resembles  a  molluscan  larva 
to  the  same  extent  as  it  does  a  larva  of  the  Polychaeta.  The 
ciliated  disc  lies  according  to  this  view  in  the  centre  of  the  velar 
area  or  prae-oral  lobe,  and  therefore  in  the  situation  in  which  a 
tuft  of  cilia  is  often  present  in  lamellibranchiate  and  other 
molluscan  larvae,  and  also  in  the  larvae  of  most  Chaetopoda.  It 
is  moreover  at  this  point  that  the  supra-cesophageal  ganglion  is 
always  formed  in  the  Mollusca  and  Chaetopoda  as  a  thickening 
of  the  epiblast  (fig.  134,  .$£-.),  so  that  the  thickening  of  the 
epiblast  in  the  ciliated  disc  of  the  Polyzoa  may  perhaps  be  a 
rudiment  of  the  supra-cesophageal  ganglion,  which  entirely 
atrophies  in  the  adult  after  the  attachment  has  been  effected  in 
the  region  of  this  disc. 

The  comparison  between  the  Polyzoon  larva  and  that  of  a 

20 — 1 


308 


POLYZOA. 


Chaetopod  becomes  very  much  strengthened  by  taking  as  types 
Mitraria1  (fig.  134)  and  Cyphonautes  (fig.  133).     The  similarity 


FIG.  134.  Two  STAGES  IN  THE  DEVELOPMENT  OF  MITRARIA.  (After  Metsch- 
nikoff.) 

m.  mouth;  an.  anus;  sg.  supra-oesophageal  ganglion;  br.  and  b.  provisional 
bristles;  pr.b.  prae-oral  ciliated  band. 

between  these  two  forms  is  so  striking  that  I  am  certainly 
inclined  to  view  the  larvae  of  the  Polyzoa  as  trochospheres  similar 
to  those  of  Chaetopods,  Rotifera,  etc.,  which  'become  fixed  in  the 
adult  by  the  extremity  of  their  prce-oral  lobe. 

The  attachment  of  the  larva  by  the  prae-oral  lobe  is  not  more 
extraordinary  than  the  attachment  of  a  Barnacle  by  its  head, 
and  after  such  a  mode  of  attachment  the  atrophy  of  the  supra- 
cesophageal  ganglion  would  be  only  natural. 

There  is  one  important  fact  which  deserves  to  be  noted  in 
the  development  of  the  Polyzoa,  viz.  that  if  the  suggestion  in  the 
text  as  to  the  mode  of  development  of  the  adult  from  the  so- 
called  larva  is  accepted,  the  Polyzoa  exhibit  universally  the 
phenomenon  of  alternations  of  generations.  The  ovum  gives  rise 
to  a  free  form  which  never  becomes  sexual,  but  produces  by 
budding  the  sexual  attached  form. 

1  The  larva  of  Mitraria  is  figured  with  the  aboral  surface  turned  upwards,  instead 
of  downwards,  as  in  the  figure  of  Cyphonautes.  The  ciliated  band  is  also  diagramma- 
tical ly  put  in  black  for  greater  distinctness. 


BIBLIOGRAPHY.  309 


BIBLIOGRAPHY. 
General. 

J.  Barrels.     Recherches  sur  P  embryologie  des  Bryozoaires.     Lille,  1877. 

Entoprocta. 

(299)  B.    Hatschek.     "Embryonalentwicklung  u.    Knospung  d.    Pedicellina 
echinata."    Zeitschrift  fur  wiss.  Zool.,  Bd.  xxix.     1877. 

(300)  M.  Salensky.     "  Etudes  sur  les  Bryozoaires  entoproctes."     Ann.  Scien. 
Nat.,  6th  Ser.  Tom.  v.     1877. 

(301)  O.  Schmidt.     "Die  Gattung  Loxosoma."      Archiv  f.  mik.  Anat.,  Bd. 
xii.     1876. 

(302)  C.  Vogt.     "Sur  le  Loxosome  des  Phascolosomes."     Archives  de  Zool. 
exper.  et  gener.,  Tom.  v.     1876. 

(303)  C.  Vogt.     "Bemerkungen  zu  Dr  Hatschek's  Aufsatz  iib.  Embryonalent- 
wicklung u.   Knospung  von  Pedicellina  echinata."     Zeit.  f.  wiss.  Zool.,  Bd.  XXX. 
1878. 

Ectoprocta. 

(304)  G.  J.  Allman.     Monograph  of  fresh  water  Polyzoa.     Ray  Society. 

(305)  G.  J.  Allman.      "  On  the  structure  of  Cyphonautes."     Quart.  J.  of  Micr. 
Scie.,  Vol.  xii.     1872. 

(306)  G.  J.  Allman.     "On  the  structure  and  development  of  the  Phylactoke- 
matous  Polyzoa."    Journal  of  'the  Linnean  Society,  Vol.  xiv.  No.  77.     1878. 

(307)  J.  Barrois.     " Le  developpement  d.  Bryozoaires  Chilostomes."     Comptes 
rendus,  Sept.  23,  1878. 

(308)  E.  Claparede.     "Beitrage  zur  Anatomic  u.  Entwicklungsgeschichte  d. 
Seebryozoen."     Zeit.fiir  wiss.  Zool.,  Bd.  xxi.     1871. 

(309)  E.   Claparede.     "Cyphonautes."     Anat.  u.  Entwick.  wirbell.    Thiere. 
Leipzig,  1864. 

(310)  R.  E.  Grant.     "Observations  on  the  structure  and  nature  of  Flustrse." 
Edinburgh  New  Philosoph.  Journal,  1827. 

(311)  B.    Hatschek.     "Embryonalentwicklung  u.    Knospung   d.    Pedicellina 
echinata"  (Description  of  Cyphonautes).     Zeit.f.  wiss.  Zool.,  Bd.  xxix.     1877. 

(312)  T.  H.  Huxley.     "Note  on  the  reproductive  organs  of  the  Cheilostome 
Polyzoa."     Quart.  Jour,  of  Micr.  Science,  Vol.  IV.     1856. 

(313)  L.  Joliet.     "Contributions  a  1'histoire  naturelle  des  Bryozoaires  des  cotes 
de  France."     Archives  de  Zoologie  Experimental,  Vol.  vi.     1877. 

(314)  E.  Metschnikoff.     " Ueber  d.  Metamorphose  einiger  Seethiere."     Got- 
tingische  Nachrichten,  1869. 

<315)     E.  Metschnikoff.     Bull,  de  tAcad.  de  St  Petersbourg,  XV.  1871,  p.  507. 

(316)  H.    Nitsche.     "Beitrage   zur   Kenntniss   d.    Bryozoen."     Zeit.  f.  wiss. 
Zool.,  Bd.  xx.     1870. 

(317)  W.   Repiachoff.     "Zur  Naturgeschichte  d.  chilostomen  Seebryozoen." 
Zeit.f.  wiss.  Zool.,  Bd.  xxvi.     1876. 


310  BIBLIOGRAPHY. 


(318)  W.  Repiachoff.     "  Ueber  die  ersten  Entwicklungsvorgange  bei  Tendra 
zostericola."    Zeit.f.  wiss.  Zool.,  Bd.  xxx.     1878.     Supplement. 

(319)  W.  Repiachoff.     "  Zur  Kenntniss  der  Bryozoen."    Zoologischer  Anzeiger, 
No.  10,  Vol.  I.     1878. 

(320)  W.  Repiachoff.     "  Bemerkungen  ub.  Cyphonautes."    Zoologischer  An- 
*eiger.  Vol.  II.     1879. 

(321)  M.  Salensky.     "  Untersuchung  an  Seebryozoen."    Zeit.  fur  wiss.  ZooL, 
Bd.  xxiv.     1874. 

(322)  A.  Schneider.     "Die   Entwicklung  u.   syst.  Stellung  d.  Bryozoen  u. 
Gephyreen."    Archiv /.  mikr.  Anat.,  Vol.  v.     1869. 

(323)  Smitt.     "  Om  Hafsbryozoernas  utveckling  och  fettkroppar. "     Aftryck  ur 
ijfvtrs.  af  Kong.  Vet.  Akad.  Fork.     Stockholm,  1865. 

(324)  T.  Hi  neks.     British  Marine  and  Polyzoa.     Van  Voorst,  1880. 
[Conf.  also  works  by  Farre,  Hincks,  Van  Beneden,  Dalyell,  Nordmann.] 


CHAPTER   XL 


BRACHIOPODA1. 

THE  observations  which  have  been  made  on  the  develop- 
mental history  of  the  Brachiopoda  have  thrown  very  consider- 
able light  on  the  systematic  position  of  this  somewhat  isolated 
group. 

Development  of  the  Layers. 

For  our  knowledge  of  the  early  stages  in  the  development  of 
the  Brachiopoda  we  are  almost  entirely  indebted  to  Kowalevsky2 
(No.  326).  His  researches  extend  to  four  forms,  Argiope, 
Terebratula,  Terebratulina,  and  Thecidium.  The  early  develop- 
ment of  the  first  three  of  these  takes  place  on  one  plan,  and 
that  of  Thecidium  on  a  second  plan. 

In  Argiope,  which  may  be  taken  as  typical  of  the  first  group, 
the  ova  are  transported  into  the  oviducts  (segmental  organs) 
where  they  undergo  their  early  development.  The  segmentation 
leads  to  the  formation  of  a  blastosphere,  which  then  becomes  a 
gastrula  by  invagination.  The  Slastopore  gradually  narrows, 
and  finally  closes,  while  at  the  same  time  the  archenteric  cavity 

1  The  classification  of  the  Brachiopoda  adopted  in  the  present  chapter  is  shewn  in 
the  subjoined  table : 

,.     ,   ,  (a.   Rhynchonellidae. 

I.  Articulata.       L    _.£. 

\b.    Terebratuhdse. 
/a.   Lingulidae. 

II.  InartiCUlata.     \b.   Craniadae. 

I  c.    Discinidae. 

2  Kowalevsky's  Memoir  is  unfortunately  written  in  Russian.     The  account  in  the 
text  is  derived  from  an  inspection  of  his  figures,  and  from  an  abstract  in  Hoffmann 
and  Schwalbe's  Jahresberichte  for  1873. 


12 


ARTICULATA. 


(fig.  135  A)  becomes  divided  into  three  lobes,  a  median  (me) 
and  two  lateral  (pv).  These  lobes  next  become  completely 
separated,  and  the  middle  one  forms  the  mesenteron,  while 
the  two  lateral  ones  give  rise  to  the  body  cavity,  their  outer 
walls  forming  the  somatic  mesoblast,  and  their  inner  the 
splanchnic  (fig.  135  B).  The  embryo  now  elongates,  and 
becomes  divided  into  three  successive  segments  (fig.  135  B), 
which  are  usually,  though  on  insufficient  grounds  (vide  Thecid- 
ium),  regarded  as  equivalent  to  the  segments  of  the  Cheetopoda. 
The  alimentary  tract  is  not  continued  into  the  hindermost  of 
them. 

In  Thecidium  the  ova  are  very  large,  and  development  takes 
place  in  a  special  incubatory  pouch  in  the  ventral  valve.     The 
embryos   are  attached   by  suspenders  to 
the  two  cirri  of  the  arms  which  immedi- 
ately adjoin  the  mouth.    There  is  a  nearly 
regular   segmentation,  and   a  very  small 
segmentation  cavity  is  developed.     There 
is  no  invagination ;  but  cells  are  budded 
off  from   the   walls  of  the  blastosphere, 
which   soon    form    a   solid  central    mass, 
enclosed   by  an  external  layer — the  epi- 
blast.     In  this  central  mass  three  cavities 
are  developed,  which  constitute  the  me- 
senteron and  the  two  halves  of  the  body 
cavity.     Around    these    cavities    distinct 
walls   become    differentiated.     The   body 
(Lacaze    Duthiers,   No.    327)   soon   after 
becomes   divided   into  two  segments,  of 
which  the  posterior  is  the  smaller.     The 
hinder  part  of  the  large  anterior  segment 
next  becomes  constricted  off  as  a  fresh 
segment,  and  subsequently  the  remaining 
part  becomes   divided    into  two,  of  which  the  anterior  is   the 
smallest.     The  embryo  thus  becomes  divided  into  four  segments, 
of  which  the  two  foremost  appear  (?)  together  to  correspond  to 
the  cephalic  segment  of  Argiope ;  but  these  segments  are  formed 
not,  as  in  Chaetopoda  and  other  truly  segmented  forms,  by  the 
addition  of  fresh  segments  between  the  last-formed  segment  and 


pv. 


FIG.  135.  Two  STAGES 

IN  THE   DEVELOPMENT  OF 

ARGIOPE.     (After    Kowa- 
levsky.) 

A.  Late  gastrula  stage. 

B.  Stage  after  the  larva 
has    become   divided   into 
three  segments. 

bl.  blastopore  ;  me.  me- 
senteron ;  pv.  body  cavity ; 
b.  temporary  bristles. 


BRACHIOPODA. 


313 


the  unsegmented  end  of  the  body,  but  by  the  interpolation  of 
fresh  segments  at  the  cephalic  end  of  the  body  as  in  Cestodes ; 
so  that  the  hindermost  segment  is  the  oldest.  Assuming  the 
correctness1  of  Lacaze  Duthiers'  observations,  the  mode  of 
formation  of  these  segments  appears  to  me  to  render  it  probable 
that  they  are  not  identical  with  the  segments  of  a  Chaetopod. 
A  suspender  is  attached  to  the  front  end  of  each  embryo.  Before 
the  four  segments  are  established  the  whole  embryo  is  covered 
with  cilia2,  and  two  and  then  four  rudimentary  eyes  are 
developed  on  the  anterior  segment  of  the  body. 

The  history  of  the  Larva  and  the  development  of  the  organs  of 

the  Adult. 

Articulata.  The  observations  of  Kowalevsky  and  Morse 
have  given  us  a  fairly  complete  history  of  the  larval  metamor- 
phosis of  some  of  the  Articulata,  while  some  of  the  later  larval 
stages  in  the  history  of  the  Inarticulata  have  been  made  known 
to  us  from  the  researches  of  Fritz  M tiller,  Brooks,  etc.  The 
embryo  of  Argiope,  which  may  be  taken 
as  the  type  for  the  Articulata,  was  left 
(fig.  135  B)  as  a  three  lobed  organism 
with  a  closed  mesenteron  and  a  body 
cavity  divided  into  two  lateral  compart- 
ments. On  the  middle  segment  of  the 
body  dorsal  and  ventral  folds,  destined 
to  form  the  mantle  lobes,  make  their 
appearance,  and  on  the  latter  two  pairs 
of  bundles  of  setae  are  present  (fig.  135  B). 
The  setae  together  with  the  mantle  folds 
grow  greatly,  and  the  setae  resemble  in  ap- 
pearance the  provisional  setae  of  many 
Chaetopods  (fig.  152).  On  the  hinder 
border  of  the  mantle  cilia  make  their 
appearance.  The  anterior  or  cephalic 
segment  assumes  a  somewhat  umbrella- 


FIG.  136.  LARVA  OF  AR- 
GIOPE. (From  Gegenbaur, 
after  Kowalevsky.) 


.  .     ,    L  m.  mantle;    b.  setae;   </. 

like  form,  and  round  its  edge  is  a  circlet     archenteron. 

1  It  should  be  stated  that  it  is  by  no  means  clear  from  Kowalevsky's  figures  that 
he  agrees  with  Lacaze  Duthiers  as  to  the  succession  of  the  segments. 

2  Kowalevsky  in  his  figures  leaves  the  penultimate  lobe  unciliated. 


3H 


ARTICULATA. 


of  long  cilia,  while  elsewhere  it  is  provided  with  a  coating  of  short 
cilia.   Two  pairs  of  eyes  also  arise  on  its  anterior  surface  (fig.  136). 

After  swimming  about  for  some  time  the  larva  becomes  fixed 
by  its  hind  lobe,  and  becomes  gradually  transformed  into  the 
adult.  The  hind  lobe  itself  becomes  the  peduncle.  After 
attachment  the  mantle  lobes  bend  forward  (fig.  137  A,  m\  and 
enclose  the  ce- 
phalic lobe. 
The  valves  of 
the  shell  are 
formed  on  their 
outer  surface  as 
two  delicate 
chitinous  plates 
(fig.  1 37  B).  At 
a  somewhat  la- 
ter stage  the 
provisional  bri- 
stles are  thrown 
off,  and  are 
eventually  re- 
placed by  per- 
manent setae 
round  the  edge 
of  the  mantle. 
The  cephalic 
lobe  becomes 
located  in  the 
dorsal  valve  of 
the  shell,  and 
the  mouth  is 
formed  near  the 
apex  of  the  ce- 
phalic lobe  im- 
mediately ven- 
tral to  the  eye-spots,  by  an  epiblastic  invagination.  The  per- 
manent muscles  are  formed  out  of  the  muscles  already  present 
in  the  embryo. 

Around  the   mouth   there   arises  a  ring  of  tentacles,  very 


FlG.  137.  TWO  STAGES  IN  THE  DEVELOPMENT  OF 
ARGIOPE,  SHEWING  THE  FOLDS  OF  THE  MANTLE  GROWING 
OVER  THE  CEPHALIC  LOBE.  (After  Kowalevsky.) 

m.  mantle  fold  ;  me.   mesenteron  ;  pd.  peduncle ;  b. 
visional  setae. 


pro- 


BRACHIOPODA. 


315 


possibly  derived  from  the  ciliated  ring  visible  in  fig.  I361.  The 
ring  of  tentacles  is  placed  obliquely,  and  the  mouth  is  situated 
near  its  ventral  side.  The  tentacles  appear  to  form  a  post-oral 
circlet,  like  that  of  Phoronis  (Actinotrocha):  they  gradually 
increase  in  number  as  the  larva  grows  older. 

Some  of  the  later  stages  in  the  development 
of  the  Terebratulidse  have  been  made  known  to 
us  by  the  observations  of  Morse  (No.  328 — 9) 
on  Terebratulina  septentrionalis. 

The  most  interesting  point  inMorse's  observa- 
tions on  the  later  stages  is  the  description  of 
the  gradual  conversion  of  the  disc  bearing  the 
circlet  of  tentacles  into  the  arms  of  the  adult. 
The  tentacles,  six  in  number,  first  form  a  ring 
round  the  edge  of  a  disc  springing  from  the 
dorsal  lobe  of  the  mantle  ;  in  their  centre  is 
the  mouth.  In  the  later  stages  calcareous 
spicula  become  developed  on  the  tentacles. 
When  the  embryo  is  far  advanced  the  tentacles 
begin  to  assume  a  horse-shoe  arrangement, 
which  bears  a  striking,  though  probably  acciden- 
tal, resemblance  to  that  of  the  tentacles  on  the 
lophophore  of  the  fresh-water  Polyzoa.  The 
disc  bearing  the  tentacles  is  prolonged  anteriorly 
into  two  processes,  the  free  ends  of  the  future 
arms.  By  this  change  of  shape  in  the  disc  the 
tentacles  form  two  rows,  one  on  the  anterior  and 
one  on  the  posterior  border  of  the  disc,  and 
eventually  become  the  cirri  of  the  arms.  The 
mouth  is  placed  between  the  two  rows  of  tenta- 
cles, where  the  two  arms  of  the  lophophore  meet 
behind.  The  position  of  the  mouth  was  the 
original  centre  of  the  ring  of  tentacles  before 
they  became  pulled  out  into  a  horse-shoe  form. 
In  front  of  the  mouth  is  a  lip.  The  arms  grow 
greatly  in  length  in  the  adult  Terebratulina.  In 
Thecidium  the  oral  disc  retains  the  horse-shoe 
form,  while  in  Argiope  the  embryonic  circular 
arrangement  of  the  tentacles  is  only  interfered 
with  by  the  appearance  of  marginal  sinuations. 


FIG.  138.  DIAGRAM  OF 
A  LONGITUDINAL  VERTICAL 
SECTION  OF  AN  ADVANCED 

EMBRYO  OFLlNGULA.  (After 

Brooks.) 

a.  end  of  valves ;  b.  thick- 
ened margin  of  mantle ;  c. 
mantle ;  d.  dorsal  median 
tentacle  ;  e.  lophophore ;  /. 
lip ;  g.  mouth ;  h.  mantle 
cavity ;  i.  body  cavity ;  k. 
wall  of  oesophagus ;  /.  oeso- 
phagus; m.  hepatic  cham- 
ber of  stomach ;  n.  intesti- 
nal chamber  of  stomach  ;  o. 
intestine ;  q.  ventral  gang- 
lion ;  r.  posterior  muscle ; 
s.  dorsal  valve  of  shell;  /. 
ventral  valve  of  shell. 


1  In  the  abstract  in  Hoffman  and  Schwalbe  Kowalevsky  is  made  to  state  that  the 
tentacles  spring  from  the  border  of  the  mantle.  This  can  hardly  be  a  correct  account 
of  what  he  states,  since  it  does  not  fit  in  with  the  adult  anatomy  of  the  parts.  The 
figures  he  gives  might  lead  to  the  supposition  that  they  sprang  from  the  edge  of  the 
cephalic  lobe,  or  perhaps  from  the  dorsal  lobe  of  the  mantle. 


316  ARTICULATA. 


The  shell  is  deposited  as  to  chitinous  plates,  which  subse- 
quently become  calcified.  It  undergoes  in  the  different  genera 
great  changes  of  form  during  its  growth. 

With  reference  to  the  larval  stages  of  other  Articulata,  a  few  points  may 
be  noted. 

The  three-lobed  larva  of  Terebratulina  septentrionalis  is  provided  with 
a  special  tuft  of  cilia  at  the  apex  of  the  front  lobe.  The  arms  appear  to 
originate,  in  Terebratulina  caput  serpentis,  as  two  processes  at  the  sides  of 
the  mouth,  on  which  the  tentacles  are  formed. 

Provisional  setae  do  not  appear  to  be  formed  in  the  lobed  embryos  of 
Thecidium  and  Terebratulina,  but  they  appear  at  a  later  stage  at  the  edge  of 
the  mantle  in  the  latter  form.  The  third  lobe  of  Thecidium  gives  rise  to  the 
dorsal  and  ventral  mantle  lobes. 

Inarticulate.  The  youngest  stages  in  the  development  of 
the  Inarticulata  are  not  known,  and  in  the  earliest  stages 
observed  the  shell  is  already  developed.  The  young  larvae  with 
shells  differ  however  from  those  of  the  Articulata  in  the  fact 
that  they  are  free  swimming,  and  that  the  peduncle  is  not 
developed. 

The  larva  of  Discina  radiata  has  been  described  by  Fritz  Miiller  (No. 
331).  It  resembles  generally  a  larva  of  the  Articulata  shortly  after  the 
tentacles  have  become  developed.  Five  pairs  of  long  provisional  setae  are 
present,  of  which  all  but  the  hindermost  are  seated  on  the  ventral  lobe  of 
the  mantle.  Shorter  setae  are  also  lodged  on  the  edge  of  the  dorsal  lobe. 
The  mouth  is  placed  on  the  ventral  side  of  a  protrusible  oral  lobe.  It  is 
imperfectly  surrounded  by  four  pairs  of  tentacles,  which  form  a  swimming 
apparatus. 

A  fuller  history  of  the  development  of  Lingula  has  been  recently  supplied 
by  Brooks  (No.  325).  The  youngest  larva  is  enveloped  in  two  nearly  similar 
plate-like  valves,  covering  the  two  mantle  lobes.  The  mouth  is  placed  at 
the  centre  of  a  disc,  attached  to  the  dorsal  valve,  on  the  margin  of  which  is 
a  ring  of  ciliated  tentacles.  The  general  position  of  the  disc  and  its 
relations  may  be  gathered  from  fig.  138,  which  represents  a  diagrammatic 
longitudinal  vertical  section  of  the  embryo. 

With  the  growth  of  the  embryo  the  tentacles  increase  in  number,  the 
new  pairs  being  always  added  between  the  odd  dorsal  tentacle  and  the  next 
pair.  There  is  an  axial  cavity  in  the  tentacles  which,  unlike  the  cavity  in 
the  tentacles  of  the  Polyzoa,  does  not  communicate  with  the  perivisceral 
cavity.  As  the  tentacles  increase  in  number,  the  lateral  parts  of  the 
tentacular  disc  grow  out  into  the  two  lateral  arms  of  the  adult,  while  the 
dorsal  margin  forms  the  median  coiled  arm.  These  changes  are  not  effected 
till  the  larva  has  become  fixed. 

The  attachment  of  the  larva  was  not  observed  ;  but  the  peduncle,  of 


BRACHIOPODA.  317 


which  there  is  no  trace  in  the  young  stages,  grows  out  as  a  simple  prolonga- 
tion of  the  hinder  end  of  the  body  while  the  larva  is  still  free.  It  had  already 
reached  a  very  great  length  in  the  youngest  fixed  larva  observed. 

Development  of  Organs. 

The  alimentary  tract  after  the  obliteration  of  the  blastopore 
forms  a  closed  sack,  which  becomes  subsequently  placed  in 
communication  with  the  exterior  by  the  stomodaeal  imagination. 
The  liver  is  formed  as  a  pair  of  dorsal  outgrowths  of  the  mesen- 
teron.  From  Brooks'  observations  on  Lingula  it  would  appear 
that  the  primitive  mesenteron  forms  the  stomach  of  the  adult 
only,  and  that  the  intestine  grows  out  from  this  as  a  solid 
process :  this  eventually  meets  the  skin,  and  here  the  anus  is 
formed.  In  the  Articulata  the  mesenteron  is  aproctous. 

The  origin  of  the  body  cavity  as  paired  archenteric  diverticula 
has  already  been  described.  Its  somatic  wall  becomes  in  Lingula 
ciliated,  and  its  cavity  filled  with  a  corpusculated  fluid,  as  in 
many  Chsetopods.  It  is  eventually  prolonged  into  the  dorsal 
and  ventral  mantle  lobes  as  a  pair  of  horn-like  prolongations 
into  each  lobe,  which  communicate  with  the  body  cavity  by 
large  ciliated  openings.  Some  incomplete  observations  of  Brooks 
on  the  development  of  the  nervous  system  in  Lingula  shew  that 
it  arises  in  the  embryo  as  a  ring  round  the  oesophagus  with 
a  ventral  sub-cesophageal  (fig.  138  q\  and  two  lateral  ganglia, 
and  two  dorsal  otocysts.  The  ventral  ganglion  is  formed  as  a 
thickening  of  the  epiblast,  with  which  it  remains  in  continuity 
for  life.  The  remainder  of  the  ring  grows  out  from  the  ventral 
ganglion  as  two  cords,  which  gradually  meet  on  the  dorsal  side 
of  the  oesophagus. 

General  observations  on  the  Affinity  of  the  Brachiopoda. 

The  larva  of  Argiope,  as  has  been  noticed  by  many  observers,  has 
undoubtedly  very  close  affinities  with  the  Chaetopoda.  It  resembles,  in  fact, 
a  mesotrochal  larval  Chaetopod  with  provisional  setae  (vide  Chapter  on 
Chaetopoda).  Lacaze  Duthiers'  observations  point  to  the  lobes  of  the  larva 
not  being  true  segments,  and  certainly  the  mesoblast  does  not  in  the  embryo 
become  segmented  as  it  ought  to  do  were  these  lobes  true  segments.  If  this 
view  is  correct  the  larva  is  to  be  compared  to  an  unsegmented  Chastopod 
larva.  In  Rhynchonella,  however,  indications  of  two  segments  are  afforded 
in  the  adult  in  the  two  pairs  of  segmental  organs. 

Though  the  larval  Brachiopod  resembles  a  mesotrochal  Chastopod  larva, 


318  ARTICULATA. 


it  does  not  appear  to  resemble  the  trochosphere  larvae  so  far  described,  or 
the  more  typical  larvae  of  the  Chaetopoda,  in  that  the  ring  of  tentacles,  which 
is  probably,  as  already  mentioned,  derived  from  the  ciliated  ring  shewn  in 
fig.  137,  is  post-oral,  and  not  prce-oral.  The  ring  of  tentacles  is  like  the 
ring  in  Actinotrocha  (the  larva  of  Phoronis)  amongst  the  Gephyrea. 
Although  there  is  no  doubt  a  striking  resemblance  between  the  tentacular 
disc  of  a  larval  Brachiopod  and  the  lophophore  of  a  Polyzoon,  which  has 
been  pointed  out  by  Lankester,  Morse,  Brooks,  etc.,  their  homology  is 
rendered,  to  my  mind,  very  doubtful  (i)  by  the  fact  that  the  lophophore  is 
prae-oral  in  Polyzoa1  and  post-oral  in  Brachiopoda  ;  and  (2)  by  the  fact  that 
the  concave  side  of  the  lophophore  is  turned  in  nearly  opposite  directions 
in  the  two  forms.  In  Brachiopods  it  is  turned  dorsalwards,  and  in  phylacto- 
laematous  Polyzoa  ventralwards. 

The  view  of  Morse,  that  the  Brachiopoda  are  degraded  tubicolous 
Chaetopods,  is  not  so  far  supported  by  any  definite  embryological  facts. 
The  development  of  the  tentacular  ring  as  well  as  its  innervation  from  the 
sub-cesophageal  ganglion  prohibit  us,  as  has  been  pointed  out  by  Gegenbaur, 
from  comparing  it  with  the  tentacles  of  tubicolous  Chaetopoda. 

BIBLIOGRAPHY. 

(325)  W.    K.    Brooks.     "Development   of   Lingula."     Chesapeake    Zoological 
Laboratory,  Scientific  Results  of  the  Session  of  1878.     Baltimore,  J.  Murphy  and  Co. 

(326)  A.  Kowalevsky.     "  Development  of  the  Brachiopoda."     Protocol  of  the 
First  Session  of  the  United  Sections  of  Anatomy,  Physiology1,  and  Comparative  Ana- 
tomy at  the  Meeting  of  Russian  Naturalists  in  Kasan,  1873.     (Russian.) 

(327)  H.  Lacaze  Duthiers.     "  Histoire  de  la  Thecidie."    Ann.  Scien.  Nat. 
etc.  Ser.  4,  Vol.  xv.     1861. 

(328)  Morse.     "  On  the  Early  Stages  of  Terebratulina  septentrionalis."     Mem. 
Boston  Soc.  Nat.  History,  Vol.  II.  1869,  also  Ann.  &  Mag.  of  Nat.  Hist.,  Series  4, 
Vol.  viii.     1871. 

(329) "On   the   Embryology   of  Terebratulina."     Mem.  Boston  Soc.  Nat. 

History,  Vol.  III.,  1873. 

(330)  -   -  "On  the  Systematic  Position  of  the  Brachiopoda."     Proceedings  of 
the  Boston  Soc.  of  Nat.  Hist  ,  1873. 

(331)  Fritz   Miiller.     "  Beschreibung  einer  Brachiopoden  Larve."     Mutter's 
Archiv,  1860. 

1  For  the  ectoproctous  Polyzoa  it  might  be  held  that  the  ciliated  ring  of  tentacles 
is  post-oral,  but  the  facts  of  development  recorded  in  the  previous  chapter  appear  to 
me  to  shew  that  this  view  is  untenable. 


CHAPTER    XII. 

CH^TOPODA1. 

Formation  of  the  Germinal  Layers. 

MOST  Chaetopoda  deposit  their  eggs  before  development. 
The  Oligochaeta  lay  them  in  peculiar  cocoons  or  sacks  formed 
by  a  secretion  of  the  integument.  Some  marine  Polychaeta  carry 
them  about  during  their  development.  Autolytus  cornutus  has 
a  special  sack  on  the  ventral  surface  in  which  they  are  hatched. 
In  Spirorbis  Pagenstecheri  they  develop  inside  the  opercular 
tentacle,  and  in  Spirorbis  spirillum  inside  the  tube  of  the 
parent. 

A  few  forms  (e.g.  Eunice  sanguinea,  Syllis  vivipara,  Nereis 
diversicolor)  are  viviparous. 

Perhaps  the  most  primitive  type  of  Chaetopod  development 
so  far  observed  is  that  of  Serpula  (Stossich,  No.  357) 2.  There  is 
a  regular  segmentation  resulting  in  the  formation  of  a  blasto- 
sphere  with  a  central  segmentation  cavity.  An  invagination  of 
the  normal  type  now  ensues.  The  blastopore  soon  narrows  to 
become  the  permanent  anus,  while  the  invaginated  hypoblast 
forms  a  small  prominence  with  an  imperfectly  developed  lumen, 
which  does  not  nearly  fill  up  the  segmentation  cavity  (fig.  139  A). 
The  embryo,  which  has  in  the  meantime  become  completely 

1  The  following  classification  of  the  Chaetopoda  is  adopted  in  the  present  section. 

I.     Achseta  (Polygordius). 

(Sedentaria. 

ii.   Polychseta.    jErrantia 
ill.    Oligochgeta. 

2  The  observations  of  Stossich  are  not  thoroughly  satisfactory. 


320  FORMATION   OF  THE   LAYERS. 

covered  with  cilia,  now  assumes  more  or  less  the  form  of  a  cone, 
at  the  apex  of  which  is  the  anus,  while  the  base  forms  the 
rudiment  of  a  large  prae-oral  lobe.  The  alimentary  sack  grows 
forwards  and  then  bends  upon  itself  nearly  at  right  angles,  and 
meets  a  stomodaeal  invagination  from  the  ventral  side  some  way 
from  the  front  end  of  the  body. 

The   alimentary  canal  soon  differentiates   itself   into   three 
regions  (i)   oesophagus,  (2)    stomach,  and  (3)  intestine.     With 


FlG.    139.      TWO  STAGES  IN  THE  DEVELOPMENT  OF  SfiRPULA.      (After  StOSsich.) 

m.  mouth  ;  an.  anus  ;  al.  archenteron. 

these  changes  the  larva,  which  in  the  meantime  becomes  hatched, 
assumes  the  characters  of  a  typical  Annelid  larva  (fig.  139  B). 
In  front  is  a  large  prae-oral  lobe,  at  the  sides  of  which  the  eye- 
spots  soon  appear.  The  primitive  segmentation  cavity  remains 
as  a  wide  space  between  the  curved  alimentary  tract  and  the 
body  walls,  and  becomes  traversed  by  muscular  fibres  passing 
between  the  two.  The  original  chorion  appears  to  serve  as 
cuticle,  and  is  perforated  by  the  cilia. 

The  further  changes  in  this  larval  form  do  not  present  features  of 
general  importance.  A  peculiar  vesicle,  which  in  anomalous  cases  is 
double,  is  formed  near  the  anus.  If  it  were  shewn  to  occur  widely 
amongst  Chaetopoda,  it  might  be  perhaps  regarded  as  homologous  with 
the  anal  vesicles  of  the  Gephyrea. 

Serpula  is  one  of  the  few  Chaetopoda  at  present  known  in 


CH^TOPODA.  321 


which  the  segmentation  is  quite  regular1.  In  other  forms  it  is 
more  or  less  unequal.  The  formation  of  the  germinal  layers 
has  been  far  more  fully  studied  in  the  Oligochaeta  than  in  the 
Polychaeta,  and  though  unfortunately  the  development  is  much 
abbreviated  in  the  former  group,  they  nevertheless  have  to  serve 
as  our  type ;  and  unless  the  contrary  is  indicated  the  statements 
in  the  remainder  of  the  section  apply  to  the  Oligochaeta.  The 
segmentation  is  nearly  regular  in  Lumbricus  agricola  (Kowa- 
levsky)  and  results  in  the  formation  of  a  flattened  blastosphere, 
one  of  the  sides  of  which  is  hypoblastic  and  the  other  epiblastic, 
the  hypoblast  cells  being  easily  distinguished  from  the  epiblast 
cells  by  their  clearer  aspect.  An  invagination  takes  place, 
in  the  course  of  which  the  hypoblast  becomes  enclosed  by  the 
epiblast,  and  a  somewhat  cylindrical  two-layered  gastrula  is 
formed.  The  opening  of  this  gastrula  at  first  extends  over 
the  whole  of  what  becomes  the  ventral  surface  of  the  future 
worm,  but  gradually  narrows  to  a  small  pore — the  permanent 
mouth — near  the  front  end.  The  central  cavity  of  the  gas- 
trula is  lined  by  hypoblast  cells,  but  the  oral  opening,  which 
leads  by  a  narrow  passage  into  the  gastric  cavity,  is  lined  by 
epiblast  cells. 

The  segmentation  of  Lumbricus  trapezoides  (Kleinenberg,  No.  341),  and 
of  Criodrilus  (Hatschek,  No.  339),  is  more  unequal  and  more  irregular  than 
that  of  Lumbricus  agricola,  and  there  is  an  invagination  which  is  inter- 
mediate between  the  embolic  and  epibolic  types. 

The  segmentation  of  Lumbricus  trapezoides  is  especially  remarkable.  It 
is  strangely  irregular  and  at  one  period  the  segmentation  cavity  communi- 
cates by  a  pore  with  the  exterior.  Before  the  completion  of  the  gastrula 
stage  the  ovum  becomes  partially  divided  into  two  halves,  each  of  which 
gives  rise  to  a  complete  embryo.  The  two  embryos  are  at  first  united 
by  an  epiblast  cord  which  connects  their  necks  (fig.  141  A),  but  this  cord  is 
very  early  ruptured,  and  the  two  embryos  then  become  quite  independent. 
Some  of  the  peculiarities  of  the  segmentation  may  no  doubt  be  explained 
by  this  remarkable  embryonic  fission. 

The  gastrula  opening  in  both  Lumbricus  trapezoides  and  Criodrilus  is 
placed  on  the  ventral  surface,  and  eventually  narrows  to  form  the  mouth 
or  possibly  (Criodrilus)  closes  at  the  position  of  the  mouth.  In  Lumbricus 
trapezoides  the  oral  opening  is  at  first  lined  by  hypoblast,  and  in  Criodrilus 
is  bounded  anteriorly  by  three  large  peculiar  epiblast  cells,  which  are 

1  According  to  Willemoes-Suhm,  Terebellides  stroemii  is  also  characterised  by  a 
regular  segmentation, 

3,  II,  21 


322  FORMATION    OF   THE    LAYERS. 

believed  by  Hatschek  to  assist  in  absorbing  the  albuminous  fluid  in  which 
the  eggs  are  suspended.  These  large  cells  are  eventually  covered  by  the 
normal  epiblast  cells  and  subsequently  disappear.  In  both  these  types 
the  hypoblast  cells  undergo,  during  their  invagination,  peculiar  changes 
connected  with  their  nutritive  function. 

In  Euaxes  (Kowalevsky)  the  segmentation  is  far  more  unequal  than  in 
the  other  types ;  a  typical  epibolic  invagination  takes  place  (fig.  140),  and 
the  blastopore  closes  completely  along  the  ventral  surface. 

In  all  the  oligochaetous  types,  with  the  exception  of  Euaxes, 
where  the  blastopore  closes  completely,  the  blastopore  becomes, 
or  coincides  with  the  mouth.  In 
Serpula  it  is  stated  (Stossich), 
as  we  have  seen,  to  coincide 
with  the  anus :  a  statement 
which  receives  confirmation 
from  the  similar  statements  of 
Willemoes-Suhm  (No.  358).  It 
is  necessary  either  to  suppose  ^ 

a  mistake  on  the  part  of  Stos- 
sirh  or  that  we  have  in  Chaeto-          FlG-     l*°'     TRANSVEKSE    SECTION 

S1Cn>01  THROUGH       THE      OVUM      OF      EUAXES 

pods  a  case  like  that  of   Gas-     DURING  AN  EARLY  STAGE  OF  DEVELOP- 
,       .  1-1  f.   1-1          MENT.     (After  Kowalevsky.) 

teropods    in   which   a   slit-like         ^  epiblast;  ms.  mesoblastic  band; 
blastopore  originally  extending    h-  hypoblast. 
along  the  ventral  surface  may  in  some  forms  become  reduced 
to  a  pore  at  the  oral,  or  in  other  forms  at  the  anal  extremity. 

So  far  only  two  germinal  layers — the  epiblast  and  the  hypo- 
blast — have  been  spoken  of.  Before  the  invagination  of  the 
hypoblast  is  completed  the  mesoblast  makes  its  appearance  in 
the  form  of  two  bands  or  streaks,  extending  longitudinally  for 
the  whole  length  of  the  embryo.  These  are  usually  spoken 
of  as  germinal  streaks,  but  to  avoid  the  ambiguity  of  this  term 
they  will  be  spoken  of  as  mesoblastic  bands. 

Their  origin  and  growth  has  been  most  fully  studied  by 
Kleinenberg  (No.  341)  in  Lum.  trapezoides.  They  commence 
in  this  species  shortly  before  the  gastrula  stage  as  two  large 
cells  on  the  surface  of  the  blastoderm,  which  may  be  called 
mesoblasts.  These  cells  lie  one  on  each  side  of  the  median 
line  at  the  hind  end  of  the  embryo.  They  soon  travel  inwards 
and  become  covered  by  the  epiblast  (fig.  141  A,  m'\  while  on 
their  inner  and  anterior  side  a  row  of  small  cells  appears  (ms). 


CH^TOPODA. 


323 


These  rows  of  cells  form  the  commencement  of  the  mesoblastic 
bands,  and  in  the  succeeding  stages  they  extend  one  on  each 
side  of  the  body  (fig.  141  B,  ms)  till  they  reach  the  sides  of  the 
mouth.  Their  forward  growth  takes  place  mainly  at  the 
expense  of  the  superjacent  epiblast  cells,  but  the  two  mesoblasts 


FIG.  141.    THREE  SECTIONS  ILLUSTRATING  THE  DEVELOPMENT  OF  LUMBRICUS 

TRAPEZOIDES.     (After  Kleinenberg.) 
ms.  mesoblastic  band  ;  m' .  mesoblast ;  al.  archenteron  ;  pp.  body  cavity. 

A.  Horizontal  and  longitudinal  section  of  an  embryo  which  is  dividing  into  two 
embryos  at  the  gastrula  stage.     It  shews  the  mesoblasts  and  the  mesoblastic  bands 
proceeding  from  them. 

B.  Transverse  section  shewing  the  two  widely-separated  mesoblastic  bands. 

C.  Transverse  section  at  a  later  stage  shewing  the  mesoblastic  bands  which  have 
approached  the  ventral  line  and  developed  a  body  cavity^/. 

at  their  hinder  extremities  probably  assist  in  their  growth. 
Each  mesoblastic  band  is  at  first  composed  of  only  a  single  row 
of  cells,  but  soon  becomes  thicker,  first  of  all  in  front,  and 
becomes  composed  of  two,  three  or  more  rows  of  cells  abreast. 
From  the  above  it  is  clear  that  the  mesoblastic  bands  have,  in 
L.  trapezoides  at  any  rate,  in  a  large  measure  an  epiblastic 
origin. 

At  first  the  two  bands  end  in  front  at  the  sides  of  the  mouth, 
but    subsequently   their   front   ends   grow   dorsalwards   at   the 

21 — 2 


324  FORMATION   OF  THE   LAYERS. 

expense   of  the   adjoining   epiblast  cells,  and  meet  above  the 
mouth,  forming  in  this  way  a  mesoblastic  dorsal  commissure. 

The  mesoblastic  bands  soon  travel  from  the  lateral  position, 
which  they  at  first  occupy,  towards  the  ventral  surface.  They 
do  not  however  meet  ventrally  for  some  time,  but  form  two 
bands,  one  on  each  side  of  the  median  ventral  line  (fig. 
141  C). 

The  usual  accounts  of  the  origin  and  growth  of  the  bands  differ  some- 
what from  the  above.  By  Kowalevsky  (No.  342)  and  Hatschek  (No.  339) 
they  are  believed  to  increase  in  Lumbricus  rubellus  and  Criodrilus  entirely 
at  the  expense  of  the  mesoblasts.  Kowalevsky  moreover  holds  that  in  L. 
rubellus  the  original  mesoblasts  spring  from  the  hypoblast.  In  some  forms, 
e.g.  Lumbricus  agricola,  the  mesoblasts  are  not  present. 

In  Euaxes  the  origin  of  the  mesoblast  bands  is  somewhat  interesting 
as  illustrating  the  relation  of  the  Chaetopod  mesoblastic  bands  to  the 
mesoblast  of  other  forms.  To  render  intelligible  the  origin  of  the  mesoblast 
in  this  form,  it  is  necessary  to  say  a  few  words  about  the  segmentation. 

By  a  somewhat  abnormal  process  of  segmentation  the  ovum  divides 
into  four  spheres,  of  which  one  is  larger  than  the  others,  and  occupies 
a  position  corresponding  with  the  future  hind  end  of  the  embryo.  The 
three  smaller  spheres  give  rise  on  their  dorsal  side  by  a  kind  of  budding 
to  small  cells,  which  become  the  epiblast ;  and  the  epiblast  is  also  partly 
formed  from  the  hinder  large  cell  in  that  this  cell  produces  by  budding 
a  small  cell,  which  again  divides  into  two.  The  anterior  of  the  two 
cells  so  formed  divides  still  further  and  becomes  incorporated  in  the 
epiblast ;  the  posterior  only  divides  into  two  which  form  the  two  mesoblasts. 
The  remainder  of  the  mesoblast  is  formed  by  further  division  of  the  three 
smaller  of  the  primitive  large  spheres,  and  at  first  forms  a  continuous 
layer  between  the  dorsal  cap  of  epiblast  and  the  four  largest  cells  which, 
after  giving  rise  to  the  epiblast  and  mesoblast,  constitute  the  hypoblast. 
As  the  epiblast  spreads  over  the  hypoblast  the  mesoblastic  sheet  gives  way 
in  the  middle,  and  the  mesoblast  remains  as  a  ridge  of  cells  at  the  edge  of 
the  epiblastic  cup.  It  forms  in  fact  a  thickening  of  the  lips  of  the  blasto- 
pore.  Behind  the  thickening  is  completed  by  the  two  mesoblasts.  The 
appearance  of  the  mesoblast  in  section  is  shewn  in  fig.  140.  As  the 
epiblast  accompanied  by  the  mesoblast  grows  round  the  hypoblast,  the 
blastopore  assumes  an  oval  form,  and  the  mesoblast  appears  as  two  bands 
forming  the  sides  of  the  oval.  The  epiblast  travels  over  the  hypoblast 
more  rapidly  than  the  mesoblast,  so  that  when  the  blastopore  becomes 
closed  ventrally  the  mesoblastic  bands  are  still  some  little  way  apart  on 
the  ventral  side. 

In  Euaxes  the  mesoblast  originates  in  a  manner  which  is  very 
similar  to  that  in  some  of  the  Gasteropoda,  e.g.  Nassa,  vide  p.  234, 
and  Vermes,  e.g.  Bonellia,  etc.  As  mentioned  in  the  chapter  on  the 


CH^TOPODA.  325 


Mollusca  the  origin  of  the  mesoblast  in  Planorbis,  p.  227,  is  very  similar  to 
that  in  Lumbricus. 

Hatschek  has  shewn  that  in  Polygordius  the  mesoblast  arises  in  funda- 
mentally the  same  way  as  in  the  Oligochaeta. 

Besides  the  mesoblast  which  arises  from  the  mesoblastic  bands,  there 
is  evidence  of  the  existence  of  further  mesoblast  in  the  larvae  of  many 
Polychaeta  in  the  form  of  muscular  fibres  which  traverse  the  space  between 
the  body  wall  and  the  wall  of  the  enteric  cavity  prior  to  the  formation 
of  the  permanent  body  cavity.  These  fibres  have  already  been  described 
in  the  embryo  of  Serpula,  and  are  probably  represented  by  stellate  cells 
in  the  cephalic  region  (pras-oral  lobe)  of  the  Oligochaeta.  These  cells  are 
probably  of  the  same  nature  as  the  amoeboid  cells  in  the  larvae  of  Echino- 
dermata,  some  Mollusca  and  other  types. 

The  Larval  form. 

True  larval  forms  are  not  found  in  the  Oligochaeta  where  the 
development  is  abbreviated.  They  occur  however  in  the  ma- 
jority of  the  marine  Polychseta. 

They  present  a  great  variety  of  characters  with  variously 
arranged  ciliated  bands.  Most  of  these  forms  can  be  more  or 
less  satisfactorily  derived  from  a  larval  form,  like  that  of  Serpula 
(fig.  139  B)  or  Polygordius  (fig.  142);  and  the  constant  recur- 
rence of  this  form  amongst  the  Chsetopoda,  combined  with  the 
fact  that  it  presents  many  points  of  resemblance  to  the  larval 
forms  of  many  Rotifers,  Molluscs,  and  Gephyreans,  seems  to 
point  to  its  being  a  primitive  ancestral  form  for  all  these 
groups. 

The  important  characters  of  this  larval  form  are  (i)  the 
division  of  the  body  into  a  large  prae-oral  lobe  and  a  relatively 
small  post-oral  region  containing  the  greater  part  of  the  alimen- 
tary tract ;  (2)  the  presence  of  a  curved  alimentary  canal 
divided  into  stomodaeum  (oesophagus),  stomach  and  intestine, 
and  opening  by  a  ventrally  placed  mouth,  and  an  anus  near  the 
hind  end  of  the  body.  To  these  may  be  added  the  frequent 
presence  of  (i)  a  ganglion  at  the  apex  of  the  prae-oral  lobe, 
(2)  a  large  cavity  between  the  wall  of  the  gut  and  the  skin, 
which  is  the  remnant  of  the  segmentation  cavity,  and  is  usually 
traversed  by  muscular  strands,  of  which  one  connecting  the  apex 
of  the  prae-oral  lobe  and  the  stomach  or  oesophagus  is  very 
commonly  present  (fig.  142). 

The  arrangement  of  the  ciliated  bands  presents  great  varia- 


326 


THE  LARVAL  FORM. 


me.p 


nph 


tions,  though  in  some  instances  it  is  constant  through  large 
groups.  In  Chaetopods  there  is  a  widely  distributed  prae-oral 
ciliated  band,  which  is  similarly  placed  to  the  ring  constantly 
found  in  the  larvae  of  Molluscs,  Rotifers,  etc.  In  many  of  these 
forms  the  band  is  practically  double,  the  opening  of  the  mouth 
being  placed  between  its  two  component  rings  (vide  fig.  142). 
The  best  introduction  to  the  study  of  the  Chaetopod  larval  forms 
will  be  the  history  of  the  changes  of  a  typical  larval  form  in 
becoming  converted  into  the  adult. 

For  this  purpose  no  better  form  can  be  selected  than  the  interesting 
larva  of  Polygordius  (vide  Agassiz,  No.  332, 
Schneider,  No.  352,  and  Hatschek,  No.  339), 
which  was  first  discovered  by  Lovdn,  and 
believed  by  him  to  be  the  larva  of  an  ordinary 
Chaetopod.  Its  true  nature  was  determined 
by  Schneider. 

At  a  very  young  stage  the  larva  has  the 
form  (fig.  142)  of  a  flattened  sphere,  with  a 
small  conical  knob  at  the  posterior  ex- 
tremity. 

At  the  equator  are  situated  two  parallel 
ciliated  bands1,  between  which  lies  the  ven- 
trally  placed  mouth  (ni).  The  more  conspicu- 
ous ciliated  band  is  formed  of  a  double  row 
of  cilia,  and  is  situated  in  front  of  the  mouth.  The  thinner  ciliated  band 
behind  the  mouth  appears  to  be  absent  in  the  American  species. 

The  mouth  leads  into  an  oesophagus,  and  this  into  a  globular  stomach 
(<?/),  which  is  continuous  with  a  rectum  terminating 
by  an  anus  (an)  placed  at  the  hind  end  of  the 
posterior  conical  knob.  The  whole  alimentary 
tract  is  ciliated.  In  the  American  form  of  larva 
there  is  a  ring  of  cilia  round  the  anus,  which  is 
developed  at  a  somewhat  later  stage  in  the  form 
observed  by  Hatschek. 

The  position  of  the  ciliated  bands  and  the 
alimentary  tract  enables  us  to  divide  the  embryo 
into  three  regions  :  a  prae-oral  region  bounded  by 
the  anterior  ciliated  band,  a  gastric  region  in 
which  the  embryonic  stomach  is  situated,  and  an 
abdominal  region  formed  of  the  posterior  conical 


FlG.      142.        POLYGORDIUS 

LARVA.     (After  Hatschek.) 

m.  mouth;  sg.  supra-ceso- 
phageal  ganglion ;  nph.  neph- 
ridion ;  me.p.  mesoblastic  band  ; 
an.  anus  ;  ol.  stomach. 


FIG.  143.  POLYGOR- 
DIUS LARVA  .  ( From  Alex . 
Agassiz.) 


1  These  two  rings  are  at  first  (Hatschek)  not  quite  closed  dorsally,  calling  to  mind 
the  early  condition  of  the  Echinoderm  larva  with  a  prae-oral  and  post-oral  ciliated 


CH^ETOPODA. 


327 


FlG.  144.  POLYGOR- 
DIUS LARVA.  (From 
Alex.  Agassiz.) 


portion,  which  by  its    subsequent    elongation    gives    rise    to    the    whole 
segmented  portion  of  the  future  Polygordius. 

At  the  front  end  of  the  prae-oral  lobe  is  situated  the  early  formed  supra- 
cesophageal  ganglion  (sg)  (first  noticed  by  Agassiz)  in  connection  with 
which  is  a  pair  of  eyes,  and  a  ramified  system  of  nerves.  The  ganglion  is 
marked  externally  by  a  crown  of  cilia. 

The  larval  epidermis  bears  a  delicate  cuticula,  and  is  separated  by  a 
considerable  interval  from  the  walls  of  the  alimentary  tract.  The  space 
between  the  two  represents  a  provisional  body  cavity,  which  is  eventually 
replaced  by  the  permanent  body  cavity  formed  between  the  two  layers 
of  the  mesoblast.  It  is  doubtful  when  the  replace- 
ment takes  place  in  the  head.  It  probably  does  so 
very  early.  The  mesoblast  is  present  in  the  usual 
form  of  two  bands  (me.p]  (germinal  streaks),  which 
are  anteriorly  continued  into  two  muscular  bands 
which  pass  through  the  embryonic  body  cavity  to 
the  front  end  of  the  prae-oral  lobe.  Another  pair  of 
contractile  bands  passes  from  the  same  region  of  the 
prse-oral  lobe  to  the  oesophagus. 

There  is  no  trace  of  the  ventral  nerve  cord.  The 
most  remarkable  organ  of  the  larva  is  a  paired  excre- 
tory organ  (npti)  discovered  by  Hatschek.  This  is  a 
ciliated  canal  with  at  first  one  and  subsequently 
several  funnel-shaped  openings  into  the  body  cavity  in  front  and  an 
external  opening  behind.  It  is  situated  immediately  anterior  to  the  lateral 
band  of  mesoblast,  and  is  parallel  with,  and  dorsal 
to,  the  contractile  band  which  passes  off  from 
this.  It  occupies  therefore  a  position  in  front 
of  the  segmented  region  of  the  adult  Polygordius. 

The  changes  by  which  this  peculiar  larval  form 
reaches  the  adult  condition  will  be  easily  gathered 
from  an  inspection  of  figs.  143 — 148.  They  con- 
sist essentially  in  the  elongation  of  what  has  been 
termed  the  abdominal  region  of  the  body,  and  the 
appearance  of  a  segmentation  in  the  mesoblast ; 
the  segments  being  formed  from  before  backwards, 
and  each  fresh  segment  being  interpolated  between 
the  anus-bearing  end  of  the  body  and  the  last 
segment. 

As  the  hind  portion  of  the  body  becomes 
elongated  the  stomach  extends  into  it,  and  gives  rise  to  the  mesenteron 
of  the  adult  (figs.  143,  144,  and  145).  For  a  long  time  the  anterior  spherical 
dilated  portion  of  the  larva  remains  very  large,  consisting  of  a  prae-oral 
lobe  and  a  post-oral  section.  The  two  together  may  be  regarded  as  con- 
stituting the  head. 

At  a  comparatively  late  stage  a  pair  of  tentacles  arises  from  the  front 


FIG.  145.  POLYGOR- 
DIUS LARVA.  (From 
Alex.  Agassiz.) 


328 


THE   LARVAL   FORM. 


end  of  the  prae-oral  lobe  (fig.  146),  and  finally  the  head  becomes  relatively 
reduced  as  compared  with  the  body,  and  gives  rise  to  the  simple  head  of 


FIG.  146.    POLYGORDIUS  LARVA.    (From  Alex. 
Agassiz.) 

the  fully  formed  worm  (fig.  148).  The  two  ciliated  bands  disappear,  the 
posterior  vanishing  first.  The  ciliated  band  at  the  hind  end  of  the  body 
also  atrophies  ;  and  just  in  front  of  it  the  ring  of  wart-like  prominences  used 
by  the  adult  to  attach  itself  becomes  developed. 

At  the  sides  of  the  head  there  is  formed  a  pair  of  ciliated  pits,  also 
found  by  Hatschek  in  the  embryo  of 
Criodrilus,  and  characteristic  of  many 
Chaetopod  larvae,  but  persistent  in  the 
adult  Polygordius,  Saccocirrus,  Poly- 
ophthalmus,  etc.  They  are  perhaps  the 
same  structures  as  the  ciliated  pits  in 
Nemertines. 

During  the  external  changes  above 
described,  by  which  the  adult  form  of 
Polygordius  is  reached,  a  series  of  in- 
ternal changes  also  takes  place  which  are 
for  the  most  part  the  same  as  in  other 
Chaetopoda ;  and  do  not  require  a  detailed 
description.  The  nervous1  and  muscular 
systems  have  precisely  the  normal  de- 
velopment. The  division  of  the  meso- 
blast  into  somites  is  not  externally  in- 
dicated. The  organs  most  worthy  of 
notice  are  the  excretory  organs. 


FIG.  147.   POLYGORDIUS  LARVA. 
(From  Alex.  Agassiz.) 


The  essential  points  in  the  above  development  of  Polygor- 
dius are  (i)  the  gradual  elongation  and  corresponding  segmenta- 
tion of  the  post-cephalic  part  of  the  body ;  and  (2)  the  relative 
reduction  in  size  of  the  prae-oral  lobe  and  its  conversion  to- 
gether with  the  oral  region  into  the  head  ;  (3)  the  atrophy  of 
the  ciliated  bands.  The  conversion  of  the  larva  into  the  adult 
takes  place  in  fact  by  the  intercalation  of  a  segmented  region 


1  The  structure  of  the  ventral  cord  in  the  adult  requires  further  elucidation. 


CH^TOPODA. 


329 


between  a  large  mouth-bearing  portion  of  the  primitive  body 
and  a  small  anus-bearing  portion1. 

The  general  mode  of  development  of  Chsetopod  larvae  is 
similar  to  the  above  except  in  details,  which  are  however  no 
doubt  often  of  great  importance.  The  history  of  the  larvae  may 


FIG.   148.    POLYGORDIUS  LARVA.    (From  Alex.  Agassiz.) 

be  conveniently  treated  under  three  heads,  (i)  The  form  of 
the  primitive  unsegmented  larva;  (2)  the  arrangement  of  the 
cilia  on  the  unsegmented  larva,  and  on  the  larva  at  later  stages  ; 
(3)  the  character  of  the  metamorphosis  and  the  development  of 
the  permanent  external  organs. 

A  larva  similar  to  the  Polygordius  larva  with  a  greatly 
developed  prae-oral  lobe  is  widely  distributed  amongst  the 
Annelids. 


An  almost  identical  form  is  that  of  Nepthys 
scolopendroides  (Claparede  and  Metschnikoff,  No. 
336)  ;  that  of  Phyllodoce  (fig.  149)  is  also  very 
similar,  and  that  of  Saccocirrus  (Metsch.  and  Clap. 
No.  336,  PL  XIII.  fig.  i),  a  very  primitive  form  most 
nearly  related  to  Polygordius,  clearly  belongs  to  the 
same  type.  Many  other  larval  forms,  such  as  that 
of  Spio  fuliginosus  (Metsch.  and  Clap.  No.  336),  Tere- 
bella,  Nerine,  etc.,  also  closely  approach  this  form. 


FIG.  149.  LARVA  OF 
PHYLLODOCE.  (From 
Alex.  Agassiz.) 


Other  really  similar  forms  at  first  sight 
appear  very  different,  but  this  is  mainly 
owing  to  the  fact  that  their  prae-oral  lobe  never  attains  a 
considerable  development.  Its  smallness,  though  obviously  of 
no  deep  morphological  significance,  at  once  produces  a  very 
different  appearance  in  a  larva. 


1  For  Semper's  view  as  to  the  intercalation  of  segments  in  the  cephalic  region, 
vide  note  on  p.  333. 


330 


THE  LARVAL  FORM. 


A  good  example  of  a  larval  form  with  a  small  prae-oral  lobe  is  afforded  by 
Capitella,  which  is  figured  by  Clap,  and  Metsch.  (No.  336,  PI.  xvn.  fig.  2). 
The  imperfect  development  of  the  prae-oral  lobe  is  also  generally  character- 
istic of  the  Oligochasta.  The  persistence  of  a  relatively  large  pras-oral  lobe 
for  so  long  a  time  as  in  Polygordius  is  very  unusual. 

The  arrangement  of  the  cilia  in  Chaetopod  larvae  has  been 
employed  as  a  means  of  classifying  them.  Although  a  classifi- 
cation so  framed  has  no  morphological  value,  yet  the  terms 
themselves  which  have  been  invented  are  convenient.  The 
terms  most  usually  employed  are  Atrochae,  Monotrochae, 
Telotrochae,  Polytrochae,  Mesotrochae.  The  polytrochae 
may  again  be  subdivided  into  Polytrochae  proper,  Nototrochae, 
Gasterotrochae,  and  Amphitrochae. 

The  atrochae  contain  forms  (fig.  139)  in  which  the  larva  is  at 
first  coated  by  an  uniform  covering  of  cilia,  which,  though  it 
may  subsequently  disappear  from  certain  areas,  does  not  break 
up  into  a  series  of  definite  bands. 

The  monotrochae  or  cephalotrochae  are  larvae  in  which  only  a 
single  prae-oral  ring  is  developed  (fig.  150  B). 

In  the  telotrochae  there  is 
present  a  prae-oral  and  a  post- 
oral,  i.e.  peri-anal  ring  (fig.  150 
A) ;  the  latter  sometimes  hav- 
ing the  form  of  a  peri-anal 
patch. 

The  polytrochae  are  seg- 
mented larvae  with  perfect  or 
imperfect  rings  of  cilia  on  the 
segments  of  the  body — usually 
one  ring  to  each  segment — 
between  the  two  characteristic 


FIG.  150.    Two  CH>ETOPOD  LARVAE. 

(From  Gegenbaur.) 

o.  mouth ;  i.  intestine ;  a.  anus ; 
•v.  pne-oral  ciliated  band ;  w.  peri-anal 
ciliated  band. 


telotrochal  rings.    When  these 

rings  are  complete  the  larvae 

are  polytrochae  proper,  when  they  are  only  half  rings  they  are 

either  nototrochae  or  gasterotrochae.     Sometimes  there  are  both 

dorsal  and  ventral  half  rings  which  do  not  however  correspond, 

such  forms  constitute  the  amphitrochae. 

In  the  mesotrochae  one  or  two  rings  are  present  in  the  middle 
of  the  body,  and  the  characteristic  telotrochal  rings  are  absent. 


CKLETOPODA.  331 


Larvae  do  not  necessarily  continue  to  belong  to  the  same  group 
at  all  ages.  A  larva  may  commence  as  a  monotrochal  form  and 
then  become  telotrochal  and  from  this  pass  into  a  polytrochal 
condition,  etc. 

The  atrochal  forms  are  to  be  regarded  as  larvae  which  never 
pass  beyond  the  primitive  stage  of  uniform  ciliation,  which  in 
other  instances  may  precede  that  of  definite  rings.  They  usually 
lose  their  cilia  early,  as  in  the  cases  of  Serpula  and  other  larvae 
described  below. 

The  atrochal  larvae  are  not  common.  The  following  history  of  an 
Eunicidan  larva  (probably  Lumbriconereis)  from  Claparede  and  Metschni- 
koff  (No.  336)  will  illustrate  their  general  history. 

In  the  earliest  stage  noticed  the  larva  has  a  spherical  form,  the  prae-oral 
lobe  not  being  very  well  marked.  In  the  interior  is  a  globular  digestive 
tract.  The  cilia  form  a  broad  central  band  leaving  free  a  narrow  space  at 
the  apex  of  the  prae-oral  lobe,  and  also  a  circumanal  space.  At  the  apex  of 
the  pras-oral  lobe  is  placed  a  bunch  of  long  cilia,  and  a  patch  of  cilia  also 
marks  out  the  anal  area. 

As  the  larva  grows  older  it  becomes  elongated,  and  the  anterior  bunch  of 
cilia  is  absorbed.  The  alimentary  canal  divides  itself  into  pharynx  and 
intestine.  The  former  opens  (?)  by  the  mouth  in  the  middle  of  the  central 
band  of  cilia,  the  latter  in  the  anal  patch.  The  setae  indicating  the  segmen- 
tation are  formed  successively  in  the  posterior  ring-like  area  free  from  cilia. 
The  cilia  disappear  after  the  formation  of  two  segments. 

In  Lumbricus,  the  embryo  of  which  ought  perhaps  to  be  grouped  with 
the  atrochae,  the  cilia  (Kleinenberg)  cover  a  ventral  tract  of  epiblast  between 
the  two  mesoblastic  cords,  and  are  continued  anteriorly  to  form  a  circle 
round  the  mouth. 

The  monotrochal  larvae  are  provided  only  with  the  important 
prae-oral  ciliated  ring  before  mentioned.  In  the  majority  of 
cases  they  are  transitional  forms  destined  very  shortly  to  become 
telotrochal,  and  in  such  instances  they  usually  have  a  more  or 
less  spherical  body  which  is  nearly  divided  into  two  equal  halves 
by  a  ciliated  ring.  In  some  few  instances,  such  as  Polynoe, 
Dasychone,  etc.,  the  monotrochal  characters  are  not  lost  till  the 
larval  cilia  are  exuviated. 

The  telotrochal  forms  (of  which  examples  are  shewn  in  figs. 
144,  150,  etc.)  may  (i)  start  as  monotrochal;  or  (2)  from  the 
first  have  a  telotrochal  character  ;  or  (3)  be  derived  from  atrochal 
forms.  The  last  mode  of  origin  probably  represents  the  ances- 
tral one. 


332  LARVAL   FORMS. 


Their  mode  of  development  is  well  illustrated  by  the  case  of  Terebella 
nebulosa  (vide  Milne-Edwards,  No.  347).  The  embryo  is  at  first  a  nearly 
spherical  ciliated  mass.  One  end  slightly  elongates  and  becomes  free  from 
cilia,  and,  acquiring  dorsally  two  eye-spots,  constitutes  a  prse-oral  lobe. 
The  elongation  continues  at  the  opposite  end,  and  near  this  is  formed  a 
narrow  area  free  from  cilia.  The  larva  now  has  the  same  characters  as  the 
atrochal  Eunicidan  larva  described  above.  It  consists  of  a  non-ciliated 
prae-oral  lobe,  followed  by  a  wide  ciliated  band,  behind  which  is  a  ring-like 
area  free  from  cilia  ;  and  behind  this  again  a  peri-anal  patch  of  cilia.  The 
ring-like  area  free  from  cilia  is,  as  in  the  Eunicidan  larva,  the  region  which 
becomes  segmented.  It  soon  becomes  longer,  and  is  then  divided  into  two 
segments  ;  a  third  and  fourth  etc.  non-ciliated  segment  becomes  succes- 
sively interposed  immediately  in  front  of  the  peri-anal  patch  ;  and,  after 
a  certain  number  of  segments  have  become  formed,  there  appear  on  some  of 
the  hinder  of  them  short  tubercles,  provided  with  single  setae  (the  notopodia), 
which  are  formed  from  before  backwards,  like  the  segments. 

The  mouth,  anus,  and  intestine  become  in  the  meantime  clearly  visible. 
The  mouth  is  on  the  posterior  side  of  the  ciliated  band,  and  the  anus  in  the 
centre  of  the  peri-anal  patch. 

The  ciliated  band  in  front  now  becomes  contracted  and  provided  with 
long  cilia.  It  passes  below  completely  in  front  of  the  mouth,  and  constitutes, 
in  fact,  a  well-marked  pras-oral  ring,  while  the  cilia  behind  constitute  an 
equally  marked  peri-anal  ring.  The  larva  has  in  fact  now  acquired  all  the 
characters  of  a  true  telotrochal  form. 

Only  a  comparatively  small  number  of  Chsetopod  larvae 
remain  permanently  telotrochal.  Of  these  Terebella  nebulosa, 
already  cited  (though  not  Terebella  conchilega),  is  one ;  Poly- 
gordius,  Saccocirrus  and  Capitella  are  other  examples  of  the 
same,  though  in  the  latter  form  the  whole  ventral  surface 
becomes  ciliated. 

The  majority  of  the  originally  telotrochal  forms  become 
polytrochal. 

In  most  cases  the  ciliated  rings  or  half  rings  of  the  polytro- 
chal forms  are  placed  at  equal  distances,  one  for  each  segment. 
They  are  especially  prominent  in  surface-swimming  larvae,  and 
are  in  rare  cases  preserved  in  the  adult.  In  some  instances 
(e.g.  Nerine  and  Spio)  the  ventral  half  rings,  instead  of  being 
segmentally  arranged,  are  somewhat  irregularly  distributed 
amongst  the  segments,  so  that  there  does  not  seem  to  be  a 
necessary  correspondence  between  the  ciliated  rings  and  the 
segments.  This  is  further  shewn  by  the  fact  that  the  ciliated 
rings  are  not  precursors  of  the  true  segmentation,  but  are 


CH^TOPODA.  333 


developed  after  the  establishment  of  the  segments,  and  thus 
seem  rather  to  be  secondarily  adapted  to  the  segments  than 
primarily  indicative  of  them. 

In  most  Polytrochae  the  rings  are  incomplete,  so  that  they 
fall  under  the  category  of  Nototrochae  or  Gasterotrochae. 

The  larva  of  Odontosyllis  is  an  example  of  the  former,  and  that  of 
Magelona  of  the  latter.  The  larvae  of  Nerine  and  Spio,  already  quoted  as 
examples  of  an  unsegmented  arrangement  of  the  ventral  ciliated  half  rings, 
are  both  amphitrochal  forms. 

As  an  example  of  a  polytrochal  form  with  complete  ciliated  rings  Oph- 
ryotrocha  puerilis  may  be  cited.  This  form,  discovered  by  Claparede  and 
Metschnikoff,  develops  a  complete  ciliated  ring  on  each  segment :  and  the 
prae-oral  ring,  though  at  first  single,  becomes  at  a  later  period  divided  into 
two.  This  form  is  further  exceptional  in  that  the  ciliated  rings  are  persistent 
in  the  adult. 

The  unimportance  of  the  character  of  the  rings  in  the  polytrochal  forms 
is  shewn  by  such  facts  as  the  absence  of  these  rings  in  Terebella  nebulosa 
and  the  presence  of  dorsal  half  rings  in  Terebella  conchilega. 

The  mesotrochal  forms  are  the  rarest  of  Chaetopod  larvae, 
and  would  seem  to  be  confined  to  the  Chaetopteridae. 

Their  most  striking  character  is  the  presence  of  one  or  two  complete 
ciliated  rings  which  girth  the  body  between  the  mouth  and  anus.  The 
whole  body  is  further  covered  with  short  cilia.  The  anus  has  a  distinct 
dorsal  situation,  while  on  its  ventral  side  there  projects  backwards  a  peculiar 
papilla. 

The  total  absence  of  the  typical  prae-oral  and  of  the  peri-anal 
bands  separates  the  mesotrochal  larvae  very  sharply  from  all  the 
previous  types. 

A  characteristic  of  many  Chaetopod  larvae  is  the  presence  of 
a  bunch  of  cilia  or  a  single  flagellum  at  the  apex  of  the  prae-oral 
lobe.  The  presence  of  such  a  structure  is  characteristic  of  the 
larval  forms  of  many  other  groups,  Turbellarians,  Nemertines, 
Molluscs,  etc. 

In  the  preceding  section  the  mode  of  multiplication  of  the 
segments  has  already  been  sufficiently  described1. 

1  It  has  been  insisted  by  Semper  (No.  355)  that  certain  of  the  anterior  segments, 
belonging  to  what  he  regards  as  the  head  region  in  opposition  to  the  trunk,  become 
interpolated  between  the  trunk  and  the  head.  The  general  evidence,  founded  on  ob- 
servations of  budding,  which  he  brings  forward,  cannot  be  discussed  here.  But  the 
special  instance  which  he  cites  (founded  on  Milne-Edwards's  (No.  347)  observations) 


334 


LARVAL  FORMS. 


FIG.  151.    LARVA  OF 
PHYLLODOCE  FROM  THE 

VENTRAL   SIDE.       (From 

Alex.  Agassiz.) 


Apart  from  the  formation  of  the  segments  the  larval  meta- 
morphosis consists  in  the  atrophy  of  the 
provisional  ciliated  rings  and  other  provi- 
sional organs,  and  in  the   acquirement  of 
the  organs  of  the  adult. 

The  great  variations  in  the  nature  of  the 
Chaetopod  appendages  render  it  impossible 
to  treat  this  part  of  the  developmental 
history  of  the  Chaetopoda  in  a  systematic 
way. 

The  mode  of  development  of  the  append- 
ages is  not  constant,  so  that  it  is  difficult  to 
draw  conclusions  as  to  the  primitive  form 
from  which  the  existing  types  of  appendages 
are  derived. 

In  a  large  number  of  cases  the  primitive  rudiments  of  the 
feet  exhibit  no  indication  of  a  division  into  notopodium  and 
neuropodium  ;  while  in  other  instances  (e.g.  Terebella  and  Nerine, 
fig.    152)  the  notopodium  is   first 
developed,  and   subsequently  the 
neuropodium  quite  independently. 

In  many  cases  the  setae  appear 
before  there  are  any  other  visible 
rudiments  of  the  feet  (e.g.  Lumbri- 
conereis)  ;  while  in  other  cases  the 
reverse  holds  good.  The  gills  arc 
usually  the  last  parts  to  appear. 

Not  only  does  the  mode  of 
development  of  the  feet  differ 
greatly  in  different  types,  but  also  the  period.  The  appearance 
of  setae  may  afford  the  first  external  indication  of  segmentation, 
or  the  rudiments  of  the  feet  may  not  appear  till  a  large  number 
of  segments  are  definitely  established. 

A  very  considerable  number  of  Chaetopod  larvae  are  provided 
with  very  long  provisional  setae  (figs.  152  and  153).     These  setae 

of  the  interpolation  of  the  head  segments,  bearing  the  gills,  in  Terebella  appears  to 
me  quite  unjustified  from  Milne-Edwards's  own  statements ;  and  is  clearly  shewn  to 
be  unfounded  by  the  careful  observations  of  Claparede  on  Ter.  conchilega,  where  the 
segments  in  question  are  demonstrated  to  be  present  from  the  first. 


FIG.  152.    LARVA  OF  NERINE, 

WITH    PROVISIONAL    SET>E.      (From 

Alex.  Agassiz.) 


CH^iTOPODA. 


335 


are  usually  placed  at  the  sides  of  the  anterior  part  of  the  body, 
immediately  behind  the  head,  and  also  sometimes  on  the 
posterior  parts  of  the  body.  In  some  instances  (e.g.  fig.  153) 


FIG.  153.     EMRRYO  CH^ETOPOU  WITH  PROVISIONAL  SEIVE.     (From  Agassiz.) 

they  form  the  only  appendages  of  the  trunk.  Alex.  Agassiz  .has 
pointed  out  that  setae  of  this  kind,  though  not  found  in  existing 
Chaetopods,  are  characteristic  of  the  fossil  forms.  Setae  of  this 
kind  are  found  in  chaetopod-like  larvae  of  some  Brachiopods 
(Argiope,  fig.  136). 

It  is  tempting  to  suppose  that  the  long  provisional  bristles 
springing  from  the  oral  region  are  the  setiform  appendages 
handed  down  from  the  unsegmented  ancestors  of  the  existing 
Chaetopod  forms.  Claparede  has  divided  Chaetopod  larvae  into 
two  great  groups  of  Metachaetae  and  Perennichaetae,  according  as 
they  possess  or  are  without  provisional  setae. 

With  reference  to  the  head  and  its  appendages  it  has  already 
been  stated  that  the  head  is  primarily  formed  of  the  prae-oral 
lobe  and  of  the  peristomial  region. 

The  embryological  facts  are  opposed  to  the  view  that  the 
prae-oral  region  either  represents  a  segment  or  is  composed  of 
segments  equivalent  to  those  of  the  trunk.  The  embryonic 
peristomial  region  may,  on  the  other  hand,  be  regarded  as  in  a 
certain  sense  the  first  segment.  Its  exact  relations  to  the 
succeeding  segments  become  frequently  more  or  less  modified  in 
the  adult.  The  prae-oral  region  is  in  most  larvae  bounded 
behind  by  the  ciliated  ring  already  described.  On  the  dorsal 
part  of  the  prae-oral  lobe  in  front  of  this  ring  are  placed  the 
eyes,  and  from  it  there  may  spring  a  variable  number  of 
processes  which  form  antennae  or  cephalic  tentacles.  The 
number  and  position  of  these  latter  are  very  variable.  They 
appear  as  simple  processes,  sometimes  arising  in  pairs,  and  at 


336  LARVAL  FORMS. 


other  times  alternating  on  the  two  sides.  There  is  frequently  a 
median  unpaired  tentacle. 

The  development  of  the  median  tentacle  in  Terebella,  where  there  is  in 
the  adult  a  great  number  of  similar  tentacles,  is  sufficiently  remarkable  to 
deserve  special  notice  ;  vide  Milne-Edwards,  Claparede,  etc.  It  arises  long 
before  any  of  the  other  tentacles  as  a  single  anterior  prolongation  of  the 
prae-oral  lobe  containing  a  parenchymatous  cavity,  which  communicates 
freely  with  the  general  perivisceral  cavity.  It  soon  becomes  partially  con- 
stricted off  at  its  base  from  the  procephalic  lobe,  but  continues  to  grow 
till  it  becomes  fully  half  as  long  as  the  remainder  of  the  body.  A  very 
characteristic  figure  of  the  larva  at  this  stage  is  given  by  Claparede  and 
Metschnikoff,  PI.  XVII.,  Fig.  I  E.  It  now  strikingly  resembles  the  larval 
proboscis  of  Balanoglossus,  and  it  is  not  easy  to  avoid  the  conclusion  that 
they  are  homologous  structures. 

Another  peculiar  cephalic  structure  which  deserves  notice  is  the  gill 
apparatus  of  the  Serpulidae. 

In  Dasychone  (Sabella)  the  gill  apparatus  arises  (Claparede  and 
Metschnikoff,  No.  336)  as  a  pair  of  membranous  wing-like  organs  on  the 
dorsal  side  of  the  prae-oral  lobe  immediately  in  front  of  the  ciliated  ring. 
Each  subsequently  becomes  divided  into 
two  rays,  and  new  rays  then  begin  to  sprout 
on  the  ventral  side  of  the  two  pairs  already 
present.  A  cartilaginous  axis  soon  becomes 
formed  in  these  rays,  and  after  this  is 
formed  fresh  rays  sprout  irregularly  from 
the  cartilaginous  skeleton. 

In  Spirorbis  spirillum  as  observed  by 
Alex.  Agassiz,  the  right  gill-tentacle  (fig. 
154,  /)  first  appears,  and  then  the  left,  and 
subsequently  the  odd  opercular  tentacle 
which  covers  the  right  original  tentacle.  FIG.  154.  LARVA  OF  SPIROR- 

The  third  and  fourth  tentacles  are  formed      BIS.     (From  Alex.  Agassiz.) 
successively  on  the  two  sides,  and  rapidly  The  first  odd  tentacle  (t)  is  shewn 

become  branched  in  the  succeeding  stages.     on  J\?eht,side- 

Behind  the  prse-oral  ciliated  ring 

With    reference    to    the    sense 

organs  it  may  be  noted  that  the  eyes,  or  at  any  rate  the  cephalic 
pigment  spots,  are  generally  more  numerous  in  the  embryo  than 
in  the  adult,  and  that  they  are  usually  present  in  the  larvae  of 
the  Sedentaria,  though  absent  in  the  adults  of  these  forms.  The 
Sedentaria  thus  pass  through  a  larval  stage  in  which  they 
resemble  the  Errantia. 

Paired  auditory  vesicles  of  a  provisional  character  have  been 
found  on  the  ventral  side  of  the  body,  in  the  fourth  segment 


CH^LTOPODA. 


337 


behind    the    mouth,    in    the     larva     of    Terebella    conchilega 
(Claparede). 

Mitraria.  A  peculiar  larval  Ch?etopod  form  known  as  Mitraria,  the 
metamorphosis  of  which  was  first  worked  out  by  Metschnikoff,  deserves  a 
special  notice. 

This  form  (fig.  155  A)  in  spite  of  its  remarkable  appearance  can  easily  be 
reduced  to  the  normal  type  of  larva. 

The  mouth  (m)  and  anus  (an)  (fig.  155  A)  are  closely  approximated,  and 
situated  within  a  vestibule  the  edge  of  which  is  lined  by  a  simple  or  lobed 
ciliated  ring.  The  shape  of  the  body  is  somewhat  conical.  The  cavity  of 


FlG.    155.      TWO   STAGES   IN   THE    DEVELOPMENT  OF   MlTRARIA. 

(After  Metschnikoff.) 

m.  mouth;  an.  anus;  sg.  supra-oesophageal  ganglion;  br.  provisional  bristles; 
pr.b.  prse-oral  ciliated  band. 

the  vestibule  forms  the  base  of  the  cone,  and  at  the  apex  is  placed  a  ciliated 
patch  (sg).  A  pair  of  lobes  (br)  bear  provisional  setae.  The  alimentary 
canal  is  formed  of  the  three  normal  parts,  oesophagus,  stomach,  and 
intestine. 

To  compare  this  larva  with  an  ordinary  Chsetopod  larva  one  must 
suppose  that  the  alimentary  canal  is  abnormally  bent,  so  that  the  post-oral 
ventral  surface  is  reduced  to  the  small  space  between  the  mouth  and  the 
anus.  The  ciliated  band  surrounding  the  vestibule  is  merely  the  usual 
prae-oral  band,  borne  on  the  very  much  extended  edge  of  the  pras-oral  lobe. 
The  apex  of  the  larva  is  the  front  end  of  the  pras-oral  lobe  with  the 
usual  ciliated  patch.  The  two  lobes  with  provisional  bristles  are  really 
dorsal  and  not  posterior. 


B.  II. 


22 


338  FORMATION  OF  ORGANS. 

The  correctness  of  the  above  interpretation  is  clearly  shewn  by  the 
metamorphosis. 

The  first  change  consists  in  the  pushing  in  of  a  fold  of  skin,  between  the 
mouth  and  anus,  towards  the  intestine,  which  at  the  same  time  rapidly 
elongates,  and  forms  the  axis  of  a  conical  projection,  which  thereupon 
becomes  segmented  and  is  thereby  shewn  to  be  the  rudiment  of  the  trunk 
(fig.  155  B).  On  the  elongation  of  the  trunk  in  this  way  the  prae-oral  lobe 
and  its  ciliated  ring  assume  an  appearance  not  very  dissimilar  to  the  same 
structures  in  Polygordius.  At  the  ciliated  apex  of  the  prae-oral  lobe  a  paired 
thickening  of  epiblast  gives  rise  to  the  supra-cesophageal  ganglia  (sg).  In 
the  further  metamorphosis,  the  prae-oral  lobe  and  its  ciliated  ring  gradually 
become  reduced,  and  finally  atrophy  in  the  normal  way,  while  the  trunk 
elongates  and  acquires  setae.  The  dorsally  situated  processes  with  provisional 
setae  last  for  some  time,  but  finally  disappear.  The  young  worm  then 
develops  a  tube  and  shews  itself  as  a  normal  tubicolous  Chaetopod. 

Formation  of  Organs. 

Except  in  the  case  of  a  few  organs  our  knowledge  of  the 
formation  of  the  organs  in  the  Chaetopoda  is  derived  from 
investigations  on  the  Oligochaeta. 

The  embryo  of  the  Oligochaeta.  has  a  more  or  less  spherical 
form,  but  it  soon  elongates,  and  becoming  segmented  acquires  a 
distinct  vermiform  character.  The  ventral  surface  is  however 
for  a  considerable  time  markedly  convex  as  compared  to  the 
dorsal. 

The  ventrally  placed  mouth  is  surrounded  by  a  well-marked 
lip,  and  in  front  of  it  is  placed  a  small  prae-oral  lobe. 

The  epiblast.  The  epiblast  cells  at  the  commencement  of 
the  gastrula  stage  become  much  flattened,  and  on  the  comple- 
tion of  the  invagination  form  an  invest- 
ment  of  flattened  cells,  only  thickened  in 
the  neighbourhood  of  the  mesoblastic 
bands  (fig.  141  B  and  C).  In  the  Poly- 
chaeta  at  any  rate  the  statements  of 
several  investigators  would  seem  to  in- 

dicate that  the  cuticle  is  derived  from  the          FJG      i  6      SFCTION 
chorion.     It    is    difficult    to    accept    this     THROUGH  THE  HEAD  OF  A 


conclusion,   but   it   deserves   further    in- 

VCStigation.  Kleinenberg.  ) 

Nervous    system.      The    most   im-         '-&•  cephalic  ganglion; 

,    ..  cc.  cephalic  portion  of  the 

portant  organ  derived  from  the  epiblast  body  cavity  ;  x.  cesophagus. 


CH^TOPODA. 


339 


FIG.  157.  SECTION  THROUGH  PART 

OF    THE     VENTRAL    WALL     OF     THE 
TRUNK   OF   AN  EMBRYO  OF   LUMBRI- 


is  the  nervous  system ;  the  origin  of  which  from  this  layer  was 
first  established  by  Kowalevsky  (No.  342). 

It  arises1  (Kleinenberg,  No.  341)  from  two  at  first  quite 
distinct  structures,  viz.  (i)  the  supra-cesophageal  rudiment 
and  (2)  the  rudiment  of  the  ventral  cord.  The  former  of 
these  takes  its  origin  as  an  unpaired  dorsal  thickening  of 
the  epiblast  at  the  front  end  of  the  head  (fig.  1 56,  e.g.],  which 
sends  two  prolongations  downwards  and  backwards  to  meet  the 
ventral  cord.  The  latter  arises  as  two  independent  thickenings 
of  the  epiblast,  one  on  each  side 
of  the  ventral  furrow  (fig.  157,  Vg]. 
These  soon  unite  underneath  the 
furrow,  in  the  median  line,  and 
after  being  differentiated  into  seg- 
mentally  arranged  ganglionic  and 
interganglionic  regions  become 
separated  from  the  epiblast.  Both 
the  supra-cesophageal  and  ven- 
tral cord  become  surrounded  by  a  £us  TRAPEZOIDES.  (After  Kleinen- 

berg.) 

layer  of  somatic  mesoblast.     The         Mm  longitudinal  muscles.  S0m  so. 

junction  between  the  tWO  parts   of      matic  mesoblast ;  sp.   splanchnic  me- 
!«  i  . '  i  soblast;   hy.   hypoblast ;    Vg.  ventral 

the  central   nervous    System  takes      nerve  cord  ;  w.  ventral  vessel. 

place  comparatively  late. 

The  mesoblast.  It  is  to  Kowalevsky  (No.  342)  and  Klein- 
enberg (No.  341)  that  we  mainly  owe  our  knowledge  of  the  history 
of  the  mesoblast.  The  fundamental  processes  which  take  place  are 
(i)  the  splitting  of  the  mesoblast  into  splanchnic  and  somatic 
layers  with  the  body  cavity  between  them,  (2)  the  transverse 
division  of  the  mesoblast  of  the  trunk  into  distinct  somites. 

The  former  process  commences  in  the  cephalic  mesoblastic 
commissure,  where  it  results  in  the  formation  of  a  pair  of  cavities 
each  with  a  thin  somatic  and  thick  splanchnic  layer  (fig.  156, 
cc) ;  and  thence  extends  gradually  backwards  into  the  trunk 
(fig.  141  C,  //).  In  the  trunk  however  the  division  into  somites 
precedes  the  horizontal  splitting  of  the  mesoblast.  The  former 
process  commences  when  the  mesoblastic  bands  form  widish 
columns  quite  separate  from  each  other.  These  columns  become 


1  For  further  details,  vide  general  chapter  on  Nervous  System. 

22 — 2 


340  FORMATION   OF   ORGANS. 

broken  up  successively  from  before  backwards  into  somewhat 
cubical  bodies,  in  the  centre  of  which  a  cavity  soon  appears. 
The  cavity  in  each  somite  is  obviously  bounded  by  four  walls, 
(i)  an  outer,  the  somatic,  which  is  the  thickest;  (2)  an  inner,  the 
splanchnic  ;  and  (3,  4)  an  anterior  and  posterior.  The  adjoining 
anterior  and  posterior  walls  of  successive  somites  unite  together 
to  form  the  transverse  dissepiments  of  the  adult,  which  subse- 
quently become  very  thin  and  are  perforated  in  numerous  places, 
thus  placing  in  communication  the  separate  compartments 
of  the  body  cavity.  The  somites,  though  at  first  confined  to  a 
small  area  on  the  ventral  side,  gradually  extend  so  as  to  meet 
their  fellows  above  and  below  and  form  complete  rings  (fig.  157) 
of  which  the  splanchnic  layer  (sp)  attaches  itself  to  the  enteric 
wall  and  the  somatic  (so)  to  the  epiblast.  In  Polygordius  and 
probably  also  Saccocirrus  and  other  forms  the  cavities  of  the 
somites  of  the  two  sides  do  not  coalesce ;  and  the  walls  which 
separate  them  constitute  dorsal  and  ventral  mesenteries.  The 
two  cavities  in  the  cephalic  commissure  unite  dorsally,  but 
ventrally  open  into  the  first  somite  of  the  trunk. 

The  mesoblastic  masses  of  the  head  are  probably  not  to  be  regarded  as 
forming  a  pair  of  somites  equivalent  to  those  in  the  trunk,  but  as  forming 
the  mesoblastic  part  of  the  pras-oral  lobe,  of  which  so  much  has  been  said  in 
the  preceding  pages.  Kleinenberg's  observations  are  however  of  great  im- 
portance as  shewing  that  the  cephalic  cavities  are  simply  an  anterior  part  of 
the  true  body  cavity. 

The  splanchnic  layer  of  the  head  cavity  gives  rise  to  the 
musculature  of  the  oesophagus. 

The  somatic  layer  of  the  trunk  somites  becomes  converted 
into  the  musculature  of  the  body  wall  and  the  external  peri- 
toneal layer  of  body  cavity.  The  first  part  of  the  muscular 
system  to  be  definitely  formed  is  the  ventral  band  of  longitudi- 
nal muscles  which  arises  on  each  side  of  the  nervous  system  in 
contact  with  the  epidermis  (fig.  157,  m).  How  the  circular 
muscles  become  subsequently  formed  outside  these  muscles  has 
not  been  made  out. 

The  splanchnic  layer  of  the  trunk  somites  gives  rise  to  the 
muscular  and  connective-tissue  wall  of  the  mcscntcron,  and  also 
to  the  walls  of  the  vascular  trunks.  The  ventral  vessel  is  first 
formed  (Kowalevsky)  as  a  solid  mass  of  cells  which  subsequently 


CILKTOPODA.  34! 


becomes  hollowed  out.  The  dorsal  vessel  in  Lumbricus  and 
Criodrilus  is  stated  by  Kowalevsky  and  Vejdovsky  to  be  formed 
by  the  coalescence  of  two  lateral  vessels ;  a  peculiarity  which  is 
probably  to  be  explained  by  the  late  extension  of  the  mesoblast 
into  the  dorsal  region. 

The  layer  from  which  the  sacks  for  the  setae  and  the 
segmental  organs  spring  is  still  doubtful.  The  sacks  for  the  setae 
are  believed  by  Kowalevsky  (No.  342)  to  be  epiblastic  invagina- 
tions,  but  are  stated  by  Hatschek  (No.  339)  to  be  mesoblastic 
products.  For  the  development  of  the  segmental  organs  the 
reader  is  referred  to  the  chapter  on  the  excretory  system. 

In  marine  Polychaeta  the  generative  organs  are  no  doubt 
mesoblastic  products,  as  they  usually  spring  from  the  peritoneal 
epithelium,  especially  the  parts  of  it  covering  the  vascular 
trunks. 

The  Alimentary  Canal,  In  Lumbricus  the  enteric  cavity 
is  formed  during  the  gastrula  stage.  In  Criodrilus  the  hypoblast 
has  at  first  no  lumen,  but  this  becomes  very  soon  established. 
In  Euaxes  on  the  other  hand,  where  there  is  a  true  epibolic 
gastrula,  the  mesenteron  is  at  first  represented  by  a  solid  mass 
of  yolk  (i.e.  hypoblastj  cells.  As  the  central  amongst  these 
become  absorbed  a  cavity  is  formed.  The  protoplasm  of  the 
yolk  cells  which  line  this  cavity  unites  into  a  continuous  polynu- 
clear  layer  containing  at  intervals  masses  of  yolk.  These  masses 
become  gradually  absorbed,  and  the  protoplasmic  wall  of  the 
mesenteron  then  breaks  up  into  a  cylindrical  glandular  epithelium 
similar  to  that  of  the  other  types. 

In  Lumbricus  and  Criodrilus  the  blastopore  remains  as  the 
mouth,  but  in  Euaxes  a  new  mouth  or  rather  stomodaeum  is 
formed  by  an  epiblastic  invagination  between  the  front  end  of 
the  two  mesoblastic  bands.  This  epiblastic  invagination  forms 
the  permanent  oesophagus;  and  in  Lumbricus  trapezoides  and 
Criodrilus,  where  the  oral  opening  is  at  first  lined  by  hypoblast, 
the  epiblast  soon  becomes  inflected  so  as  to  line  the  cesophageal 
region.  The  splanchnic  mesoblast  of  the  cephalic  region  subse- 
quently invests  the  oesophagus,  and  some  of  its  cells  penetrating 
between  the  adjoining  epiblast  cells  give  rise  to  a  thick  wall  for 
this  part  of  the  alimentary  tract ;  the  original  epiblast  cells  being 
reduced  to  a  thin  membrane.  This  mesoblastic  wall  is  sharply 


342  ALTERNATIONS   OF   GENERATIONS. 

separated  from  the  muscular  wall  outside,  which  is  also  formed 
of  splanchnic  mesoblast. 

The  anus  is  a  late  formation. 

Alternations  of  generations. 

Amongst  Chaetopoda  a  considerable  number  of  forms  exhibit 
the  phenomenon  of  alternations  of  generations,  which  in  the 
same  general  way  as  in  the  case  of  the  Ccelenterata,  is  second- 
arily caused  by  budding  or  fission. 

The  process  of  fission  essentially  consists  in  the  division  of  a 
parent  form  into  two  zooids  by  the  formation  of  a  zone  of  fission 
between  two  old  rings,  which  becomes  differentiated  (i)  into  an 
anal  zone  in  front  which  forms  the  anal  region  of  the  anterior 
zooid,  and  (2)  into  a  cephalic  zone  behind  which  forms  the  head 
and  some  of  the  succeeding  segments  of  the  posterior  zooid. 
The  anal  zone  is  capable,  by  growth  and  successive  segmenta- 
tion, of  giving  rise  to  an  indefinite  number  of  fresh  segments. 

In  Protula  Dysteri,  as  shewn  by  Huxley,  there  is  a  simple 
fission  into  two  in  the  way  described.  Sexual  reproduction  does 
not  take  place  at  the  same  time  as  reproduction  by  fission, 
but  both  zooids  produced  are  quite  similar  and  multiply 
sexually. 

In  the  freshwater  forms  Nais  and  Chaetogaster  a  more  or 
less  similar  phenomenon  takes  place.  By  a  continual  process  of 
growth  in  the  anal  zones,  and  the  formation  of  fresh  zones  of 
fission  whenever  four  or  five  segments  are  added  in  front  of  an 
anal  zone,  complicated  chains  of  adhering  zooids  are  produced, 
each  with  a  small  number  of  segments.  As  long  as  the  process 
of  fission  continues  sexual  products  are  not  developed,  but  even- 
tually the  chains  break  up,  the  individuals  derived  from  them 
cease  to  go  on  budding,  and,  after  developing  a  considerably 
greater  number  of  segments  than  in  the  asexual  state,  reproduce 
themselves  sexually.  The  forms  developed  from  the  ovum  then 
repeat  again  the  phenomenon  of  budding,  etc.,  and  so  the  cycle 
is  continued1. 

The  phenomena  so  far  can  hardly  be  described  as  cases  of 

1  Reproduction  by  budding  and  formation  of  the  sexual  products  to  some  extent 
overlap. 


CH/ETOPODA.  343 


alternation  of  generations.  The  process  is  however  in  certain 
types  further  differentiated.  In  Syllis  (Quatrefages)  fission 
takes  place,  the  parent  form  dividing  into  two,  of  which  only  the 
posterior  after  its  detachment  develops  sexual  organs.  The 
anterior  asexual  zooid  continues  to  produce  fresh  sexual  zooids 
by  fission.  In  Myrianida  also,  where  a  chain  of  zooids  is  formed, 
the  sexual  elements  seem  to  be  confined  to  the  individuals 
produced  by  budding. 

The  cases  of  Syllis  and  Myrianida  seem  to  be  genuine 
examples  of  alternations  of  generations,  but  a  still  better 
instance  is  afforded  by  Autolytus  (Krohn,  No.  343,  and  Agassiz, 
No.  333). 

In  Autolytus  cornutus  the  parent  stock,  produced  directly 
from  the  egg,  acquires  about  40 — 45  segments,  and  then  gives 
rise  by  fission,  with  the  production  of  a  zone  of  fission  between 
about  the  I3th  and  I4th  rings,  to  a  fresh  zooid  behind.  This 
after  becoming  fully  developed  into  either  a  male  or  a  female  is 
detached  from  the  parent  stock,  from  which  it  very  markedly 
differs.  The  males  and  females  are  moreover  very  different  from 
each  other.  In  the  female  zooid  the  eggs  are  carried  into  a 
kind  of  pouch  where  they  undergo  their  development  and  give 
rise  to  asexual  parent  stocks.  After  the  young  are  hatched  the 
female  dies.  The  asexual  stock,  after  budding  off  one  asexual 
zooid,  elongates  again  and  buds  off  a  second  zooid.  It  never 
develops  generative  organs. 

The  life  history  of  some  species  of  the  genus  Nereis  presents  certain  very 
striking  peculiarities  which  have  not  yet  been  completely  elucidated. 

As  was  first  shewn  by  Malmgren  asexual  examples  of  various  species  of 
Nereis  may  acquire  the  characters  of  Heteronereis  and  become  sexually 
mature. 

The  metamorphosis  of  Nereis  Dumerilii  has  been  investigated  by 
Claparede,  who  has  arrived  at  certain  very  remarkable  conclusions.  He 
finds  that  there  are  two  distinct  sexual  generations  of  the  Nereis  form  of 
this  species,  and  two  distinct  sexual  generations  of  the  Heteronereis  form. 

One  sexual  Nereis,  characterized  by  its  small  size,  is  dioecious,  the  other 
discovered  by  Metschnikoff  is  hermaphrodite. 

Of  the  Heteronereis  sexual  forms,  both  are  dioecious,  one  is  small,  and 
swims  on  the  surface,  the  other  is  larger  and  lives  at  the  bottom. 

How  these  various  generations  are  mutually  related  has  not  been  made 
out ;  but  Claparede  traced  the  passage  of  large  asexual  examples  of  the 
Nereis  form  into  the  large  Heteronereis  form. 


344  CH^iTOPODA. 


BIBLIOGRAPHY. 

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Lyceum  Nat.  Hist,  of  New  York,  Vol.  vin.  1866. 

(333)  Alex.  Agassiz.     "  On  the  embr}'ology  of  Autolytus  cornutus  and  alterna- 
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(334)  W.  Busch.     Beobachtnngcn  ii.  Anat.  u.  Entwick.  einiger  wirbel loser  See- 
thiere, 1851. 

(335)  Ed.  Claparede.     Beobachlungen  ii.  Anat.  it.  Entwick.  wirbelloser  Thiere 
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(336)  Ed.  Claparede  u.  E.  Metschnikoff.     "  Beitrage  z.  Kenntniss  iib.  Ent- 
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(337)  E.  Grube.     Untersuchungen  iib.  Entwicklung  d.  Anneliden.     Konigsberg, 
1844- 

(338)  B.  Hatschek.     "Beitrage  z.  Entwick.  u.  Morphol.  d.  Anneliden."    Sitz. 
d.  k.  Akad.  Wiss.  Wien,  Vol.  LXXIV.  1876. 

(339)  B.  Hatschek.     "  Studien  iiber  Entwicklungsgeschichte  der  Anneliden." 
Arbeiten  aus  d.  zoologischen  Institute  d.  Universitdt  Wien.      Von  C.  Claus.     Heft  III. 
1878. 

(340)  Th.  H.  Huxley.    "On  hermaphrodite  and  fissiparous  species  of  tubicolar 
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(341)  N.  Kleinenberg.     "The  development  of  the  earthworm  Lumbricus  tra- 
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cus  trapezoides.     Napoli,  1878. 

(342)  A.  Kowalevsky.     "  Embryologische  Studien  an  Wurmern  u.  Arthropo- 
den."     Mem.  Acad.  Petersbourg,  Series  VII.  Vol.  XVI.  1871. 

(343)  A.  Krohn.     "  Ueber  die  Erscheinungen  bei  d.  Fortpflanzung  von  Syllis 
prolifera  u.  Autolytus  prolifer."     Archiv f.  Naturgesch.  1852. 

(344)  R.  Leuckart.     "  Ueb.  d.  Jugendzustande  ein.  Anneliden,  etc."    Archiv 
f.  Naturgesch.  1855. 

(345)  S.   Loven.      "  Beobachtungen   u.    die   Metamorphose  von   Anneliden." 
Weigmann's  Archiv,  1842. 

(346)  E.  Metschnikoff.     "  Ueber  die  Metamorphose  einiger  Seethiere  (Mitra- 
ria)."    Zeit.f.  wiss.  Zool.  Vol.  xxi.  1871. 

(347)  M.   Milne-Edwards.      "  Recherches    zoologiques,    etc."     Ann.    Scie. 
Natttr.  in.  Serie,  Vol.  in.  1845. 

(348)  J.   M  tiller.     "  Ueb.  d.   Jugendzustande  einiger  Seethiere."    Monats.  d. 
k.  Akad.  Wiss.     Berlin,  1851. 

(349)  Max  Muller.     "Ueber  d.  weit.  Entwick.  von  Mesotrocha  sexoculata." 
Muller's  Archiv,  1855. 

(350)  Quatrefages.     "  Me"moire  s.  1'embryogenie  des  Annelides."    Ann.  Scie. 
Natur.  in.  Serie,  Vol.  x.  1848. 

(351)  M.  Sars.     "Zur  Entwicklung  d.  Anneliden."     A  re hiv  f.  Naturgeschichte, 
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(352)  A.  Schneider.    "Ueber  Bau  u.  Entwicklung  von  Polygordius."    Muller's 
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CHyfcTOPODA.  345 


(354)  M.  Schultze.     Ueb.  die  Entwicklitng  von  Arenicola  piscatorum  u.  anderer 
Kiemenwiirmer .     Halle,  1856. 

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(358)  R.   v.    Willemoes-Suhm.      "  Biologische   Beobachtungen   U.   niedrige 
Meeresthiere."     Zeit.  f.  wiss.  Zool.  Bd.  xxi.  1871. 


CHAPTER    XIII. 

DISCOPHORA1. 

THE  eggs  of  the  Discophora,  each  enclosed  in  a  delicate 
membrane,  are  enveloped  in  a  kind  of  mucous  case  formed  by  a 
secretion  of  the  integument,  which  hardens  into  a  capsule  or 
cocoon.  In  each  cocoon  there  are  a  limited  number  of  eggs 
surrounded  by  albumen.  The  cocoons  are  attached  to  water- 
plants,  etc.  In  Clepsine  the  embryos  leave  the  cocoon  very 
soon  after  they  get  rid  of  the  egg  membrane,  but  in  Nephelis 
they  remain  within  the  cocoon  for  a  very  much  longer  period 
(27 — 28  days  after  hatching).  The  young  of  Clepsine,  after 
their  liberation,  attach  themselves  to  the  ventral  surface  of  their 
parent. 

Our  knowledge  of  the  development  of  the  Discophora  is  in  a 
very  unsatisfactory  state ;  but  sufficient  is  known  to  shew  that  it 
has  very  many  points  in  common  with  that  of  the  Oligochaeta, 
and  that  the  Discophora  are  therefore  closely  related  to  the 
Chaetopoda.  In  Clepsine  there  is  an  epibolic  gastrula,  and 
mesoblastic  bands  like  those  in  Euaxes  are  also  formed.  In 
Nephelis  however  the  segmentation  is  very  abnormal,  and  the 
formation  of  the  germinal  layers  cannot  easily  be  reduced  to  an 
invaginate  gastrula  type,  though  probably  it  is  modified  from 
such  a  type.  Mesoblastic  bands  similar  to  those  in  the  Oligo- 
chaeta occur  in  this  form  also. 

The  embryology  of  Clepsine,  which  will  serve  as  type  for  the 
Leeches  without  jaws  (Rhyncobdellidae),  has  recently  been 
studied  by  Whitman  (No.  365),  and  that  of  Nephelis,  which  will 

1  The  Discophora  are  divided  into  the  following  groups. 
I.    Rhyncobdellidse. 
II.    Gnathobdellidae. 
III.    Branchiobdellidffi. 


DISCOPHORA. 


347 


serve  as  type  for  the  Leeches  with  jaws  (Gnathobdellidae),  has 
been  studied  by  Butschli  (No.  359).  The  early  history  of  both 
types  is  imperfectly  known1. 

Formation  of  the  layers. 

Clepsine.  It  is  necessary  to  give  a  full  account  of  the  segmentation 
of  Clepsine,  as  the  formation  of  the  germinal  layers  would  be  otherwise 
unintelligible. 

Segmentation  commences  with  the  division  of  the  ovum  into  two  unequal 
spheres  by  a  vertical  cleavage  passing  from  the  animal  to  the  vegetative 
pole.  By  a  second  vertical  cleavage  the  large  segment  is  divided  into  two 
unequal  parts,  and  the  small  one  into  two  equal  parts.  Of  the  four  segments 
so  produced  three  are  relatively  small,  and  one,  placed  at  the  posterior  end, 
is  large.  Each  of  the  four  segments  next  gives  rise  to  a  small  cell  at  the 
animal  pole.  These  small  cells  form  the  commencement  of  the  epiblast, 
and,  according  to  Whitman,  the  mouth  is  eventually  placed  in  their  centre. 
Such  a  position  for  the  mouth,  at  the  animal  pole,  is  extremely  unusual,  and 
the  statements  on  this  head  require  further  confirmation. 

The  posterior  large  segment  now  divides  into  two,  one  of  which  is  dorsal, 
and  the  other  and  larger  ven- 
tral.  The  former  I  shall  call 
with  Whitman  the  neuroblast, 
and  the  latter  the  mesoblast. 
The  mesoblast  very  shortly 
divides  again.  During  the  for- 
mation of  the  neuroblast  and 
mesoblast  additional  epiblastic 
small  cells  are  added  from  the 
three  spheres  which  give  rise 
to  three  of  the  primitive  epi- 
blast cells,  which  may  now  be 
called  the  vitelline  spheres. 

The  neuroblast  next  divides 
into  ten  cells,  of  which  the  two 
smaller  are  soon  broken  up 
into  epiblastic  cells,  while  the 
remaining  eight  arrange  them- 
selves in  two  groups  of  four 
each,  one  group  on  each  side 
at  the  posterior  border  of  the  epiblastic  cap.  The  two  mesoblasts  also  take  up 
a  position  on  the  right  and  left  sides  immediately  ventral  to  the  four  neuro- 
blasts  of  each  side.  The  neuroblasts  and  mesoblasts  now  commence  to 


FlG.    158.      TWO   VIEWS   OF  THE  LARVA  OF 

CLEPSINE.     (After  Whitman.) 

o.  oral  extremity ;   m  mouth ;  pr.  germinal 
streak. 

A.  This  figure  shews  the  blastoderm  (shaded) 
with  a  thickened  edge  formed  by  the  primitive 
(i.e.  mesoblastic)  streaks  with  the  four  so-called 
neuroblasts   posteriorly.     The  vitelline  spheres 
are  left  without  shading. 

B.  represents  an  embryo  in  which  the  blas- 
toderm has  enclosed  the  yolk,  and  in  which  the 
division  into  segments  has  taken  place.     At  the 
hind   end  are  shewn  the  so-called  neuroblasts 
forming  the  termination  of  the  germinal  streak. 


1  Hoffmann's  account  (No.  36)  is  so  different  from  that  of  other  observers  that 
I  have  been  unable  to  make  any  use  of  it. 


348  CLEPSINE. 


proliferate  at  their  anterior  border,  and  produce  on  each  side  a  thickened 
band  of  cells  underneath  the  edge  of  the  cap  of  epiblast  cells.  Each  of  these 
bands  is  formed  of  a  superficial  quadruple1  row  of  neuroblasts  budded  off 
from  the  four  primary  neuroblasts,  and  a  deeper  row  of  mesoblasts.  The 
compound  streaks  so  formed  may  be  called  the  germinal  streaks. 

The  general  appearance  of  the  embryo  as  seen  from  the  dorsal  surface, 
after  the  appearance  of  the  two  germinal  streaks,  may  be  gathered  from 
fig.  158  A.  The  epiblastic  cap  in  this  figure  is  shaded.  The  epiblastic  cap, 
accompanied  by  the  germinal  streaks,  now  rapidly  extends  and  encloses  the 
three  vitelline  spheres  by  a  process  equivalent  to  that  of  an  ordinary  epibolic 
gastrula;  but  the  front  and  hind  ends  of  the  streaks  remain  practically 
stationary.  Owing  to  this  mode  of  growth  the  edges  of  the  epiblastic  cap 
and  the  germinal  streaks  meet  in  a  linear  fashion  along  the  ventral  surface 
of  the  embryo  (fig.  159,  A  and  B).  The  germinal  streaks  first  meet  anteriorly 
(B)  and  their  junction  is  then  gradually  continued  backwards.  The  process 
is  completed  at  about  the  time  of  hatching. 

During  the  above  changes  the  nuclei  of  the  vitelline  spheres  pass  to  the 
surface  and  rapidly  divide.  Eventually,  together  with  part  of  the  protoplasm 
of  the  vitelline  spheres,  they  appear  to  give  rise  to  a  layer  of  hypoblastic 
cells.  This  layer  encloses  the  remains  of  the  vitelline  spheres,  which 
become  the  yolk. 

At  the  front  end  of  the  germinal  streaks,  in  a  position  corresponding  with  that 
of  the  four  original  epiblast  cells, 
two  depressions  appear  which 
coalesce  to  form  the  single  oral 
invagination ;  in  the  centre  of 
which  are  formed  the  mouth  and 
pharynx  by  a  second  epiblastic 
invagination. 

The  most  important  point  in  FIG.  159.    Two  EMBRYOS  OF  CLEPSINE  IN 

connection  with  the  above  history  WHICH  THE  GERMINAL  STREAKS  HAVE  PARTI- 
is  the  fate  of  what  have  been  ALLY^MET  ALONG  THE  VENTRAL  LINE.  (After 

called  the  germinal  streaks.    Ac-  ^  germinal,  i.e.  mesoblastic  streaks, 

cording  to   Whitman    they   are  The  area  covered  by  epiblast  is  shaded. 

composed  of  two  kinds  of  cells,  The  so-called  neuroblasts  at  the  end  of  the 
viz.  four  rows  of  smaller  super-  germinal  streaks  are  shewn  in  B. 
ficial  cells,  which  he  calls  neuroblasts,  and,  in  the  later  stages  at  any  rate,  a 
row  of  deeper  large  cells,  which  he  calls  mesoblasts.  As  to  the  eventual  fate 
of  these  cells  he  states  that  the  neuroblasts  uniting  together  in  the  median 
line  form  the  rudiment  of  the  ventral  ganglionic  chain,  while  the  mesoblasts 
equally  coalesce  and  give  rise  to  the  mesoblast.  Such  a  mode  of  origin  for  a 
ventral  ganglionic  chain  is,  so  far  as  I  know,  without  a  parallel  in  the  whole 
animal  kingdom ;  and  whatever  evidence  Whitman  may  have  that  the  cells 

1  According  to  Robin  it  is  more  usual  for  there  to  be  only  a  triple  row  of  primary 
neuroblasts. 


DISCOPHORA.  349 


in  question  really  do  give  rise  to  the  nervous  system  he  has  not  thought  fit  to 
produce  it  in  his  paper.  He  figures  a  section  with  the  eight  neuroblastic  cells 
in  the  middle  ventral  line,  and  in  the  next  stage  described  the  nervous 
system  is  divided  up  into  ganglia !  The  first  stage,  in  which  the  so-called 
nervous  system  has  the  form  of  a  single  row  of  eight  cells,  is  quite  unlike 
any  rudiment  of  the  nervous  system  such  as  is  usually  met  with  in  the 
Chaetopoda,  and  not  a  single  stage  between  this  and  a  ganglionated  cord  is 
described  or  figured.  Whitman,  whose  views  seem  to  have  been  influenced 
by  a  peculiar,  and  in  my  opinion  erroneous,  theory  of  Rauber's  about  the 
relation  of  the  neural  groove  of  Vertebrata  to  the  blastopore,  does  not  seem 
to  be  aware  that  his  determination  of  the  fate  of  his  neuroblasts  requires  any 
special  support. 

He  quotes  the  formation  of  these  parts  in  Euaxes  (vide  preceding 
Chapter,  p.  324)  as  similar  to  that  in  Clepsine.  In  this  comparison  it 
appears  to  me  probable  that  he  may  be  quite  correct,  but  the  result  of  the 
comparison  would  be  to  shew  that  the  neuroblasts  and  mesoblasts  composed 
together  a  mesoblastic  band  similar  to  that  of  the  Oligochaeta.  Till  more 
evidence  is  brought  forward  by  Whitman  or  some  other  observer  in  support 
of  the  view  that  the  so-called  neuroblasts  have  any  share  in  forming  the 
nervous  system,  they  must  in  my  opinion  be  regarded  as  probably  forming, 
in  conjunction  with  the  mesoblasts,  two  simple  mesoblastic  bands.  Kowa- 
levsky  has  moreover  briefly  stated  that  he  has  satisfied  himself  that  the 
nervous  system  in  Clepsine  originates  from  the  epiblast — a  statement  which 
certainly  could  not  be  brought  into  harmony  with  Whitman's  account. 

Nephelis.  Nephelis  will  form  my  type  of  the  Gnathobdellidae.  The 
segmentation  of  this  form  has  not  yet  been  thoroughly  investigated,  but 
Biitschli's  (No.  359)  observations  are  probably  the  most  trustworthy. 

The  ovum  first  divides  into  two,  and  then  into  four  segments  of  which 
two  are  slightly  smaller  than  the  others.  Four  small  cells  which  form  the 
commencement  of  the  epiblast  are  now  formed.  Three  of  them  are  derived 
by  budding  from  the  two  larger  and  one  of  the  smaller  of  the  four  cells, 
and  the  fourth  from  a  subsequent  division  of  one  of  the  larger  cells1. 
The  three  cells  which  assisted  in  the  formation  of  the  epiblast  cells  again 
give  rise  each  to  a  small  cell ;  and  the  small  cells  so  formed  constitute  a 
layer  underneath  the  epiblast  which  is  the  commencement  of  the  hypoblast, 
while  the  cells  from  which  they  originated  form  the  vitelline  spheres. 
Shortly  after  the  formation  of  the  hypoblast,  the  large  sphere  which  has 
hitherto  been  quiescent  divides  into  two,  one  of  which  then  gives  rise 
in  succession  to  two  small  epiblastic  elements. 

The  two  large  spheres,  resulting  from  the  division  of  the  originally 
quiescent  sphere,  next  divide  again  on  the  opposite  side  of  the  embryo, 
and  form  a  layer  of  epiblast  there  ;  so  that  there  is  now  on  one  side  of 
the  embryo  (the  ventral  according  to  Robin)  a  layer  of  epiblast  formed 

1  Doubts  have  been  cast  by  Whitman  on  the  above  account  of  the  origin  of  the 
four  epiblast  cells. 


350  CLEPSINE. 


of  six  cells,  and  on  the  opposite  side  a  layer  formed  of  four  cells.  The 
two  layers  meet  at  the  front  border  of  the  embryo  and  between  them  are 
placed  the  three  large  vitelline  spheres.  The  two  patches  of  epiblast  cells 
now  rapidly  increase,  and  gradually  spread  over  the  three  large  vitelline 
spheres.  Except  where  they  meet  -each  other  at  the  front  edge  they  leave 
uncovered  a  large  part  of  the  margin  of  the  vitelline  spheres. 

While  these  changes  have  been  taking  place  on  the  exterior,  the 
hypoblast  cells  have  increased  in  number  (additional  cells  being  probably 
derived  from  the  three  large  vitelline  spheres)  and  fill  up  in  a  column-like 
fashion  a  space  which  is  bounded  behind  by  the  three  vitelline  spheres,  and 
in  front  by  the  epiblast  of  the  anterior  end  of  the  embryo.  At  the  sides  of 
the  hypoblast  the  mesoblast  has  become  established,  probably  as  two  lateral 
bands.  The  origin  of  the  cells  forming  it  has  not  yet  been  determined. 
The  hypoblast  cells  in  the  succeeding  stage  arrange  themselves  round  a 
central  archenteric  cavity,  and  at  the  same  time  rapidly  increase  in  size 
and  become  filled  with  a  secondary  deposit  of  food-yolk.  Shortly  after- 
wards a  mouth  and  thick-walled  oesophagus  are  formed,  probably  from  an 
epiblastic  invagination.  The  mesoblast  now  forms  two  curved  lateral 
bands  at  the  two  sides  of  the  body,  equivalent  to  the  mesoblastic  bands 
of  the  Chaetopoda.  The  three  vitelline  spheres,  still  largely  uncovered  by 
the  epiblast,  lie  at  the  posterior  end  of  the  body.  The  embryo  grows 
rapidly,  especially  anteriorly,  and  the  three  vitelline  spheres  become 
covered  by  a  layer  of  flattened  epiblast  cells.  Around  the  oesophagus  a 
cavity  traversed  by  muscular  fibres  is  established.  Elsewhere  there  is  no 
trace  of  such  a  cavity.  The  cephalic  region  becomes  ciliated,  and  the 
dorsal  part  of  it,  which  represents  a  rudimentary  prae-oral  lobe,  is  especially 
prominent.  The  cilia  of  the  oral  region  are  continued  into  the  lumen  of 
the  oesophagus,  and  at  a  later  period  are  prolonged,  as  in  Lumbricus,  along 
the  median  line  of  the  ventral  surface. 

The  mesoblastic  bands  would  seem  from  Biitschli's  observations,  which 
receive  confirmation  from  Kleinenberg's  researches  on  Lumbricus,  to  be  pro- 
longed dorsally  to  the  oesophagus  into  the  cephalic  region.  Posteriorly  they 
abut  on  the  large  vitelline  spheres,  which  were  supposed  by  Kowalevsky 
to  give  origin  to  them,  and  to  play  the  same  part  as  the  large  meso- 
blasts  in  Lumbricus.  It  has  already  been  shewn  that  the  function  of  the 
large  cells  in  Lumbricus  has  been  exaggerated,  and  Biitschli  denies  to 
them  in  Nephelis  any  share  in  the  production  of  the  mesoblast.  It  seems 
in  fact  probable  that  they  are  homologous  with  the  three  vitelline  spheres 
of  Clepsine  ;  and  that  their  primitive  function  is  to  give  origin  to  the 
hypoblast.  They  are  visible  for  a  long  time  at  the  hind  end  of  the  embryo, 
but  eventually  break  up  into  smaller  cells,  the  fate  of  which  is  unknown. 

The  embryo  of  Hirudo  would  appear  from  the  researches  of  Robin 
to  develop  in  nearly  the  same  way  as  that  of  Nephelis.  The  anterior 
part  is  not  however  ciliated.  The  three  large  posterior  cells  disappear 
relatively  early. 


DISCOPHORA.  351 


General  history  of  the  larva. 

The  larva  of  Clepsine,  at  the  time  when  the  mesoblastic 
bands  have  met  along  the  ventral  line,  is  represented  in  fig. 
158  B.  It  is  seen  to  be  already  segmented,  the  process  having 
proceeded  pari  passu  with  the  ventral  coalescence  of  the  meso- 
blastic bands.  The  segments  are  formed  from  before  backwards 
as  in  Chaetopoda.  The  dorsal  surface  is  flat  and  short,  and  the 
ventral  very  convex.  The  embryo  about  this  time  leaves  its 
capsule,  and  attaches  itself  to  its  parent.  It  rapidly  elongates, 
and  the  dorsal  surface,  growing  more  rapidly  than  the  ventral, 
becomes  at  last  the  more  convex.  Eventually  thirty-three  post- 
oral  segments  become  formed  ;  of  which  the  eight  last  coalesce 
to  form  the  posterior  sucker. 

The  general  development  of  the  body  of  Nephelis  and 
Hirudo  is  nearly  the  same  as  that  of  Clepsine.  The  embryo 
passes  from  a  spherical  to  an  oval,  and  then  to  a  vermiform 
shape.  For  full  details  the  reader  is  referred  to  Robin's 
memoir. 

The  presence  of  a  well-marked  protuberance  above  the 
oesophagus,  which  forms  the  rudiment  of  a  prae-oral  lobe,  has 
already  been  mentioned  as  characteristic  of  the  embryo  of 
Nephelis  ;  no  such  structure  is  found  in  Clepsine. 

History  of  the  germinal  layers  and  development  of  organs. 

The  epiblast.  The  epiblast  is  formed  of  a  single  layer  of 
cells  and  early  develops  a  delicate  cuticle  which  is  clearly  formed 
quite  independently  of  the  egg  membrane.  It  becomes  raised 
into  a  series  of  transverse  rings  which  bear  no  relation  to  the 
true  somites  of  the  mesoblast. 

The  nervous  system.  The  nervous  system  is  probably 
derived  from  the  epiblast,  but  its  origin  still  requires  further 
investigation.  The  ventral  cord  breaks  up  into  a  series  of 
ganglia,  which  at  first  correspond  exactly  with  the  somites  of 
the  mesoblast.  Of  these,  four  or  perhaps  three  eventually  coal- 
esce to  form  the  sub-cesophageal  ganglion,  and  seven  or  eight 
become  united  in  the  posterior  sucker. 

It  would  appear  from  Biitschli's  statements  that  the  supra- 


352  NEPHELIS. 


cesophageal  ganglion  arises,  as  in  Oligochaeta,  independently  of 
the  ventral  cord. 

Mesoblast.  It  has  already  been  indicated  that  the  meso- 
blast  probably  takes  its  origin  both  in  Nephelis  and  Clepsine 
from  the  two  mesoblastic  bands  which  unite  in  the  median 
ventral  line.  The  further  history  of  these  bands  is  only  im- 
perfectly known.  They  become  segmented  from  before  back- 
wards. The  somites  formed  by  the  segmentation  gradually 
grow  upwards  and  meet  in  the  dorsal  line.  Septa  are  formed 
between  the  somites  probably  in  the  same  way  as  in  the 
Oligochaeta. 

In  Clepsine  the  mesoblastic  bands  are  stated  by  Kowalevsky  to  be- 
come split  into  somatic  and  splanchnic  layers,  between  which  are  placed 
the  so-called  lateral  sinuses.  These  sinuses  form,  according  to  Whitman, 
a  single  continuous  tube  investing  the  alimentary  tract ;  a  tube  which 
differs  therefore  to  a  very  small  extent  from  the  normal  body  cavity  of 
the  Chaetopoda.  The  somatic  layer  of  mesoblast  no  doubt  gives  rise  to 
the  circular  and  longitudinal  muscular  layers  of  the  embryo.  The  former 
is  stated  to  appear  the  earliest,  while  the  latter,  as  in  the  Oligochaeta, 
first  takes  its  origin  on  the  ventral  side. 

A  delicate  musculature,  formed  mainly  of  transverse  but  also  of  longi- 
tudinal fibres,  would  appear  to  be  developed  independently  of  the  meso- 
blastic bands  in  Nephelis  and  Hirudo  (Rathke,  Leuckart,  Robin,  and 
Biitschli).  It  develops  apparently  from  certain  stellate  cells  which  are 
found  between  the  walls  of  the  alimentary  tract  and  the  skin,  and  which 
probably  correspond  to  the  system  of  contractile  fibres  which  pass  from 
the  body  wall  to  the  alimentary  tract  through  the  segmentation  cavity  in 
the  larva  of  Chaetopoda,  various  Vermes  and  Mollusca1. 

The  mesoblast,  so  far  as  is  known,  gives  rise,  in  addition  to 
the  parts  already  mentioned,  to  the  excretory  organs,  generative 
organs,  vascular  system,  etc. 

Excretory  organs.  There  are  found  in  the  embryo  of  Nephe- 
lis and  Hirudo  certain  remarkable  provisional  excretory  organs 
the  origin  and  history  of  which  is  not  yet  fully  made  out.  In 
Nephelis  they  appear  as  one  (according  to  Robin,  No.  364),  or 
(according  to  Biitschli,  No.  359)  as  two  successive  pairs  of 

1  According  to  Robin  this  system  of  muscles  becomes  gradually  strengthened  and 
converted  into  the  permanent  system.  Rathke  on  the  other  hand  states  that  it  is 
provisional,  and  that  it  is  replaced  by  the  muscles  developed  from  the  mesoblastic 
somites.  It  is  possible  to  suppose  that  it  may  really  become  incorporated  in  the  latter 
system. 


DISCOPHORA.  353 


convoluted  tubes  on  the  dorsal  side  of  the  embryo,  which  are 
stated  by  the  latter  author  to  develop  from  the  scattered  meso- 
blast  cells  underneath  the  skin.  At  their  fullest  development 
they  extend,  according  to  Robin,  from  close  to  the  head  to  near 
the  ventral  sucker.  Each  of  them  is  U-shaped,  with  the  open 
end  forwards,  each  limb  of  the  U  being  formed  by  two  tubes 
united  in  front.  No  external  opening  has  been  clearly  made 
out.  Semper  believed  that  the  tubes  were  continuous  with 
the  three  posterior  vitelline  cells,  but  this  has  been  shewn  not 
to  be  the  case.  Fiirbringer1  is  inclined  from  his  own  re- 
searches to  believe  that  they  open  laterally.  They  contain  a 
clear  fluid. 

In  Hirudo,  Leuckart  (No.  362)  has  described  three  similar 
pairs  of  organs  the  structure  of  which  he  has  fully  elucidated. 
They  are  situated  in  the  posterior  part  of  the  body,  and  each  of 
them  commences  with  an  enlargement  from  which  a  convoluted 
tube  is  continued  for  some  distance  backwards  ;  it  then  turns 
forwards  again  and  afterwards  bends  upon  itself  to  open  to  the 
exterior.  The  anterior  part  is  broken  up  into  a  kind  of  laby- 
rinthic  network. 

The  true  segmental  organs  are  found  in  a  certain  number 
of  the  segments  and  are  stated  (Whitman)  to  develop  from 
groups  of  mesoblast  cells.  Their  origin  requires  however  further 
investigation. 

A  double  row  of  colossal  cells  on  each  side  of  the  body  has  been 
described  in  Clepsine  by  Whitman  as  derived  from  the  mesoblastic  plates. 
These  cells  (fig.  58  B),  which  he  calls  segment-cells,  lie  opposite  the  walls  of 
the  septa.  The  inner  row  is  stated  to  be  connected  with  the  segmental 
organs.  Their  eventual  history  is  unknown,  but  they  are  conjectured 
by  Whitman  to  be  the  mother  cells  of  the  testes. 

The  alimentary  tract.  This  is  formed  primitively  of  two 
parts — the  epiblastic  stomodaeum — forming  mouth,  pharynx, 
and  oesophagus,  and  the  hypoblastic  mesenteron.  The  anus  is 
formed  very  late  as  a  simple  perforation  immediately  dorsal  to 
the  posterior  sucker. 

In  Clepsine,  where  there  is  an  epibolic  gastrula,  the  rudiment 

1  Morphologisches  Jahrbuch,  Vol.  iv.  p.  676.  He  further  speaks  of  the  tube  as 
"  feinverzweigt  u.  netzformig  verastelt,"  but  whether  from  his  own  observations  is 
not  clear. 

B.  II.  23 


354  DEVELOPMENT  OF  ORGANS. 

of  the  mesenteron  is  at  first  formed  of  the  three  vitelline 
spheres,  from  the  surface  of  which  a  true  hypoblastic  layer 
enclosing  a  central  yolk  mass  becomes  differentiated,  as  already 
described.  The  mesenteric  sack  so  formed  is  constricted  by  the 
growth  of  the  mesoblastic  septa  into  a  series  of  lobes,  while  the 
posterior  part  forms  a  narrow  and  at  first  very  short  tube  open- 
ing by  the  anus. 

The  lobed  region  forms  the  sacculated  stomach  of  the  adult. 
The  sacculations  of  the  stomach  by  their  mode  of  origin  neces- 
sarily correspond  with  the  segments.  In  the  adult  however  the 
anterior  lobe  is  really  double  and  has  two  divisions  for  the  two 
segments  it  fills,  while  the  posterior  lobe,  which,  as  is  well 
known,  extends  backwards  parallel  with  the  rectum,  is  composed 
of  five  segmental  sacculations.  In  connection  with  the  stomo- 
daeum  a  protrusible  pharynx  is  developed. 

In  Hirudo  and  Nephelis  the  mesenteron  has  from  the  first  a 
sack-like  form.  The  cells  which  compose  the  sack  give  rise  to  a 
secondary  deposit  of  food-yolk.  The  further  changes  are  prac- 
tically the  same  as  in  Clepsine.  In  Hirudo  the  posterior  saccu- 
lation  of  the  stomach  is  primitively  unpaired.  The  jaws  are 
formed  at  about  the  same  time  as  the  eyes  as  protuberances  on 
the  wall  of  the  oral  cavity. 

BIBLIOGRAPHY. 

(359)  O.  Biitschli.     "  Entwicklungsgeschichtliche  Beitrage  (Nephelis)."     Zeit. 
f.  wiss.  Zool.  Vol.  xxix.  1877. 

(360)  E.  Grube.     Untersuchnngen  iib.  d.  Entwicklung  d.  Aniiclidcu.     Konigs- 
l)crg,  1844. 

(361)  C.K.Hoffmann.     "  Zur  Entwicklungsgeschichte  d.  Clepsineen."     Nie- 
derldnd.  Archiv  f.  Zool.  Vol.  iv.  1877. 

(362)  R.    Leuckart.     Die  mcnschlichen   Parasiten   (Hirudo),   Vol.   i.   |>.   686, 
et  seq. 

(363)  II.  Rathke.     Beit.  z.  Entwicklungsgesch.  d.  Hirudineen.     Leipzig,  1862. 

(364)  Ch.  Robin.     Mfm.  sur  le  Dhjeloppcment  embryogenique  des  Hirudwccs. 
1'aris,  1875. 

(365)  C.  O.  Whitman.     "  Embryology  of  Clepsine."      Quart.   J.  of  Micro. 
Science,  Vol.  xvm.  1878. 

[Vide  also  C.  Semper  (No.  355)  and  Kowalevsky  (No.  342)  for  isolated  observations.] 


CHAPTER  XIV. 

GEPHYREA1. 

IT  is  convenient  for  the  purposes  of  embryology  to  divide 
the  Gephyrea  into  two  groups,  viz.  (i)  Gephyrea  nuda  or  true 
Gephyrea;  and  (2)  Gephyrea  tubicola  formed  by  the  genus 
Phoronis. 

GEPHYREA  NUDA. 
Segmentation  and  formation  of  the  layers. 

An  embolic  or  epibolic  gastrula  is  characteristic  of  the 
Gephyrea,  and  the  blastopore  appears,  in  some  cases  at  any  rate 
(Phascolosoma,  Thalassema),  to  become  the  mouth. 

Bonellia.  In  Bonellia  (Spengel,  No.  370)  the  segmentation 
is  unequal  but  complete,  and,  as  in  many  Molluscs  etc.,  the 
ovum  exhibits  before  its  commencement  a  distinction  into  a 
protoplasmic  and  a  yoke  pole.  The  ovum  first  divides  into  four 
equal  segments,  each  of  them  formed  of  the  same  constituents  as 
the  original  ovum.  At  the  animal  pole  four  small  cells,  entirely 
formed  of  protoplasm,  are  next  formed  by  an  equatorial  furrow. 
They  soon  place  themselves  in  the  intervals  between  the  large 
spheres.  Four  small  cells  are  again  budded  off  from  the  large 
spheres  and  the  eight  small  cells  then  divide.  By  a  further 
continuation  of  the  division  of  the  existing  small  cells,  and  the 
formation  of  fresh  ones  from  the  large  spheres,  a  layer  of  small 

1  The  following  scheme  shews  the  classification  of  the  Gephyrea  adopted  in  the 
present  chapter  :  — 


i.    Gephyrea  nuda.    {« 
ii.    Gephyrea  tubicola  (Phoronis). 

23—2 


356 


SEGMENTATION. 


cells  is  eventually  formed,  which  completely  envelops  the  four 
large  spheres  except  for  a  small  blastopore  at  the  vegetative  pole 
of  the  ovum  (fig.  160  A).  The  large  spheres  continue  to  give 
rise  to  smaller  cells  which  however  no  longer  take  a  superficial 
position  but  lie  within  the  layer  of  small  cells,  and  give  rise  to 
the  hypoblast  (fig.  r6o  B).  The  small  cells  become  the  epiblast, 
and  at  the  blastopore  they  curl  inwards  (fig.  160  B)  and  give 


FIG.  160.     EPIBOLIC  GASTRULA  OF  BONELLIA.    (After  Spengel.) 

A.  Stage  when  the  four  hypoblast  cells  are  nearly  enclosed. 

B.  Stage  after  the  formation  of  the  mesoblast  has  commenced  by  an  infolding  of 
the  lips  of  the  blastopore. 

ep.  epiblast ;  me.  mesoblast ;  bl.  blastopore. 

rise  to  a  layer  of  cells,  which  appears  to  extend  as  an  unbroken 
sheet  between  the  epiblast  and  hypoblast,  and  to  form  the 
mesoblast.  The  blastopore  now  closes  up,  but  its  position  in 
relation  to  the  parts  of  the  embryo  has  not  been  made  out. 

In  Phascolosoma  (Selenka,  No.  369)  the  ovum,  enclosed  in  a 
porous  zona  radiata,  divides  into  two  unequal  spheres,  of  which 
the  smaller  next  divides  into  two  and  then  into  four.  An 
invagination  takes  place  which  is  intermediate  between  the 
embolic  and  the  epibolic  types.  The  small  cells,  the  number  of 
which  is  increased  by  additions  from  the  large  sphere,  divide,  and 
grow  round  the  large  sphere.  The  latter  in  the  meantime  also 
divides,  and  the  cells  produced  from  it  form  on  the  one  hand  a 
small  sack  which  opens  by  the  blastopore,  and  on  the  other  they 
fill  up  the  segmentation  cavity,  and  become  the  mesoblast  and 
blood  corpuscles.  Tin-  Mastoporc  becomes  the  permanent 
mouth. 


GEPHYREA. 


357 


Larval  forms  and  development  of  organs. 

Amongst  the  Gephyrea  armata  the  larva  has  as  a  rule 
(Thalassema,  Echiurus)  the  characters  of  a  trochosphere,  and 
closely  approaches  the  typical  form  characteristic  of  the  larva  of 
Polygordius,  often  known  as  Loven's  larva.  In  Bonellia  this 
larval  form  is  less  perfectly  preserved. 

Echiurus.  In  Echiurus  (Salensky,  No.  368)  the  youngest 
known  larva  has  all  the  typical  trochosphere  characters  (fig.  161). 
It  is  covered  with  cilia  and  divided  into  a  prae-oral  lobe  and 
post-oral  region  of  nearly  equal  dimensions.  There  is  a  double 
ciliated  ring  which  separates  the  two  sections  of  the  body  as  in 
the  larva  of  Polygordius :  the  mouth  (m)  opens  between  its  two 
elements.  The  alimentary  canal  is  divided  into  a  stomodaeum 
with  a  ventral  opening,  a  large  stomach,  and  a  short  intestine 
opening  by  a  terminal  anus  (an).  Connecting  the  oesophagus 
with  the  apex  of  the  prae-oral  lobe  is  the  usual  contractile  band, 
and  at  the  insertion  of  this  band -is  a  thickening  of  the  epiblast 
which  probably  represents  the  rudiment  of  the  supra-oesophageal 
ganglion.  A  ventral  nerve  cord  is  stated  by  Salensky  to  be 
present,  but  his  observations  on  this  point  are  not  quite  satis- 
factory. 

The  metamorphosis  is  accompanied  by  the  loss  of  swimming 
power,  and  consists  in  the 
enlargement  of  the  post-oral 
portion  of  the  trunk,  and  in 
the  simultaneous  reduction 
of  the  prae-oral  lobe,  which 
remains  however  perma- 
nently as  the  cylindrical 
proboscis.  A  groove  which 
terminates  posteriorly  at  the 
mouth  is  very  early  formed 
on  its  ventral  side.  The 
ciliated  rings  gradually  dis- 
appear during  the  metamor- 
phosis. 


FIG.  161.    LARVA  OF  ECHIURUS. 
(After  Salensky.) 


„  _  m.   mouth ;    an.    anus  ;    sg.    supra-ceso 

Of  the   further  external     phageal  ganglion  (?). 


358 


LARVAL  FORMS. 


changes  the  most  important  are  (i)  the  early  appearance 
round  the  anal  end  of  the  body  of  a  ring  of  bristles ;  and  (2)  the 
appearance  of  a  pair  of  ventral  setae  in  the  anterior  part  of  the 
body.  The  anterior  ring  of  bristles  characteristic  of  the  adult 
Echiurus  does  not  appear  till  a  late  period. 

Of  the  internal  changes  the  earliest  is  the  formation  of  the 
anal  respiratory  sacks.  With  the  growth  of  the  posterior  part 
of  the  trunk  the  intestine  elongates,  and  becomes  coiled. 

Bonellia.  The  embryo  of  Bonellia,  while  still  within  the 
egg,  retains  a  spherical  form  and  acquires  an  equatorial  band  of 
cilia,  behind  which  a  second  narrower  band  is  soon  established, 
while  in  front  of  the  first  one  a  pair  of  eye-spots  becomes 


KM;.  162.     THREE  STAGES  IN  THE  DEVELOPMENT  OF  BONELLIA.    (After  Spengel.) 

A.  Larva  with  two  ciliated  bands  and  two  eye-spots. 

B.  Ripe  larva  from  the  dorsal  surface. 

C.  Young  female  Bonellia  from  the  side. 

a/,  alimentary  tract ;  m.  mouth ;  sc.  provisional  excretory  tube ;  s.  ventral  hook  ; 
an.-',  anal  vesicle. 

formed  (fig.  162  A).  The  embryo  on  becoming  hatched  rapidly 
elongates,  while  at  the  same  time  it  becomes  dorso-vcntrally 
flattened  and  acquires  a  complete  coating  of  cilia  (fig.  162  B). 
According  to  Spengel  it  resembles  at  this  time  in  its  form  and 
habits  a  rhabdoccelous  Turbcllarian.  The  anterior  part  is 
however  somewhat  swollen  and  presents  an  indication  of  a 
pre-oral  lobe. 


GEPHYREA.  359 


During  the  above  changes  important  advances  are  made  in  the  forma- 
tion of  the  organs  from  the  embryonic  layers. 

The  epiblast  acquires  a  superficial  cuticula,  which  is  perhaps  directly 
derived  from  the  vitelline  membrane.  The  nervous  system  is  also  formed, 
probably  from  the  epiblast.  The  band-like  supra-cesophageal  ganglion  is 
the  first  part  of  the  nervous  system  formed,  and  appears  to  be  undoubtedly 
derived  from  the  epiblast.  The  ventral  cord  arises  somewhat  later,  but  the 
first  stages  in  its  development  have  not  been  satisfactorily  traced.  It  is 
continuous  with  the  supra-cesophageal  band  which  completely  girths  the 
oesophagus  without  exhibiting  any  special  dorsal  enlargement.  After  the 
ventral  cord  has  become  completely  separated  from  the  epiblast  a  central 
fibrous  mass  becomes  differentiated  in  it,  while  the  lateral  parts  are  composed 
of  ganglion  cells.  In  the  arrangement  of  its  cells  it  presents  indications  of 
being  composed  of  two  lateral  halves.  It  is,  however,  without  ganglionic 
swellings. 

The  mesoblast,  though  at  first  very  thin,  soon  exhibits  a  differentiation 
into  a  splanchnic  and  somatic  layer — though  the  two  do  not  become 
distinctly  separated  by  a  body  cavity.  The  somatic  layer  rapidly  becomes 
thicker,  and  enlarges  laterally  to  form  two  bands  united  dorsally  and 
ventrally  by  narrow,  thinner  bands.  The  outermost  parts  of  each  of  these 
bands  become  differentiated  into  an  external  circular  and  an  internal 
longitudinal  layer  of  muscles.  In  the  pras-oral  lobe  the  mesoblast  assumes 
a  somewhat  vacuolated  character. 

The  hypoblast  cells  form  a  complete  layer  round  the  four  yolk  cells  from 
which  they  arise  (fig.  162  B,  al\  but  at  first  no  alimentary  lumen  is  developed. 
The  oesophagus  appears  during  this  period  as  an,  at  first  solid,  but  subse- 
quently hollow,  outgrowth  of  the  hypoblast  towards  the  epiblast. 

The  metamorphosis  of  the  larva  into  the  adult  female 
Bonellia  commences  with  the  conversion  of  many  of  the  in- 
different mesoblast  cells  into  blood  corpuscles,  and  the  intro- 
duction into  the  body  cavity  of  a  large  amount  of  fluid,  which 
separates  the  splanchnic  and  somatic  layers  of  mesoblast.  The 
fluid  is  believed  by  Spengel  to  be  sea-water,  introduced  by  two 
anal  pouches,  the  development  of  which  is  described  below. 

The  body  cavity  is  lined  by  a  peritoneum,  and  very  soon 
distinct  vessels,  formed  by  folds  of  the  peritoneum,  become 
established.  Of  these  there  are  three  trunks,  two  lateral  and  a 
median  in  the  prae-oral  lobe  (proboscis),  and  in  the  body  a 
ventral  trunk  above  the  nerve  cord,  and  an  intestinal  trunk 
opening  anteriorly  into  the  ventral  one.  The  vessels  appear 
to  communicate  with  the  body  cavity. 

In  the  course  of  the  above  changes  the  two  ciliated  bands 


360  LARVAL   FOR. MX 


disappear,  the  hinder  one  first.  The  cilia  covering  the  general 
surface  become  atrophied,  with  the  exception  of  those  on  the 
ventral  side  of  the  prae-oral  lobe.  The  latter  structure  becomes 
more  prominent ;  the  stellate  mesoblast  cells,  which  fill  up  its 
interior,  become  contractile,  and  it  gives  rise  to  the  proboscis 
(fig.  162  C). 

At  the  point  where  the  cesophageal  protuberance  joined  the  epiblast  at 
a  previous  stage  the  mouth  becomes  established  (fig.  162  C,  ;//),  and  though 
it  is  formed  subsequently  to  the  atrophy  of  the  anterior  ciliated  band,  yet 
there  is  evidence  that  it  is  potentially  situated  behind  this  band.  The  lumen 
of  the  alimentary  canal  becomes  established  by  the  absorption  of  the 
remains  of  the  four  central  cells.  The  anus  is  formed  on  the  ventral  side 
of  the  posterior  end  of  the  body,  and  close  to  it  the  pouches  already  noticed 
grow  out  from  the  hindermost  part  of  the  alimentary  tract  (fig.  162  C,  an.v\ 
They  are  at  first  simple  blind  pouches,  but  subsequently  open  into  the 
body  cavity1.  They  become  the  anal  pouches  of  the  adult.  There  is  present 
when  the  mouth  is  first  formed  a  peculiar  process  of  the  alimentary  tract 
projecting  into  the  prae-oral  lobe,  which  appears  to  atrophy  shortly  after- 
wards. 

After  the  formation  of  the  mouth,  there  are  formed  on  the  ventral  side  of 
and  slightly  behind  it  (i)  anteriorly  a  pair  of  tubes,  which  appear  to  be 
provisional  excretory  organs  and  soon  disappear  (fig.  162  C,  sc}\  and  (2) 
behind  them  a  pair  of  bristles  (s)  which  remain  in  the  adult.  The  formation 
of  the  permanent  excretory  (?)  organ  (oviduct  and  uterus)  has  not  been 
followed  out.  The  ovary  appears  very  early  as  a  differentiation  of  the 
epithelium  lining  the  ventral  vessel. 

The  larvae,  which  become  the  minute  parasitic  males,  undergo 
a  very  different  and  far  less  complete  metamorphosis  than  those 
which  become  females.  They  attach  themselves  to  the  pro- 
boscis of  an  adult  female,  and  lose  their  ciliated  bands.  Germi- 
nal cells  make  their  appearance  in  the  mesoblast,  which  form 
spherical  masses,  and,  like  the  germinal  balls  in  the  female 
ovary,  consist  of  a  central  cell,  and  an  epithelium  around  it. 
The  central  cell  becomes  very  large,  while  the  peripheral  cells 
give  rise  to  the  spermatozoa.  A  body  cavity  becomes  developed 
in  the  larvae,  into  which  the  spermatic  balls  are  dehisced. 
Neither  mouth  nor  anus  is  formed.  The  further  changes  have 
not  been  followed  out. 

1  The  fact  that  these  pouches  are  outgrowths  of  the  alimentary  tract  appears  to 
preclude  the  possibility  of  their  being  homologous  with  excretory  tubes  of  the  Plaly- 
elminthes  and  Rotifera. 


GEPHYREA.  361 


The  larval  males  make  their  way  into  the  oesophagus  of  the 
female,  where  they  no  doubt  live  for  some  time,  and  probably 
become  mature,  though  the  seminal  pouch  of  the  adult  is  not 
found  in  many  of  the  males  living  in  the  oesophagus.  When 
mature  the  males  leave  the  oesophagus,  and  pass  into  the 
uterus. 

Phascolosoma.  Cilia  appear  in  Phascolosoma  (Selenka, 
No.  369)  while  the  ovum  is  still  segmenting.  After  segmentation 
they  form  a  definite  band  immediately  behind  the  mouth,  which 
divides  the  Jarva  into  two  hemispheres — a  prae-oral  and  a  post- 
oral.  A  prae-oral  band  of  cilia  is  soon  formed  close  to  the  post- 
oral  band,  and  at  the  apex  of  the  prae-oral  lobe  a  tuft  of  cilia 
also  appears. 

The  larva  has  now  the  characters  of  a  trochosphere,  but 
differs  from  the  typical  trochosphere  in  the  post-oral  part  of  the 
ciliated  equatorial  ring  being  more  important  than  the  prae-oral, 
and  in  the  absence  of  an  anus. 

The  metamorphosis  commences  very  early.  The  trunk 
rapidly  elongates,  and  the  prae-oral  lobe  becomes  relatively  less 
and  less  conspicuous.  The  zona  radiata  becomes  the  larval 
cuticle. 

Three  pairs  of  bristles  are  formed  on  the  trunk,  of  which  the 
posterior  pair  appears  first,  then  the  anterior,  and  finally  the 
middle  pair :  an  order  of  succession  which  clearly  proves  they 
can  have  no  connection  with  a  true  segmentation. 

The  tentacles  become  developed  between  the  two  parts  of  the 
ciliated  ring,  and  finally  the  prse-oral  lobe,  unlike  what  takes 
place  in  the  Gephyrea  armata,  nearly  completely  vanishes. 

The  anus  appears  fairly  late  on  the  dorsal  surface,  and  the 
ventral  nerve  cord  is  established  as  an  unganglionated  thickening 
of  the  ventral  epiblast. 

GEPHYREA  TUBICOLA. 

The  larva  of  Phoronis  was  known  as  Actinotrocha  long 
before  its  connection  with  Phoronis  was  established  by  Kowa- 
levsky  (No.  372).  There  is  a  complete  segmentation  leading  to 
the  formation  of  a  blastosphere,  which  is  followed  by  an  invagi- 
nation,  the  opening  of  which  is  said  by  Kowalevsky  to  remain  as 


362  ACTINOTROCHA. 


the  mouth1.  It  is  at  first  terminal,  but  on  the  development  of  a 
large  prse-oral  lobe  it  assumes  a  ventral  position.  The  anus 
is  formed  at  a  later  period  at  the  posterior  end  of  the  body. 


FlG.    163.      A   SERIES  OF  STAGES   IN   THE   DEVELOPMENT   OF   PlIORONIS   FROM 

ACTINOTROCHA.     (After  Metschnikoff.) 

A.  Young  larva. 

H.  Larva  after  the  formation  of  post-oral  ring  of  tentacles. 

C.  Larva  with  commencing  invagination  to  form  the  body  of  Phoronis. 

D.  Invagination  partially  everted. 

E.  Invagination  completely  everted. 

m.  mouth  ;  an.  anus;  iv.  invagination  to  form  the  body  of  Phoronis. 

The  youngest  free  larva  observed  by  Metschnikoff  (No.  373) 
was  less  developed  than  the  oldest  larva  found  by  Kowalevsky. 

1  Kowalevsky  states  that  whnt  I  have  called  the  mouth  is  the  anus,  but  his  sub- 
sequent descriptions  shew  that  he  has  transposed  the  mouth  and  anus  in  the  embryo, 
and  that  the  opening,  which  he  asserts  to  be  the  anus,  is  in  reality  the  mouth. 


GEPHYREA.  363 


It  probably  belongs  to  a  different  species.  The  body  is  uni- 
formly ciliated  (fig.  163  A).  There  is  a  large  contractile  prse-oral 
lobe,  and  the  body  ends  behind  in  two  processes.  The  mouth 
(m)  is  ventral,  and  the  anus  (an)  dorsal,  and  not  terminal  as  in 
Kowalevsky's  larva. 

The  alimentary  tract  is  divided  into  stomodseum,  stomach 
and  intestine.  The  two  processes  at  the  hind  end  of  the  body 
are  the  rudiments  of  the  first-formed  pair  of  the  arms  which  are 
so  characteristic  of  the  fully  developed  Actinotrocha.  A  second 
pair  of  arms  next  become  established  on  the  dorsal  side  of  the 
previously  existing  pair,  and  the  region  where  the  anus  is  placed 
grows  out  as  a  special  process.  New  pairs  of  arms  continue  to 
be  formed  in  succession  dorsalwards  and  forwards,  and  soon 
constitute  a  complete  oblique  post-oral  ring  (fig.  163  B).  They  are 
covered  by  long  cilia.  Round  the  anal  process  a  very  conspicuous 
ciliated  ring  also  becomes  established. 

At  the  period  when  five  pairs  of  arms  are  present  a  delicate  membrane 
becomes  visible  on  the  ventral  side  of  the  intestine  which  joins  the  somatic 
mesoblast  anteriorly.  This  membrane  is  the  rudiment  of  the  future  ventral 
vessel.  The  somatic  mesoblast  is  present  even  before  this  period  as  a 
delicate  layer  of  circular  muscular  fibres. 

When  six  pairs  of  arms  have  become  formed  an  involution 
(fig.  163  C,  iv)  appears  on  the  ventral  side,  immediately  behind 
the  ring  of  arms.  This  involution  consists  both  of  the  epiblast 
and  somatic  mesoblast.  It  grows  inwards  towards  the  intestine, 
and,  increasing  greatly  in  length,  becomes  at  the  same  time 
much  folded. 

When  it  has  reached  its  full  development  the  critical  period 
of  the  metamorphosis  of  Actinotrocha  into  Phoronis  is  reached, 
and  is  completed  in  about  a  quarter  of  an  hour.  The  ventral 
involution  becomes  evoluted  (fig.  163  D),  just  as  one  might  turn 
out  the  finger  of  a  glove  which  had  been  pulled  inwards.  When 
the  involution  has  been  to  a  certain  extent  everted,  the  alimen- 
tary canal  passes  into  it,  and  at  the  same  time  the  body  of  the 
larva  becomes  violently  contracted.  By  the  time  the  evagi- 
nation  is  completed  it  forms  (fig.  162  E)  a  long  conical  body, 
containing  the  greater  part  of  the  alimentary  tract,  and  consti- 
tuting the  body  of  the  young-  Phoronis.  The  original  anal  process 
remains  on  the  dorsal  side  as  a  small  papilla  (fig.  162  E,  an). 


364  \<  T1NOTROCHA. 


While  these  changes  have  been  taking  place  the  prae-oral 
lobe  has  become  much  contracted,  and  partly  withdrawn  into 
the  stomodajum.  At  the  same  time  the  arms  have  become  bent 
forward,  so  as  to  form  a  ring  round  the  mouth.  Their  bases 
become  much  thickened.  The  metamorphosis  is  completed  by 
the  entire  withdrawal  of  the  prae-oral  lobe  within  the  oesophagus, 
and  by  the  casting  off  of  the  ends  of  the  arms,  their  bases 
remaining  as  the  circumoral  ring  of  tentacles,  which  form 
however  a  lophophore  rather  than  a  complete  ring.  The 
perianal  ring  of  cilia  is  also  thrown  off,  and  the  anal  process 
withdrawn  into  the  body  of  the  young  Phoronis.  There  are  now 
three  longitudinal  vascular  trunks,  united  anteriorly  by  a  circular 
vessel  which  is  prolonged  into  the  tentacles. 

General  Considerations. 

The  development  of  Phoronis  is  so  different  from  that  of  the 
other  Gephyrea  that  further  investigations  are  required  to  shew 
whether  Phoronis  is  a  true  Gephyrean.  Apart  from  its  peculiar 
metamorphosis  Actinotrocha  is  a  very  interesting  larval  form,  in 
that  it  is  without  a  prae-oral  ciliated  ring,  and  that  the  tentacles 
of  the  adult  are  derived  from  a  true  post-oral  ring,  prolonged 
into  arm-like  processes. 

The  other  Gephyrea  present  in  their  development  an  obvious 
similarity  to  the  normal  Chaetopoda,  but  their  development  stops 
short  of  that  of  the  Chaetopoda,  in  that  they  are  clearly  without 
any  indications  of  a  true  segmentation.  In  the  face  of  what  is 
known  of  their  development  it  is  hardly  credible  that  they  can 
represent  a  degenerate  Chaetopod  phylum  in  which  segmentation 
has  become  lost.  Further  than  this  the  Gephyrea  armata  seem 
in  one  respect  to  be  a  very  primitive  type  in  that  they  retain 
through  life  a  well-developed  pra-oral  lobe,  which  constitutes 
their  proboscis.  In  almost  all  other  forms,  except  Balanoglossus, 
the  larval  prae-oral  lobe  becomes  reduced  to  a  relatively  in- 
significant anterior  part  of  the  head. 

BIBLIOGRAPHY. 
Gephyrea  nuda. 

(366)  A.  Kowalevsky.  Sitz.  d.  zool.  Abth.  d.  III.  Vcrsam.  russ.  Naturj. 
(Thalasscma).  Zeit.f.  wiss.  Zool.  Vol.  xxn.  1872,  p.  284. 


GEPHYREA.  365 


(367)  A.  Krohn.     "Ueb.  d.  Larve  d.  Sipunculus  nudus  nebst  Bemerkungen," 
etc.     Miiller's  Archiv,  1857. 

(368)  M.  Salensky.    "Ueber  die  Metamorphose  d.  Echiurus."    Morphologisches 
Jahrbuch,  Bd.  11. 

(369)  E.  Selenka.     "Eifurchung  u.  Larvenbildung  von  Phascolosoma  elonga- 
tum."    Zeit.f.  wiss.  ZooL  1875,  Bd.  xxv.  p.  i. 

(370)  J.  W.  Spengel.    "Beitrage  z.  Kenntniss  d.  Gephyreen  (Bonellia)."    Mit- 
theil.  a.  d.  zool.  Station  z.  Neapel,  Vol.  I.  1879. 

Gephyrea  tubicola  (Actinotrocha). 

(371)  A.  Krohn.     " Ueb.  Pilidium  u.  Actinotrocha."     Miiller's  Archiv,  1858. 

(372)  A.  Kowalevsky.     "On  anatomy  and  development  of  Phoronis,"  Peters- 
bourg,  1867.     2  PI.     Russian.      Vide  Leuckart's  Bericht,  1866-7. 

(373)  E.  Metschnikoff.     "  Ueber  d.  Metamorphose  einiger  Seethiere  (Actino- 
trocha)."    Zeit.f.  wiss.  Zool.  Bd.  xxi.  1871. 

(374)  J.  Miiller.     " Bericht  lib.  ein.  Thierformen  d.  Nordsee."    Miiller's  Archiv, 
1846. 

(375)  An.  Schneider.     "Ueb.  d.  Metamorphose  d.  Actinotrocha  branchiata." 
Miiller's  Arch.  1862. 


CHAPTER   XV. 


CH/ETOGNATHA,    MYZOSTOMEA   AND   GASTROTRICHA. 

THE  present  chapter  deals  with  three  small  isolated  groups, 
which  only  resemble  each  other  in  that  the  systematic  position  of 
all  of  them  is  equally  obscure. 

Chatognatha. 

The  discoveries  of  Kowalevsky  (No.  378)  confirmed  by 
Btitschli  (No.  376)  with  reference  to  the  development  of  Sagitta, 
though  they  have  not  brought  us  nearer  to  a  knowledge  of  the 
systematic  position  of  this  remarkable  form,  are  nevertheless  of 


FIG.  164.  THREE  STAGES  IN  THE  DEVELOPMENT  OF  SAGITTA.  (A  and  C  after 
Hiitschli  and  B  after  Kowalevsky.)  The  three  embryos  are  represented  in  the  same 
positions. 

A.     The  gastrula  stage. 

li.  A  succeeding  stage  in  which  the  primitive  archenteron  is  commencing  to  be 
divided  into  three  parts,  the  two  lateral  of  which  are  destined  to  form  the  body 
cavity. 

C.  A  later  stage  in  which  the  mouth  involution  (/;/)  has  become  continuous  with 
the  alimentary  tract,  and  the  blastopore  has  become  closed. 

m.  mouth;  al.  alimentary  canal ;  ae.  archenteron ;  bl.p.  blastopore;  pv.  perivisceral 
cavity;  sf>.  splanchnopleuric  mesoblast;  so.  somatopleuric  mesoblast ;  ge.  generative 
organs. 


CH^TOGNATHA.  367 


great  value  for  the  more  general  problems  of  embryology.  The 
development  commences  after  the  eggs  are  laid.  The  segmen- 
tation is  uniform,  and  a  blastosphere,  formed  of  a  single  layer  of 
columnar  cells,  is  the  product  of  it.  An  invagination  takes 
place,  the  opening  of  which  narrows  to  a  blastopore  situated 
at  the  pole  of  the  embryo  opposite  that  at  which  the  mouth 
subsequently  appears  (fig.  164  A).  The  simple  archenteron  soon 
becomes  anteriorly  divided  into  three  lobes,  which  communicate 
freely  with  the  still  single  cavity  behind  (fig.  164  B).  The  two 
lateral  lobes  are  destined  to  form  the  body  cavity,  and  the 
median  lobe  the  alimentary  tract  of  the  adult.  An  invagination 
soon  arises  at  the  opposite  pole  of  the  embryo  to  the  blastopore 
and  forms  the  mouth  and  oesophagus  (fig.  164  B  and  C,  m). 

At  the  gastrula  stage  there  is  formed  a  paired  mass  destined 
to  give  rise  to  the  generative  organs.  It  arises  as  a  prominence 
of  six  cells,  projecting  from  the  hypoblast  at  the  anterior  pole  of 
the  archenteron,  and  soon  separates  itself  as  a  mass,  or  probably 
a  pair  of  masses,  lying  freely  in  the  cavity  of  the  archenteron 
(fig.  164  A.yge).  When  the  folding  of  the  primitive  cavity  takes 
place  the  generative  rudiment  is  situated  at  the  hind  end  of  the 
median  lobe  of  the  archenteron  in  the  position  represented 
in  fig.  164  C,  ge. 

An  elongation  of  the  posterior  end  of  the  embryo  now  takes 
place,  and  the  embryo  becomes  coiled  up  in  the  egg,  and  when 
eventually  hatched  sufficiently  resembles  the  adult  to  be  recog- 
nisable as  a  young  Sagitta. 

Before  hatching  takes  place  various  important  changes 
become  manifest.  The  blastopore  disappears  after  being  carried 
to  the  ventral  surface.  The  middle  section  of  the  trilobed  region 
of  the  archenteron  becomes  separated  from  the  unpaired 
posterior  part,  and  forms  a  tube,  blind  behind,  but  opening 
in  front  by  the  mouth  (fig.  165  A,  al).  It  constitutes  the  perma- 
nent alimentary  tract,  and  is  formed  of  a  pharyngeal  epiblastic 
invagination,  and  a  posterior  hypoblastic  section  derived  from 
the  primitive  archenteron.  The  anus  is  apparently  not  formed 
till  comparatively  late.  After  the  isolation  of  the  alimentary 
tract  the  remainder  of  the  archenteron  is  formed  of  two  cavities 
in  front,  which  open  freely  into  a  single  cavity  behind  (fig. 
165  A).  The  whole  of  it  constitutes  the  body  cavity  and  its  walls 


368 


CH/ETOGNATHA. 


f/ic  mesoblast.  The  anterior  paired  part  becomes  partitioned  off 
into  a  head  section  and  a  trunk  section  (fig.  165  A  and  B).  The 
former  constitutes  a  pair  of  distinct  cavities  (c.pv)  in  the  head, 
and  the  latter  two  cavities  opening  freely  into  the  unpaired 
portion  behind.  At  the  junction  of  the  paired  cavities  with  the 
unpaired  cavity  are  situated  the  generative  organs  (ge).  The 
inner  wall  of  each  of  the  paired  cavities  forms  the  splanchno- 
pleuric  mesoblast,  and  the  outer  wall  of  the  whole  the  somatic 
mesoblast.  The  inner  walls  of  the  posterior  cavities  unite  above 
and  below  the  alimentary  tract,  and  form  the  dorsal  and  ventral 
mesenteries,  which  divide  the  body  cavity  into  two  compartments 
in  the  adult.  Before  the  hatching  of  the  embryo  takes  place 
this  mesentery  is  continued  backwards  so  as  to  divide  the 
primitively  unpaired  caudal  part  of  the  body  cavity  in  the 
same  way. 

From  the  somatic  mesoblast  of  the  trunk  is  derived  the 
single  layer  of  longitudinal  muscles  of  Sagitta,  and  part  of  the 
epithelioid  lining  of  the  body  cavity.  The  anterior  termination 
of  the  trunk  division  of  the  body  cavity  is  marked  in  the  adult 
by  the  mesentery  dividing  into  two  laminae,  which  bend  outwards 
to  join  the  body  wall. 

The  cephalic  sec- 
tion of  the  body  cavity 
seems  to  atrophy,  and 
its  walls  to  become  con- 
verted into  the  compli- 
cated system  of  muscles 
present  in  the  head  of 
the  adult  Sagitta. 

In  the  presence  of 
a  section  of  the  body 
cavity  in  the  head  the 
embryo  of  Sagitta  re 
sembles  Lumbricus, 
Spiders,  etc. 

The  generative  ru- 
diment of  each  side 
divides  into  an  anterior 
and  a  posterior  part 


In;.  [65.  Two  VIEWS  OF  A  LATE  EMBRYO  OF 
SV.ITTA.  A.  from  the  dorsal  surface.  I?,  from  the 
tide.  (After  15iitschli.) 

m.  mouth ;  al.  alimentary  canal ;  v.g.  ventral 
ganglion  (thickening  of  epiblast) ;  rp.  epiblast  ;  c.pv, 
cephalic  section  of  body  cavity;  so.  somatopleure ; 
s/>.  splanchnopleure  ;  ,;v.  generative 


CH^ETOGNATHA.  369 


(fig.  165,  ge].  The  former  constitutes  the  ovary,  and  is  situated 
in  front  of  the  septum  dividing  the  tail  from  the  body  ;  and  the 
latter,  in  the  caudal  region  of  the  trunk,  forms  the  testis. 

The  nervous  system  originates  from  the  epiblast.  There  is  a 
ventral  thickening  (fig.  165  B,  v.g)  in  the  anterior  region  of  the 
trunk,  and  a  dorsal  one  in  the  head.  The  two  are  at  first 
continuous,  and  on  becoming  separated  from  the  epiblast  remain 
united  by  thin  cords. 

The  ventral  ganglion  is  far  more  prominent  during  embryonic 
life  than  in  the  adult.  Its  position  and  early  prominence  in  the 
embryo  perhaps  indicate  that  it  is  the  homologue  of  the  ventral 
cord  of  Chaetopoda1. 

BIBLIOGRAPHY. 

(376)  O.  Biitschli.      "Zur  Entwicklungsgeschichte  der  Sagitta."     Zeitschrift f. 
wiss.  Zoo!.,  Vol.  xxni.  1873. 

(377)  C.  Gegenbaur.     "  Uber  die  Entwicklung  der  Sagitta."     Abhand.  d.  na- 
turforschenden  Gesellschaft  in  Halle,  1857. 

(378)  A.  Kowalevsky.     " Embryologische  Studien  an  Wiirmern  u.  Arthropo- 
den."     Mem.  Acad.  Petersbourg,  VII.  ser.,  Tom.  XVI.,  No.  12.     1871. 

MYZOSTOMEA. 

The  development  of  these  peculiar  parasites  on  Crinoids  has  been 
investigated  by  Metschnikoff  (No.  380),  Semper  (No.  381),  and  Graff 
(No.  379). 

The  segmentation  is  unequal,  and  would  appear  to  be  followed  by  an 
epibolic  invagination.  The  outer  layer  of  cells  (epiblast)  becomes  covered 
with  cilia,  and  the  inner  is  transformed  into  a  non-cellular  (?)  central  yolk 
mass.  At  this  stage  the  larva  is  hatched,  and  commences  to  lead  a  free 
existence.  In  the  next  stage  observed  by  Metschnikoff,  the  mouth,  oeso- 
phagus, stomach,  and  anus  had  become  developed  ;  and  two  pairs  of  feet 
were  present.  In  both  of  these  feet  Chaetopod-like  setae  were  present,  which 
in  the  hinder  pair  were  simple  fine  bristles  without  a  terminal  hook.  The 
papilliform  portion  of  the  foot  is  at  first  undeveloped.  The  feet  become 
successively  added,  like  Chaetopod  segments,  and  the  stomach  does  not 
become  dendriform  till  the  whole  complement  of  feet  (5  pairs)  are  present. 

In  the  primitive  covering  of  cilia,  combined  with  a  subsequent  indication 

1  Langerhans  has  recently  made  some  important  investigations  on  the  nervous 
system  of  Sagitta,  and  identifies  the  ventral  ganglion  with  the  parieto-splanchnic  gan- 
glia of  Molluscs,  while  he  has  found  a  pair  of  new  ganglia,  the  development  of  which 
is  unknown,  which  he  calls  the  suboesophageal  or  pedal  ganglia.  The  embryolo- 
gical  facts  do  not  appear  to  be  in  favour  of  these  interpretations. 

B.  II.  24 


3/0  MYZOSTOMEA. 


of  segments  in  the  formation  of  the  feet  and  setae,  the  larva  of  the  Myzo- 
stomea  shews  an  approximation  to  the  Chaetopoda,  and  the  group  is 
probably  to  be  regarded  as  an  early  Chactopod  type  specially  modified  in 
connection  with  its  parasitic  habits. 

BIBLIOGRAPHY. 

(379)  L.Graff.     Das  Genus  Myzostoma.     Leipzig,  1877. 

(380)  E.    Metschnikoff.      "Zur    Entwicklungsgeschichte   d.    Myzostomum." 
Zfit.f.  wiss.  Zool.y  Vol.  XVI.     1866. 

(381)  C.  Semper.    "Z.  Anat.  u.  Entwick.  d.  Gat.  Myzostomum."    Ztit.f.  wiss. 
Zool.,  Vol.  ix.     1858. 

GASTROTRICHA. 

A  few  observations  of  Ludwig  on  the  winter  eggs  of  Ichthydium  larus 
shew  that  the  segmentation  is  a  total  and  apparently  a  regular  one.  It 
leads  to  the  formation  of  a  solid  morula.  The  embryo  has  a  ventral 
curvature,  and  the  caudal  forks  are  early  formed  as  cuticular  structures.  By 
the  time  the  embryo  leaves  the  egg,  it  has  almost  reached  the  adult  state. 
The  ventral  cilia  arise  some  little  time  prior  to  the  hatching. 

BIBLIOGRAPHY. 

(382)  H.  Ludwig.     "  Ueber  die  Ordnung  Gastrotricha  Mctschn"    Zeit.  f.  wiss. 
Zool.,  Vol.  xxvi.     1876. 


CHAPTER   XVI. 

NEMATELMINTHES  AND  ACANTHOCEPHALA. 

NEM  ATELMINTHES  '. 

Nematoidea.  Although  the  ova  of  various  Nematodes  have 
formed  some  of  the  earliest,  as  well  as  the  most  frequent  objects 
of  embryological  observation,  their  development  is  still  but  very 
imperfectly  known.  Both  viviparous  and  oviparous  forms  are 
common,  and  in  the  case  of  the  oviparous  forms  the  eggs  are 
usually  enveloped  in  a  hard  shell.  The  segmentation  is  total 
and  nearly  regular,  though  the  two  first  segments  are  often 
unequal.  The  relation  of  the  segmentation  spheres  to  the 
germinal  layers  is  however  only  satisfactorily  established  (through 
the  researches  of  Butschli  (No.  383))  in  the  case  of  Cucullanus 
elegans,  a  form  parasitic  in  the  Perch2. 

The  early  development  of  this  embryo  takes  place  within 
the  body  of  the  parent,  and  the  egg  is  enveloped  in  a  delicate 
membrane.  After  the  completion  of  the  early  stages  of  seg- 
mentation the  embryo  acquires  the  form  of  a  thin  flat  plate 
composed  of  two  layers  of  cells  (fig.  166  A  and  B).  The  two 
layers  of  this  plate  give  rise  respectively  to  the  epiblast  and 
hypoblast,  and  at  a  certain  stage  the  hypoblastic  layer  ceases  to 

1  The  following  classification  of  the  Nematoda  is  employed  in  this  chapter : 

r  Ascaridae. 
Strongylidae. 

Trichinidse.  II.    Gordioidea. 

I.    Nematoidea. ,  Filarid8B.  m.    Chaetosomoidea. 

Mermithidae. 
[_  Anguillulidse. 

2  The  ova  of  Anguillula  aceti  are  stated  by  Hallez  to  undergo  a  similar  develop- 
ment to  those  of  Cucullanus. 

24—2 


372 


CUCULLANUS. 


grow,  while  the  growth  of  the  epiblastic  layer  continues.  As  a 
consequence  of  this  the  sides  of  the  plate  begin  to  fold  over 
towards  the  side  of  the  hypoblast  (fig.  166  D.)  This  folding 
results  in  the  formation  of  a  remarkably  constituted  gastrula, 
which  has  the  form  of  a  hollow  two-layered  cylinder  with  an 
incompletely  closed  slit  on  one  side  (fig.  166  E,  bl.p}.  This  slit 
has  the  value  of  a  blastopore.  It  becomes  closed  by  the  coales- 
cence of  the  two  edges,  a  process  which  commences  posteriorly, 


FIG.  166. 


A. 
B. 
C. 


VARIOUS   STAGES   IN   THE   DEVELOPMENT  OF  CUCULLANUS   ELEGANS. 

(From  Biitschli.) 

Surface  view  of  flattened  embryo  at  an  early  stage  in  the  segmentation. 
Side  view  of  an  embryo  at  a  somewhat  later  stage,  in  optical  section. 
Flattened  embryo  at  the  completion  of  segmentation. 

D.  Embryo  at  the  commencement  of  the  gastrula  stage. 

E.  Embryo  when  the  blastopore  is  reduced  to  a  mere  slit. 

F.  Vermiform  embryo  after  the  division  of  the  alimentary  tract  into  oesophageal 
and  glandular  divisions. 

m.  mouth;  ep.  epiblast;  hy.  hypoblast;  me.  mesoblast;  a?,  oesophagus;  bl.p.  blas- 
topore. 

and  then  gradually  extends  forwards.  In  front  the  blastopore 
never  becomes  completely  closed,  but  remains  as  the  permanent 
mouth.  The  embryo  after  these  changes  has  a  worm-like  form, 
which  becomes  the  more  obvious  as  it  grows  in  length  and 
becomes  curved  (fig.  166  F). 

The  hypoblast  of  the  embryo  gives  rise  to  the  alimentary 


NEMATELM1NTHES.  373 


canal,  and  soon  becomes  divided  into  an  cesophageal  section 
(fig.  1 66  F,  ce)  formed  of  granular  cells,  and  a  posterior  division 
formed  of  clear  cells.  The  mesoblast  (fig.  166,  me)  takes  its 
origin  from  certain  special  hypoblast  cells  around  the  mouth, 
and  thence  grows  backwards  towards  the  posterior  end  of  the 
body. 

The  young  Cucullanus  becomes  hatched  while  still  in  the 
generative  ducts  of  its  parent,  and  is  distinguished  by  the 
presence  of  a  remarkable  thread-like  tail.  On  the  dorsal  surface 
is  a  provisional  boring  apparatus  in  the  form  of  a  conical  papilla. 
A  firm  cuticle  enveloping  the  body  is  already  present.  In  this 
condition  it  leaves  its  parent  and  host,  and  leads  for  a  time  a 
free  existence  in  the  water.  Its  metamorphosis  is  dealt  with  in 
another  section. 

The  ova  of  the  Oxyuridae  parasitic  in  Insects  are  stated  by  Galeb  (No. 
386)  to  take  the  form  of  a  blastosphere  at  the  close  of  segmentation.  An 
inner  layer  is  then  formed  by  delamination.  What  the  inner  layer  gives  rise 
to  is  not  clear,  since  the  whole  alimentary  canal  is  stated  to  be  derived  from 
two  buds,  which  arise  at  opposite  ends  of  the  body,  and  grow  inwards  till 
they  meet. 

The  generative  organs.  The  study  of  the  development  of 
the  generative  organs  of  Nematodes  has  led  to  some  interesting 
results.  In  the  case  of  both  sexes  the  generative  organs  origi- 
nate (Schneider,  No.  390)  from  a  single  cell.  This  cell  elongates 
and  its  nuclei  multiply.  After  assuming  a  somewhat  columnar 
form,  it  divides  into  (i)  a  superficial  investing  layer,  and  (2)  an 
axial  portion. 

In  the  female  the  superficial  layer  is  only  developed  dis- 
tinctly in  the  median  part  of  the  column.  In  the  course  of  the 
further  development  the  two  ends  of  the  column  become  the 
blind  ends  of  the  ovary,  and  the  axial  tissue  they  contain  forms 
the  germinal  tissue  of  nucleated  protoplasm.  The  superficial 
layer  gives  rise  to  the  epithelium  of  the  uterus  and  oviduct. 
The  germinal  tissue,  which  is  originally  continuous,  is  interrupted 
in  the  middle  part  (where  the  superficial  layer  gives  rise  to  the 
uterus  and  oviduct),  and  is  confined  to  the  two  blind  extremities 
of  the  tube. 

In  the  male  the  superficial  layer,  which  gives  rise  to  the  epi- 
thelium of  the  vas  deferens,  is  only  formed  at  the  hinder  end  of 


374  METAMORPHOSIS. 


the  original  column.     In  other  respects  the  development  takes 
place  as  in  the  female. 

Gordioidea.  The  ovum  of  Gordius  undergoes  a  regular  segmentation. 
According  to  Villot  (No.  391)  it  forms  at  the  close  of  segmentation  a  morula, 
which  becomes  two-layered  by  delamination.  The  embryo  is  at  first 
spherical,  but  soon  becomes  elongated. 

By  an  invagination  at  the  anterior  extremity  the  head  is  formed.  It 
consists  of  a  basal  portion,  armed  with  three  rings  of  stylets,  and  a  conical 
proboscis,  armed  with  three  large  stylets.  When  the  larva  becomes  free 
the  head  becomes  everted,  though  it  remains  retractile.  By  the  time  the 
embryo  is  hatched  a  complete  alimentary  tract  is  formed  with  an  oral  opening 
at  the  end  of  the  proboscis,  and  a  subterminal  ventral  anal  opening.  It  is 
divided  into  an  oesophagus  and  stomach,  and  a  large  gland  opens  into  it  at 
the  base  of  the  proboscis. 

The  body  has  a  number  of  transverse  folds,  which  give  it  a  ringed 
appearance. 

Metamorphosis  and  life  history. 

Nematoidea.  Although  a  large  number  of  Nematodes  have 
a  free  existence  and  simple  life  history,  yet  the  greater  number 
of  known  genera  are  parasitic,  and  undergo  a  more  or  less  com- 
plicated metamorphosis1.  According  to  this  metamorphosis 
they  may  be  divided  into  two  groups  (which  by  no  means 
closely  correspond  with  the  natural  divisions),  viz.  those  which 
have  a  single  host,  and  those  with  two  hosts.  Each  of  these 
main  divisions  may  be  subdivided  again  into  two. 

In  the  first  group  with  one  host  the  simplest  cases  are  those 
in  which  the  adult  sexual  form  of  parasite  lays  its  eggs  in  the 
alimentary  tract  of  its  host,  and  the  eggs  are  thence  transported 
to  the  exterior.  The  embryo  still  in  the  egg,  if  favoured  by 
sufficient  warmth  and  moisture,  completes  its  development  up 
to  a  certain  point,  and,  if  then  swallowed  by  an  individual  of  the 
species  in  which  it  is  parasitic  in  the  adult  condition,  it  is 
denuded  of  its  shell  by  the  action  of  the  gastric  juice,  and 
develops  directly  into  the  sexual  form. 

Leuckart  has  experimentally  established  this  metamorphosis  in  the  case 
of  Trichocephalus  affinis,  Oxyurus  ambigua,  and  Heterakis  vermicularis. 
The  Oxyuridae  of  Blatta  and  Hydrophilus  have  a  similar  life  history 

1  The  following  facts  are  mainly  derived  from  Leuckart's  exhaustive  treatise 
(No.  388). 


NEMATELMINTHES.  375 


(Caleb,  No.  386),  and  it  is  almost  certain  that  the  metamorphosis  of  the 
human  parasites,  Ascaris  lumbricoides  and  Oxyurus  vermicularis,  is  of  this 
nature. 

A  slightly  more  complicated  metamorphosis  is  common  in 
the  genera  Ascaris  and  Strongylus.  In  these  cases  the  egg-shell 
is  thin,  and  the  embryo  becomes  free  externally,  and  enjoys  for 
a  shorter  or  longer  period  a  free  existence  in  water  or  moist 
earth.  During  this  period  it  grows  in  size,  and  though  not 
sexual  usually  closely  resembles  the  adult  form  of  the  perma- 
nently free  genus  Rhabditis.  In  some  cases  the  free  larva 
becomes  parasitic  in  a  freshwater  Mollusc,  but  without  thereby 
undergoing  any  change.  It  eventually  enters  the  alimentary 
tract  of  its  proper  host  and  there  become  sexual. 

As  examples  of  this  form  of  development  worked  out  by  Leuckart  may 
be  mentioned  Uochmius  trigonocephalus,  parasitic  in  the  dog,  and  Ascaris 
acuminata,  in  the  frog.  The  human  parasite  Dochmius  duodenale  under- 
goes the  same  metamorphosis  as  Dochmius  trigonocephalus. 

A  remarkable  modification  of  this  type  of  metamorphosis  is  found  in 
Ascaris  (Rhabdonema)  nigrovenosa,  which  in  its  most  developed  condition 
is  parasitic  in  the  lungs  of  the  frog  (Metschnikoff,  Leuckart,  No.  388).  The 
embryos  pass  through  their  first  developmental  phases  in  the  body  of  the 
parent.  They  have  the  typical  Rhabditis  form,  and  make  their  way  after 
birth  into  the  frog's  rectum.  From  this  they  pass  to  the  exterior,  and  then 
living  either  in  moist  earth,  or  the  faeces  of  the  frog,  develop  into  a  sexual 
form,  but  are  very  much  smaller  than  in  the  adult  condition.  The  sexes  are 
distinct,  and  the  males  are  distinguished  from  the  females  by  their  smaller 
size,  shorter  and  rounded  tails,  and  thinner  bodies.  The  females  have 
paired  ovaries  with  a  very  small  number  of  eggs,  but  the  testis  of  the  males 
is  unpaired.  Impregnation  takes  place  in  the  usual  way,  and  in  summer 
time  about  four  embryos  are  developed  in  each  female,  which  soon  burst 
their  egg-capsules,  and  then  move  freely  in  the  uterus.  Their  active  move- 
ments soon  burst  the  uterine  walls,  and  they  then  come  to  lie  freely  in  the 
body  cavity.  The  remaining  viscera  of  the  mother  are  next  reduced  to  a 
finely  granular  material,  which  serves  for  the  nutrition  of  the  young  forms 
which  continue  to  live  in  the  maternal  skin.  The  larvae  eventually  become 
free,  and  though  in  many  respects  different  from  the  parent  form  which  gave 
rise  to  them,  have  nevertheless  the  Rhabditis  form.  They  live  in  water  or 
slime,  and  sometimes  become  parasitic  in  water-snails ;  in  neither  case  how- 
ever do  they  undergo  important  changes  unless  eventually  swallowed  by  a 
frog.  They  then  pass  down  the  trachea  into  the  lungs  and  there  rapidly 
develop  into  the  adult  form.  No  separate  males  have  been  found  in  the 
lungs  of  the  frog,  but  it  has  been  shewn  by  Schneider  (No.  390)  that  the 
so-called  females  are  really  hermaphrodites  ;  the  same  gland  giving  origin 


376  METAMORPHOSIS. 


to  both  spermatozoa  and  ova,  the  former  being  developed  before  the  latter1. 
The  remarkable  feature  of  the  above  life  history  is  the  fact  that  in  the  stage 
corresponding  with  the  free  larval  stage  of  the  previous  forms  the  larvae  of 
this  species  become  sexual,  and  give  rise  to  a  second  free  larval  generation, 
which  develops  into  the  adult  form  on  again  becoming  parasitic  in  the 
original  host.  It  constitutes  a  somewhat  exceptional  case  of  heterogamy  as 
defined  in  the  introduction. 

Amongst  the  Nematodes  with  but  a  single  host  a  remarkable  parasite  in 
wheat  has  its  place.  This  form,  known  as  Anguillula  scandens,  inhabits  in 
the  adult  condition  the  ears  of  wheat,  in  which  it  lays  its  eggs.  After 
hatching,  the  larvae  become  encysted,  but  become  free  on  the  death  of  the 
plant.  They  now  inhabit  moist  earth,  but  eventually  make  their  way  into 
the  ears  of  the  young  wheat  and  become  sexually  mature. 

The  second  group  of  parasitic  Nematodes  with  two  hosts 
may  be  divided  into  two  groups,  according  to  whether  the  larva 
has  a  free  existence  before  passing  into  its  first  or  intermediate 
host,  or  is  taken  into  it  while  still  in  the  egg.  In  the  majority 
of  cases  the  larval  forms  live  in  special  connective  tissue  cap- 
sules, or  sometimes  free  in  the  tissues  of  their  intermediate 
hosts ;  but  the  adults,  as  in  the  cases  of  other  parasitic  Nema- 
todes, inhabit  the  alimentary  tract. 

The  life  history  of  Spiroptera  obtusa  may  be  cited  as  an  example  of  a 
Nematode  with  two  hosts  in  which  the  embryo  is  transported  into  its 
intermediate  host  while  still  within  the  egg.  The  adult  of  this  form  is 
parasitic  in  the  mouse,  and  the  ova  pass  out  of  the  alimentary  tract  with  the 
excreta,  and  may  commonly  be  found  in  barns,  etc.  If  one  of  the  ova  is 
now  eaten  by  the  meal-worm  (larva  of  Tenebrio),  it  passes  into  the  body 
cavity  of  this  worm  and  undergoes  further  development.  After  about  five 
weeks  it  becomes  encapsuled  between  the  '  fat  bodies '  of  the  meal-worm. 
It  then  undergoes  an  ecdysis,  and,  if  the  meal-worm  with  its  parasites  is 
now  eaten  by  the  mouse,  the  parasites  leave  their  capsule  and  develop  into 
the  sexual  form. 

As  examples  of  life  histories  in  which  a  free  state  intervenes  before  the 
intermediate  host,  Cucullanus  elegans  and  Dracunculus  may  be  selected. 
The  adult  Cucullanus  elegans  is  parasitic  in  the  alimentary  tract  of  the  Perch 
and  other  freshwater  fishes.  It  is  a  viviparous  form,  and  the  young  after 
birth  pass  out  into  the  water.  They  next  become  parasitic  in  Cyclops, 
passing  in  through  the  mouth,  so  into  the  alimentary  tract,  and  thence  into 
the  body  cavity.  They  soon  undergo  an  ecdysis,  in  the  course  of  which  the 
oesophagus  becomes  divided  into  a  muscular  pharynx  and  true  glandular 

1  Leuckart  does  not  appear  to  be  satisfied  as  to  the  hermaphroditism  of  these 
forms ;  and  holds  that  it  is  quite  possible  that  the  ova  may  develop  parthenogeneti- 
cally. 


NEMATELMINTHES.  377 


oesophagus.  They  then  grow  rapidly  in  length,  and  at  a  second  ecdysis 
acquire  a  peculiar  beaker-like  mouth  cavity  approaching  that  of  the  adult. 
They  do  not  become  encapsuled.  No  further  development  of  the  worm 
takes  place  so  long  as  it  remains  in  the  Cyclops,  but,  if  the  Cyclops  is  now 
swallowed  by  a  Perch,  the  worm  undergoes  a  further  ecdysis,  and  rapidly 
attains  to  sexual  maturity. 

The  observations  of  Fedschenko  on  Dracunculus  medinensis1,  which  is 
parasitic  in  the  subcutaneous  connective  tissue  in  Man,  would  seem  to  shew 
that  it  undergoes  a  metamorphosis  very  similar  to  that  of  Cucullanus.  There 
is  moreover  a  striking  resemblance  between  the  larvae  of  the  two  forms. 
The  larvae  of  Dracunculus  become  transported  into  water,  and  then  make 
their  way  into  the  body  cavity  of  a  Cyclops  by  boring  through  the  soft  skin 
between  the  segments  on  the  ventral  surface  of  the  body.  In  the  body  cavity 
the  larvae  undergo  an  ecdysis  and  further  development.  But  on  reaching 
a  certain  stage  of  development,  though  they  remain  a  long  time  in  the 
Cyclops,  they  grow  no  further.  The  remaining  history  is  unknown,  but 
probably  the  next  host  is  man,  in  which  the  larva  comes  to  maturity.  In  the 
adult  condition  only  females  of  Dracunculus  are  known,  and  it  has  been 
suggested  by  various  writers  that  the  apparent  females  are  in  reality  herma- 
phrodites, like  Ascaris  nigrovenosa,  in  which  the  male  organs  come  to 
maturity  before  the  female. 

Another  very  remarkable  human  parasite  belonging  to  the  same  group 
as  Dracunculus  is  the  form  known  as  Filaria  sanguinis  hominis,  or  Filaria 
Bancrofti2. 

The  sexual  form  is  parasitic  in  warm  climates  in  the  human  tissues,  and 
produces  multitudes  of  larvae  which  pass  into  the  blood,  and  are  sometimes 
voided  with  the  urine.  The  larvae  in  the  blood  do  not  undergo  a  further  deve- 
lopment, and  unless  transported  to  an  intermediate  host  die  before  very  long. 
Some,  though  as  yet  hardly  sufficient,  evidence  has  been  brought  forward  to 
shew  that  if  the  blood  of  an  infected  patient  is  sucked  by  a  mosquito  the 
larvae  develop  further  in  the  alimentary  tract  of  the  mosquito,  pass  through  a 
more  or  less  quiescent  stage,  and  eventually  grow  considerably  in  size,  and 
on  the  death  of  the  mosquito  pass  into  the  water.  From  the  water  they  are 
probably  transported  directly  or  indirectly  into  the  human  intestines,  and 
then  bore  their  way  into  the  tissues  in  which  they  are  parasitic,  and  become 
sexually  mature. 

The  well-known  Trichina  spiralis  has  a  life  history  unlike  that  of  other 
known  Nematodes,  though  there  can  be  little  doubt  that  this  form  should 
be  classified  in  respect  to  its  life  history  with  the  last- described  forms. 
The  peculiarity  of  the  life  history  of  Trichina  is  that  the  embryos  set  free 
in  the  alimentary  canal  pass  through  the  walls  into  the  muscular  tissues  and 
there  encyst ;  but  do  not  in  a  general  way  pass  out  from  the  alimentary 

1  Vide  Leuckart,  D.  men.  Par.,  Vol.  II.  p.  704. 

2  Vide   D.   P.  Manson,   "  On   the   development   of  Filaria   sanguinis  hominis." 
Journal  of  the  Linnean  Society,  Vol.  xiv.  No.  75. 


378  MKTAMORPHOSIS. 


canal  of  one  host  and  thence  into  a  fresh  host  to  encyst.  It  occasionally 
however  happens  that  this  migration  does  take  place,  and  the  life  history 
of  Trichina  spiralis  then  becomes  almost  identical  with  that  of  some  of  the 
forms  of  the  third  type.  Trichina  is  parasitic  in  man,  and  in  swine,  and 
also  in  the  rat,  mouse,  cat,  fox  and  other  forms  which  feed  upon  them. 
Artificially  it  can  be  introduced  into  various  herbivorous  forms  (rabbit, 
guinea-pig,  horse)  and  even  birds. 

The  sexual  form  inhabits  the  alimentary  canal.  The  female  is  vivi- 
parous, and  produces  myriads  of  embryos,  which  pass  into  the  alimentary 
canal  of  their  host,  through  the  walls  of  which  they  make  their  way,  and 
travelling  along  lines  of  connective  tissue  pass  into  the  muscles.  Here  the 
embryos,  which  are  born  in  a  very  imperfect  condition,  rapidly  develop, 
and  eventually  assume  a  quiescent  condition  in  a  space  inclosed  by  sarco- 
lemma. Within  the  sarcolemma  a  firm  capsule  is  developed  for  each  larva, 
which  after  some  months  becomes  calcified  ;  and  after  the  atrophy  of  the 
sarcolemma  a  connective  tissue  layer  is  formed  around  it.  Within  its 
capsule  the  larva  can  live  for  many  years,  even  ten  or  more,  without 
undergoing  further  development,  but  if  at  last  the  infected  flesh  is  eaten  by 
a  suitable  form,  e.g.  the  infected  flesh  of  the  pig  by  man,  the  quiescent 
state  of  the  larva  is  brought  to  a  close,  and  sexual  maturity  is  attained  in 
the  alimentary  tract  of  the  new  host. 

Gordioidea.  The  free  larva  of  Gordius  already  described  usually  pene- 
trates into  the  larva  of  Chironomus  where  it  becomes  encysted.  On  the 
Chironomus  being  eaten  by  some  fish  (Villot,  No.  39)  (Phoxinus  laevis  or 
Cobitis  barbatula),  it  penetrates  into  the  wall  of  the  intestine  of  its  second 
host,  becomes  again  encysted  and  remains  quiescent  for  some  time.  Event- 
ually in  the  spring  it  leaves  its  capsule,  and  enters  the  intestine,  and 
passes  to  the  exterior  with  the  faeces.  It  then  undergoes  a  gradual  meta- 
morphosis, in  the  course  of  which  it  loses  its  ringed  structure  and  cephalic 
armature,  grows  in  length,  acquires  its  ventral  cord,  and  on  the  develop- 
ment of  the  generative  organs  loses  the  greater  part  of  its  alimentary  tract. 

Young  examples  of  Gordius  have  often  been  found  in  various  terrestrial 
carnivorous  Insecta,  but  the  meaning  of  this  fact  is  not  yet  clear. 


BIBLIOGRAPHY. 

(383)  O.  Biitschli.     "Entwicklungsgeschichte  d.  Cucullanus  elegans."     Zdt.j. 
wiss.  Zool.,  B.  xxvi.     1876. 

(384)  T.  S.  Cobbold.     Entozoa.     Groombridge  and  Son,  1864. 

(385)  T.   S.  Cobbold.     Parasites;  A    Treatise  on   the  Entozoa  of  Man  mn/ 
Animals.     Churchill,  1879. 

(386)  O.  Galeb.     "Organisation  et  developpement  des  Oxyurides,"  &c.     Arch- 
ives de  Zool.  expcr.  et  getter. ,  Vol.  vn.     1878. 

(387)  R.  Leu ck  art.     Untcrsufkutigcn  itb.  Trichina  spiralis.    2nd  ed.    Leip/ig, 
1866. 

(388)  R.  Leuckart.     Die  tnenschlichcn  Parasitcn,  Bd.  II.  1876. 


NEMATELMINTHES.  379 


(389)  H.  A.  Pagenstecher.    Die  Trichinen  nach  Versitchen  dargestellt.    Leip- 
zig, 1865. 

(390)  A.Schneider.     Monographic  d.  Nemaioden.     Berlin,  1866. 

(391)  A.  Villot.     "Monographic  des  Dragoneaux"  (Gordioidea).     Archives  de 
Zool.  exper.  et gener.,  Vol.  ill.     1874. 

ACANTHOCEPHALA. 

The  Acanthocephala  appear  to  be  always  viviparous.  At  the  time  of 
impregnation  the  ovum  is  a  naked  cell,  and  undergoes  in  this  condition  the 
earlier  phases  of  segmentation. 

The  segmentation  is  unequal  (Leuckart,  No.  393),  but  whether  there  is  an 
epibolic  gastrula  has  not  clearly  been  made  out. 

Before  segmentation  is  completed  there  are  formed  round  the  ovum 
thick  protecting  membranes,  which  are  usually  three  in  number,  the  middle 
one  being  the  strongest.  After  segmentation  the  central  cells  of  the  ovum 
fuse  together  to  give  rise  to  a  granular  mass,  while  the  peripheral  cells  at  a 
slightly  later  period  form  a  more  transparent  syncytium.  At  the  anterior 
end  of  the  embryo  there  appears  a  superficial  cuticle  bearing  in  front  a  ring 
of  hooks. 

The  embryo  is  now  carried  out  with  the  excreta  from  the  intestine  of 
the  vertebrate  host  in  which  its  parent  lives.  It  is  then  swallowed  by  some 
invertebrate  host1. 

In  the  intestine  of  the  invertebrate  host  the  larva  is  freed  from  its 
membranes,  and  is  found  to  have  a  somewhat  elongated  conical  form,  ter- 
minating anteriorly  in  an  obliquely  placed  disc,  turned  slightly  towards  the 
ventral  surface  and  armed  with  hooks.  Between  this  disc  and  the  granular 
mass,  already  described  as  formed  from  the  central  cells  of  the  embryo,  is  a 
rather  conspicuous  solid  body.  Leuckart  supposes  that  this  body  may  re- 
present a  rudimentary  functionless  pharynx,  while  the  granular  mass  in 
his  opinion  is  an  equally  rudimentary  and  functionless  intestine.  The  body 
wall  is  formed  of  a  semifluid  internal  layer  surrounding  the  rudimentary 
intestine,  if  such  it  be,  and  of  a  firmer  outer  wall  immediately  within  the  cuticle. 
The  adult  Echinorhyncus  is  formed  by  a  remarkable  process  of  develop- 
ment within  the  body  of  the  larva,  and  the  skin  is  the  only  part  of  the 
larva  which  is  carried  over  to  the  adult. 

In  Echinorhyncus  proteus  the  larva  remains  mobile  during  the  forma- 
tion of  the  adult,  but  in  other  forms  the  metamorphosis  takes  place  during 
a  quiescent  condition  of  the  larva. 

The  organs  of  the  adult  are  differentiated  from  a  mass  of  cells  which 
appears  to  be  a  product  of  the  central  embryonic  granular  mass,  and  is 

1  Echin.  proteus,  which  is  parasitic  in  the  adult  state  in  many  freshwater  fish, 
passes  through  its  larval  condition  in  the  body  cavity  of  Gammarus  pulex.  Ech. 
angustatus,  parasitic  in  the  Perch,  is  found  in  the  larval  condition  in  the  body  cavity 
of  Asellus  aquaticus.  Ech.  gigas,  parasitic  in  swine,  is  stated  by  Schneider  (No.  394) 
to  pass  through  its  larval  stages  in  maggots. 


380  ACANTHOCEPHALA. 


called  by  Leuckart  the  embryonic  nucleus.  The  embryonic  nucleus  becomes 
divided  into  four  linearly  arranged  groups  of  cells,  of  which  the  hindermost 
but  one  is  the  largest,  and  very  early  differentiates  itself  into  (i)  a  peripheral 
layer,  and  (2)  a  central  mass  formed  of  two  distinct  bodies.  The  peripheral 
layer  of  this  segment  grows  forwards  and  backwards,  and  embraces  the 
other  segments,  with  the  exception  of  the  front  end  of  the  first  one  which 
is  left  uncovered.  The  envelope  so  formed  gives  rise  to  the  splanchnic  and 
somatic  mesoblast  of  the  adult  worm.  Of  the  four  groups  of  cells  within  it 
the  anterior  gives  rise  to  the  proboscis,  the  next  to  the  nerve  ganglion,  the 
third,  formed  of  two  bodies,  to  the  paired  generatives,  and  the  fourth  to  the 
generative  ducts.  The  whole  of  the  above  complex  rapidly  elongates,  and 
as  it  does  so  the  enveloping  membrane  becomes  split  into  two  layers  ;  of 
which  the  outer  forms  the  muscular  wall  of  the  body  (somatic  mesoblast), 
and  the  inner  the  muscular  sheath  of  the  proboscis  and  the  so-called  gene- 
rative ligament  enveloping  the  generative  organs.  The  inner  layer  may  be 
called  the  splanchnic  mesoblast  in  spite  of  the  absence  of  an  intestine. 
The  cavity  between  the  two  mesoblastic  layers  forms  the  body  cavity. 

The  various  parts  of  the  adult  continue  to  differentiate  themselves  as 
the  whole  increases  in  size.  The  generative  masses  very  early  shew  traces 
of  becoming  differentiated  into  testes  or  ovaries.  In  the  male  the  two 
generative  masses  remain  spherical,  but  in  the  female  become  elongated  : 
the  rudiment  of  the  generative  ducts  becomes  divided  into  three  sections 
in  both  sexes.  The  most  remarkable  changes  are,  however,  those  undergone 
by  the  rudiment  of  the  proboscis. 

In  its  interior  there  is  formed  a  cavity,  but  the  wall  bounding  the  front 
end  of  the  cavity  soon  disappears.  By  the  time  that  this  has  taken  place 
the  body  of  the  adult  completely  fills  up  the  larval  skin,  to  which  it  very 
soon  attaches  itself.  The  hollow  rudiment  of  the  proboscis  then  becomes 
everted,  and  forms  a  papilla  at  the  end  of  the  body,  immediately  ad- 
joining the  larval  skin.  This  papilla,  with  the  larval  skin  covering  it, 
constitutes  the  permanent  proboscis.  The  original  larval  cuticle  is  either 
now  or  at  an  earlier  period  thrown  off  and  a  fresh  cuticle  developed.  The 
hooks  of  the  proboscis  are  formed  from  cells  of  the  above  papilla,  which 
grow  through  the  larval  skin  as  conical  prominences,  on  the  apex  of  which 
a  chitinous  hook  is  modelled.  The  remainder  of  the  larval  skin  forms  the 
skin  of  the  adult,  and  at  a  later  period  develops  in  its  deeper  layer  the 
peculiar  plexus  of  vessels  so  characteristic  of  the  Acanthocephala.  The 
anterior  oval  appendages  of  the  adult  cutis,  known  as  the  lemnisci,  are 
outgrowths  from  the  larval  skin. 

The  Echinorhyncus  has  with  the  completion  of  these  changes  practically 
acquired  its  adult  structure  ;  but  in  the  female  the  ovaries  undergo  at  this 
period  remarkable  changes,  in  that  they  break  up  into  a  number  of  spherical 
masses,  which  lie  in  the  lumen  of  the  generative  ligaments,  and  also  make 
their  way  into  the  body  cavity. 

The  young  Echinorhyncus  requires  to  be  transported  to  its  permanent 
host,  which  feeds  on  its  larval  host,  before  attaining  to  sexual  maturity. 


ACANTHOCEPHALA.  381 


BIBLIOGRAPHY. 

(392)  R.  Greeff.     "  Untersuchungen  ii.  d.  Bau  u.  Entwicklung  des  Echin.  milia- 
rius."     Archiv  f.  Naturgesch.     1864. 

(393)  R.   Leuckart.     Die  menschlichen   Parasiten.     Vol.   n.  p.  80 1    et   seq. 
1876. 

(394)  An.  Schneider.      "  Ueb.  d.  Bau  d.  Acanthocephalen."     Archiv  f.  Anat. 
u,  Phys.     1868. 

(395)  G.  R.  Wagener.     Beitrdge  z.  Entwicklungsgeschichte  d.  Eingeweidewiir- 
mer.     Haarlem,  1865. 


CHAPTER  XVII. 

TRACHEATA. 

PROTOTRACH  EAT  A. 
THE  remarkable  researches  of  Moseley  (No.  396)  on  Peripatus 


FIG.  167.    ADULT  EXAMPLE  OF  PERIPATUS  CAPENSIS,  natural  size. 
(From  Moseley.) 

capensis  have  brought  clearly  to  light  the  affinities  of  this  form 
with  the  tracheate  Arthropoda ;    and    its    numerous    primitive 


FIG.  168.     Two  STAGES  IN  THE  DEVELOPMENT  OF  PERIPATUS  CAPENSIS. 
(After  Moseley.) 

A.  Youngest  stage  hitherto  observed  before  the  appearance  of  the  legs. 

B.  Later  stage  after  the  legs  and  antennae  have  become  developed. 
Both  figures  represent  the  larva  as  it  appears  within  the  egg. 

i  and  i.     First  and  second  post-oral  appendages. 

characters,  such  as  the  generally  distributed  tracheal  apertures, 
the  imperfectly  segmented   limbs,  the  diverging  ventral  nerve 


TRACHEATA. 


383 


cords  with  imperfectly  marked  ganglia,  and  the  nephridia  (seg- 
mental  organs1),  would  render  its  embryology  of  peculiar  in- 
terest. Unfortunately  Moseley  was  unable,  from  want  of 
material,  to  make  so  complete  a  study  of  its  development  as  of 
its  anatomy.  The  youngest  embryo  observed  was  in  part 
distinctly  segmented,  and  coiled  up  within  the  egg  (fig.  168  A). 
The  procephalic  lobes  resemble  those  of  the  Arthropoda  gene- 
rally, and  are  unlike  the  prae-oral  lobe  of 
Chaetopods  or  Discophora.  They  are  not 
marked  off  by  a  transverse  constriction 
from  the  succeeding  segments.  The  three 
embryonic  layers  are  differentiated,  and 
the  interior  is  filled  with  a  brownish  mass 
— the  remnant  of  the  yolk — which  is  pro- 
bably enclosed  in  a  distinct  intestinal  wall, 
and  is  lobed  in  correspondence  with  the 
segmentation  of  the  body.  The  mouth 
invagination  is  not  present,  and  but  two 
pairs  of  slight  prominences  mark  the  rudi- 
ments of  the  two  anterior  post-oral  ap- 
pendages. 

The  single  pair  of  antennae  is  formed 
in  the  next  stage,  and  is  followed  by  the 
remaining  post-oral  appendages,  which 
arise  in  succession  from  before  backwards 
somewhat  later  than  the  segments  to  which 
they  appertain. 

The  posterior  part  of  the  embryo  be- 
comes uncoiled,  and  the  whole  embryo 
bent  double  in  the  egg  (fig.  168  B). 

The  mouth  appears  as  a  slit-like  open- 
ing between  and  below  the  procephalic 
lobes.  On  each  side  and  somewhat  behind  it  there  grows  out 
an  appendage — the  first  post-oral  pair  (fig.  169,  i) — while  in 
front  and  behind  it  are  formed  the  upper  and  lower  lips.  These 
two  appendages  next  turn  inwards  towards  the  mouth,  and  their 


FIG.  169.  EMBRYO 
OF  PERIPATUS  CAPENSIS. 
Slightly  older  than  A  in 
fig.  168;  unrolled.  (After 
Moseley.) 

a.  antennae  ;  o.  mouth  ; 
i.  intestine  ;  c.  procephalic 
lobe,  i,  2,  3,  etc.,  post- 
oral  appendages. 


1  F.  M.  Balfour,   "On    certain  points  in  the   Anatomy  of  Peripatus  capensis." 
Quart.  Journ.  of  Micros.  Science,  Vol.  xix.  1879. 


PROTOTRACHEATA. 


bases  become  gradually  closed  over  by  two  processes  of  the 
procephalic  region  (fig.  170,  m) 
The  whole  of  these  structures 
assist  in  forming  a  kind  of 
secondary  mouth  cavity,  which 
is  at  a  later  period  further 
completed  by  the  processes  of 
the  procephalic  region  meeting 
above  the  mouth,  covering  over 
the  labrum,  and  growing  back- 
wards to  near  the  origin  of  the 
second  pair  of  post-oral  appen- 
dages. 

The  antennae  early  become 
jointed,  and  fresh  joints  con- 
tinue to  be  added  throughout 
embryonic  life ;  in  the  adult 
there  are  present  fully  thirty 
joints.  It  appears  to  me  probable  (though  Mr  Moseley  takes 
the  contrary  view)  from  the  late  development  of  the  paired 
processes  of  the  procephalic  lobes,  which  give  rise  to  the  circular 
lip  of  the  adult,  that  they 
are  not  true  appendages. 
The  next  pair  therefore 
to  the  antennae  is  the  first 
post-oral  pair.  It  is  the 
only  pair  connected  with 
the  mouth.  At  their  ex- 
tremities there  is  formed  a 
pair  of  claws  similar  to 
those  of  the  ambulatory 

legs  (fig.  171).     The  next          FIG.  171.    HEAD  OF  AN  EMBRYO  PERIPA- 
,  .  .       r  TUS.     (From  Moseley.) 

and  largest  pair  of  appen-         The  figure  shews  the  jaws  (mamlil)lcs)>  and 

dagCS    in    the    embryo   are      close    to    them    epiblastic    involutions,    which 


FIG.  170.  VENTRAL  VIEW  OF  THE 
HEAD  OF  AN  EMBRYO  OF  PERIPATUS  CA- 
PENSIS  AT  A  LATE  STAGE  OF  DEVELOP- 
MENT. 

/.  thickening  of  epiblast  of  procepha- 
lic lobe  to  form  supra-oesophageal  gan- 
glion ;  ///.  process  from  procephalic  lobe 
growing  over  the  first  post-oral  appen- 
dage ;  o.  mouth;  e.  eye;  i  and  2,  first 
and  second  pair  of  post-oral  appendages. 


the   oral    papillae.      They 


grow  into  the  supra-oesophageal  ganglia.     The 
antennae,  oral  cavity,  and  oral  papilhe  are  also 

are  chiefly  remarkable  for     shewn. 

containing  the  ducts  of  the  slime  glands  which  open  at  their 
bases.  They  are  without  claws.  The  succeeding  appendages 
become  eventually  imperfectly  five-jointed  ;  two  claws  are 


TRACHEATA. 


385 


formed  as  cuticular  investments  of  papillae  in  pockets  of  the 
skin  at  the  ends  of  their  terminal  joints. 

I  have  been  able  to  make  a  few  observations  on  the  internal  structure  of 
the  embryos  from  specimens  supplied  to  me  by  Moseley.  These  are  so  far 
confined  to  a  few  stages,  one  slightly  earlier,  the  others  slightly  later,  than 
the  embryo  represented  in  fig.  168  B.  The  epiblast  is  formed  of  a  layer  of 
columnar  cells,  two  deep  on  the  ventral  surface,  except  along  the  median  line 
where  there  is  a  well-marked  groove  and  the  epiblast  is  much  thinner  (fig.  172). 

The  ventral  cords  of  the  trunk  are  formed  as  two  independent  epiblastic 
thickenings.  In  my  earlier  stage  these  are  barely  separated  from  the 
epiblast,  but  in  the  later  ones  are  quite  independent  (fig.  172,  v.n),  and 
partly  surrounded  by  mesoblast. 

The  supra-cesophageal  ganglia  are  formed  as  thickenings  of  the  epiblast 
of  the  ventral  side  of  the  procephalic  lobes  in  front  of  the  stomodaeum. 
They  are  shewn  at  /  in  fig.  170.  The  thickenings  of  the  two  sides  are  at 
first  independent.  At  a  somewhat  later  period  an  invagination  of  the 
epiblast  grows  into  each  of  these  lobes.  The  openings  of  these  invaginations 
extend  from  the  oral  cavity  forwards;  and  they  are  shewn  in  fig.  171 l. 
Their  openings  become  closed,  and  the  walls  of  the  invaginations  constitute 
a  large  part  of  the  embryonic  supra-cesophageal  ganglia. 

Similar  epiblastic  invaginations  assist  in  forming  the  supra-cesophageal 
ganglia  of  other  Tracheata. 
They  are  described  in  the  sequel 
for  Insects,  Spiders  and  Scor- 
pions. The  position  of  the  supra- 
cesophageal  ganglia  on  the  ven- 
tral side  of  the  procephalic  lobes 
is  the  same  as  that  in  other 
Tracheata. 

The  mesoblast  is  formed,  in 
the  earliest  of  my  embryos,  of 
scattered  cells  in  the  fairly  wide 
space  between  the  mesenteron 
and  the  epiblast.  There  are  two 
distinct  bands  of  mesoblast  on 
the  outer  sides  of  the  nervous 
cords.  In  the  later  stage  the 
mesoblast  is  divided  into  dis- 
tinct somatic  and  splanchnic  lay- 
ers, both  very  thin  ;  but  the  two 
layers  are  connected  by  trans- 
verse strands  (fig.  172).  There 


sp.w 


$.m 


FIG.  172.  SECTION  THROUGH  THE  TRUNK 
OF  AN  EMBRYO  OF  PERIPATUS.  The  embryo 
from  which  the  section  is  taken  was  somewhat 
younger  than  fig.  171. 

sp.m.  splanchnic  mesoblast. 

s.m.    somatic  mesoblast. 

me.     median  section  of  body  cavity. 

k.       lateral  section  of  body  cavity. 

v.n.     ventral  nerve  cord. 

me.     mesenteron. 


1  This  figure  is  taken  from  Moseley.  The  epiblastic  invaginations  are  represented 
in  it  very  accurately,  and  though  not  mentioned  in  the  text  of  the  paper,  Moseley 
informs  me  that  he  has  long  been  aware  of  the  homology  of  these  folds  with  those  in 
various  other  Tracheata. 


B.  II. 


25 


386  PROTOTRACHEATA. 


are  two  special  longitudinal  septa  dividing  the  body  cavity  into  three 
compartments,  a  median  (me),  containing  the  mesenteron,  and  two  lateral 
(Ic)  containing  the  nerve  cords.  This  division  of  the  body  cavity  persists, 
as  I  have  elsewhere  shewn,  in  the  adult.  A  similar  division  is  found  in 
some  Chaetopoda,  e.g.  Polygordius. 

I  failed  to  make  out  that  the  mesoblast  was  divided  into  somites,  and 
feel  fairly  confident  that  it  is  not  so  in  the  stages  I  have  investigated. 

There  is  a  section  of  the  body  cavity  in  the  limbs  as  in  embryo  Myria- 
pods,  Spiders,  etc. 

In  the  procephalic  lobe  there  is  a  well-developed  section  of  the  body 
cavity,  which  lies  dorsal  to  and  in  front  of  the  rudiment  of  the  supra- 
cesophageal  ganglia. 

The  alimentary  tract  is  formed  of  a  mesenteron  (fig.  172),  a  stomo- 
daeum,  and  proctodaeum.  The  wall  of  the  mesenteron  is  formed,  in  the 
stages  investigated  by  me,  of  a  single  layer  of  cells  with  yolk  particles, 
and  encloses  a  lumen  free  from  yolk.  The  forward  extension  of  the 
mesenteron  is  remarkable. 

The  stomodaeum  in  the  earlier  stage  is  a  simple  pit,  which  meets  but  does 
not  open  into  the  mesenteron.  In  the  later  stage  the  external  opening  of 
the  pit  is  complicated  by  the  structures  already  described.  The  procto- 
daeum is  a  moderately  deep  pit  near  the  hinder  end  of  the  body. 

The  existence  of  a  tracheal  system1  is  in  itself  almost  sufficient  to 
demonstrate  the  affinities  of  Peripatus  with  the  Tracheata,  in  spite  of  the 
presence  of  nephridia.  The  embryological  characters  of  the  procephalic 
lobes,  of  the  limbs  and  claws,  place  however  this  conclusion  beyond 
the  reach  of  scepticism.  If  the  reader  will  compare  the  figure  of  Peripatus 
with  that  of  an  embryo  Scorpion  (fig.  196  A)  or  Spider  (fig.  200  C)  or  better 
still  with  Metschnikoffs  figure  of  Geophilus  (No.  399)  PI.  xxi.  fig.  u,he 
will  be  satisfied  on  this  point. 

The  homologies  of  the  anterior  appendages  are  not  very  easy 
to  determine  ;  but  since  there  does  not  appear  to  me  to  be  suffi- 
cient evidence  to  shew  that  any  of  the  anterior  appendages  have 
become  aborted,  the  first  post-oral  appendages  embedded  in  the 
lips  may  provisionally  be  regarded  as  equivalent  to  the  mandibles, 
and  the  oral  papillae  to  the  first  pair  of  maxillae,  etc.  Moseley  is 
somewhat  doubtful  about  the  homologies  of  the  appendages, 
and  hesitates  between  considering  the  oral  papillae  as  equivalent 
to  the  second  pair  of  maxillae  (on  account  of  their  containing  the 
openings  of  the  mucous  glands,  which  he  compares  with  the 
spinning  glands  of  caterpillars),  or  to  the  poison  claws  (fourth 

1  The  specimens  shewing  tracheae  which  Moseley  has  placed  in  my  hands  are 
quite  sufficient  to  leave  no  doubt  whatever  in  my  mind  as  to  the  general  accuracy  of 
his  description  of  the  tracheal  system. 


TRACHEATA.  387 


post-oral  appendages)  of  the  Chilopoda  (on  account  of  the 
poison-glands  which  he  thinks  may  be  homologous  with  the 
mucous  glands). 

The  arguments  for  either  of  these  views  do  not  appear  to  me  con- 
clusive. There  are  glands  opening  into  various  anterior  appendages  in 
the  Tracheata,  such  as  the  poison  glands  in  the  Chelicerae  (mandibles)  of 
Spiders,  and  there  is  some  evidence  in  Insects  for  the  existence  of  a  gland 
belonging  to  the  first  pair  of  maxillae,  which  might  be  compared  with  the 
mucous  gland  of  Peripatus.  For  reasons  already  stated  I  do  not  regard 
the  processes  of  the  cephalic  lobes,  which  form  the  lips,  as  a  pair  of  true 
appendages. 

BIBLIOGRAPHY. 

(396)  H.  N.  Moseley.  "On  the  Structure  and  Development  of  Peripatus 
capensis."  Phil.  Trans.  Vol.  164,  1874. 

MYRIAPODA1. 

Chilognatha.  The  first  stages  in  the  development  of  the 
Chilognatha  have  been  investigated  by  Metschnikoffand  Stecker, 
but  their  accounts  are  so  contradictory  as  hardly  to  admit  of 
reconciliation. 

According  to  Metschnikoff,  by  whom  the  following  four 
species  have  been  investigated,  viz.,  Strongylosoma  Guerinii, 
Polydesmus  complanatus,  Polyxenus  lagurus,  and  Julus  Mone- 
letei,  the  segmentation  is  at  first  regular  and  complete,  but, 
when  the  segments  are  still  fairly  large,  the  regular  segmentation 
is  supplemented  by  the  appearance  of  a  number  of  small  cells  at 
various  points  on  the  surface,  which  in  time  give  rise  to  a 
continuous  blastoderm. 

The  blastoderm  becomes  thickened  on  the  ventral  surface, 
and  so  forms  a  ventral  plate2. 

1  The  classification  of  the  Myriapoda  employed  in  the  present  section  is 

I.     Chilognatha.     (Millipedes.) 
II.     Chilopoda.     (Centipedes.) 

2  Stecker's  (No.  400)  observations  were  made  on  the  eggs  of  Julus  fasciatus,  Julus 
fcetidus,  Craspedosoma  marmoratum,  Polydesmus  complanatus,  and  Strongylosoma 
pallipes,  and  though  carried  on  by  means  of  sections,  still  leave  some  points  very 
obscure,  and  do  not  appear  to  me  deserving  of  much  confidence.     The  two  species  of 
Julus  and  Craspedosoma  undergo,  according  to  Stecker,  a  nearly  identical  develop- 
ment.    The  egg  before  segmentation  is  constituted  of  two  substances,  a  central  proto- 
plasmic, and  a  peripheral  deutoplastic.     It  first  divides  into  two  equal  segments,  and 
coincidentally  with  their  formation  part  of  the  central  protoplasm  travels  to  the 

25—2 


388  CHILOGNATHA. 


FIG.  173.    THREE  STAGES  IN  THE  DEVELOPMENT  OF  STRONGYLOSOMA  GUERINII. 
(After  Metschnikoff.) 

A.  Embryo  on  eleventh  day  with  commencing  ventral  flexure  (*). 

B.  Embryo  with  three  pairs  of  post-oral  appendages. 

C.  Embryo  with  five  pairs  of  post-oral  appendages. 

gs.  ventral  plate;  at.  antenme;  1—5  post-oral  appendages;  x.  point  of  flexure  of 
the  ventral  plate. 

surface  as  two  clear  fluid  segments.  The  ovum  is  thus  composed  of  two  yolk  segments 
to  two  protoplasmic  segments.  The  two  former  next  divide  into  four,  with  the  pro- 
duction of  two  fresh  protoplasmic  segments.  The  four  protoplasmic  segments  now 
constitute  the  upper  or  animal  pole  of  the  egg,  and  occupy  the  position  of  the  future 
ventral  plate.  The  yolk  segments  form  the  lower  pole,  which  is  however  dorsal  in 
relation  to  the  future  animal.  The  protoplasmic  segments  increase  in  number  by  a 
regular  division,  and  arrange  themselves  in  three  rows,  of  which  the  two  outermost 
rapidly  grow  over  the  yolk  segments.  A  large  segmentation  cavity  is  stated  to  be 
present  in  the  interior  of  the  ovum. 

It  would  appear  from  Stecker's  description  that  the  yolk  segments  (hypoblast) 
next  become  regularly  invaginated,  so  as  to  enclose  a  gastric  cavity,  opening  externally 
by  a  blastopore;  but  it  is  difficult  to  believe  that  a  typical  gastrula,  such  as  that 
represented  by  Sleeker,  really  comes  into  the  cycle  of  development  of  the  Chilo- 
gnatha. 

The  mesoblast  is  stated  to  be  derived  mainly  from  the  epiblast.  This  layer  in  the 
region  of  the  future  ventral  plate  becomes  reduced  to  two  rows  of  cells,  and  the  inner 
of  these  by  the  division  of  its  constituent  elements  gives  rise  to  the  mesoblast.  The 
development  of  Polydesmus  and  Strongylosoma  is  not  very  different  from  that  of  Julus. 
The  protoplasm  at  the  upper  pole  occupies  from  the  first  a  superficial  position. 
Segmentation  commences  at  the  lower  pole,  where  the  food  yolk  is  mainly  present ! 
The  gastrula  is  stated  to  be  similar  to  that  of  Julus,  The  mesoblast  is  formed  in 
Polydesmus  as  a  layer  of  cells  split  off  from  the  epiblast,  but  in  Strongylosoma  as  an 
outgrowth  from  the  lips  of  the  blastopore.  Stecker,  in  spite  of  the  statements  in  his 
paper  as  to  the  origin  of  the  mesoblast  from  the  epiblast,  sums  up  at  the  end  to  the 
effect  that  both  the  primary  layers  have  a  share  in  the  formation  of  the  mesoblast, 
which  originates  by  a  process  of  endogenous  cell-division ! 

It  may  be  noted  that  the  closure  of  the  blastopore  takes  place,  according  to 
Stecker,  on  the  dorsal  side  of  the  embryo. 


TRACHEATA.  389 


The  most  important  sources  of  information  for  the  general 
embryology  of  the  Chilognatha  are  the  papers  of  Newport  (No. 
397)  and  Metschnikoff  (No.  398).  The  development  of  Strongy- 
losoma  may  be  taken  as  fairly  typical  for  the  group ;  and  the 
subsequent  statements,  unless  the  reverse  is  stated,  apply  to  the 
species  of  Strongylosoma  investigated  by  Metschnikoff. 

After  the  segmentation  and  formation  of  the  layers  the  first 
observable  structure  is  a  transverse  furrow  in  the  thickening  of 
the  epiblast  on  the  ventral  surface  of  the  embryo.  This  furrow 
rapidly  deepens,  and  gives  rise  to  a  ventral  flexure  of  the  embryo 
(fig.  173  A,  x\  which  is  much  later  in  making  its  appearance  in 
Julus  than  in  Strongylosoma  and  Polyxenus.  A  pair  of  ap- 
pendages, which  become  the  antennae,  makes  its  appearance 
shortly  after  the  formation  of  the  transverse  furrow,  and  there 
soon  follow  in  order  the  next  three  pairs  of  appendages.  All 
these  parts  are  formed  in  the  infolded  portion  of  the  ventral 
thickening  of  the  blastoderm  (fig.  173  B).  The  ventral  thicken- 
ing has  in  the  meantime  become  marked  by  a  longitudinal 
furrow,  but  whether  this  is  connected  with  the  formation  of 
the  nervous  system,  or  is  equivalent  to  the  mesoblastic  furrow  in 
Insects,  and  connected  with  the  formation  of  the  mesoblast,  has 
not  been  made  out.  Shortly  after  the  appearance  of  the  three 
pairs  of  appendages  behind  the  antennae  two  further  pairs  become 
added,  and  at  the  same  time  oral  and  anal  invaginations  become 
formed '(fig-  173  Q.  In  front  of  the  oral  opening  an  unpaired 
upper  lip  is  developed.  The  prse-oral  part  of  the  ventral  plate 
develops  into  the  bilobed  procephalic  lobes,  the  epiblast  of 
which  is  mainly  employed  in  the  formation  of  the  supra-cesopha- 
geal  ganglia.  The  next  important  change  which  takes  place  is 
the  segmentation  of  the  body  of  the  embryo  (fig.  174  A),  the 
most  essential  feature  in  which  is  the  division  of  the  mesoblast 
into  somites.  Segments  are  formed  in  order  from  before  back- 
wards, and  soon  extend  to  the  region  behind  the  appendages. 
On  the  appearance  of  segmentation  the  appendages  commence 
to  assume  their  permanent  form.  The  two  anterior  pairs  of 
post-oral  appendages  become  jaws  ;  and  the  part  of  the  embryo 
which  carries  them  and  the  antennae  is  marked  off  from  the 
trunk  as  the  head.  The  three  following  pairs  of  appendages 
grow  in  length  and  assume  a  form  suited  for  locomotion.  Behind 


390  CHILOGNATHA. 


the  three  existing  pairs  of  limbs  there  are  developed  three  fresh 
pairs,  of  whicJi  tJie  two  anterior  belong  to  a  single  primitive  seg- 
ment. While  the  above  changes  take  place  in  the  appendages 
the  embryo  undergoes  an  ecdysis,  which  gives  rise  to  a  cuticular 
membrane  within  the  single  egg-membrane  (chorion,  Metschni- 
koff\  On  this  cuticle  a  tooth-like  process  is  developed,  the 
function  of  which  is  to  assist  in  the  hatching  of  the  embryo 
(fig.  174  A). 

In  Polyxenus  a  cuticular  membrane  is  present  as  in  Strongylosoma, 
but  it  is  not  provided  with  a  tooth-like  process.  In  the  same  form  amoeboid 
cells  separate  themselves  from  the  blastoderm  at  an  early  period.  These 
cells  have  been  compared  to  the  embryonic  envelopes  of  Insects  described 
below. 

In  Julus  two  cuticular  membranes  are  present  at  the  time  of  hatching  : 
the  inner  one  is  very  strongly  developed  and  encloses  the  embryo  after 
hatching.  After  leaving  the  chorion  the  embryo  Julus  remains  connected 
with  it  by  a  structureless  membrane  which  is  probably  the  outer  of  the  two 
cuticular  membranes. 

At  the  time  when  the  embryo  of  Strongylosoma  is  hatched 
(fig.  174  B)  nine  post-cephalic  segments  appear  to  be  present. 


FlG.    174.      TWO   STAGES   IN   THE    DEVELOPMENT  OF  STRONGYLOSOMA    GUEKINll. 

(After  Metschnikoff.) 

A.  A  seventeen  days'  embryo,  already  segmented. 

B.  A  just-hatched  larva. 

Of  these  segments  the  second  is  apparently  (from  MetschnikofT's 
figure,   174  B)  without  a  pair  of   appendages;    the   third   and 


TRACHEATA.  391 


fourth  are  each  provided  with  a  single  functional  pair  of  limbs ; 
the  fifth  segment  is  provided  with  two  pairs  of  rudimentary 
limbs,  which  are  involuted  in  a  single  sack  and  not  visible  with- 
out preparation,  and  therefore  not  shewn  in  the  figure.  The 
sixth  segment  is  provided  with  but  a  single  pair  of"  appendages, 
though  a  second  pair  is  subsequently  developed  on  it1. 

Julus,  at  the  time  it  leaves  the  chorion,  is  imperfectly  segmented,  but  is 
provided  with  antennas,  mandibles,  and  maxillae,  and  seven  pairs  of  limbs, 
of  which  the  first  three  are  much  more  developed  than  the  remainder. 
Segmentation  soon  makes  its  appearance,  and  the  head  becomes  distinct 
from  the  trunk,  and  on  each  of  the  three  anterior  trunk  segments  a  single 
pair  of  limbs  is  very  conspicuous  (Metschnikoff)2.  Each  of  the  succeeding 
segments  bears  eventually  two  pairs  of  appendages.  At  the  time  when 
the  inner  embryonic  cuticle  is  cast  off,  the  larva  appears  to  be  hexapodous, 
like  the  young  Strongylosoma,  but  there  are  in  reality  four  pairs  of  rudi- 
mentary appendages  behind  the  three  functional  pairs.  The  latter  only 
appear  on  the  surface  after  the  first  post-embryonic  ecdysis.  Pauropus 
(Lubbock)  is  hexapodous  in  a  young  stage.  At  the  next  moult  two  pairs 
of  appendages  are  added,  and  subsequently  one  pair  at  each  moult. 

There  appear  to  be  eight  post-oral  segments  in  Julus  at  the 
time  of  hatching.  According  to  Newport  fresh  segments  are 
added  in  post-embryonic  life  by  successive  budding  from  a 
blastema  between  the  penultimate  segment  and  that  in  front  of 
it.  They  arise  in  batches  of  six  at  the  successive  ecdyses,  till 
the  full  number  is  completed.  A  functional,  though  not  a  real 
hexapodous  condition,  appears  to  be  characteristic  of  Chilognatha 
generally  at  the  time  of  hatching. 

The  most  interesting  anatomical  feature  of  the  Chilognatha 
is  the  double  character  of  their  segments,  the  feet  (except  the 
first  three  or  four,  or  more),  the  circulatory,  the  respiratory,  and 
the  nervous  systems  shewing  this  peculiarity.  Newport's  and 

1  Though  the  superficially  hexapodous  larva  of  Strongylosoma  and  other  Chilo- 
gnatha has  a  striking  resemblance  to  some  larval  Insects,  no  real  comparison  is  pos- 
sible between  them,  even  on  the  assumption  that  the  three  functional  appendages  of 
both  are  homologous,  because  Embryology  clearly  proves  that  the  hexapodous  Insect 
type  has  originated  from  an  ancestor  with  numerous  appendages  by  the  atrophy  of 
those  appendages,  and  not  from  an  hexapodous  larval  form  prior  to  the  development 
of  the  full  number  of  adult  appendages. 

2  Newport  states  however  that  a  pair  of  limbs  is  present  on  the  first,  second,  and 
fourth  post-oral   segments,  but  that  the  third  segment  is  apodous ;  and  this  is  un- 
doubtedly the  case  in  the  adult. 


392 


CHILOPODA. 


Metschnikoff's  observations  have  not  thrown  as  much  light  on 
the  nature  of  the  double  segments  as  might  have  been  hoped, 
but  it  appears  probable  that  they  have  not  originated  from  a 
fusion  of  two  primitively  distinct  segments,  but  from  a  later 
imperfect  division  of  each  of  the  primitive  segments  into  two, 
and  the  supply  to  each  of  the  divisions  of  a  primitive  segment  of 
a  complete  set  of  organs. 

Chilopoda.  Up  to  the  present  time  the  development  of  only 
one  type  of  Chilopoda,  viz.  that  of  Geophilus,  has  been  worked 
out.  Most  forms  lay  their  eggs,  but  Scolopendra  is  viviparous. 


a  u .  i 


FlG.  175.      TWO  STAGES   IN   THE   DEVELOPMENT   OF  GEOPHILUS. 

(After  Metschnikoff.) 

A.  Side-view  of  embryo  at  the  stage  when  the  segments  are  beginning  to  be  formed. 

B.  Later  stage  after  the  appendages  have  become  established. 

at.  antenna.-;  an.t.  proctodseum. 

The  segmentation  appears  to  resemble  that  in  the  Chilognatha, 
and  at  its  close  there  is  present  a  blastoderm  surrounding  a 
central  mass  of  yolk  cells.  A  ventral  thickening  of  the  blasto- 
derm is  soon  formed.  It  becomes  divided  into  numerous  seg- 
ments, which  continue  to  be  formed  successively  from  the 
posterior  unsegmented  part.  The  antennae  are  the  first  append- 
ages to  appear,  and  are  well  developed  when  eighteen  segments 
have  become  visible  (fig.  175  A).  The  post-oral  appendages 
are  formed  slightly  later,  and  in  order  from  before  backwards. 
As  the  embryo  grows  in  length,  and  fresh  segments  continue  to 
be  formed,  the  posterior  part  of  it  becomes  bent  over  so  as  to 
face  the  ventral  surface  of  the  anterior,  and  it  acquires  an 


TRACHEATA.  393 


appearance  something  like  that  of  many  embryo  Crustaceans 
(fig.  175  B).  Between  forty  and  fifty  segments  are  formed  while 
the  embryo  is  still  in  the  egg.  The  appendages  long  remain 
unjointed.  The  fourth  post-oral  appendage,  which  becomes  the 
poison-claw,  is  early  marked  out  by  its  greater  size  :  on  the 
third  post-oral  there  is  formed  a  temporary  spine  to  open  the 
egg  membrane. 

It  does  not  appear,  from  Metschnikoff's  figures  of  Geophilus,  that  any 
of  the  anterior  segments  are  without  appendages,  and  it  is  very  probable 
that  Newport  is  mistaken  in  supposing  that  the  embryo  has  a  segment  with- 
out appendages  behind  that  with  the  poison  claws,  which  coalesces  with  the 
segment  of  the  latter.  It  also  appears  to  me  rather  doubtful  whether  the 
third  pair  of  post-oral  appendages,  i.e.  those  in  front  of  the  poison-claws,  can 
fairly  be  considered  as  forming  part  of  the  basilar  plate.  The  basilar  plate 
is  really  the  segment  of  the  poison-claws,  and  may  fuse  more  or  less  com- 
pletely with  the  segment  in  front  and  behind  it,  and  the  latter  is  sometimes 
without  a  pair  of  appendages  (Lithobius,  Scutigera). 

Geophilus,  at  the  time  of  birth,  has  a  rounded  form  like  that 
of  the  Chilognatha. 

The  young  of  Lithobius  is  born  with  only  six  pairs  of  limbs. 

General  observation  on  the  homologies  of  the  appendages  of 
Myriapoda. 

The  chief  difficulty  in  this  connection  is  the  homology  of  the  third  pair  of 
post-oral  appendages. 

In  adult  Chilognatha  there  is  present  behind  the  mandibles  a  four-lobed 
plate,  which  is  usually  regarded  as  representing  two  pairs  of  appendages, 
viz.  the  first  and  second  pairs  of  maxillae  of  Insects.  Metschnikoff's  ob- 
servations seem  however  to  shew  that  this  plate  represents  but  a  single 
pair  of  appendages,  which  clearly  corresponds  with  the  first  pair  of  maxillae 
in  Insects.  The  pair  of  appendages  behind  this  plate  is  ambulatory,  but 
turned  towards  the  head  ;  it  is  in  the  embryo  the  foremost  of  the  three 
functional  pairs  of  legs  with  which  the  larva  is  born.  Is  it  equivalent  to 
the  second  pair  of  maxillae  of  Insects  or  to  the  first  pair  of  limbs  of  Insects? 
In  favour  of  the  former  view  is  the  fact  (i)  that  in  embryo  Insects  the 
second  pair  of  maxillae  sometimes  resembles  the  limbs  rather  than  the 
jaws,  so  that  it  might  be  supposed  that  in  Chilognatha  a  primitive 
ambulatory  condition  of  the  third  pair  of  appendages  has  been  retained  ; 
(2)  that  the  disappearance  of  a  pair  of  appendages  would  have  to  be 
postulated  if  the  second  alternative  is  adopted,  and  that  if  Insects  are 
descended  from  forms  related  to  the  Myriapods  it  is  surprising  to  find  a 
pair  of  appendages  always  present  in  the  former,  absent  in  the  latter. 


394 


MYRIAPODA. 


The  arguments  which  can  be  urged  for  the  opposite  view  do  not  appear 
to  me  to  have  much  weight,  so  that  the  homology  of  the  appendages  in 
question  with  the  second  pair  of  maxillae  may  be  provisionally  assumed. 

The  third  pair  of  post-oral  appendages  of  the  Chilopoda  may  probably 
also  be  assumed  to  be  equivalent  to  the  second  pair  of  maxillae,  though  they 
are  limb-like  and  not  connected  with  the  head.  The  subjoined  table  shews 
the  probable  homologies  of  the  appendages. 


CHILOGNATHA(Strongylo- 

so  ma  at  time  of  birth). 

CHILOPODA  (Scolopendra 
adult). 

Pre-oral  region. 

Antennae. 

Antennas. 

ist  Post-oral  segment. 

Mandibles. 

Mandibles. 

2nd         ,,             ,, 

Maxillae  i.       (Four-lobed 
plate  in  adult,  but  a  sim- 
ple pair  of  appendages 
in  embryo). 

Maxillie  i. 
(Palp  and  bilobed  median 
process). 

3rd         „ 
(probably   equivalent  to 
segment  bearing  2nd  pair 
of  maxillae  in  Insects). 

ist    pair    of    ambulatory 
limbs. 

Limb-like  appendages  with 
basal  parts  in  contact. 

4th         ,,             ,, 

(?)  Apodous. 

Poison  claws. 

5th 

2nd    pair    of   ambulatory 
limbs. 

ist    pair     of    ambulatory 
limbs. 

6th 

3rd         „         ,,         „ 

2nd         „         ,, 

7th 

4th  and  sth     „         „ 
(rudimentary.  ) 

3rd-       „         „         „ 

8th        ,,            ,, 

6th 
(the  7th  pair  is  developed 
in  this  segment  later). 

4th          „         „         „ 

9th 

Apodous. 

5th 

loth      ,, 

,,          (last  segment  in 
embryo). 

6th 

The  germinal  layers  and  formation  of  organs. 

The  development  of  the  organs  of  the  Myriapoda,  and  the  origin  of  the 
germinal  layers,  are  very  imperfectly  known :  Myriapoda  appear  however 
to  be  closely  similar  to  Insects  in  this  part  of  their  development,  and  the 
general  question  of  the  layers  will  be  treated  more  fully  in  connection  with 
that  group. 

The  greater  part  of  the  blastoderm  gives  rise  to  the  epiblast,  which 
furnishes  the  skin,  nervous  system,  tracheal  system,  and  the  stomodacum 
and  proctodaeum. 


TRACHEATA.  395 


The  mesoblast  arises  in  connection  with  the  ventral  thickening  of  the 
blastoderm,  but  the  details  of  its  formation  are  not  known.  Metschnikoff 
describes  a  longitudinal  furrow  which  appears  very  early  in  Strongylosoma, 
which  is  perhaps  equivalent  to  the  mesoblastic  furrows  of  Insects,  and  so 
connected  with  the  formation  of  the  mesoblast. 

The  mesoblast  is  divided  up  into  a  series  of  protovertebra-like  bodies — 
the  mesoblastic  somites — the  cavities  of  which  become  the  body  cavity  and 
the  walls  the  muscles  and  probably  the  heart.  They  are  (Metschnikoff) 
prolonged  into  the  legs,  though  the  prolongations  become  subsequently 
segmented  off  from  the  main  masses.  The  splanchnic  mesoblast  is, 
according  to  Metschnikoff,  formed  independently  of  the  somites,  but  this 
point  requires  further  observation. 

The  origin  of  the  hypoblast  remains  uncertain,  but  it  appears  probable 
that  it  originates,  in  a  large  measure  at  least,  from  the  yolk  segments.  In 
the  Chilognatha  the  mesenteron  is  formed  in  the  interior  of  the  yolk  seg- 
ments, so  that  those  yolk  segments  which  are  not  employed  in  the  formation 
of  the  alimentary  canal  lie  freely  in  the  body  cavity.  In  the  relation  of 
the  yolk  segments  to  the  alimentary  canal  the  Chilopoda  present  a  strong 
contrast  to  the  Chilognatha,  in  that  the  greater  part  of  the  yolk  lies 
within  their  mesenteron.  The  mesenteron  is  at  first  a  closed  sack,  but  is 
eventually  placed  in  communication  with  the  stomodaeum  and  the  procto- 
dasum.  The  Malpighian  bodies  arise  as  outgrowths  from  the  blind  ex- 
tremity of  the  latter. 

BIBLIOGRAPHY. 

(397)  G.  N  e  wp  or  t.     "  On  the  Organs  of  Reproduction  and  Development  of  the 
Myriapoda."     Philosophical  Transactions,  1841. 

(398)  E.  Metschnikoff.    ' ' Embryologie der  doppeltflissigen  Myriapoden  (Chi- 
lognatha)."    Zeit.f.  wiss.  Zool.,  Vol.  xxiv.     1874. 

(399)     ' '  Embryologisches  iiber  Geophilus."     Zeit.  f.  wiss.  ZooLy  Vol.  xxv. 

1875- 

(400)  Anton  Stecker.     "Die  Anlage  d.  Keimblatter  bei  den  Diplopoden." 
Archivf.  mik.  Anatomie,  Bd.  xiv.     1877. 

INSECTA1. 

The  formation  of  the  embryonic  layers  in  Insects  has  not 
been  followed  out  in  detail  in  a  large  number  of  types ;  but,  as 

1  The  following  classification  of  the  Insecta  is  employed  in  this  chapter, 
((i)     Collembola. 

I.    Aptera.    |(a)    Thysanura. 

!(i)     Orthoptera  genuina  (Blatta,  Locusta,  etc.). 
(2)  „  pseudoneuroptera    (Termes,   Ephemera, 

Libellula). 

!(i)     Hemiptera  heteroptera  (Cimex,  Notonecta,  etc.). 
(2)  ,,          homoptera  (Aphis,  Cicada,  etc.). 

(3)  ,,          parasita  (Pediculus,  etc.). 


396 


INSECTA. 


in  so  many  other  instances,  some  of  the  most  complete  histories 
we  have  are  due  to  Kowalevsky  (No.  416).     The  development 


FiG.    176.      FOUR    EMBRYOS   OF    llYDROPHlLUS   P1CEUS    VIEWED    FROM    THE 

VENTRAL  SURFACE.     (After  Kowalevsky.) 
The  upper  end  is  the  anterior,    gg.  germinal  groove;  am.  amnion. 

of  Hydrophilus  has  been  worked  out  by  him  more  fully  than 
that  of  any  other  form,  and  will  serve  as  a  type  for  comparison 
with  other  forms. 

The  segmentation  has  not  been  studied,  but  no  doubt  belongs 
to  the  centrolecithal  type  (vide  pp.  no — 120).  At  its  close 
there  is  an  uniform  layer  of  cells  enclosing  a  central  mass  of 
yolk.  These  cells,  in  the  earliest  observed  stage,  were  flat  on 
the  dorsal,  but  columnar  on  part  of  the  ventral  surface  of  the 
egg,  where  they  form  a  thickening  which  will  be  called  the  ven- 
tral plate.  At  the  posterior  part  of  the  ventral  plate  two  folds, 
with  a  furrow  between  them,  make  their  appearance.  They  form 
a  structure  which  may  be  spoken  of  as  the  germinal  groove  (fig. 

!(i)     Diptera  genuina  (Musca,  Tipula,  etc.). 
(2)        „         aphaniptera  (Pulex,  etc.). 
(3)         ,,         pupipara  (Braula,  etc.). 

v      ..  ( (i)     Neuroptera  planipennia  (Myrniclcon,  etc.)- 

TOptera.      j  (a)  ^  trichoptera  (Phryganea,  etc.). 

VI.    Coleoptera. 
VII.    Lepidoptera. 

(i)     Hymenoptera  aculeata  (Apis,  Formica,  etc.). 
(a)  ,,  entomophaga  (Ichneumon,  Platy- 

gaster,  etc). 
(3)  ,,  phytophaga  ( Tenthredo,  Sirex,  etc.). 


VIII.    Hymenoptera. 


TRACHEATA. 


397 


y* 


FlG.  177.      TWO  TRANSVERSE  SECTIONS  THROUGH 

EMBRYOS  OF  HvDROPHiLUS  piCEUS.     (After  Kowa- 
levsky.) 

A.  Section  through  an  embryo  of  the  stage  repre- 
sented in   fig.    176   B,   at   the  point   where  the   two 
germinal  folds  most  approximate. 

B.  Section   through   an   embryo   somewhat   later 
than  the  stage  fig.  176  D,  through  the  anterior  region 
where  the  amnion  has  not  completely  closed  over  the 
embryo. 


).  The  cells 
which  form  the  floor 
of  the  groove  are  far 
more  columnar  than 
those  of  other  parts 
of  the  blastoderm  (fig. 
177  A).  The  two 
folds  on  each  side  of 
it  gradually  approach 
each  other.  They  do 
so  at  first  behind,  and 
then  in  the  middle; 
from  the  latter  point 
the  approximation 
gradually  extends 
backwards  and  for- 
wards (fig.  176  B  and 
C).  In  the  middle 
and  hinder  parts  of 
the  ventral  plate  the 
groove  becomes,  by 
the  coalescence  of  the  folds,  converted  into  a  canal  (fig.  178  A, 
gg),  the  central  cavity  of  which  soon  disappears,  while  at  the 
same  time  the  cells  of  the  wall  undergo  division,  become  more 
rounded,  and  form  a  definite  layer  (me} — the  mesoblast — beneath 
the  columnar  cells  of  the  surface.  Anteriorly  the  process  is 
slightly  different,  though  it  leads  to  the  similar  formation  of 
mesoblast  (fig.  177  B).  The  flat  floor  of  the  groove  becomes  in 
front  bodily  converted  into  the  mesoblast,  but  the  groove  itself 
is  never  converted  into  a  canal.  The  two  folds  simply  meet 
above,  and  form  a  continuous  superficial  layer. 

During  the  later  stages  of  the  process  last  described  remark- 
able structures,  eminently  characteristic  of  the  Insecta,  have 
made  their  first  appearance.  These  structures  are  certain 
embryonic  membranes  or  coverings,  which  present  in  their  mode 
of  formation  and  arrangement  a  startling  similarity  to  the  true 
and  false  amnion  of  the  Vertebrata.  They  appear  as  a  double 
fold  of  the  blastoderm  round  the  edge  of  the  germinal  area, 
which  spreads  over  the  ventral  plate,  from  behind  forwards,  in  a 


gg.    germinal   groove ; 
nion ;  yk.  yolk. 


me.   mesoblast ;    am.   am- 


INSECT  A. 


general  way  in  the  same 
manner  as  the  amnion  in, 
for  instance,  the  chick. 
The  folds  at  their  origin 
are  shewn  in  surface  view 
in  fig.  176  D,  am,  and  in 
section  in  fig.  177  B,  am. 
The  folds  eventually 
meet,  coalesce  (fig.  178, 
am)  and  give  rise  to  two 
membranes  covering  the 
ventral  plate,  viz.  an 
inner  one,  which  is  con- 
tinuous with  the  edge  of 
the  ventral  plate ;  and 
an  outer,  continuous  with 
the  remainder  of  the 
blastoderm.  The  verte- 
brate nomenclature  may 
be  conveniently  employ- 
ed for  these  membranes. 
The  inner  limb  of  the 
fold  will  therefore  be  spoken  of  as  the  amnion,  and  the  outer 
one,  including  the  dorsal  part  of  the  blastoderm,  as  the 
serous  envelope1.  A  slight  consideration  of  the  mode  of 
formation  of  the  membranes,  or  an  inspection  of  the  figures 
illustrating  their  formation,  makes  it  at  once  clear  that  the  yolk 
can  pass  in  freely  between  the  amnion  and  serous  envelope  (vide 
fig.  181).  At  the  hind  end  of  the  embryo  this  actually  takes 
place,  so  that  the  ventral  plate  covered  by  the  amnion  appears  to 
become  completely  imbedded  in  the  yolk:  elsewhere  the  two 
membranes  are  in  contact.  At  first  (fig.  176)  the  ventral  plate 
occupies  but  a  small  portion  of  the  ventral  surface  of  the  egg,  but 
during  the  changes  above  described  it  extends  over  the  whole 
ventral  surface,  and  even  slightly  on  the  dorsal  surface  both  in 
front  and  behind.  It  becomes  at  the  same  time  (fig.  179)  divided 


FIG.  178.   SECTIONS  THROUGH  TWO  EMBRYOS 
OF  HYDROPHILUS  PICEUS.     (After  Kowalevsky.) 

A.  Section  through  the  posterior  part  of  the 
embryo  fig.  1 76  D,  shewing  the  completely  closed 
amnion  and  the  germinal  groove. 

B.  Section  through  an  older  embryo  in  which 
the  mesoblast  has  grown  out  into  a  continuous 
plate  beneath  the  epiblast. 

gg.  germinal  groove ;  am.  amnion  ;  yk.  yolk  ; 
cp.  epiblast. 


1  The  reverse  nomenclature  to  this  is  rather  inconveniently  employed  by  Metsch- 
nikoff. 


TRACHEATA. 


399 


FIG.  179.  EMBRYO  OF 
HYDROPHILUS     PICEUS 

VIEWED  FROM  THE  VEN- 

TRAL  SURFACE.     (After 
Kowalevsky.) 

pc.L  procephalic  lobe. 


by  a  series  of  transverse  lines  into  segments,  which  increase  in 
number  and  finally  amount  in  all  to  seven- 
teen, not  including  the  most  anterior  section, 
which  gives  off  as  lateral  outgrowths  the 
two  procephalic  lobes  (pc.l).  The  changes 
so  far  described  are  included  within  what 
Kowalevsky  calls  his  first  embryonic  period; 
at  its  close  the  parts  contained  within  the 
chorion  have  the  arrangement  shewn  in  fig. 
178  B.  The  whole  of  the  body  of  the 
embryo  is  formed  from  the  ventral  plate, 
and  no  part  from  the  amnion  or  serous 
envelope. 

The  general  history  of  the  succeeding 
stages  may  be  briefly  told. 

The  appendages  appear  as  very  small 
rudiments  at  the  close  of  the  last  stage,  but 
soon  become  much  more  prominent  (fig. 
1 80  A).  They  are  formed  as  outgrowths  of  both  layers,  and 
arise  nearly  simultaneously.  There 
are  in  all  eight  pairs  of  appendages. 
The  anterior  or  antennae  (at)  spring 
from  the  procephalic  lobes,  and 
the  succeeding  appendages  from 
the  segments  following.  The  last 
pair  of  embryonic  appendages, 
which  disappears  very  early,  is 
formed  behind  the  third  pair  of 
the  future  thoracic  limbs.  Paired 
epiblastic  involutions,  shewn  as  pits 
in  the  posterior  segments  in  fig. 
1 80  A,  give  rise  to  the  tracheae; 
and  the  nervous  system  is  formed 
as  two  lateral  epiblastic  thicken- 
ings, one  on  each  side  of  the  mid- 
ventral  line.  These  eventually  be- 
come split  off  from  the  skin  ;  while 
between  them  there  passes  in  a 
median  invagination  of  the  skin 


FlG.   1 80.      TWO  STAGES  IN  THE 
DEVELOPMENT     OF     HYDROPHILUS 

PICEUS.      (From    Gegenbaur,    after 
Kowalevsky.) 

Is.    labrum ;    at.   antenna ;    md. 


400  INSECTA. 


(fig.  189  C).  The  two  nervous  strands  are  continuous  in  front 
with  the  supra-oesophageal  ganglia,  which  are  formed  of  the 
epiblast  of  the  procephalic  lobes.  These  plates  gradually  grow 
round  the  dorsal  side  of  the  embryo,  and  there  is  formed 
immediately  behind  them  an  oral  invagination,  in  front  of  which 
there  appears  an  upper  lip  (fig.  180,  Is).  A  proctodaeum  is  formed 
at  the  hind  end  of  the  body  slightly  later  than  the  stomodaeum. 
The  mesoblast  cells  become  divided  into  two  bands,  one  on 
each  side  of  the  middle  line  (fig.  189  A),  and  split  into 
splanchnic  and  somatic  layers.  The  central  yolk  mass  at  about 
the  stage  represented  in  fig.  179  begins  to  break  up  into 
yolk  spheres.  The  hypoblast  is  formed  first  on  the  ventral 
side  at  the  junction  of  the  mesoblast  and  the  yolk,  and 
gradually  extends  and  forms  a  complete  sack-like  mesenteron, 
enveloping  the  yolk  (fig.  185  al).  The  amnion  and  serous 
membrane  retain  their  primitive  constitution  for  some  time,  but 
gradually  become  thinner  on  the  ventral  surface,  where  a  rupture 
appears  eventually  to  take  place.  The  greater  part  of  them 
disappears,  but  in  the  closure  of  the  dorsal  parietes  the  serous 
envelope  plays  a  peculiar  part,  which  is  not  yet  understood.  It 
is  described  on  p.  404.  The  heart  is  formed  from  the  mesoblas- 
tic  layers,  where  they  meet  in  the  middle  dorsal  line  (fig.  185  C, 
hi].  The  somatic  mesoblast  gives  rise  to  the  muscles  and 
connective  tissue,  and  the  splanchnic  mesoblast  to  the  muscular 
part  of  the  wall  of  the  alimentary  tract,  which  accompanies  the 
hypoblast  in  its  growth  round  the  yolk.  The  proctodaeum 
forms  the  rectum  and  Malpighian  bodies1,  and  the  stomodseum 
the  oesophagus  and  proventriculus.  The  two  epiblastic  sections 
of  the  alimentary  tract  are  eventually  placed  in  communication 
with  the  mesenteron. 

The  development  of  Hydrophilus  is  a  fair  type  of  that  of 
Insects  generally,  but  it  is  necessary  to  follow  with  somewhat 
greater  detail  the  comparative  history  of  the  various  parts  which 
have  been  briefly  described  for  this  type. 

TJte  embryonic  membranes  and  the  formation  of  the  layers. 

All  Insects  have  at  the  close  of  segmentation  a  blastoderm 
formed  of  a  single  row  of  cells  enclosing  a  central  yolk  mass, 

1  This  has  not  been  shewn  in  the  case  of  Hydrophilus, 


TRACHEATA. 


401 


which  usually  contains  nuclei,  and  in  the  Poduridae  is  divided  up 
in  the  ordinary  segmentation  into  distinct  yolk  cells.  The  first 
definite  structure  formed  is  a  thickening  of  the  blastoderm, 
which  forms  a  ventral  plate. 

The  ventral  plate  is  very  differently  situated  in  relation  to  the  yolk  in 
different  types.  In  most  Diptera,  Hymenoptera  and  (?)  Neuroptera  (Phry- 
ganea)  it  forms  from  the  first  a  thickening  extending  over  nearly  the 
whole  ventral  surface  of  the  ovum,  and  in  many  cases  extends  in  its  sub- 
sequent growth  not  only  over  the  whole  ventral  surface,  but  over  a  con- 
siderable part  of  the  apparent  dorsal  surface  as  well  (Chironomus,  Simulia, 
Gryllotalpa,  etc.).  In  Coleoptera,  so  far  as  is  known,  it  commences  as  a  less 
extended  thickening  either  of  the  central  part  (Donacia)  or  posterior  part 
(Hydrophilus)  of  the  ventral  surface,  and  gradually  grows  in  both  directions, 
passing  over  to  the  dorsal  surface  behind. 

Embryonic  membranes.     In  the  majority  of  Insects  there 
are  developed  enveloping  membranes  like  those  of  Hydrophilus. 

The  typical  mode  of  formation  of  these  membranes  is  repre- 
sented diagrammatically  in  fig.  181  A  and  B.  A  fold  of  the 
blastoderm  arises  round  the  edge  of  the  ventral  plate.  This 
fold,  like  the  am- 
niotic  fold  of  the 
higher  Vertebrata, 
is  formed  of  two 
limbs,  an  outer, 
the  serous  mem- 
brane (se),  and  an 
inner,  the  true  am- 
nion  (am).  Both 
limbs  extend  so 
as  to  cover  over 
the  ventral  plate, 
and  finally  meet 
and  coalesce,  so 
thatadouble  mem- 
brane is  present 
over  the  ventral 
plate.  At  the  same 
time  (fig.  181  B) 
the  point  where  the  fold  originates  is  carried  dorsalwards  by  the 
B.  II.  26 


Sf 


FIG.  181.  DIAGRAMMATIC  LONGITUDINAL  SECTIONS 
OF  AN  INSECT  EMBRYO  AT  TWO  STAGES  TO  SHEW  THE 

DEVELOPMENT  OF  THE  EMBRYONIC  ENVELOPES. 

In  A  the  amniotic  folds  have  not  quite  met  so  as  to 
cover  the  ventral  plate.  The  yolk  is  represented  as  divided 
into  yolk  cells.  In  B  the  sides  of  the  ventral  plate  have 
extended  so  as  nearly  to  complete  the  dorsal  integument. 
The  mesenteron  is  represented  as  a  closed  sack  filled  with 
yolk  cells,  am.  amnion;  se.  serous  envelope;  v.p.  ven- 
tral plate  ;  d.  i.  dorsal  integument ;  me.  mesenteron  ;  st. 
stomodaeum  ;  an  i.  proctodaeum. 


4O2  INSECTA. 


dorsal  extension  of  the  edges  of  the  ventral  plate,  which  give 
rise  to  the  dorsal  integument  (d.i).  This  process  continues 
till  the  whole  dorsal  surface  is  covered  by  the  integument. 
The  amnion  then  separates  from  the  dorsal  integument,  and  the 
embryo  becomes  enveloped  in  two  membranes — an  inner,  the 
amnion,  and  an  outer,  the  serous  membrane.  In  fig.  181  B  the 
embryo  is  represented  at  the  stage  immediately  preceding  the 
closure  of  the  dorsal  surface. 

By  the  time  that  these  changes  are  effected,  the  serous 
membrane  and  amnion  are  both  very  thin  and  not  easily 
separable.  The  amnion  appears  to  be  usually  absorbed  before 
hatching;  but  in  hatching  both  membranes,  if  present,  are  either 
absorbed,  or  else  ruptured  and  thrown  off. 

The  above  mode  of  development  of  the  embryonic  membranes  has  been 
especially  established  by  the  researches  of  Kowalevsky  (No.  416)  and  Graber 
(No.  412)  for  various  Hymenoptera  (Apis),  Diptera  (Chironomus\  Lepido- 
ptera  and  Coleoptera  (Melolontha,  Lino). 

Considerable  variations  in  the  development  of  the  enveloping  membranes 
are  known. 

When  the  fold  which  gives  rise  to  the  membranes  is  first  formed,  there 
is,  as  is  obvious  in  fig.  181  A,  a  perfectly  free  passage  by  which  the  yolk  can 
pass  in  between  the  amnion  and  serous  membrane.  Such  a  passage  of  the 
yolk  between  the  two  membranes  takes  place  posteriorly  in  Hydrophilus  and 
Donacia:  in  Lepidoptera  the  yolk  passes  in  everywhere,  so  that  in  this  form 
the  ventral  plate  becomes  first  of  all  imbedded  in  the  yolk,  and  finally,  on  the 
completion  of  the  dorsal  integument,  the  embryo  is  enclosed  in  a  complete 
envelope  of  yolk  contained  between  the  amnion  and  the  serous  membrane. 
During  the  formation  of  the  dorsal  integument  the  external  yolk  sack  com- 
municates by  a  dorsally  situated  umbilical  canal  with  the  yolk  cavity  within 
the  body.  On  the  rupture  of  the  amnion  the  embryo  is  nourished  at  the 
expense  of  the  yolk  contained  in  the  external  yolk  sack. 

In  the  Hemiptera  and  the  Libellulidae  the  ventral  plate  also  becomes 
imbedded  in  the  yolk,  but  in  a  somewhat  different  fashion  to  the  Lepido- 
ptera, which  more  resembles  on  an  exaggerated  scale  what  takes  place  in 
Hydrophilus. 

In  the  Libellulidas  (Calopteryx)  there  is  first  of  all  formed  (Brandt,  No. 
403)  a  small  ventral  and  posterior  thickening  of  the  blastoderm  (fig.  182  A). 
The  hinder  part  of  this  becomes  infolded  into  the  yolk  as  a  projection  (fig. 
182  B),  which  consists  of  two  laminae,  an  anterior  and  a  posterior,  continuous 
at  the  apex  of  the  invagination.  The  whole  structure,  which  is  completely 
imbedded  within  the  yolk,  rapidly  grows  in  length,  and  turns  towards 
the  front  end  of  the  egg  (fig.  182  C).  Its  anterior  lamina  remains  thick  and 
gives  rise  to  the  ventral  plate  (ps),  the  posterior  (am)  on  the  other  hand 


TRACHEATA. 


403 


becomes  very  thin,  and 
forms  a  covering  corre- 
sponding with  the  amnion 
of  the  more  ordinary  types. 
The  remainder  of  the  blas- 
toderm covering  the  yolk 
(se)  forms  the  homologue 
of  the  serous  membrane 
of  other  types.  The  ven- 
tral surface  of  the  ventral 
plate  is  turned  towards 
the  dorsal  side  (retaining 
the  same  nomenclature  as 
in  ordinary  cases)  of  the 
egg,  and  the  cephalic 
extremity  is  situated  at 
the  point  of  origin  of  the 
infolding. 

The  further  history  is 
however  somewhat  pecu- 
liar. The  amnion  is  at  first 
(fig.  182  C)  continuous  with 
the  serous  envelope  on  the 
posterior  side  only,  so  that 
the  serous  envelope  does 
not  form  a  continuous  sack, 
but  has  an  opening  close 
to  the  head  of  the  embryo. 
In  the  Hemiptera  parasita  this  opening  (Melnikow,  No.  422)  remains  per- 
manent, and  the  embryo,  after  it  has  reached  a  certain  stage  of  development, 
becomes  everted  through  it,  while  the  yolk,  enclosed  in  the  continuous  mem- 
brane formed  by  the  amnion  and  serous  envelope,  forms  a  yolk  sack  on  the 
dorsal  surface.  In  the  Libellulidae  however  and  most  Hemiptera,  a  fusion  of 
the  two  limbs  of  the  serous  membrane  takes  place  in  the  usual  way,  so  as  to 
convert  it  into  a  completely  closed  sack  (fig.  183  A).  After  the  formation  of 
the  appendages  a  fusion  takes  place  between  the  amnion  and  serous  enve- 
lope over  a  small  area  close  to  the  head  of  the  embryo.  In  the  middle  of 
this  area  a  rupture  is  then  effected,  and  the  head  of  the  embryo  followed  by 
the  body  is  gradually  pushed  through  the  opening  (fig.  183  B  and  C).  The 
embryo  becomes  in  the  process  completely  rotated,  and  carried  into  a 
position  in  the  egg-shell  identical  with  that  of  the  embryos  of  other  orders  of 
Insects  (fig.  183  C). 

Owing  to  the  rupture  of  the  embryonic  envelopes  taking  place  at  the 
point  where  they  are  fused  into  one,  the  yolk  does  not  escape  in  the  above 
process,  but  is  carried  into  a  kind  of  yolk  sack,  on  the  dorsal  surface  of  the 
embryo,  formed  of  the  remains  of  the  amnion  and  serous  envelope.  The 

26—2 


FIG.  182.    THREE  STAGES  IN  THE  DEVELOPMENT 

OF  THE  EMBRYO  OF  CALOPTERYX.      (After  Brandt.) 

The  embryo  is  represented  in  the  egg-shell. 

A.  Embryo  with  ventral  plate. 

B.  Commencing  involution  of  ventral  plate. 

C.  Involution  of  ventral  plate  completed. 

ps.  vefitral  plate;   g.  edge  of  ventral  plate;   am. 
amnion ;  se-  serous  envelope. 


404 


INSECTA. 


walls  of  the  yolk  sack  either 
assist  in  forming  the  dorsal 
parietes  of  the  body,  or  are 
more  probably  enclosed 
within  the  body  by  the 
growth  of  the  dorsal  pari- 
etes from  the  edge  of  the 
ventral  plate. 

In  Hydrophilus  and 
apparently  in  the  Phry- 
ganidae  also,  there  are  cer- 
tain remarkable  peculiari- 
ties in  the  closure  of  the 
dorsal  surface.  The  fullest 
observations  on  the  subject 
have  been  made  by  Kowa- 
levsky  (No.  416), but  Dohrn 
(No.  408)  has  with  some 
probability  thrown  doubts 
on  Kowalevsky's  interpreta- 
tions. According  to  Dohrn 
the  part  of  the  serous  enve- 
lope which  covers  the  dor- 
sal surface  becomes  thick- 
ened, and  gives  rise  to  a 
peculiar  dorsal  plate  which 
is  shewn  in  surface  view  in 
ventral  parts  of  the  amnion 
and  serous  membrane  have 
either  been  ruptured  or 
have  disappeared.  While 
the  dorsal  plate  is  being 
formed,  the  mesoblast,  and 
somewhat  later  the  lateral 
parts  of  the  epiblast  of  the 
ventral  plate  gradually 
grow  towards  the  dorsal 
side  and  enclose  the  dorsal 
plate,  the  wall  of  which  in 
the  process  appears  to  be 
folded  over  so  as  first  of 
all  to  form  a  groove  and 
finally  a  canal.  The  stages 
in  this  growth  are  shewn 
from  the  surface  in  fig.  184 
B  and  C  and  in  section  in 


FlG.   183.    THREE  STAGES  IN  THE  DEVELOPMENT 

OF  CALOPTERYX.     (After  Brandt.) 

The  embryo  is  represented  in  the  egg-shell;  B. 
and  C.  shew  the  inversion  of  the  embryo. 

sf.  serous  envelope ;  am.  amnion ;  ab.  abdomen ; 
v.  anterior  end  of  head ;  at.  antennae ;  md.  mandible ; 
mxl.  maxilla  i ;  mx*.  maxilla  2 ;  p1—^.  three  pairs 
of  legs;  oe.  oesophagus. 

fig.  184  A,  doi  and  in  section  in  fig.  185  A,  do.    The 


FIG.  184.  THREE  LARVAL  STAGES  OF  HYDRO- 
PHILUS FROM  THE  DORSAL  SIDE,  SHEWING  THE 
GRADUAL  CLOSING  IN  OF  THE  DORSAL  REGION  WITH 
THE  FORMATION  <  >!•  THK  I'l.CULIAR  DORSAL  ORGAN 

do.     (After  Kowalevsky.) 

do.  dorsal  organ ;  at.  antennae. 


TRACHEATA. 


405 


fig.  185  B,  do.  The  canal  is  buried  on  the  dorsal  part  of  the  yolk,  but  for 
some  time  remains  open  by  a  round  aperture  in  front  (fig.  184  C).  The 
whole  structure  is  known  as  the  dorsal  canal.  It  appears  to  atrophy  without 
leaving  a  trace.  The  heart  when  formed  lies  immediately  dorsal  to  it1. 


A. 
B. 
C. 


vn 


FIG.  185.     THREE  TRANSVERSE  SECTIONS  THROUGH  ADVANCED 

EMBRYOS  OF  HYDROPHILUS. 

Section  through  the  posterior  part  of  the  body  of  the  same  age  as  fig.  184  A. 
Section  through  the  embryo  of  the  same  age  as  fig.  184  C. 
Section  through  a  still  older  embryo. 
do.  dorsal  plate ;  vn.  ventral  nerve  cord ;  al.  mesenteron ;  ht.  heart. 
The  large  spaces  at  the  sides  are  parts  of  the  body  cavity. 

In  the  Poduridas  the  embryonic  membranes  appear  to  be  at  any  rate 
imperfect.  Metschnikoff  states  in  his  paper  on  Geophilus  that  in  some  ants 
no  true  embryonic  membranes  are  found,  but  merely  scattered  cells  which 
take  their  place.  In  the  Ichneumonidas  the  existence  of  two  embryonic 
membranes  is  very  doubtful. 

Formation  of  the  embryonic  layers.  The  formation  of  the 
layers  has  been  studied  in  sections  by  Kowalevsky  (No.  416), 

1  According  to  Kowalevsky  the  history  of  the  dorsal  plate  is  somewhat  different. 
He  believes  that  on  the  absorption  of  the  amnion  the  ventral  plate  unites  with  the 
serous  membrane,  and  that  the  latter  directly  gives  rise  to  the  dorsal  integument, 
while  the  thickened  part  of  it  becomes  involuted  to  form  the  dorsal  tube  already 
described. 


406  INSECTA. 


Hatschek  (No.  414),  and  Graber  (No.  412),  etc.  From  their 
researches  it  would  appear  that  the  formation  of  the  mesoblast 
always  takes  place  in  a  manner  closely  resembling  that  in 
Hydrophilus.  The  essential  features  of  the  process  (figs.  177 
and  178)  appear  to  be  that  a  groove  is  formed  along  the  median 
line  of  the  ventral  plate,  and  that  the  sides  of  this  groove  either 
(i)  simply  close  over  like  the  walls  of  the  medullary  groove  in 
Vertebrates,  and  so  convert  the  groove  into  a  tube,  which  soon 
becomes  solid  and  forms  a  mass  or  plate  of  cells  internal  to  the 
epiblast ;  or  (2)  that  the  cells  on  each  side  of  the  groove  grow 
over  it  and  meet  in  the  middle  line,  forming  a  layer  external 
to  the  cells  which  lined  the  groove.  The  former  of  these 
processes  is  the  most  usual ;  and  in  the  Muscidae  the  dimensions 
of  the  groove  are  very  considerable  (Graber,  No.  411).  In  both 
cases  the  process  is  fundamentally  the  same,  and  causes  the 
ventral  plate  to  become  divided  into  two  layers1.  The  external 
layer  or  epiblast  is  an  uniform  sheet  forming  the  main  part  of 
the  ventral  plate  (fig.  178  B,  ep).  It  is  continuous  at  its  edge 
with  the  amnion.  The  inner  layer  or  mesoblast  constitutes  an 
independent  plate  of  cells  internal  to  the  epiblast  (fig.  178  B,  me). 
The  mesoblast  soon  becomes  divided  into  two  lateral  bands. 

The  origin  of  the  hypoblast  is  still  in  dispute.  It  will  be 
remembered  (vide  pp.  1 14  and  1 16)  that  after  the  segmentation  a 
number  of  nuclei  remain  in  the  yolk  ;  and  that  eventually  a 
secondary  segmentation  of  the  yolk  takes  place  around  these 
nuclei,  and  gives  rise  to  a  mass  of  yolk  cells,  which  fill  up  the 
interior  of  the  embryo.  These  cells  are  diagrammatically  shewn 
in  figs.  181  and  189,  and  it  is  probable  that  they  constitute  the 
true  hypoblast.  Their  further  history  is  given  below. 

Formation  of  the  organs  and  their  relation  to  the  germinal 

layers. 

The  segments  and  appendages.  One  of  the  earliest 
phenomena  in  the  development  is  the  appearance  of  transverse 
lines  indicating  segmentation  (fig.  186).  The  transverse  lines 
are  apparently  caused  by  shallow  superficial  grooves,  and  also  in 

1  Tichomiroff  (No.  420)  denies  the  existence  of  a  true  invagination  to  form  the 
mesoblast,  and  also  asserts  that  a  separation  of  mesoblast  cells  from  the  epiblast  can 
take  place  at  other  parts  besides  the  median  ventral  line. 


TRACHEATA. 


407 


many  cases  by  the  division  of  the  mesoblastic  bands  into 
separate  somites.  The  most  anterior  line  marks  off  a  prae-oral 
segment,  which  soon  sends  out  two  lateral  wings — the  procephalic 
lobes.  The  remaining  segments  are  at  first  fairly  uniform. 
Their  number  does  not,  however,  appear  to  be  very  constant. 
So  far  as  is  known  they  never  exceed  seventeen,  and  this 
number  is  probably  the  typical  one  (figs.  186  and  187). 

In  Diptera  the  number  appears  to  be  usually  fifteen  though  it  may  be 
only  fourteen.  In  Lepidoptera  and  in  Apis  there  appear  to  be  sixteen 
segments.  These  and  other  variations  affect  only  the  number  of  the  segments 
which  form  the  abdomen  of  the  adult. 

The  appendages  arise  as  paired  pouch- 
like  outgrowths  of  the  epiblast  and  meso- 
blast ;  and  their  number  and  the  order  of 
their  appearance  are  subject  to  considerable 
variation,  the  meaning  of  which  is  not  yet 
clear.  As  a  rule  they  arise  subsequently  to 
the  segmentation  of  the  parts  of  the  body 
to  which  they  belong.  There  is  always 
formed  one  pair  of  appendages  which  spring 
from  the  lateral  lobes  of  the  procephalic 
region,  or  from  the  boundary  line  between 
these  and  the  median  ventral  part  of  this 
region.  These  appendages  are  the  antennae. 
They  have  in  the  embryo  a  distinctly  ven- 
tral position  as  compared  to  that  which 
they  have  in  the  adult. 

In  the  median  ventral  part  of  the  pro- 
cephalic  region  there  arises  the  labrum  (fig.  187,  Is}.  It  is  formed 
by  the  coalescence  of  a  pair  of  prominences  very  similar  to  true 
appendages,  though  it  is  probable  that  they  have  not  this 
value1. 

1  If  these  structures  are  equivalent  to  appendages,  they  may  correspond  to  one  of 
the  pairs  of  antennae  of  Crustacea.  From  a  figure  by  Fritz  Miiller  of  the  larva  of 
Calotermes  (Jenaische  Zeit.  Vol.  XI.  pi.  n,  fig.  12)  it  would  appear  that  they  lie  in 
front  of  the  true  antennae,  and  would  therefore  on  the  above  hypothesis  correspond  to 
the  first  pair  of  antennae  of  Crustacea.  Biitschli  (No.  405)  describes  in  the  Bee  a  pair 
of  prominences  immediately  in  front  of  the  mandibles  which  eventually  unite  to  form 
a  kind  of  underlip ;  they  in  some  ways  resemble  true  appendages. 


FIG.  1 86.     EMBRYO 
OF    HYDROPHILUS    PI- 

CEUS  VIEWED  FROM  THE 
VENTRAL  SURFACE. 

(After  Kowalevsky.) 
pc.  I.  procephalic  lobe. 


408 


INSECTA. 


The  antennae  themselves  can  hardly  be  considered  to  have 
the  same  morphological  value  as  the  succeeding  appendages. 
They  are  rather  equivalent  to  paired  processes  of  the  prae-oral 
lobes  of  the  Chaetopoda. 

From  the  first  three  post-oral  segments  there  grow  out  the 
mandibles  and  two  pairs  of  maxillae,  and  from  the  three  following 
segments  the  three  pairs  of  thoracic  appendages.  In  many 
Insects  (cf.  Hydrophilus)  a  certain  .number  of  appendages  of  the 
same  nature  as  the  anterior  ones  are  visible  in  the  embryo  on 
the  abdominal  segments,  a  fact  which  shews  that  Insects  are 
descended  from  ancestors  with  more  than  three  pairs  of  ambu- 
latory appendages. 

In  Apis  according  to  Biitschli  (No.  405)  all  the  abdominal  segments  are 
provided  with  appendages,  which  always 
remain  in  a  very  rudimentary  condition. 
All  trace  of  them  as  well  as  of  the  thoracic 
appendages  is  lost  by  the  time  the  embryo 
is  hatched.  In  the  phytophagous  Hy- 
menoptera  the  larva  is  provided  with 
9 — ii  pairs  of  legs. 

In  the  embryo  of  Lepidoptera  there 
would  appear  from  Kowalevsky's  figures 
to  be  rudiments  of  ten  pairs  of  post-tho- 
racic appendages.  In  the  caterpillar  of 
this  group  there  are  at  the  maximum  five 
pairs  of  such  rudimentary  feet,  viz.  a  pair 
on  the  3rd,  4th,  5th,  and  6th,  and  on  the 
last  abdominal  segment.  The  embryos 
of  Hydrophilus  (fig.  187),  Mantis,  etc.  are 
also  provided  with  additional  appendages. 
In  various  Thysanura  small  prominences 
are  present  on  more  or  fewer  of  the  abdo- 
minal segments  (fig.  192),  which  may 
probably  be  regarded  as  rudimentary 
feet. 

Whether  all  or  any  of  the  appendages 
of  various  kinds  connected  with  the 
hindermost  segments  belong  to  the  same 
category  as  the  legs  is  very  doubtful.  Their  usual  absence  in  the  embryo  or 
in  any  case  their  late  appearance  appears  to  me  against  so  regarding  them ; 
but  Biitschli  is  of  opinion  that  in  the  Bee  the  parts  of  the  sting  are  related 
genetically  to  the  appendages  of  the  penultimate  and  antepenultimate  abdo- 
minal segments,  and  this  view  is  to  some  extent  supported  by  more  recent 


FlG.  187.  TWO  STAGES  IN  THE 
DEVELOPMENT  OF  HYDROPHILUS 

PICEUS.      (From    Gegenbaur,    after 
Kowalevsky. ) 

Is.  labrum;  at.  antenna;  tnd. 
mandible;  nix.  maxilla  I.;  li.  max- 
illa II.;  //>"/"•  feet;  a.  anus. 


TRACHEATA. 


409 


observations  (Kraepelin,  etc.),  and  if  it  holds  true  for  the  Bee  must  be  regarded 
as  correct  for  other  cases  also. 

As  to  the  order  of  the  appearance  of  the  appendages  observations  are  as 
yet  too  scanty  to  form  any  complete  scheme.  In  many  cases  all  the  appen- 
dages appear  approximately  at  the  same  moment,  e.g.  Hydrophilus,  but 
whether  this  holds  good  for  all  Coleoptera  is  by  no  means  certain.  In  Apis 
the  appendages  are  stated  by  Biitschli  to  arise  simultaneously,  but  according 
to  Kowalevsky  the  two  mouth  appendages  first  appear,  then  the  antennae, 
and  still  later  the  thoracic  appendages.  In  the  Diptera  the  mouth  appen- 
dages are  first  formed,  and  either  simultaneously  with  these,  or  slightly  later, 
the  antennae.  In  the  Hemiptera  and  Libellulidae  the  thoracic  appendages 
are  the  first  to  be  formed,  and  the  second  pair  of  maxillae  makes  its  appear- 
ance before  the  other  cephalic  appendages. 

The  history  of  the  changes  in  the  embryonic  appendages  during  the 
attainment  of  the  'adult  con-  . 

dition  is  beyond  the  scope 
of  this  treatise,  but  it  may 
be  noted  that  the  second 
pair  of  maxillae  are  rela- 
tively very  large  in  the 
embryo,  and  not  infre- 
quently (Libellula,  etc.) 
have  more  resemblance  to 
the  ambulatory  than  to  the 
masticatory  appendages. 

The  exact  nature  of  the 
wings  and  their  relation  to 
the  other  segments  is  still 
very  obscure.  They  ap- 
pear as  dorsal  leaf-like  ap- 
pendages on  the  2nd  and 
3rd  thoracic  segments,  and 
are  in  many  respects  simi- 
lar to  the  tracheal  gills 
of  the  larvae  of  Epheme- 
ridae  and  Phryganidae  (fig. 
1 88  A),  of  which  they  are 
supposed  by  Gegenbaur 
and  Lubbock  to  be  modifi- 
cations. The  undoubtedly 
secondary  character  of  the 
closed  tracheal  system  of 
larvae  with  tracheal  gills 
tells  against  this  view. 
Fritz  Miiller  finds  in  the 
larvae  of  Calotermes  ru- 


FIG.  188.  FIGURES  ILLUSTRATING  AQUATIC  RE- 
SPIRATION IN  INSECTS.     (After  Gegenbaur.) 

A.  Hinder  portion   of  the  body  of  Ephemera 
vulgata.     a.  longitudinal  tracheal  trunks;  b.  alimen- 
tary canal ;  c.  tracheal  gills. 

B.  Larva  of  ^Eschna  grandis.     a.  superior  longi- 
tudinal tracheal  trunks ;  b.  their  anterior  end ;  c.  por- 
tion branching  on  proctodaeum  ;  o.  eyes. 

C.  Alimentary  canal  of  the  same  larva  from  the 
side,     a,  b,  and  c.  as  in  B  ;  d.  inferior  tracheal  trunk  ; 
e.   transverse  branches  between   upper    and    lower 
tracheal  trunks. 


410  INSECTA. 


gosus  (one  of  the  Termites)  that  peculiar  and  similar  dorsal  appendages  are 
present  on  the  two  anterior  of  the  thoracic  segments.  They  are  without 
tracheae.  The  anterior  atrophies,  and  the  posterior  acquires  tracheas  and  gives 
rise  to  the  first  pair  of  wings.  The  second  pair  of  wings  is  formed  from 
small  processes  on  the  third  thoracic  segment  like  those  on  the  other  two. 
Fritz  Miiller  concludes  from  these  facts  that  the  wings  of  Insects  are 
developed  from  dorsal  processes  of  the  body,  not  equivalent  to  the  ventral 
appendages.  What  the  primitive  function  of  these  appendages  was  is  not 
clear.  Fritz  Miiller  suggests  that  they  may  have  been  employed  as  respira- 
tory organs  in  the  passage  from  an  aqueous  to  a  terrestrial  existence,  when 
the  Termite  ancestors  lived  in  moist  habitations — a  function  for  which  pro- 
cesses supplied  with  blood-channels  would  be  well  adapted.  The  undoubted 
affinity  of  Insects  to  Myriapods,  coupled  with  the  discovery  by  Moseley  of  a 
tracheal  system  in  Peripatus,  is  however  nearly  fatal  to  the  view  that  Insects 
can  have  sprung  directly  from  aquatic  ancestors  not  provided  with  tracheae. 
But  although  this  suggestion  of  Fritz  Miiller  cannot  be  accepted,  it  is  still 
possible  that  the  processes  discovered  by  him  may  have  been  the  earliest 
rudiments  of  wings,  which  were  employed  first  as  organs  of  propulsion  by  a 
water-inhabiting  Insect  ancestor  which  had  not  yet  acquired  the  power  of 
flying. 

The  nervous  system.  The  nervous  system  arises  entirely 
from  the  epiblast;  but  the  development  of  the  prae-oral  and 
post-oral  sections  may  be  best  considered  separately. 

The  post-oral  section,  or  ventral  cord  of  the  adult,  arises  as 
two  longitudinal  thickenings  of  the  epiblast,  one  on  each  side  of 
the  median  line  (fig.  189  B,  vn),  which  are  subsequently  split  ofif 
from  the  superficial  skin  and  give  rise  to  the  two  lateral  strands 
of  the  ventral  cord.  At  a  later  period  they  undergo  a  differenti- 
ation into  ganglia  and  connecting  cords. 

Between  these  two  embryonic  nerve  cords  there  is  at  first  a  shallow 
furrow,  which  soon  becomes  a  deep  groove  (fig.  189  C).  At  this  stage  the 
differentiation  of  the  lateral  elements  into  ganglia  and  commissures  takes 
place,  and,  according  to  Hatschek  (No.  414),  the  median  groove  becomes  in 
the  region  of  the  ganglia  converted  into  a  canal,  the  walls  of  which  soon  fuse 
with  those  of  the  ganglionic  enlargements  of  the  lateral  cords,  and  connect 
them  across  the  middle  line.  Between  the  ganglia  on  the  other  hand  the 
median  groove  undergoes  atrophy,  becoming  first  a  solid  cord  interposed 
between  the  lateral  strands  of  the  nervous  system,  and  finally  disappearing 
without  giving  rise  to  any  part  of  the  nervous  system.  It  is  probable  that 
Hatschek  is  entirely  mistaken  about  the  entrance  of  a  median  element  into 
the  ventral  cord,  and  that  the  appearances  he  has  described  are  due  to 
shrinkage.  In  Spiders  the  absence  of  a  median  element  can  be  shewn  with 
great  certainty,  and,  as  already  stated,  this  element  is  not  present  in 


TRACHEATA. 


411 


Peripatus.  Hatschek  states  that  in  the  mandibular  segment  the  median 
element  is  absorbed,  and  that  the  two  lateral  cords  of  that  part  give  rise  to 
the  oesophageal  commissures,  while  the  sub-cesophageal  ganglion  is  formed 
from  the  fusion  of  the  ganglia  of  the  two  maxillary  segments. 

The  prae-oral  portion  of  the  nervous  system  consists  entirely 
of  the  supra-cesophageal  ganglion.  It  is  formed,  according  to 
Hatschek,  of  three  parts.  Firstly  and  mainly,  of  a  layer  sepa- 


FIG.  189.    THREE  TRANSVERSE  SECTIONS  THROUGH  THE  EMBRYO  OF 
HYDROPHILUS.     (After  Kowalevsky.) 

A.  Transverse  section  through  the  larva  represented  in  fig.  187  A. 

B.  Transverse  section  through  a  somewhat  older  embryo  in  the  region  of  one  of 
the  stigmata. 

C.  Transverse  section  through  the  larva  represented  in  fig.  187  B. 

vn.  ventral  nerve  cord;  am.  amnion  and  serous  membrane ;  me.  mesoblast ;  me.s. 
somatic  mesoblast ;  hy.  hypoblast  (?) ;  yk.  yolk  cells  (true  hypoblast) ;  st.  stigma  of 
trachea. 

rated  from  the  thickened  inner  part  of  the  cephalic  lobe  on  each 
side ;  secondly,  of  an  anterior  continuation  of  the  lateral  cords ; 
and  thirdly,  of  a  pit  of  skin  invaginated  on  each  side  close  to  the 


412  IN  SECT  A. 


dorsal  border  of  the  antennae.  This  pit  is  at  first  provided  with 
a  lumen,  which  is  subsequently  obliterated;  while  the  walls  of 
the  pit  become  converted  into  true  ganglion  cells.  The  two 
supra-cesophageal  ganglia  remain  disconnected  on  the  dorsal 
side  till  quite  the  close  of  embryonic  life. 

The  tracheae  and  salivary  glands.  The  tracheae,  as  was 
first  shewn  by  Butschli  (No.  405),  arise  as  independent  segment- 
ally  arranged  paired  invaginations  of  the  epiblast  (fig.  189  B  and 
C,  st).  Their  openings  are  always  placed  on  the  outer  sides  of 
the  appendages  of  their  segments,  where  such  are  present. 

Although  in  the  adult  stigmata  are  never  found  in  the  space 
between  the  prothorax  and  head1,  in  the  embryo  and  the  larva 
tracheal  invaginations  may  be  developed  in  all  the  thoracic  (and 
possibly  in  the  three  jaw-bearing  segments)  and  in  all  the 
abdominal  segments  except  the  two  posterior. 

In  the  embryo  of  the  Lepidoptera,  according  to  Hatschek  (No.  414), 
there  are  14  pairs  of  stigmata,  belonging  to  the  14  segments  of  the  body 
behind  the  mouth ;  but  Tichomiroff  states  that  Hatschek  is  in  error  in 
making  this  statement  for  the  foremost  post-oral  segments.  The  last  two 
segments  are  without  stigmata.  In  the  larvae  of  Lepidoptera  as  well  as  those 
of  many  Hymenoptera,  Coleoptera  and  Diptera,  stigmata  are  present  on  all 
the  postcephalic  segments  except  the  2nd  and  3rd  thoracic  and  the  two  last 
abdominal.  In  Apis  there  are  eleven  pairs  of  tracheal  invaginations  accord- 
ing to  Kowalevsky  (No.  416),  but  according  to  Butschli  (No.  405)  only  ten, 
the  prothorax  being  without  one.  In  the  Bee  they  appear  simultaneously, 
and  before  the  appendages. 

The  blind  ends  of  the  tracheal  invaginations  frequently  (e.g. 
Apis)  unite  together  into  a  common  longitudinal  canal,  which 
forms  a  longitudinal  tracheal  stem.  In  other  cases  (eg.  Gryllo- 
talpa,  Dohrn,  No.  408)  they  remain  distinct,  and  each  tracheal 
stem  has  a  system  of  branches  of  its  own. 

The  development  of  the  tracheae  strongly  supports  the  view, 
arrived  at  by  Moseley  from  his  investigations  on  Peripatus,  that 
they  are  modifications  of  cutaneous  glands. 

The  salivary  and  spinning  glands  are  epiblastic  structures, 
which  in  their  mode  of  development  are  very  similar  to  the 
tracheae,  and  perhaps  have  a  similar  origin.  The  salivary  glands 

1  In  Smynthurus,  one  of  the  Collembola,  there  are,  according  to  Lubbock,  only 
two  stigmata,  which  are  placed  on  the  head. 


TRACHEATA.  413 


arise  as  paired  epiblastic  imaginations,  not,  as  might  be 
expected,  of  the  Stomodaeum,  but  of  the  ventral  plate  behind 
the  mouth  on  the  inner  side  of  the  mandibles.  At  first  indepen- 
dent, they  eventually  unite  in  a  common  duct,  which  falls  into 
the  mouth.  The  spinning  glands  arise  on  the  inner  side  of  the 
second  pair  of  maxillae  in  Apis  and  Lepidoptera,  and  form 
elongated  glands  extending  through  nearly  the  whole  length 
of  the  body.  They  are  very  similar  in  their  structure  and  deve- 
lopment to  salivary  glands,  and  are  only  employed  during  larval 
life.  They  no  doubt  resemble  the  mucous  glands  of  the  oral 
papillae  of  Peripatus,  with  which  they  have  been  compared  by 
Moseley.  The  mucous  glands  of  Peripatus  may  perhaps  be  the 
homologous  organs  of  the  first  pair  of  maxillae,  for  the  existence 
of  which  there  appears  to  be  some  evidence  amongst  Insects. 

Mesoblast.  It  has  been  stated  that  the  mesoblast  becomes 
divided  in  the  region  of  the  body  into  two  lateral  bands  (fig.  189 
A).  These  bands  in  many,  if  not  all  forms,  become  divided 
into  a  series  of  somites  corresponding  with  the  segments  of  the 
body.  In  each  of  them  a  cavity  appears — the  commencing 
perivisceral  cavity — which  divides  them  into  a  somatic  plate  in 
contact  with  the  epiblast,  and  a  splanchnic  plate  in  contact  with 
the  hypoblast  (fig.  189).  In  the  interspaces  between  the 
segments  the  mesoblast  is  continuous  across  the  median  ventral 
line.  The  mesoblast  is  prolonged  into  each  of  the  appendages 
as  these  are  formed,  and  in  the  appendages  there  is  present  a 
central  cavity.  By  Metschnikoff  these  cavities  are  stated  to  be 
continuous,  as  in  Myriapods  and  Arachnida,  with  those  of  the 
somites ;  but  by  Hatschek  (No.  414)  they  are  stated  to  be 
independent  of  those  in  the  somites  and  to  be  open  to  the  yolk. 

The  further  details  of  the  history  of  the  mesoblast  are  very  imperfectly 
known,  and  the  fullest  account'  we  have  is  that  by  Dohrn  (No.  408)  for 
Gryllotalpa.  It  would  appear  that  the  mesoblast  grows  round  and  encloses 
the  dorsal  side  of  the  yolk  earlier  than  the  epiblast.  In  Gryllotalpa  it  forms 
a  pulsating  membrane.  As  the  epiblast  extends  dorsalwards  the  median 
dorsal  part  of  the  membrane  is  constricted  off  as  a  tube  which  forms  the 
heart.  At  the  same  time  the  free  space  between  the  pulsating  membrane 
and  the  yolk  is  obliterated,  but  transverse  passages  are  left  at  the  lines 
between  the  somites,  through  which  the  blood  passes  from  the  ventral  part  of 
the  body  to  corresponding  openings  in  the  wall  of  the  heart.  The  greater 
part  of  the  membrane  gives  rise  to  the  muscles  of  the  trunk. 


414  INSECTA. 


Ventrally  the  mesoblastic  bands  soon  meet  across  the  median  line.  The 
cavities  in  the  appendages  become  obliterated  and  their  mesoblastic  walls 
form  the  muscles,  etc.  The  cavities  in  the  separate  mesoblastic  somites  also 
cease  to  be  distinctly  circumscribed. 

The  splanchnic  mesoblast  follows  the  hypoblast  in  its  growth,  and  gives 
rise  to  the  connective  tissue  and  muscular  parts  of  the  walls  of  the  aliment- 
ary tract.  The  mesoblastic  wall  of  the  proctodaeum  is  probably  formed 
independently  of  the  mesoblastic  somites.  In  the  head  the  mesoblast  is 
stated  to  form  at  first  a  median  ventral  mass,  which  does  not  pass  into  the 
procephalic  lobe  ;  though  it  assists  in  forming  both  the  antennae  and  upper 
lip. 

The  alimentary  canal.  The  alimentary  tract  of  Insects  is 
formed  of  three  distinct  sections  (fig.  181) — a  mesenteron  or 
middle  section  (me),  a  stomodaeum  (st)  and  a  proctodaeum  (an). 
The  stomodaeum  and  proctodaeum  are  invaginations  of  the 
epiblast,  while  the  mesenteron  is  lined  by  the  hypoblast.  The 
distinction  between  the  three  is  usually  well  marked  in  the  adult 
by  the  epiblastic  derivatives  being  lined  by  chitin.  The  stomo- 
daeum consists  of  mouth,  oesophagus,  crop,  and  proventriculus  or 
gizzard,  when  such  are  present.  The  mesenteron  includes  the 
stomach,  and  is  sometimes  (Orthoptera,  etc.)  provided  at  its 
front  end  with  pyloric  diverticula — posteriorly  it  terminates  just 
in  front  of  the  Malpighian  bodies.  These  latter  fall  into  the 
proctodaeum,  which  includes  the  whole  of  the  region  from  their 
insertion  to  the  anus. 

The  oral  invagination  appears  nearly  coincidently.  with  the 
first  formation  of  segments  at  the  front  end  of  the  groove 
between  the  lateral  nerve  cords,  and  the  anal  invagination 
appears  slightly  later  at  the  hindermost  end  of  the  ventral  plate. 

The  Malpighian  bodies  arise  as  two  pairs  of  outgrowths  of  the 
epiblast  of  t/te  proctodceum,  whether  solid  at  first  is  not  certain. 
The  subsequent  increase  which  usually  takes  place  in  their 
number  is  due  to  sproutings  (at  first  solid)  of  the  two  original 
vessels. 

The  glandular  walls  of  the  mesenteron  are  formed  from  the  hypoblast ; 
but  the  exact  origin  of  the  layer  has  not  been  thoroughly  worked  out  in  all 
cases.  In  Hydrophilus  it  is  stated  by  Kowalevsky  (No.  416)  to  appear  as 
two  sheets  split  off  from  the  lateral  masses  of  mesoblast,  which  gradually 
grow  round  the  yolk,  and  a  similar  mode  of  formation  would  seem  to  hold 
good  for  Apis.  Tichomiroff  (No.  420)  confirms  Kowalevsky  on  this  point, 


TR  ACHE  AT  A.  415 


and  further  states  that  these  two  masses  meet  first  ventrally  and  much  later 
on  the  dorsal  side.  In  Lepidoptera,  on  the  other  hand,  Hatschek  finds  that 
the  hypoblast  arises  as  a  median  mass  of  polygonal  cells  in  the  anterior  part 
of  the  ventral  plate.  These  cells  increase  by  absorbing  material  from  the 
yolk,  and  then  gradually  extend  themselves  and  grow  round  the  yolk. 

Dohrn  (No.  408)  believes  that  the  yolk  cells,  the  origin  of  which  has 
already  been  spoken  of,  give  rise  to  the  hypoblastic  walls  of  the  mesenteron, 
and  this  view  appears  to  be  shared  by  Graber  (No.  412),  though  the  latter 
author  holds  that  some  of  the  yolk  cells  are  derived  by  budding  from  the 
blastoderm1. 

From  the  analogy  of  Spiders  I  am  inclined  to  accept  Dohrn's  and 
Graber's  view.  It  appears  to  me  probable  that  Kowalevsky's  observations 
are  to  be  explained  by  supposing  that  the  hypoblast  plates  which  he  believes 
to  be  split  off  from  the  mesoblast  are  really  separated  from  the  yolk. 

.It  will  be  convenient  to  add  here  a  few  details  to  what  has  already  been 
stated  as  to  the  origin  of  the  yolk  cells.  As  mentioned  above,  the  central 
yolk  breaks  up  at  a  period,  which  is  not  constant  in  the  different  forms,  into 
polygonal  or  rounded  masses,  in  each  of  which  a  nucleus  has  in  many 
instances  been  clearly  demonstrated  although  in  others  such  nuclei  have  not 
been  made  out.  It  is  probable  however  that  nuclei  are  in  all  cases  really 
present,  and  that  these  masses  must  be  therefore  regarded  as  cells.  They 
constitute  in  fact  the  yolk  cells.  The  periphery  of  the  yolk  breaks  up  into 
cells  while  the  centre  is  still  quite  homogeneous. 

The  hypoblastic  walls  of  the  mesenteron  appear  to  be  formed 
in  the  first  instance  laterally  (fig.  189  B  and  C,  hy).  They  then 
meet  ventrally  (fig.  185  A  and  B),  and  finally  close  in  the 
mesenteron  on  the  dorsal  side. 

The  mesenteron  is  at  first  a  closed  sack,  independent  of  both 
stomodaeum  and  proctodaeum  ;  and  in  the  case  of  the  Bee  it  so 
remains  even  after  the  close  of  embryonic  life.  The  only  gland- 
ular organs  of  the  mesenteron  are  the  not  unfrequent  pyloric 
tubes,  which  are  simple  outgrowths  of  its  anterior  end.  It  is 
possible  that  in  some  instances  they  may  be  formed  in  situ 
around  the  lateral  parts  of  the  yolk. 

In  many  instances  the  whole  of  the  yolk  is  enclosed  in  the  walls  of  the 
mesenteron,  but  in  other  cases,  as  in  Chironomus  and  Simulia  (Weismann, 
No.  430 ;  Metschnikoff,  No.  423),  part  of  the  yolk  may  be  left  between  the 
ventral  wall  of  the  mesenteron  and  the  ventral  plate.  In  Chironomus  the 

1  Graber's  view  on  this  point  may  probably  be  explained  by  supposing  that  he  has 
mistaken  a  passage  of  yolk  cells  into  the  blastoderm  for  a  passage  of  blastoderm  cells 
into  the  yolk.  The  former  occurrence  takes  place,  as  I  have  found,  largely  in  Spiders, 
and  probably  therefore  also  occurs  in  Insects. 


41 6  INSECTA. 


mass  of  yolk  external  to  the  mesenteron  takes  the  form  of  a  median  and  two 
lateral  streaks.  Some  of  the  yolk  cells  either  prior  to  the  establishment  of 
the  mesenteron,  or  derived  from  the  unenclosed  portions  of  the  yolk,  pass 
into  the  developing  organs  (Dohrn,  408)  and  serve  as  a  kind  of  nutritive  cell. 
They  also  form  blood  corpuscles  and  connective-tissue  elements.  Such  yolk 
cells  may  be  compared  to  the  peculiar  bodies  described  by  Reichenbach  in 
Astacus,  which  form  the  secondary  mesoblast.  Similar  cells  play  a  very 
important  part  in  the  development  of  Spiders. 

Generative  organs.  The  observations  on  the  development  of  the 
generative  organs  are  somewhat  scanty.  In  Diptera  certain  cells — known 
as  the  pole  cells — are  stated  by  both  Metschnikoff  (No.  423)  and  Leuckart  to 
give  rise  to  the  generative  organs.  The  cells  in  question  (in  Chironomus 
and  Musca  vomitoria,  Weismann,  No.  430)  appear  at  the  hinder  end  of  the 
ovum  before  any  other  cells  of  the  blastoderm.  They  soon  separate  from 
the  blastoderm  and  increase  by  division.  In  the  embryo,  produced  by  the 
viviparous  larva  of  Cecidomyia,  there  is  at  first  a  single  pole  cell,  which 
eventually  divides  into  four,  and  the  resulting  cells  become  enclosed  within 
the  blastoderm.  They  next  divide  into  two  masses,  which  are  stated  by 
Metschnikoff  (No.  423)  to  become  surrounded  by  indifferent  embryonic  cells1. 
Their  protoplasm  then  fuses,  and  their  nuclei  divide,  and  they  give  rise  to 
the  larval  ovaries,  for  which  the  enclosing  cells  form  the  tunics. 

In  Aphis  Metschnikoff  (No.  423)  detected  at  a  very  early  stage  a  mass 
of  cells  which  give  rise  to  the  generative  organs.  These  cells  are  situated 
at  the  hind  end  of  the  ventral  plate ;  and,  except  in  the  case  of  one  of  the 
cells  which  gives  rise  by  division  to  a  green  mass  adjoining  the  fat  body, 
the  protoplasm  of  the  separate  cells  fuses  into  a  syncytium.  Towards  the 
close  of  embryonic  life  the  syncytium  assumes  a  horse-shoe  form.  The  mass 
is  next  divided  into  two,  and  the  peripheral  layer  of  each  part  gives  rise 
to  the  tunic,  while  from  the  hinder  extremity  of  each  part  an  at  first  solid 
duct— the  egg- tube — grows  out.  The  masses  themselves  form  the  ger- 
mogens.  The  oviduct  is  formed  by  a  coalescence  of  the  ducts  from  each 
germogen. 

Ganin  derives  the  generative  organs  in  Platygaster  (vide  p.  347)  from 
the  hind  end  of  the  ventral  plate  close  to  the  proctodaeum  ;  while  Suckow 
states  that  the  generative  organs  are  outgrowths  of  the  proctodicum. 
According  to  these  two  sets  of  observations  the  generative  organs  would 
appear  to  have  an  epiblastic  origin — an  origin  which  is  not  incompatible 
with  that  from  the  pole  cells. 

In  Lepidoptera  the  genital  organs  are  present  in  the  later  periods  of 
embryonic  life  as  distinct  paired  organs,  one  on  each  side  of  the  heart,  in 
the  eighth  postcephalic  segment.  They  are  elliptical  bodies  with  a  duct 
passing  off  from  the  posterior  end  in  the  female  or  from  the  middle  in  the 
male.  The  egg-tubes  or  seminal  tubes  are  outgrowths  of  the  elliptical 
bodies. 

1  This  point  requires  further  observation. 


TRACHEATA. 


417 


In  other  Insects  the  later  stages  in  the  development  of  the  generative 
organs  closely  resemble  those  in  the  Lepidoptera,  and  the  organs  are  usually 
distinctly  visible  in  the  later  stages  of  embryonic  life. 

It  may  probably  be  laid  down,  in  spite  of  some  of  Metschnikoff's 
observations  above  quoted,  that  the  original  generative  mass  gives  rise  to 
both  the  true  genital  glands  and  their  ducts.  It  appears  also  to  be  fairly 
clear  that  the  genital  glands  of  both  sexes  have  an  identical  origin. 

Special  types  of  larva. 

Certain  of  the  Hymenopterous  forms,  which  deposit  their  eggs  in  the 
eggs  or  larvae  of  other  Insects,  present  very  peculiar  modifications  in  their 
development.  Platygaster,  which  lays  its  egg  in  the  larvae  of  Cecidomyia, 
undergoes  perhaps  the  most  remarkable  development  amongst  these  forms. 
It  has  been  studied  especially  by  Ganin  (No.  410),  from  whom  the  following 
account  is  taken. 

The  very  first  stages  are  unfortunately  but  imperfectly  known,  and  the 
interpretations  offered  by  Ganin  do  not  in  all  cases  appear  quite  satis- 
factory. In  the  earliest  stage  after  being  laid  the  egg  is  enclosed  in  a 
capsule  produced  into  a  stalk  (fig.  190  A).  In  the  interior  of  the  egg 
there  soon  appears  a  single  spherical  body,  regarded  by  Ganin  as  a  cell 
(fig.  190  B).  In  the  next  stage  three  similar  bodies  appear  in  the  vitellus, 
no  doubt  derived  from  the  first  one  (fig.  190  C).  The  central  one  presents 
somewhat  different  characters  to  the  two  others,  and,  according  to  Ganin, 
gives  rise  to  the  whole  embryo.  The  two  peripheral  bodies  increase  by 
division,  and  soon  ap- 
pear as  nuclei  imbed- 
ded in  a  layer  of  pro- 
toplasm (fig.  190  D, 
E,  F).  The  layer  so 
formed  serves  as  a 
covering  for  the  em- 
bryo, regarded  by 
Ganin  as  equivalent 
to  the  amnion  (?  se- 
rous membrane)  of 
other  Insect  em- 
bryos. In  the  em- 
bryo cell  new  cells 
are  stated  to  be 
formed  by  a  process 
of  endogenous  cell  formation  (fig.  190  D,  E).  It  appears  probable  that 
Ganin  has  mistaken  nuclei  for  cells  in  the  earlier  stages,  and  that  a  blasto- 
derm is  formed  as  in  other  Insects,  and  that  this  becomes  divided  in  a  way 
not  explained  into  a  superficial  layer  which  gives  rise  to  the  serous 
envelope,  and  a  deeper  layer  which  forms  the  embryo.  However  this 

B.  II.  27 


FlG.    190.      A  SERIES  OF  STAGES  IN  THE  DEVELOPMENT 

OF  PLATYGASTER.    (From  Lubbock  ;  after  Ganin.) 


41 8  INSECTA. 


may  be,  a  differentiation  into  an  epiblastic  layer  of  columnar  cells  and 
a  hypoblastic  layer  of  more  rounded  cells  soon  becomes  apparent  in  the  body 
of  the  embryo.  Subsequently  to  this  the  embryo  grows  rapidly,  till  by  a 
deep  transverse  constriction  on  the  ventral  surface  it  becomes  divided  into  an 
anterior  cephalothoracic  portion  and  a  posterior  caudal  portion  (fig.  190  F). 
The  cephalothorax  grows  in  breadth,  and  near  its  anterior  end  an  in- 
vagination  appears,  which  gives  rise  to  the  mouth  and  cesophagus.  On 
the  ventral  side  of  the  cephalothorax  there  is  first  formed  a  pair  of 
claw-like  appendages  on  each  side  of  the  mouth,  then  a  posterior  pair  of 
appendages  near  the  junction  of  the  cephalothorax  and  abdomen,  and 
lastly  a  pair  of  short  conical  antennae  in  front. 

At  the  same  time  the  hind  end  of  the  abdomen  becomes  bifid,  and  gives 
rise  to  a  fork-like  caudal  appendage  ;  and  at  a  slightly  later  period  four 
grooves  make  their  appearance  in  the  caudal  region,  and  divide  this  part  of 
the  embryo  into  successive  segments.  While  these  changes  have  been 
taking  place  in  the  general  form  of  the  embryo,  the  epiblast  has  given  rise 
to  a  cuticle,  and  the  hypoblastic  cells  have  become  differentiated  into  a 
central  hypoblastic  axis — the  mesenteron — and  a  surrounding  layer  of 
mesoblast,  some  of  the  cells  of  which  form  longitudinal  muscles. 

With  this  stage  closes  what  may  be  regarded  as  the  embryonic  develop- 
ment of  Platygaster.  The  embryo  becomes  free  from  the  amnion,  and  pre- 
sents itself  as  a  larva,  which  from  its  very  remarkable  characters  has  been 
spoken  of  as  the  Cyclops  larva  by  Ganin. 

The  larvae  of  three  species  have  been  described  by  Ganin,  which  are  repre- 
sented in  fig.  1 9 1  A,  B,  C.  These  larvae  are  strangely  dissimilar  to  the  ordinary 
Hexapod  type,  whether  larval  or  adult.  They  are  formed  of  a  cephalothoracic 
shield  with  the  three  pairs  of  appendages  (a,  kf,  lfg\  the  development  of 
which  has  already  been  described,  and  of  an  abdomen  formed  of  five  seg- 
ments, the  last  of  which  bears  the  somewhat  varying  caudal  appendages. 
The  nervous  system  is  as  yet  undeveloped. 

The  larvae  move  about  in  the  tissues  of  their  hosts  by  means  of  their 
claws. 

The  first  larval  condition  is  succeeded  by  a  second  with  very  different 
characters,  and  the  passage  from  the  first  to  the  second  is  accompanied  by 
an  ecdysis. 

The  ecdysis  commences  at  the  caudal  extremity,  and  the  whole  of  the 
last  segment  is  completely  thrown  off.  As  the  ecdysis  extends  forwards 
the  tail  loses  its  segmentation  and  becomes  strongly  compressed,  the 
appendages  of  the  cephalothorax  are  thrown  off,  and  the  whole  embryo 
assumes  an  oval  form  without  any  sharp  distinction  into  different  regions 
and  without  the  slightest  indication  of  segmentation  (fig.  191  D).  Of  the 
internal  changes  which  take  place  during  the  shedding  of  the  cuticle,  the 
first  is  the  formation  of  a  proctodaeum  (gfi)  by  an  invagination,  which  ends 
blindly  in  contact  with  the  mesenteron.  Shortly  after  this  a  thickening  of 
the  epiblast  (bsm}  appears  along  the  ventral  surface,  which  gives  rise  mainly 
to  the  ventral  nerve  cord  ;  this  thickening  is  continuous  behind  with  the 


TRACHEATA. 


419 


epiblast  which  is  invaginated  to  form  the  proctodaeum,  and  in  front  is  pro- 
longed on  each  side  into  two  procephalic  lobes,  in  which  there  are  also 
thickenings  of  the  epiblast  (gsae),  which  become  converted  into  supra- 
oesophageal  ganglia,  and  possibly  other  parts. 

Towards  the  close  of  the  second  larval  period  the  muscles  (/;«)  become 
segmentally  arranged,  and  give   indications    of   the    segmentation   which 


FlG.  191.      A  SERIES  OF  STAGES  IN  THE  DEVELOPMENT  OF  PLATYGASTER. 

(From  Lubbock  ;  after  Ganin.) 

A.  B.  C.     Cyclops  larvae  of  three  species  of  Platygaster. 
D.     Second  larval  stage.     E.     Third  larval  stage. 

mo.  mouth  ;  a.  antenna ;  kf.  hooked  feet ;  Ifg.  lateral  feet ;  /.  branches  of  tail ; 
ul.  lower  lip ;  slkf.  oesophagus ;  gsae.  supra- oesophageal  ganglion ;  bsm.  ventral  epi- 
blastic  plate ;  Im.  lateral  muscles  (the  letters  also  point  in  D  to  the  salivary  glands) ; 
gh.  proctodseum ;  ga.  generative  organs ;  md.  mandibles ;  ag.  ducts  of  salivary  glands ; 
sp.  (in  E)  salivary  glands ;  mis.  stomach ;  ed.  intestine ;  ew.  rectum ;  ao.  anus ; 
tr.  tracheae  ;  fk.  fat  body. 

becomes  apparent  in  the  third  larval  period.  The  third  and  last  larval 
stage  (fig.  191  E)  of  Platygaster,  during  which  it  still  remains  in  the  tissues 
of  its  host,  presents  no  very  peculiar  features.  The  passage  from  the  second 
to  the  third  form  is  accompanied  by  an  ecdysis. 

Remarkable  as  are  the  larvae  just  described,  there  can  I  think  be 
no  reason,  considering  their  parasitic  habits,  for  regarding  them  as  ancestral. 

27—2 


420  INSECTA. 


Metamorphosis  and  heterogamy. 

Metamorphosis.  The  majority  of  Insects  are  born  in  a 
condition  in  which  they  obviously  differ  from  their  parents.  The 
extent  of  this  difference  is  subject  to  very  great  variations,  but 
as  a  rule  the  larvae  pass  through  a  very  marked  metamorphosis 
before  reaching  the  adult  state.  The  complete  history  of  this 
metamorphosis  in  the  different  orders  of  Insects  involves  a  far 
too  considerable  amount  of  zoological  detail  to  be  dealt  with  in 
this  work  ;  and  I  shall  confine  myself  to  a  few  observations  on 
the  general  characters  and  origin  of  the  metamorphosis,  and  of 
the  histological  processes  which  take  place  during  its  occur- 
rence1. 

In  the  Aptera  the  larva  differs  from  the  adult  only  in  the 
number  of  facets  in  the  cornea  and  joints  in  the  antennae. 

In  most  Orthoptera  and  Hemiptera  the  larvae  differ  from  the 
adult  in  the  absence  of  wings  and  in  other  points.  The  wings, 
etc.,  are  gradually  acquired  in  the  course  of  a  series  of  successive 
moultings.  In  the  Ephemeridae  and  Libellulidae,  however,  the 
metamorphosis  is  more  complicated,  in  that  the  larvae  have 
provisional  tracheal  gills  which  are  exuviated  before  the  final 
moult.  In  the  Ephemeridae  there  are  usually  a  great  number  of 
moultings  ;  the  tracheal  gills  appear  after  the  second  moult,  and 
the  rudiments  of  the  wings  when  the  larva  is  about  half  grown. 
Larval  life  may  last  for  a  very  long  period. 

In  all  the  other  groups  of  Insects,  viz.  the  Diptera,  Neuro- 
ptera,  Coleoptera,  Lepidoptera,  and  Hymenoptera,  the  larva 
passes — with  a  few  exceptions — through  a  quiescent  stage,  in 
which  it  is  known  as  a  pupa,  before  it  attains  the  adult  stage. 
These  forms  are  known  as  the  Holometabola. 

In  the  Diptera  the  larvae  are  apodous.  In  the  true  flies  (Muscidae)  they 
are  without  a  distinct  head  and  have  the  jaws  replaced  by  hooks.  In  the 
Tipulidae  there  is  on  the  other  hand  a  well-developed  head  with  the  normal 
appendages.  The  pupae  of  the  Muscidae  are  quiescent,  and  are  enclosed  in 
the  skin  of  the  larva  which  shrinks  and  forms  a  firm  oval  case.  In  the 

1  For  a  systematic  account  of  this  subject  the  reader  is  referred  to  Lubbock  (No. 
420)  and  to  Graber  (No.  411).  He  will  find  in  Weismann  (Nos.  430  and  431)  a  detailed 
account  of  the  internal  changes  which  take  place. 


TRACHEATA.  42 1 


Tipulidae  the  larval  skin  is  thrown  off  at  the  pupa  stage,  and  in  some  cases 
the  pupae  continue  to  move  about. 

The  larvae  of  the  Neuroptera  are  hexapodous  voracious  forms.  When  the 
larva  becomes  a  pupa  all  the  external  organs  of  the  imago  are  already 
established.  The  pupa  is  often  invested  in  a  cocoon.  It  is  usually  quiescent, 
though  sometimes  it  begins  to  move  about  shortly  before  the  imago  emerges. 

In  the  Coleoptera  there  is  considerable  variety  in  the  larval  forms.  As  a 
rule  the  larvae  are  hexapodous  and  resemble  wingless  Insects.  But  some 
herbivorous  larvae  (e.g.  the  larva  of  Melolontha)  closely  resemble  true 
caterpillars,  and  there  are  also  grub-like  larvae  without  feet  (Curculio)  which 
resemble  the  larvae  of  Hymenoptera.  The  pupa  is  quiescent,  but  has  all 
the  parts  of  the  future  beetle  plainly  visible.  The  most  interesting  larvae 
among  the  Coleoptera  are  those  of  Sitaris,  one  of  the  Meloidae  (Fabre,  No. 
409).  They  leave  the  egg  as  active  hexapodous  larvae  which  attach  them- 
selves to  the  bodies  of  Hymenoptera,  and  are  thence  transported  to  a  cell 
filled  with  honey.  Here  they  eat  the  ovum  of  the  Hymenopterous  form. 
They  then  undergo  an  ecdysis,  in  which  they  functionally  lose  their  append- 
ages, retaining  however  small  rudiments  of  them,  and  become  grubs.  They 
feed  on  the  honey  and  after  a  further  ecdysis  become  pupae. 

In  the  Lepidoptera  the  larva  has  the  well-known  form  of  a  caterpillar. 
The  caterpillars  have  strong  jaws,  adapted  for  biting  vegetable  tissues, 
which  are  quite  unlike  the  oral  appendages  of  the  adult.  They  have  three 
pairs  of  jointed  thoracic  legs,  and  a  variable  number  (usually  five)  of  pairs 
of  rudimentary  abdominal  legs — the  so-called  pro-legs.  The  larva  undergoes 
numerous  ecdyses,  and  the  external  parts  of  the  adult  such  as  the  wings,  etc., 
are  formed  underneath  the  chitinous  exoskeleton  before  the  pupa  stage. 
The  pupa  is  known  as  a  chrysalis  and  in  some  Lepidoptera  is  enveloped  in 
a  cocoon. 

The  Hymenoptera  present  considerable  variations  in  the  character  of  the 
larvae.  In  the  Aculeata,  many  Entomophaga,  the  Cynipidae,  etc.,  the  larvae 
are  apodous  grubs,  incapable  of  going  in  search  of  their  food ;  but  in  the 
Siricidse  they  are  hexapodous  forms  like  caterpillars,  which  are  sometimes 
even  provided  with  pro-legs.  In  some  of  the  Entomophaga  the  larvae  have 
very  remarkable  characters  which  have  already  been  described  in  a  special 
section,  'vide  pp.  418,  419. 

Before  proceeding  to  the  consideration  of  the  value  of  the 
various  larval  forms  thus  shortly  enumerated,  it  is  necessary  to 
say  a  few  words  as  to  the  internal  changes  which  take  place 
during  the  occurrence  of  the  above  metamorphosis.  In  the 
simplest  cases,  such  as  those  of  the  Orthoptera  and  Hemiptera, 
where  the  metamorphosis  is  confined  to  the  gradual  formation 
of  the  wings,  etc.  in  a  series  of  moults,  the  wings  first  appear  as 
two  folds  of  the  epidermis  beneath  the  cuticle  on  the  two 
posterior  thoracic  segments.  At  the  next  moult  these  processes 


422  INSECTA. 


become  covered  by  the  freshly  formed  cuticle,  and  appear  as 
small  projections.  At  every  successive  moult  these  projections 
become  more  prominent  owing  to  a  growth  in  the  epidermis 
which  has  taken  place  in  the  preceding  interval.  Accompanying 
the  formation  of  such  organs  as  the  wings,  internal  changes 
necessarily  take  place  in  the  arrangement  of  the  muscles,  etc.  of 
the  thorax,  which  proceed  pari  passu  with  the  formation  of  the 
organs  to  which  they  belong.  The  characters  of  the  metamor- 
phosis in  such  forms  as  the  Ephemeridae  only  differ  from  the 
above  in  the  fact  that  provisional  organs  are  thrown  off  at  the 
same  time  that  the  new  ones  are  formed. 

In  the  case  of  the  Holometabola  the  internal  phenomena  of 
the  metamorphosis  are  of  a  very  much  more  remarkable  cha- 
racter. The  details  of  our  knowledge  are  largely  due  to  Weis- 
mann  (Nos.  430  and  431).  The  larvae  of  the  Holometabola  have 
for  the  most  part  a  very  different  mode  of  life  to  the  adults. 
A  simple  series  of  transitions  between  the  two  is  impossible, 
because  intermediate  forms  would  be  for  the  most  part  incapable 
of  existing.  The  transition  from  the  larval  to  the  adult  state  is 
therefore  necessarily  a  more  or  less  sudden  one,  and  takes  place 
during  the  quiescent  pupa  condition.  Many  of  the  external 
adult  organs  are  however  formed  prior  to  the  pupa  stage,  but  do 
not  become  visible  on  the  surface.  The  simplest  mode  of  Holo- 
metabolic  metamorphosis  may  be  illustrated  by  the  development 
of  Corethra  plumicornis,  one  of  the  Tipulidae.  This  larva,  like 
that  of  other  Tipulidae,  is  without  thoracic  appendages,  but 
before  the  last  larval  moult,  and  therefore  shortly  before  the 
pupa  stage,  certain  structures  are  formed,  which  Weismann  has 
called  imaginal  discs.  These  imaginal  discs  are  in  Corethra 
simply  invaginations  of  the  epidermis.  There  are  in  the  thorax 
six  pairs  of  such  structures,  three  dorsal  and  three  ventral.  The 
three  ventral  are  attached  to  the  terminations  of  the  sensory 
nerves,  and  the  limbs  of  the  imago  are  formed  as  simple 
outgrowths  of  them,  which  as  they  grow  in  length  take  a  spiral 
form.  In  the  interior  of  these  outgrowths  are  formed  the 
muscles,  tracheae,  etc.,  of  the  limbs;  which  are  believed  by 
Weismann  (it  appears  to  me  without  sufficient  ground)  to  be 
derived  from  a  proliferation  of  the  cells  of  the  neurilemma. 
The  wings  are  formed  from  the  two  posterior  dorsal  imaginal 


TRACHEATA.  423 


discs.  The  hypodermis  of  the  larva  passes  directly  into  that  of 
the  imago. 

The  pupa  stage  of  Corethra  is  relatively  very  short,  and  the 
changes  in  the  internal  parts  which  take  place  during  it  are  not 
considerable.  The  larval  abdominal  muscles  pass  for  the  most 
part  unchanged  into  those  of  the  imago,  while  the  special 
thoracic  muscles  connected  with  the  wings,  etc.,  develop  directly 
during  the  latest  larval  period  from  cords  of  cells  already  formed 
in  the  embryo. 

In  the  Lepidoptera  the  changes  in  the  passage  from  the 
larval  to  the  adult  state  are  not  very  much  more  considerable 
than  those  in  Corethra.  Similar  imaginal  discs  give  rise  during 
the  later  larval  periods  to  the  wings,  etc.  The  internal  changes 
during  the  longer  pupa  period  are  somewhat  more  considerable. 
Important  modifications  and  new  formations  arise  in  connec- 
tion with  the  alimentary  tract,  the  nervous  and  muscular 
systems. 

The  changes  which  take  place  in  the  true  flies  (Muscidse)  are 
far  more  complicated  than  either  those  in  Corethra  or  in  the 
Lepidoptera.  The  abdomen  of  the  larva  of  Musca  becomes 
bodily  converted  into  the  abdomen  of  the  imago  as  in  the  above 
types,  but  the  whole  epidermis  and  appendages  of  the  head  and 
thorax  are  derived  from  imaginal  discs  which  are  formed  within 
and  (so  far  as  is  known)  independently  of  the  epidermis  of  the 
larva  or  embryo.  These  imaginal  discs  are  simple  masses  of 
apparently  indifferent  cells,  which  for  the  most  part  appear  at 
the  close  of  embryonic  life,  and  are  attached  to  nerves  or 
tracheae.  They  grow  in  size  during  larval  life,  but  during  the 
relatively  long  pupa  stage  they  unite  together  to  give  rise  to  a 
continuous  epidermis,  from  which  the  appendages  grow  out  as 
processes.  The  epidermis  of  the  anterior  part  of  the  larva  is 
simply  thrown  off,  and  has  no  share  in  forming  the  epidermis  of 
the  adult. 

There  are  a  pair  of  cephalic  imaginal  discs  and  six  pairs  of 
thoracic  discs.  Two  pairs,  a  dorsal  and  a  ventral,  give  rise  to 
each  thoracic  ring,  and  the  appendages  attached  to  it. 

Though,  as  mentioned  above,  no  evidence  has  yet  been 
produced  to  shew  that  the  imaginal  discs  of  Musca  are  derived 
from  the  embryonic  epiblast,  yet  their  mode  of  growth  and 


424  1NSECTA. 


eventual  fate  proves  beyond  the  shadow  of  a  doubt  that  they  are 
homologous  with  the  imaginal  discs  of  Corethra.  Their  earliest 
origin  is  well  worth  further  investigation. 

The  metamorphosis  of  the  internal  organs  is  still  more 
striking  than  that  of  the  external.  There  is  a  disruption,  total 
or  partial,  of  all  the  internal  organs  except  the  generative 
organs.  In  the  case  of  the  alimentary  tract,  the  Malpighian 
vessels,  the  heart  and  the  central  nervous  system,  the  disruption 
is  of  a  partial  kind,  which  has  been  called  by  Weismann 
histolysis.  The  cells  of  these  organs  undergo  a  fatty  degenera- 
tion, the  nuclei  alone  in  some  cases  remaining.  The  kind  of 
plasma  resulting  from  this  degeneration  retains  the  shape  of  the 
organs,  and  finally  becomes  built  up  again  into  the  correspond- 
ing organs  of  the  imago.  The  tracheae,  muscles  and  peripheral 
nerves,  and  an  anterior  part  of  the  alimentary  tract,  are  entirely 
disrupted.  They  seem  to  be  formed  again  from  granular  cells 
derived  from  the  enormous  fat  body. 

The  phenomena  of  the  development  of  the  Muscidse  are  undoubtedly  of 
rather  a  surprising  character.  Leaving  for  the  moment  the  question  of  the 
origin  of  the  pupa  stage  to  which  I  return  below,  it  will  be  admitted  on  all 
hands  that  during  the  pupa  stage  the  larva  undergoes  a  series  of  changes 
which,  had  they  taken  place  by  slow  degrees,  would  have  involved,  in  such  a 
case  as  Musca,  a  complete  though  gradual  renewal  of  the  tissues.  Such 
being  the  case,  the  cells  of  the  organs  common  to  the  larva  and  the  imago 
would,  in  the  natural  course  of  things,  not  be  the  same  cells  as  those  of  the 
larva  but  descendants  of  them.  We  might  therefore  expect  to  find  in  the 
rapid  conversion  of  the  larval  organs  into  those  of  the  adult  some  condensa- 
tion, so  to  speak,  of  the  process  of  ordinary  cell  division.  Such  condensations 
are  probably  represented  in  the  histolysis  in  the  case  of  the  internal  organs, 
and  in  the  formation  of  imaginal  discs  in  the  case  of  the  external  ones,  and 
I  think  it  probable  that  further  investigation  will  shew  that  the  imaginal 
discs  of  the  Muscidae  are  derivatives  of  the  embryonic  epiblast.  The  above 
considerations  by  no  means  explain  the  whole  of  Weismann's  interesting 
observations,  but  an  explanation  is  I  believe  to  be  found  by  following  up 
these  lines. 

More  or  less  parallel  phenomena  to  those  in  Insects  are  found  in  the 
development  of  the  Platyelminthes  and  Echinoderms.  The  four  disc-like 
invaginations  of  the  skin  in  many  larval  Nemertines  (vide  p.  198),  which 
give  rise  to  the  permanent  body  wall  of  the  Nemertine,  may  be  compared  to 
the  imaginal  discs.  The  subsequent  throwing  off  of  the  skin  of  Pilidium  or 
larva  of  Desor  is  a  phenomenon  like  the  absorption  of  part  of  the  larval 
skin  of  Musca.  The  formation  of  an  independent  skin  within  the  first  larval 


TR  ACHE  AT  A. 


425 


form  in  the  Distomeaeand  in  the  Cestoda  may  be  compared  to  the  apparently 
independent  formation  of  the  imaginal  discs  in  Musca. 

The  fact  that  in  a  majority  of  instances  it  is  possible  to  trace 
an  intimate  connection  between  the  surroundings  of  a  larva  and 
its  organization  proves  in  the  clearest  way  that  the  characters  of 
the  majority  of  existing  larval  forms  of  Insects  have  owed  their 
origin  to  secondary  adaptations.  A  few  instances  will  illustrate 
this  point. 

In  the  simplest  types  of  metamorphosis,  e.g.  those  of  the 
Orthoptera  genuina,  the  larva  has  precisely  the  same  habits  as 
the  adult.  We  find  that  a  caterpillar 
form  is  assumed  by  phytophagous  larvae 
amongst  the  Lepidoptera,  Hymenoptera 
and  Coleoptera.  Where  the  larva  has 
not  to  go  in  search  of  its  nutriment  the 
grub-like  apodous  form  is  assumed.  The 
existence  of  such  an  apodous  larva  is 
especially  striking  in  the  Hymenoptera, 
in  that  rudiments  of  thoracic  and  abdo- 
minal appendages  are  present  in  the 
embryo  and  disappear  again  in  the  larva. 
The  case  of  the  larva  of  Sitaris,  already 
described  (p.  421),  affords  another  very 
striking  proof  that  the  organization  of 
the  larva  is  adapted  to  its  habits. 

It  follows  from  the  above  that  the 
development  of  such  forms  as  the  Or- 
thoptera genuina  is  more  primitive  than 
that  of  the  holometabolous  forms;  a 
conclusion  which  tallies  with  the  fact 


FIG.  102. 


ANTERIOR 


HALF  OF  CAMPODEA  FRAGI- 
LIS.  (From  Gegenbaur;  af- 
ter Palmen.) 

a.  antennae ;  p.  feet ;  j> ', 
post-tho 
feet;  s. 


stigma. 

that  both  palaeontological   and  anatomical  evidence  shew  the 
Orthoptera  to  be  a  very  primitive  group  of  Insects. 

The  above  argument  probably  applies  with  still  greater  force 
to  the  case  of  the  Thysanura ;  and  it  seems  to  be  probable  that 
this  group  is  more  nearly  related  than  any  other  to  the  primitive 
wingless  ancestors  of  Insects1.  The  characters  of  the  oral 

1  Brauer  and  Lubbock  (No.  421)  have  pointed  out  the  primitive  characters  of  these 
forms,  especially  of  Campodea. 


426  INSECTA. 


appendages  in  this  group,  the  simplicity  of  their  metamorphosis, 
and  the  presence  of  abdominal  appendages  (fig.  192),  all  tell  in 
favour  of  this  view,  while  the  resemblance  of  the  adult  to  the 
larvae  of  the  Pseudoneuroptera,  etc.,  points  in  the  same  direction. 
The  Thysanura  and  Collembola  are  not  however  to  be  regarded 
as  belonging  to  the  true  stock  of  the  ancestors  of  Insects,  but  as 
degenerated  relations  of  this  stock  ;  much  as  Amphioxus  and 
the  Ascidians  are  degenerate  relations  of  the  ancestral  stock  of 
Vertebrates,  and  Peripatus  of  that  of  the  Tracheata.  It  is 
probable  that  all  these  forms  have  succeeded  in  retaining  their 
primitive  characters  from  their  degenerate  habits,  which  pre- 
vented them  from  entering  into  competition  in  the  struggle  for 
existence  with  their  more  highly  endowed  relatives.  While  in  a 
general  way  it  is  clear  that  the  larval  forms  of  Insects  cannot  be 
expected  to  throw  much  light  on  the  nature  of  Insect  ancestors, 
it  does  nevertheless  appear  to  me  probable  that  such  forms  as 
the  caterpillars  of  the  Lepidoptera  are  not  without  a  meaning  in 
this  respect.  It  is  easy  to  conceive  that  even  a  secondary  larval 
form  may  have  been  produced  by  the  prolongation  of  one  of  the 
embryonic  stages  ;  and  the  general  similarity  of  a  caterpillar  to 
Peripatus,  and  the  retention  by  it  of  post-thoracic  appendages,  are 
facts  which  appear  to  favour  this  view  of  the  origin  of  the  cater- 
pillar form. 

The  two  most  obscure  points  which  still  remain  to  be  dealt 
with  in  the  metamorphosis  of  Insects  are  (i)  the  origin  of  the 
quiescent  pupa  stage  ;  (2)  the  frequent  dissimilarity  between  the 
masticatory  apparatus  of  the  larva  and  adult. 

These  two  points  may  be  conveniently  dealt  with  together, 
and  some  valuable  remarks  about  them  will  be  found  in  Lubbock 
(No.  420). 

On  grounds  already  indicated  it  may  be  considered  certain 
that  the  groups  of  Insects  without  a  pupa  stage,  and  with  a  larva 
very  similarly  organised  to  the  adult,  preceded  the  existing 
holometabolic  groups.  The  starting-point  in  the  metamorphosis 
of  the  latter  groups  was  therefore  something  like  that  of  the 
Orthoptera.  Suppose  it  became  an  advantage  to  a  species  that 
the  larva  and  adult  should  feed  in  a  somewhat  different  way,  a 
difference  in  the  character  of  their  mouth  parts  would  soon  make 
itself  manifest ;  and,  since  an  intermediate  type  of  mouth  parts 


TRACHEATA.  427 


would  probably  be  disadvantageous,  there  would  be  a  tendency 
to  concentrate  into  a  single  moult  the  transition  from  the  larval 
to  the  adult  form  of  mouth  parts.  At  each  ordinary  moult  there 
is  a  short  period  of  quiescence,  and  this  period  of  quiescence 
would  naturally  become  longer  in  the  important  moult  at  which 
the  change  in  the  mouth  parts  was  effected.  In  this  way  a 
rudimentary  pupa  stage  might  be  started.  The  pupa  stage, 
once  started,  might  easily  become  a  more  important  factor  in 
the  metamorphosis.  If  the  larva  and  imago  diverged  still  more 
from  each  other,  a  continually  increasing  amount  of  change 
would  have  to  be  effected  at  the  pupa  stage.  It  would  probably 
be  advantageous  to  the  species  that  the  larva  should  not  have 
rudimentary  functionless  wings ;  and  the  establishment  of  the 
wings  as  external  organs  would  therefore  become  deferred  to 
the  pupa  stage.  The  same  would  probably  apply  to  other 
organs. 

Insects  usually  pass  through  the  pupa  stage  in  winter  in  cold 
climates  and  during  the  dry  season  in  the  tropics,  this  stage 
serving  therefore  apparently  for  the  protection  of  the  species 
during  the  inclement  season  of  the  year.  These  facts  are  easily 
explained  on  the  supposition  that  the  pupa  stage  has  become 
secondarily  adapted  to  play  a  part  in  the  economy  of  the 
species  quite  different  from  that  to  which  it  owes  its  origin. 

Heterogamy.  The  cases  of  alternations  of  generations 
amongst  Insects  all  fall  under  the  heading  already  defined  in 
the  introduction  as  Heterogamy.  Heterogamy  amongst  Insects 
has  been  rendered  possible  by  the  existence  of  parthenogenesis, 
which,  as  stated  in  the  introduction,  has  been  taken  hold  of  by 
natural  selection,  and  has  led  to  the  production  of  generations  of 
parthenogenetic  forms,  by  which  a  clear  economy  in  reproduction 
is  effected.  Parthenogenesis  without  heterogamy  occurs  in  a 
large  number  of  forms.  In  Bees,  Wasps,  and  a  Sawfly  (Nematus 
ventricosus)  the  unfertilized  ova  give  rise  to  males.  In  two 
Lepidopterous  genera  (Psyche  and  Solenobia)  the  unfertilized 
ova  give  rise  mainly,  if  not  entirely,  to  females.  Heterogamy 
occurs  in  none  of  the  above  types,  but  in  Psyche  and  Solenobia 
males  are  only  occasionally  found,  so  that  a  series  of  generations 
producing  female  young  from  unfertilized  ova  are  followed  by  a 
generation  producing  young  of  both  sexes  from  fertilized  ova.  It 


428  INSECTA. 


would  be  interesting  to  know  if  the  unimpregnated  female  would 
not  after  a  certain  number  of  generations  give  rise  to  both  males 
and  females ;  such  an  occurrence  might  be  anticipated  on 
grounds  of  analogy.  In  the  cases  of  true  heterogamy  partheno- 
genesis has  become  confined  to  special  generations,  which  differ 
in  their  character  from  the  generations  which  reproduce  them- 
selves sexually.  The  parthenogenetic  generations  generally 
flourish  during  the  season  when  food  is  abundant;  while  the 
sexual  generations  occur  at  intervals  which  are  often  secondarily 
regulated  by  the  season,  supply  of  food,  etc. 

A  very  simple  case  of  this  kind  occurs,  if  we  may  trust  the 
recent  researches  of  Lichtenstein1,  in  certain  Gall  Insects 
(Cynipidae).  He  finds  that  the  female  of  a  form  known  as 
Spathegaster  baccarum,  of  which  both  males  and  females  are 
plentiful,  pricks  a  characteristic  gall  in  certain  leaves,  in  which 
she  deposits  the  fertilized  eggs.  The  eggs  from  these  galls  give 
rise  to  a  winged  and  apparently  adult  form,  which  is  not,  how- 
ever, Spathegaster,  but  is  a  species  considered  to  belong  to  a 
distinct  genus  known  as  Neuroterus  ventricularis.  Only  females 
of  Neuroterus  are  found,  and  they  lay  unfertilized  ova  in  peculiar 
galls  which  develop  into  Spathegaster  baccarum.  Here  we  have 
a  true  case  of  heterogamy,  the  females  which  produce  partheno- 
genetically  having  become  differentiated  from  those  which  pro- 
duce sexually.  Another  interesting  type  of  heterogamy  is  that 
which  has  been  long  known  in  the  Aphides.  In  the  autumn 
impregnated  eggs  are  deposited  by  females,  which  give  rise  in 
the  course  of  the  spring  to  females  which  produce  partheno- 
genetically  and  viviparously.  The  viviparous  females  always 
differ  from  the  females  which  lay  the  fertilized  eggs.  The  gene- 
rative organs  are  of  course  differently  constituted,  and  the  ova  of 
the  viviparous  females  are  much  smaller  than  those  of  the  ovi- 
parous females,  as  is  generally  the  case  in  closely  allied  vivi- 
parous and  oviparous  forms;  but  in  addition  the  former  are 
usually  without  wings,  while  the  latter  are  winged.  The  reverse 
is  however  occasionally  the  case.  An  indefinite  number  of  gene- 
rations of  viviparous  females  may  be  produced  if  they  are  arti- 
ficially kept  warm  and  supplied  with  food ;  but  in  the  course  of 

1  Petites  Nouvelles  Entomolog iyues,  May,  1878. 


TRACHEATA.  429 


nature  the  viviparous  females  produce  in  the  autumn  males  and 
females  which  lay  eggs  with  firm  shells,  and  so  preserve  the 
species  through  the  winter.  The  heterogamy  of  the  allied 
Coccidae  is  practically  the  same  as  that  of  the  Aphidae.  In  the 
case  of  Chermes  and  Phylloxera  the  parthenogenetic  generations 
lay  their  eggs  in  the  normal  way. 

The  complete  history  of  Phylloxera  quercus  has  been  worked 
out  by  Balbiani  (No.  401).  The  apterous  females  during  the 
summer  lay  eggs  developing  parthenogenetically  into  apterous 
females,  which  continue  the  same  mode  of  reproduction.  In  the 
autumn,  however,  the  eggs  which  are  laid  give  rise  in  part  to 
winged  forms  and  in  part  to  apterous  forms.  Both  of  these 
forms  lay  smaller  and  larger  eggs,  which  develop  respectively 
into  very  minute  males  and  females  without  digestive  organs. 
The  fertilized  eggs  laid  by  these  forms  probably  give  rise  to  the 
parthenogenetic  females. 

A  remarkable  case  of  heterogamy  accompanied  by  paedo- 
genesis  was  discovered  by  Wagner  to  take  place  in  certain 
species  of  Cecydomyia  (Miastor),  a  genus  of  the  Diptera.  The 
female  lays  a  few  eggs  in  the  bark  of  trees,  etc.  These  eggs 
develop  in  the  winter  into  larvae,  in  which  ovaries  are  early 
formed.  The  ova  become  detached  into  the  body  cavity, 
surrounded  by  their  follicles,  and  grow  at  the  cost  of  the 
follicles.  They  soon  commence  to  undergo  a  true  development, 
and  on  becoming  hatched  they  remain  for  some  time  in  the 
body  cavity  of  the  parent,  and  are  nourished  at  the  expense  of 
its  viscera.  They  finally  leave  the  empty  skin  of  their  parent, 
and  subsequently  reproduce  a  fresh  batch  of  larvae  in  the  same 
way.  After  several  generations  the  larvae  undergo  in  the 
following  summer  a  metamorphosis,  and  develop  into  the  sexual 
form. 

Another  case  of  paedogenesis  is  that  of  the  larvae  of  Chiro- 
nomus,  which  have  been  shewn  by  Grimm  (No.  413)  to  lay  eggs 
which  develop  exactly  in  the  same  way  as  fertilized  eggs  into 
larvae. 

BIBLIOGRAPHY. 

(401)  M.  Balbiani.  "  Observations  s.  la  reproduction  d.  Phylloxera  du  Chene." 
An.  Sc.  Nat.  Ser.  v.  Vol.  xix.  1874. 


430  INSECTA. 


(402)  E.  Bess  els.     "  Studien  u.  d.  Entwicklung  d.  Sexualdriisen  bei  den  Lepi- 
doptera."    Ztit.f.  wiss.  Zool.  Bd.  xvii.     1867. 

(403)  Alex.  Brandt.     "Beitrage  zur  Entwicklungsgeschichte  d.  Libellulida  u. 
Hemiptera,  mil  besonderer  Berucksichtigung  d.  Embryonalhiillen  derselben."     Mem. 
Ac.  Petersbourg,  Ser.  vn.  Vol.  xm.  1869. 

(404)  Alex.  Brandt.     Ueber  das  Ei  u.  seine  Bildungsstdttt.     Leipzig,  1878. 

(405)  O.  Biitschli.     "Zur  Entwicklungsgeschichte  d.  Biene."     Zeit.  f.  wiss. 
Zool.  Bd.  xx.  1870. 

(406)  H.  Dewitz.    "Bau  u.  Entwicklung  d.  Stachels,  etc."    Zeit.f.  wiss.  Zool. 
Vols.  xxv.  and  xxvin.     1875  and  1877. 

(407)  H.   Dewitz.     "Beitrage  zur  Kenntniss  d.  Postembryonalentwicklung  d. 
Gliedmassen  bei  den  Insecten."     Zeit.f.  wiss.  Zool.  xxx.     Supplement.     1878. 

(408)  A.  Dohrn.    "Notizen  zur  Kenntniss  d.  Insectenentwicklung."    Zeitschrift 
f.  wiss.  Zool.  Bd.  xxvi.     1876. 

(409)  M.  Fabre.    "  L'hypermetamorphose  et  lesmoeursdes  Meloides."   An.Sci. 
Nat.  Series  iv.  Vol.  vn.  1857. 

(410)  Ganin.   "  Beitrage  zur  Erkenntniss  d.  Entwicklungsgeschichte  d.  Insecten." 
Zeit.f.  wiss.  Zool.  Bd.  xix.     1869. 

(411)  V.  Graber.    Die  Insecten.    MUnchen,  1877. 

(412)  V.  Graber.     "Vorlauf.   Ergeb.    lib.    vergl.   Embryologie  d.   Insecten." 
Archivf.  mikr.  Anat.  Vol.  XV.    1878. 

(413)  O.  v.  Grimm.   "  Ungeschlechtliche  Fortpflanzung  einer  Chironomus  Art-u. 
deren  Entwicklung  aus  dem  unbefruchteten  Ei."     Mem.  Acad.  Petersbourg.     1870. 

(414)  B.  Hatschek.    "  Beitrage  zur  Entwicklung  d.  Lepidopteren."    Jenaische 
Zeitschrift,  Bd.  XI. 

(415)  A.  K  6 1 1  i  k  e  r.    "  Observationes  de  prima  insectorum  genese,  etc. "   Ann.  Sc. 
Nat.  Vol.  xx.     1843. 

(416)  A.  Kowalevsky.  "  Embryologische  Studien  an  Wurmern  u.  Arthropoden." 
Mem.  Ac.  imp.  Petersbourg,  Ser.  vn.  Vol.  xvi.     1871. 

(417)  C.  Kraepelin.     4 '  Untersuchungen  Ub.  d.  Bau,  Mechanismus  u.  d.  Ent- 
wick.  des  Stachels  d.  bienartigen  Thiere."     Zeit.f.  wiss.  Zool.  Vol.  xxni.  1873. 

(418)  C.  Kupffer.     "Faltenblatt  an  d.  Embryonen  d.  Gattung  Chironomus." 
Arch.f.  mikr.  Anat.  Vol.  u.     1866. 

(419)  R.  Leuckart.     Zur  Kenntniss  d.  Generationswechsels  u.  d.  Parthenogenese 
b.  d.  Insecten.     Frankfurt,  1858. 

(420)  Lubbock.     Origin  and  Metamorphosis  of  Insects.     1874. 

(421)  Lubbock.    Monograph  on  Collembola  and  Thysanura.   Ray  Society,  1873. 

(422)  Melnikow.     "  Beitrage  z.  Embryonalentwicklung  d.  Insecten."    Archiv 
f.  Naturgeschichte,  Bd.  xxxv.  1869. 

(423)  E.  Metschnikoff.     "Embryologische   Studien   an  Insecten."    Zeit.  f. 
wiss.  Zool.  Bd.  xvi.  1866. 

(424)  P.  Meyer.     "Ontogenie  und  Phylogenie  d.  Insecten."    Jenaische  Zeit- 
schrift, Vol.  x.  1876. 

(425)  FritzMiiller.     "  Beitrage  z.  Kenntniss  d.  Termiten."    Jenaische  Zeit- 
schrift, Vol.  IX.     1875. 

(426)  A.  S.  Packard.     " Embryological  Studies  on  Diplex,  Perithemis,  and 
the  Thysanurous  genus  Isotoma."     Mem.  Peabody  Acad.  Science,  I.  i.     1871. 

(427)  Suckow.     "  Geschlechtsorgane  d.  Insecten."     Ileusinger's  Zeitschrift  f. 
organ.  Physik,  Bd.  n.    1828. 


TRACHEATA. 


431 


(428)  Tichomiroff.     "  Ueber  die  Entwicklungsgeschichte  des  Seidenwiirms." 
Zoologischer  Anzeiger,  n.  Jahr.  No.  20  (Preliminary  Notice). 

(429)  Aug.  Weismann.     "Zur  Embryologie  d.  Insecten."     Archiv  f.  Anat. 
und  Phys.  1864. 

(430)  Aug.   Weismann.     "  Entwicklung  d.  Dipteren."    Zeit.  f.    wiss.   Zool. 
Vols.  xin.  and  xiv.     Leipzig,  1863 — 4. 

(431)  Aug.  Weismann.     "  Die  Metamorphose  d.  Corethra  plumicornis. "    Zeit. 
f.  wiss.  Zool.  Vol.  xvi.     1866. 

(432)  N.  Wagner.     "Beitrag  z.  Lehre  d.  Fortpflanzung  d.  Insectenlarven." 
Zeit.f.  wiss.  Zool.  Vol.  xin.    1860. 

(433)  Zaddach.     Untersuchungen  iib.  d.  Bau  u.  d.  Entwicklungd.  Gliederthiere. 
Berlin,  1854. 

ARACHNIDA1. 

The  development  of  several  divisions  of  this  interesting 
group  has  been  worked  out ;  and  it  will  be  convenient  to  deal  in 
the  first  instance  with  the  special  history  of  each  of  these 
divisions,  and  then  to  treat  in  a 
separate  section  the  develop- 
ment of  the  organs  for  the 
whole  group. 

Scorpionidae.  The  embry- 
onic development  always  takes 
place  within  the  female  Scor- 
pion. In  Buthus  it  takes  place 
within  follicle-like  protuber- 
ances of  the  wall  of  the  ovary. 
In  Scorpio  also  development 
commences  while  the  egg  is 
still  in  the  follicle,  but  when  the 
trunk  becomes  segmented  the 
embryo  passes  into  the  ovarian 
tube.  The  chief  authority  for 
the  development  of  the  Scorpio- 
nidae is  Metschnikoff  (No.  434). 

At  the  pole  of  the  ovum  facing  the  ovarian  tube  there  is 


FIG.  193.  OVUM  OF  SCORPION  WITH 
THE  ALREADY -FORMED  BLASTODERM 
SHEWING  THE  PARTIAL  SEGMENTATION. 
(After  Metschnikoff.) 

bl.  blastoderm. 


1  The  classification  of  the  Arachnida  adopted  in  the  present  work  is  shewn  below. 
c  Scorpionidse.  .  .  ( Tetrapneumones. 

Pedipalpi.  IL   Aranema-     JDipneumones. 

I.     ArthrOgastra.     \  Pseudoscorpionidae. 

I  Soiifugse.  in.  Acarina, 

^  Phalangidse. 


432 


SCORPIONID^E. 


formed  a  germinal  disc  which  undergoes  a  partial  segmentation 
(fig.  193  bl).  A  somewhat  saucer-shaped  one-layered  blasto- 
derm is  then  formed,  which  soon  becomes  thickened  in  the 
centre  and  then  divided  into  two  layers.  The  outer  of  these 
is  the  epiblast,  the  inner  the  mesoblast.  Beneath  the  mesoblast 
there  subsequently  appear  granular  cells,  which  form  the 
commencement  of  the  hypoblast1. 

During  the  formation  of  the  blastoderm  a  cellular  envelope  is  formed 
round  the  embryo.  Its  origin  is  doubtful,  though  it  is  regarded  by 
Metschnikoff  as  probably  derived  from  the  blastoderm  and  homologous 
with  the  amnion  of  Insects.  It  becomes  double  in  the  later  stages  (fig.  195). 

During  the  differentiation  of  the  three  embryonic  layers  the 
germinal  disc  becomes  somewhat  pyriform,  the  pointed  end 
being  the  posterior.  At  this  extremity  there  is  a  special  thick- 
ening which  is  perhaps 
equivalent  to  the  prim- 
itive cumulus  of  Spiders. 
The  germinal  disc  con- 
tinues gradually  to  spread 
over  the  yolk,  but  the 
original  pyriform  area  is 
thicker  than  the  remain- 
der, and  is  marked  off 
anteriorly  and  posterior- 
ly by  a  shallow  furrow. 
It  constitutes  a  structure 
corresponding  with  the 
ventral  plate  of  other 
Tracheata.  It  soon  be- 
comes grooved  by  a  FIG.  194.  THREE  SURFACE  VIEWS  OF  THE 

.A      ,.        ,    f  VENTRAL    PLATE   OF    A    DEVELOPING    SCORPION. 

shallow  longitudinal  fur-     (After  Metschnikoff.) 

A.  Before  segmentation. 

B.  After  five  segments  have  become  formed. 

C.  After  the  appendages  have  begun  to  be 

formed. 


row  (fig.  194  A)  which 
subsequently  becomes 
less  distinct.  It  is  then 
divided  by  two  transverse  lines  into  three  parts2. 


1  The  origin  of  the  hypoblast  cells,  if  such  these  cells  are,  is  obscure.  Metschnikoff 
doubtfully  derives  them  from  the  blastoderm  cells  ;  from  my  investigations  on  Spiders 
it  appears  to  me  more  probable  that  they  originate  in  the  yolk. 

*  The   exact  fate   of  the   three  original   segments  is  left  somewhat  obscure  by 


TRACHEATA. 


433 


In  succeeding  stages  the  anterior  of  the  three  parts  is  clearly 
marked  out  as  the  procephalic  lobe,  and  soon  becomes  somewhat 
broader.  Fresh  segments  are  added  from  before  backwards, 
and  the  whole  ventral  plate  increases  rapidly  in  length  (fig. 
194  B). 

When  ten  segments  have  become  formed,  appendages  appear 
as  paired  outgrowths  of  the  nine  posterior  segments  (fig.  194  C). 
The  second  segment  bears  the  pedipalpi,  the  four  succeeding 
segments  the  four  ambulatory  appendages,  and  the  four  hinder- 
most  segments  smaller  provisional  appendages  which  subse- 
quently disappear,  with  the  possible  exception  of  the  second. 
The  foremost  segment,  immediately  behind  the  procephalic 
lobes,  is  very  small,  and  still  without  a  rudiment  of  the  cheli- 
cerae, which  are  subsequently  formed  on  it.  It  would  appear 
from  Metschnikoff's  figures  to 
be  developed  later  than  the 
other  post-oral  segments  pre- 
sent at  this  stage.  The  still 
unsegmented  tail  has  become 
very  prominent  and  makes  an 
angle  of  180°  with  the  re- 
mainder of  the  body,  over  the 
ventral  surface  of  which  it  is 
flexed. 

By  the  time  that  twelve 
segments  are  definitely  form- 
ed, the  procephalic  region  is 
distinctly  bilobed,  and  in  the 
median  groove  extending 
along  it  the  stomodaeum  has 
become  formed  (fig.  196  A). 
The  chelicerae  (ck)  appear  as 
small  rudiments  on  the  first 
post-oral  segment,  and  the 


FlG.  195.  A  FAIRLY-ADVANCED  EM- 
BRYO OF  THE  SCORPION  ENVELOPED  IN 

ITS  MEMBRANES.     (After  Metschnikoff. ) 

ch.  chelicerae ;  pd.  pedipalpi ;  p^—p4. 
ambulatory  appendages ;  al>.  post-abdomen 
(tail). 


Metschnikoff.  He  believes  however  that  the  anterior  segment  forms  the  procephalic 
lobes,  the  posterior  probably  the  telson  and  five  adjoining  caudal  segments,  and 
the  middle  one  the  remainder  of  the  body.  This  view  does  not  appear  to  me  quite 
satisfactory,  since  on  the  analogy  of  Spiders  and  other  Arthropoda  the  fresh  somites 
ought  to  be  added  by  a  continuous  segmentation  of  the  posterior  lobe. 

B.  II.  28 


434  1  SEUDOSCORPIONID^E. 

nerve  cords  are  distinctly  differentiated  and  ganglionated.  In 
the  embryonic  state  there  is  one  ganglion  for  each  segment. 
The  ganglion  in  the  first  segment  (that  bearing  the  chelicerse)  is 
very  small,  but  is  undoubtedly  post-oral. 

At  this  stage,  by  a  growth  in  which  all  the  three  germinal 
layers  have  a  share,  the  yolk  is  completely  closed  in  by  the 
blastoderm.  It  is  a  remarkable  fact  with  only  few  parallels,  and 
those  amongst  the  Arthropoda,  that  the  blastopore,  or  point 
where  the  embryonic  membranes  meet  in  closing  in  the  yolk,  is 
situated  on  the  dorsal  surface  of  the  embryo. 

The  general  relations  of  the  embryo  at  about  this  stage  are 
shewn  in  fig.  195,  where  the  embryo  enclosed  in  its  double 
cellular  membrane  is  seen  in  a  side  view.  This  embryo  is  about 
the  same  age  as  that  seen  from  the  ventral  surface  in  fig.  196  A. 

The  general  nature  of  the  further  changes  may  easily  be 
gathered  from  an  inspection  of  fig.  196  B  and  C,  but  a  few 
points  may  be  noted. 

An  upper  lip  or  labrum  is  formed  as  an  unpaired  organ  in 
the  line  between  the  procephalic  lobes.  The  pedipalpi  become 
chelate  before  becoming  jointed,  and  the  chelicerae  also  early 
acquire  their  characteristic  form.  Rudimentary  appendages 
appear  on  the  six  segments  behind  the  ambulatory  legs,  five  of 
which  are  distinctly  shewn  in  fig.  195  ;  they  persist  only  on  the 
second  segment,  where  they  appear  to  form  the  comb-like 
organs  or  pectines.  The  last  abdominal  segment,  Le.  that  next 
the  tail,  is  without  provisional  appendages.  The  embryonic  tail 
is  divided  into  six  segments  including  the  telson  (fig.  196  C,  ab). 
The  lungs  (st)  are  formed  by  paired  invaginations,  the  walls  of 
which  subsequently  become  plicated,  on  the  four  last  segments 
which  bear  rudimentary  limbs,  and  simultaneously  with  the 
disappearance  of  the  rudimentary  limbs. 

PseudoscorpionidaB.  The  development  of  Qielifer  has  been  in- 
vestigated by  Metschnikoff  (436),  and  although  (except  that  it  is  provided 
with  tracheae  instead  of  pulmonary  sacks)  it  might  be  supposed  to  be  closely 
related  to  Scorpio,  yet  in  its  development  is  strikingly  different. 

The  eggs  after  being  laid  are  carried  by  the  female  attached  to  the  first 
segment  of  the  abdomen.  The  segmentation  (vide  p.  93)  is  intermediate 
between  the  types  of  complete  and  superficial  segmentation.  The  ovum, 
mainly  formed  of  food-yolk,  divides  into  two,  four,  and  eight  equal  segments 


TRACHEATA. 


435 


(fig.  197  A).  There  then  appear  one  or  more  clear  segments  on  the  surface 
of  these,  and  finally  a  complete  layer  of  cells  is  formed  round  the  central 
yolk  spheres  (fig.  197  B),  which  latter  subsequently  agglomerate  into  a 
central  mass.  The  superficial  cells  form  what  may  be  called  a  blastoderm, 
which  soon  becomes  divided  into  two  layers  (fig.  197  C).  There  now 
appears  a  single  pair  of  appendages  (the  pedipalpi)  (fig.  198  A,/^/),  while  at 
the  same  time  the  front  end  of  the  embryo  grows  out  into  a  remarkable 
proboscis-like  prominence— a  temporary  upper  lip  (concealed  in  the  figure 


flf 


ab- 


FIG.  196.    THREE  STAGES  IN  THE  DEVELOPMENT  OF  THE  SCORPION.     THE 

EMBRYOS   ARE   REPRESENTED   AS    IF   SEEN   EXTENDED   ON   A  PLANE. 

(After  Metschnikoff.) 

ch.  chelicerae  ;  pd.   pedipalpi ;  pl — />*.  ambulatory  appendages ;  pe.  pecten ;   st. 
stigmata ;  ab.  post  abdomen  (tail). 

behind  the  pedipalpus),  and  the  abdomen  (ab)  becomes  bent  forwards  to- 
wards the  ventral  surface.  In  this  very  rudimentary  condition,  after  under- 
going an  ecdysis,  the  larva  is  hatched,  although  it  still  remains  attached  to 
its  parent.  After  hatching  it  grows  rapidly,  and  becomes  filled  with  a 
peculiar  transparent  material.  The  first  pair  of  ambulatory  appendages  is 
formed  behind  the  pedipalpi  and  then  the  three  suceeding  pairs,  while  at  the 
same  time  the  chelicerae  appear  as  small  rudiments  in  front.  External  signs 
of  segmentation  have  not  yet  appeared,  but  about  this  period  the  nervous 
system  is  formed.  The  supra-cesophageal  ganglia  are  especially  distinct, 
and  provided  with  a  central  cavity,  probably  formed  by  an  invagination,  as 
in  other  Arachnida.  In  the  succeeding  stages  (fig.  198  B)  four  provisional 

28—2 


ARANETNA. 


pairs  of  appendages  (shewn  as  small  knobs  at  ati]  appear  behind  the  ambu- 
latory feet.  The  abdomen  is  bent  forwards  so  as  to  reach  almost  to  the 
pedipalpi.  In  the  later  stages  (fig.  198  C)  the  adult  form  is  gradually 
attained.  The  enormous  upper  lip  persists  for  some  time,  but  subsequently 
atrophies  and  is  replaced  by  a  normal  labrum.  The  appendages  behind  the 


FIG.  igj.    SEGMENTATION  AND  FORMATION  OF  THE  BLASTODERM  IN  CHELIFER. 

(After  Metschnikoff.) 

In  A  the  ovum  is  divided  into  a  number  of  separate  segments.  In  B  a  number  of 
small  cells  have  appeared  (bl)  which  form  a  blastoderm  enveloping  the  large  yolk 
spheres.  In  C  the  blastoderm  has  become  divided  into  two  layers. 

ambulatory  feet  atrophy,  and  the  tail  is  gradually  bent  back  into  its  final 
position.  The  segmentation  and  the  gradual  growth  of  the  limbs  do  not  call 
for  special  description,  and  the  formation  of  the  organs,  so  far  as  is  known, 
agrees  with  other  types. 

The  segmentation  of  Chthonius  is  apparently  similar  to  that  of  Chelifer 
(Stecker,  No.  437). 

Phalangidae.  Our  knowledge  of  the  development  of  Phalangium  is 
unfortunately  confined  to  the  later  stages  (Balbiani,  No.  438).  These  stages 
do  not  appear  however  to  differ  very  greatly  from  those  of  true  Spiders. 

Araneina.  The  eggs  of  true  Spiders  are  either  deposited  in 
nests  made  specially  for  them,  or  are  carried  about  by  the 
females.  Species  belonging  to  a  considerable  number  of  genera, 
viz.  Pholcus,  Epeira,  Lycosa,  Clubione,  Tegenaria  and  Agelcna 


TRACHEATA. 


437 


have  been  studied  by  Claparede  (No.  442),  Balbiani  (No.  439), 
Barrois  (No.  441)  and  myself  (No.  440),  and  the  close  similarity 
between  their  embryos  leaves  but  little  doubt  that  there  are  no 
great  variations  in  development  within  the  group. 

The  ovum  is  enclosed  in  a  delicate  vitelline  membrane, 
enveloped  in  its  turn  by  a  chorion  secreted  by  the  walls  of  the 
oviduct.  The  chorion  is  covered  by  numerous  rounded  promi- 
nences, and  occasionally  exhibits  a  pattern  corresponding  with 
the  areas  of  the  cells  which  formed  it.  The  segmentation  has 
already  been  fully  described,  pp.  1 18  and  1 19.  At  its  close  there 
is  present  an  enveloping  blastoderm  formed  of  a  single  layer  of 
large  flattened  cells.  The  yolk  within  is  formed  of  a  number  of 


'  °r.°v-ii~-cr^     ^H1— 

o°o°o£°^  o°^*afe 


Cll 


ab 


FIG.  198.    THREE  STAGES  IN  THE  DEVELOPMENT  OF  CHELIFKR. 

(After  Metschnikoff.) 
pd.  pedipalpi ;  ab.  abdomen ;   an.i.  anal  invagination  ;   c/i.  chelicerse. 

large  polygonal  segments  ;  each  of  which  is  composed  of  large 
yolk  spherules,  and  contains  a  nucleus  surrounded  by  a  layer  of 
protoplasm,  which  is  prolonged  into  stellate  processes  holding 
together  the  yolk  spherules.  The  nucleus,  surrounded  by  the 
major  part  of  the  protoplasm  of  each  yolk  cell,  appears,  as  a  rule, 


438  ARANEINA. 


to  be  situated  not  at  the  centre,  but  on  one  side  of  its  yolk 
segment. 

The  further  description  of  the  development  of  Spiders  applies 
more  especially  to  Agelena  labyrinthica,  the  species  which 
formed  the  subject  of  my  own  investigations. 

The  first  differentiation  of  the  blastoderm  consists  in  the 
cells  of  nearly  the  whole  of  one  hemisphere  becoming  somewhat 
more  columnar  than  those  of  the  other  hemisphere,  and  in  the 
cells  of  a  small  area  near  one  end  of  the  thickened  hemisphere 
becoming  distinctly  more  columnar  than  elsewhere,  and  two 
layers  thick.  This  area  forms  a  protuberance  on  the  surface  of 
the  ovum,  originally  discovered  by  Claparede,  and  called  by  him 
the  primitive  cumulus.  In  the  next  stage  the  cells  of  the 
thickened  hemisphere  of  the  blastoderm  become  still  more 
columnar;  and  a  second  area,  at  first  connected  by  a  whitish 
streak  with  the  cumulus,  makes  its  appearance.  In  the  second 
area  the  blastoderm  is  also  more  than  one  cell  deep  (fig.  199). 
It  will  be  noticed  that  the  blastoderm,  though  more  than  one 
cell  thick  over  a  large  part  of  the  ventral  surface,  is  not  divided 
into  distinct  layers.  The  second  area  appears  as  a  white  patch 
and  soon  becomes  more  distinct,  while  the  streak  continued  to 
it  from  the  cumulus  is  no  longer  visible.  It  is  shewn  in  surface 
view  in  fig.  200  A.  Though  my  observations  on  this  stage  are 
not  quite  satisfactory,  yet  it  appears  to  me  probable  that  there 
is  a  longitudinal  thickened  ridge  of  the  blastoderm  extending 
from  the  primitive  cumulus  to  the  large  white  area.  The  section 
represented  in  fig.  199,  which  I  believe  to  be  oblique,  passes 
through  this  ridge  at  its  most  projecting  part. 

The  nuclei  of  the  yolk  cells  during  the  above  stages  multiply 
rapidly,  and  cells  are  formed  in  the  yolk  which  join  the  blasto- 
derm ;  there  can  however  be  no  doubt  that  the  main  increase  in 
the  cells  of  the  blastoderm  has  been  due  to  the  division  of  the 
original  blastoderm  cells. 

In  the  next  stage  I  have  been  able  to  observe  there  is,  in  the 
place  of  the  previous  thickened  half  of  the  blastoderm,  a  well 
developed  ventral  plate  with  a  procephalic  lobe  in  front,  a 
caudal  lobe  behind,  and  an  intermediate  region  marked  by 
about  three  transverse  grooves,  indicating  a  division  into 
segments.  This  plate  is  throughout  two  or  more  rows  of 


TRACHEATA. 


439 


FIG.  199.  SECTION  THROUGH  THE  EM- 
BRYO OF  AGELENA  LABYRINTHICA. 

The  section  is  from  an  embryo  of  the 
same  age  as  fig.  200  A,  and  is  represented 
with  the  ventral  plate  upwards.  In  the 
ventral  plate  is  seen  a  keel-like  thickening, 
which  gives  rise  to  the  main  mass  of  the 
mesoblast. 

yk.  yolk  divided  into  large  polygonal 
cells,  in  several  of  which  nuclei  are  shewn. 


cells  thick,  and  the  cells 
which  form  it  are  divided  into 
two  distinct  layers — a  colum- 
nar superficial  layer  of  epiblast 
cells,  and  a  deeper  layer  of 
mesoblast  cells  (fig.  203  A). 
In  the  latter  layer  there  are 
several  very  large  cells  which 
are  in  the  act  of  passing  from 
the  yolk  into  the  blastoderm. 
The  identification  of  the  struc- 
tures visible  in  the  previous 
stage  with  those  visible  in 
the  present  stage  is  to  a 
great  extent  a  matter  of 
guess-work,  but  it  appears 
to  me  probable  that  the 
primitive  cumulus  is  still  present  as  a  slight  prominence  visible 
in  surface  views  on  the  caudal  lobe,  and  that  the  other  thickened 
patch  persists  as  the  procephalic  lobe.  However  this  may  be, 
the  significance  of  the  primitive  cumulus  appears  to  be  that  it  is 
the  part  of  the  blastoderm  where  two  rows  of  cells  become  first 
established  \ 

The  whole  region  of  the  blastoderm  other  than  the  ventral 
plate  is  formed  of  a  single  row  of  flattened  epiblast  cells.  The 
yolk  retains  its  original  constitution. 

By  this  stage  the  epiblast  and  mesoblast  are  distinctly 
differentiated,  and  the  homologue  of  the  hypoblast  is  to  be 
sought  for  in  the  yolk-cells.  The  yolk-cells  are  not  however 
entirely  hypoblastic,  since  they  continue  for  the  greater  part  of 
the  development  to  give  rise  to  fresh  cells  which  join  the  meso- 
blast. 

The  Spider's  blastoderm  now  resembles  that  of  an  Insect 
(except  for  the  amnion)  after  the  establishment  of  the  mesoblast, 
and  the  mode  of  origin  of  the  mesoblast  in  both  groups  is  very 
similar,  in  that  the  longitudinal  ridge-like  thickening  of  the 

1  Various  views  have  been  put  forward  by  Claparfede  and  Balbiani  about  the 
position  and  significance  of  the  primitive  cumulus.  For  a  discussion  of  which  vide 
self,  No.  440. 


440  AKANEINA. 


mesoblast  shewn  in  fig.  199  is  probably  the  homologue  of  the 
mesoblastic  groove  of  the  Insects'  blastoderm. 

The  ventral  plate  continues  to  grow  rapidly,  and  at  a  some- 
what later  stage  (fig.  200  B)  there  are  six  segments  interposed 
between  the  procephalic  and  caudal  lobes.  The  two  anterior  of 
these  (ch  and  pd),  especially  the  foremost,  are  less  distinct  than 
the  remainder ;  and  it  is  probable  that  both  of  them,  and  in  any 
case  the  anterior  one,  are  formed  later  than  the  three  segments 
following.  These  two  segments  are  the  segments  of  the  chelicenc 
and  pedipalpi.  The  four  segments  following  belong  to  the  four 
pairs  of  ambulatory  legs.  The  segments  form  raised  transverse 
bands  separated  by  transverse  grooves.  There  is  at  this  stage  a 
faintly  marked  groove  extending  along  the  median  line  of  the 
ventral  plate.  This  groove  is  mainly  caused  by  the  originally 
single  mesoblastic  plate  having  become  divided  throughout  the 
whole  region  of  the  ventral  plate,  except  possibly  the  procephalic 
lobes,  into  two  bands,  one  on  each  side  of  the  middle  line  (fig. 
203  B). 

The  segments  continue  to  increase  in  number  by  the  con- 
tinuous addition  of  fresh  segments  between  the  one  last  formed 
and  the  caudal  lobe.  By  the  stage  with  nine  segments  the  first 
rudiments  of  the  limbs  make  their  appearance.  The  first 
rudiments  to  appear  are  those  of  the  pedipalpi  and  four  ambu- 
latory limbs  :  the  chelicerae,  like  the  segment  to  which  they 
belong,  lag  behind  in  development.  The  limbs  appear  as  small 
protuberances  at  the  borders  of  their  segments.  By  the  stage 
when  they  are  formed  the  procephalic  region  has  become 
bilobed,  and  the  two  lobes  of  which  it  is  composed  are  separated 
by  a  shallow  groove. 

By  a  continuous  elongation  the  ventral  plate  comes  to  form 
a  nearly  complete  equatorial  ring  round  the  ovum,  the  pro- 
cephalic  and  caudal  lobes  being  only  separated  by  a  very  narrow 
space,  the  undeveloped  dorsal  region  of  the  embryo.  This  is 
shewn  in  longitudinal  section  in  fig.  204.  In  this  condition  the 
embryo  may  be  spoken  of  as  having  a  dorsal  flexure.  By  the 
time  that  this  stage  is  reached  (fig.  200  C)  the  full  number  of 
segments  and  appendages  has  become  established.  There  are 
in  all  sixteen  segments  (including  the  caudal  lobe).  The  first 
six  of  these  bear  the  permanent  appendages  of  the  adult ;  the 


TRACHEATA.  44! 


next  four  are  provided  with  provisional  appendages  ;  while  the 
last  six  are  without  appendages.  The  further  features  of  this 
stage  which  deserve  notice  are  (i)  the  appearance  of  a  shallow 
depression  (st) — the  rudiment  of  the  stomodaeum — between  the 
hinder  part  of  the  two  procephalic  lobes  ;  (2)  the  appearance  of 


FIG.  aoo.     FOUR  STAGES  IN  THE  DEVELOPMENT  OF  AGELENA  LABYRINTHICA. 

A.  Stage  when  the  ventral  plate  is  very  imperfectly  differentiated,  pr.c.  primitive 
cumulus. 

B.  Ovum  viewed  from  the  side  when  the  ventral  plate  has  become  divided  into 
six  segments,     ch.  segment  of  chelicerae  imperfectly  separated  from  procephalic  lobe  ; 
pd.  segment  of  pedipalpi. 

C.  Ventral   plate   ideally  unrolled   after   the    full    number    of   segments    and 
appendages    are    established,     st.    stomodoeum   between    the    two    proe-oral    lobes. 
Behind  the  six  pairs  of  permanent  appendages  are  seen  four  pairs   of  provisional 
appendages. 

D  and  E.  Two  views  of  an  embryo  at  the  same  stage.  D  ideally  unrolled, 
E  seen  from  the  side.  st.  stomodseum  ;  ch.  chelicerse  ;  on  their  inner  side  is  seen 
the  ganglion  belonging  to  them.  pd.  pedipalpi ;  pr.p.  provisional  appendages. 

raised  areas  on  the  inner  side  of  the  six  anterior  appendage- 
bearing  segments.  These  are  the  rudiments  of  the  ventral 
ganglia.  It  deserves  to  be  especially  noted  that  the  segment  of 


442  AKANEINA. 


the  chelicera,  like  the  succeeding  segments,  is  provided  with 
ganglia ;  and  that  the  ganglia  of  the  chelicerae  are  quite  distinct 
from  the  supra-cesophageal  ganglia  derived  from  the  procephalic 
lobes.  (3)  The  pointed  form  of  the  caudal  lobe.  In  Pholcus 
(Claparede,  No.  442)  the  caudal  lobe  forms  a  projecting  structure 
which,  like  the  caudal  lobe  of  the  Scorpion,  bends  forward  so  as 
to  face  the  ventral  surface  of  the  part  of  the  body  immediately 
in  front.  In  most  Spiders  such  a  projecting  caudal  lobe  is  not 
found.  While  the  embryo  still  retains  its  dorsal  flexure  con- 
siderable changes  are  effected  in  its  general  constitution.  The 
appendages  (fig.  200  D  and  E)  become  imperfectly  jointed,  and 
grow  inwards  so  as  to  approach  each  other  in  the  middle  line. 
Even  in  the  stage  before  this,  the  ventral  integument  between 
the  rudiments  of  the  ganglia  had  become  very  much  thinner, 
and  had  in  this  way  divided  the  ventral  plate  into  two  halves. 
At  the  present  stage  the  two  halves  of  the  ventral  plate  are  still 
further  separated,  and  there  is  a  wide  space  on  the  ventral  side 
only  covered  by  a  delicate  layer  of  epiblast.  This  is  shewn  in 
surface  view  (fig.  200  D)  and  in  section  in  fig.  203  C. 

The  stomodaeum  (j/)  is  much  more  conspicuous,  and  is 
bounded  in  front  by  a  prominent  upper  lip,  and  by  a  less 
marked  lip  behind.  The  upper  lip  becomes  less  conspicuous  in 
later  stages,  and  is  perhaps  to  be  compared  with  the  provisional 
upper  lip  of  Chelifer.  Each  procephalic  lobe  is  now  marked  by 
a  deep  semicircular  groove. 

The  next  period  in  the  development  is  characterised  by  the 
gradual  change  in  the  flexure  of  the  embryo  from  a  dorsal  to  a 
ventral  one  ;  accompanied  by  the  division  of  the  body  into  an 
abdomen  and  cephalo-thorax,  and  the  gradual  assumption  of  the 
adult  characters. 

The  change  in  the  flexure  of  the  embryo  is  caused  by  the 
elongation  of  the  dorsal  region,  which  has  hitherto  been  hardly 
developed.  Such  an  elongation  increases  the  space  on  the 
dorsal  surface  between  the  procephalic  and  caudal  regions,  and 
therefore  necessarily  separates  the  caudal  and  procephalic  lobes ; 
but,  since  the  ventral  plate  does  not  become  shortened  in  the 
process,  and  the  embryo  cannot  straighten  itself  in  the  egg-shell, 
it  necessarily  becomes  ventrally  flexed. 

If  there  were  but  little  food  yolk  this  flexure  would  naturally 


TRACHEATA.  443 


cause  the  whole  embryo  to  be  bent  in  so  as  to  have  the  ventral 
surface  concave.  But  instead  of  this  the  flexure  is  at  first  con- 
fined to  the  two  bands  which  form  the  ventral  plate.  These 
bands,  as  shewn  in  fig.  201  A,  acquire  a  true  ventral  flexure,  but 
the  yolk  forms  a  projection — a  kind  of  yolk  sack  as  Barrois 
(No.  441)  calls  it — distending  the  thin  integument  between  the 
two  ventral  bands.  This  yolk  sack  is  shewn  in  surface  view  in 


FlG.  201.      TWO  LATE  STAGES  IN  THE  DEVELOPMENT  OF  AGELENA  LABYRINTHICA. 

A.  Embryo  from  the  side  at  the  stage  when  there  is  a  large  ventral  protuberance 
of  yolk.     The  angle  between  the  line  of  insertion  of  the  permanent  and  provisional 
appendages  shews  the  extent  of  the  ventral  flexure. 

B.  Embryo  nearly  ready  to  be  hatched.     The   abdomen  which  has   not   quite 
acquired  its  permanent  form  is  seen  to  be  pressed  against  the  ventral  side  of  the 
thorax. 

prJ.  procephalic  lobe;  pd.  pedipalpi ;  ch.  chelicerae ;  c,L  caudal  lobe;  pr.p.  pro- 
visional appendages. 

fig.  20 1  A  and  in  section  in  fig.  206.  At  a  later  period,  when 
the  yolk  has  become  largely  absorbed,  the  true  nature  of  the 
ventral  flexure  becomes  quite  obvious,  since  the  abdomen  of  the 
young  Spider,  while  still  in  the  egg,  is  found  to  be  bent  over  so 
as  to  press  against  the  ventral  surface  of  the  thorax  (fig.  201  B). 
The  general  character  of  the  changes  which  take  place 
during  this  period  in  the  development  is  shewn  in  fig.  201  A  and 
B  representing  two  stages  in  it.  In  the  first  of  these  stages 
there  is  no  constriction  between  the  future  thorax  and  abdomen. 


444  ACARINA. 


The  four  pairs  of  provisional  appendages  exhibit  no  signs  of 
atrophy  ;  and  the  extent  of  the  ventral  flexure  is  shewn  by  the 
angle  formed  between  the  line  of  their  insertion  and  that  of  the 
appendages  in  front.  The  yolk  has  enormously  distended  the 
integument  between  the  two  halves  of  the  ventral  plate,  as  is 
illustrated  by  the  fact  that,  at  a  somewhat  earlier  stage  than 
that  figured,  the  limbs  cross  each  other  in  the  median  ventral 
line,  while  at  this  stage  they  do  not  nearly  meet  The  limbs 
have  acquired  their  full  complement  of  joints,  and  the  pedipalpi 
bear  a  cutting  blade  on  their  basal  joint. 

The  dorsal  surface  between  the  prominent  caudal  lobe  and 
the  procephalic  lobes  forms  more  than  a  semicircle.  The  terga 
are  fully  established,  and  the  boundaries  between  them,  especially 
in  the  abdomen,  are  indicated  by  transverse  markings.  A  large 
lower  lip  now  bounds  the  stomodaeum,  and  the  upper  lip  has 
somewhat  atrophied.  In  the  later  stage  (fig.  201  B)  the  greater 
part  of  the  yolk  has  passed  into  the  abdomen,  which  is  now  to 
some  extent  constricted  off  from  the  cephalo-thorax.  The 
appendages  of  the  four  anterior  abdominal  somites  have  dis- 
appeared, and  the  caudal  lobe  has  become  very  small.  In  front 
of  it  are  placed  two  pairs  of  spinning  mammillae.  A  delicate 
cuticle  has  become  established,  which  is  very  soon  moulted. 

Acarina.  The  development  of  the  Acarina,  which  has  been  mainly 
investigated  by  Claparede  (No.  446),  is  chiefly  remarkable  from  the  frequent 
occurrence  of  several  larval  forms  following  each  other  after  successive 
ecdyses.  The  segmentation  (vide  p.  116)  ends  in  the  formation  of  a  blasto- 
derm of  a  single  layer  of  cells  enclosing  a  central  yolk  mass. 

A  ventral  plate  is  soon  formed  as  a  thickening  of  the  blastoderm,  in  which 
an  indistinct  segmentation  becomes  early  observable.  In  Myobia,  which  is 
parasitic  on  the  common  mouse,  the  ventral  plate  becomes  divided  by  five 
constrictions  into  six  segments  (fig.  202  A),  from  the  five  anterior  of  which 
paired  appendages  very  soon  grow  out  (fig.  202  B)  The  appendages  are  the 
chelicerae  (ch}  and  pedipalpi  (pd]  and  the  first  three  pairs  of  limbs  (p^—fi1}. 
On  the  dorsal  side  of  the  chelicerae  a  thickened  prominence  of  the  ventral 
plate  appears  to  correspond  to  the  procephalic  lobes  of  other  Arachnida. 
The  part  of  the  body  behind  the  five  primitive  appendage-bearing  segments 
appears  to  become  divided  into  at  least  two  segments.  In  other  mites  the 
same  appendages  are  formed  as  in  Myobia,  but  the  preceding  segmentation 
of  the  ventral  plate  is  not  always  very  obvious. 

In  Myobia  two  moultings  take  place  while  the  embryo  is  still  within  the 
primitive  egg-shell.  The  first  of  these  is  accompanied  by  the  apparently 
total  disappearance  of  the  three  pediform  appendages,  and  the  complete 


TRACK  EAT  A. 


445 


coalescence  of  the  two  gnathiform  appendages  into  a  proboscis  (fig.  202  C). 
The  feet  next  grow  out  again,  and  a  second  ecdysis  then  takes  place.  The 
embryo  becomes  thus  inclosed  within  three  successive  membranes,  viz.  the 
original  egg-shell  and  two  cuticular  membranes  (fig.  202  D).  After  the 
second  ecdysis  the  appendages  assume  their  final  form,  and  the  embryo 
leaves  the  egg  as  an  hexapodous  larva.  The  fourth  pair  of  appendages  is 


FIG.  202.     FOUR  SUCCESSIVE  STAGES  IN  THE  DEVELOPMENT  OF  MYOBIA  MUSCULI.- 

(After  Claparede.) 

J1 — j4.  post-oral  segments  ;  ch.  chelicerae ;  pd.  pedipalpi ;  pr.  proboscis  formed  by 
the  coalescence  of  the  chelicerse  and  pedipalpi ;  pl,  /*,  etc.  ambulatory  appendages. 

acquired  by  a  post-embryonic  metamorphosis.  From  the  proboscis  are 
formed  the  rudimentary  palpi  of  the  second  pair  of  appendages,  and  two 
elongated  needles  representing  the  chelicerae. 

In  the  cheese  mite  (Tyroglyphus)  the  embryo  has  two  ecdyses  which  are 
not  accompanied  by  the  peculiar  changes  observable  in  Myobia :  the 
cheliceras  and  pedipalpi  fuse  however  to  form  the  proboscis.  The  first 
larval  form  is  hexapodous,  and  the  last  pair  of  appendages  is  formed  at  a 
subsequent  ecdysis. 

In  Atax  Bonzi,  a  form  parasitic  on  Unio,  the  development  and  meta- 
morphosis are  even  more  complicated  than  in  Myobia.  The  first  ecdysis 
occurs  before  the  formation  of  the  limbs,  and  shortly  after  the  ventral  plate 
has  become  divided  into  segments.  Within  the  cuticular  membrane  resulting 
from  the  first  ecdysis  the  anterior  five  pairs  of  limbs  spring  out  in  the  usual 
fashion.  They  undergo  considerable  differentiation ;  the  chelicerae  and 
pedipalpi  approaching  each  other  at  the  anterior  extremity  of  the  body,  and 
the  three  ambulatory  legs  becoming  segmented  and  clawed.  An  oesophagus, 
a  stomach,  and  an  oesophageal  nerve-ring  are  also  formed.  When  the  larva 


446  ACARINA. 


has  attained  this  stage  the  original  egg-shell  is  split  into  two  valves  and 
eventually  cast  off,  but  the  embryo  remains  enclosed  within  the  cuticular 
membrane  shed  at  the  first  ecdysis.  This  cuticular  membrane  is  spoken  of 
by  Claparede  as  the  deutovum.  In  the  deutovum  the  embryo  undergoes 
further  changes ;  the  chelicerae  and  pedipalpi  coalesce  and  form  the 
proboscis  ;  a  spacious  body  cavity  with  blood  corpuscles  appears  ;  and  the 
alimentary  canal  enclosing  the  yolk  is  formed. 

The  larva  now  begins  to  move,  the  cuticular  membrane  enclosing  it  is 
ruptured,  and  the  larva  becomes  free.  It  does  not  long  remain  active,  but 
soon  bores  its  way  into  the  gills  of  its  host,  undergoes  a  fresh  moult,  and 
becomes  quiescent.  The  cuticular  membrane  of  the  moult  just  effected 
swells  up  by  the  absorption  of  water  and  becomes  spherical.  Peculiar 
changes  take  place  in  the  tissues,  and  the  limbs  become,  as  in  Myobia, 
nearly  absorbed,  remaining  however  as  small  knobs.  The  larva  swims 
about  as  a  spherical  body  within  its  shell.  The  feet  next  grow  out  afresh, 
and  the  posterior  pair  is  added.  From  the  proboscis  the  palpi  (of  the 
pedipalpi)  grow  out  below.  The  larva  again  becomes  free,  and  amongst 
other  changes  the  chelicerae  grow  out  from  the  proboscis.  A  further  ecdysis, 
with  a  period  of  quiescence,  intervenes  between  this  second  larval  form  and 
the  adult  state. 

The  changes  in  the  appendages  which  appear  common  to  the  Mites 
generally  are  (i)  the  late  development  of  the  fourth  pair  of  appendages,  which 
results  in  the  constant  occurrence  of  an  hexapodous  larva  ;  and  (2)  the  early 
fusion  of  the  chelicerae  and  pedipalpi  to  form  a  proboscis  in  which  no  trace 
of  the  original  appendages  can  be  discerned.  In  most  instances  palpi  and 
stilets  of  variable  form  are  subsequently  developed  in  connexion  with  the 
proboscis,  and,  as  indicated  in  the  above  descriptions,  are  assumed  to  cor- 
respond with  the  two  original  embryonic  appendages. 

TJie  history  of  tJie  germinal  layers. 

It  is  a  somewhat  remarkable  fact  that  each  of  the  groups  of 
the  Arachnida  so  far  studied  has  a  different  form  of  segmenta- 
tion. The  types  of  Chelifer  and  the  Spiders  are  simple  modi- 
fications of  the  centrolecithal  type,  while  that  of  Scorpio,  though 
apparently  meroblastic,  is  probably  to  be  regarded  in  the  same 
light  (vide  p.  120  and  p.  434).  The  early  development  begins  in 
the  Scorpion  and  Spiders  with  the  formation  of  a  ventral  plate, 
and  there  can  be  but  little  doubt  that  Chelifer  is  provided 
with  an  homologous  structure,  though  very  probably  modified, 
owing  to  the  small  amount  of  food-yolk  and  early  period  of 
hatching. 

The  history  of  the  layers  and  their  conversion  into  the  organs 
has  been  studied  in  the  case  of  the  Scorpion  (Metschnikoff,  No. 


TRACHEATA.  447 


434),  and  of  the  Spiders ;  and  a  close  agreement  has  been  found 
to  obtain  between  them. 

It  will  be  convenient  to  take  the  latter  group  as  type,  and 
simply  to  call  attention  to  any  points  in  which  the  two  groups 
differ. 

The  epiblast.  The  epiblast,  besides  giving  rise  to  the  skin 
(hypodermis  and  cuticle),  also  supplies  the  elements  for  the 
nervous  system  and  organs  of  sense,  and  for  the  respiratory 
sacks,  the  stomodaeum  and  proctodaeum. 

At  the  period  when  the  mesoblast  is  definitely  established, 
the  epiblast  is  formed  of  a  single  layer  of  columnar  cells  in  the 
region  of  the  ventral  plate,  and  of  a  layer  of  flat  cells  over  other 
parts  of  the  yolk. 

When  about  six  segments  are  present  the  first  changes  take 
place.  The  epiblast  of  the  ventral  plate  then  becomes  somewhat 
thinner  in  the  median  line  than  at  the  two  sides  (fig.  203  B).  In 
succeeding  stages  the  contrast  between  the  median  and  the 
lateral  parts  becomes  still  more  marked,  so  that  the  epiblast 
becomes  finally  constituted  of  two  lateral  thickened  bands,  which 
meet  in  front  in  the  procephalic  lobes,  and  behind  in  the  caudal 
lobe,  and  are  elsewhere  connected  by  a  very  thin  layer  (fig. 
203  C).  Shortly  after  the  appendages  begin  to  be  formed,  the 
first  rudiments  of  the  ventral  nerve-cord  become  established  as 
epiblastic  thickenings  on  the  inner  side  of  each  of  the  lateral 
bands.  The  thickenings  of  the  epiblast  of  the  two  sides  are 
quite  independent,  as  may  be  seen  in  fig.  203  C,  vn,  taken  from  a 
stage  somewhat  subsequent  to  their  first  appearance.  They  are 
developed  from  before  backwards,  but  either  from  the  first,  or  in 
any  case  very  soon  afterwards,  cease  to  form  uniform  thickenings, 
but  constitute  a  linear  series  of  swellings — the  future  ganglia — 
connected  by  very  short  less  prominent  thickenings  of  the  epi- 
blast (fig.  200  C).  The  rudiments  of  the  ventral  nerve-cord  are 
for  a  long  time  continuous  with  the  epiblast,  but  shortly  after  the 
establishment  of  the  dorsal  surface  of  the  embryo  they  become 
separated  from  the  epiblast  and  constitute  two  independent 
cords,  the  histological  structure  of  which  is  the  same  as  in  other 
Tracheata  (fig.  206,  vn\ 

The  ventral  cords  are  at  first  composed  of  as  many  ganglia 
as  there   are  segments.     The  foremost  pair,  belonging  to  the 


448 


ARACHNIDA. 


segment  of  the  chelicerae,  lie  immediately  behind  the  stomodaeum, 
and  are  as  independent  of  each  other  as  the  remaining  ganglia. 
Anteriorly  they  border  on  the  supra-cesophageal  ganglia.  When 
the  yolk  sack  is  formed  in  connection  with  the  ventral  flexure  of 
the  embryo,  the  two  nerve-cords  become  very  widely  separated 
(fig.  206,  vn)  in  their  middle  region.  At  a  later  period,  at  the 
stage  represented  in  fig.  201  B,  they  again  become  approximated 
in  the  ventral  line,  and  delicate  commissures  are  formed  uniting 


FIG.  203.    TRANSVERSE  SECTIONS  THROUGH  THE  VENTRAL  PLATE  OF  AGELBNA 
LABYRINTHICA  AT  THREE  STAGES. 

A.  Stage  when  about  three  segments  are  formed.     The  mesoblastic  plate  is  not 
divided  into  two  bands. 

B.  Stage  when  six  segments  are  present  (fig.  ?oo  B).    The  mesoblast  is  now 
divided  into  two  bands. 

C.  Stage  represented  in  fig.  200  D.    The  ventral  cords  have  begun  to  be  formed 
on  thickenings  of  the  epiblast,  and  the  limbs  are  established. 

ep.  epiblast ;   me.  mesoblast ;  me.s.  mesoblastic  somite  ;   7>n.  ventral  nerve-cord  ; 
yk.  yolk. 

the  ganglia  of  the  two  sides,  but  there  is  no  trace  at  this  or  any 
other  period  of  a  median  invagination  of  epiblast  between  the 
two  cords,  such  as  Hatschek  and  other  observers  have  attempted 
to  establish  for  various  Arthropoda  and  Chaetopoda.  At  the 
stage  represented  in  fig.  201  A  the  nerve  ganglia  are  still  present 
in  the  abdomen,  though  only  about  four  ganglia  can  be  distin- 
guished. At  a  later  stage  these  ganglia  fuse  into  two  continuous 


TRACHEATA.  449 


cords,  united  however  by  commissures  corresponding  with  the 
previous  ganglia. 

The  ganglia  of  the  chelicerae  have,  by  the  stage  represented 
in  fig.  20 1  B,  completely  fused  with  the  supra-oesophageal  ganglia 
and  form  part  of  the  oesophageal  commissure.  The  cesophageal 
commissure  is  however  completed  ventrally  by  the  ganglia  of 
the  pedipalpi. 

The  supra-cesophageal  ganglia  are  formed  independently  of 
the  ventral  cords  as  two  thickenings  of  the  procephalic  lobes  (fig. 
205).  The  thickenings  of  the  two  lobes  are  independent,  and 
each  of  them  becomes  early  marked  out  by  a  semicircular  groove 
(fig.  200  D)  running  outwards  from  the  upper  lip.  Each  thick- 
ening eventually  becomes  detached  from  the  superficial  epiblast, 
but  before  this  takes  place  the  two  grooves  become  deeper, 
and  on  the  separation  of  the  ganglia  from  the  epiblast,  the 
cells  lining  the  grooves  become  involuted  and  detached  from 
the  skin,  and  form  an  integral  part  of  the  supra-oesophageal 
ganglia. 

At  the  stage  represented  in  fig.  201  B  the  supra-oesophageal  ganglia 
are  completely  detached  from  the  epiblast,  and  are  constituted  of  the 
following  parts  :  (i)  A  dorsal  section  formed  of  two  hemispherical  lobes, 
mainly  formed  of  the  invaginated  lining  of  the  semicircular  grooves.  The 
original  lumen  of  the  groove  is  still  present  on  the  outer  side  of  these 
lobes.  (2)  Two  central  masses,  one  for  each  ganglion,  formed  of  puncti- 
form  tissue,  and  connected  by  a  transverse  commissure.  (3)  A  ventral 
anterior  lobe.  (4)  The  original  ganglia  of  the  chelicerae,  which  form  the 
ventral  parts  of  the  ganglia1. 

The  later  stages  in  the  development  of  the  nervous  system  have  not 
been  worked  out. 

The  development  of  the  nervous  system  in  the  Scorpion  is  almost 
identical  with  that  in  Spiders,  but  Metschnikoff  believes,  though  without 
adducing  satisfactory  evidence,  that  the  median  integument  between  the 
two  nerve  cords  assists  in  forming  the  ventral  nerve  cord.  Grooves  are 
present  in  the  supra-cesophageal  ganglia  similar  to  those  in  Spiders. 

The  mesoblast.  The  history  of  the  mesoblast,  up  to  the 
formation  of  a  ventral  plate  subjacent  to  the  thickened  plate  of 
epiblast,  has  been  already  given.  The  ventral  plate  is  shewn 
in  fig.  203  A.  It  is  seen  to  be  formed  mainly  of  small  cells, 

1  For  further  details  vide  self,  No.  440. 
B.  II.  29 


45O  ARACHNIDA. 


but  some  large  cells  are  shewn  in  the  act  of  passing  into  it 
from  the  yolk.  During  a  considerable  section  of  the  subse- 
quent development  the  mesoblast  is  confined  to  the  ventral 
plate. 

The   first   important   change   takes   place   when   about    six 
somites   are  established  ;   the  mesoblast  then  becomes  divided 


f/0 


FIG.  204.     LONGITUDINAL  SECTION  THROUGH  AN  EMBRYO  OF  AGELENA 

LABYRINTHICA. 

The  section  is  through  an  embryo  of  the  same  age  as  that  represented  in  fig. 
200  C,  and  is  taken  slightly  to  one  side  of  the  middle  line  so  as  to  shew  the  relation 
of  the  mesoblastic  somites  to  the  limbs.  In  the  interior  are  seen  the  yolk  segments 
and  their  nuclei. 

i — 16.  the  segments;  pr.l.  procephalic  lobe  ;  do.  dorsal  integument. 

into  two  lateral  bands,  shewn  in  section  in  fig.  203  B,  which  meet 
however  in  front  in  the  procephalic  lobes,  and  behind  in  the 
caudal  lobes.  Very  shortly  afterwards  these  bands  become 
broken  up  into  a  number  of  parts  corresponding  to  the  segments, 
each  of  which  soon  becomes  divided  into  two  layers,  which 
enclose  a  cavity  between  them  (vide  fig.  204  and  fig.  207).  The 
outer  layer  (somatic)  is  thicker  and  attached  to  the  epiblast, 
and  the  inner  layer  (splanchnic)  is  thinner  and  mainly,  if  not 
entirely,  derived  (in  Agelena)  from  cells  which  originate  in  the 
yolk.  These  structures  constitute  the  mesoblastic  somites.  In 
the  appendage-bearing  segments  the  somatic  layer  of  each  of 
them,  together  with  a  prolongation  of  the  cavity,  is  continued 


TRACHEATA. 


451 


into  the  appendage  (fig.  203  C).  Since  the  cavity  of  the  meso- 
blastic  somites  is  part  of  the  body  cavity,  all  the  appendages 
contain  prolongations  of  the  body  cavity.  Not  only  is  a  pair  of 
mesoblastic  somites  formed  for  each  segment  of  the  body,  but 
also  for  the  procephalic  lobes  (fig.  205).  The  mesoblastic  somites 
for  these  lobes  are  established  somewhat  later  than  for  the  true 
segments,  but  only  differ  from  them  in  the  fact  that  the  somites 
of  the  two  sides  are  united  by  a  median  bridge  of  undivided  meso- 
blast.  The  development  of  a  somite  for  the  procephalic  lobes 
is  similar  to  what  has  been  described  by  Kleinenberg  for  Lum- 
bricus  (p.  339), 
but  must  not  be 
necessarily  sup- 
posed to  indicate 
that  the  procepha- 
lic lobes  form  a 
segment  equiva- 
lent to  the  seg- 
ments of  the  trunk. 
They  are  -rather 
equivalent  to  the 


ce.s 


FIG.   205.     SECTION    THROUGH    THE    PROCEPHALIC 
LOBES  OF  AN  EMBRYO  OF  AGELENA  LABYRINTHICA. 

The  section  is  taken  from  an  embryo  of  the  same  age 
as  fig.  200  D. 


Drae  oral     lobe    of     groove 


stomodseum ;    gr.    section    through    semi-circular 
procephalic  lobe  ;  ce.s.  cephalic  section  of  body 
cavitv. 

Chaetopod  larvae. 
When  the  dorsal  surface  of  the  embryo  is  established  a  thick 
layer  of  mesoblast  becomes  formed  below  the  epiblast.  This 
layer  is  not  however  derived  from  an  upgrowth  of  the  mesoblast 
of  the  somites,  but  from  cells  which  originate  in  the  yolk.  The 
first  traces  of  the  layer  are  seen  in  fig.  204,  do,  and  it  is  fully 
established  as  a  layer  of  large  round  cells  in  the  stage  shewn  in 
fig.  206.  This  layer  of  cells  is  seen  to  be  quite  independent  of 
the  mesoblastic  somites  (ine.s).  The  mesoblast  of  the  dorsal 
surface  becomes  at  the  stage  represented  in  fig.  201  B  divided 
into  splanchnic  and  somatic  layers,  and  in  the  abdomen  at  any 
rate  into  somites  continuous  with  those  of  the  ventral  part  of  the 
mesoblast.  At  the  lines  of  junction  of  successive  somites  the 
splanchnic  layer  of  mesoblast  dips  into  the  yolk,  and  forms  a 
number  of  transverse  septa,  which  do  not  reach  the  middle  of 
the  yolk,  but  leave  a  central  part  free,  in  which  the  mesenteron 
is  subsequently  formed.  At  the  insertion  of  these  septa  there 

29 — 2 


452 


ARACHNIDA. 


me.s 


are  developed  widish  spaces  between  the  layers  of  somatic 
and  splanchnic  mesoblast,  which  form  transversely  directed 
channels  passing 
from  the  heart  out- 
wards. They  are 
probably  venous. 
At  a  later  stage 
the  septa  send  out 
lateral  offshoots, 
and  divide  the 
peripheral  part  of 
the  abdominal  cav- 
ity into  a  number 
of  compartments 
filled  with  yolk.  It 
is  probable  that 
the  hepatic  diverti- 
cula  are  eventually 
formed  in  these 
compartments. 

The       somatic 
layer  of  mesoblast 


FIG.  206.  TRANSVERSE  SECTION  THROUGH  THE  THO- 
RACIC REGION  OF  AN  EMBRYO  OF  AGELENA  LABYRINTHICA. 

The  section  is  taken  from  an  embryo  of  the  same  age 
as  fig.  201  A,  and  passes  through  the  maximum  pro- 
tuberance of  the  ventral  yolk  sack. 

vn.  ventral  nerve  cord ;  yk.  yolk ;  me.s.  mesoblastic 
somite ;  ao.  aorta. 


is  converted  into  the  muscles,  both  of  the  limbs  and  trunk,  the 
superficial  connective  tissue,  nervous  sheath,  etc.  It  probably 
also  gives  rise  to  the  three  muscles  attached  to  the  suctorial 
apparatus  of  the  oesophagus. 

The  heart  and  aorta  are  formed  as  a  solid  rod  of  cells  of  the 
dorsal  mesoblast,  before  it  is  distinctly  divided  into  splanchnic 
and  somatic  layers.  Eventually  the  central  cells  of  the  heart 
become  blood  corpuscles,  while  its  walls  are  constituted  of  an 
outer  muscular  and  inner  epithelioid  layer.  It  becomes  func- 
tional, and  acquires  its  valves,  arterial  branches,  etc.,  by  the 
stage  represented  in  fig.  201  B. 

The  history  of  the  mesoblast,  more  especially  of  the  mesoblastic  somites, 
of  the  Scorpion  is  very  similar  to  that  in  Spiders  :  their  cavity  is  continued 
in  the  same  way  into  the  limbs.  The  general  character  of  the  somites 
in  the  tail  is  shewn  in  fig.  207.  The  caudal  aorta  is  stated  by  MetschnikofT 
to  be  formed  from  part  of  the  mesenteron,  but  this  is  too  improbable  to  be 
accepted  without  further  confirmation. 


TRACHEATA. 


453 


The  hypoblast  and  alimentary  tract.  It  has  already 
been  stated  that  the  yolk  is  to  be  regarded  as  corresponding  to 
the  hypoblast  of  other  types. 

For  a  considerable  period  it  is  composed  of  the  polygonal 
yolk  cells  already  described  and  shewn  in  figs.  203,  204,  and  205. 
The  yolk  cells  divide  and  be- 
come somewhat  smaller  as  de- 
velopment proceeds ;  but  the 
main  products  of  the  division 
of  the  yolk  nuclei  and  the  pro- 
toplasm around  them  are  un- 
doubtedly cells  which  join  the 
mesoblast  (fig.  203  A).  The 
permanent  alimentary  tract  is 
formed  of  three  sections,  viz. 
stomodaeum,  proctodaeum,  and 
mesenteron.  The  stomodaeum 
and  proctodaeum  are  both 
formed  before  the  mesenteron. 
The  stomodaeum  is  formed  as 
an  epiblastic  pit  between  the 
two  procephalic  lobes  (figs.  200 
and  205,  st).  It  becomes 
deeper,  and  by  the  latest  stage 
figured  is  a  deep  pit  lined  by  a 
cuticle  and  ending  blindly.  To 
its  hinder  section,  which  forms 
the  suctorial  apparatus  of  the  adult,  three  powerful  muscles  (a 
dorsal  and  two  lateral)  are  attached. 

The  proctodaeum  is  formed  considerably  later  than  the 
stomodaeum.  It  is  a  comparatively  shallow  involution,  which 
forms  the  rectum  of  the  adult.  It  is  dilated  at  its  extremity,  and 
two  Malpighian  vessels  early  grow  out  from  it. 

The  mesenteron  is  formed  in  the  interior  of  the  yolk.  Its 
walls  are  derived  from  the  cellular  elements  of  the  yolk,  and  the 
first  section  to  be  formed  is  the  hinder  extremity,  which  appears 
as  a  short  tube  ending  blindly  behind  in  contact  with  the  procto- 
daeum, and  open  to  the  yolk  in  front.  The  later  history  of  the 
mesenteron  has  not  been  followed,  but  it  undoubtedly  includes 


FlG.  207.  TAIL  OF  AN  ADVANCED  EM- 
BRYO OF  THE  SCORPION  TO  ILLUSTRATE 
THE  STRUCTURE  OF  THE  MESOBLASTIC 

SOMITES.     (After  Metschnikoff.) 

al.  alimentary  tract;  an.i.  anal  in- 
vagination ;  ep.  epiblast ;  me.s.  meso- 
blastic  somite. 


454  ARACHNIDA. 


the  whole  of  the  abdominal  section  of  the  alimentary  canal  of 
the  adult,  except  the  rectum,  and  probably  also  the  thoracic 
section.  The  later  history  of  the  yolk  which  encloses  the  mesen- 
teron  has  not  been  satisfactorily  studied,  though  it  no  doubt 
gives  rise  to  the  hepatic  tubes,  and  probably  also  to  the  thoracic 
diverticula  of  the  alimentary  tract. 

The  general  history  of  the  alimentary  tract  in  Scorpio  is  much  the  same 
as  in  Spiders.  The  hypoblast,  the  origin  of  which  as  mentioned  above  is 
somewhat  uncertain,  first  appears  on  the  ventral  side  and  gradually  spreads 
so  as  to  envelop  the  yolk,  and  form  the  wall  of  the  mesenteron,  from 
which  the  liver  is  formed  as  a  pair  of  lateral  outgrowths.  The  procto- 
daeum  and  stomodseum  are  both  short,  especially  the  former  (vide  fig.  207). 

Summary  and  general  conclusions. 

The  embryonic  forms  of  Scorpio  and  Spiders  are  very 
similar,  but  in  spite  of  the  general  similarity  of  Chelifer  to 
Scorpio,  the  embryo  of  the  former  differs  far  more  from  that  of 
Scorpio  than  the  latter  does  from  Spiders.  This  peculiarity  is 
probably  to  be  explained  by  the  early  period  at  which  Chelifer 
is  hatched  ;  and  though  a  more  thorough  investigation  of  this 
interesting  form  is  much  to  be  desired,  it  does  not  seem  probable 
that  its  larva  is  a  primitive  type. 

The  larvae  of  the  Acarina  with  their  peculiar  ecdyses  are  to 
be  regarded  as  much  modified  larval  forms.  It  is  not  however 
easy  to  assign  a  meaning  to  the  hexapodous  stage  through 
which  they  generally  pass. 

With  reference  to  the  segments  and  appendages,  some  inter- 
esting points  are  brought  out  by  the  embryological  study  of 
these  forms. 

The  maximum  number  of  segments  is  present  in  the 
Scorpion,  in  which  nineteen  segments  (not  including  the  prae- 
oral  lobes,  but  including  the  telson)  are  developed.  Of  these  the 
first  twelve  segments  have  traces  of  appendages,  but  the  append- 
ages of  the  six  last  of  these  (unless  the  pecten  is  an  appendage) 
atrophy.  In  Spiders  there  are  indications  in  the  embryo  of 
sixteen  segments ;  and  in  all  the  Arachnida,  except  the  Acarina, 
at  the  least  four  segments  bear  appendages  in  the  embryo 
which  are  without  them  in  the  adult.  The  morphological  bear- 
ings of  this  fact  are  obvious. 


TRACHEATA.  455 


It  deserves  to  be  noted  that,  in  both  Scorpio  and  the  Spider, 
the  chelicerae  are  borne  in  the  embryo  by  the  first  post-oral 
segment,  and  provided  with  a  distinct  ganglion,  so  that  they 
cannot  correspond  (as  they  are  usually  supposed  to  do)  with  the 
antennae  of  Insects,  which  are  always  developed  on  the  prae-oral 
lobes,  and  never  supplied  by  an  independent  ganglion. 

The  chelicerae  would  seem  probably  to  correspond  with  the 
mandibles  of  Insects,  and  the  antennae  to  be  absent.  In  favour 
of  this  view  is  the  fact  that  the  embryonic  ganglion  of  the 
mandibles  of  Insects  is  stated  (cf.  Lepidoptera,  Hatschek,  p.  340) 
to  become,  like  the  ganglion  of  the  chelicerae,  converted  into 
part  of  the  cesophageal  commissure. 

If  the  above  considerations  are  correct,  the  appendages  of 
the  Arachnida  retain  in  many  respects  a  very  much  more  prim- 
itive condition  than  those  of  Insects.  In  the  first  place,  both  the 
chelicerae  and  pedipalpi  are  much  less  differentiated  than  the 
mandibles  and  first  pair  of  maxillae  with  which  they  correspond. 
In  the  second  place,  the  first  pair  of  ambulatory  limbs  must  be 
equivalent  to  the  second  pair  of  maxillae  of  Insects,  which,  for 
reasons  stated  above,  were  probably  originally  ambulatory.  It 
seems  in  fact  a  necessary  deduction  from  the  arguments  stated 
that  the  ancestors  of  the  present  Insecta  and  Arachnida  must 
have  diverged  from  a  common  stem  of  the  Tracheata  at  a  time 
when  the  second  pair  of  maxillae  were  still  ambulatory  in 
function. 

With  reference  to  the  order  of  the  development  of  the  appendages 
and  segments,  very  considerable  differences  are  noticeable  in  the  different 
Arachnoid  types.  This  fact  alone  appears  to  me  to  be  sufficient  to  prove 
that  the  order  of  appearance  of  the  appendages  is  often  a  matter  of 
embryonic  convenience,  without  any  deep  morphological  significance.  In 
Scorpio  the  segments  develop  successively,  except  perhaps  the  first  post- 
oral,  which  is  developed  after  some  of  the  succeeded  segments  have 
been  formed.  In  Spiders  the  segment  of  the  chelicerae,  and  probably  also 
of  the  pedipalpi,  appears  later  than  the  next  three  or  four.  In  both  these 
types  the  segments  arise  before  the  appendages,  but  the  reverse  appears  to 
be  the  case  in  Chelifer.  The  permanent  appendages,  except  the  chelicerae, 
appear  simultaneously  in  Scorpions  and  Spiders.  The  second  pair  appears 
long  before  the  others  in  Chelifer,  then  the  third,  next  the  first,  and  finally 
the  three  hindermost. 


456  ARACHNIDA. 


BIBLIOGRAPHY. 

Scorpionidcz. 

(434)  El.  Metschnikoff.     "  Embryologie  des  Scorpions."    Zeit.f.wiss.  Zool. 
Bd.  xxi.     1870. 

(435)  H.  Rathke.     Reisebemerkungen  aus  Taurien  (Scorpio),  Leipzig,  1837. 

Pseudoscorpionidce. 

(436)  El.  Metschnikoff.    "  Entwicklungsgeschichte  d.  Chelifer."   Zeit.f.wiss. 
Zool.,  Bd.  xxi.     1870. 

(437)  A.  Stecker.     "  Entwicklung  der  Chthonius-Eier  im  Mutterleibe  und  die 
Bildung  des  Blastoderms."     Sitzung.  konigl.  bohmisch.  Gesellschaft  Wissensch.,  1876, 
3.  Heft,  and  Aimed,  and  Mag.  Nat.  History,  1876,  xvm.  197. 

Phalangida. 

(438)  M.  Balbiani.     "  Memoire  sur  le  developpement  des  Phalangides."    Ann. 
Scien.  Nat.  Series  v.  Vol.  xvi.     1872. 

A  raneina. 

(439)  M.  Balbiani.     "Memoire  sur  le  developpement  des  Araneides."    Ann. 
Scien.  Nat.  Series  v.  Vol.  xvn.     1873. 

(440)  F.  M.  Balfour.     "Notes  on  the  development  of  the  Araneina."     Quart. 
Journ.  of  Micr.  Science,  Vol.  xx.     1880. 

(441)  J.  Barrois.     "  Recherches  s.  1.  developpement  des  Araigndes. "    Journal 
de  1'Anat.  et  de  la  Physiol.     1878. 

(442)  E.  Claparede.     Recherches  s.  t evolution  des  Araignees.     Utrecht,  1862. 

(443)  Hero  Id.     De  generatione  Araneorum  in  Ovo.     Marburg,  1824. 

(444)  H.  Ludwig.     "Ueber  die  Bildung  des  Blastoderms  bei  den  Spinnen." 
Zeit.f.  wiss.  Zool.,  Vol.  xxvi.     1876. 

Acarina. 

(445)  P.  van  Beneden.     "  Developpement  de  1'Atax  ypsilophora."  Acad.  Bru- 
xelles,  t.  xxiv. 

(446)  Ed.  Claparede.     "Studien  iiber  Acarinen."    Zeit.f.  wiss.  Zool.,  Bd. 
xvm.  1868. 

Formation   of   the  layers   and    the  embryonic   envelopes    in    the 

Tracheata. 

There  is  a  striking  constancy  in  the  mode  of  formation  of 
the  layers  throughout  the  group.  In  the  first  place  the  hypo- 
blast  is  not  formed  by  a  process  which  can  be  reduced  to 
invagination :  in  other  words,  there  is  no  gastrula  stage. 


TRACHEATA.  457 


Efforts  have  been  made  to  shew  that  the  mesoblastic  groove  of  Insects 
implies  a  modified  gastrula,  but  since  it  is  the  essence  of  a  gastrula  that  it 
should  directly  or  indirectly  give  rise  to  the  archenteron,  the  groove  in 
question  cannot  fall  under  this  category.  Although  the  mesoblastic  groove 
of  Insects  is  not  a  gastrula,  it  is  quite  possible  that  it  is  the  rudiment  of  a 
blastopore,  the  gastrula  corresponding  to  which  has  now  vanished  from 
the  development.  It  would  thus  be  analogous  to  the  primitive  streak  of 
Vertebrates1. 

The  growth  of  the  blastoderm  over  the  yolk  in  Scorpions  admits  no 
doubt  of  being  regarded  as  an  epibolic  gastrula.  The  blastopore  would 
however  be  situated  dorsally,  a  position  which  it  does  not  occupy  in  any 
gastrula  type  so  far  dealt  with.  This  fact,  coupled  with  the  consideration 
that  the  partial  segmentation  of  Scorpio  can  be  derived  without  difficulty 
from  the  ordinary  Arachnidan  type  (vide  p.  120),  seems  to  shew  that  there 
is  no  true  epibolic  invagination  in  the  development  of  Scorpio. 

On  the  formation  of  the  blastoderm  traces  of  two  embryonic 
layers  are  established.  The  blastoderm  itself  is  essentially  the 
epiblast,  while  the  central  yolk  is  the  hypoblast.  The  formation 
of  the  embryo  commences  in  connection  with  a  thickening  of  the 
blastoderm,  known  as  the  ventral  plate.  The  mesoblast  is 
formed  as  an  unpaired  plate  split  off  from  the  epiblast  of  the 
ventral  plate.  This  process  takes  place  in  at  any  rate  two  ways. 
In  Insects  a  groove  is  formed,  which  becomes  constricted  off  to 
form  the  mesoblastic  plate :  in  Spiders  there  is  a  keel-like 
thickening  of  the  blastoderm,  which  takes  the  place  of  the 
groove. 

The  unpaired  mesoblastic  plate  becomes  in  all  forms  very 
soon  divided  into  two  mesoblastic  bands. 

The  mesoblastic  bands  are  very  similar  to,  and  probably 
homologous  with,  those  of  Chaetopoda ;  but  the  different  modes 
by  which  they  arise  in  these  two  groups  are  very  striking,  and 
probably  indicate  that  profound  modifications  have  taken  place 
in  the  early  development  of  the  Tracheata.  In  the  Chaetopoda 
the  bands  are  from  the  first  widely  separated,  and  gradually 
approach  each  other  ventrally,  though  without  meeting.  In  the 
Tracheata  they  arise  from  the  division  of  an  unpaired  ventral 
plate. 

The  further  history  of  the  mesoblastic  bands  is  nearly  the 

1  The  primitive  streak  of  Vertebrates,  as  will  appear  in  the  sequel,  has  no  con- 
nection with  the  medullary  groove,  and  is  the  rudiment  of  the  blastopore. 


458  TRACHEATA. 


same  for  all  the  Tracheata  so  far  investigated,  and  is  also  very 
much  the  same  as  for  the  Chaetopoda.  There  is  a  division  into 
somites;  each  containing  a  section  of  the  body  cavity.  In  the 
cephalic  section  of  the  mesoblastic  bands  a  section  of  the  body 
cavity  is  also  formed.  In  Arachnida,  Myriapoda,  and  probably 
also  Insecta,  the  body  cavity  is  primitively  prolonged  into  the 
limbs. 

In  Spiders  at  any  rate,  and  very  probably  in  the  other  groups 
of  the  Tracheata,  a  large  part  of  the  mesoblast  is  not  derived 
from  the  mesoblastic  plate,  but  is  secondarily  added  from  the 
yolk-cells. 

In  all  Tracheata  the  yolk-cells  give  rise  to  the  mesenteron 
which,  in  opposition,  as  will  hereafter  appear,  to  the  mesenteron 
of  the  Crustacea,  forms  the  main  section  of  the  permanent 
alimentary  tract. 

One  of  the  points  which  is  still  most  obscure  in  connection 
with  the  embryology  of  the  Tracheata  is  the  origin  of  the 
embryonic  membranes.  Amongst  Insects,  with  the  exception 
of  the  Thysanura,  such  membranes  are  well  developed.  In  the 
other  groups  definite  membranes  like  those  of  Insects  are  never 
found,  but  in  the  Scorpion  a  cellular  envelope  appears  to  be 
formed  round  the  embryo  from  the  cells  of  the  blastoderm,  and 
more  or  less  similar  structures  have  been  described  in  some 
Myriapods  (vide  p.  390).  These  structures  no  doubt  further 
require  investigation,  but  may  provisionally  be  regarded  as 
homologous  with  the  amnion  and  serous  membrane  of  Insects. 
In  the  present  state  of  our  knowledge  it  does  not  seem  easy  to 
give  any  explanation  of  the  origin  of  these  membranes,  but  they 
may  be  in  some  way  derived  from  an  early  ecdysis. 


CHAPTER    XVIII 


CRUSTACEA1. 


History  of  the  larval  forms1 '. 

THE  larval  forms  of  the  Crustacea  appear  to  have  more  faith- 
fully preserved  their  primitive  characters  than  those  of  almost 
any  other  group. 

BRANCHIOPODA. 

The  Branchiopoda,  comprising  under  that  term  the  Phyllo- 
poda  and  Cladocera,  contain  the  Crustacea  with  the  maximum 
number  of  segments  and  the  least  differentiation  of  the  separate 
appendages.  This  and  other  considerations  render  it  probable 
that  they  are  to  be  regarded  as  the  most  central  group  of  the 
Crustaceans,  and  as  in  many  respects  least  modified  from  the 
ancestral  type  from  which  all  the  groups  have  originated. 

1  The  following  is  the  classification  of  the  Crustacea  employed  in  the  present 
chapter. 

i Phyllopoda.  ( Natantia. 

I.  Branchiopoda.    ciadocenu  III.  Copepoda.    Euc°PeP°da  Iparasita. 

( Branchiura 
T  Nebaliadse.  jThoracica. 

M  f     Sat-  <v-  wdi,  paminai ia 

II.  Malacostraca.   ]  Stomatopoda.  ULocephaia. 

I  Cumacese.  v.  Ostracoda. 

I  Edriophthalmata. 

2  The  importance  of  the  larval  history  of  the  Crustacea,  coupled  with  our  compara- 
tive ignorance  of  the  formation  of  the  layers,  has  rendered  it  necessary  for  me  to 
diverge  somewhat  from  the  general  plan  of  the  work,  and  to  defer  the  account  of  the 
formation  of  the  layers  till  after  that  of  the  larval  forms. 


460  PHYLLOPODA. 


The  free  larval  stages  when  such  exist  commence  with  a 
larval  form  known  as  the  Nauplius. 

The  term  Nauplius  was  applied  by  O.  F.  Muller  to  certain 
larval  forms  of  the  Copepoda  (fig.  229)  in  the  belief  that  they 
were  adult. 

The  term  has  now  been  extended  to  a  very  large  number  of 
larvae  which  have  certain  definite  characters  in  common.  They 
are  provided  (fig.  208  A)  with  three  pairs  of  appendages,  the 
future  two  pairs  of  antennae  and  mandibles.  The  first  pair  of 
antennae  (an1)  is  uniramous  and  mainly  sensory  in  function,  the 
second  pair  of  antennae  (an*)  and  mandibles  (md)  are  biramous 


A      qn 


FlG.  208.      TWO  STAGES  IN  THE  DEVELOPMENT  OF  APUS  CANCRIFORM1S. 

(After  Claus.) 

A.  Nauplius  stage  at  the  time  of  hatching. 

B.  Stage  after  first  ecdysis. 

an1,  and  a«2.   First  and  second  antennae  ;  md.  mandible  ;  MX.  maxilla  ;  /.  labrum; 
fr.  frontal  sense  organ  ;  /.  caudal  fork  ;  s.  segments. 

swimming  appendages,  and  the  mandibles  are  without  the  future 
cutting  blade.  The  Nauplius  mandibles  represent  in  fact  the 
palp.  The  two  posterior  appendages  are  both  provided  with 
hook-like  prominences  on  their  basal  joints,  used  in  mastication. 
The  body  in  most  cases  is  unsegmented,  and  bears  anteriorly  a 
single  median  eye.  There  is  a  large  upper  lip,  and  an  aliment- 
ary canal  formed  of  cesophagus,  stomach  and  rectum.  The  anus 
opens  near  the  hind  end  of  the  body.  On  the  dorsal  surface 
small  folds  of  skin  frequently  represent  the  commencement  of  a 
dorsal  shield.  One  very  striking  peculiarity  of  the  Nauplius 
according  to  Claus  and  Dohrn  is  the  fact  that  the  second  pair 
of  antennae  is  innervated  from  a  sub-oesophageal  ganglion.  A 
larval  form  with  the  above  characters  occurs  with  more  or  less 
frequency  in  all  the  Crustacean  groups.  In  most  instances  it 


CRUSTACEA.  461 


does  not  exactly  conform  to  the  above  type,  and  the  divergences 
are  more  considerable  in  the  Phyllopods  than  in  most  other 
groups.  Its  characters  in  each  case  are  described  in  the  sequel. 
Phyllopoda.  For  the  Phyllopoda  the  development  of  Apus 
cancriformis  may  conveniently  be  taken  as  type  (Claus,  No.  454). 
The  embryo  at  the  time  it  leaves  the  egg  (fig.  208  A)  is  some- 
what oval  in  outline,  and  narrowed  posteriorly.  There  is  a 
slight  V-shaped  indentation  behind,  at  the  apex  of  which  is 
situated  the  anus.  The  body,  unlike  that  of  the  typical 
Nauplius,  is  already  divided  into  two  regions,  a  cephalic  and 
post-cephalic.  On  the  ventral  side  of  the  cephalic  region  there 
are  present  the  three  normal  pairs  of  appendages.  Foremost 
there  are  the  small  anterior  antennae  (an1),  which  are  simple 
unjointed  rod-like  bodies  with  two  moveable  hairs  at  their 
extremities.  They  are  inserted  at  the  sides  of  the  large  upper- 
lip  or  labrum  (/).  Behind  these  are  the  posterior  antennae,  which 
are  enormously  developed  and  serve  as  the  most  important 
larval  organs  of  locomotion.  They  are  biramous,  being  formed 
of  a  basal  portion  with  a  strong  hook-like  bristle  projecting 
from  its  inner  side,  an  inner  unjointed  branch  with  three  bristles, 
and  an  outer  large  imperfectly  five-jointed  branch  with  five  long 
lateral  bristles.  The  hook-like  organ  attached  to  this  pair  of 
appendages  would  seem  to  imply  that  it  served  in  some  ancestral 
form  as  jaws  (Claus).  This  character  is  apparently  universal  in 
the  embryos  of  true  Phyllopods,  and  constantly  occurs  in  the 
Copepoda,  etc. 

The  third  pair  of  appendages  or  mandibles  (md)  is  attached 
close  below  the  upper  lip.  They  are  as  yet  unprovided  with 
cutting  blades,  and  terminate  in  two  short  branches,  the  inner 
with  two  and  the  outer  with  three  bristles. 

At  the  front  of  the  head  there  is  the  typical  unpaired  eye. 
On  the  dorsal  surface  there  is  already  present  a  rudiment  of  the 
cephalic  shield,  continuous  anteriorly  with  the  labrum  (/)  or 
upper  lip,  the  extraordinary  size  of  which  is  characteristic  of  the 
larvae  of  Phyllopods.  The  post-cephalic  region,  which  afterwards 
becomes  the  thorax  and  abdomen,  contains  underneath  the  skin 
rudiments  of  the  five  anterior  thoracic  segments  and  their 
appendages,  and  presents  in  this  respect  an  important  variation 
from  the  typical  Nauplius  form.  After  the  first  ecdysis  the 


462  PHYLLOPODA. 


larva  (fig.  208  B)  loses  its  oval  form,  mainly  owing  to  the  elong- 
ation of  the  hinder  part  of  the  body  and  the  lateral  extension  of 
the  cephalic  shield,  which  moreover  now  completely  covers  over 
the  head  and  has  begun  to  grow  backwards  so  as  to  cover  over 
the  thoracic  region.  At  the  second  ecdysis  there  appears  at  its 
side  a  rudimentary  shell  gland.  In  the  cephalic  region  two 
small  papillae  (fr)  are  now  present  at  the  front  of  the  head  close 
to  the  unpaired  eye.  They  are  of  the  nature  of  sense  organs, 
and  may  be  called  the  frontal  sense  papillae.  They  have  been 
shewn  by  Claus  to  be  of  some  phylogenetic  importance.  The 
three  pairs  of  Nauplius  appendages  have  not  altered  much,  but 
a  rudimentary  cutting  blade  has  grown  out  from  the  basal  joint 
of  the  mandible.  A  gland  opening  at  the  base  of  the  antennae 
is  now  present,  which  is  probably  equivalent  to  the  green  gland 
often  present  in  the  Malacostraca.  Behind  the  mandibles  a  pair 
of  simple  processes  has  appeared,  which  forms  the  rudiment  of 
the  first  pair  of  maxillae  (mx). 

In  the  thoracic  region  more  segments  have  been  added 
posteriorly,  and  the  appendages  of  the  three  anterior  segments 
are  very  distinctly  formed.  The  tail  is  distinctly  forked.  The 
heart  is  formed  at  the  second  ecdysis,  and  then  extends  to  the 
sixth  thoracic  segment :  the  posterior  chambers  are  successively 
added  from  before  backwards. 

At  the  successive  ecdyses  which  the  larva  undergoes  new 
segments  continue  to  be  formed  at  the  posterior  end  of  the  body, 
and  limbs  arise  on  the  segments  already  formed.  These  limbs 
probably  represent  the  primitive  form  of  an  important  type  of 
Crustacean  appendage,  which  is  of  value  for  interpreting  the 
parts  of  the  various  malacostracan  appendages.  They  consist 
(fig.  209)  of  a  basal  portion  (protopodite  of  Huxley)  bearing  two 
rami.  The  basal  portion  has  two  projections  on  the  inner  side. 
To  the  outer  side  of  the  basal  portion  there  is  attached  a 
dorsally  directed  branchial  sack  (br)  (epipodite  of  Huxley).  The 
outer  ramus  (ex)  (exopodite  of  Huxley)  is  formed  of  a  single  plate 
with  marginal  setae.  The  inner  one  (en)  (endopodite  of  Huxley) 
is  four-jointed,  and  a  process  similar  to  those  of  the  basal  joint 
is  given  off  from  the  inner  side  of  the  three  proximal  joints. 

At  the  third  ecdysis  several  new  features  appear  in  the 
cephalic  region,  which  becomes  more  prominent  in  the  succeeding 


CRUSTACEA.  463 


stages.    In  the  first  place  the  paired  eyes  are  formed  at  each  side 

of  and  behind  the  unpaired  eye,  second- 

ly  the   posterior    pair    of    maxillae    is 

formed  though  it  always  remains  very 

rudimentary.     The  shell  gland  becomes 

fully  developed  opening  at  the  base  of 

the  first  pair  of  maxillae.     The  dorsal 

shield  gradually  grows  backwards  till  it 

covers  its  full  complement  of  segments. 

After  the  fifth  ecdysis  the  Nauplius          FIG.  209.  TYPICAL  PHYL- 

.  ,  .  ,  ,  LOPOD  APPENDAGE.      (Copied 

appendages  undergo  a   rapid   atrophy.     from  ciaus.) 

The  second  pair  of  antennae  especially         ex.  exopodite  ;  en.  endo- 

becomes  reduced  in  size,  and  the  man- 


dibular    palp  —  the     primitive    Nauplius      portion  bearing  the  two  proxi- 
r     .  .....  ,       mal  projections  is  not  sharply 

portion  of  the  mandible—  is  contracted  separated  from  the  endopo- 
to  a  mere  rudiment,  which  eventually  dite- 
completely  disappears,  while  the  blade  is  correspondingly  en- 
larged and  also  becomes  toothed.  The  adult  condition  is  only 
gradually  attained  after  a  very  large  number  of  successive  changes 
of  skin. 

The  chief  point  of  interest  in  the  above  development  is  the 
fact  of  the  primitive  Nauplius  form  becoming  gradually  convert- 
ed without  any  special  metamorphosis  into  the  adult  condition1. 

Branchipus  like  Apus  is  hatched  as  a  somewhat  modified  Nauplius, 
which  however  differs  from  that  of  Apus  in  the  hinder  region  of  the  body 
having  no  indications  of  segments.  It  goes  through  a  very  similar  meta- 
morphosis, but  is  at  no  period  of  its  metamorphosis  provided  with  a  dorsal 
shield  :  the  second  pair  of  antennae  does  not  abort,  and  in  the  male  is  pro- 
vided with  clasping  organs,  which  are  perhaps  remnants  of  the  embryonic 
hooks  so  characteristic  of  this  pair  of  antennas. 

The  larva  of  Estheria  when  hatched  has  a  Nauplius  form,  a  large 
upper  lip,  caudal  fork  and  single  eye.  There  are  two  functional  pairs  of 
swimming  appendages  —  the  second  pair  of  antennae  and  mandibles.  The 
first  pair  of  antennae  has  not  been  detected,  and  a  dorsal  mantle  to  form 
the  shell  is  not  developed.  At  the  first  moult  the  anterior  pair  of 
antennae  arises  as  small  stump-like  structures,  and  a  small  dorsal  shield 
is  also  formed.  Rudiments  of  six  or  seven  pairs  of  appendages  sprout 

1  Nothing  appears  to  be  known  with  reference  to  the  manner  in  which  it  comes 
about  that  more  than  one  appendage  is  borne  on  each  of  the  segments  from  the 
eleventh  to  the  twentieth.  An  investigation  of  this  point  would  be  of  some  interest 
with  reference  to  the  meaning  of  segmentation- 


464 


CLADOCERA. 


out  in  the  usual  way,  and  continue  to  increase  in  number  at  successive 
moults  :  the  shell  is  rapidly  developed.  The  chief  point  of  interest  in 
the  development  of  this  form  is  the  close  resemblance  of  the  young  larva 
to  a  typical  adult  Cladocera  (Claus).  This  is  shewn  in  the  form  of  the 
shell,  which  has  not  reached  its  full  anterior  extension,  the  rudimentary 
anterior  antennae,  the  large  locomotor  second  pair  of  antennas,  which  differ 
however  from  the  corresponding  organs  in  the  Cladocera  in  the  presence 
of  typical  larval  hooks.  Even  the  abdomen  resembles  that  of  Daphnia. 
These  features  perhaps  indicate  that  the  Cladocera  are  to  be  derived 
from  some  Phyllopod  form  like  Estheria  by  a  process  of  retrogressive 
metamorphosis.  The  posterior  antennas  in  the  adult  Estheria  are  large 
biramous  appendages,  and  are  used  for  swimming ;  and  though  they 
have  lost  the  embryonic  hook,  they  still  retain  to  a  larger  extent  than 
in  other  Phyllopod  families  their  Nauplius  characteristics. 

The  Nauplius  form  of  the  Phyllopods  is  marked  by  several 
definite  peculiarities.  Its  body  is  distinctly  divided  into  a  ceph- 
alic and  post-cephalic  region.  The  upper  lip  is  extraordinarily 
large,  relatively  very  much  more  so  than  at  the  later  stages. 
The  first  pair  of  antennae  is  usually  rudimentary  and  sometimes 
even  absent ;  while  the  second  pair  is  exceptionally  large,  and 
would  seem  to  be  capable  of  functioning  not  only  as  a  swimming 
organ,  but  even  as  a  masticating  organ.  A  dorsal  shield  is 
nearly  or  quite  absent. 

Cladocera.  The  probable  derivation  of  the  Cladocera  from  a  form 
similar  to  Estheria  has  already  been  mentioned,  and  it  might  have  been 
anticipated  that  the  deve- 
lopment would  be  similar 
to  that  of  the  Phyllopods. 
The  development  of  the  ma- 
jority of  the  Cladocera  takes 
place  however  in  the  egg, 
and  the  young  when  hatched 
closely  resembles  their  pa- 
rents, though  in  the  egg  they 
pass  through  a  Nauplius 
stage  (Dohrn).  An  excep- 
tion to  the  general  rule  is 
however  offered  by  the  case 
of  the  winter  eggs  of  Lepto- 
dora,  one  of  the  most  primi- 
tive of  the  Cladoceran 


families.   The  summer  eggs      after  Sars.) 


FIG.  709  A.    NAUPLIUS  LARVA  OF  LEPTODORA 
IIYAI.INA  FROM  wiNTKR  EGG.  (Copied  from  Bronn ; 


develop  without  metamor- 


«;/'.    antenna   of  first   pair;    an*,    antenna   of 


phosis,  but  Sars  (No.  461)      second  pair;  ntd.  mandible;/  caudal  fork. 


CRUSTACEA. 


465 


has  discovered  that  the  larva  leaves  the  winter  eggs  in  the  form  of  a 
Nauplius  (fig.  209).  This  Nauplius  closely  resembles  that  of  the  Phyllopods. 
The  body  is  elongated  and  in  addition  to  normal  Nauplius  appendages 
is  marked  by  six  pairs  of  ridges — the  indications  of  the  future  feet.  The 
anterior  antennae  are  as  usual  small ;  the  second  large  and  biramous, 
but  the  masticatory  bristle  characteristic  of  the  Phyllopods  is  not  present. 
The  mandibles  are  without  a  cutting  blade.  A  large  upper  lip  and  unpaired 
eye  are  present. 

The  adult  form  is  attained  in  the  same  manner  as  amongst  the  Phyllo- 
pods after  the  third  moult. 


MALACOSTRACA. 

Owing  to  the  size  and  importance  of  the  various  forms 
included  in  the  Malacostraca,  greater  attention  has  been  paid  to 
their  embryology  than  to  that  of  any  other  division  of  the 
Crustacea ;  and  the  proper  interpretation  of  their  larval  forms 
involves  some  of  the  most  interesting  problems  in  the  whole 
range  of  Embryology. 

The  majority  of  Malacostraca  pass  through  a  more  or  less 
complicated  metamorphosis,  though  in  the  Nebaliadae,  the 
Cumaceae,  some  of  the  Schizopoda,  a  few  Decapoda  (Astacus, 
Gecarcinus,  etc.),  and  in  the  Edriophthalmata,  the  larva  on 
leaving  the  egg  has  nearly  the  form  of  the  adult.  In  contradis- 
tinction to  the  lower  groups  of  Crustacea  the  Nauplius  form  of 
larva  is  rare,  though  it  occurs  in  the  case  of  one  of  the  Schizopods 
(Euphausia,  fig.  212),  in  some  of  the  lower  forms  of  the  Decapods 
(Penaeus,  fig.  214),  and 
perhaps  also,  though  this 
has  not  been  made  out,  in 
some  of  the  Stomatopoda. 

In  the  majority  of  the 
Decapoda  the  larva  leaves 
the  egg  in  a  form  known 
as  the  Zoaea  (fig.  210). 
This  larval  form  is 
characterised  by  the  pre- 
sence of  a  large  cephalo- 

thoracic    t  shield      usually     FIG.  210.  ZO^EAOFTHIAPOLITA.  (After'Claus.) 
,         ., ,      ,    ,        ,  mxp*.   second  maxillipede. 

armed   with    lateral,    an- 
terior, and  dorsal  spines.     The  caudal  segments  are  well  de- 

B.  II.  30 


466  SCHIZOPODA. 


veloped,  though  wit/tout  appendages,  and  the  tail,  which  functions 
in  swimming,  is  usually  forked.  The  six  posterior  thoracic  seg- 
ments are,  on  the  other  hand,  rudimentary  or  non-existent.  There 
are  seven  anterior  pairs  of  appendages  shewn  in  detail  in  fig.  21 1, 
viz.  the  two  pairs  of  antennae  (At.  I.  and  At.  II.),  neither  of  them 
used  as  swimming  organs,  the  mandibles  without  a  palp  (ma7), 
well-developed  maxillae  (two  pairs,  mx  I  and  mx  2),  and  two  or 
sometimes  (Macrura)  three  pairs  of  biramous  natatory  maxilli- 
peds  (mxp  I  and  mxp  2).  Two  lateral  compound  stalked  eyes 
are  present,  together  with  a  median  Nauplius  eye.  The  heart 
has  in  the  majority  of  cases  only  one  or  two  (Brachyura)  pairs  of 
ostia. 

The  Zoaea  larva,  though  typically  developed  in  the  Decapoda, 
is  not  always  present  (e.g.  Astacus  and  Homarus),  and  some- 


FIG.  211.    THE  APPENDAGES  OF  A  CRAB  Z<VEA. 

.-//./.  first  antenna  ;  At.  I  I.  second  antenna ;  md.  mandible  (without  a  palp);  mx. 
\.  first  maxilla;  mx.  i.  second  maxilla;  mxp.  \.  first  maxilliped  ;  mxp.  i.  second 
maxilliped. 

ex.  exopodite ;  en.  endopodite. 

times  occurs  in  a  very  modified  form.     It  makes  its  appearance 
in  an  altered  garb  in  the  ontogeny  of  some  of  the  other  groups. 

The  two  Malacostracan  forms,  amongst  those  so  far  studied, 
in  which  the  phylogenetic  record  is  most  fully  preserved  in  the 
ontogeny,  are  Euphausia  amongst  the  Schizopods  and  Penaeus 
amongst  the  Decapods. 

Schizopoda.  Euphausia  leaves  the  egg  (MetschnikofT,  No.  468—9) 
as  a  true  Nauplius  with  only  three  pairs  of  appendages,  the  two  hinder 


CRUSTACEA. 


467 


biramous,  and  an  unsegmented  body.  The  second  pair  of  antennae  has  not 
however  the  colossal  dimensions  so  common  in  the  lower  types.  A  mouth  is 
present,  but  the  anus  is  undeveloped. 

After  the  first  moult  three  pairs  of  prominences — the  rudiments  of  the 
two  maxillae  and  ist  maxillipeds  arise  behind  the  Nauplius  appendages 
(fig.  212).  At  the  same  time  an  anus  appears  between  the  two  limbs  of 
a  rudimentary  caudal  fork  ;  and  an  unpaired  eye  and  upper  lip  appear  in 
front.  After  another  moult  (fig.  212)  a  lower  lip  is  formed  (UL)  as  a 
pair  of  prominences  very  similar  to  true  appendages  ;  and  a  delicate 
cephalo-thoracic  shield  also  becomes  developed.  Still  later  the  cutting  blade 
of  the  mandible  is  formed,  and  the  palp  (Nauplius  appendage)  is  greatly 


FIG.  212.    NAUPLIUS  OF  EUPHAUSIA.     (From  Glaus;  after  Metschnikoff.) 
The  Nauplius  is   represented  shortly  before  an  ecdysis,  and  in  addition  to  the 

proper  appendages  rudiments  of  the  three  following  pairs  are  present. 

OL.  upper  lip  ;    UL.  lower  lip  ;   Md.  mandible ;   MX',  and  MX",  two  pairs  of 

maxillae ;  mf .  maxilliped  i . 

reduced.      The  cephalo-thoracic   shield  grows   over  the  front  part  of  the 
embryo,  and  becomes  characteristically  toothed  at  its  edge.     There  are  also 

30—2 


468  SCHIZOPODA. 


two  frontal  papillae  very  similar  to  those  already  described  in  the  Phyllopod 
larvae.  Rudiments  of  the  compound  eyes  make  their  appearance,  and 
though  no  new  appendages  are  added,  those  already  present  undergo  further 
differentiations.  They  remain  however  very  simple ;  the  maxillipeds 
especially  are  very  short  and  resemble  somewhat  Phyllopod  appendages. 

Up  to  this  stage  the  tail  has  remained  rudimentary  and  short,  but 
after  a  further  ecdysis  (Claus)  it  grows  greatly  in  length.  At  the  same 
time  the  cephalo-thoracic  shield  acquires  a  short  spine  directed  backwards. 
The  larva  is  now  in  a  condition  to  which  Claus  has  given  the  name  of 
Protozoasa  (fig.  213  A). 

Very  shortly  afterwards  the  region  immediately  following  the  segments 
already  formed  becomes  indistinctly  segmented,  while  the  tail  is  still  with- 
out a  trace  of  segmentation.  The  region  of  the  thorax  proper  soon  be- 
comes distinctly  divided  into  seven  very  short  segments,  while  at  the  same 
time  the  now  elongated  caudal  region  has  become  divided  into  its  normal 
number  of  segments  (fig.  213  B).  By  this  stage  the  larva  has  become 


FIG.  213.     LARVAE  OF  EUPHAUSIA.     (After  Claus.)     From  the  side. 

A.     Protozorea  larva.  B.     Zonea  larva. 

mx'.  and  tux",  maxillre  I  and  2  ;  mxp^.  maxilliped  r. 

a  true  Zoaea — though  differing  from  the  normal  Zoaea  in  the  fact  that 
the  thoracic  region  is  segmented,  and  in  the  absence  of  a  second  pair  of 
maxillipeds. 

The  adult  characters  are  very  gradually  acquired  in  a  series  of  suc- 
cessive moults  ;  the  later  development  of  Euphausia  resembling  in  this 
respect  that  of  the  Phyllopods.  On  the  other  hand  Euphausia  differs  from 
that  group  in  the  fact  that  the  abdominal  (caudal)  and  thoracic  appendages 
develop  as  two  independent  series  from  before  backwards,  of  which  the 
abdominal  series  is  the  earliest  to  attain  maturity. 


CRUSTACEA. 


469 


This  is  shewn  in  the  following  table  compiled  from  Claus'  observations. 


LENGTH  OF  LARVA. 

APPENDAGES   OF  THORACIC 
REGION  ;  viz.  the  2nd  and 
3rd  maxilliped  and  5  ambu 
latory  appendages. 

APPENDAGES  OF  ABDOMEN. 

3  —  3^  mm. 

2nd  maxilliped,  rudimentary. 

ist  abdominal  appendage. 

3£—  4  mm. 

•2nd  maxilliped,  biramous. 
3rd  rudimentary, 
ist  and  2nd   ambulatory   ap- 
pendages, rudimentary. 

2nd  and  3rd  abdominal  ap- 
pendages. 
4th  and  5th  rudimentary. 

4^  —  5  mm. 

3rd  maxilliped,  biramous. 

4th,  5th,  and  6th  fully  de- 
veloped. 

5—5^  mm. 

3rd  and   4th  ambulatory   ap- 
pendages. 

6  mm. 

5th  ambulatory  appendage. 

All  the  appendages  following  the  second  pair  of  maxillas  are  biramous, 
and  the  first  eight  of  these  bear  branched  gills  as  their  epipodites.  It  is 
remarkable  that  the  epipodite  is  developed  on  all  the  appendages  anteriorly 
in  point  of  time  to  the  outer  ramus  (exopodite). 

Although  in  Mysis  there  is  no  free  larval  stage,  and  the  development 
takes  place  in  a  maternal  incubatory  pouch,  yet  a  stage  may  be  detected 
which  clearly  corresponds  with  the  Nauplius  stage  of  Euphausia  (E.  van 
Beneden,  No.  465).  At  this  stage,  in  which  only  the  three  Nauplius 
appendages  are  developed,  the  Mysis  embryo  is  hatched.  An  ecdysis 
takes  place,  but  the  Nauplius  skin  is  not  completely  thrown  off,  and 
remains  as  an  envelope  surrounding  the  larva  during  its  later  develop- 
ment. 

Decapoda.  Amongst  the  Decapoda  the  larva  usually  leaves 
the  egg  in  the  Zoaea  form,  but  a  remarkable  exception  to  this 
general  rule  is  afforded  by  the  case  of  one  or  more  species  of 
Penseus.  Fritz  M  tiller  was  the  first  to  shew  that  the  larva  of 
these  forms  leaves  the  egg  as  a  typical  Nauplius,  and  it  is 
probable  that  in  the  successive  larval  stages  of  these  forms  the 
ancestral  history  of  the  Decapoda  is  most  fully  preserved1. 

The  youngest  known  larva  of  Penaeus  (fig.  214)  has  a  some- 
what oval  unsegmented  body.  There  spring  from  it  the  three 
typical  pairs  of  Nauplius  appendages.  The  first  is  uniramous, 
the  second  and  third  are  biramous,  and  both  of  them  adapted 


1  The  doubts  which  have  been  thrown  upon  Miiller's  observations  appear  to  be 
quite  unfounded. 


470  DECAPODA. 


for  swimming,  and  the  third  of  them  (mandibles)  is  without  a 
trace  of  the  future  blade.  The  body  has  no  carapace,  and  bears 
anteriorly  a  single  median  simple  eye.  Posteriorly  it  is  produced 
into  two  bristles. 

After  the  first  moult  the  larva  has  a  rudiment  of  a  forked 
tail,  while  a  dorsal  fold  of  skin  indicates  the  commencement  of 


FIG.  214.    NAUPLIUS  STAGE  OF  PEN^EUS.    (After  Fritz  Miiller.) 

the  cephalo-thoracic  shield.  A  large  provisional  helmet-shaped 
upper  lip  like  that  in  Phyllopods  has  also  appeared.  Behind 
the  appendages  already  formed  there  are  stump-like  rudiments 
of  the  four  succeeding  pairs  (two  pairs  of  maxillae  and  two  pairs 
of  maxillipeds) ;  and  in  a  slightly  older  larva  the  formation  of 
the  mandibular  blade  has  commenced,  together  with  the  atrophy 
of  the  palp  or  Nauplius  appendage. 

Between  this  and  the  next  observed  stage  there  is  possibly  a 
slight  lacuna.  The  next  stage  (fig.  215)  at  any  rate  represents 
the  commencement  of  the  Zoaea  series.  The  cephalo-thoracic 
shield  has  greatly  grown,  and  eventually  acquires  the  usual 
dorsal  spine.  The  posterior  region  of  the  body  is  prolonged 
into  a  tail,  which  is  quite  as  long  as  the  whole  of  the  remainder 
of  the  body.  The  four  appendages  which  were  quite  functionless 
at  the  last  stage  have  now  sprouted  into  full  activity.  The 


CRUSTACEA. 


471 


region  immediately  be- 
hind them  is  divided  (fig. 
215)  into  six  segments 
(the  six  thoracic  seg- 
ments) without  appen- 
dages, while  somewhat 
later  the  five  anterior 
abdominal  segments  be- 
come indicated,  but  are 
equally  with  the  thoracic 
segments  without  feet. 
The  mode  of  appearance 
of  these  segments  shews 
that  the  thoracic  and 
abdominal  segments  de- 
velop in  regular  succes- 
sion from  before  back- 
wards (Claus).  Of  the 
palp  of  the  mandibles, 
as  is  usual  amongst  Zosea 
forms,  not  a  trace  remains, 
though  in  the  youngest 
Zoaea  caught  by  Fritz 
Miiller  a  very  small  rudiment  of  the  palp  was  present.  The 
first  pair  of  antennae  is  unusually  long,  and  the  second  pair 
continues  to  function  as  a  biramous  swimming  organ ;  the 
outer  ramus  is  multiarticulate.  The  other  appendages  are  fully 
jointed,  and  the  two  maxillipeds  biramous.  On  the  dorsal 
surface  of  the  body  the  unpaired  eye  is  still  present,  but  on  each 
side  of  it  traces  of  the  stalked  eyes  have  appeared.  Frontal 
sense  organs  like  those  of  Phyllopods  are  also  present. 

From  the  Protozoaea  form  the  larva  passes  into  that  of  a  true 
Zoaea  with  the  usual  appendages  and  spines,  characterised  how- 
ever by  certain  remarkable  peculiarities.  Of  these  the  most 
important  are  (i)  the  large  size  of  the  two  pairs  of  antennae  and 
the  retention  of  its  Nauplius  function  by  the  second  of  them ; 
(2)  the  fact  that  the  appendages  of  the  six  thoracic  segments 
appear  as  small  biramous  Schizopod  legs,  while  the  abdominal 
appendages,  with  the  exception  of  the  sixth,  are  still  without 


FIG. 


215. 


PROTOZO^EA  STAGE  OF  PEN/EUS. 
(After  Fritz  Miiller.) 


472  DECAPODA. 


their  swimming  feet.  The  early  appearance  of  the  appendages 
of  the  sixth  abdominal  segment  is  probably  correlated  with 
their  natatory  function  in  connection  with  the  tail.  As  a  point 
of  smaller  importance  which  may  be  mentioned  is  the  fact  that 
both  pairs  of  maxillae  are  provided  with  small  respiratory  plates 
(exopodites)  for  regulating  the  flow  of  water  under  the  dorsal 
shield.  From  the  Zoaea  form  the  larva  passes  into  a  Mysis  or 
Schizopod  stage  (fig.  216),  characterised  by  the  thoracic  feet  and 
maxillipeds  resembling  in  form  and  function  the  biramous  feet 
of  Mysis,  the  outer  ramus  being  at  first  in  many  cases  much 
larger  than  the  inner.  The  gill  pouches  appear  at  the  base  of 
these  feet  nearly  at  the  same  time  as  the  endopodites  become 
functional.  At  the  same  time  the  antennae  become  profoundly 
modified.  The  anterior  antennae  shed  their  long  hairs,  and  from 
the  inner  side  of  the  fourth  joint  there  springs  a  new  process, 


FIG.  216.    PEN^EUS  LARVA  IN  THE  MYSIS  STAGE.    (After  Claus.) 

which  eventually  elongates  and  becomes  the  inner  flagellum. 
The  outer  ramus  of  the  posterior  antennae  is  reduced  to  a  scale, 
while  the  flagellum  is  developed  from  a  stump-like  rudiment  of 
the  inner  ramus  (Claus).  A  palp  sprouts  on  the  mandible  and 
the  median  eye  disappears. 

The  abdominal  feet  do  not  appear  till  the  commencement  of 
the  Mysis  stage,  and  hardly  become  functional  till  its  close. 

From  the  Mysis  stage  the  larva  passes  quite  simply  into  the 
adult  form.  The  outer  ramus  of  the  thoracic  feet  is  more  or  less 
completely  lost.  The  maxillipeds,  or  the  two  anterior  pairs  at 
any  rate,  lose  their  ambulatory  function,  cutting  plates  develop 
on  the  inner  side  of  their  basal  joints,  and  the  two  rami  persist 


CRUSTACEA. 


473 


as  small  appendages  on   their   outer   side.     Gill   pouches   also 
sprout  from  their  outer  side. 

The  respiratory  plate  of  the  second  maxilla  attains  its  full 
development  and  that  on  the  first  maxilla  disappears1.  The 
Nauplius,  so  far  as  is  known,  does  not  occur  in  any  other 
Decapod  form  except  Penaeus. 


The  next  most  primitive 
larval  history  known  is 
that  which  appears  in  the 
Sergestidae.  The  larval 
history,  which  has  been 
fully  elucidated  by  Claus, 
commences  with  a  Proto- 
zoaea  form  (fig.  217),  which 
develops  into  a  remarkable 
Zoaea  first  described  by 
Dohrn  as  Elaphocaris. 
This  develops  into  a  form 
originally  described  by 
Claus  as  Acanthosoma, 
and  this  into  a  form  known 
as  Mastigopus  (fig.  218) 
from  which  it  is  easy  to 
pass  to  the  adult. 

The  remarkable  Proto- 
zoaea  (fig.  217)  is  charac- 
terised by  the  presence  on 
the  dorsal  shield  of  a  fron- 
tal, dorsal  and  two  lateral 
spikes,  each  richly  armed 
with  long  side  spines.  The 


FIG.  217.    LATEST  PROTOZO^A  STAGE  OF  SEK- 

GESTES  LARVA  (ELAPHOCARIS).      (After  Claus.) 


mxp'" '.  third  pair  of  maxillipeds. 

normal  Zoasa  appendages  are  present,  and  in  addition  to  them  a  small  third 
pair  of  maxillipeds.  The  thoracic  region  is  divided  into  five  short  rings,  but 
the  abdomen  is  unsegmented.  The  tail  is  forked  and  provided  with  long 
spines.  The  antennae,  like  those  of  Penasus,  are  long — the  second  pair 
biramous  ;  the  mandibles  unpalped.  Both  pairs  of  maxillae  are  provided 
with  respiratory  plates  ;  the  second  pair  is  footlike,  and  has  at  its  base  a 
glandular  mass  believed  by  Claus  to  be  the  equivalent  of  the  Entomostracan 
shell-gland.  The  maxillipeds  have  the  usual  biramous  characters.  A 


1  From  Claus'  observations  (No.  448)  it  would  appear  that  the  respiratory  plate 
is  only  the  exopodite  and  not,  as  is  usually  stated,  the  coalesced  exopodite  and 
epipodite.  Huxley  in  his  Comparative  Anatomy  reserves  this  point  for  embryological 
elucidation. 


474 


DECAPOD  A. 


FIG.  218.    MASTIGOPUS  STAGE 
OF  SERGESTES.    (From  Claus.) 
Mf".  maxilliped  3. 

helmet-shaped  upper  lip  like 
that  of  a  typical  Nauplius  is 
present,  and  the  eyes  are  situa- 
ted on  very  long  stalks. 

In  the  true  Zoaea  stage  there 
appear    on    the    five    thoracic 

segments  pouch-like  biramous  rudiments  of  the  limbs.  The 
tail  becomes  segmented;  but  the  segments,  with  the 
exception  of  the  sixth,  remain  without  appendages.  On 
the  sixth  a  very  long  bilobed  pouch  appears  as  the  com- 
mencement of  the  swimming  feet  of  this  segment.  The 
segments  of  the  abdomen  are  armed  with  lateral  spines. 

From  the  Zoaea  stage  the  larva  passes  into  the  form 
known  as  Acanthosoma,  which  represents  the  Mysis  stage 
of  Penaeus.  The  complex  spikes  on  the  dorsal  shield  of 
the  Zoaea  stage  are  reduced  to  simple  spines,  but  the 
spines  of  the  tail  still  retain  their  full  size.  In  the  appendages  the  chief 
changes  consist  (i)  in  the  reduction  of  the  jointed  outer  ramus  of  the 
second  pair  of  antennae  to  a  stump  representing  the  scale,  and  the  elon- 
gation of  the  inner  one  to  the  flagellum  ;  (2)  in  the  elongation  of  the  five 
ambulatory  thoracic  appendages  into  biramous  feet,  like  the  maxillipeds, 
and  in  the  sprouting  forth  of  rudimentary  abdominal  feet. 


CRUSTACEA. 


475 


The  most  obvious  external  indica- 
tions of  the  passage  from  the  Acantho- 
soma  to  the  Mastigopus  stage  (fig.  218) 
are  to  be  found  in  the  elongation  of  the 
abdomen,  the  reduction  and  flattening 
of  the  cephalo-thoracic  shield,  and  the 
nearly  complete  obliteration  of  all  the 
spines  but  the  anterior.  The  eyes  on 
their  elongated  stalks  are  still  very 
characteristic,  and  the  elongation  of 
the  flagellum  of  the  second  pair  of 
antennae  is  very  striking. 

The  maxillae  and  maxillipeds  un- 
dergo considerable  metamorphosis,  the 
abdominal  feet  attain  their  adult  form, 
and  the  three  anterior  thoracic  ambu- 
latory legs  lose  their  outer  rami.  The 
most  remarkable  change  of  all  concerns 
the  two  last  pairs  of  thoracic  appen- 
dages, which,  instead  of  being  meta- 
morphosed like  the  preceding  ones,  are 
completely  or  nearly  completely  thrown 
off  in  the  moult  which  inaugurates  the 
Mastigopus  stage,  and  are  subsequently 
redeveloped.  With  the  reappearance 
of  these  appendages,  and  the  changes 
in  the  other  appendages  already  indi- 
cated, the  adult  form  is  practically 
attained. 


FIG.  219.    LARVA  OF  HIPPOLYTE 
IN  ZO/EA  STAGE.     (From  Claus.) 

MX',  and  MX",  maxillae  i  and   2 ; 
Mf.  Mf.  Mf".  maxillipeds. 


FIG.  220. 


OLDER  LARVA  OF  HIPPOLYTE  AFTER  THE  THORACIC  APPENDAGES  HAVE 
BECOME  FORMED.     (From  Claus.) 


476  DECAPODA. 


With  reference  to  the  development  of  the  majority  of  the 
Carabidae,  Penaeinae,  Palaemoninae,  Crangoninae,  it  may  be  stated 
generally  that  they  leave  the  egg  in  the  Zoaea  stage  (fig.  219) 
with  anterior  appendages  up  to  the  third  pair  of  maxillipeds. 
The  thorax  is  unsegmented  and  indeed  almost  unrepresented, 
but  the  abdomen  is  long  and  divided  into  distinct  segments. 
Both  thoracic  and  abdominal  appendages  are  absent,  and  the 
tail  is  formed  by  a  simple  plate  with  numerous  bristles,  not 
forked,  as  in  the  case  of  the  Zoaea  of  Fritz  M tiller's  Penaeus  and 
Sergestes.  A  dorsal  spine  is  frequently  found  on  the  second 
abdominal  segment.  From  the  Zoaea  form  the  embryo  passes 
into  a  Mysis  stage  (fig.  220),  during  which  the  thoracic  ap- 
pendages gradually  appear  as  biramous  swimming  feet;  they 


FIG.  221.  NEWLY-HATCHED  LARVA  OF  THE  AMERICAN  LOBSTER.  (After  Smith.) 
are  all  developed  before  any  of  the  abdominal  appendages, 
except  the  last.  In  some  cases  the  development  is  still  further 
abbreviated.  Thus  the  larvae  of  Crangon  and  Palaemonetes 
(Faxon,  No.  476)  possess  at  hatching  the  rudiments  of  the  two 
anterior  pairs  of  thoracic  feet,  and  Palaemon  of  three  pairs'. 

Amongst  the  other  Macrura  the  larva  generally  leaves  the 
egg  as  a  Zoaea  similar  to  that  of  the  prawns.     In  the  case  of  the 

1  Fritz  Miiller  has  recently  (Zoologisrher  Anzeiger^  No.  52)  described  a  still  more 
abbreviated  development  of  a  Pala-mon  living  in  brooks  near  Blumenau. 


CRUSTACEA.  477 


Thalassinidae  and  Paguridae  a  Mysis  stage  has  disappeared. 
The  most  remarkable  abbreviations  of  the  typical  development 
are  presented  on  the  one  hand  by  Homarus  and  Astacus,  and  on 
the  other  by  the  Loricata. 

The  development  of  Homarus  has  been  fully  worked  out  by  S.  J.  Smith 
(No.  491)  for  the  American  lobster  (Homarus  americanus).  The  larva  (fig. 
221)  leaves  the  egg  in  an  advanced  Mysis  stage.  The  cephalo-thoracic 
shield  is  fully  developed,  and  armed  with  a  rostrum  in  front.  The  first  pair 
of  antennae  is  unjointed  but  the  second  is  biramous,  the  outer  ramus  forming 
a  large  Mysis-like  scale.  The  mandibles,  which  are  palped,  the  maxillae, 
and  the  two  anterior  maxillipeds  differ  only  in  minor  details  from  the  same 
appendages  of  the  adult.  The  third  pair  of  maxillipeds  is  Mysis-like  and 
biramous,  and  the  five  ambulatory  legs  closely  resemble  them,  the  endopo- 
dite  of  the  first  being  imperfectly  chelate.  The  abdomen  is  well  developed 
but  without  appendages.  The  second,  third,  fourth  and  fifth  segments  are 
armed  with  dorsal  and  lateral  spines. 

In  the  next  stage  swimming  feet  have  appeared  on  the  second,  third, 
fourth  and  fifth  abdominal  segments,  and  the  appendages  already  present 
have  approached  their  adult  form.  Still  later,  when  the  larva  is  about  half 
an  inch  in  length,  the  approach  to  the  adult  form  is  more  marked,  and  the 
exopodites  of  the  ambulatory  legs  though  present  are  relatively  much 
reduced  in  size.  The  swimmerets  of  the  sixth  abdominal  segment  are 
formed.  In  the  next  stage  observed  the  larva  has  entirely  lost  its  Schizopod 
characters,  and  though  still  retaining  its  free  swimming  habits  differs  from 
the  adult  form  only  in  generic  characters. 

As  has  been  already  stated,  no  free  larval  stages  occur  in  the  develop- 
ment of  Astacus,  but  the  young  is  hatched  in  a  form  in  which  it  differs  only 
in  unimportant  details  from  the  adult. 

The  peculiar  larval  form  of  the  Loricata  (Scyllarus,  Palinurus)  has  long 
been  known  under  the  name  Phyllosoma  (fig.  222  C),  but  its  true  nature  was 
first  shewn  by  Couch  (No.  474)  [Couch  did  not  however  recognise  the 
identity  of  his  larva  with  Phyllosoma  ;  this  was  first  done  by  Gerstacker] 
and  shortly  afterwards  by  Gerbe  and  Coste.  These  observations  were 
however  for  a  long  time  not  generally  accepted,  till  Dohrn  (No.  477) 
published  his  valuable  memoir  giving  an  account  of  how  he  succeeded  in 
actually  rearing  Phyllosoma  from  the  eggs  of  Scyllarus  and  Palinurus,  and 
shewing  that  some  of  the  most  remarkable  features  of  the  metamorphosis  of 
the  Loricata  occur  before  the  larva  is  hatched. 

The  embryo  of  Scyllarus  in  the  egg  first  of  all  passes  through  the  usual 
Nauplius  stage,  and  then  after  the  formation  of  a  cuticle  develops  an 
elongated  thoracico-abdominal  region  bent  completely  over  the  anterior 
part  of  the  body.  There  appear  moreover  a  number  of  appendages  and  the 
rudiments  of  various  organs  ;  and  the  embryo  passes  into  a  form  which  may 
be  described  as  the  embryonic  Phyllosoma  stage.  In  this  stage  there  are 
present  on  the  anterior  part  of  the  body,  in  front  of  the  ventral  flexure,  two 


478  DECAPODA. 


pairs  of  antennae,  mandibles,  two  pairs  of  maxillae,  the  second  commencing 
to  be  biramous,  and  a  small  stump  representing  the  first  pair  of  maxillipeds. 
The  part  of  the  body  bent  over  consists  of  a  small  quadrate  caudal  plate, 
and  an  appendage-bearing  region  to  which  are  attached  anteriorly  three 
pairs  of  biramous  appendages— the  second  and  third  maxillipeds,  and  the 
anterior  pair  of  ambulatory  legs — and  two  pairs  of  undivided  appendages — 
the  second  and  third  pairs  of  ambulatory  legs.  In  a  slightly  later  stage  the 
first  pair  of  maxillae  becomes  biramous,  as  also  does  the  first  pair  of  maxilli- 
peds in  a  very  rudimentary  fashion.  The  second  and  third  pairs  of  ambu- 
latory legs  become  biramous,  while  the  second  and  third  maxilliped  nearly 
completely  lose  their  outer  ramus.  Very  small  rudiments  of  the  two  hinder 
ambulatory  legs  become  formed.  If  the  embryo  is  taken  at  this  stage  (vide 
fig.  222  A,  which  represents  a  nearly  similar  larva  of  Palinurus)  out  of  the 
egg,  it  is  seen  to  consist  of  (i)  an  anterior  enlargement  with  a  vaulted  dorsal 
shield  enclosing  the  yolk,  two  stalked  eyes,  and  a  median  eye  ;  (2)  a  thoracic 
region  in  which  the  indications  of  segmentation  are  visible  with  the  two 


FIG.  222.    LARWE  OF  THE  LORICATA.     (After  Claus.) 

A.  Embryo  of  Palinurus  shortly  before  hatching. 

B.  Young  Phyllosoma  larva  of  Scyllarus,  without  the  first  maxilliped,  the  two 
last  thoracic  appendages,  or  the  abdominal  appendages. 

C.  Fully-grown  Phyllosoma  with  all  the  Decapod  appendages. 

at*,  antenna  of  first  pair  ;  at*,  antenna  of  second  pair ;  md.  mandible ;  ntx1.  first 
maxilla;  mx1.  second  maxilla;  mx^—mxf.  maxillipeds;  /1— /3.  thoracic 
appendages. 

posterior  pairs  of  maxillipeds  (mxfp  and  wr/3)  and  the  ambulatory  legs  (/l); 
(3)  an  abdominal  region  distinctly  divided  into  segments  and  ending  in  a  fork. 
Before  the  embryo  becomes  hatched  the  first  pair  of  maxillipeds  becomes 
reduced  in  size  and  finally  vanishes.  The  second  pair  of  maxillae  becomes 
reduced  to  simple  stumps  with  a  few  bristles,  the  second  pair  of  antennae 


CRUSTACEA.  479 


also  appears  to  undergo  a  retrogressive  change,  while  the  two  last  thoracic 
segments  cease  to  be  distinguishable.  It  thus  appears  that  during  embryonic 
life  the  second  pair  of  antennae,  the  second  pair  of  maxillae,  and  the  second 
and  third  pair  of  maxillipeds  and  the  two  hinder  ambulatory  appendages 
undergo  retrogressive  changes,  while  the  first  pair  of  maxillipeds  is  completely 
obliterated ! 

The  general  form  of  the  larva  when  hatched  (fig.  222  B)  is  not  very 
different  from  that  which  it  has  during  the  later  stages  within  the  egg.  The 
body  is  divided  into  three  regions:  (i)  an  anterior  cephalic;  (2)  a  middle 
thoracic,  and  (3)  a  small  posterior  abdominal  portion  ;  and  all  of  them  are 
characterised  by  their  extreme  dorso-ventral  compression,  so  that  the  whole 
animal  has  the  form  of  a  three-lobed  disc,  the  strange  appearance  of  which 
is  much  increased  by  its  glass-like  transparency. 

The  cephalic  portion  is  oval  and  projects  slightly  behind  so  as  to  overlap 
the  thorax.  Its  upper  surface  constitutes  the  dorsal  shield,  from  which  there 
spring  anteriorly  the  two  compound  eyes  on  long  stalks,  between  which  is  a 
median  Nauplius  eye.  The  mouth  is  situated  about  the  middle  of  the  under 
surface  of  the  anterior  disc.  It  leads  into  a  stomach  from  which  an  anterior 
and  a  lateral  hepatic  diverticulum  springs  out  on  each  side.  The  former 
remains  as  a  simple  diverticulum  through  larval  life,  but  the  latter  becomes 
an  extremely  complicated  glandular  structure. 

At  the  front  border  of  the  disc  is  placed  the  unjointed  but  elongated 
first  pair  of  antennae  (rt/1).  Externally  to  and  behind  these  there  spring  the 
short  posterior  antennae  (at'*}.  At  the  base  of  which  the  green  gland  is 
already  formed.  Surrounding  the  mouth  are  the  mandibles  (md)  and  anterior 
pair  of  maxillae  (mx1),  and  some  distance  behind  the  second  pair  of  maxillae 
(mx*),  consisting  of  a  cylindrical  basal  joint  and  short  terminal  joint  armed 
with  bristles.  The  first  pair  of  maxillipeds  is  absent. 

The  thoracic  region  is  formed  of  an  oval  segmented  disc  attached  to  the 
under  surface  of  the  cephalic  disc.  From  its  front  segment  arises  the  second 
pair  of  maxillipeds  (inxpl£}  as  single  five-jointed  appendages,  and  from  the 
next  segment  springs  the  five-jointed  elongated  but  uniramous  third  pair 
of  maxillipeds  (mxfl3},  and  behind  this  there  arise  three  pairs  of  six-jointed 
ambulatory  appendages  (p\  /2,  p3,  of  which  only  the  basal  joint  is  represented 
in  the  figure)  with  an  exopodite  springing  from  their  second  joint.  The  two 
posterior  thoracic  rings  and  their  appendages  cannot  be  made  out. 

The  abdomen  is  reduced  to  a  short  imperfectly  segmented  stump,  ending 
in  a  fork,  between  the  prongs  of  which  the  anus  opens.  Even  the  youngest 
larval  Phyllosoma,  such  as  has  just  been  described,  cannot  be  compared  with  a 
Zoaea,  but  belongs  rather,  in  the  possession  of  biramous  thoracic  feet,  to  a 
Mysis  stage.  In  the  forked  tail  and  Nauplius  eye  there  appear  however  to 
be  certain  very  primitive  characters  carried  on  to  this  stage. 

The  passage  of  this  young  larva  to  the  fully  formed  Phyllosoma  (fig. 
222  C)  is  very  simple.  It  consists  essentially  in  the  fresh  development  of 
the  first  pair  of  maxillipeds  and  the  two  last  ambulatory  appendages,  the 
growth  and  segmentation  of  the  abdomen,  and  the  sprouting  on  it  of  biramous 


480  DECAPODA. 


swimming  feet.  In  the  course  of  these  changes  the  larva  becomes  a  true 
Decapod  in  the  arrangement  and  number  of  its  appendages  ;  and  indeed  it 
was  united  with  this  group  before  its  larval  character  was  made  out.  In 
addition  to  the  appearance  of  new  appendages  certain  changes  take  place  in 
those  already  present.  The  two  posterior  maxillipeds,  in  the  Palinurus 
Phyllosoma  at  any  rate,  acquire  again  an  exopodite,  and  together  with  the 
biramous  ambulatory  feet  develop  epipodites  in  the  form  of  gill  pouches. 

The  mode  of  passage  of  the  Phyllosoma  to  the  adult  is  not  known,  but 
it  can  easily  be  seen  from  the  oldest  Phyllosoma  forms  that  the  dorsal 
cephalic  plate  grows  over  the  thorax,  and  gives  rise  to  the  cephalo-thoracic 
shield  of  the  adult. 

There  are  slight  structural  differences,  especially  in  the  antennae,  between 
the  Phyllosoma  of  Scyllarus  and  that  of  Palinurus,  but  the  chief  difference 
in  development  is  that  the  first  pair  of  maxillipeds  of  the  Palinurus  embryo, 
though  reduced  in  the  embryonic  state,  does  not  completely  vanish,  at  any 
rate  till  after  the  free  larval  state  has  commenced  ;  and  it  is  doubtful  if 
it  does  so  even  then.  The  freshly  hatched  Palinurus  Phyllosoma  is  very 
considerably  more  developed  than  that  of  Scyllarus. 

Brachyura.  All  the  Brachyura,  with  the  exception  of  one  or 
more  species  of  land  crabs1,  leave  the  egg  in  the  Zoaia  condition, 
and  though  there  are  slight  variations  of  structure,  yet  on  the 


FIG.  223.    THE  APPENDAGES  OF  A  CRAB  ZOJEA. 

At.  I.  first  antenna  ;  At.  //.  second  antenna ;  md.  mandible  (without  a  palp) ;  mx. 
i.  first  maxilla  ;  mx.  i.  second  maxilla  ;  w.r.  3.  third  maxilla  ;  mxp.  i.  first  maxilliped  ; 
mxp.  i.  second  maxilliped. 

ex.  exopodite  ;  ett.  cndopodite. 

whole  the  Crab  Zoaea  is  a  very  well  marked  form.     Immediately 
after  leaving  the  egg  (fig.  210)  it  has  a  somewhat  oval  shape 

1  It  has  been  clearly  demonstrated  that  the  majority  of  land-crabs  leave  the  egg  in 
the  7.oxa.  form. 


CRUSTACEA.  481 


with  a  long  distinctly-segmented  abdomen  bent  underneath  the 
thorax.  The  cephalo-thoracic  shield  covers  over  the  front  part 
of  the  body,  and  is  prolonged  into  a  long  frontal  spine  pointing 
forwards,  and  springing  from  the  region  between  the  two  eyes ; 
a  long  dorsal  spine  pointing  backwards ;  and  two  lateral  spines. 

To  the  under  surface  of  the  body  are  attached  the  anterior 
appendages  up  to  the  second  maxilliped,  while  the  six  following 
pairs  of  thoracic  appendages  are  either  absent  or  represented 
only  in  a  very  rudimentary  form.  The  abdomen  is  without 
appendages. 

The  anterior  antennae  are  single  and  unjointed,  but  provided 
at  their  extremity  with  a  few  olfactory  hairs  (only  two  in 
Carcinus  Mcenas)  and  one  or  two  bristles.  The  rudiment  of  the 
secondary  flageltum  appears  in  very  young  Zoaeae  on  the  inner 
side  of  the  antennules  (fig.  223  At.  /.).  The  posterior  antennae 
are  without  the  flagellum,  but  are  provided  with  a  scale  repre- 
senting the  exopodite  (fig.  223  At.  II.  ex]  and  usually  a  spinous 


FIG.  224.     CRAB  ZO^EA  AFTER  TH..  THIRD  PAIR  OF  MAXILLIPEDS  AND  THE 

THORACIC  AND  ABDOMINAL  APPENDAGES   HAVE  BECOME  DEVELOPED. 

at1,  antenna  of  first  pair ;  atz.  antenna  of  second  pair ;  mxl.  first  maxilla ;  mop. 
second  maxilla ;  mxp1.  first  maxilliped  ;  mxjP.  second  maxilliped ;  mxf.  third  max- 
illiped ;  oc.  eye ;  ht.  heart. 

process.  The  flagellum  is  very  early  developed  and  is  repre- 
sented in  fig.  223,  At.  II.  en.  The  mandibles  (md)  are  large  but 
without  a  palp.  The  anterior  maxillae  (mx  i)  have  a  short  two- 
jointed  endopodite  (palp)  with  a  few  hairs,  and  a  basal  portion 
B.  II.  31 


--• 


482  DECAPODA. 


with  two  blades,  of  which  the  distal  is  the  largest,  both  armed 
with  stiff  bristles.  The  posterior  maxillae  have  a  small  respira- 
tory plate  (exopodite),  an  endopodite  (palp)  shaped  like  a 
double  blade,  and  two  basal  joints  each  continued  into  a  double 
blade.  The  two  maxillipeds  (inxp  i  and  mxp  2)  have  the  form 
and  function  of  biramous  swimming  feet.  The  exopodite  of 
both  is  two-jointed  and  bears  long  bristles  at  its  extremity ;  the 
endopodite  of  the  anterior  is  five-jointed  and  long,  that  of  the 
second  is  three-jointed  and  comparatively  short. 

In  the  six-jointed  tail  the  second  segment  has  usually  two 
dorsally  directed  spines,  and  the  three  succeeding  segments  each 
of  them  two  posteriorly  directed.  The  telson  or  swimming  plate 
is  not  at  first  separated  from  the  sixth  segment ;  on  each  side  it 
is  prolonged  into  two  well-marked  prongs  ;  and  to  each  prong 
three  bristles  are  usually  attached  (fig.  224).  The  heart  (fig. 
224  ht)  lies  under  the  dorsal  spine  and  is  prolonged  into  an 
anterior,  posterior,  and  dorsal  aorta.  It  has  only  two  pairs  of 
venous  ostia. 

During  the  Zoaea  stage  the  larva  rapidly  grows  in  size,  and 
undergoes  considerable  changes  in  its  appendages  which  reach 
the  full  Decapod  number  (fig.  224).  On  both  pairs  of  antennae 
a  flagellum  becomes  developed  and  grows  considerably  in  length. 
Before  the  close  of  the  Zoaea  condition  a  small  and  unjointed 
palp  appears  on  the  mandible.  Behind  the  second  maxilliped 
the  third  maxilliped  (inxp*}  early  appears  as  a  small  biramous 
appendage,  and  the  five  ambulatory  feet  become  distinctly 
formed  as  uniramous  appendages — the  exopodites  not  being 
present.  The  third  pair  of  maxillipeds  and  three  following 
ambulatory  appendages  develop  gill  pouches.  The  abdominal 
feet  are  formed  on  the  second  to  the  sixth  segments  of  the  tail 
as  simple  pouches. 

The  oldest  Zoaea  is  transmuted  at  its  moult  into  a  form 
known  as  Megalopa,  which  is  really  almost  identical  with  an 
anomurous  Decapod.  No  Schizopod  stage  is  intercalated,  which 
shews  that  the  development  is  in  many  respects  greatly  abbrevi- 
ated. The  essential  characters  of  the  Megalopa  are  to  be  found 
in  (i)  the  reduction  of  the  two  anterior  maxillipeds,  which 
cease  to  function  as  swimming  feet,  and  together  with  the 
appendages  in  front  of  them  assume  the  adult  form  ;  (2)  the  full 


CRUSTACEA.  483 


functional  development  of  the  five  ambulatory  appendages  ; 
(3)  the  reduction  of  the  forked  telson  to  an  oval  swimming 
plate,  and  the  growth  in  size  of  the  abdominal  feet,  which 
become  large  swimming  plates  and  are  at  the  same  time 
provided  with  short  endopodites  which  serve  to  lock  the  feet  of 
the  two  sides. 

With  these  essential  characters  the  form  of  the  Megalopa  differs  con- 
siderably in  different  cases.  In  some  instances  (e.g.  Carcinus  mcenas)  the 
Zoasa  spines  of  the  youngest  Megalopa  are  so  large  that  the  larva  appears 
almost  more  like  a  Zoasa  than  a  Megalopa  (Spence  Bate,  No.  470).  In  other 
cases,  e.g.  that  represented  on  fig.  225,  the  Zoasa  spines  are  still  present  but 
much  reduced;  and  the  cephalo-thoracic  shield  has  very  much  the  adult 
form.  In  other  cases  again  (e.g.  Portunus)  the  Zoasa  spines  are  completely 
thrown  off  at  the  youngest  Megalopa  stage. 

There  is  a  gradual  passage  from  the  youngest  Megalopa  to 
the  adult  form  by  a  series  of  moults. 

Some  of  the  brachyu- 
rous  Zoasa  forms  exhibit 
considerable  divergences 
from  the  described  type, 
more  espcially  in  the  ar- 
mature of  the  shield.  In 
some  forms  the  spines  are 
altogether  absent,  e.g.  Maja 
(Couch,  No.  474)  and  Eu- 
rynome.  In  other  forms 
the  frontal  spine  may  be 
much  reduced  or  absent 
(Inachus  and  Achasus). 
The  dorsal  spine  may  also 
be  absent,  and  in  one  form 
described  by  Dohrn  (No. 
478)  there  is  a  long  frontal 
spine  and  two  pairs  of 
lateral  spines,  but  no  dorsal  ^  MEGALOPA  STAGE  OF  CRAB  LARVA. 

spine.      Both    dorsal  and 

frontal  spines  may  attain  enormous  dimensions  and  be  swollen  at  their  extre- 
mities (Dohrn).  A  form  has  been  described  by  Claus  as  Pterocaris  in  which 
the  cephalo-thoracic  shield  is  laterally  expanded  into  two  wing-like  processes. 

The  Zoasa  of  Porcellana  presents  on  the  whole  the  most  remarkable 
peculiarities  and,  as  might  be  anticipated  from  the  systematic  position  of  the 
adult,  is  in  some  respects  intermediate  between  the  macrurous  Zoasa  and  that 
of  the  Brachyura.  It  is  characterized  by  the  oval  form  of  the  body,  and  by 

31-2 


484  STOMATOPODA. 


the  presence  of  one  enormously  long  frontal  spine  and  two  posterior  spines. 
The  usual  dorsal  spine  is  absent.  The  tail  plate  is  rounded  and  has  the 
character  of  the  tail  of  a  macrurous  Zoaea,  but  in  the  young  Zoasa  the  third 
pair  of  maxillipeds  is  absent  and  the  appendages  generally  have  a  brachyu- 
rous  character.  A  Megalopa  stage  is  hardly  represented,  since  the  adult 
may  almost  be  regarded  as  a  permanent  Megalopa. 

Stomatopoda.  The  history  of  the  larval  forms  of  the  Stomatopoda 
(Squilla  etc.)  has  not  unfortunately  been  thoroughly  worked  out,  but  what  is 
known  from  the  researches  of  Fritz  Miiller  (No.  495)  and  Claus  (No.  494)  is 
of  very  great  importance.  There  are  it  appears  two  types,  both  of  which 
used  to  be  described  as  adult  forms  under  the  respective  names  Erichthus 
and  Alima. 

The  youngest  known  Erichthus  form  is  about  two  millimetres  in  length, 
and  has  the  characters  of  a  modified  Zoaea  (fig.  226).  The  body  is  divided 
into  three  regions,  an  anterior  unsegmented  region  to  which  are  attached 
two  pairs  of  antennas,  mandibles,  and  maxillae  (two  pairs).  This  portion 
has  a  dorsal  shield  covering  the  next  or  middle  region,  which  consists  of 
five  segments  each  with  a  pair  of  biramous  appendages.  These  appendages 
represent  the  five  maxillipeds  of  the  adult1.  The  portion  of  the  body 
behind  this  is  without  appendages.  It  consists  of  three  short  anterior 
segments,— the  three  posterior  thoracic  segments  of  the  adult, — and  a  long 
unsegmented  tail.  The  three  footless  thoracic  segments  are  covered  by  the 
dorsal  shield.  Both  pairs  of  antennae  are  uniramous  and  comparatively 
short.  The  mandibles,  like  those  of  Phyllopods,  are  without  palps,  and  the 
two  following  pairs  of  maxillae  are  small.  The  five  maxillipeds  have  the 
characters  of  normal  biramous  Zoaea  feet.  From  the  front  of  the  head 
spring  a  pair  of  compound  eyes  with  short  stalks,  which  grow  longer 
in  the  succeeding  stages  ;  between  them  is  a  median  eye.  The  dorsal 
shield  is  attached  just  behind  this  eye,  and  is  provided,  as  in  the  typical 
Zoaea,  with  a  frontal  spike — while  its  hinder  border  is  produced  into  two 
lateral  spikes  and  one  median.  In  a  larva  of  about  three  millimetres  a  pair 
of  biramous  appendages  arises  behind  the  three  footless  thoracic  segments. 
It  is  the  anterior  pair  of  ab- 
dominal feet  (fig.  226).  The 
inner  ramus  of  the  second  pair 
of  maxillipeds  soon  grows 
greatly  in  length,  indicating 
its  subsequent  larger  size  and 
prehensile  form  (fig.  227  g). 
When  the  larva  after  one  or 

two  moults  attains  a  length  FlG-  ™6-  SECOND  STAGE  OF  ERICHTHUS 
of  six  millimetres  Cfitr  227 1  LARVA  OFSQUII.LA  WITH  FIVE  MAXILLIPEDS  AND 

(tig.    227)        THE    FIRST   PAIR    OF   ABDOMINAL    APPENDAGES. 

the  abdomen  has  six  segments     (From  Claus.) 

1  These  five  maxillipeds  correspond  with  the  three  maxillipeds  and  two  anterior 
ambulatory  appendages  of  the  Decapoda. 


CRUSTACEA. 


485 


(the  sixth  hardly  differentiated),  each  with  a  pair  of  appendages  (the  two 
hindermost  still  rudimentary)  which  have  become  gradually  developed  from 
before  backwards.  The  three  hindermost  thoracic  segments  are  still  without 
appendages. 

Some  changes  of  importance  have  occurred  in  the  other  parts.     Both 
antennas  have  acquired  a  second  flagellum,  but  the  mandible  is  still  without 


FIG.  227.     ADVANCED  ERICHTHUS  LARVA  OF  SQUILLA  WITH  FIVE  PAIRS  OF 

ABDOMINAL  APPENDAGES.      (From  Claus.) 

f.  first  maxilliped  ;  g.  second  maxilliped. 

a  palp.  The  first  and  second  pair  of  maxillipeds  have  both  undergone 
important  modifications.  Their  outer  ramus  (exopodite)  has  been  thrown 
off,  and  a  gill-plate  (epipodite)  has  appeared  as  an  outgrowth  from  their 
basal  joint.  Each  of  them  is  composed  of  six  joints.  The  three  following 
biramous  appendages  have  retained  their  earlier  characters  but  have  become 
much  reduced  in  size.  In  the  subsequent  moults  the  most  remarkable  new 
features  concern  the  three  posterior  maxillipeds,  which  undergo  atrophy,  and 
are  either  completely  lost  or  reduced  to  mere  unjointed  sacks  (fig.  228).  In 


FIG.  228.  ADVANCED  ERICHTHUS  LARVA  OF  SQUILLA  WHEN  THE  THREE 
POSTERIOR  MAXILLIPEDS  HAVE  BECOME  REDUCED  TO  MINUTE  POUCHES. 

(From  Claus.) 

the  stage  where  the  complete  Erichthus  type  has  been  reached,  these 
three  appendages  have  again  sprouted  forth  in  their  permanent  form  and 
each  of  them  is  provided  with  a  gill-sack  on  its  coxal  joint.  Behind  them 
the  three  ambulatory  appendages  of  the  thorax  have  also  appeared,  first 
as  simple  buds,  which  subsequently  however  become  biramous.  On  their 
development  the  full  number  of  adult  appendages  is  acquired. 

The  most  noteworthy  points  in  the  developmental  history  detailed  above 
are  the  following : 

(i)  The  thoracic  and  abdominal  segments  (apart  from  their  appendages) 
develop  successively  from  before  backwards. 


486  STOMATOPODA. 


(2)  The  three  last  maxillipeds  develop  before  the  abdominal  feet,  as 
biramous  appendages,  but  subsequently  completely  atrophy,  and  then  sprout 
out  again  in  their  permanent  form. 

(3)  The  abdominal  feet  develop  in  succession  from  before  backwards, 
and  the  whole  series  of  them  is  fully  formed  before  a  trace  of  the  appendages 
of   the   three  hindermost   thoracic   segments   has  appeared.      It   may   be 
mentioned  as  a  point  of  some  importance  that  the  Zoaea  of  Squilla  has 
an  elongated  many-chambered   heart,  and  not  the  short   compact  heart 
usually  found  in  the  Zoaea. 

The  younger  stages  of  the  Alima  larva  are  not  known1,  but  the  earliest 
stage  observed  is  remarkable  for  presenting  no  trace  of  the  three  posterior 
pairs  of  maxillipeds,  or  of  the  three  following  pairs  of  thoracic  appendages. 
The  segments  belonging  to  these  appendages  are  however  well  developed. 
The  tail  has  its  full  complement  of  segments  with  the  normal  number  of 
well  developed  swimming  feet.  The  larva  represents  in  fact  the  stage  of 
the  Erichthus  larva  when  the  three  posterior  pairs  of  maxillipeds  have 
undergone  atrophy  ;  but  it  is  probable  that  these  appendages  never  become 
developed  in  this  form  of  larva. 

Apart  from  the  above  peculiarities  the  Alima  form  of  larva  closely 
resembles  the  Erichthus  form. 

Nebaliadae.  The  development  of  Nebalia  is  abbreviated,  but  from 
MetschnikofFs  figures2  may  be  seen  to  resemble  closely  that  of  Mysis. 
The  abdomen  has  comparatively  little  yolk,  and  is  bent  over  the  ventral 
surface  of  the  thorax.  There  is  in  the  egg  a  Nauplius  stage  with  three 
appendages,  and  subsequently  a  stage  with  the  Zoaea  appendages. 

The  larva  when  it  leaves  the  egg  has  the  majority  of  its  appendages 
formed,  but  is  still  enveloped  in  a  larval  skin,  and  like  Mysis  bends  its 
abdomen  towards  the  dorsal  side.  When  the  larva  is  finally  hatched  it  does 
not  differ  greatly  from  the  adult. 

Cum  ace  ae.  The  development  of  the  Cumaceae  takes  place  for  the 
most  part  within  the  egg,  and  has  been  shewn  by  Dohrn  (No.  496)  to 
resemble  in  many  points  that  of  the  Isopods.  A  dorsal  organ  is  present, 
and  a  fold  is  formed  immediately  behind  this  which  gives  to  the  embryo  a 
dorsal  flexure.  Both  of  these  features  are  eminently  characteristic  of  the 
Isopoda. 

The  formation  of  the  two  pairs  of  antennie,  mandibles,  and  two  pairs 
of  maxillae  and  the  following  seven  pairs  of  appendages  takes  place  very 
early.  The  pair  of  appendages  behind  the  second  maxilku  assumes  an 
ambulatory  form,  and  exhibits  a  Schizopod  character  very  early,  differing 
in  both  these  respects  from  the  homologous  appendages  in  the  Isopoda. 
The  cephalo-thoracic  shield  commences  to  be  formed  when  the  appendages 
are  still  quite  rudimentary  as  a  pair  of  folds  in  the  maxillary  region.  The 

1  The  observations  of  Brooks  (No.  493)  render  it  probable  that  the  Alima  larva 
leaves  the  egg  in  a  form  not  very  dissimilar  to  the  youngest  known  larva. 
3  His  paper  is  unfortunately  in  Russian. 


CRUSTACEA.  487 


eyes  are  formed  slightly  later  on  each  side  of  the  head,  and  only  coalesce  at 
a  subsequent  period  to  form  the  peculiar  median  sessile  eye  of  the  adult. 

The  two  pairs  of  appendages  behind  the  second  maxillae  become  con- 
verted into  maxillipeds,  and  the  exopodite  of  the  first  of  them  becomes  the 
main  ramus,  while  in  the  externally  similar  second  maxilliped  the  exopodite 
atrophies  and  the  endopodite  alone  remains. 

The  larva  is  hatched  without  the  last  pair  of  thoracic  limbs  or  the 
abdominal  appendages  (which  are  never  developed  in  the  female),  but  in 
other  respects  closely  resembles  the  adult.  Before  hatching  the  dorsal 
flexure  is  exchanged  for  a  ventral  one,  and  the  larva  acquires  a  character 
more  like  that  of  a  Decapod. 

COPEPODA. 

Natantia.  The  free  Copepoda  are  undoubtedly  amongst 
the  lowest  forms  of  those  Crustacea  which  are  free  or  do  not 
lead  a  parasitic  existence.  Although  some  features  of  their 
anatomy,  such  for  instance  as  the  frequent  absence  of  a  heart, 
may  be  put  down  to  a  retrogressive  development,  yet,  from  their 
retention  of  the  median  frontal  eye  of  the  Nauplius  as  the  sole 
organ  of  vision1,  their  simple  biramous  swimming  legs,  and  other 
characters,  they  may  claim  to  be  very  primitive  forms,  which 
have  diverged  to  no  great  extent  from  the  main  line  of  Crus- 
tacean development.  They  supply  a  long  series  of  transitional 
steps  from  the  Nauplius  stage  to  the  adult  condition. 

While  still  within  the  egg-shell  the  embryo  is  divided  by  two 
transverse  constrictions  into  three  segments,  on  which  the  three 
Nauplius  appendages  are  developed,  viz.  the  two  pairs  of 
antennae  and  the  mandibles.  When  the  embryo  is  hatched  the 
indication  of  a  division  into  segments  has  vanished,  but  the 
larva  is  in  the  fullest  sense  a  typical  Nauplius2.  There  are 
slight  variations  in  the  shape  of  the  Nauplius  in  different  genera, 
but  its  general  form  and  character  are  very  constant.  It  has 
(fig.  229  A)  an  oval  unsegmented  body  with  three  pairs  of 
appendages  springing  from  the  ventral  surface.  The  anterior  of 
these  (at  i)  is  uniramous,  and  usually  formed  of  three  joints 
which  bear  bristles  on  their  under  surface.  The  two  posterior 

1  The  Pontellidse  form  an  exception  to  this  statement,  in  that  they  are  provided 
with  paired  lateral  eyes  in  addition  to  the  median  one. 

2  The  term  Nauplius  was  applied  to  the  larva  of  Cyclops  and  allied  organisms  by 
O.  F.  Muller  under  the  impression  that  they  were  adult  forms. 


488 


COPEPODA. 


pairs  of  appendages  are  both  biramous.  The  second  pair  of 
antennae  (at  2)  is  the  largest.  Its  basal  portion  (protopodite) 
bears  on  its  inner  side  a  powerful  hook-like  bristle.  The  outer 
ramus  is  the  longest  and  many-jointed  ;  the  inner  ramus  has 
only  two  joints.  The  mandibles  (md),  though  smaller  than  the 
second  pair  of  antennae,  have  a  nearly  identical  structure.  No 
blade-like  projection  is  as  yet  developed  on  their  protopodite. 
Between  the  points  of  insertion  of  the  first  pair  of  antennae  is 
the  median  eye  (oc),  which  originates  by  the  coalescence  of  two 
distinct  parts.  The  mouth  is  ventral,  and  placed  in  the  middle 
line  between  the  second  pair  of  antennae  and  the  mandibles  :  it 


FIG.  229.    SUCCESSIVE  STAGES  IN  THE  DEVELOPMENT  OF  CYCLOPS  TENUICORMS. 

(Copied  from  Bronn ;  after  Claus.) 

A.  B.  and  C.     Nauplius  stages.     D.  Youngest  Copepod  stage.     In  this  figure  maxillae 
and  the  two  rami  of  the  maxilliped  are  seen  immediately  behind  the  mandible  md. 

oc.  eye  ;  at1,  first  pair  of  antennae  ;  a/8,  second  pair  of  antennre  ;  md.  mandible ; 
/*.  first  pair  of  feet ;  /2.  second  pair  of  feet ;  f.  third  pair  of  feet ;  //.  excretory  con- 
cretions in  the  intestine. 

is  provided  with  an  unpaired  upper  lip.  There  are  two  bristles 
at  the  hind  end  of  the  embryo  between  which  the  anus  is  placed, 
and  in  some  cases  there  is  at  this  part  a  slight  indication  of  the 
future  caudal  fork. 

The  larva  undergoes  a  number  of  successive  ecdyses,  at  each 
of  which  the  body  becomes  more  elongated,  and  certain  other 


CRUSTACEA.  489 


changes  take  place.  First  of  all  a  pair  of  appendages  arises 
behind  the  mandibles,  which  form  the  maxillae  (fig.  229  B) ;  at 
the  same  time  the  basal  joint  of  the  maxillae  develops  a  cutting- 
blade.  Three  successive  pairs  of  appendages  (fig.  229  C)  next 
become  formed — the  so-called  maxillipeds  (the  homologues  of 
the  second  pair  of  maxillae),  and  the  two  first  thoracic  limbs. 
Each  of  these  though  very  rudimentary  is  nevertheless  bifid. 
The  body  becomes  greatly  elongated,  and  the  caudal  fork  more 
developed. 

Up  to  this  stage  of  development  the  Nauplius  appendages 
have  retained  their  primitive  character  almost  unaltered  ;  but 
after  a  few  more  ecdyses  a  sudden  change  takes  place ;  a  cephalo- 
thoracic  shield  becomes  fully  developed,  and  the  larva  comes  to 
resemble  in  character  an  adult  Copepod,  from  which  it  mainly 
differs  in  the  smaller  number  of  segments  and  appendages.  In 
the  earliest  'Cyclops'  stage  the  same  number  of  appendages  are 
present  as  in  the  last  Nauplius  stage.  There  (fig.  229  D)  is  a 
well  developed  cephalo-thorax,  and  four  free  segments  behind  it. 
To  the  cephalo-thoracic  region  the  antennae,  mandibles,  maxillae, 
the  now  double  pair  of  maxillipeds  (derived  from  the  original 
single  pair  of  appendages),  and  first  pair  of  thoracic  appendages 
(pl)  are  attached.  The  second  pair  of  thoracic  appendages  (/2) 
is  fixed  to  the  first  free  segment,  and  the  rudiment  of  a  third 
pair  (/3)  projects  from  the  second  free  segment.  The  first  pair 
of  antennae  has  grown  longer  by  the  addition  of  new  joints,  and 
continues  to  increase  in  length  in  the  following  ecdyses  till  it 
attains  its  full  adult  development,  and  then  forms  the  chief 
organ  of  locomotion.  The  second  pair  of  antennae  is  much 
reduced  and  has  lost  one  of  its  rami.  The  two  rami  of  the 
mandibles  are  reduced  to  a  simple  palp,  while  the  blade  has 
assumed  its  full  importance.  The  maxillae  and  following  appen- 
dages have  greatly  increased  in  size.  They  are  all  biramous, 
though  the  two  rami  are  not  as  yet  jointed.  The  adult  state  is 
gradually  attained  after  a  number  of  successive  ecdyses,  at 
which  new  segments  and  appendages  are  added,  while  new 
joints  are  formed  for  those  already  present. 

Parasita.  The  earliest  developmental  stages  of  the  parasitic  types 
of  Copepoda  closely  resemble  those  of  the  free  forms,  but,  as  might  be 
expected  from  the  peculiarly  modified  forms  of  the  adult,  they  present  a 


490 


COPEPODA. 


large  number  of  secondary  characters.     So  far  as  is  known  a  more  or  less 
modified  Nauplius  larva  is  usually  preserved. 

The  development  of  Achtheres  percarum,  one  of  the  Lernaeopoda  parasitic 
in  the  mouth,  etc.  of  the  common  Perch,  may  be  selected  to  illustrate  the 
mode  of  development  of  these  forms.  The  larva  leaves  the  egg  as  a  much 
simplified  Nauplius  (fig.  230  A).  It  has  an  oval  body  with  only  the  two 
anterior  pairs  of  Nauplius  appendages  ;  both  of  them  in  the  rudimentary 
condition  of  unjointed  rods.  The  usual  median  eye  is  present,  and  there  is 
also  found  a  peculiar  sternal  papilla,  on  which  opens  a  spiral  canal  filled 
with  a  glutinous  material,  which  is  probably  derived  from  a  gland  which 
disappears  on  the  completion  of  the  duct.  The  probable  function  of  this 


FIG.  330.  SUCCESSIVE  STAGES  IN  THE  DEVELOPMENT  OF  ACHTHERES  PERCARUM. 
(Copied  from  Bronn  ;  after  Claus. ) 

A.  Modified  Nauplius  stage.  B.  Cyclops  stage.  C.  Late  stage  of  male 
embryo.  D.  Sexually  mature  female.  E.  Sexually  mature  male. 

at1,  first  pair  of  antennae;  at3,  second  pair  of  antennae;  tnd.  mandible;  tnx. 
maxillae  ;  ptn1.  outer  pair  of  maxillipeds  ;  ftn^.  inner  pair  of  maxillipeds ;  J>1.  first  pair 
of  legs ;  /*.  second  pair  of  legs  ;  z.  frontal  organ  ;  i.  intestine ;  o.  larval  eye  ;  b. 
glandular  body  ;  t.  organ  of  touch  ;  ov.  ovary  ;  /.  rod  projecting  from  coalesced  maxil- 
lipeds ;  g.  cement  gland ;  rs.  receptaculurn  seminis ;  n.  nervous  system  ;  te.  testis ;  v. 
vas  deferens. 

organ  is  to  assist  at  a  later  period  in  the  attachment  of  the  parasite  to  its 
host.  Underneath  the  Nauplius  skin  a  number  of  appendages  are  visible, 
which  become  functional  after  the  first  ecdysis.  This  takes  place  within  a 
few  hours  after  the  hatching  of  the  Nauplius,  and  the  larva  then  passes  from 


CRUSTACEA.  491 


this  rudimentary  Nauplius  stage  into  a  stage  corresponding  with  the  Cyclops 
stage  of  the  free  forms  (fig.  230  B).  In  the  Cyclops  stage  the  larva  has  an 
elongated  body  with  a  large  cephalo-thoracic  shield,  and  four  free  posterior 
segments,  the  last  of  which  bears  a  forked  tail. 

There  are  now  present  eight  pairs  of  appendages,  viz.  antennae  (two 
pairs),  mandibles,  maxillae,  maxillipeds,  and  three  pairs  of  swimming  feet. 
The  Nauplius  appendages  are  greatly  modified.  The  first  pair  of  antennae  is 
three-jointed,  and  the  second  biramous.  The  outer  ramus  is  the  longest,  and 
bears  a  claw-like  bristle  at  its  extremity.  This  pair  of  appendages  is  used 
by  the  larva  for  fixing  itself.  The  mandibles  are  small  and  connected  with 
the  proboscidiform  mouth ;  and  the  single  pair  of  maxillae  is  small  and  palped. 
The  maxillipeds  (pm*  and  flm2)  are  believed  by  Claus  to  be  primitively 
a  single  biramous  appendage,  but  early  appear  as  two  distinct  structures1, 
the  outer  and  larger  of  which  becomes  the  main  organ  by  which  the  larva  is 
fixed.  Both  are  at  this  stage  simple  two-jointed  appendages.  The  two 
anterior  pairs  of  swimming  feet  have  the  typical  structure,  and  consist  of  a 
protopodite  bearing  an  unjointed  exopodite  and  endopodite.  The  first  pair 
is  attached  to  the  cephalo-thorax  and  the  second  (p*}  to  the  first  free  thoracic 
segment.  The  third  pair  is  very  small  and  attached  to  the  second  free 
segment.  The  mouth  is  situated  at  the  end  of  a  kind  of  proboscis  formed 
by  prolongations  of  the  upper  and  lower  lips.  The  alimentary  tract  is  fairly 
simple,  and  the  anus  opens  between  the  caudal  forks. 

Between  this  and  the  next  known  stage  it  is  quite  possible  that  one 
or  more  may  intervene.  However  this  may  be  the  larva  in  the  next  stage 
observed  (fig.  230  C)  has  already  become  parasitic  in  the  mouth  of  the  Perch, 
and  has  acquired  an  elongated  vermiform  aspect.  The  body  is  divided  into 
two  sections,  an  anterior  unsegmented,  and  a  posterior  formed  of  five 
segments,  of  which  the  foremost  is  the  first  thoracic  segment  which  in  the 
earlier  stage  was  fused  with  the  cephalo-thorax.  The  tail  bears  a  rudimen- 
tary fork  between  the  prongs  of  which  the  anus  opens.  The  swimming  feet 
have  disappeared,  so  also  has  the  eye  and  the  spiral  duct  of  the  embryonic 
frontal  organ.  The  outer  of  the  two  divisions  of  the  maxilliped  have  under- 
gone the  most  important  modification,  in  that  they  have  become  united  at 
their  ends,  where  they  form  an  organ  from  which  an  elongated  rod  (_/) 
projects,  and  attaches  the  larva  to  the  mouth  or  gills  of  its  host.  The 
antennae  and  jaws  have  nearly  acquired  their  adult  form.  The  nervous 
system  consists  of  supra-  and  infra-cesophageal  ganglia  and  two  lateral 
trunks  given  off  from  the  latter.  At  this  stage  the  males  and  females  can 
already  be  distinguished,  not  only  by  certain  differences  in  the  rudimen- 
tary generative  organs,  but  also  by  the  fact  that  the  outer  branch  of  the 
maxillipeds  is  much  longer  in  the  female  than  in  the  male,  and  projects 
beyond  the  head. 

In  the  next  ecdysis  the  adult  condition  is  reached.     The  outer  maxilli- 

1  Van  Beneden  (No.  506)  in  the  genera  investigated  by  him  finds  that  the  two 
maxillipeds  are  really  distinct  pairs  of  appendages. 


492  CIRRIPEDIA. 


peds  of  the  male  (fig.  230  £,/>#*)  separate  again  ;  while  in  the  female  (fig. 
230  D)  they  remain  fused  and  develop  a  sucker.  The  male  is  only  about 
one-fifth  the  length  of  the  female.  In  both  sexes  the  abdomen  is  much 
reduced. 

In  the  genera  Anchorella,  Lernaeopoda,  Brachiella  and  Hessia,  Ed.  van 
BenecUn  (No.  506)  has  shewn  that  the  embryo,  although  it  passes  through 
a  crypto-Nauplius  stage  in  the  egg,  is  when  hatched  already  in  the  Cyclops 
stage. 

Branchiura.  The  peculiar  parasite  Argulus,  the  affinities  of  which 
with  the  Copepoda  have  been  demonstrated  by  Claus  (No.  511),  is  hatched 
in  a  Cyclops  stage,  and  has  no  Nauplius  stage.  At  the  time  of  hatching  it 
closely  resembles  the  adult  in  general  form.  Its  appendages  are  however 
very  nearly  those  of  a  typical  larval  Copepod.  The  body  is  composed  of 
a  cephalo-thorax  and  free  region  behind  this.  The  cephalo-thorax  bears 
on  its  under  surface  antennae  (two  pairs),  mandibles,  maxillipeds,  and  the 
first  pair  of  thoracic  feet. 

The  first  pair  of  antennae  is  three-jointed,  but  the  basal  joint  bears  a 
hook.  The  second  pair  is  biramous,  the  inner  ramus  terminating  in  a  hook. 
The  mandible  is  palped,  but  the  palp  is  completely  separated  from  the 
cutting  blade1.  The  maxilla  would,  according  to  Claus,  appear  to  be 
absent. 

The  two  typical  divisions  of  the  Copepod  maxillipeds  are  present,  viz.  an 
outer  and  anterior  larger  division,  and  an  inner  and  posterior  smaller  one. 
The  first  pair  of  thoracic  feet,  as  is  usual  amongst  Copepoda,  is  attached 
to  the  cephalo-thorax.  It  has  not  the  typical  biramous  Copepod  character. 
There  are  four  free  segments  behind  the  cephalo-thorax,  the  last  of  which 
ends  in  a  fork.  Three  of  them  bear  appendages,  which  are  rudimentary  in 
this  early  larval  stage.  On  the  dorsal  surface  are  present  paired  eyes  as 
well  as  an  unpaired  median  eye. 

Between  the  larval  condition  and  that  of  the  adult  a  number  of  ecdyses 
intervene. 

CIRRIPEDIA. 

The  larvae  of  all  the  Cirripedia,  with  one  or  two  exceptions, 
leave  the  egg  in  the  Nauplius  condition.  The  Nauplii  differ 
somewhat  in  the  separate  groups,  and  the  post-nauplial  stages 
vary  not  inconsiderably. 

It  will  be  most  convenient  to  treat  successively  the  larval 

1  It  seems  not  impossible  that  the  appendage  regarded  by  Claus  as  the  mandibular 
palp  may  really  represent  the  maxilla,  which  would  otherwise  seem  to  be  absent. 
This  mode  of  interpretation  would  bring  the  appendages  of  Argulus  into  a  much 
closer  agreement  with  those  of  the  parasitic  Copepoda.  It  does  not  seem  incompatible 
with  the  existence  of  the  stylet-like  maxillse  detected  by  Claus  in  the  adult. 


CRUSTACEA.  493 


history  of  the  four  sub-orders,  viz.  Thoracica,  Abdominalia, 
Apoda,  and  Rhizocephala. 

Thoracica.  The  just  hatched  larvae  at  once  leave  the  egg 
lamellae  of  their  parent.  They  pass  out  through  an  opening  in 
the  mantle  near  the  mouth,  and  during  this  passage  the  shell  of 
the  parent  is  opened  and  the  movements  of  the  cirriform  feet 
cease. 

The  larval  stages  commence  with  a  Nauplius1  which,  though 
regarded  by  Claus  as  closely  resembling  the  Copepod  Nauplius 
(figs.  231  and  232  A),  certainly  has  very  marked  pecularities  of 
its  own,  and  in  some  respects  approaches  the  Phyllopod 
Nauplius.  It  is  in  the  youngest  stage  somewhat  triangular  in 
form,  and  covered  on  the  dorsal  side  by  a  very  delicate  and 
hardly  perceptible  dorsal  shield,  which  is  prolonged  laterally 
into  two  very  peculiar  conical  horns  (fig.  231  Ik),  which  are  the 
most  characteristic  structures  of  the  Cirriped  Nauplius.  They 
are  connected  with  a  glandular  mass,  the  secretion  from  which 
passes  out  at  their  apex.  Anteriorly  the  dorsal  shield  has  the 
same  extension  as  the  body,  but  posteriorly  it  projects  slightly. 

An  unpaired  eye  is  situated  on  the  ventral  surface  of  the 
head,  and  immediately  behind  it  there  springs  a  more  or  less 
considerable  upper  lip  (Ib),  which  resembles  the  Phyllopod 
labrum  rather  than  that  of  the  Copepoda.  Both  mouth  and 
anus  are  present,  and  the  hind  end  of  the  body  is  slightly  forked 
in  some  forms,  but  ends  in  others,  e.g.  Lepas  fascicularis,  in  an 
elongated  spine.  The  anterior  of  the  three  pairs  of  Nauplius 
appendages  (At*)  is  uniramous,  and  the  two  posterior  (Af  and 
md)  are  biramous.  From  the  protopodites  of  both  the  latter 
spring  strong  hooks  like  those  of  the  Copepod  and  Phyllopod 
Nauplii.  In  some  Nauplii,  e.g.  that  of  Balanus,  the  appendages 
are  at  first  not  jointed,  but  in  other  Nauplii,  e.g.  that  of  Lepas 
fascicularis,  the  jointing  is  well  marked.  In  Lepas  fascicularis 
the  earliest  free  Nauplius  is  enveloped  in  a  larval  skin,  which  is 
thrown  off  after  a  few  hours.  The  Nauplii  of  all  the  Thoracica 
undergo  a  considerable  number  of  moults  before  their  appendages 
increase  in  number  or  segmentation  of  the  body  appears.  During 
these  moults  they  grow  larger,  and  the  posterior  part  of  the 

1  Alepas  squalicola  is  stated  by  Koren  and  Danielssen  to  form  an  exception  to 
this  rule,  and  to  leave  the  egg  with  six  pairs  of  appendages. 


494 


CIRRIPEDIA. 


body — the  future  thoracic  and  abdominal  region — grows  re- 
latively in  length.  There  also  appear  close  to  the  sides  of  the 
unpaired  eye  two  conical  bodies,  which  correspond  with  the 
frontal  sense  organs  of  the  Phyllopods.  During  their  growth 
the  different  larvae  undergo  changes  varying  greatly  in  degree. 

In  Balanus  the  changes  consist  for  the  most  part  in  the  full 
segmentation  of  the  appendages  and  the  growth  and  distinctness 


FIG.  231.    NAUPLIUS  LARVA  OF  LEPAS  FASCICULARIS  VIEWED  FROM  THE  SIDE. 
oc.  eye  ;  At.  i.  antenna  of  first  pair  ;  At.  2.  antenna  of  second  pair  ;  md.  mandible ; 
Ib.    labrum ;   an.   anus;   me.   mesenteron;    d.sp.    dorsal   spine;   c.sp.    caudal   spine; 
Vp.  ventral  spine  ;  Ih.  lateral  horns. 

of  the  dorsal  shield,  which  forms  a  somewhat  blunt  triangular 
plate,  broadest  in  front,  with  the  anterior  horns  very  long,  and 
two  short  posterior  spines.  The  tail  also  becomes  produced  into 
a  long  spine. 

In  Lepas  fascicularis  the  changes  in  appearance  of  the 
Nauplius,  owing  to  a  great  spinous  development  on  its  shield, 
are  very  considerable  ;  and,  together  with  its  enormous  size, 
render  it  a  very  remarkable  form.  Dohrn  (No.  520),  who  was 
the  first  to  describe  it,  named  it  Archizoaea  gigas. 


CRUSTACEA.  495 


The  dorsal  shield  of  the  Nauplius  of  Lepas  fascicularis  (fig.  231)  becomes 
somewhat  hexagonal,  and  there  springs  from  the  middle  of  the  dorsal  surface 
an  enormously  long  spine  (d,sp],  like  the  dorsal  spine  of  a  Zoa^a.  The  hind 
end  of  the  shield  is  also  produced  into  a  long  caudal  spine  (c.sfi]  between 
which  and  the  dorsal  spine  are  some  feather-like  processes.  From  its  edge 
there  spring  in  addition  to  the  primitive  frontal  horns  three  main  pairs  of 
horns,  one  pair  anterior,  one  lateral,  and  one  posterior,  and  smaller  ones  in 
addition.  All  these  processes  (with  the  exception  of  the  dorsal  and  posterior 
spines)  are  hollow  and  open  at  their  extremities,  and  like  the  primitive 
frontal  horns  contain  the  ducts  of  glands  situated  under  the  shield.  On  the 
under  surface  of  the  larva  is  situated  the  unpaired  eye  (pc]  on  each  side  of 
which  spring  the  two-jointed  frontal  sense  organs.  Immediately  behind 
these  is  the  enormous  upper  lip  (lb]  which  covers  the  mouth1.  At  the  sides 
of  the  lip  lie  the  three  pairs  of  Nauplius  appendages,  which  are  very 
characteristic  but  present  no  special  peculiarities.  Posteriorly  the  body  is 
produced  into  a  long  ventral  spine-like  process  ( Vfi)  homologous  with  that 
of  other  more  normal  Nauplii.  At  the  base  of  this  process  large  moveable 
paired  spines  appear  at  successive  moults,  six  pairs  being  eventually  formed. 
These  spines  give  to  the  region  in  which  they  are  situated  a  segmented 
appearance,  and  perhaps  similar  structures  have  given  rise  to  the  appear- 
ance of  segmentation  in  Spence  Bate's  figures.  The  anus  is  situated  on 
the  dorsal  side  of  this  ventral  process,  and  between  it  and  the  caudal 
spine  of  the  shield  above.  The  fact  that  the  anus  occupies  this  position 
appears  to  indicate  that  the  ventral  process  is  homologous  with  the 
caudal  fork  of  the  Copepoda,  on  the  dorsal  side  of  which  the  anus  so 
often  opens2. 

From  the  Nauplius  condition  the  larvae  pass  at  a  single 
moult  into  an  entirely  different  condition  known  as  the  Cypris 
stage.  In  preparation  for  this  condition  there  appear,  during 
the  last  Nauplius  moults,  the  rudiments  of  several  fresh  organs, 
which  are  more  or  less  developed  in  different  types.  In  the 
first  place  a  compound  eye  is  formed  on  each  side  of  the 
median  eye.  Secondly  there  appears  behind  the  mandibles  a 
fourth  pair  of  appendages — the  first  pair  of  maxillae — and 
internal  to  these  a  pair  of  small  prominences,  which  are  perhaps 

1  Willemoes  Suhm  (No.  530)  states  that  the  mouth  is  situated  at  the  free  end  of  the 
upper  lip,  and  that  the  oesophagus  passes  through  it.     From  an  examination  of  some 
specimens  of  this  Nauplius,  for  which  I  am  indebted  to  Moseley,  I  am  inclined  to 
think  that  this  is  a  mistake,  and  that  a  groove  on  the  surface  of  the  upper  lip  has  been 
taken  by  Suhm  for  the  oesophagus. 

2  The  enormous  spinous  development  of  the  larva  of  Lepas  fascicularis  is  probably 
to  be  explained  as  a  secondary  protective  adaptation,  and  has  no  genetic  connection 
with  the  somewhat  similar  spinous  armature  of  the  Zosea. 


496"  CIRRIPEDIA. 


equivalent  to  the  second  pair  of  maxillae,  and  give  rise  to  the 
third  pair  of  jaws  in  the  adult  (sometimes  spoken  of  as  the 
lower  lip). 

Behind  these  appendages  there  are  moreover  formed  the  rudi- 
ments of  six  pairs  of  feet.  Under  the  cuticle  of  the  first  pair  of 
antennae  there  may  be  seen  just  before  the  final  moult  the  four- 
jointed  antennae  of  the  Cypris  stage  with  the  rudiment  of  a  disc 
on  the  second  joint  by  which  the  larvae  eventually  become 
attached. 

By  the  free  Cypris  stage,  into  which  the  larva  next  passes,  a 
very  complete  metamorphosis  has  been  effected.  The  median 
and  paired  eyes  are  present  as  before,  but  the  dorsal  shield  has 
become  a  bivalve  shell,  the  two  valves  of  which  are  united  along 
their  dorsal,  anterior,  and  posterior  margins.  The  two  valves 
are  further  kept  in  place  by  an  adductor  muscle  situated  close 
below  the  mouth.  Remains  of  the  lateral  horns  still  persist.  The 
anterior  antennae  have  undergone  the  metamorphosis  already 
indicated.  They  are  four-jointed,  the  two  basal  joints  being 
long,  and  the  second  provided  with  a  suctorial  disc,  in  the  centre 
of  which  is  the  opening  of  the  duct  of  the  so-called  antennary  or 
cement  gland,  which  is  a  granular  mass  lying  on  the  ventral 
side  of  the  anterior  region  of  the  body.  The  gland  arises 
(Willemoes  Suhm)  during  the  Nauplius  stage  in  the  large  upper 
lip.  The  two  distal  joints  of  the  antennae  are  short,  and  the 
last  of  them  is  provided  with  olfactory  hairs.  The  great  upper 
lip  and  second  pair  of  antennae  and  mandibles  have  disappeared, 
but  a  small  papilla,  forming  the  commencement  of  the  adult 
mandibles,  is  perhaps  developed  in  the  base  of  the  Nauplius 
mandibles.  The  first  pair  of  maxillae  have  become  small  papillae 
and  the  second  pair  probably  remain.  The  six  posterior  pairs 
of  appendages  have  grown  out  as  functional  biramous  swimming 
feet,  which  can  project  beyond  the  shell  and  are  used  in  the 
locomotion  of  the  larva.  They  are  composed  of  two  basal 
joints,  and  two  rami  with  swimming  hairs,  each  two-jointed. 
These  feet  resemble  Copepod  feet,  and  form  the  main  ground 
for  the  views  of  Claus  and  others  that  the  Copepoda  and 
Cirripedia  are  closely  related.  They  are  regarded  by  Claus  as 
representing  the  five  pairs  of  natatory  feet  of  Copepoda,  and  the 
generative  appendages  of  the  segment  behind  these.  Between 


CRUSTACEA. 


497 


the  natatory  feet  are  delicate  chitinous  lamellae,  in  the  spaces 
between  which  the  cirriform  feet  of  the  adult  become  developed. 
The  ventral  spinous  process  of  the  Nauplius  stage  is  much  reduced, 
though  usually  three-jointed.  It  becomes  completely  aborted 
after  the  larva  is  fixed. 

In  addition  to  the  antennary  gland  there  is  present,  near  the 
dorsal  side  of  the  body  above  the  natatory  feet,  a  peculiar  paired 
glandular  mass,  the  origin  of  which  has  not  been  clearly  made 
out,  but  which  is  perhaps  equivalent  to  the  entomostracan  shell 
gland.  It  probably  supplies  the  material  for  the  shell  in  suc- 
ceeding stages1. 

The  free  Cypris  stage  is  not  of  long  duration ;  and  during  it 
the  larva  does  not  take  food.  It  is  succeeded  by  a  stage  known 
as  the  pupa  stage  (fig.  232  B),  in  which  the  larva  becomes  fixed, 
while  underneath  the  larval  skin  the  adult  structures  are  de- 
veloped. This  stage  fully  deserves  its  name,  since  it  is  a  quies- 
cent stage  during  which  no  nutriment  is  taken.  The  attachment 
takes  place  by  the  sucker  of  the  antennae,  and  the  cement  gland 
(/)  supplies  the  cementing  material  for  effecting  it.  A  retro- 
gressive metamorphosis  of  a  large  number  of  the  organs  sets  in, 
while  at  the  same  time  the  for- 
mation of  new  adult  structures 
is  proceeded  with.  The  eyes 
become  gradually  lost,  but  the 
Nauplius  eye  is  retained,though 
in  a  rudimentary  state,  and  the 
terminal  joints  of  the  antennae 
with  their  olfactory  hairs  are 
thrown  off.  The  bivalve  shell 
is  moulted  about  the  same  time 
as  the  eyes,  the  skin  below  it 
remaining  as  the  mantle.  The 
caudal  process  becomes  abor- 
ted. Underneath  the  natatory 


FIG.  232.  LARVAL  FORMS  OF  THE 
THORACICA.  (From  Huxley.) 

A.  Nauplius  of  Balanus  balanoides. 
(After  Sp.  Bate.)  B.  Pupa  stage  of  Lepas 
australis.  (After  Darwin.) 

n.  antennary  apodemes ;  /.  cement 
gland  with  duct  to  antenna. 


1  There  is  considerable  confusion  about  the  shell  gland  and  antennary  gland.  In 
my  account  Willemoes  Suhm  has  been  followed.  Claus  however  regards  what  I  have 
called  the  antennary  gland  as  the  shell  gland,  and  states  that  it  does  not  open  into  the 
antennae  till  a  later  period.  He  does  not  clearly  describe  its  opening,  nor  the  organ 
which  I  have  called  the  shell  gland. 

B.  II.  32 


498  CIRRIPEDIA. 


feet,  and  between  the  above-mentioned  chitinous  lamellae,  the 
cirriform  feet  are  formed ;  and  on  their  completion  the  natatory 
feet  become  thrown  off  and  replaced  by  the  permanent  feet.  In 
the  Lepadidae,  in  which  the  metamorphosis  of  the  pupa  stages 
has  been  most  fully  studied,  the  anterior  part  of  the  body  with 
the  antennae  gradually  grows  out  into  an  elongated  stalk,  into 
which  pass  the  ovaries,  which  are  formed  during  the  Cypris 
stage.  At  the  base  of  the  stalk  is  the  protuberant  mouth,  the 
appendages  of  which  soon  attain  a  higher  development  than  in 
the  Cypris  stage.  At  the  front  part  of  it  a  large  upper  lip 
becomes  formed.  Above  the  mantle  and  between  it  and  the 
shell  there  are  formed  in  the  Lepadidae  the  provisional  valves  of 
the  shell.  These  valves  are  chitinous,  and  have  a  fenestrated 
structure,  owing  to  the  chitin  being  deposited  round  the  margin 
of  the  separate  epidermis  (hypodermis)  cells.  These  valves  in 
the  Lepadidae  "  prefigure  in  shape,  size,  and  direction  of  growth, 
the  shelly  valves  to  be  formed  under  and  around  them"  (Darwin, 
No.  519,  p.  129). 

Whatever  may  be  the  number  of  valves  in  the  adult  the  provisional 
valves  never  exceed  five,  viz.  the  two  scuta,  the  two  terga  and  the  carina. 
They  are  relatively  far  smaller  than  the  permanent  valves  and  are  therefore 
separated  by  considerable  membranous  intervals.  They  are  often  preserved 
for  a  long  time  on  the  permanent  calcareous  valves.  In  the  Balanidce 
the  embryonic  valves  are  membranous  and  do  not  overlap,  but  do  not 
present  the  peculiar  fenestrated  structure  of  the  primordial  valves  of  the 
Lepadidae. 

In  connection  with  the  moult  of  the  pupa  skin,  and  the 
conversion  of  the  pupa  into  the  adult  form,  a  remarkable  change 
in  the  position  takes  place.  The  pupa  lies  with  the  ventral  side 
parallel  to  and  adjoining  the  surface  of  attachment,  while  the 
long  axis  of  the  body  of  the  young  Cirriped  is  placed  nearly  at 
right  angles  to  the  surface  of  attachment.  This  change  is 
connected  with  the  ecdyses  of  the  antennary  apodemes  («), 
which  leave  a  deep  bay  on  the  ventral  surface  behind  the 
peduncle.  The  chitinous  skin  of  the  Cirriped  passes  round 
the  head  of  this  bay,  but  on  the  moult  of  the  pupa  skin 
taking  place  becomes  stretched  out,  owing  to  the  posterior 
part  of  the  larva  bending  dorsalwards.  It  is  this  flexure  which 
causes  the  change  in  the  position  of  the  larva. 


CRUSTACEA. 


499 


In  addition  to  the  remarkable  external  metamorphosis 
undergone  during  the  pupa  stage,  a  series  of  hardly  less  con- 
siderable internal  changes  take  place,  such  as  the  atrophy  of 
the  muscles  of  the  antennae,  a  change  in  the  position  of  the 
stomach,  etc. 

Abdominalia.  In  the  Alcippidae  the  larva  leaves  the  egg  as  a 
Nauplius,  and  this  stage  is  eventually  followed  by  a  pupa  stage  closely 
resembling  that  of  the  Thoracica.  There  are  six  pairs  of  thoracic  natatory 
legs  (Darwin,  No.  519).  Of  these  only  the  first  and  the  last  three  are  pre- 
served in  the  adult,  the  first  being  bent  forward  in  connection  with  the 
mouth.  The  body  moreover  partially  preserves  its  segmentation,  and  the 
mantle  does  not  secrete  calcareous  valves. 

The  very  remarkable  genus  Cryptophialus,  the  development  of  which  is 
described  by  Darwin  (No.  519)  in  his  classical  memoir,  is  without  a  free 
Nauplius  stage.  The  embryo  is  at  first  oval  but  soon  acquires  two  anterior 
processes,  apparently  the  first  pair  of  antennae,  and  a  posterior  prominence, 
the  abdomen.  In  a  later  stage  the  abdominal  prominence  disappears,  and 
the  antennary  processes,  within  which  the  true  antennas  are  now  visible,  are 
carried  more  towards  the  ventral 
surface.  The  larva  next  passes  into 
the  free  Cypris  stage,  during  which  it 
creeps  about  the  mantle  cavity  of  its 
parent.  It  is  enveloped  in  a  bivalve 
shell,  and  the  antennae  have  the  nor- 
mal cirriped  structure.  There  are  no 
other  true  appendages,  but  posteriorly 
three  pairs  of  bristles  are  attached  to 
a  rudimentary  abdomen.  Paired  com- 
pound eyes  are  present.  During  the 
succeeding  pupa  stage  the  metamor- 
phosis into  the  adult  form  takes  place, 
but  this  has  not  been  followed  out  in 
detail. 

In  Kochlorine,  a  form  discovered 
by  Noll  (No.  526)  and  closely  related 
to  Cryptophialus,  the  larvae  found 
within  the  mantle  represent  ap- 
parently two  larval  stages,  similar  to 
two  of  the  larval  stages  described  by 
Darwin. 

Rhizocephala.      The  Rhizo- 
cephala,  as  might  have  been  antici- 


FIG.  233.  STAGES  IN  THE  DEVELOP- 
MENT OF  THE  RHIZOCEPHALA.  (From 
Huxley,  after  Fritz  Miiller.) 

A.  Nauplius  of  Sacculina  purpurea. 
B.  Cypris  stage  of  Lernseodiscus  por- 
cellanae.  C.  Adult  of  Peltogaster  paguri. 

II,  III.  IV.  Two  pairs  of  antennae 
and  mandibles;  cp.  carapace;  a.  anterior 
end  of  body;  b.  generative  aperture;  c. 
root-like  processes. 


pated  from  their  close  relationship  to  Anelasma  squalicola  amongst  the 
Thoracica,  undergo  a  development  differing  much  less  from  the  type  of  the 
Thoracica  than  that  of  Cryptophialus  and  Kochlorine. 

32—2 


5oo 


OSTRACODA. 


Sacculina  leaves  the  egg  as  a  Nauplius  (fig.  233  A),  which  differs  from 
the  ordinary  type  mainly  (i)  in  the  large  development  of  an  oval  dorsal 
shield  (eft]  which  projects  far  beyond  the  edge  of  the  body,  but  is  provided 
with  the  typical  sternal  horns,  etc.  ;  and  (2)  in  the  absence  of  a  mouth. 
The  Cypris  and  pupa  stages  of  Sacculina  and  other  Rhizocephala  (fig.  233  B) 
are  closely  similar  to  those  of  the  Thoracica,  but  the  paired  eye  is  absent. 
The  attachment  takes  place  in  the  usual  way,  but  the  subsequent  metamor- 
phosis leads  to  the  loss  of  the  thoracic  feet  and  generally  to  retrogressive 
changes. 

OSTRACODA. 

Our  knowledge  of  the  development  of  this  remarkable  group  is  entirely 
due  to  the  investigations  of  Claus. 

Some  forms  of  Cythere  are  viviparous,  and  in  the  marine  form  Cypridina 
the  embryo  develops  within  the  valves  of  the  shell.  Cypris  attaches  its 
eggs  to  water  plants.  The  larvae  of  Cypris  are  free,  and  their  development 
is  somewhat  complicated.  The  whole  development  is  completed  in  nine 
ecdyses,  each  of  them  accompanied  by  more  or  less  important  changes  in 
the  constitution  of  the  larva. 

In  the  earliest  free  stage  the  larva  has  the  characters  of  a  true  Nauplius 
with  three  pairs  of  appendages  (fig.  234  A).  The  Nauplius  presents  how- 
B  A 


-A' 


MX  SM 


FlG.  234.      TWO   STAGES    IN   THE   DEVELOPMENT  OF   CYPRIS.      (From  ChlUS.) 

A.  Earliest  (Nauplius)  stage.  B.  Second  stage. 

A'.  A".     First  and   second  pairs  of  antennae ;   Md.   mandibles ;    OL.  labrum ; 
MX,',  first  pair  of  maxilla;  /".  first  pair  of  feet. 

ever  one  or  two  very  marked  secondary  characters.  In  the  first  place  it  is 
completely  enveloped  in  a  fully  formed  bivalve  shell,  differing  in  unessential 
points  from  the  shell  of  the  adult.  An  adductor  muscle  (SM]  for  the  shell 
is  present.  Again  the  second  and  third  appendages,  though  locomotive  in 
function  are  neither  of  them  biramous,  and  the  third  one  already  contains 
a  rudiment  of  the  future  mandibular  blade,  and  terminates  in  an  anteriorly 
directed  hook-like  bristle.  The  first  pair  of  antenna?  is  moreover  very 
similar  to  the  second  and  is  used  in  progression.  Neither  of  the  pairs  of 


CRUSTACEA. 


501 


antennae  become  much  modified  in  the  subsequent  metamorphosis.  The 
Nauplius  has  a  single  median  eye,  as  in  the  adult  Cypris,  and  a  fully 
developed  alimentary  tract. 

The  second  stage  (fig.  234  B),  inaugurated  by  the  first  moult,  is  mainly 
characterized  by  the  appearance  of  two  fresh  pairs  of  appendages,  viz.  the 
first  pair  of  maxillae  and  the  first  pair  of  feet ;  the  second  pair  of  maxillae 
not  being  developed  till  later.  The  first  pair  appear  as  leaf-like  curved 


FIG.  235.     STAGES  IN  THE  DEVELOPMENT  OF  CYPRIS.    (From  Claus.) 

A.  Fourth  stage.  B.   Fifth  stage. 

MX',  first  maxilla ;  MX",  and/',  second  maxilla  ;  /".   first  pair  of  feet ;  L.  liver. 

plates  (Mx'}  more  or  less  like  Phyllopod  appendages  (Claus)  though  at  this 
stage  without  an  exopodite.  The  first  pair  of  feet  (/"}  terminates  in  a 
curved  claw  and  is  used  for  adhering.  The  mandibles  have  by  this  stage 
fully  developed  blades,  and  have  practically  attained  their  adult  form,  con- 
sisting of  a  powerful  toothed  blade  and  a  four-jointed  palp. 

During  the  third  and  fourth  stages  the  first  pair  of  maxillae  acquire 
their  pectinated  gill  plate  (epipodite)  and  four  blades  ;  and  in  the  fourth 
stage  (fig.  235  A)  the  second  pair  of  maxillae  (Mx"}  arises,  as  a  pair  of 
curved  plates,  similar  to  the  first  pair  of  maxillae  at  their  first  appearance. 
The  forked  tail  is  indicated  during  the  fourth  stage  by  two  bristles.  During 
the  fifth  stage  (fig.  235  B)  the  number  of  joints  of  the  first  pair  of  antennae 
becomes  increased,  and  the  posterior  maxillae  develop  a  blade  and  become 


502 


PHYLOGENY   OF  THE  CRUSTACEA. 


four-jointed  ambulatory  appendages  terminating  in  a  hook.    The  caudal  fork 
becomes  more  distinct. 

In  the  sixth  stage  (fig.  236)  the  second  and  hindermost  pair  of  feet  be- 
comes formed  (/"')  and  the  maxillae  of  the  second  pair  lose  their  ambulatory 
function,  and  begin  to  be  converted  into  definite  masticatory  appendages  by 
the  reduced  jointing  of  their  palp,  and  the  increase  of  their  cutting  blades. 
By  the  seventh  stage  all  the  appendages  have  practically  attained  their 


Fu 


FIG.  236.     SIXTH  STAGE  IN  THE  DEVELOPMENT  OF  CYPRIS.    (From  Claus.) 
MX!,  first  maxilla ;  Mx".f.  second  maxilla;  /'.  and/"',  first  and  second  pair  oi 
feet ;  Fu.  caudal  fork  ;  L.  liver ;  S.D.  shell  gland. 

permanent  form  ;  the  second  pair  of  maxillae  has  acquired  small  branchial 
plates,  and  the  two  following  feet  have  become  jointed.  In  the  eighth  and 
ninth  stages  the  generative  organs  attain  their  mature  form. 

The  larva  of  Cythere  at  the  time  of  birth  has  rudiments  of  all  the  limbs, 
but  the  mandibular  palp  still  functions  as  a  limb,  and  the  three  feet  (2nd 
pair  of  maxillae  and  two  following  appendages)  are  very  rudimentary. 

The  larvae  of  Cypridina  are  hatched  in  a  condition  which  to  all  intents 
and  purposes  resembles  the  adult. 


Phylogeny  of  the  Crustacea. 

The  classical  work  of  Fritz  Miiller  (No.  452)  on  the  phylogeny  of  the 
Crustacea  has  given  a  great  impetus  to  the  study  of  their  larval  forms,  and 
the  interpretations  of  these  forms  which  he  has  offered  have  been  the  subject 
of  a  very  large  amount  of  criticism  and  discussion.  A  great  step  forward 
in  this  discussion  has  been  recently  made  by  Claus  in  his  memoir  (No.  448). 

The  most  fundamental  question  concerns  the  meaning  of  the  Nauplius. 
Is  the  Nauplius  the  ancestral  form  of  the  Crustacea,  as  is  believed  by  Fritz 
Miiller  and  Claus,  or  are  its  peculiarities  and  constant  occurrence  due  to 
some  other  cause  ?  The  most  plausible  explanation  on  the  second  hypothesis 


CRUSTACEA.  503 


would  seem  to  be  the  following.  The  segments  with  their  appendages  of 
Arthropoda  and  Annelida  are  normally  formed  from  before  backwards, 
therefore  every  member  of  these  two  groups  with  more  than  three  segments 
must  necessarily  pass  through  a  stage  with  only  three  segments,  and  the  fact 
that  in  a  particular  group  this  stage  is  often  reached  on  the  larva  being 
hatched  is  in  itself  no  proof  that  the  ancestor  of  the  group  had  only  three 
segments  with  their  appendages.  This  explanation  appears  to  me,  so  far 
as  it  goes,  quite  valid ;  but  though  it  relieves  us  from  the  necessity  of 
supposing  that  the  primitive  Crustacea  had  only  three  pairs  of  appendages, 
it  does  not  explain  several  other  peculiarities  of  the  Nauplius1.  The  more 
important  of  these  are  the  following. 

1.  That  the  mandibles  have  the  form  of  biramous  swimming  feet  and 
are  not  provided  with  a  cutting  blade. 

2.  That  the  second  pair  of  antennae  are  biramous  swimming  feet  with  a 
hook  used  in  mastication,  and  are  innervated  (?)  from  the  subcesophageal 
ganglion. 

3.  The  absence  of  segmentation  in  the  Nauplius  body.     An  absence 
which  is  the  more  striking  in  that  before  the  Nauplius  stage  is  fully  reached 
the  body  of  the  embryo   is   frequently  divided  into   three  segments,  e.g. 
Copepoda  and  Cirripedia 

4.  The  absence  of  a  heart. 

5.  The  presence  of  a  median  single  eye  as  the  sole  organ  of  vision. 

Of  these  points  the  first,  second,  and  fifth  appear  only  to  be  capable  of 
being  explained  phylogenetically,  while  with  reference  to  the  absence  of  a 
heart  it  appears  very  improbable  that  the  ancestral  Crustacea  were  without 
a  central  organ  of  circulation.  If  the  above  positions  are  accepted  the 
conclusion  would  seem  to  follow  that  in  a  certain  sense  the  Nauplius  is 
an  ancestral  form — but  that,  while  it  no  doubt  had  its  three  anterior  pairs 
of  appendages  similar  to  those  of  existing  Nauplii,  it  may  perhaps  have 
been  provided  with  a  segmented  body  behind  provided  with  simple  biramous 
appendages.  A  heart  and  cephalo-thoracic  shield  may  also  have  been 
present,  though  the  existence  of  the  latter  is  perhaps  doubtful.  There  was 
no  doubt  a  median  single  eye,  but  it  is  difficult  to  decide  whether  or  no 
paired  compound  eyes  were  also  present.  The  tail  ended  in  a  fork  between 
the  prongs  of  which  the  anus  opened ;  and  the  mouth  was  protected  by  a 
large  upper  lip.  In  fact,  it  may  very  probably  turn  out  that  the  most 
primitive  Crustacea  more  resembled  an  Apus  larva  at  the  moult  immediately 
before  the  appendages  lose  their  Nauplius  characters  (fig.  208  B),  or  a 
Cyclops  larva  just  before  the  Cyclops  stage  (fig.  229),  than  the  earliest 
Nauplius  of  either  of  these  forms. 

If  the  Nauplius  ancestor  thus  reconstructed  is  admitted  to  have  existed, 
the  next  question  in  the  phylogeny  of  the  Crustacea  concerns  the  relations 
of  the  various  phyla  to  the  Nauplius.  Are  the  different  phyla  descended 
from  the  Nauplius  direct,  or  have  they  branched  at  a  later  period  from 

1  For  the  characters  of  Nauplius  vide  p.  460. 


504  PHYLOGENY  OF  THE   CRUSTACEA. 

some  central  stem?  It  is  perhaps  hardly  possible  as  yet  to  give  a  full  and 
satisfactory  answer  to  this  question,  which  requires  to  be  dealt  with  for  each 
separate  phylum ;  but  it  may  probably  be  safely  maintained  that  the  existing 
Phyllopods  are  members  of  a  group  which  was  previously  much  larger,  and 
the  most  central  of  all  the  Crustacean  groups;  and  which  more  nearly 
retains  in  the  characters  of  the  second  pair  of  antennae  etc.  the  Nauplius 
peculiarities.  This  view  is  shared  both  by  Claus  and  Dohrn,  and  appears 
to  be  in  accordance  with  all  the  evidence  we  have  both  palaeontological  and 
morphological.  Claus  indeed  carries  this  view  still  further,  and  believes 
that  the  later  Nauplius  stages  of  the  different  Entomostracan  groups  and 
the  Malacostraca  (Penaeus  larva)  exhibit  undoubted  Phyllopod  affinities. 
He  therefore  postulates  the  earlier  existence  of  a  Protophyllopod  form,  which 
would  correspond  very  closely  with  the  Nauplius  as  reconstructed  above, 
from  which  he  believes  all  the  Crustacean  groups  to  have  diverged. 

It  is  beyond  the  scope  of  this  work  to  attempt  to  grapple  with  all  the 
difficulties  which  arise  in  connection  with  the  origin  and  relationships  of  the 
various  phyla,  but  I  confine  myself  to  a  few  suggestions  arising  out  of  the 
developmental  histories  recorded  above. 

Malacostraca.  In  attempting  to  reconstitute  from  the  evidence  in 
our  possession  the  ancestral  history  of  the  Malacostraca  we  may  omit  from 
consideration  the  larval  history  of  all  those  types  which  leave  the  egg  in 
nearly  the  adult  form,  and  confine  our  attention  to  those  types  in  which  the 
larval  history  is  most  completely  preserved. 

There  are  three  forms  which  are  of  special  value  in  this  respect,  viz. 
Euphausia,  Penaeus  and  Squilla.  From  the  history  of  these  which  has 
already  been  given  it  appears  that  in  the  case  of  the  Decapoda  four  stages 
(Claus)  may  be  traced  in  the  best  preserved  larval  histories. 

1.  A  Nauplius  stage  with  the  usual  Nauplius  characters. 

2.  A  Protozoaea  stage  in  which  the  maxillae  and  first  pair  of  maxillipeds 
are  formed  behind  the  Nauplius  appendages  ;  but  in  which  the  tail  is  still 
unsegmented.     This  stage  is  comparatively  rarely  preserved  and  usually  not 
very  well  marked. 

3.  A  Zoaea  stage  the  chief  features  of  which  have  already  been  fully 
characterised  (vide  p.  465).     Three  more  or  less  distinct  types  of  Zosea  are 
distinguished  by  Claus.     (a)  That  of  Penaeus,  in  which  the  appendages  up 
to  the  third  pair  of  maxillipeds  are  formed,  and  the  thorax  and  abdomen  are 
segmented,  the  former  being  however  very  short.     The  heart  is  oval,  with 
one  pair  of  ostia.     From  this  type  the  Zoaea  forms  of  the  other  Decapoda 
are  believed  by  Claus  to  be  derived,     (b}  That  of  Euphausia,  with  but  one 
pair  of  maxillipeds  and  those  short  and  Phyllopod-like.     The  heart  oval 
with   one  pair  of  ostia.     (c)   That   of  Squilla,    with   an  elongated  many- 
chambered  heart,  two  pairs  of  maxillipeds  and  the  abdominal  appendages  in 
full  activity. 

4.  A  Mysis  stage,  which   is   only  found   in   the  macrurous   Decapod 
larvie. 

The  embryological  questions  requiring  to  be  settled  concern  the  value 


CRUSTACEA.  50$ 


of  the  above  stages.  Do  they  represent  stages  in  the  actual  evolution  of 
the  present  types,  or  have  their  characters  been  secondarily  acquired  in 
larval  life  ? 

With  reference  to  the  first  stage  this  question  has  already  been  discussed, 
and  the  conclusion  arrived  at,  that  the  Nauplius  does  in  a  much  modified 
form  represent  an  ancestral  type.  As  to  the  fourth  stage  there  can  be  no 
doubt  that  it  is  also  ancestral,  considering  that  it  is  almost  the  repetition  of 
an  actually  existing  form. 

The  second  stage  can  clearly  only  be  regarded  as  an  embryonic  prepara- 
tion for  the  third  ;  and  the  great  difficulty  concerns  the  third  stage. 

The  natural  view  is  that  this  stage  like  the  others  has  an  ancestral  value, 
and  this  view  was  originally  put  forward  by  Fritz  Miiller  and  has  been 
argued  for  also  by  Dohrn.  On  the  other  hand  the  opposite  side  has  been 
taken  by  Claus,  who  has  dealt  with  the  question  very  ably  and  at  great 
length,  and  has  clearly  shewn  that  some  of  Fritz  Miiller's  positions  are 
untenable.  Though  Claus'  opinion  is  entitled  to  very  great  weight,  an 
answer  can  perhaps  be  given  to  some  of  his  objections.  The  view  adopted 
in  this  section  can  best  be  explained  by  setting  forth  the  chief  points  which 
Claus  urges  against  Fritz  Miiller's  view. 

The  primary  question  which  needs  to  be  settled  is  whether  the  Malacos- 
traca  have  diverged  very  early  from  the  Nauplius  root,  or  later  in  the  history 
of  the  Crustacea  from  the  Phyllopod  stem.  On  this  question  Claus1  brings 
arguments,  which  appear  to  me  very  conclusive,  to  shew  that  the  Malacos- 
traca  are  derived  from  a  late  Protophyllopod  type,  and  Claus'  view  on  this 
point  is  shared  also  by  Dohrn.  The  Phyllopoda  present  so  many  characters 
(not  possessed  by  the  Nauplius)  in  common  with  the  Malacostraca  or  their 
larval  forms,  that  it  is  incredible  that  the  whole  of  these  should  have 
originated  independently  in  the  two  groups.  The  more  important  of  these 
characters  are  the  following. 

1.  The  compound  eyes,  so  often  stalked  in  both  groups. 

2.  The  absence  of  a  palp  on  the  mandible,  a  very  marked  character  of 
the  Zoasa  as  well  as  of  the  Phyllopoda. 

3.  The  presence  of  a  pair  of  frontal  sense  knobs. 

4.  The  Phyllopod  character  of  many  of  the  appendages.    Cf.  first  pair  of 
maxillipeds  of  the  Euphausia  Zosea. 

1  Claus  speaks  of  the  various  Crustacean  phyla  as  having  sprung  from  a  Protophyl- 
lopod form,  and  it  might  be  supposed  that  he  considered  that  they  all  diverged  from 
the  same  form.  It  is  clear  however  from  the  context  that  he  regards  the  Protophyl- 
lopod type  from  which  the  Malacostraca  originated  as  far  more  like  existing  Phyl- 
lopods  than  that  from  which  the  Entomostracan  groups  have  sprung.  It  is  not  quite 
easy  to  get  a  consistent  view  of  his  position  on  the  question,  since  he  states  (p.  77)  that 
the  Malacostraca  and  the  Copepods  diverged  from  a  similar  form,  which  is  represented 
in  their  respective  developments  by  the  Protozosea  and  earliest  Cyclops  stage.  Yet  if 
I  understand  him  rightly,  he  does  not  consider  the  Protozosea  stage  to  be  the  Proto- 
phyllopod stage  from  which  the  Malacostraca  have  diverged,  but  states  on  p.  71  that 
it  was  not  an  ancestral  form  at  all. 


506  PHYLOGENY   OF  THE  CRUSTACEA. 

5.  The  presence  of  gill  pouches  (epipodites)  on  many  of  the  append- 
ages1. 

In  addition  to  these  points,  to  which  others  might  be  added,  Claus 
attempts  to  shew  that  Nebalia  must  be  regarded  as  a  type  intermediate 
between  the  Phyllopods  and  Malacostraca.  This  view  seems  fairly  esta- 
blished, and  if  true  is  conclusive  in  favour  of  the  Phyllopod  origin  of  the 
Malacostraca.  If  the  Protophyllopod  origin  of  the  Malacostraca  is  admitted, 
it  seems  clear  that  the  ancestral  forms  of  the  Malacostraca  must  have  de- 
veloped their  segments  regularly  from  before  backwards,  and  been  provided 
with  nearly  similar  appendages  on  all  the  segments.  This  however  is  far 
from  the  case  in  existing  Malacostraca,  and  Fritz  Miiller  commences  his 
summary  of  the  characters  of  the  Zoaea  in  the  following  words2.  "The 
middle  body  with  its  appendages,  those  five  pairs  of  feet  to  which  these 
animals  owe  their  name,  is  either  entirely  wanting  or  scarcely  indicated." 
This  he  regards  as  an  ancestral  character  of  the  Malacostraca,  and  is  of 
opinion  that  their  thorax  is  to  be  regarded  as  a  later  acquirement  than  the 
head  or  abdomen.  Claus'  answer  on  this  point  is  that  in  the  most  primitive 
Zoasas,  viz.  those  already  spoken  of  as  types,  the  thoracic  and  abdominal 
segments  actually  develop,  in  regular  succession  from  before  backwards, 
and  he  therefore  concludes  that  the  late  development  of  the  thorax  in  the 
majority  of  Zoaea  forms  is  secondary  and  not  an  ancestral  Phyllopod 
peculiarity. 

This  is  the  main  argument  used  by  Claus  against  the  Zosea  having  any 
ancestral  meaning.  His  view  as  to  the  meaning  of  the  Zoaea  may  be 
gathered  from  the  following  passage.  After  assuming  that  none  of  the 
existing  Zoaea  types  could  have  been  adult  animals,  he  says—"  Much  more 
"probably  the  process  of  alteration  of  the  metamorphosis,  which  the  Mala- 
"  costracan  phylum  underwent  in  the  course  of  time  and  in  conjunction 
"  with  the  divergence  of  the  later  Malacostracan  groups,  led  secondarily 
"  to  the  three  different  Zoaea  configurations  to  which  probably  later  modifica- 
"  tions  were  added,  as  for  instance  in  the  young  form  of  the  Cumaceae.  We 
"might  with  the  same  justice  conclude  that  adult  Insects  existed  as  cater- 
"  pillars  or  pupae  as  that  the  primitive  form  of  the  Malacostraca  was  a 
"  Protozoaea  or  Zoaea." 

Granting  Claus'  two  main  positions,  viz.  that  the  Malacostraca  are 
derived  from  Protophyllopods,  and  that  the  segments  were  in  the  primary 
ancestral  forms  developed  from  before  backwards,  it  does  not  appear  im- 
possible that  a  secondary  and  later  ancestral  form  may  have  existed  with  a 
reduced  thorax.  This  reduction  may  only  have  been  partial,  so  that  the 
Zoaea  ancestor  would  have  had  the  following  form.  A  large  cephalo-thorax 
and  well-developed  tail  (?)  with  swimming  appendages.  The  appendages  up 
to  the  second  pair  of  maxillipeds  fully  developed,  but  the  thorax  very 

1  Claus  appears  to  consider  it  doubtful  whether  the  Malacostracan  gills  can  be 
compared  with  the  Phyllopod  gill-pouches. 
3  Facts  for  Darwin,  p.  49. 


CRUSTACEA.  507 


imperfect  and  provided  only  with  delicate  foliaceous  appendages  not  pro- 
jecting beyond  the  edge  of  the  cephalo-thoracic  shield. 

Another  hypothesis  for  which  there  is  perhaps  still  more  to  be  said  is 
that  there  was  a  true  ancestral  Zoaea  stage  in  which  the  thoracic  appendages 
were  completely  aborted.  Claus  maintains  that  the  Zoaea  form  with 
aborted  thorax  is  only  a  larval  form ;  but  he  would  probably  admit  that  its 
larval  characters  were  acquired  to  enable  the  larva  to  swim  better.  If  this 
much  be  admitted  it  is  not  easy  to  see  why  an  actual  member  of  the 
ancestral  series  of  Crustacea  should  not  have  developed  the  Zoaea  pecu- 
liarities when  the  mud-dwelling  habits  of  the  Phyllopod  ancestors  were 
abandoned,  and  a  swimming  mode  of  life  adopted.  This  view,  which 
involves  the  supposition  that  the  five  (or  six  including  the  third  maxillipeds) 
thoracic  appendages  were  lost  in  the  adult  (for  they  may  be  supposed  to 
have  been  retained  in  the  larva)  for  a  series  of  generations,  and  reappeared 
again  in  the  adult  condition,  at  a  later  period,  may  at  first  sight  appear  very 
improbable,  but  there  are,  especially  in  the  larval  history  of  the  Stomatopoda, 
some  actual  facts  which  receive  their  most  plausible  explanation  on  this 
hypothesis. 

These  facts  consist  in  cases  of  the  actual  loss  of  appendages  during 
development,  and  their  subsequent  reappearance.  The  two  most  striking 
cases  are  the  following. 

1.  In  the  Erichthus  form  of  the  Squilla  larva  the  appendages  corre- 
sponding to  the  third  pair  of  maxillipeds  and  first  two  pairs  of  ambulatory 
appendages  of  the  Decapoda  are  developed  in  the  Protozosea  stage,  but 
completely  aborted  in  the  Zoasa  stage,  and  subsequently  redeveloped. 

2.  In  the  case  of  the  larva  of  Sergestes  in  the  passage  from  the  Acan- 
thosoma  (Mysis)  stage  to  the  Mastigopus  stage  the  two  hindermost  thoracic 
appendages  become  atrophied  and  redevelop  again  later. 

Both  of  these  cases  clearly  fit  in  very  well  with  the  view  that  there  was  an 
actual  period  in  the  history  of  the  Malacostraca  in  which  the  ancestors  of 
the  present  forms  were  without  the  appendages  which  are  aborted  and 
redeveloped  again  in  these  larval  forms.  Claus'  hypothesis  affords  no 
explanation  of  these  remarkable  cases. 

It  is  however  always  possible  to  maintain  that  the  loss  and  reappearance 
of  the  appendages  in  these  cases  may  have  no  ancestral  meaning ;  and  the 
abortion  of  the  first  pair  of  maxillipeds  and  reduction  of  some  of  the  other 
appendages  in  the  case  of  the  Loricata  is  in  favour  of  this  explanation. 
Similar  examples  of  the  abortion  and  reappearance  of  appendages,  which 
cannot  be  explained  in  the  way  attempted  above,  are  afforded  by  the  Mites 
and  also  by  the  Insects,  e.g.  Bees. 

On  the  other  hand  there  is  almost  a  conclusive  indication  that  the  loss 
of  the  appendages  in  Sergestes  has  really  the  meaning  assigned  to  it,  in  that 
in  the  allied  genius  Leucifer  the  two  appendages  in  question  are  actually 
absent  in  the  adult,  so  that  the  stage  with  these  appendages  absent  is 
permanently  retained  in  an  adult  form.  In  the  absence  of  the  mandibular 
palp  in  all  the  Zoaea  forms,  its  actual  atrophy  in  the  Penaeus  Zoasa,  and  its 


508  PHYLOGENY  OF  THE  CRUSTACEA. 

universal  reappearance  in  adult  Malacostraca,  are  cases  which  tell  in  favour 
of  the  above  explanation.  The  mandibular  palp  is  permanently  absent  in 
Phyllopods,  which  clearly  shews  that  its  absence  in  the  Zoaea  stage  is  due  to 
the  retention  of  an  ancestral  peculiarity,  and  that  its  reappearance  in  the 
adult  forms  was  a  late  occurrence  in  the  Malacostracan  history. 

The  chief  obvious  difficulty  of  this  view  is  the  redevelopment  of  the 
thoracic  feet  after  their  disappearance  for  a  certain  number  of  generations. 
The  possibility  of  such  an  occurrence  appears  to  me  however  clearly  demon- 
strated by  the  case  of  the  mandibular  palp,  which  has  undoubtedly  been 
reacquired  by  the  Malacostraca,  and  by  the  case  of  the  two  last  thoracic 
appendages  of  Sergestes  just  mentioned.  The  above  difficulty  may  be 
diminished  if  we  suppose  that  the  larvae  of  the  Zoaea  ancestors  always 
developed  the  appendages  in  question.  Such  appendages  might  first  only 
partially  atrophy  in  a  particular  Zoaea  form  and  then  gradually  come  to 
be  functional  again  ;  so  that,  as  a  form  with  functional  thoracic  limbs 
came  to  be  developed  out  of  the  Zoaea,  we  should  find  in  the  larval  history 
of  this  form  that  the  limbs  were  developed  in  the  pre-zoaeal  larval  stages, 
partially  atrophied  in  the  Zoaea  stage,  and  redeveloped  in  the  adult.  From 
this  condition  it  would  not  be  difficult  to  pass  to  a  further  one  in  which  the 
development  of  the  thoracic  limbs  became  deferred  till  after  the  Zoaea  stage. 

The  general  arguments  in  favour  of  a  Zoaea  ancestor  with  partially  or 
completely  aborted  thoracic  appendages  having  actually  existed  in  the  past 
appear  to  me  very  powerful.  In  all  the  Malacostracan  groups  in  which 
the  larva  leaves  the  egg  in  an  imperfect  form  a  true  Zoaea  stage  is  found. 
That  the  forms  of  these  Zoaeas  should  differ  considerably  is  only  what  might 
be  expected,  considering  that  they  lead  a  free  existence  and  are  liable  to 
be  acted  upon  by  natural  selection,  and  it  is  probable  that  none  of  those 
at  present  existing  closely  resemble  the  ancestral  form.  The  spines  from 
their  carapace,  which  vary  so  much,  were  probably  originally  developed, 
as  suggested  by  Fritz  Miiller,  as  a  means  of  defence.  The  simplicity  of 
the  heart— so  different  from  that  of  Phyllopods— in  most  forms  of  Zoaea 
is  a  difficulty,  but  the  reduction  in  the  length  of  the  heart  may  very 
probably  be  a  secondary  modification  ;  the  primitive  condition  being  retained 
in  the  Squilla  Zoaea.  In  any  case  this  difficulty  is  not  greater  on  the 
hypothesis  of  the  Zoaea  being  an  ancestral  form,  than  on  that  of  its  being  a 
purely  larval  one. 

The  points  of  agreement  in  the  number  and  character  of  the  appendages, 
form  of  the  abdomen,  etc.  between  the  various  types  of  Zoaea  appear  to  me 
too  striking  to  be  explained  in  the  manner  attempted  by  Claus.  It  seems 
improbable  that  a  peculiarity  of  form  acquired  by  the  larva  of  some  ancestral 
Malacostracan  should  have  been  retained  so  permanently  in  so  many  groups l 

1  A  secondary  larval  form  is  less  likely  to  be  repeated  in  development  than  an 
ancestral  adult  stage,  because  there  is  always  a  strong  tendency  for  the  former,  which 
is  a  secondarily  intercalated  link  in  the  chain,  to  drop  out  by  the  occurrence  of  a 
reversion  to  the  original  type  of  development. 


CRUSTACEA.  509 


—more  permanently  indeed  than  undoubtedly  ancestral  forms  like  that  of 
Mysis — and  it  would  be  still  more  remarkable  that  a  Zoaea  form  should 
have  been  two  or  more  times  independently  developed. 

There  are  perhaps  not  sufficient  materials  to  reconstruct  the  characters  of 
the  Zoaea  ancestor,  but  it  probably  was  provided  with  the  anterior  appen- 
dages up  to  the  second  pair  of  maxillipeds,  and  (?)  with  abdominal  swim- 
ming feet.  The  heart  may  very  likely  have  been  many-chambered. 
Whether  gill  pouches  were  present  on  the  maxillipeds  and  abdominal  feet 
does  not  appear  to  me  capable  of  being  decided.  The  carapace  and  general 
shape  were  probably  the  same  as  in  existing  Zoaeas.  It  must  be  left  an  open 
question  whether  the  six  hindermost  thoracic  appendages  were  absent  or 
only  very  much  reduced  in  size. 

On  the  whole  then  it  may  be  regarded  as  probable  that  the  Malacostraca 
are  descended  from  Protophyllopod  forms,  in  which,  on  the  adoption  of 
swimming  habits,  six  appendages  of  the  middle  region  of  the  body  were 
reduced  or  aborted,  and  a  Zoaea  form  acquired,  and  that  subsequently  the 
lost  appendages  were  redeveloped  in  the  descendants  of  these  forms,  and 
have  finally  become  the  most  typical  appendages  of  the  group. 

The  relationship  of  the  various  Malacostracan  groups  is  too  difficult 
a  subject  to  be  discussed  here,  but  it  seems  to  me  most  likely  that  in 
addition  to  the  groups  with  a  Zoaea  stage  the  Edriophthalmata  and  Cumaceae 
are  also  post-zoaeal  forms  which  have  lost  the  Zoasa  stage.  Nebalia  is 
however  very  probably  to  be  regarded  as  a  prae-zoaeal  form  which  has 
survived  to  the  present  day ;  and  one  might  easily  fancy  that  its  eight  thin 
thoracic  segments  with  their  small  Phyllopod-like  feet  might  become  nearly 
aborted. 

Copepoda.  The  Copepoda  certainly  appear  to  have  diverged  very 
early  from  the  main  stem,  as  is  shewn  by  their  simple  biramous  feet  and  the 
retention  of  the  median  eye  as  the  sole  organ  of  vision.  It  may  be  argued 
that  they  have  lost  the  eye  by  retrogressive  changes,  and  in  favour  of  this 
view  cases  of  the  Pontellidae  and  of  Argulus  may  be  cited.  It  is  however 
more  than  doubtful  whether  the  lateral  eyes  of  the  Pontellidae  are  related  to 
the  compound  Phyllopod  eye,  and  the  affinities  of  Argulus  are  still  uncertain. 
It  would  moreover  be  a  great  paradox  if  in  a  large  group  of  Crustacea  the 
lateral  eyes  had  been  retained  in  a  parasitic  form  only  (Argulus),  but  lost  in 
all  the  free  forms. 

Cirripedia.  The  Cirripedia  are  believed  by  Claus  to  belong  to  the 
same  phylum  as  the  Copepoda.  This  view  does  not  appear  to  be  completely 
borne  out  by  their  larval  history.  The  Nauplius  differs  very  markedly  from 
that  of  the  Copepoda,  and  this  is  still  more  true  of  the  Cypris  stage.  The 
Copepod-like  appendages  of  this  stage  are  chiefly  relied  upon  to  support  the 
above  view,  but  this  form  of  appendages  was  probably  very  primitive 
and  general,  and  the  number  (without  taking  into  consideration  the  doubtful 
case  of  Cryptophialus)  does  not  correspond  to  that  in  Copepoda.  On  the 
other  hand  the  paired  eyes  and  the  bivalve  shell  form  great  difficulties  in  the 
way  of  Claus'  view.  It  is  clear  that  the  Cypris  stage  represents  more  or  less 


510 


PHYLOGENY  OF  THE  CRUSTACEA. 


closely  an  ancestral  form  of  the  Cirripedia,  and  that  both  the  large  bivalve 
shell  and  the  compound  eyes  were  ancestral  characters.  These  characters 
would  seem  incompatible  with  Copepod  affinities,  but  point  to  the  indepen- 
dent derivation  of  the  Cirripedia  from  some  early  bivalve  Phyllopod  form. 

Ostracoda.  The  independent  origin  of  the  Ostracoda  from  the  main 
Crustacean  stem  seems  probable.  Claus  points  out  that  the  Ostracoda 
present  by  no  means  a  simple  organisation,  and  concludes  that  they  were 
not  descended  from  a  form  with  a  more  complex  organisation  and  a  larger 
number  of  appendages.  Some  simplifications  have  however  undoubtedly 
taken  place,  as  the  loss  of  the  heart,  and  of  the  compound  eyes  in  many 
forms.  These  simplifications  are  probably  to  be  explained  (as  is  done  by 
Claus)  as  adaptations  due  to  the  small  size  of  body  and  its  enclosure  in  a 
thick  bivalve  shell.  Although  Claus  is  strongly  opposed  to  the  view  that 


I) 


FIG.  737.  FIGURES  ILLUSTRATING  THE  DEVELOPMENT  OF  ASTACUS.  (From 
Parker  ;  after  Reichenbach.) 

A.  Section  through  part  of  the  ovum  during  segmentation,  n.  nuclei ;  w.y.  white 
yolk  ;  y.p.  yolk  pyramids ;  c.  central  yolk  mass. 

B  and  C.  Longitudinal  sections  during  the  gastrula  stage,  a.  archenteron  ; 
b.  blastopore ;  ms.  mesoblast ;  ec.  epiblast ;  en.  hypoblast  distinguished  from  epiblast 
by  shading. 

I '.  Highly  magnified  view  of  the  anterior  lip  of  blastopore  to  shew  the  origin  of 
the  primary  mesoblast  from  the  wall  of  the  archenteron.  p.ms.  primary  mesoblast ; 
ec.  epiblast ;  en.  hypoblast. 

I  Two  hypoblast  cells  to  shew  the  amoeba-like  absorption  of  yolk  spheres. 
y.  yolk  ;  ».  nucleus  ;  /.  pseudopodial  process. 

F.  Hypoblast  cells  giving  rise  endogenously  to  the  secondary  mesoblast  (s.nts.). 
tt.  nuclei. 


CRUSTACEA.  5 1 


the  number  of  the  appendages  has  been  reduced,  yet  the  very  fact  of  the 
(in  some  respects)  complex  organisation  of  this  group  might  seem  to  indicate 
that  it  cannot  have  diverged  from  the  Phyllopod  stem  at  so  early  a  stage  as 
(on  Claus'  view  of  the  Nauplius)  would  seem  to  be  implied  by  the  very  small 
number  of  appendages  which  is  characteristic  of  it,  and  it  therefore  appears 
most  probable  that  the  present  number  may  be  smaller  than  that  of  the 
ancestral  forms. 

The  formation  of  the  germinal  layers. 

The  formation  of  the  germinal  layers  has  been  more  fully 
studied  in  various  Malacostraca,  more  especially  in  the  Decapoda, 
than  in  other  groups. 

Decapoda.  To  Bobretzky  (No.  472)  is  due  the  credit  of 
having  been  the  pioneer  in  this  line  of  investigation  ;  and  his 
researches  have  been  followed  up  and  enlarged  by  Haeckel, 
Reichenbach  (No.  488),  and  Mayer  (No.  482).  The  segmentation 
is  centrolecithal  and  regular  (fig.  237  A).  At  its  close  the 
blastoderm  is  formed  of  a  single  uniform  layer  of  lens-shaped 
cells  enclosing  a  central  sphere  of  yolk,  in  which  as  a  rule  all 
trace  of  the  division  into  columns,  present  during  the  earlier 
stages  of  segmentation,  has  disappeared ;  though  in  Palaemon 
the  columns  remain  for  a  long  period  distinct.  The  cells  of  the 
blastoderm  are  at  first  uniform,  but  in  Astacus,  Eupagurus, 
and  most  Decapoda,  soon  become  more  columnar  for  a  small 
area,  and  form  a  circular  patch.  The  whole  patch  either 
becomes  at  once  invaginated  (Eupagurus,  Palaemon,  fig.  239  A) 
or  else  the  edge  of  it  is  invaginated  as  a  roughly  speaking 
circular  groove  deeper  anteriorly  than  posteriorly,  within  which 
the  remainder  of  the  patch  forms  a  kind  of  central  plug,  which 
does  not  become  invaginated  till  a  somewhat  later  period 
(Astacus,  fig.  237  B  and  C).  After  the  invagination  of  the 
above  patch  the  remainder  of  the  blastoderm  cells  form  the 
epiblast. 

The  invaginated  sack  appears  to  be  the  archenteron  and  its 
mouth  the  blastopore.  The  mouth  finally  becomes  closed1,  and 
the  sack  itself  then  forms  the  mesenteron. 

In  Astacus  the  archenteron  gradually  grows  forwards,  its 
opening  is  at  first  wide,  but  becomes  continuously  narrowed 

1  Bobretzky  first  stated  that  the  invagination  remained  open,  but  subsequently 
corrected  himself.  Zeit.  /.  Wiss.  Zool.,  Bd.  xxiv.  p.  186. 


512 


FORMATION   OF  THE    LAYERS. 


and  is  finally  obliterated.  Very  shortly  after  this  occurrence 
there  is  formed,  slightly  in  front  of  the  point  where  the  last  trace 
of  the  blastopore  was  observable,  a  fresh  epiblastic  invagination, 
which  gives  rise  to  the  proctodaeum,  and  the  opening  of  which 
remains  as  the  definite  anus.  The  proctodaeum  (fig.  238  A,  kg) 
is  very  soon  placed  in  communication  with  the  mesenteron  (mg). 
The  stomodaeum  (fg)  is  formed  during  the  same  stage  as  the 
proctodaeum.  It  gives  rise  to  the  oesophagus  and  stomach. 
The  hypoblast  cells  which  form  the  wall  of  the  archenteron 
grow  with  remarkable  rapidity  at  the  expense  of  the  yolk ;  the 
spherules  of  which  they  absorb  and  digest  in  an  amceba-like 
fashion  by  means  of  their  pseudopodia.  They  become  longer 
and  longer,  and  finally,  after  ab- 
sorbing the  whole  yolk,  acquire 
a  form  almost  exactly  similar  to 
that  of  the  yolk  pyramids  dur- 
ing segmentation  (fig.  238  B). 
They  enclose  the  cavity  of  the 
mesenteron,  and  their  nuclei 
and  protoplasm  are  situated  ex- 
ternally. The  cells  of  the  me- 
senteron close  to  its  junction 
with  the  proctodaeum  differ 
from  those  elsewhere  in  being 
nearly  flat. 

In  Palaemon  (Bobretzky) 
the  primitive  invagination  (fig. 
239  A)  has  far  smaller  dimen- 
sions than  in  Astacus,  and  ap- 
pears before  the  blastoderm 
cells  have  separated  from  the 
yolk  pyramids.  The  cells  which 
are  situated  at  the  bottom  of  it 
pass  into  the  yolk,  increase  in 
number,  and  absorb  the  whole 
yolk,  forming  a  solid  mass  of 
hypoblast  in  which  the  outlines 
of  the  individual  cells  would 
seem  at  first  not  to  be  distinct. 


FlG.    238.      TWO  LONGITUDINAL    SEC- 
TIONS   OF    THE    EMBRYO    OF     ASTACUS. 

(From  Parker  ;  after  Bobretzky.) 

A.  Nauplius  stage.  B.  Stage  after 
the  hypoblast  cells  have  absorbed  the 
food  yolk.  The  ventral  surface  is  turned 
upwards,  fg.  stomodseum ;  hg.  procto- 
dccum  ;  an.  anus ;  m.  mouth  ;  mg.  me- 
senteron ;  abd.  abdomen  ;  h.  heart. 

The  blastopore  in  the  mean- 


CRUSTACEA.  513 

time  becomes  closed.  Some  of  the  nuclei  now  pass  to  the 
periphery  of  the  yolk  mass  ;  the  cells  appertaining  to  them 
gradually  become  distinct  and  assume  a  pyramidal  form  (fig. 
239  B,  hy\  the  inner  ends  of  the  cells  losing  themselves  in  a 
central  mass  of  yolk,  in  the  interior  of  which  nuclei  are  at  first 
present  but  soon  disappear.  The  mesenteron  thus  becomes 
constituted  of  a  layer  of  pyramidal  cells  which  merge  into 
a  central  mass  of  yolk.  Some  of  the  hypoblast  cells  adjoining 
the  junction  of  the  proctodaeum  and  mesenteron  become 
flattened,  and  in  the  neighbourhood  of  these  cells  a  lumen 


FlG.  239.      TWO  STAGES  IN  THE  DEVELOPMENT  OF  PAL^MON  SEEN  IN  SECTION. 

(After  Bobretzky.) 

A.  Gastrula  stage. 

B.  Longitudinal  section  through  a  late  stage,     hy.  hypoblast ;  sg.  supra-resopha- 
geal  ganglion  ;  vg.  ventral  nerve  cord  ;  hd.  proctodseum ;  st.  stomodseum. 

first  appears.  The  stomodaeum  and  proctodaeum  are  formed  as 
in  Astacus.  Fig.  239  B  shews  the  relative  positions  of  the 
proctodaeum,  stomodaeum,  and  mesenteron.  Although  the 
process  of  formation  of  the  hypoblast  and  mesenteron  is 
essentially  the  same  in  Astacus  and  Palaemon,  yet  the  differences 
between  these  two  forms  are  very  interesting,  in  that  the  yolk  is 
external  to  the  mesenteron  in  Astacus,  but  enclosed  within  it  in 
Palaemon.  This  difference  in  the  position  of  the  yolk  is  rendered 
possible  by  the  fact  that  the  invaginated  hypoblast  cells  in 
Palaemon  do  not,  at  first,  form  a  continuous  layer  enclosing  a 
central  cavity,  while  they  do  so  in  Astacus. 

The  mesoblast  appears  to  be  formed  of  cells  budded  off 
from  the  anterior  wall  of  the  archenteron  (Astacus,  fig.  237  D), 
B.  II.  33 


514  FORMATION  OF  THE  LAYERS. 

or  from  its  lateral  walls  generally  (Palaemon).  They  make 
their  first  appearance  soon  after  the  imagination  of  the  hypo- 
blast  has  commenced.  The  mesoblast  cells  are  at  first  spherical, 
and  gradually  spread,  especially  in  an  anterior  direction,  from 
their  point  of  origin. 

According  to  Reichenbach  there  are  formed  in  Astacus  at  the  Nauplius 
stage  a  number  of  peculiar  cells  which  he  speaks  of  as  *  secondary  mesoblast 
cells.'  His  account  is  not  very  clear  or  satisfactory,  but  it  appears  that  they 
originate  (fig.  237  F)  in  the  hypoblast  cells  by  a  kind  of  endogenous  growth, 
and  though  they  have  at  first  certain  peculiar  characters  they  soon  become 
indistinguishable  from  the  remaining  mesoblast  cells. 

Towards  the  end  of  the  Nauplius  period  the  secondary  mesoblast  cells 
aggregate  themselves  into  a  rod  close  to  the  epiblast  in  the  median  ventral 
line,  and  even  bifurcate  round  the  mouth  and  extend  forwards  to  the 
extremity  of  the  procephalic  lobes.  This  rod  of  cells  very  soon  vanishes, 
and  the  secondary  mesoblast  cells  become  indistinguishable  from  the 
primary.  Reichenbach  believes,  on  not  very  clear  evidence,  that  these  cells 
have  to  do  with  the  formation  of  the  blood. 

General  form  of  the  body.  The  ventral  thickening  of  epitlast 
or  ventral  plate,  continuous  with  the  invaginated  patch  already 
mentioned,  forms  the  first  indication  of  the  embryo.  It  is  at 
first  oval,  but  soon  becomes  elongated  and  extended  anteriorly 
into  two  lateral  lobes — the  procephalic  lobes.  Its  bilateral 
symmetry  is  further  indicated  by  a  median  longitudinal  furrow. 
The  posterior  end  of  the  ventral  plate  next  becomes  raised  into 
a  distinct  lobe — the  abdomen — which  in  Astacus  at  first  lies  in 
front  of  the  still  open  blastopore.  This  lobe  rapidly  grows  in  size, 
and  at  its  extremity  is  placed  the  narrow  anal  opening.  It  soon 
forms  a  well-marked  abdomen  bent  forwards  over  the  region  in 
front  (figs.  239  B,  and  240  A  and  B).  Its  early  development 
as  a  distinct  outgrowth  causes  it  to  be  without  yolk ;  and  so  to 
contrast  very  forcibly  with  the  anterior  thoracic  and  cephalic 
regions  of  the  body.  In  most  cases  this  process  corresponds  to 
the  future  abdomen,  but  in  some  cases  (Loricata)  it  appears  to 
include  part  of  the  thorax.  Before  it  has  reached  a  considerable 
development,  three  pairs  of  appendages  spring  from  the  region 
of  the  head,  viz.  two  pairs  of  antennae  and  the  mandibles,  and 
inaugurate  a  so-called  Nauplius  stage  (fig.  240  A).  These  three 
appendages  are  formed  nearly  simultaneously,  but  the  hinder- 
most  appears  to  become  visible  slightly  before  the  two  others 


CRUSTACEA.  515 


(Bobretzky).  The  mouth  lies  slightly  behind  the  anterior  pair 
of  antennae,  but  distinctly  in  front  of  the  posterior  pair.  The 
other  appendages,  the  number  of  which  at  the  time  of  hatching 
varies  greatly  in  the  different  Decapods  (vide  section  on  larval 
development),  sprout  in  succession  from  before  backwards  (fig. 
240  B).  The  food  yolk  in  the  head  and  thoracic  region 
gradually  becomes  reduced  in  quantity  with  the  growth  of  the 
embryo,  and  by  the  time  of  hatching  the  disparity  in  size  between 
the  thorax  and  abdomen  has  ceased  to  exist. 

Isopoda.  The  early  embryonic  phases  of  the  Isopoda  have 
been  studied  by  means  of  sections  by  Bobretzky  (No.  498)  and 
Bullar  (No.  499)  and  have  been  found  to  present  considerable 


FlG.  240.      TWO   STAGES   IN   THE   DEVELOPMENT  OF 

A.  Nauplius  stage. 

B.  Stage  with  eight  pairs  of  appendages,   op.  eyes  ;  at1,  and  at*,  first  and  second 
antennae;  md.  mandibles;  mxl,  mx2.  first  and  second  maxillae;  mxp*.  third  maxilli- 
peds  ;  Ib.  upper  lip. 

variations.  When  laid  the  egg  is  enclosed  in  a  chorion,  but 
shortly  after  the  commencement  of  segmentation  (Ed.  van 
Beneden  and  Bullar)  a  second  membrane  appears,  which  is 
probably  of  the  nature  of  a  larval  membrane. 

In  all  the  forms  the  segmentation  is  followed  by  the 
formation  of  a  blastoderm,  completely  enclosing  the  yolk,  and 
thickened  along  an  area  which  will  become  the  ventral  surface  of 
the  embryo.  In  this  area  the  blastoderm  is  formed  of  at  least 
two  layers  of  cells — an  external  columnar  epiblast,  and  an 
internal  layer  of  scattered  cells  which  form  the  mesoblast  and 
probably  in  part  also  the  hypoblast  (Oniscus,  Bobretzky  ;  Cymo- 
thoa,  Bullar). 

33—2 


516  FORMATION   OF  THE  LAYERS. 

In  Asellus  aquaticus  there  is  a  centrolecithal  segmentation, 
ending  in  the  formation  of  a  blastoderm,  which  appears  first 
on  the  ventral  surface  and  subsequently  extends  to  the  dorsal. 

In  Oniscus  murarius,  and  Cymothoa  the  segmentation  is 
partial  [for  its  peculiarities  and  relationship  vide  p.  120]  and  a 
disc,  formed  of  a  single  layer  of  cells,  appears  at  a  pole  of  the 
egg  which  corresponds  to  the  future  ventral  surface  (Bobretzky). 
This  layer  gradually  grows  round  the  yolk  partly  by  division  of 
its  cells,  though  a  formation  of  fresh  cells  from  the  yolk  may 
also  take  place.  Before  it  has  extended  far  round  the  yolk,  the 
central  part  of  it  becomes  two  or  more  layers  deep,  and  the  cells 
of  the  deeper  layers  rapidly  increase  in  number,  and  are  destined 
to  give  rise  to  the  mesoblast  and  probably  also  to  part  or  the 
whole  of  the  hypoblast.  In  Cymothoa  this  layer  does  not  at 
first  undergo  any  important  change,  but  in  Oniscus  it  becomes 
very  thick,  and  its  innermost  cells  (Bobretzky)  become  imbedded 
in  the  yolk,  which  they  rapidly  absorb;  and  increasing  in 
number  first  of  all  form  a  layer  in  the  periphery  of  the  yolk,  and 
finally  fill  up  the  whole  of  the  interior  of  the  yolk  (fig.  241  A), 
absorbing  it  in  the  process. 

It  appears  possible  that  these  cells  do  not,  as  Bobretzky  believes,  origin- 
ate from  the  blastoderm,  but  from  nuclei  in  the  yolk  which  have  escaped 
his  observation.  This  mode  of  origin  would  be  similar  to  that  by  which  yolk 
cells  originate  in  the  eggs  of  the  Insecta,  etc.  If  Bobretzky's  account  is 
correct  we  must  look  to  Palaemon,  as  he  himself  suggests,  to  find  an  explana- 
tion of  the  passage  of  the  hypoblast  cells  into  the  yolk.  The  thickening  of 
the  primitive  germinal  disc  would,  according  to  this  view,  be  equivalent  to 
the  invagination  of  the  archenteron  in  Astacus,  Palaemon,  etc. 

Whatever  may  be  the  origin  of  the  cells  in  the  yolk  they  no 
doubt  correspond  to  the  hypoblast  of  other  types.  In  Cymothoa 
nothing  similar  to  them  has  been  met  with,  but  the  hypoblast 
has  a  somewhat  different  origin  ;  being  apparently  formed  from 
some  of  the  indifferent  cells  below  the  epiblast,  which  collect  as 
a  solid  mass  on  the  ventral  surface,  and  then  divide  into  two 
masses  which  become  hollow  and  give  rise  to  the  liver  caeca. 
Their  fate,  as  well  as  that  of  the  hypoblast  in  Oniscus,  is  dealt 
with  in  connection  with  the  alimentary  tract.  The  completion 
of  the  enclosure  of  the  yolk  by  the  blastoderm  takes  place  on 
the  dorsal  surface.  In  all  the  Isopods  which  have  been  carefully 


CRUSTACEA.  517 


studied,  there  appears  before  any  other  organ  a  provisional 
structure  formed  from  the  epiblast  and  known  as  the  dorsal 
organ.  An  account  of  it  is  given  in  connection  with  the  de- 
velopment of  the  organs.  The  general  external  changes  under- 
gone by  the  larva  in  its  development  are  as  follows.  The 
ventral  thickened  area  of  the  blastoderm  (ventral  plate)  shapes 
itself  and  girths  nearly  the  whole  circumference  of  the  ovum  in 
Oniscus  (fig.  241  A)  but  is  relatively  much  shorter  in  Cymothoa. 
Anteriorly  it  dilates  into  the  two  procephalic  lobes.  In 
Cymothoa  it  next  becomes  segmented;  and  the  anterior  seg- 
ments are  formed  nearly  simultaneously,  and  those  of  the 
abdomen  somewhat  later.  At  the  same  time  a  median  depres- 


FlG.  241.      TWO  LONGITUDINAL  SECTIONS   THROUGH  THE  EMBRYO  OF  ONISCUS 

MURARIUS.     (After  Bobretzky.) 

st.  stomodaeum ;  pr.  proctodseum  ;  hy.  hypoblast  formed  of  large  nucleated  cells 
imbedded  in  the  yolk  ;  m.  mesoblast ;  vg.  ventral  nerve  cord  ;  sg.  supra- oesophageal 
ganglion ;  li.  liver ;  do.  dorsal  organ  ;  zp.  rudiment  of  masticatory  apparatus  ;  ol.  upper 
lip. 

sion  appears  dividing  the  blastoderm  longitudinally  into  two 
halves.  The  appendages  are  formed  later  than  their  segments, 
and  the  whole  of  them  are  formed  nearly  simultaneously,  with 
the  exception  of  the  last  thoracic,  which  does  not  appear  till 
comparatively  late  after  the  hatching  of  the  embryo.  The  late 
development  of  the  seventh  thoracic  segment  and  appendage  is 
a  feature  common  to  the  majority  of  the  Isopoda  (Fritz  Miiller). 
In  Oniscus  the  limbs  are  formed  in  nearly  the  same  way  as  in 
Cymothoa,  but  in  Asellus  they  do  not  arise  quite  simultaneously. 
First  of  all,  the  two  antennae  and  mandibles  (the  future  palp) 
appear,  inaugurating  a  stage  often  spoken  of  as  the  Nauplius 
stage,  which  is  supposed  to  correspond  with  the  free  Nauplius 


5l8  FORMATION   OF  THE  LAYERS. 

stage  of  Penaeus  and  Euphausia.  At  this  stage  a  cuticle  is  shed 
(Van  Beneden)  which  remains  as  an  envelope  surrounding  the 
larva  till  the  time  of  hatching.  Similar  cuticular  envelopes  are 
formed  in  many  Isopoda.  Subsequently  the  appendages  of  the 
thorax  appear,  and  finally  those  of  the  abdomen.  Later  than 
the  appendages  there  arise  behind  the  mouth  two  prominences 
which  resemble  appendages,  but  give  rise  to  a  bilobed  lower  lip 
(Dohrn). 

In  Asellus  and  Oniscus  the  ventral  plate  moulds  itself  to  the 
shape  of  the  egg,  and  covers  the  greater  part  of  the  dorsal  as 
well  as  of  the  ventral  side  (fig.  241  A).  As  a  result  of  this  the 
ventral  surface  of  the  embryo  is  throughout  convex ;  and  in 
Asellus  a  deep  fold  appears  on  the  back  of  the  embryo,  so  that 
the  embryo  appears  coiled  up  within  the  egg  with  its  ventral 
side  outwards  and  its  head  and  tail  in  contact.  In  Oniscus  the 
ventral  surface  is  convex,  but  the  dorsal  surface  is  never  bent  in 
as  in  Asellus.  In  Cymothoa  the  egg  is  very  big  and  the 
ventral  plate  does  not  extend  nearly  so  far  round  to  the  dorsal 
side  as  in  Asellus,  in  consequence  of  which  the  ventral  surface 
is  not  nearly  so  convex  as  in  other  Isopoda.  At  the  same  time 
the  telson  is  early  formed,  and  is  bent  forwards  so  as  to  lie 
on  the  under  side  of  the  part  of  the  blastoderm  in  front.  In 
having  this  ventral  curvature  of  the  telson  Cymothoa  forms 
an  exception  amongst  Isopods ;  and  in  this  respect  is  interme- 
diate between  the  embryos  of  Asellus  and  those  of  the 
Amphipoda. 

Amphipoda.  Amongst  the  Amphipoda  the  segmentation 
is  usually  centrolecithal.  In  the  case  of  Gammarus  locusta 
(Ed.  van  Beneden  and  Bessels,  No.  503)  it  commences  with 
an  unequal  but  total  segmentation  like  that  of  the  Frog  (vide  p. 
97),  and  the  separation  of  a  central  yolk  mass  is  a  late  occur- 
rence ;  and  it  is  noticeable  that  the  part  of  the  egg  with  the 
small  segments  eventually  becomes  the  ventral  surface.  In  the 
fresh-water  species  of  Gammarus  (G.  pulex  and  fluviatilis)  the 
segmentation  is  more  like  that  of  Insects  ;  the  blastoderm  cells 
being  formed  nearly  simultaneously  over  a  large  part  of  the 
surface  of  the  egg. 

Both  forms  of  segmentation  give  rise  to  a  blastoderm  cover- 
ing the  whole  egg,  which  soon  becomes  thickened  on  the  ventral 


CRUSTACEA.  519 


surface.  There  is  formed,  as  in  the  Isopoda,  a  larval  membrane 
at  about  the  time  when  the  blastoderm  is  completed.  Very 
soon  after  this  the  egg  loses  its  spherical  shape,  and  becomes 
produced  into  a  pointed  extremity — the  future  abdomen — which 
is  immediately  bent  over  the  ventral  surface  of  the  part  in  front. 
The  ventral  curvature  of  the  hinder  part  of  the  embryo  at  so 
early  an  age  stands  in  marked  contrast  to  the  usual  condition  of 
Isopod  embryos,  and  is  only  approached  in  this  group,  so  far  as 
is  known,  in  the  case  of  Cymothoa. 

At  the  formation  of  the  first  larval  membrane  the  blastoderm 
cells  separate  themselves  from  it,  except  at  one  part  on  the 
dorsal  surface.  The  patch  of  cells  adherent  at  this  part  gives  rise 
to  a  dorsal  organ,  comparable  with  that  in  Oniscus,  connecting 
the  embryo  and  its  first  larval  skin.  A  perforation  appears  in  it 
at  a  later  period. 

The  segments  and  limbs  of  the  Amphipoda  are  all  formed 
before  the  larva  leaves  the  egg. 

Cladocera.  The  segmentation  (Grobben,  No.  455)  takes  place  on  the 
normal  centrolecithal  type,  but  is  somewhat  unequal.  Before  the  close  of 
the  segmentation  there  may  be  seen  at  the  apex  of  the  vegetative  pole  one 
cell  marked  off  from  the  remainder  by  its  granular  aspect.  It  gives  rise 
to  the  generative  organs.  One  of  the  cells  adjoining  it  gives  rise  to  the 
hypoblast,  and  the  other  cells  which  surround  it  form  the  commencement 
of  the  mesoblast.  The  remaining  cells  of  the  ovum  form  the  epiblast.  By 
a  later  stage  the  hypoblast  cell  is  divided  into  thirty-two  cells  and  the  genital 
cell  into  four,  while  the  mesoblast  forms  a  circle  of  twelve  cells  round  the 
genital  mass. 

The  hypoblast  soon  becomes  involuted  ;  the  blastopore  probably  closes, 
and  the  hypoblast  forms  a  solid  cord  of  cells  which  eventually  becomes  the 
mesenteron.  The  stomodaeum  is  said  to  be  formed  at  the  point  of  closure 
of  the  blastopore.  The  mesoblast  passes  inwards  and  forms  a  mass  ad- 
joining the  hypoblast,  and  somewhat  later  the  genital  mass  also  becomes 
covered  by  the  epiblast.  The  proctodseum  appears  to  be  formed  later  than 
the  stomodasum. 

The  embryo  as  first  shewn  by  Dohrn  passes  through  a  Nauplius  stage 
in  the  brood-pouch,  but  is  hatched,  except  in  the  case  of  the  winter  eggs  of 
Leptodora,  in  a  form  closely  resembling  the  adult. 

Copepoda.  Amongst  the  free  Copepoda  the  segmentation  and 
formation  of  the  layers  have  recently  been  investigated  by  Hoek  (No.  512). 
He  finds  that  there  is,  in  both  the  fresh-water  and  marine  forms  studied 
by  him,  a  centrolecithal  segmentation  similar  to  that  of  Palaemon  and 
Pagurus  (vide  p.  112),  which  might  from  the  surface  be  supposed  to  be 


520  FORMATION  OF  THE  LAYERS. 

complete  and  nearly  regular.  After  the  formation  of  the  blastoderm  an 
invagination  of  some  of  its  cells  takes  place  and  is  completed  in  about  a 
quarter  of  an  hour.  The  opening  becomes  closed.  This  invagination  is 
compared  by  Hoek  to  the  invagination  in  Astacus,  and  is  believed  by  him 
to  give  rise  to  the  mesenteron.  Its  point  of  closing  corresponds  with  the 
hind  end  of  the  embryo.  On  the  ventral  surface  there  appear  two  trans- 
verse furrows  dividing  the  embryo  into  three  segments,  and  a  median 
longitudinal  furrow  which  does  not  extend  to  the  front  end  of  the  foremost 
segment.  The  three  pairs  of  Nauplius  appendages  and  upper  lip  become 
subsequently  formed  as  outgrowths  from  the  sides  of  the  ventral  blasto- 
dermic  thickening. 

Amongst  the  parasitic  Copepoda  there  are  found  two  distinct  types  of 
segmentation,  analogous  to  those  in  the  Isopoda.  In  the  case  of  Condra- 
canthus  the  segmentation  is  somewhat  irregular,  but  on  the  type  of  Eupa- 
gurus,  etc.  (vide  p.  112).  In  the  other  group  (Anchorella,  Clavella,  Congeri- 
cola,  Caligus,  Lerneopoda)  the  segmentation  nearly  resembles  the  ordinary 
meroblastic  type  (vide  p.  120),  and  is  to  be  explained  in  the  same  manner  as 
in  the  cases  of  Oniscus  and  Cymothoa.  The  first  blastodermic  cells  some- 
times appear  in  a  position  corresponding  with  the  head  end  of  the  embryo 
(Anchorella),  at  other  times  at  the  hind  end  (Clavella),  and  sometimes  in  the 
middle  of  the  ventral  surface.  The  dorsal  surface  of  the  yolk  is  always 
the  latest  to  be  inclosed  by  the  blastoderm  cells.  A  larval  cuticle  similar 
to  that  of  the  Isopoda  is  formed  at  the  same  time  as  the  blastoderm.  At 
the  sides  of  the  ventral  thickening  of  the  blastoderm  there  grow  out  the 
Nauplius  appendages,  of  which  only  the  first  two  appear  in  Anchorella. 
In  Anchorella  and  Lerneopoda  the  embryos  are  not  hatched  at  the 
Nauplius  stage,  but  after  the  Nauplius  appendages  have  been  formed 
a  fresh  cuticle — the  Nauplius  cuticle — is  shed,  and  within  it  the  embryo 
develops  till  it  reaches  the  so-called  Cyclops  stage  (vide  p.  490).  The 
embryo  within  the  egg  has  its  abdomen  curved  dorsalwards  as  amongst  the 
Isopoda. 

Cinipedia.  The  segmentation  of  Balanus  and  Lepas  commences  by 
the  segregation  of  the  constituents  of  the  egg  into  a  more  protoplasmic 
portion,  and  a  portion  formed  mainly  of  food  material.  The  former  sepa- 
rates from  the  latter  as  a  distinct  segment,  and  then  divides  into  two  not 
quite  equal  portions.  The  division  of  the  protoplasmic  part  of  the  embryo 
continues,  and  the  resulting  segments  grow  round  the  single  yolk  segment. 
The  point  where  they  finally  enclose  it  is  situated  on  the  ventral  surface 
(Lang)  at  about  the  position  of  the  mouth  (?). 

After  being  enclosed  by  the  protoplasmic  cells  the  yolk  divides,  and  gives 
rise  to  a  number  of  cells,  which  probably  supply  the  material  for  the  walls  of 
the  mesenteron.  The  external  layer  of  protoplasm  forms  the  so-called 
blastoderm,  and  soon  (Arnold,  Lang)  becomes  thickened  on  the  dorsal 
surface. 

The  embryo  is  next  divided  by  two  constrictions  into  three  segments ; 
and  there  are  formed  the  three  appendages  corresponding  to  these,  which  are 


CRUSTACEA.  52! 


at  first  simple.     The  two  posterior  soon  become  biramous.    The  larva  leaves 
the  egg  before  any  further  appendages  become  formed. 

Comparative  development  of  the  organs. 

Central  nervous  system.  The  ventral  nerve  cord  of  the 
Crustacea  develops  as  a  thickening  of  the  epiblast  along  the 
median  ventral  line ;  the  differentiation  of  which  commences  in 
front,  and  thence  extends  backwards.  The  ventral  cord  is  at 
first  unsegmented.  The  supra-oesophageal  ganglia  originate  as 
thickenings  of  the  epiblast  of  the  procephalic  lobes. 

The  details  of  the  above  processes  are  still  in  most  cases  very  imper- 
fectly known.  The  fullest  account  we  have  is  that  of  Reichenbach  (No.  488) 
for  Astacus.  He  finds  that  the  supra- cesophageal  ganglia  and  ventral  cord 
arise  as  a  continuous  formation,  and  not  independently  as  would  seem  to  be 
the  case  in  Chsetopoda.  The  supra-cesophageal  ganglia  are  formed  from  the 
procephalic  lobes.  The  first  trace  of  them  is  visible  in  the  form  of  a  pair  of 
pits,  one  on  each  side  of  the  middle  line.  These  pits  become  in  the 
Nauplius  stage  very  deep,  and  their  walls  are  then  continued  into  two  ridges 
where  the  epiblast  is  several  cells  deep,  which  pass  backwards  one  on  each 
side  of  the  mouth.  The  walls  of  the  pits  are  believed  by  Reichenbach  to 
give  rise  to  the  optic  portions  of  the  supra-cesophageal  ganglia,  and  the 
epiblastic  ridges  to  the  remainder  of  the  ganglia  and  to  the  circum-cesopha- 
geal  commissures.  At  a  much  later  stage,  when  the  ambulatory  feet  have 
become  formed,  a  median  involution  of  epiblast  in  front  of  the  mouth  and 
between  the  two  epiblast  ridges  gives  rise  to  a  central  part  of  the  supra- 
cesophageal  ganglia.  Five  elements  are  thus  believed  by  Reichenbach  to  be 
concerned  in  the  formation  of  these  ganglia,  viz.  two  epiblast  pits,  two 
epiblast  ridges,  and  an  involution  of  epiblast  between  the  latter.  It  should 
be  noted  however  that  the  fate  neither  of  the  pair  of  pits,  nor  of  the  median 
involution,  appears  to  have  been  satisfactorily  worked  out.  The  two 
epiblast  ridges,  which  pass  back  from  the  supra-cesophageal  ganglia  on 
each  side  of  the  mouth,  are  continued  as  a  pair  of  thickenings  of  the  epiblast 
along  the  sides  of  a  median  ventral  groove.  This  groove  is  deep  in  front 
and  shallows  out  posteriorly.  The  thickenings  on  the  sides  of  this  groove 
no  doubt  give  rise  to  the  lateral  halves  of  the  ventral  cord,  and  the  cells  of 
the  groove  itself  are  believed  by  Reichenbach,  but  it  appears  to  me  without 
sufficient  evidence,  to  become  invaginated  also  and  to  assist  in  forming  the 
ventral  cord.  When  the  ventral  cord  becomes  separated  from  the  epiblast 
the  two  halves  of  it  are  united  in  the  middle  line,  but  it  is  markedly  bilobed 
in  section. 

In  the  Isopoda  it  would  appear  both  from  Bobretzky's  and  Bullar's 
observations  that  the  ventral  nerve  cord  arises  as  an  unpaired  thickening  of 
the  epiblast  in  which  there  is  no  trace  of  anything  like  a  median  involution. 
After  this  thickening  has  become  separated  from  the  epiblast  a  slight 


522  DEVELOPMENT  OF  ORGANS. 

median  furrow  indicates  its  constitution  out  of  two  lateral  cords.  The 
supra-oesophageal  ganglia  are  stated  to  be  developed  quite  simply  as  a  pair 
of  thickenings  of  the  procephalic  lobes,  but  whether  they  are  from  the 
first  continuous  with  the  ventral  cord  does  not  appear  to  have  been  deter- 
mined. 

The  later  stages  in  the  differentiation  of  the  ventral  cord  are, 
so  far  as  is  known,  very  similar  throughout  the  Crustacea.  The 
ventral  cord  is,  as  has  been  stated,  at  first  unsegmented  (fig.  241 
A,  vg\  but  soon  becomes  divided  by  a  series  of  constrictions  into 
as  many  ganglia  as  there  are  pairs  of  appendages  or  segments 
(fig.  241  B,  vg). 

There  appears  either  on  the  ventral  side  (Oniscus)  or  in  the 
centre  (Astacus,  Palaemon)  of  the  two  halves  of  each  segment  or 
ganglion  a  space  filled  with  finely  punctuated  material,  which  is 
the  commencement  of  the  commissural  portion  of  the  cords. 
The  commissural  tissue  soon  becomes  continuous  through  the 
length  of  the  ventral  cord,  and  is  also  prolonged  into  the  supra- 
cesophageal  ganglia. 

After  the  formation  of  the  commissural  tissue  the  remaining 
cells  of  the  cord  form  the  true  ganglion  cells.  A  gradual 
separation  of  the  ganglia  next  takes  place,  and  the  cells  become 
confined  to  the  ganglia,  which  are  finally  only  connected  by  a 
double  band  of  commissural  tissue.  The  commissural  tissue  not 
only  gives  rise  to  the  longitudinal  cords  connecting  the  successive 
ganglia,  but  also  to  the  transverse  commissures  which  unite  the 
two  halves  of  the  individual  ganglia. 

The  ganglia  usually,  if  not  always,  appear  at  first  to  corre- 
spond in  number  with  the  segments,  and  the  smaller  number  so 
often  present  in  the  adult  is  due  to  the  coalescence  of  originally 
distinct  ganglia. 

Organs  of  special  sense.  Comparatively  little  is  known  on 
this  head.  The  compound  eyes  are  developed  from  the  coales- 
cence of  two  structures,  both  however  epiblastic,  viz.  (i)  part  of 
the  superficial  epiblast  of  the  procephalic  lobes ;  (2)  part  of  the 
supra-cesophageal  ganglia.  The  former  gives  rise  to  the  corneal 
lenses,  the  crystalline  cones,  and  the  pigment  surrounding 
them ;  the  latter  to  the  rhabdoms  and  the  cells  which  encircle 
them.  Between  these  two  parts  a  mesoblastic  pigment  is  inter- 
posed. 


CRUSTACEA. 


523 


Of  the  development  of  the  auditory  and  olfactory  organs 
almost  nothing  is  known. 

Dorsal  organ.  In  a  considerable  number  of  the  Malacostraca 
and  Branchiopoda  a  peculiar  organ  is  developed  from  the  epiblast 
in  the  anterior  dorsal  region.  This  organ  has  been  called  the 
dorsal  organ.  It  appears  to  be  of  a  glandular  nature,  and  is 
usually  very  large  in  the  embryo  or  larva  and  disappears  in  the 
adult ;  but  in  some  Branchiopoda  it  persists  through  life.  In 
most  cases  it  is  unpaired,  but  in  some  instances  a  paired  organ 
appears  to  take  its  place. 

Various  views  as  to  its  nature  have  been  put  forward.  There 
is  but  little  doubt  of  its  being  glandular,  and  it  is  possible  that  it 
is  a  provisional  renal  organ,  though  so  far  as  I  know  concretions 
have  not  yet  been  found  in  it. 

Its  development  has  been  most  fully  studied  in  the  Isopoda. 

In  Cymothoa  (Bullar,  No.  499)  there  appears  on  the  dorsal  surface,  in  the 
region  which  afterwards  becomes  the  first  thoracic  segment,  an  unpaired 
linear  thickening  of  the  blastoderm.  This  soon  becomes  a  circular  patch, 
the  central  part  of  which  is  inva- 
ginated  so  as  to  communicate 
with  the  exterior  by  a  narrow 
opening  only  (fig.  242).  It  be- 
comes at  the  same  time  attached 
to  the  inner  egg  membrane.  It 
retains  this  condition  till  the  close 
of  larval  life. 

In  Oniscus  (Dohrn,  No.  500 ; 
Bobretzky,  No.  498)  there  appears 
very  early  a  dorsal  patch  of  thick- 
ened cells.  These  cells  become 
attached  at  their  edge  to  the 
inner  egg  membrane  and  gradu- 
ally separated  from  the  embryo, 
with  which  they  finally  only  re-  ,  FlG-  W-  DIAGRAMMATIC  SECTION  OF 
.  ,  ...  CYMOTHOA  SHEWING  THE  DORSAL  ORGAN. 
main  in  connection  by  a  hollow  (From  Bullar.) 

column  of  cells  (fig.  241  A,  do). 

The  original  patch  now  gradually  spreads  over  the  inner  egg  membrane,  and 
forms  a  transverse  saddle-shaped  band  of  flattened  cells  which  engirths  the 
embryo  on  all  but  the  ventral  side. 

In  the  Amphipods  the  epiblast  cells  remain  attached  for  a  small  area  on 
the  dorsal  surface  to  the  first  larval  skin,  when  this  is  formed.  This  patch 
of  cells,  often  spoken  of  as  a  micropyle  apparatus,  forms  a  dorsal  organ 
equivalent  to  that  in  Oniscus.  A  perforation  is  formed  in  it  at  a  later 


524 


DEVELOPMENT  OF  ORGANS. 


period.    A  perhaps  homologous  structure  is  found  in  the  embryos  of  Euphau- 
sia,  Cuma,  etc. 

In  many  Branchiopoda  a  dorsal  organ  is  found.  Its  development  has 
been  studied  by  Grob- 
ben  in  Moina.  It 
persists  in  the  adult 
in  Branchipus,  Lim- 
nadia,  Estherea,  etc. 

In  the  Copepoda 
a  dorsal  organ  is 
sometimes  found  in 
the  embryo ;  Grob- 
ben  at  any  rate  be- 
lieves that  he  has 
detected  an  organ  of 
this  nature  in  the 
embryo  of  Cyclops 
serrulatus. 

A  paired  organ 
which  appears  to  be 


FIG.  243.  DIAGRAMMATIC  SECTION  OF  AN  EMBRYO 
OF  ASELLUS  AQUATICUS  TO  SHEW  THE  PAIRED  DORSAL 
ORGAN.  (From  Bullar ;  after  E.  van  Beneden.) 


of  the  same  nature 
has  been  found  in 
Asellus  and  Mysis. 

In  Asellus  (Rathke  (No.  501),  Dohrn  (No.  500),  Van  Beneden  (No.  497)) 
this  organ  originates  as  two  cellular  masses  at  the  sides  of  the  body  just 
behind  the  region  of  the  procephalic  lobes.  Each  of  them  becomes  trifoliate 
and  bends  towards  the  ventral  surface.  In  each  of  their  lobes  a  cavity 
arises  and  finally  the  three  cavities  unite,  forming  a  trilobed  cavity  open  to 
the  yolk.  This  organ  eventually  becomes  so  large  that  it  breaks  through  the 
egg  membranes  and  projects  at  the  sides  of  the  embryo  (fig.  243\  Though 
formed  before  the  appendages  it  does  not  attain  its  full  development  till 
considerably  after  the  latter  have  become  well  established. 

In  Mysis  it  appears  during  the  Nauplius  stage  as  a  pair  of  cavities  lined 
by  columnar  cells,  which  atrophy  very  early. 

Various  attempts  have  been  made  to  identify  organs  in  other  Arthropod 
embryos  with  the  dorsal  organ  of  the  Crustacea,  but  the  only  organ  at  all 
similar  which  has  so  far  been  described  is  one  found  in  the  embryo  of  Lingu- 
atula  (vide  Chapter  XIX.),  but  there  is  no  reason  to  think  that  this  organ  is 
really  homologous  with  the  dorsal  organ  of  the  Crustacea. 


The  mesoblast.  The  mesoblast  in  the  types  so  far  investi- 
gated arises  from  the  same  cells  as  the  hypoblast,  and  appears 
as  a  somewhat  irregular  layer  between  the  epiblast  and  the 
hypoblast.  It  gives  rise  to  the  same  parts  as  in  other  forms,  but 
it  is  remarkable  that  it  does  not,  in  most  Decapods  and  Isopods 


CRUSTACEA.  525 


(and  so  far  we  do  not  know  about  other  forms),  become  divided 
into  somites,  at  any  rate  with  the  same  distinctness  that  is  usual 
in  Annelids  and  Arthropods.  Not  only  so,  but  there  is  at  first 
no  marked  division  into  a  somatic  and  splanchnic  layer  with  an 
intervening  body  cavity.  Some  of  the  cells  become  differentiated 
into  the  muscles  of  the  body  wall  and  limbs ;  and  other  cells, 
usually  in  the  form  of  a  very  thin  layer,  into  the  muscles  of  the 
alimentary  tract.  In  the  tail  of  Palcsmon  Bobretzky  noticed 
that  the  cells  about  to  form  the  muscles  of  the  body  were 
imperfectly  divided  into  cubical  masses  corresponding  with  the 
segments ;  which  however,  in  the  absence  of  a  central  cavity, 
differed  from  typical  mesoblastic  somites.  In  Mysis  Metschni- 
koff  states  that  the  mesoblast  becomes  broken  up  into  distinct 
somites.  Further  investigations  on  this  subject  are  required. 
The  body  cavity  has  the  form  of  irregular  blood  sinuses  amongst 
the  internal  organs. 

Heart.  The  origin  and  development  of  the  heart  and  vascular  system 
are  but  very  imperfectly  known. 

In  Phyllopods  (Branchipus)  Claus  (No.  454)  has  shewn  that  the  heart  is 
formed  by  the  coalescence  of  the  lateral  parts  of  the  mesoblast  of  the  ventral 
plates.  The  chambers  are  formed  successively  as  the  segments  to  which 
they  belong  are  established,  and  the  anterior  chambers  are  in  full  activity 
while  the  posterior  are  not  yet  formed. 

In  Astacus  and  Palaemon,  Bobretzky  finds  that  at  the  stage  before  the 
heart  definitely  appears  there  may  be  seen  a  solid  mass  of  mesoblast  cells 
in  the  position  which  it  eventually  occupies1;  and  considers  it  probable  that 
the  heart  originates  from  this  mass.  At  the  time  when  the  heart  can  first 
be  made  out  and  before  it  has  begun  to  beat,  it  has  the  form  of  an  oval  sack 
with  delicate  walls  separated  from  the  mesenteron  by  a  layer  of  splanchnic 
mesoblast.  Its  cavity  is  filled  with  a  peculiar  plasma  which  also  fills  up  the 
various  cavities  in  the  mesoblast.  Around  it  a  pericardial  sack  is  soon 
formed,  and  the  walls  of  the  heart  become  greatly  thickened.  Four  bands 
pass  off  from  the  heart,  two  dorsalwards  which  become  fixed  to  the 
integument,  and  two  ventralwards.  There  is  also  a  median  band  of  cells 
connecting  the  heart  with  the  dorsal  integument.  The  main  arteries  arise 
as  direct  prolongations  of  the  heart.  Dohrn's  observations  on  Asellus 
greatly  strengthen  the  view  that  the  heart  originates  from  a  solid  meso- 
blastic mass,  in  that  he  was  able  to  observe  the  hollowing  out  of  the  mass  in 

1  Reichenbach  describes  these  cells,  and  states  that  there  is  a  thickening  of  the 
epiblast  adjoining  them.  In  one  place  he  states  that  the  heart  arises  from  this  thicken- 
ing of  epiblast,  and  in  another  that  it  arises  from  the  mesoblast.  An  epiblastic  origin 
of  the  heart  is  extremely  improbable. 


526  DEVELOPMENT  OF  ORGANS. 

the  living  embryo  (cf.  the  development  of  the  heart  in  Spiders).  Some  of  the 
central  cells  (nuclei,  Dohrn)  become  blood  corpuscles.  The  formation  of 
these  is  not,  according  to  Dohrn,  confined  to  the  heart,  but  takes  place  in 
situ  in  all  the  parts  of  the  body  (antennae,  appendages,  etc.).  The  corpuscles 
are  formed  as  free  nuclei  and  are  primarily  derived  from  the  yolk,  which  at 
first  freely  communicates  with  the  cavities  of  the  appendages. 

Alimentary  tract.  In  Astacus  the  formation  of  the  mesenteron  by 
invagination,  and  the  absorption  of  the  yolk  by  the  hypoblast  cells,  have 
already  been  described.  On  the  absorption  of  the  yolk  the  mesenteron  has 
the  form  of  a  sack,  the  walls  of  which  are  formed  of  immensely  long  cells — 
the  yolk  pyramids — at  the  base  of  which  the  nucleus  is  placed  (fig.  238  B). 
This  sack  gives  rise  both  to  the  portion  of  the  alimentary  canal  between  the 
abdomen  and  the  stomach  and  to  the  liver.  The  epithelial  wall  of  both  of 
these  parts  is  formed  by  the  outermost  portions  of  the  pyramids  with  the 
nuclei  and  protoplasm  becoming  separated  off  from  the  yolk  as  a  layer  of 
flat  epithelial  cells.  The  yolk  then  breaks  up  and  forms  a  mass  of  nutritive 
material  filling  up  the  cavity  of  the  mesenteron. 

The  differentiation  both  of  the  liver  and  alimentary  tract  proper  first 
takes  place  on  the  ventral  side,  and  commences  close  to  the  point  where  the 
proctodasum  ends,  and  extends  forward  from  this  point.  A  layer  of  epithelial 
cells  is  thus  formed  on  the  ventral  side  of  the  mesenteron  which  very  soon 
becomes  raised  into  a  series  of  longitudinal  folds,  one  of  which  in  the 
middle  line  is  very  conspicuous.  The  median  fold  eventually,  by  uniting 
with  a  corresponding  fold  on  the  dorsal  side,  gives  rise  to  the  true  mesente- 
ron ;  while  the  lateral  folds  form  parallel  hepatic  cylinders,  which  in  front 
are  not  constricted  off  from  the  alimentary  tract.  The  lateral  parts  of  the 
dorsal  side  of  the  mesenteron  similarly  give  rise  to  hepatic  cylinders.  The 
yolk  pyramids  of  the  anterior  part  of  the  mesenteron,  which  projects 
forwards  as  a  pair  of  diverticula  on  each  side  to  the  level  of  the  stomach,  are 
not  converted  into  hepatic  cylinders  till  after  the  larva  is  hatched. 

The  proctodasum  very  early  opens  into  the  mesenteron,  but  the  stomo- 
daeum  remains  closed  till  the  differentiation  of  the  mid-gut  is  nearly 
completed.  The  proctodaeum  gives  rise  to  the  abdominal  part  of  the  intes- 
tine, and  the  stomodaeum  to  the  oesophagus  and  stomach.  The  commence- 
ment of  the  masticatory  apparatus  in  the  latter  appears  very  early  as  a 
dorsal  thickening  of  the  epithelium. 

The  primitive  mesenteron  in  Palaemon  differentiates  itself  into  the 
permanent  mid-gut  and  liver  in  a  manner  generally  similar  to  that  in 
Astacus,  though  the  process  is  considerably  less  complicated.  A  distinct 
layer  of  cells  separates  itself  from  the  outer  part  of  the  yolk  pyramids, 
and  gives  rise  to  the  glandular  lining  both  of  the  mid-gut  and  of  the  liver. 
The  differentiation  of  this  layer  commences  behind,  and  the  mid-gut  very 
soon  communicates  freely  with  the  proctodasum.  The  lateral  parts  of 
the  primitive  mesenteron  become  constricted  into  four  wings,  two  directed 
forwards  and  two  backwards  ;  these,  after  the  yolk  in  them  has  become 
absorbed,  constitute  the  liver.  The  median  part  simply  becomes  the  me- 


CRUSTACEA.  527 


senteron.  The  stomachic  end  of  the  stomodaeum  lies  in  contact  with  the 
mesenteron  close  to  the  point  where  it  is  continued  into  the  hepatic 
diverticula,  and,  though  the  partition-wall  between  the  two  becomes  early 
very  thin,  a  free  communication  is  not  established  till  the  yolk  has  been 
completely  absorbed. 

The  alimentary  tract  in  the  Isopoda  is  mainly  if  not  entirely  formed 
from  the  proctodaeum  and  stomodaeum,  both  of  which  arise  before  any  other 
part  of  the  alimentary  system  as  epiblastic  invaginations,  and  gradually 
grow  inwards  (fig.  244).  In  Oniscus  the  liver  is  formed  as  two  discs 
at  the  surface  of  the  yolk  on  each  side  of  the  anterior  part  of  the  body. 
Their  walls  are  composed  of  cubical  cells  derived  from  the  yolk  cells,  the 


pr 

sr  "aqcagga»w.rt-j_ .-.  f.i~T':. -^a^Mi^  •  .    - ..         >va^^^ 

Vff 

FlG.    244.      TWO   LONGITUDINAL   SECTIONS  THROUGH    THE    EMBRYO    OF    ONISCUS 

MURARIUS.     (After  Bobretzky.) 

st.  stomodaeum  ;  pr.  proctodseum  ;  hy.  hypoblast  formed  of  large  nucleated  cells 
imbedded  in  yolk  ;  m.  mesoblast ;  vg.  ventral  nerve  cord  ;  jr^.  supra- oesophageal  gan- 
glion ;  li.  liver;  do.  dorsal  organ;  zp.  rudiment  of  masticatory  apparatus. 

origin  of  which  was  spoken  of  on  p.  516.  These  two  discs  gradually  take 
the  form  of  sacks  (fig.  244  B,  li.)  freely  open  on  their  inner  side  to  the 
yolk.  As  these  sacks  continue  to  grow  the  stomodaeum  and  proctodaeum 
do  not  remain  passive.  The  stomodaeum,  which  gives  rise  to  the  oesophagus 
and  stomach  of  the  adult,  soon  exhibits  a  posterior  dilatation  destined  to 
become  the  stomach,  on  the  dorsal  wall  of  which  a  well-marked  prominence 
— the  earliest  trace  of  the  future  armature — is  soon  formed  (fig.  244  B, 
xp}.  The  proctodaeum  (pr)  grows  with  much  greater  rapidity  than  the 
stomodaeum,  and  its  end  adjoining  the  yolk  becomes  extremely  thin  or  even 
broken  through.  In  the  earliest  stages  it  was  surrounded  by  the  yolk  cells, 
but  in  its  later  growth  the  yolk  cells  become  gradually  reduced  in  number 
and  appear  to  recede  before  it — so  much  so  that  one  is  led  to  conclude 
that  the  later  growth  of  the  proctodaeum  takes  place  at  the  expense  of  the 
yolk  cells. 

The  liver  sacks  become  filled  with  a  granular  material  without  a  trace 
of  cells  ;  their  posterior  wall  is  continuous  with  the  yolk  cells,  and  their 
anterior  lies  close  behind  the  stomach.  The  proctodaeum  continually 
grows  forwards  till  it  approaches  close  to  the  stomodaeum,  and  the  two 


528  DEVELOPMENT  OF  ORGANS. 

liver  sacks,  now  united  into  one  at  their  base,  become  directly  continuous 
with  the  proctodaeum.  By  the  stage  when  this  junction  is  effected  the  yolk 
cells  have  completely  disappeared.  It  seems  then  that  in  Oniscus  the  yolk 
cells  (hypoblast)  are  mainly  employed  in  giving  rise  to  the  walls  of  the 
liver ;  but  that  they  probably  also  supply  the  material  for  the  later  growth 
of  the  apparent  proctodaeum.  It  becomes  therefore  necessary  to  conclude 
that  the  latter,  which  might  seem,  together  with  the  stomodasum,  to  form 
the  whole  alimentary  tract,  does  in  reality  correspond  to  the  proctodaeum 
and  mesenteron  together,  though  the  digestive  fluids  are  no  doubt  mainly 
secreted  not  in  the  mesenteron  but  in  the  hepatic  diverticula.  The  procto- 
daeum and  stomodaeum  at  first  meet  each  other  without  communicating,  but 
before  long  the  partition  between  the  two  is  broken  through. 

In  Cymothoa  (Bullar,  No.  499)  the  proctodaeum  and  stomodaeum 
develop  in  the  same  manner  as  in  Oniscus,  but  the  hypoblast  has  quite 
a  different  form.  The  main  mass  of  the  yolk,  which  is  much  greater  than 
in  Oniscus,  is  not  contained  in  definite  yolk  cells,  but  the  hypoblast  is 
represented  by  (i)  two  solid  masses  of  cells,  derived  apparently  from  the 
inner  layer  of  blastoderm  cells,  which  give  rise  to  the  liver  ;  and  (2)  by  a 
membrane  enclosing  the  yolk  in  which  nuclei  are  present. 

The  two  hepatic  masses  lie  on  the  surface  of  the  yolk,  and  each  of  them 
becomes  divided  into  three  short  caecal  tubes  freely  open  to  the  yolk. 
The  stomodaeum  soon  reaches  its  full  length,  but  the  proctodaeum  grows 
forwards  above  the  yolk  till  it  meets  the  stomodaeum.  By  the  time  this 
takes  place  the  liver  caeca  have  grown  into  three  large  tubes  filled  with 
fluid,  and  provided  with  a  muscular  wall.  They  now  lie  above  the  yolk, 
and  no  longer  communicate  directly  with  the  cavity  of  the  yolk  sack, 
but  open  together  with  the  yolk  sack  into  the  point  of  junction  of  the 
proctodaeum  and  stomodaeum.  The  yolk  sack  of  Cymothoa  no  doubt 
represents  part  of  the  mesenteron,  but  there  is  no  evidence  in  favour  of 
any  part  of  the  apparent  proctodaeum  representing  it  also,  though  it  is 
quite  possible  that  it  may  do  so.  The  relations  of  the  yolk  sack  and  hepatic 
diverticula  in  Cymothoa  appear  to  hold  good  for  Asellus  and  probably  for 
most  Isopoda. 

The  differences  between  the  Decapods  and  Isopods  in  the  development 
of  the  mesenteron  are  not  inconsiderable,  but  they  are  probably  to  be 
explained  by  the  relatively  larger  amount  of  food  yolk  in  the  latter  forms. 
The  solid  yolk  in  the  Isopods  on  this  view  represents  the  primitive  mesen- 
teron of  Decapods  after  the  yolk  has  been  absorbed  by  the  hypoblast  cells. 
Starting  from  this  standpoint  we  find  that  in  both  groups  the  lateral  parts  of 
the  mesenteron  become  the  liver.  In  Decapods  the  middle  part  becomes 
directly  converted  into  the  mid-gut,  the  differentiation  of  it  commencing 
behind  and  proceeding  forwards.  In  the  Isopods,  owing  to  the  mesenteron 
not  having  a  distinct  cavity,  the  differentiation  of  it,  which  proceeds  forwards 
as  in  Decapods,  appears  simply  like  a  prolongation  forwards  of  the  procto- 
da?um,  the  cells  for  the  prolongation  being  probably  supplied  from  the  yolk. 
In  Cymothoa  the  food  yolk  is  so  bulky  that  a  special  yolk  sack  is  developed 


CRUSTACEA. 


529 


for  its  retention,  which  is  not  completely  absorbed  till  some  time  after  the 
alimentary  canal  has  the  form  of  a  continuous  tube.  The  walls  of  this  yolk 
sack  are  morphologically  a  specially  developed  part  of  the  mesenteron. 

BIBLIOGRAPHY. 

General  Works. 

(447)  C.  Spence  Bate.     "  Report  on  the  present  state  of  our  knowledge  of  the 
Crustacea."     Report  of  the  British  Association  for  1878. 

(448)  C.  Claus.     Untersuchungen  zur  Erforschung der  genealogischen  Grundlage 
des  Crustaceen- Systems.     Wien,  1876. 

(449)  A.    Dohrn.     "Geschichte  des   Krebsstammes. "     Jenaische    Zeitschrift, 
Vol.  VI.  1871. 

(450)  A.  Gerstaecker.     Bronris  Thierreich,  Bd.  v.    Arthropoda,  1866. 

(451)  Th.    H.    Huxley.      The  Anatomy  of  Invertebrated  Animals.     London, 
1877. 

(452)  Fritz  Mliller.    Fur  Darwin,   1864.     Translation,  Facts  for  Darwin. 
London,  1869. 

Branchiopoda. 

(453)  Brauer.     "Vorlaufige  Mittheilung  iiber  die  Entwicklung  u.  Lebensweise 
des  Lepidurus  (Apus)  productus."     Sitz.  der  Ak.  d.  Wiss.  Wien,  Vol.  LXIX.,  1874. 

(454)  C.  Claus.     "Zur  Kenntniss  d.  Baues  u.  d.  Entwicklung  von  Branchipus 
stagnalisu.  Apus  cancriformis."  Abh.  d.  kb'nig.  Gesell.  der  Wiss.  Gb'ttingen,  Vol.  XVIII. 
1873. 

(455)  C.    Grobben.     "Zur    Entwicklungsgeschichte    d.    Moina    rectirostris." 
Arbeit,  a.  d.  zoologisch.  Institute  Wien,  Vol.  II.,  1879. 

(456)  E.  Grube.     " Bemerkungen  uber  die  Phyllopoden  nebst  einer  Uebersicht 
etc."     Archivf.  Naturgeschichte,  Vol.  xix.,  1853. 

(457)  N.  Joly.    "  Histoire  d'un  petit  Crustace  (Artemia  salina,  Leach}  etc."    An- 
nales  d.  Sciences  Natur.,  2nd  ser.,  Vol.  xiii.,  1840. 

(458)  N.  Joly.    "  Recherches  zoologiques   anatomiques   et   physiologiques  sur 
1'Isaura  cycladoides  (  =  Estheria)  nouveau  genre,  etc."    Annales  d.  Sciences  Nat.,  2nd 
ser.,  Vol.  xvii.,  1842. 

(459)  Lereboullet.    "  Observations  sur  la  generation  et  le  developpement  de  la 
Ltmnadia  de  Hermann."     Annales  d.  Sciences  Natur.,  <$th  ser.,  Vol.  v.,  1866. 

(460)  F.  Leydig.     "  Ueber  Artemia  salina  u.  Branchipus  stagnalis."     Zeit.  f. 
wiss.  ZooL,  Vol.  in.,  1851. 

(461)  G.  O.  Sars.     "  Om  en  dimorph  Udvikling  samt   Generationsvexel  hos 
Leptodora."     Vidensk.  Selskab.  Forhand,  1873. 

(462)  G.  Zaddach.     De  apodis  cancreformis  Schaeff.  anatome  et  historia  evolu- 
tionis.     Dissertatio  inanguralis  zootomica.     Bonnse,  1841. 

Nebaliadce. 

(463)  C.   Claus.     "  Ueber  den  Bau  u.  die  systematische  Stellung  von  Nebalia." 
Zeit.f.  wiss.  Zool.,  Bd.  xxn.  1872. 

(464)  E.  Metschnikoff.     Development  of  Nebalia  (Russian),  1868. 

B.  II.  34 


530  BIBLIOGRAPHY. 


Schizopoda. 

(465)  E.  van  Beneden,  "  Recherches  sur  1'Embryogenie  des  Crustaces.  n. 
DeVeloppement  des  Mysis."  Bullet,  de  rAcadtmie  roy.  de  Belgique,  second  series, 
Tom.  xxvin.  1869. 

(46G)     C.  Glaus.     "  Ueber  einige  Schizopoden  u.  niedere  Malakostraken."    Zett. 
f.  wiss.  Zoologie,  Bd.  XII I.,  1863. 

(467)  A.  Dohrn.    "  Untersuchungen  Ub.  Bau  u.  Entwicklung  d.  Arthropoden." 
Zeit.f.  wiss.  Zool.y  Bd.  XXL,  1871,  .p.  375.     Peneus  zoaea  (larva  of  Euphausia). 

(468)  E.  Metschnikoff.     "  Ueber  ein  Larvenstadium  von  Euphausia."     Zeit. 
fiir  wiss.  Zool.,  Bd.  xix.,  1869. 

(469)  E.  Metschnikoff.    "  Ueber  den  Naupliuszustand  von  Euphausia. "    Zeit. 
fiir  wiss.  Zool.,  Bd.  XXI.,  1871. 

Decapoda. 

(470)  S pence   Bate.     "On  the  development  of  Decapod  Crustacea."     Phil. 
Trans.,  1858. 

(471)  Spence  Bate.    "  On  the  development  of  Pagurus."  Ann.  and  Mag.  Nat. 
History,  Series  4,  Vol.  II.,  1868. 

(472)  N.    Bobretzky.     Development  of  Astacus  and  Palamon.     Kiew,  1873. 
(Russian.) 

(473)  C.  Glaus.     "Zur  Kenntniss  d.  Malakostrakenlarven. "     Wiirzb.  naturw. 
Zeitschrift,  1861. 

(474)  R.  Q.  Couch.     "On  the  Metamorphosis  of  the  Decapod  Crustaceans." 
Report  Cornwall  Polyt.  Society.  1848. 

(475)  Du  Cane.     "On  the  Metamorphosis  of  Crustacea."    Ann.  and  Mag.  of 
Nat.  History,  1839. 

(476)  Walter  Faxon.    "  On  the  development  of  Palsemonetes  vulgaris."    Bull, 
of  the  Mus.  of  Camp.  Anat.  Harvard,  Cambridge,  Mass.,  Vol.  v.,  1879. 

(477)  A.  Dohrn.    "  Untersuchungen  lib.  Bau  u.  Entwicklung  d.  Arthropoden." 
"  Zur   Entwicklungsgeschichte  der   Panzerkrebse.     Scyllarus  Palinurus."    Zeit.  f. 
wiss.  Zool.,  Bd.  xx.,  1870. 

(478)  A.  Dohrn.     "Untersuchungen  lib.  Bau  u.  Entwicklung  d.  Arthropoden. 
Erster  Beitrag  z.  Kenntniss  d.  Malacostraken  u.  ihrer  Larven  Amphion  Reynaudi, 
Lophogaster,  Portunus,  Porcellanus,  Elaphocaris. "    Zeit.  f.  wiss.  Zool.,  Bd.   xx., 
1870. 

(479)  A.  Dohrn.     "Untersuchungen  lib.  Bau  u.  Entwicklung  d.  Arthropoden. 
Zweiter  Beitrag,  etc."     Zeit.f.  wiss.  Zool.,  Bd.  xxi.,  1871. 

(480)  N.  Joly.     "  Sur  la  Caridina  Desmarestii."    Ann.  Scien.  Nat.,  Tom.  xix., 
1843. 

(481)  Lereboullet.    "  Recherches  d .  1'embryologie  comparee  sur le  developpement 
du  Brochet,  de  la  Perche  et  de  1'Ecrevisse."  Mem.  Savans  ktrang.  Paris,  Vol.  xvn., 
1862. 

(482)  P.    Mayer.     "Zur   Entwicklungsgeschichte  d.  Dekapoden."    Jenaische 
Zeitschrift,  Vol.  XI.,  1877. 

(483)  F  r i  t  z  M  u  1 1  e  r.    "  Die  Verwandlung  der  Porcellana."    Archivf.  Natnrge- 
schichte,  1862. 


CRUSTACEA.  531 


(484)  Fritz  Muller.    "  Die  Verwandlungen  d.  Garneelen,"    Archiv  f.  Natur- 
gesch.,  Tom.  xxix. 

(485)  Fritz  Muller.     "  Ueber  die  Naupliusbrut  d.  Garneelen."     Zeit  f.  wiss. 
Zool.,  Bd.  xxx.,  1878. 

(486)  T.  J.  Parker.     "An  account  of  Reichenbach's  researches  on  the  early 
development  of  the  Fresh-water  Crayfish."     Quart.  J.  of  M.  Science,  Vol.  xvin., 
1878. 

(487)  H.  Rathke.     Ueber  die  Bildung  u.  Entivicklung  d.  Flusskrebses.     Leip- 
zig, 1829. 

(488)  H.   Reichenbach.     "  Die  Embryoanlage  u.  erste  Entwicklung  d.  Fluss- 
krebses."    Zeit.f.  wiss.  Zool.,  Vol.  xxix.,  1877. 

(489)  F.  Richters.     "  Ein  Beitrag  zur  Entwicklungsgeschichte  d.  Loricaten." 
Zeit.f.  wiss.  Zool.,  Bd.  xxiil.,  1873. 

(490)  G.  O.  Sars.     "  Om  Hummers  posiembryonale  Udvikling. "     Vidensk  Selsk. 
Fork.     Christiania,  1874. 

(491)  Sidney  J.  Smith.    "  The  early  stages  of  the  American  Lobster. "    Trans, 
of  the  Connecticut  Acad.  of  Arts  and  Sciences,  Vol.  n.,  Part  2,  1873. 

(492)  R.  v.  Willemoes  Suhm.    "  Preliminary  note  on  the  development  of  some 
pelagic  Decapoda."    Proc.  of  Royal  Society,  1876. 

Stomatopoda. 

(493)  W.  K.  Brooks.     "  On  the  larval  stages  of  Squilla  empusa."     Chesapeake 
Zoological  Laboratory,  Scientific  results  of  the  Session  of  1878.     Baltimore,  1879- 

(494)  C.  Claus.     "Die  Metamorphose  der  Squilliden."     Abhand.  der  kbnigl. 
Gesell.  der  Wiss.  zu  Gbttingen,  1871. 

(495)  Fr.  Muller.    "  Bruchstuck  a.  der  Entwicklungsgeschichte  d.  Maulfusser  I. 
und  II."    Archiv  f.  Naturgeschichte,  Vol.  xxvin.,  1862,  and  Vol.  xxix.,  1863. 

Cumacea. 

(496)  A.  Dohrn.     "  Ueber  den  Bau  u.  Entwicklung  d.  Cumaceen."    Jenaische 
Zeitschrift,  Vol.  v.,  1870. 

Isopoda. 

(497)  Ed.  van  Beneden.     "  Recherches  sur  1'Embryogenie  des  Crustaces.     I. 
Asellus  aquaticus."     Bull,  de  FAcad.  roy  Belgique,  2me  serie,  Tom.  xxvni.,  No.  7, 
1869. 

(498)  N.   Bobretzky.     "  Zur  Embryologie  des  Oniscus  murarius."     Zeit.  fur 
wiss.  Zool.,  Bd.  xxiv.,  1874. 

(499)  J.  F.  Bullar.     "On  the  development  of  the  parasitic  Isopoda."     Phil. 
Trans.,  Part  II.,  1878. 

(500)  A.  Dohrn.     "  Die  embryonale  Entwicklung  des  Asellus  aquaticus."    Zeit. 
f.  wiss.  Zool.,  Vol.  xvn.,  1867. 

(501)  H.  Rathke.     Untersuchungen  iiber  die  Bildung  tmd  Entwicklung  der 
Wasser-Assel.     Leipzig,  1832. 

(502)  H.  Rathke.    Zur  Morphologic.    Reisebemerkungen  aus  Taurien.     Riga  u. 
Leipzig,  1837.     (Bopyrus,  Idothea,  Ligia,  lanira.) 

34—2 


532  BIBLIOGRAPHY. 


A  mphipoda. 

(503)  Ed.  van   Beneden  and   E.  Bessels.     "M&noire  sur   la  formation  du 
blastoderme  chez  les  Amphipodes,  les  Lerneens  et  les  Cope"podes."    Classe  des  Sciences 
deTAcad.  roy.  de  Belgique,  Vol.  xxxiv.,  1868. 

(504)  De  la  Valletta  St  George.   "  Studien  iiber  die  Entwicklung  der  Amphi- 
poden."    Abhand.  d.  naturfor.  Gesell.  zu  Halle,  Bd.  v.,  1860. 

Copepoda. 

(505)  E.  van  Beneden  and  E.  Bessels.     "  Me*moire  sur  la  formation  du  blas- 
toderme chez  les  Amphipodes,  les  Lerndens  et  Copepodes."     Classe  des  Sciences  de 
FAcad.  roy.  de  Belgique,  Vol.  xxxiv.,  1868. 

(506)  E.  van  Beneden.    " Recherches sur  1'Embryogenie des Crustaces iv.    An- 
chorella,  Lerneopoda,  Branchiella,  Hessia."    Bull,  de  FAcad.  roy.  de  Belgique,  2me 
serie,  T.  xxix.,  1870. 

(507)  C.  Claus.     Zur  Anatomie  u.  Entwicklungsgeschichte  d.  Copepoden. 

(508)  C.  Claus.     "  Untersuchungen  Uber  die  Organisation  u.  Verwandschaft  d. 
Copepoden."     Wiirzburger  naturwiss.  Zeitschrift,  Bd.  III.,  1862. 

(509)  C.  Claus.     "  Ueber  den  Bau  u.  d.  Entwicklung  von  Achtheres  percarum." 
Zeit.f.  wiss.  Zool.,  Bd.  XL,  1862. 

(510)  C.  Claus.    Die  freilebenden  Copepoden  mit  besonderer  Berucksichtigung  der 
Fauna  Deutschlands,  des  Nordsee  u.  des  Mittelmeeres.     Leipzig,  1863. 

(511)  C.  Claus.    "  Ueber  d.  Entwicklung,  Organisation  u.  systematische  Stellung 
d.  Argulidse."     Zeit.f.  wiss.  Zool.,  Bd.  xxv.,  1875. 

(512)  P.  P.  C.  Hoek.    "  Zur  Entwicklungsgeschichte  d.  Entomostracen."    Nie- 
derldndisches  Archiv,  Vol.  IV.,  1877. 

(513)  N  o  r  d  m  a  n  n.    Mikrographische  Beitrdge  zur  Naturgeschichte  der  ivirbellosen 
l^hiere.    Zweites  Heft.     1832. 

(514)  Salensky.     "  Sphseronella  Leuckartii."    Archivf.  Naturgeschichte,  1868. 

(515)  F.  Vejdovsky.    "Untersuchungen  Ub.  d.  Anat.  u.  Metamorph.  v.  Trache- 
liastes  polycolpus."    Zeit.f.  wiss.  Zool.,  Vol.  xxix.,  1877. 

Cirripedia. 

(516)  C.  Spence  Bate.     "On  the  development  of  the  Cirripedia."    Annals 
and  Mag.  of  Natur.  History.     Second  Series,  Vin.,  1851. 

(517)  E.  van  Beneden.     "  DeVeloppement  des  Sacculines."    Bull,  de  I" Acad. 
roy.  de  Belg.,  1870. 

(518)  C.  Claus.     Die  Cypris-dhnliche  Larve  der  Cifripedien.     Marburg,  1869. 

(519)  Ch.    Darwin.     A    monograph  of  the  sub-class  Cirripedia,  i  Vols.,  Ray 
Society,  1851—4. 

(520)  A.  Dohrn.     •'  Untersuchungen  iiber  Bau  u.  Entwicklung  d.  Arthropoden 
ix.     Eine  neue  Naupliusform   (Archizoea  gigas)."    Zeit.  f.  wiss.  Zool.,  Bd.    xx., 
1870. 

(521)  P.   P.   C.   Hoek.     "Zur  Entwicklungsgeschichte  der  Entomostraken  i. 
Kinbryologie  von  Balanus."     Niederldndisches  Archiv  fur  Zoologie,  Vol.  III.,  1876 — 7. 

(522)  R.  Kossmann.    "Suctoria  u.  Lepadidoc."    Arbeiten  a.  d.  zool.-zoot.  Insti- 
tuted. Univer.  Wiirz.,  Vol.  I.,  1873. 


CRUSTACEA.  533 


(523)  Aug.  Krohn.     "  Beobachtungen  iiber  die  Entwicklung  der  Cirripedien." 
Wiegmanris  Archiv  fur  Naturgesch.,  xxvi.,  1860. 

(524)  E.  Metschnikoff.   Sitzungsberichte d.  Versammlung  deutscher  Naturfors- 
cher  zu  Hannover,  1865.    (Balanus  balanoides.) 

(525)  Fritz    Muller.      "Die    Rhizocephalen."      Archiv  f.    Naturgeschichte, 
1862-3. 

(526)  F.  C.  Noll.    "  Kochlorine  hamata,  ein  bohrendes  Cirriped."    Zeit.f.  wiss. 
Zool.,  Bd.  xxv.,  1875. 

(527)  A.  Pagenstecher.    "  Beitrage  zur  Anatomic  und  Entwicklungsgeschichte 
von  Lepas  pectinata."     Zeit.f.  wiss.  ZooL,  Vol.  xni.,  1863. 

(528)  J.  V.  Thompson.    Zoological  Researches  and  Illustrations,  Vol.  I.,  Part  I. 
Memoir  IV.    On  the  Cirripedes  or  Barnacles.     8vo.  Cork,  1830. 

(529)  J.  V.  Thompson.     "  Discovery  of  the  Metamorphosis  in  the  second  type 
of  the  Cirripedes,  viz.  the  Lepades  completing  the  natural  history  of  these  singular 
animals,  and  confirming  their  affinity  with  the  Crustacea."     Phil.  Trans.  1835.     Part 
n. 

(530)  R.  von  Willemoes  Suhm.    "On  the  development  of  Lepas  fascicularis." 
Phil.  Trans.,  Vol.  166,  1876. 

Ostracoda. 

(531)  C.  Glaus.    "  Zur  naheren  Kenntniss  der  Jugendformen  von  Cypris  ovum." 
Zeit.f.  wiss.  ZooL,  Bd.  xv.,  1865. 

(532)  C.  Glaus.     "Beitrage  zur  Kenntniss  d.  Ostracoden.     Entwicklungsges- 
chichte von  Cypris  ovum."     Schriften  d.  Gesell.  zur  Befdrderung  d.  gesamm.  Natur- 
wiss.  zu  Marburg,  Vol.  IX.,  1868. 


CHAPTER  XIX. 


PCECILOPODA,  PYCNOGONIDA,  TARDIGRADA,  AND  LIN- 
GUATULIDA;  AND  COMPARATIVE  SUMMARY  OF 
ARTHROPODAN  DEVELOPMENT. 

THE  groups  dealt  with  in  the  present  Chapter  undoubtedly 
belong  to  the  Arthropoda.  They  are  not  closely  related,  and  in 
the  case  of  each  group  it  is  still  uncertain  with  which  of  the 
main  phyla  they  should  be  united.  It  is  possible  that  they  may 
all  be  offshoots  from  the  Arachnidan  phylum. 

PCECILOPODA. 

The  development  of  Limulus  has  been  studied  by  Dohrn  (No.  533)  and 
Packard  (No.  534).  The  ova  are  laid  in  the  sand  near  the  spring-tide 
marks.  They  are  enveloped  in  a  thick  chorion  formed  of  several  layers  ; 
and  (during  the  later  stages  of  development  at  any  rate)  there  is  a  mem- 
brane within  the  chorion  which  exhibits  clear  indications  of  cell  outlines1. 

There  is  a  centrolecithal  segmentation,  which  ends  in  the  formation  of 
a  blastoderm  enclosing  a  central  yolk  mass.  A  ventral  plate  is  then 
formed,  which  is  thicker  in  the  region  where  the  abdomen  is  eventually 
developed.  Six  segments  soon  become  faintly  indicated  in  the  cephalo- 
thoracic  region,  the  ends  of  which  grow  out  into  prominent  appendages 
(fig.  245  A)  ;  of  these  there  are  six  pairs,  which  increase  in  size  from  before 
backwards.  A  stomodaeum  (m)  is  by  this  time  established  and  is  placed  well 
in  front  of  the  foremost  pair  of  appendages'*-. 

In  the  course  of  the  next  few  days  the  two  first  appendages  of  the 
abdominal  region  become  formed  (vide  fig.  245  C  shewing  those  abdominal 
appendages  at  a  later  stage),  and  have  a  very  different  shape  and  direction 
to  those  of  the  cephalothorax.  The  appendages  of  the  latter  become 

1  The  nature  of  the  inner  membrane  is  obscure.     It  is  believed  by  Packard  to  be 
moulted  after  the  formation  of  the  limbs,  and  to  be  equivalent  to  the  amnion  of  Insects, 
while  by  Dohrn  it  is  regarded  as  a  product  of  the  follicle  cells. 

2  Dohrn  finds  at  first  only  five  appendages,  but  thinks  that  the  sixth  (the  anterior 
one)  may  have  been  present  but  invisible. 


PCECILOPODA. 


535 


flexed  in  the  middle  in  such  a  way  that  their  ends  become  directed  towards 
the  median  line  (fig.  245  B).  The  body  of  the  embryo  (fig.  245  B)  is 
now  distinctly  divided  into  two  regions— the  cephalothoracic  in  front,  and 
the  abdominal  behind,  both  divided  into  segments. 


FIG.  245.  THREE  STAGES  IN  THE  DEVELOPMENT  OF  LIMULUS  POLYPHEMUS. 
(Somewhat  modified  from  Packard.) 

A.  Embryo  in  which  the  thoracic  limbs  and  mouth  have  become  developed  on 
the  ventral  plate.     The  outer  line  represents  what  Packard  believes  to  be  the  amnion. 

B.  Later  embryo  from  the  ventral  surface. 

C.  Later  embryo,  just  before  the  splitting  of  the  chorion  from  the  side.     The  full 
number  of  segments  of  the  abdomen,  and  three  abdominal  appendages,  have  become 
established ;  m.  mouth  ;  I — IX.  appendages. 

Round  the  edge  of  the  ventral  plate  there  is  a  distinct  ridge — the 
rudiment  of  the  cephalothoracic  shield. 

With  the  further  growth  of  the  embryo  the  chorion  becomes  split 
and  cast  off,  the  embryo  being  left  enclosed  within  the  inner  membrane. 
The  embryo  has  a  decided  ventral  flexure,  and  the  abdominal  region 
grows  greatly  and  forms  a  kind  of  cap  at  the  hinder  end,  while  its 
vaulted  dorsal  side  becomes  divided  into  segments  (fig.  245  C).  Of  these 
there  are  according  to  Dohrn  seven,  but  according  to  Packard  nine,  of 
which  the  last  forms  the  rudiment  of  the  caudal  spine. 

In  the  thoracic  region  the  nervous  system  is  by  this  stage  formed  as 
a  ganglionated  cord  (Dohrn),  with  no  resemblance  to  the  peculiar  cesopha- 
geal  ring  of  the  adult.  The  mouth  is  stated  by  Dohrn  to  lie  between  the 
second  pair  of  limbs,  so  that,  if  the  descriptions  we  have  are  correct,  it  must 
have  by  this  stage  changed  its  position  with  reference  to  the  appendages. 
Between  the  thorax  and  abdomen  two  papillae  have  arisen  which  form  the 


536 


PCEC1LOPODA. 


so-called  lower  lip  of  the  adult,  but  from  their  position  and  late  development 
they  can  hardly  be  regarded  as  segmental  appendages.  In  the  course  of 
further  changes  all  the  parts  become  more  distinct,  while  the  membrane  in 
which  the  larva  is  placed  becomes  enormously  distended  (fig.  246  A).  The 
rudiments  of  the  compound  eyes  are  formed  on  the  third  (Packard)  or  fourth 
(Dohrn)  segment  of  the  cephalothorax,  and  the  simple  eyes  near  the  median 
line  in  front.  The  rudiments  of  the  inner  process  of  the  chelae  of  the  cepha- 
lothoracic  appendages  arise  as  buds.  The  abdominal  appendages  become 
more  plate-like,  and  the  rudiments  of  a  third  pair  appear  behind  the  two 
already  present.  The  heart  appears  on  the  dorsal  surface. 

An  ecdysis  now  takes  place,  and  in  the  stage  following  the  limbs  have 
approached  far  more  closely  to  their  adult  state  (fig.  246  A).  The 
cephalothoracic  appendages  become  fully  jointed ;  the  two  anterior  ab- 
dominal appendages  (vn.)  have  approached,  and  begin  to  resemble  the  oper- 


ce. 


VIII 


FlO.  246.      TWO  STAGES  IN  THE  DEVELOPMENT  OF  LlMULUS  POLYPHEMUS. 

(After  Dohrn.) 

A.  An  advanced  embryo  enveloped  in  the  distended  inner  membrane  shortly 
before  hatching  ;  from  the  ventral  side. 

B.  A  later  embryo  at  the  Trilobite  stage,  from  the  dorsal  side. 
I.,  vii.,  VIII.     First,  seventh,  and  eight  appendages. 

cs.  caudal  spine  ;  se.  simple  eye  ;  ce.  compound  eye. 

culum  of  the  adult,  and  on  the  second  pair  is  formed  a  small  inner  ramus. 
The  segmentation  of  the  now  vaulted  cephalothorax  becomes  less  obvious, 
though  still  indicated  by  the  arrangement  of  the  yolk  masses  which  form 
the  future  hepatic  diverticula. 

Shortly  after  this  stage  the  embryo  is  hatched,  and  at  about  the  time  of 
hatching  acquires  a  form  (fig.  246  B)  in  which  it  bears,  as  pointed  out  by 
Dohrn  and  Packard,  the  most  striking  resemblance  to  a  Trilobite. 

Viewed  from  the  dorsal  surface  (fig.  246  B)  it  is  divided  into  two 
distinct  regions,  the  cephalothoracic  in  front  and  the  abdominal  behind. 
The  cephalothoracic  has  become  much  flatter  and  wider,  has  lost  all  trace 
of  its  previous  segmentation,  and  has  become  distinctly  trilobed.  The 


PCECILOPODA.  537 


central  lobe  forms  a  well-marked  keel,  and  at  the  line  of  insertion  of  the 
rim-like  edge  of  the  lateral  lobes  are  placed  the  two  pairs  of  eyes  (se  and 
ce).  The  abdominal  region  is  also  distinctly  trilobed  and  divided  into  nine 
segments  ;  the  last,  which  is  merely  formed  of  a  median  process,  being  the 
rudiment  of  the  caudal  spine.  The  edges  of  the  second  to  the  seventh  are 
armed  with  a  spine.  The  changes  in  the  appendages  are  not  very  con- 
siderable. The  anterior  pair  nearly  meet  in  the  middle  line  in  front  or 
the  mouth ;  and  the  latter  structure  is  completely  covered  by  an  upper 
lip.  Each  abdominal  appendage  of  the  second  pair  is  provided  with  four 
gill-lamellas,  attached  close  to  its  base. 

Three  weeks  after  hatching  an  ecdysis  takes  place,  and  the  larva  passes 
from  a  trilobite  into  a  limuloid  form.  The  segmentation  of  the  abdomen 
has  become  much  less  obvious,  and  this  part  of  the  embryo  closely  resem- 
bles its  permanent  form.  The  caudal  spine  is  longer,  but  is  still  relatively 
short.  A  fourth  pair  of  abdominal  appendages  is  established,  and  the  first 
pair  have  partially  coalesced,  while  the  second  and  third  pairs  have  become 
jointed,  their  outer  ramus  containing  four  and  their  inner  three  joints. 
Additional  gill-lamellae  attached  to  the  two  basal  joints  of  the  second  and 
third  abdominal  appendages  have  appeared. 

The  further  changes  are  not  of  great  importance.  They  are  effected  in 
a  series  of  successive  moults.  The  young  larvae  swim  actively  at  the 
surface. 

Our,  in  many  respects,  imperfect  knowledge  of  the  development  of 
Limulus  is  not  sufficient  to  shew  whether  it  is  more  closely  related  to  the 
Crustacea  or  to  the  Arachnida,  or  is  an  independent  phylum. 

The  somewhat  Crustacean  character  of  biramous  abdominal  feet,  etc. 
is  not  to  be  denied,  but  at  the  same  time  the  characters  of  the  embryo 
appear  to  me  to  be  decidedly  more  arachnidan  than  crustacean.  The 
embryo,  when  the  appendages  are  first  formed,  has  a  decidedly  arach- 
nidan facies.  It  will  be  remembered  that  when  the  limbs  are  first  formed 
they  are  all  post-oral.  They  resemble  in  this  respect  the  limbs  of  the 
Arachnida,  and  it  seems  to  be  probable  that  the  anterior  pair  is  equivalent 
to  the  cheliceras  of  Arachnida,  which,  as  shewn  in  a  previous  section,  are 
really  post-oral  appendages  in  no  way  homologous  with  antennae1. 

The  six  thoracic  appendages  may  thus  be  compared  with  the  six 
Arachnidan  appendages;  which  they  resemble  in  their  relation  to  the 
mouth,  their  basal  cutting  blades,  etc. 

The  existence  of  abdominal  appendages  behind  the  six  cephalothoracic 
does  not  militate  against  the  Arachnidan  affinities  of  Limulus,  because  in 
the  Arachnida  rudimentary  abdominal  appendages  are  always  present  in 
the  embryo.  The  character  of  the  abdominal  appendages  is  probably 

1  Dohrn  believes  that  he  has  succeeded  in  shewing  that  the  first  pair  of  appendages 
of  Limulus  is  innervated  in  the  embryo  from  the  supra-cesophageal  ganglia.  His 
observations  do  not  appear  to  me  conclusive,  and,  arguing  from  what  we  know  of  the 
development  of  the  Arachnida,  the  innervation  of  these  appendages  in  the  adult  can  be 
of  no  morphological  importance. 


538  PYCNOGONIDA. 


secondarily  adapted  to  an  aquatic  respiration,  since  it  is  likely  (for  the 
reasons  already  mentioned  in  connection  with  the  Tracheata)  that  if  Limulus 
has  any  affinities  with  the  stock  of  the  Tracheata  it  is  descended  from  air- 
breathing  forms,  and  has  acquired  its  aquatic  mode  of  respiration.  The 
anastomosis  of  the  two  halves  of  the  generative  glands  is  an  Arachnidan 
character,  and  the  position  of  the  generative  openings  in  Limulus  is  more 
like  that  in  the  Scorpion  than  in  Crustacea. 

A  fuller  study  of  the  development  would  be  very  likely  to  throw 
further  light  on  the  affinities  of  Limulus,  and  if  Packard's  view  about  the 
nature  of  the  inner  egg  membrane  were  to  be  confirmed,  strong  evidence 
would  thereby  be  produced  in  favour  of  the  Arachnidan  affinities. 

(533)  A.  Dohrn.     "Untersuch.  Ub.  Bau  u.  Entwick.  d.  Arthropoden  (Limulus 
polyphemus)."     Jenaische  Zeitschrift,  Vol.  vi.,  1871. 

(534)  A.  S.  Packard.     "The  development  of  Limulus  polyphemus."    Mem. 
Boston  Soc.  Nat.  History,  Vol.  II.,  1872. 

PYCNOGONIDA. 

The  embryos,  during  the  first  phases  of  their  development,  are  always 
carried  by  the  male  in  sacks  which  are  attached  to  a  pair  of  appendages 
(the  third)  specially  formed  for  this  purpose.  The  segmentation  of  the 
ovum  is  complete,  and  there  is  in  most  forms  developed  within  the  egg- 
shell a  larva  with  three  pairs  of  two-jointed  appendages,  and  a  rostrum 
placed  between  the  front  pair. 

It  will  be  convenient  to  take  Achelia  kevis,  studied  by  Dohrn  (No.  536), 
as  type. 

The  larva  of  Achelia  when  hatched  is  provided  with  the  typical  three 
pairs  of  appendages.  The  foremost  of  them  is  chelate,  and  the  two  follow- 
ing pairs  are  each  provided  with  a  claw.  Of  the  three  pairs  of  larval- 
appendages  Dohrn  states  that  he  has  satisfied  himself  that  the  anterior  is 
innervated  by  the  supra-cesophageal  ganglion,  and  the  two  posterior  by 
separate  nerves  coming  from  two  imperfectly  united  ventral  ganglia.  The 
larva  is  provided  with  a  median  eye  formed  of  two  coalesced  pigment 
spots,  and  with  a  simple  stomach. 

The  gradual  conversion  of  the  larva  into  the  adult  takes  place  by  the 
elongation  of  the  posterior  end  of  the  body  into  a  papilla,  and  the  forma- 
tion there,  at  a  later  period,  of  the  anus ;  while  at  the  two  sides  of  the 
anal  papilla  rudiments  of  a  fresh  pair  of  appendages — the  first  pair  of  am- 
bulatory limbs  of  the  adult — make  their  appearance.  The  three  remaining 
pairs  of  limbs  become  formed  successively  as  lateral  outgrowths,  and  their 
development  is  accomplished  in  a  number  of  successive  ecdyses.  As  they 
are  formed  caeca  from  the  stomach  become  prolonged  into  them.  For  each 
of  them  there  appears  a  special  ganglion.  While  the  above  changes  are 
taking  place  the  three  pairs  of  larval  appendages  undergo  considerable 
reduction.  The  anterior  pair  singly  becomes  smaller,  the  second  loses 
its  claw,  and  the  third  becomes  reduced  to  a  mere  stump.  In  the  adult  the 


PENTASTOMIDA.  539 


second  pair  of  appendages  becomes  enlarged  again  and  forms  the  so-called 
palpi,  while  the  third  pair  develops  in  the  male  into  the  egg-carrying  append- 
ages, but  is  aborted  in  the  female.  The  first  pair  form  appendages  lying 
parallel  to  the  rostrum,  which  are  sometimes  called  pedipalpi  and  some- 
times antennae. 

The  anal  papilla  is  a  rudimentary  abdomen,  and,  as  Dohrn  has  shewn, 
contains  rudiments  of  two  pairs  of  ganglia. 

The  larvae  of  Phoxichilidium  are  parasitic  in  various  Hydrozoa  (Hydrac- 
tinia,  etc.).  After  hatching  they  crawl  into  the  Hydractinia  stock.  They 
are  at  first  provided  with  the  three  normal  pairs  of  larval  appendages.  The 
two  hinder  of  these  are  soon  thrown  off,  and  the  posterior  part  of  the  trunk, 
with  the  four  ambulatory  appendages  belonging  to  it,  becomes  gradually 
developed  in  a  series  of  moults.  The  legs,  with  the  exception  of  the  hinder- 
most  pair,  are  fully  formed  at  the  first  ecdysis  after  the  larva  has  become 
free.  In  the  genus  Pallene  the  metamorphosis  is  abbreviated,  and  the' 
young  are  hatched  with  the  full  complement  of  appendages. 

The  position  of  the  Pycnogonida  is  not  as  yet  satisfactorily  settled. 
The  six-legged  larva  has  none  of  the  characteristic  features  of  the  Nauplius, 
except  the  possession  of  the  same  number  of  appendages. 

The  number  of  appendages  (7)  of  the  Pycnogonida  does  not  coincide 
with  that  of  the  Arachnida.  On  the  other  hand,  the  presence  of  chelate 
appendages  innervated  in  the  adult  by  the  supra-cesophageal  ganglia  rather 
points  to  a  common  phylum  for  the  Pycnogonida  and  Arachnida  ;  though  as 
shewn  above  (p.  455)  all  the  appendages  in  the  embryo  of  true  Arachnida 
are  innervated  by  post-oral  ganglia.  The  innervation  of  these  appendages 
in .  the  larvae  of  Pycnogonida  requires  further  investigation.  Against 
such  a  relationship  the  extra  pair  of  appendages  in  the  Pycnogonida  is 
no  argument,  since  the  embryos  of  most  Arachnida  are  provided  with  four 
such  extra  pairs.  The  two  groups  must  no  doubt  have  diverged  very 
early. 

BIBLIOGRAPHY. 

(535)  G.  Cavanna.     "  Studie  e  ricerche  sui  Picnogonidi."    Pubblicazioni  del  R. 
Institute  di  Studi  stiperiori  in  Firenze,  1877. 

(536)  An.  Dohrn.     "  Ueber  Entwickhuig  u.  Baud.  Pycnogoniden."    Jenaische 
Zeitschrift,  Vol.  v.  1870,  and  "  Neue  Untersuchungen  lib.  Pycnogoniden."    Mitthdl. 
a.  d.  zoologischen  Station  zu  Neafel,  Bd.  I.  1878. 

(537)  G.  Hodge.     "  Observations  on  a  species  of  Pycnogon,  etc."    Annal.  and 
Mag.  of  Nat.  Hist.  Vol.  ix.  1862. 

(538)  C.  Semper.     "  Ueber  Pycnogoniden  u.  ihre  in  Hydroiden  schmarotzenden 
Larvenformen."     Arbeiten  a.  d.  zool.-zoot.  Instit.  Wiirzburg,  Vol.  I.  1874. 

PENTASTOMIDA. 

The  development  and  metamorphosis  of  Pentastomum  taenoides  have 
been  thoroughly  worked  out  by  Leuckart  (No.  540)  and  will  serve  as  type 
for  the  group. 


540  PENTASTOMIDA. 


In  the  sexual  state  it  inhabits  the  nasal  cavities  of  the  dog.  The  early 
embryonic  development  takes  place  as  the  ovum  gradually  passes  down  the 
uterus.  The  segmentation  appears  to  be  complete  ;  and  gives  rise  to  an 
oval  mass  in  which  the  separate  cells  can  hardly  be  distinguished.  This 
gradually  differentiates  itself  into  a  characteristic  embryo,  divided  into  a  tail 
and  trunk.  The  tail  is  applied  to  the  ventral  surface  of  the  trunk,  and  on 
the  latter  two  pairs  of  stump-like  unsegmented  appendages  arise,  each 
provided  with  a  pair  of  claws.  At  the  anterior  extremity  of  the  body  is 
formed  the  mouth,  with  a  ventral  spine  and  lateral  hook,  which  are  perhaps 
degenerated  jaws.  The  spine  functions  as  a  boring  apparatus,  and  an 
apparatus  with  a  similar  function  is  formed  at  the  end  of  the  tail.  A  larval 
cuticle  now  appears,  which  soon  becomes  detached  from  the  embryo,  except 
on  the  dorsal  surface,  where  it  remains  firmly  united  to  a  peculiar  papilla. 
This  papilla  becomes  eventually  divided  into  two  parts,  one  of  which  remains 
attached  to  the  cuticle,  while  the  part  connected  with  the  embryo  forms  a 
raised  cross  placed  in  a  cup- shaped  groove.  The  whole  structure  has  been 
compared,  on  insufficient  grounds,  to  the  dorsal  organ  of  the  Crustacea. 

The  eggs,  containing  the  embryo  in  the  condition  above  described,  are 
eventually  carried  out  with  the  nasal  slime,  and,  if  transported  thence  into 
the  alimentary  cavity  of  a  rabbit  or  hare,  the  embryos  become  hatched  by 
the  action  of  the  gastric  juice.  From  the  alimentary  tract  of  their  new  host 
they  make  their  way  into  the  lungs  or  liver.  They  here  become  enveloped 
in  a  cyst,  in  the  interior  of  which  they  undergo  a  very  remarkable  metamor- 
phosis. They  are,  however,  so  minute  and  delicate  that  Leuckart  was 
unable  to  elucidate  their  structure  till  eight  weeks  after  they  had  been 
swallowed.  At  this  period  they  are  irregularly-shaped  organisms,  with  a 
most  distant  resemblance  to  the  earlier  embryos.  They  are  without  their 
previous  appendages,  but  the  alimentary  tract  is  now  distinctly  differentiated. 
The  remains  of  two  cuticles  in  the  cyst  seem  to  shew  that  the  above  changes 
are  effected  in  two  ecdyses. 

In  the  course  of  a  series  of  ecdyses  the  various  organs  of  the  larval  form 
known  as  Pentastomum  denticulatum  continue  to  become  differentiated. 
After  the  first  (= third)  ecdysis  the  cesophageal  nerve-ring  and  sexually 
undifferentiated  generative  organs  are  developed.  At  the  fourth  (=sixth) 
ecdysis  the  two  pairs  of  hooks  of  the  adult  are  formed  in  pockets  which 
appeared  at  a  somewhat  earlier  stage  ;  and  the  body  acquires  an  annulated 
character.  At  a  somewhat  earlier  period  rudiments  of  the  external  genera- 
tive organs  indicate  the  sex  of  the  larva. 

After  a  number  of  further  ecdyses,  which  are  completed  in  about  six 
months  after  the  introduction  of  the  embryos  into  the  intermediate  host,  the 
larva  attains  its  full  development,  and  acquires  a  form  in  which  it  has  long 
been  known  as  Pentastomum  denticulatum.  It  now  leaves  its  cyst  and 
begins  to  move  about.  It  is  in  a  state  fit  to  be  introduced  into  its  final  host ; 
but  if  it  be  not  so  introduced  it  may  become  encysted  afresh. 

If  the  part  of  a  rabbit  or  hare  infected  by  a  Pentastomum  denticulatum 
be  eaten  by  a  dog  or  wolf,  the  parasite  passes  into  the  nasal  cavity  of  the 


TARDIGRADA.  541 


latter,  and  after  further  changes  of  cuticle  becomes  a  fully-developed  sexual 
Pentastomum  taenioides,  which  does  not  differ  to  any  very  marked  extent 
from  P.  denticulatum. 

In  their  general  characters  the  larval  migrations  of  Pentastomum  are 
similar  to  those  of  the  Cestodes. 

The  internal  anatomy  of  the  adult  Pentastomum,  as  well  as  the 
characters  of  the  larva  with  two  pairs  of  clawed  appendages,  are  perhaps 
sufficient  to  warrant  us  in  placing  it  with  the  Arthropoda,  though  it  would 
be  difficult  to  shew  that  it  ought  not  to  be  placed  with  such  a  form  as 
Myzostomum  (vide  p.  369).  There  do  not  appear  to  be  any  sufficient 
grounds  to  justify  its  being  placed  with  the  Mites  amongst  the  Arachnida. 
If  indeed  the  rings  of  the  body  of  the  Pentastomida  are  to  be  taken  as 
implying  a  true  segmentation,  it  is  clear  that  the  Pentastomida  cannot  be 
associated  with  the  Mites. 

BIBLIOGRAPHY. 

(539)  P.  J.  van  Beneden.    "  Recherches  s.  1'organisation  et  le  developpement  d. 
Linguatules."     Ann.  d.  Sden.  Nat.,  3  Ser.,  Vol.  XI. 

(540)  R.  Leuckart.    "  Bau  u.  Entwicklungsgeschichte  d.  Pentastomen."  Leipzig 
and  Heidelberg.     1860. 

TARDIGRADA. 

Very  little  is  known  with  reference  to  the  development  of  the  Tardigrada. 
A  complete  and  regular  segmentation  (von  Siebold,  Kaufmann,  No.  541)  is 
followed  by  the  appearance  of  a  groove  on  the  ventral  side  indicating  a 
ventral  flexure.  At  about  the  time  of  the  appearance  of  the  groove  the  cells 
become  divided  into  an  epiblastic  investing  layer  and  a  central  hypoblastic 
mass. 

The  armature  of  the  pharynx  is  formed  very  early  at  the  anterior 
extremity,  and  the  limbs  arise  in  succession  from  before  backwards. 

The  above  imperfect  details  throw  no  light  on  the  systematic  position  of 
this  group. 

Tardigrada. 

(541)  J.  Kaufmann.     "  Ueber  die  Entwicklung  u.  systematische  Stellung  d. 
Tardigraden."    Zeit.f.  wiss,  ZooL,  Bd.  HI.  1851. 

Summary  of  Arthropodan  Development. 
The  numerous  characters  common  to  the  whole  of  the 
Arthropoda  led  naturalists  to  unite  them  in  a  common  phylum, 
but  the  later  researches  on  the  genealogy  of  the  Tracheata  and 
Crustacea  tend  to  throw  doubts  on  this  conclusion,  while  there 
is  not  as  yet  sufficient  evidence  to  assign  with  certainty  a 
definite  position  in  either  of  these  classes  to  the  smaller  groups 
described  in  the  present  chapter.  There  seems  to  be  but  little 


542  SUMMARY. 


doubt  that  the  Tracheata  are  descended  from  a  terrestrial  Anne- 
lidan  type  related  to  Peripatus.  The  affinities  of  Peripatus  to 
the  Tracheata  are,  as  pointed  out  in  a  previous  chapter  (p.  386), 
very  clear,  while  at  the  same  time  it  is  not  possible  to  regard 
Peripatus  simply  as  a  degraded  Tracheate,  owing  to  the  fact 
that  it  is  provided  with  such  distinctly  Annelidan  organs  as 
nephridia,  and  that  its  geographical  distribution  shews  it  to  be  a 
very  ancient  form. 

The  Crustacea  on  the  other  hand  are  clearly  descended  from 
a  Phyllopod-like  ancestor,  which  can  be  in  no  way  related  to 
Peripatus. 

The  somewhat  unexpected  conclusion  that  the  Arthropoda 
have  a  double  phylum  is  on  the  whole  borne  out  by  the  anatomy 
of  the  two  groups.  Without  attempting  to  prove  this  in  detail, 
it  may  be  pointed  out  that  the  Crustacean  appendages  are 
typically  biramous,  while  those  of  the  Tracheata  are  never  at 
any  stage  of  development  biramous1;  and  the  similarity  between 
the  appendages  of  some  of  the  higher  Crustacea  and  those  of 
many  Tracheata  is  an  adaptive  one,  and  could  in  no  case  be 
used  as  an  argument  for  the  affinity  of  the  two  groups. 

The  similarity  of  many  organs  is  to  be  explained  by  both 
groups  being  descendants  of  Annelidan  ancestors.  The  simi- 
larity of  the  compound  eye  in  the  two  groups  cannot  however 
be  explained  in  this  way,  and  is  one  of  the  greatest  difficulties 
of  the  above  view.  It  is  moreover  remarkable  that  the  eye  of 
Peripatus2  is  formed  on  a  different  type  to  either  the  single  or 
compound  eyes  of  most  Arthropoda. 

The  conclusion  that  the  Crustacea  and  Tracheata  belong  to 
two  distinct  phyla  is  confirmed  by  a  consideration  of  their 
development.  They  have  no  doubt  in  common  a  centrolecithal 
segmentation,  but,  as  already  insisted  on,  the  segmentation  is 
no  safe  guide  to  the  affinities. 

In  the  Tracheata  the  archenteron  is  never,  so  far  as  we 
know,  formed  by  an  invagination3,  while  in  Crustacea  the 

1  The  biflagellate  antennae  of  Pauropus  amongst  the  Myriapocls  can  hardly  be 
considered  as  constituting  an  exception  to  this  rule. 

3  I  hope  to  shew  this  in  a  paper  I  am  preparing  on  the  anatomy  of  Peripatus. 

8  Stecker's  description  of  an  invagination  in  the  Chilognatha  cannot  be  accepted 
without  further  confirmation  ;  -vide  p.  388. 


SUMMARY.  543 


evidence  is  in  favour  of  such  an  invagination  being  the  usual, 
and,  without  doubt,  the  primitive,  mode  of  origin. 

The  mesoblast  in  the  Tracheata  is  formed  in  connection  with 
a  median  thickening  of  the  ventral  plate.  The  unpaired  plate 
of  mesoblast  so  formed  becomes  divided  into  two  bands,  one  on 
each  side  of  the  middle  line. 

In  both  Spiders  and  Myriopods,  and  probably  Insects,  the 
two  plates  of  mesoblast  are  subsequently  divided  into  somites, 
the  lumen  of  which  is  continued  into  the  limbs. 

In  Crustacea  the  mesoblast  usually  originates  from  the  walls 
of  the  invagination,  which  gives  rise  to  the  mesenteron. 

It  does  not  become  divided  into  two  distinct  bands,  but 
forms  a  layer  of  scattered  cells  between  the  epiblast  and  hypo- 
blast,  and  does  not  usually  break  up  into  somites  ;  and  though 
somites  are  stated  in  some  cases  to  be  found  they  do  not 
resemble  those  in  the  Tracheata. 

The  proctodaeum  is  usually  formed  in  Crustacea  before  and 
rarely  later1  than  the  stomodaeum.  The  reverse  is  true  for  the 
Tracheata.  In  Crustacea  the  proctodseum  and  stomodaeum, 
especially  the  former,  are  very  long,  and  usually  give  rise  to  the 
greater  part  of  the  alimentary  tract,  while  the  mesenteron  is 
usually  short. 

In  the  Tracheata  the  mesenteron  is  always  considerable,  and 
the  proctodaeum  is  always  short.  The  derivation  of  the  Mal- 
pighian  bodies  from  the  proctodaeum  is  common  to  most 
Tracheata.  Such  diverticula  of  the  proctodaeum  are  not  found 
in  Crustacea. 

1  This  is  stated  to  be  the  case  in  Moina  (Grobben). 


CHAPTER   XX. 


ECHINODERMATA1. 

THE  development  of  the  Echinodermata  naturally  falls  into 
two  sections: — 

(i)  The  development  of  the  germinal  layers  and  of  the 
systems  of  organs;  (2)  the  development  of  the  larval  appendages 
and  the  metamorphosis. 

The  Development  of  the  Germinal  Layers  and  of  tJie  Systems 

of  Organs. 

The  development  of  the  systems  of  organs  presents  no  very 
important  variations  within  the  limits  of  the  group. 

Holothuroidea.  The  Holothurians  have  been  most  fully 
studied  (Selenka,  No.  563),  and  may  be  conveniently  taken  as 
type. 

The  segmentation  is  nearly  regular,  though  towards  its  close, 
and  in  some  instances  still  earlier,  a  difference  becomes  apparent 
between  the  upper  and  the  lower  poles. 

At  the  close  of  segmentation  (fig.  247  A)  the  egg  has  a 
nearly  spherical  form,  and  is  constituted  of  a  single  layer  of 
columnar  cells  enclosing  a  small  segmentation  cavity.  The 
lower  pole  is  slightly  thickened,  and  the  egg  rotates  by  means  of 
fine  cilia. 

An  invagination  now  makes  its  appearance  at  the  lower 
pole  (fig.  247  B),  and  simultaneously  there  become  budded  off 
from  tJie  cells  undergoing  the  invagination  amoeboid  cells,  which 

1  The  following  classification  of  the  Echinodermata  is  employed  in  this  chapter. 

I.    Holothuroidea.  IV.    Echinoidea. 

II.    Asteroidea.  V.    Crinoidea. 

III.    Ophiuroidea. 


ECHINODERMATA.  545 


eventually  form  the  muscular  system  and  the  connective  tissue. 
These  cells  very  probably  have  a  bilaterally  symmetrical  origin. 
This  stage  represents  the  gastrula  stage  which  is  common  to  all 
Echinoderms.  The  invaginated  sack  is  the  archenteron.  As  it 
grows  larger  one  side  of  the  embryo  becomes  flattened,  and  the 
other  more  convex.  On  the  flattened  side  a  fresh  invagination 


FIG.    247.      TWO    STAGES    IN     THE    DEVELOPMENT     OF     HOLOTHURIA     TUBULOSA 

VIEWED  IN  OPTICAL  SECTION.     (After  Selenka.) 
A.  Blastosphere  stage  at  the  close  of  segmentation.     B.  Gastrula  stage. 
mr.  micropyle ;  //.  chorion;  s.c.  segmentation  cavity;  bl.  blastoderm;  ep.  epiblast; 
hy.  hypoblast;  ms.  amoeboid  cells  derived  from  hypoblast ;  a.e.  archenteron. 

arises,  the  opening  of  which  forms  the  permanent  mouth,  the 
opening  of  the  first  invagination  remaining  as  the  permanent 
anus  (fig.  248  A). 

These  changes  give  us  the  means  of  attaching  definite  names 
to  the  various  parts  of  the  embryo.  It  deserves  to  be  noted  in 
the  first  place  that  the  embryo  has  assumed  a  distinctly  bilateral 
form.  There  is  present  a  more  or  less  concave  surface  ex- 
tending from  the  mouth  to  near  the  anus,  which  will  be  spoken 
of  as  the  ventral  surface.  The  anus  is  situated  at  the  posterior 
extremity.  The  convex  surface  opposite  the  ventral  surface 
forms  the  dorsal  surface,  which  terminates  anteriorly  in  a 
rounded  prse-oral  prominence. 

It  will  be  noticed  in  fig.  248  A  that  in  addition  to  the 
primitive  anal  invagination  there  is  present  a  vesicle  (?/.). 
This  vesicle  is  directly  formed  by  a  constriction  of  the  primitive 
B.  II.  35 


546 


HOLOTHUROIDEA. 


archenteron  (fig.  249  Vpv.),  and  is  called  by  Selenka  the  vaso- 
peritoneal  vesicle.  It  gives  origin  to  the  epithelioid  lining  of 
the  body  cavity  and  water-vascular  system  of  the  adult1.  In  the 
parts  now  developed  we  have  the  rudiments  of  all  the  adult  organs. 
The  mouth  and  anal  involutions  (after  the  separation  of  the 
vaso-peritoneal  vesicle)  meet  and  unite,  a  constriction  indicating 
their  point  of  junction  (fig.  248  B).  Eventually  the  former  gives 


FIG.  248.     THREE  STAGES  IN  THE  DEVELOPMENT  OF  HOLOTHURIA  TUBULOSA 

VIEWED  FROM  THE  SIDE  IN  OPTICAL  SECTION.    (After  Selenka.) 
tn.  mouth;  oe.  oesophagus;  st.   stomach;  i.  intestine;  a.  anus;   I.e.  longitudinal 
ciliated  band;   v.p.  vaso-peritoneal  vesicle;  p.v.  peritoneal  vesicle;  p.r.  right  peri- 
toneal vesicle ;  //.  left  peritoneal  vesicle ;  w.v.  water- vascular  vesicle ;  p.  dorsal  pore 
of  water- vascular  system ;  ms.  muscle  cells. 

rise  to  the  mouth  and  cesophagus,  and  the  latter  to  the  re- 
mainder of  the  alimentary  canal2. 

The  vaso-peritoneal  vesicle  undergoes  a  series  of  remarkable 
changes.  After  its  separation  from  the  archenteron  it  takes 
up  a  position  on  the  left  side  of  this,  elongates  in  an  antero- 
posterior  direction,  and  from  about  its  middle  sends  a  narrow 
diverticulum  towards  the  dorsal  surface  of  the  body,  where  an 

1  The  origin  of  the  vaso-peritoneal  vesicle  is  not  quite  the  same  in  all  the  species. 
In  Holothuria  tubulosa  it  is  separated  from  the  csecal  end  of  the  archenteron;  the 
remainder  of  which  then  grows  towards  the  oral  invagination.     In  Cucumaria  the 
archenteron  forks  (fig.  249) ;  and  one  fork  forms  the  vaso-peritoneal  vesicle,  and  the 
other  the  major  part  of  the  mesenteron. 

2  There  appears  to  be  some  uncertainty  as  to  how  much  of  the  larval  cesophagus  is 
derived  from  the  stomodaeal  invagination. 


ECHINODERMATA. 


547 


opening  to  the  exterior  becomes  formed  (fig.  248  B,  /.).  The 
diverticulum  becomes  the  madreporic  canal,  and  the  opening 
the  dorsal  pore. 

The  vaso-peritoneal  vesicle  next  divides  into  two,  an  an- 
terior vesicle  (fig.  248  B,  w.v.),  from  which  is  derived  the 
epithelium  of  the  water-vascular  system,  and  a  posterior  (fig. 
248  B, /.?;.),  which  gives  rise  to  the  epithelioid  lining  of  the  body 
cavity.  The  anterior  vesicle  (fig.  248  C,  w.v.)  becomes  five- 
lobed,  takes  a  horseshoe-shaped  form,  and  grows  round  the 
oesophagus  (fig.  256,  w.v.r).  The  five  lobes  form  the  rudiments 
of  the  water-vascular  prolongations  into  the  tentacles.  The 
remaining  parts  of  the  water-vascular  system  are  also  developed 
as  outgrowths  of  the  original  vesicle.  Five  of  these,  alternating 
with  the  original  diverticula,  form  the  five  ambulacral  canals, 
from  which  diverticula  are  produced  into  the  ambulacral  feet ;  a 
sixth  gives  rise  to  the  Polian  vesicle.  The  remaining  parts  of 
the  original  vesicle  form  the  water-vascular  ring. 

We  must  suppose  that  eventually  the  madreporic  canal  loses 
its  connection  with  the  exterior  so  as  to  hang  loosely  in  the 
interior,  though  the  steps  of  this  process  do  not  appear  to 
have  been  made  out. 

The  original  hinder  peri- 
toneal vesicle  grows  rapidly, 
and  divides  into  two  (fig.  248  C, 
pi.  and  pr.},  which  encircle  the 
two  sides  of  the  alimentary 
canal,  and  meet  above  and 
below  it.  The  outer  wall  of 
each  of  them  attaches  itself  to 
the  skin,  and  the  inner  one  to 
the  alimentary  canal  and  water- 
vascular  system  ;  in  both  cases 
the  walls  remain  separated 
from  the  adjacent  parts  by  a 
layer  of  the  amoeboid  cells 
already  spoken  of.  The  cavity 
of  the  peritoneal  vesicles  be- 
comes the  permanent  body 
cavity.  Where  the  walls  of 


-ME 


FIG.  249.    LONGITUDINAL  SECTION 

THROUGH  AN  EMBRYO  OF  CUCUMARIA 
DOLIOLUM  AT  THE  END  OF  THE  FOURTH 
DAY. 

Vpv.  vaso-peritoneal  vesicle;  ME. 
mesenteron;  Blp.,  Ptd.  blastopore,  proc- 
todaeum. 

35—2 


548  HOLOTHUROIDEA. 


the  two  vesicles  meet  on  the  dorsal  side,  a  mesentery,  suspend- 
ing the  alimentary  canal  and  dividing  the  body  cavity  longitu- 
dinally, is  often  formed.  In  other  parts  the  partition  walls 
between  the  two  sacks  appear  to  be  absorbed. 

The  amoeboid  cells,  which  were  derived  from  the  invaginated 
cells,  arrange  themselves  as  a  layer  round  all  the  organs  (fig. 
249).  Some  of  them  remain  amoeboid,  attach  themselves  to  the 
skin,  and  form  part  of  the  cutis;  and  in  these  cells  the  cal- 
careous spicula  of  the  larva  and  adult  are  formed.  Others 
form  the  musculature  of  the  larval  alimentary  tract,  while  the 
remainder  give  rise  to  the  musculature  and  connective  tissue  of 
the  adult. 

The  development  of  the  vascular  system  is  not  known,  but  the  discovery 
of  Kowalevsky,  confirmed  by  Selenka,  that  from  the  walls  of  the  water- 
vascular  system  corpuscles  are  developed,  identical  with  those  in  the  blood- 
vessels, indicates  that  it  probably  develops  in  connection  with  the  water- 
vascular  system.  The  observations  of  Hoffmann  and  Perrier  on  the  commu- 
nication of  the  two  systems  in  the  Echinoidea  point  to  the  same  conclusion. 
Though  nothing  very  definite  is  known  with  reference  to  the  development  of 
the  nervous  system,  Metschnikoff  suggests  that  it  develops  in  connection 
with  the  thickened  bands  of  epiblast  which  are  formed  by  a  metamorphosis 
of  the  ciliated  bands  of  the  embryo,  and  accompany  the  five  radial  tubes 
(vide  p.  555).  In  any  case  its  condition  in  the  adult  leaves  no  doubt  of  its 
being  a  derivative  of  the  epiblast. 

From  the  above  description  the  following  general  conclusions 
may  be  drawn  : — 

(1)  The  blastosphere  stage  is  followed  by  a  gastrula  stage. 

(2)  The  gastrula  opening  forms  the  permanent  anus,  and  the 
mouth  is  formed  by  a  fresh  invagination. 

(3)  The  mesoblast  arises  entirely  from  the  invaginated  cells, 
but  in  two  ways  : — 

(a)  As  scattered  amoeboid  cells,  which  give  origin  to  the 
muscles  and  connective  tissue  (including  the  cutis)  of  the  body 
wall  and  alimentary  tract. 

(&)  As  a  portion  separated  off  from  the  archenteron, 
which  gives  rise  both  to  the  epithelioid  lining  of  the  body  cavity, 
and  of  the  water-vascular  system. 

(4)  The  oesophagus  is  derived  from  an  invagination  of  the 
epiblast,  and  the  remainder  of  the  alimentary  canal  from  the 
archenteron. 


ECHINODERMATA.  549 


(5)  The  embryonic  systems  of  organs  pass  directly  into  those 
of  the  adult. 

The  development  of  Synapta  diverges,  as  might  be  expected,  to  a  very 
small  extent  from  that  of  Holothuria. 

Asteroidea.  In  Asterias  the  early  stages  of  development  conform  to 
our  type.  There  arise,  however,  two  bilaterally  symmetrical  vaso-peritoneal 
diverticula  from  the  archenteron.  These  diverticula  give  rise  both  to  the 
lining  of  the  body  cavity  and  water-vascular  system.  With  reference  to 
the  exact  changes  they  undergo  there  is,  however,  some  difference  of  opinion. 
Agassiz  (543)  maintains  that  both  vesicles  are  concerned  in  the  formation  of 
the  water-vascular  system,  while  Metschnikoff  (560)  holds  that  the  water- 
vascular  system  is  entirely  derived  from  the  anterior  part  of  the  larger  left 
vesicle,  while  the  right  and  remainder  of  the  left  vesicle  form  the  body 
cavity.  MetschnikofFs  statements  appear  to  be  the  most  probable.  The 
anterior  part  of  the  left  vesicle,  after  separating  from  the  posterior,  grows 
into  a  five-lobed  rosette  (fig.  260,  /),  and  a  madreporic  canal  (h]  with  a  dorsal 
pore  opening  to  the  exterior.  The  rosette  appears  not  to  grow  round  the 
oesophagus,  as  in  the  cases  hitherto  described.  But  the  latter  is  stated  to 
disappear,  and  a  new  oesophagus  to  be  formed,  which  pierces  the  rosette, 
and  places  the  old  mouth  in  communication  with  the  stomach.  Except 
where  the  anus  is  absent  in  the  adult,  the  larval  anus  probably  persists. 

Ophiuroidea.  The  early  development  of  the  Ophiuroidea  is  not  so 
fully  known  as  that  of  other  types.  Most  species  have  a  free-swimming 
larva,  but  some  (Amphiura)  are  viviparous. 

The  early  stages  of  the  free-swimming  larvae  have  not  been  described, 
but  I  have  myself  observed  in  the  case  of  Ophiothrix  fragilis  that  the 
segmentation  is  uniform,  and  is  followed  by  the  normal  invagination.  The 
opening  of  this  no  doubt  remains  as  the  larval  anus,  and  there  are  probably 
two  outgrowths  from  this  to  form  the  vaso-peritoneal  vesicles.  Each  of  these 
divides  into  two  parts,  an  anterior  lying  close  to  the  oesophagus,  and  a 
posterior  close  to  the  stomach.  The  anterior  on  the  right  side  aborts  ;  that 
on  the  left  side  becomes  the  water-vascular  vesicle,  early  opens  to  the 
exterior,  and  eventually  grows  round  the  oesophagus,  which,  as  in  Holothu- 
rians,  becomes  the  oesophagus  of  the  adult.  The  posterior  vesicles  give  rise 
to  the  lining  of  the  body  cavity,  but  are  stated  by  Metschnikoff  to  be  at  first 
solid,  and  only  subsequently  to  acquire  a  cavity— the  permanent  body  cavity. 
The  anus  naturally  disappears,  since  it  is  absent  in  the  adult.  In  the 
viviparous  type  the  first  stages  are  imperfectly  known,  but  it  appears  that 
the  blastopore  vanishes  before  the  appearance  of  the  mouth.  The  develop- 
ment of  the  ^vaso-peritoneal  bodies  takes  place  as  in  the  free-swimming 
larvae. 

Echinoidea.  In  the  Echinoidea  (Agassiz,  No.  542,  Selenka,  No.  564) 
there  is  a  regular  segmentation  and  the  normal  invagination  (fig.  250  A). 
The  amoeboid  mesoblast  cells  arise  as  two  laterally  placed  masses,  and  give 
rise  to  the  usual  parts.  The  archenteron  grows  forward  and  bends  towards 


550 


CRINOIDEA. 


the  ventral  side  (fig.  250  B).  It  becomes  (fig.  250  C)  divided  into  three 
chambers,  of  which  the  two  hindermost  (d  and  c)  form  the  stomach  and 
intestine ;  while  the  anterior  forms  the  oesophagus,  and  gives  rise  to  the 


FIG.  250.    THREE  SIDE  VIEWS  OF  EARLY  STAGES  IN  THE  DEVELOPMENT  OF 

STRONGYLOCENTRUS.    (From  Agassiz.) 

a,  anus  (blastopore) ;  d.  stomach ;  o.  oesophagus ;  c .  rectum ;  w.  vaso-peritoneal 
vesicle ;  v.  ciliated  ridge ;  r.  calcareous  rod. 

vaso-peritoneal  vesicles.  These  latter  appear  as  a  pair  of  outgrowths 
(fig.  251),  but  become  constricted  off  as  a  single  two-horned  vesicle,  which 
subsequently  divides  into  two.  The  left  of  these 
is  eventually  divided,  as  in  Asteroids,  into  a 
peritoneal  and  water-vascular  sack,  while  the 
right  forms  the  right  peritoneal  sack.  An  oral 
invagination  on  the  flattened  ventral  side  meets 
the  mesenteron  after  its  separation  from  the 
vaso-peritoneal  vesicle.  The  larval  anus  per- 
sists, as  also  does  the  larval  mouth,  but  owing 
to  the  manner  in  which  the  water-vascular 
rosette  is  established  the  larval  oesophagus  ap- 
pears to  be  absorbed,  and  to  be  replaced  by  a 
fresh  oesophagus. 

Crinoidea.  Antedon,  the  only  Crinoid 
so  far  studied  (Gotte,  No.  549),  presents  some 
not  inconsiderable  variations  from  the  usual 
Echinoderm  type.  The  blastopore  is  placed  on 
the  somewhat  flattened  side  of  the  oval  blasto- 
sphere,  and  not,  as  is  usual,  at  the  hinder  end. 

The  blastopore  completely  closes,  and  is  not  converted  into  the  perma- 
nent anus.  The  archenteron  gives  rise  to  the  epithelioid  lining  of  both  body 
cavity  and  water-vascular  system.  These  parts  do  not,  however,  appear  as 
a  single  or  paired  outgrowth  from  the  archenteron,  but  as  three  distinct 
outgrowths  which  are  not  formed  contemporaneously.  Two  of  them  are  first 


FIG.  -251.  DORSO-VEN- 
TRAL  VIEW  OF  AN  EARLY 
LARVA  OF  STRONGYLOCEN- 
TRUS. (From  Agassiz.) 

a.  anus ;  d.  stomach  ;  o. 
oesophagus ;  w.  vaso-perito- 
neal vesicle;  r.  calcareous 
rod. 


ECHINODERMATA. 


551 


formed  and  become  the  future  body  cavity;  but  their  lumens  remain  distinct. 
Jngmally  appearing  as  lateral  outgrowths,  the  right  one  assumes  a  dorsal 
position  and  sends  a  prolongation  into  the  stalk  (fig.  252  rp'\  and 
the  left  one  assumes  first  a  ventral,  and  then  an  oral  position  (fur 
252  lp\ 

The  third  outgrowth  of  the  archenteron  gives  rise  to  the  water-vascular 
vesicle.  It  first  grows  round  the  region  of  the  future  oesophagus  and  so 
forms  the  water-vascular  ring. 
The  wall  of  the  ring  then 
grows  towards  the  body  wall 
so  as  to  divide  the  oral  (left) 
peritoneal  vesicle  into  two 
distinct  vesicles,  an  anterior 
and  a  posterior,  shewn  in  fig. 
253,  lp'  and  lp.  Before  this 
division  is  completed,  the 
water-vascular  ring  is  pro- 
duced in  front  into  five  pro- 


FIG.  252.  LONGITUDINAL  SECTION  THROUGH 
AN  ANTEDON  LARVA.  (From  Carpenter:  after 
Gotte.) 


al.  mesenteron  ;  -wv.  water-  vascular  ring  ; 
lp.  left  (oral)  peritoneal  vesicle;  rp.  right  peri- 
toneal vesicle  ;  rp'.  continuation  of  right  vesicle 
into  the  stalk  ;  st.  stalk. 


cesses—the  future  tentacles 
(fig.  252,  wv)— which  project 
into  the  cavity  of  the  oral 
vesicle  (lp\  After  the  oral 
peritoneal  space  has  become 
completely  divided  into  two  parts,  the  anterior  dilates  (fig.  253,  //)  greatly, 
and  forms  a  large  vestibule  at  the  anterior  end  of  the  body.  This  vestibule 
(lp'}  next  acquires  a  communication  with  the  mesenteron,  shewn  in  fig.  253 
at  m.  The  anterior  wall  of  this  vestibule  is  finally  broken  through.  By  this 
rupture  the  mesenteron  is  placed  in  communication  with  the  exterior  by  the 
opening  at  m,  while  at  the  same  time  the  tentacles  of  the  water-vascular  ring 
(/)  project  freely  to  the  exterior.  Such  is  Gotte's  account  of  the  prge-oral 
body  space,  but,  as  he  himself  points  out,  it  involves  our  believing  that  the 
lining  of  the  diverticulum  derived  from  the  primitive  alimentary  vesicle 
becomes  part  of  the  external  skin.  This  occurrence  is  so  remarkable,  that 
more  evidence  appears  to  me  requisite  before  accepting  it. 

The  formation  of  the  anus  occurs  late.  Its  position  appears  to  be  the 
same  as  that  of  the  blastopore,  and  is  indicated  by  a  papilla  of  the  mesente- 
ron attaching  itself  to  the  skin  on  the  ventral  side  (fig.  253,  an).  It  event- 
ually becomes  placed  in  an  interradial  space  within  the  oral  disc  of  the  adult. 
The  water-vascular  ring  has  no  direct  communication  with  the  exterior,  but 
the  place  of  the  madreporic  canal  of  other  types  appears  to  be  taken  in 
the  larva  by  a  single  tube  leading  from  the  exterior  into  the  body  cavity,  the 
external  opening  of  which  is  placed  on  one  of  the  oral  plates  (vide  p.  571)  in 
the  next  interradial  space  to  the  right  of  the  anus,  and  a  corresponding 
diverticulum  of  the  water-vascular  ring  opening  into  the  body  cavity.  The 
line  of  junction  between  the  left  and  right  peritoneal  vesicles  forms  in  the 
larva  a  ring-like  mesentery  dividing  the  oral  from  the  aboral  part  of  the  body 


552 


CRINOIDEA. 


cavity.  In  the  adult1  the  oral  section  of  the  larval  body  cavity  becomes  the 
ventral  part  of  the  circumvisceral  division  of  the  body  cavity,  and  the 
subtentacular  canals  of  the  arms  and  disc ;  while  the  aboral  section  becomes 
the  dorsal  part  of  the  circumvisceral  division  of  the  body  cavity,  the  cceliac 
canals  of  the  arms,  and  the  cavity  of  the  centro-dorsal  piece.  The  primitive 


,+wr 


FIG.  253.    LONGITUDINAL  SECTION  THROUGH  THE  CALYX  OF  AN  ADVANCED 
PENTRACRINOID  ANTEDON  LARVA  WITH  CLOSED  VESTIBULE. 

(From  Carpenter ;  after  Gotte.) 

ae.  epithelium  of  oral  vestibule;  ;//.  mouth;  al.  mesenteron;  an.  rudiment  of 
permanent  anus;  lp.  posterior  part  of  left  (oral)  peritoneal  sack;  lp' '.  anterior  part  of 
left  (oral)  peritoneal  sack;  wr.  water-vascular  ring;  /.  tentacle;  mt.  mesentery; 
rp.  right  peritoneal  sack;  rp '.  continuation  of  right  peritoneal  sack  into  the  stalk; 
r.  roof  of  tentacular  vestibule. 

distinction  between  the  sections  of  the  larval  body  cavity  becomes  to  a  large 
extent  obliterated,  while  the  axial  and  intervisceral  sections  of  the  body- 
cavity  of  the  adult  are  late  developments. 

The  more  important  points  in  the  development  indicated  in 
the  preceding  pages  are  as  follows : 

(i)  The  blastosphere  is  usually  elongated  in  the  direction 
of  the  axis  of  invagination,  but  in  Comatula  it  is  elongated 
transversely  to  this  axis. 

1  Vide  P.  H.  Carpenter,  "On  the  genus  Actinometra."  Linnean  Trans.,  and 
Series,  Zoology,  Vol.  n.,  Part  I.,  1879. 


ECHINODERMATA.  553 


(2)  The   blastopore  usually  becomes  the  permanent  anus, 
but  it  closes  at  the  end  of  larval  life  (there  being  no  anus  in  the 
adult)  in  Ophiuroids  and  some  Asteroids,  while  in  Comatula  it 
closes  very  early,  and  a  fresh  anus  is  formed  at  the  point  where 
the  blastopore  was  placed. 

(3)  The   larval  mouth  always  becomes  the  mouth  of  the 
adult. 

(4)  The  archenteron  always  gives  rise  to  outgrowths  which 
form  the  peritoneal  membrane  and  water-vascular  systems.     In 
Comatula  there  are  three  such  outgrowths,  two   paired,  which 
form  the  peritoneal  vesicles,  and  one  unpaired,  which  forms  the 
water-vascular  vesicle.     In  Asteroids  and  Ophiuroids  there  are 
two  outgrowths.     In  Ophiuroids  both  of  these  are  divided  into  a 
peritoneal    and    a  water-vascular  vesicle,  but  the   right  water- 
vascular  vesicle  atrophies.     In  Asteroids  only  one  water-vascular 
vesicle  is  formed,  which  is  derived  from  the  left  peritoneal  vesicle. 
In  Echinoids  and  Holothuroids  there  is  a  single  vaso-peritoneal 
vesicle. 

(5)  The   water- vascular    vesicle    grows    round    the    larval 
oesophagus    in    Holothuroids,    Ophiuroids,    and    Comatula ;    in 
these  cases  the  larval  oesophagus  is  carried  on  into  the  adult. 
In  other  forms  the  water-vascular  vesicle  forms  a  ring  which 
does   not   enclose    the  cesophagus   (Asteroids   and   Echinoids); 
in  such  cases  a  new  oesophagus  is  formed,  which  perforates  this 
ring. 

Development  of  the  larval  appendages  and  metamorphosis. 

Holothuroidea.  The  young  larva  of  Synapta,  to  which  J. 
Muller  gave  the  name  Auricularia  (fig.  255),  is  in  many  respects 
the  simplest  form  of  Echinoderm  larva.  With  a  few  exceptions 
the  Auricularia  type  of  larva  is  common  to  the  Holothuria. 

It  is  (fig.  254  A  and  fig.  255)  bilaterally  symmetrical,  pre- 
senting a  flattened  ventral  surface,  and  a  convex  dorsal  one. 
The  anus  (an)  is  situated  nearly  at  the  hinder  pole,  and  the 
mouth  (m)  about  the  middle  of  the  ventral  surface.  In  front 
of  the  mouth  is  a  considerable  process,  the  prae-oral  lobe. 
Between  the  mouth  and  anus  is  a  space,  more  or  less  concave 
according  to  the  age  of  the  embryo,  interrupted  by  a  ciliated 


554 


AURICULARIA. 


A  similar  ciliated  ridge  is 
A  E 


ridge  a  little  in  front  of  the  anus, 
present  on  the  ventral  surface 
of  the  prae-oral  lobe  immedi- 
ately in  front  of  the  mouth. 
The  anal  and  oral  ridges  are 
connected  by  two  lateral  cili- 
ated bands,  the  whole  forming 
a  continuous  band,  which, 
since  the  mouth  lies  in  the 
centre  of  it  (fig.  255),  may  be 
regarded  as  a  ring  completely 
surrounding  the  body  behind 
the  mouth,  or  more  naturally 
as  a  longitudinal  ring. 

The  bilateral  Auricularia 
is  developed  from  a  slightly 
elongated  gastrula  with  an  uniform  covering  of  cilia.  The 
gastrula  becomes  flattened  on  the  oral  side.  At  the  same  time 
the  cilia  become  specially  developed  on  the  oral  and  anal  ridges, 
and  then  on  the  remainder  of  the  ciliated  ring,  while  they  are 


FIG.  254.  A.  THE  LARVA  OF  A  HOLO- 
THUROID.  B.  THE  LARVA  OF  AN  ASTER- 
OID. 

;//.  mouth;  st.  stomach;  a.  anus;  l.c> 
primitive  longitudinal  ciliated  band;  pr.c. 
prae-oral  ciliated  band. 


FIG.  155.  DIAGRAMMATIC  FIGURES  REPRESENTING  THE  EVOLUTION  OF  AN 
AURICULARIA  FROM  THE  SIMPLEST  ECHINODERM  LARVAL  FORM.  (Copied  from 
MUller.) 

The  black  line  represents  the  ciliated  ridge.  The  shaded  part  is  the  oral  side  of 
the  ring,  the  clear  part  the  aboral  side. 

/;;.  mouth;  an.  anus. 

simultaneously  obliterated  elsewhere  ;  and  so  a  complete  Auricu- 
laria is  developed.  The  water-vascular  ring  in  the  fully-developed 
larva  has  already  considerably  advanced  in  the  growth  round  the 
oesophagus  (fig.  256  w.v.r). 

Most  Holothurian  larvae,  in  their  transformation  from  the 
bilateral  Auricularia  form  to  the  radial  form  of  the  adult,  pass 
through  a  stage  in  which  the  cilia  form  a  number  of  transverse 


ECHINODERMATA. 


555 


-2>.v 


rings,  usually  five  in  number,  surrounding  the  body.  The 
stages  in  this  metamorphosis  are  shewn  in  figs.  256,  257,  and 
258. 

The  primitive  ciliated  band, 
at  a  certain  stage  of  the  meta- 
morphosis, breaks  up  into  a 
number  of  separate  portions 
(fig.  256),  the  whole  of  which  are 
placed  on  the  ventral  surface. 
Four  of  these  (fig.  257  A  and  B) 
arrange  themselves  in  the  form 
of  an  angular  ring  round  the 
mouth,  which  at  this  period  pro- 
jects considerably.  The  remain- 
ing portions  of  the  primitive 
band  change  their  direction  from 
a  longitudinal  one  to  a  trans- 
verse (fig.  257  B),  and  eventually 
grow  into  complete  rings  (fig. 
2570).  Of  these  there  are  five. 
The  middle  one  (257  B)  is  the 
first  to  develop,  and  is  formed 
from  the  dorsal  parts  of  the 
primitive  ring.  The  two  hinder 
rings  develop  next,  and  last  of 
all  the  two  anterior  ones,  one  of 
which  appears  to  be  in  front  of  the  mouth  (fig.  257  C). 

The  later  development  of  the  mouth,  and  of  the  ciliated  ridge 
surrounding  it,  is  involved  in  some  obscurity.  It  appears  from 
Metschnikoff  (No.  560)  that  an  invagination  of  the  oesophagus 
takes  place,  carrying  with  it  the  ciliated  ridge  around  the  mouth. 
This  ridge  becomes  eventually  converted  into  the  covering  for 
the  five  tentacular  outgrowths  of  the  water- vascular  ring  (fig. 
258),  and  possibly  also  forms  the  nervous  system. 

The  opening  of  the  cesophageal  invagination  is  at  first  behind 
the  foremost  ciliated  ring,  but  eventually  comes  to  lie  in  front  of 
it,  and  assumes  a  nearly  terminal  though  slightly  ventral  position 
(fig.  258).  No  account  has  been  given  of  the  process  by  which 
this  takes  place,  but  the  mouth  is  stated  by  Metschnikoff  (though 


FIG.  256.  FULL-GROWN  LARVA  OF 
SYNAPTA.  (After  Metschnikoff.) 

m.  mouth ;  st.  stomach  ;  a.  anus ; 
p.v.  left  division  of  perivisceral  cavity, 
which  is  still  connected  with  the  water- 
vascular  system  ;  w.v.r.  water-vascular 
ring  which  has  not  yet  completely  en- 
circled the  oesophagus;  I.e.  longitudinal 
part  of  ciliated  band ;  pr.c.  prae-oral  part 
of  ciliated  band. 


556 


BIPINNARIA. 


Miiller  differs  from  him  on  this  point)  to  remain  open  through- 
out. The  further  changes  in  the  metamorphosis  are  not  con- 
siderable. The  ciliated  bands  disappear,  and  a  calcareous  ring 
of  ten  pieces,  five  ambulacral  and  five  interambulacral,  is  formed 
round  the  oesophagus.  A  provisional  calcareous  skeleton  is  also 
developed. 

All   the   embryonic   systems   of   organs   pass   in   this   case 
directly  into  those  of  the  adult. 

The  metamorphosis  of  most  Holothuroidea  is  similar  to  that  just 
described.  In  Cucumaria  (Selenka)  there  is  however  no  Auricularia  stage, 
and  the  uniformly  ciliated  stage  is  succeeded  by  one  with  five  transverse 


FIG.  257.  THREE  STAGES  IN  THE  DEVELOPMENT  OF  SYNAPTA.  A  and  B 
are  viewed  from  the  ventral  surface,  and  C  from  the  side.  (After  Metschnikoff.) 

m.  mouth;  oe.  oesophagus;  pv.  walls  of  the  perivisceral  cavity;  wv.  longitudinal 
vessel  of  the  water- vascular  system;  p.  dorsal  pore  of  water-vascular  system; 
cr.  ciliated  ring  formed  round  the  mouth  from  parts  of  the  primitive  ciliated 
band. 

bands  of  cilia,  and  a  prae-oral  and  an  anal  ciliated  cap.  The  mouth  is  at 
first  situated  ventrally  behind  the  prse-oral  cap  of  cilia,  but  the  prae-oral 
cap  becomes  gradually  absorbed,  and  the  mouth  assumes  a  terminal 
position. 

In  Psolinus  (Kowalevsky)  there  is  no  embryonic  ciliated  stage,  and  the 
adult  condition  is  attained  without  even  a  metamorphosis.  There  appear  to 


ECHINODERMATA. 


557 


be  five  plates  surrounding  the 
mouth,  which  are  developed  before 
any  other  part  of  the  skeleton,  and 
are  regarded  by  P.  H.  Carpenter 
(No.  548)  as  equivalent  to  the  five 
oral  plates  of  the  Crinoidea.  The 
larval  condition  with  ciliated  bands 
is  often  spoken  of  as  the  pupa  stage, 
and  during  it  the  larvae  of  Holo- 
thurians  proper  use  their  embryonic 
tube  feet  to  creep  about. 

Asteroidea.  The  com- 
monest and  most  thoroughly 
investigated  form  of  Asteroid 
larva  is  a  free  swimming  form 
known  as  Bipinnaria. 

This  form  in  passing  from 
the  spherical  to  the  bilateral 
condition  passes  through  at 
first  almost  identical  changes 
to  the  Auricularian  larva. 
The  cilia  become  at  an  early 
period  confined  to  an  oral 
and  anal  ridge. 

The  anal  ridge  gradually  extends  dorsalwards,  and  finally 
forms  a  complete  longitudinal  post-oral  ring  (fig.  259  A)  ;  the 
oral  ridge  also  extends  dorsalwards,  and  forms  a  closed  prae-oral 
ring  (fig.  259  A),  the  space  within  which  is  left  unshaded  in  my 
figure. 

The  presence  of  two  rings  instead  of  one  distinguishes  the 
Bipinnaria  from  the  Auricularia.  The  two  larvae  are  shewn  side 
by  side  in  fig.  254,  and  it  is  obvious  that  the  two  bands  of  the 
Bipinnaria  are  (as  pointed  out  by  Gegenbaur)  equivalent  to  the 
single  band  of  the  Auricularia  divided  into  two.  Ontologically, 
however,  the  two  bands  of  Bipinnaria  do  not  appear  to  arise 
from  the  division  of  a  single  band. 

As  the  Bipinnaria  grows  older,  a  series  of  arms  grows  out 
along  lines  of  the  two  ciliated  bands  (fig.  259  C),  and,  in  many 
cases,  three  special  arms  are  formed,  not  connected  with  the 
ciliated  bands,  and  covered  with  warts.  These  latter  arms  are 


FlG.  258.  A  LATE  STAGE  IN  THE  DE- 
VELOPMENT OF  SYNAPTA.  (After  Metschni- 
koff.) 

The  figure  shews  the  vestibular  cavity 
with  retracted  tentacles  ;  the  ciliated  bands  ; 
the  water-vascular  system,  etc. 

p.  dorsal  pore  of  water-vascular  system  ; 
pv.  walls  of  perivisceral  cavity;  ms.  amoe- 
boid cells. 


558 


BIPINNARIA. 


known  as  brachiolar  arms,  and  the  larvae  provided  with  them 
as  Brachiolaria  (fig.  259  D). 

As  a  rule  the  following  arms  can  be  distinguished  (fig.  259  C  and  D),  on 
the  hinder  ring  (Agassiz'  nomenclature)  a  median  anal  pair,  a  dorsal  anal 
pair,  and  a  ventral  anal  pair,  a  dorsal  oral  pair,  and  an  unpaired  anterior 
dorsal  arm  ;  on  the  prae-oral  ring  a  ventral  oral  pair,  and  sometimes  (Miiller) 
an  unpaired  anterior  ventral  arm. 

The  three  brachiolar  arms  arise  as  processes  from  the  base  of  the 
unpaired  dorsal  arm,  and  the  two  ventral  oral  arms.  The  extent  of  the 
development  of  the  arms  varies  with  the  species. 


FIG.   259.    DIAGRAMMATIC  REPRESENTATION  OF  VARIOUS  FORMS  OF  ASTEROID 
LARWE.     A,  B,  C,  BIPINNARIA;   D,  BRACHIOLARIA.     (Copied  from  Muller.) 
The  black  lines  represent  the  ciliated  bands ;  and  the  shading  the  space  between 
the  prae-oral  and  the  post-oral  bands. 

m.  mouth;  an.  anus. 

The  changes  by  which  the  Bipinnaria  or  Brachiolaria  becomes 
converted  into  the  adult  starfish  are  very  much  more  complicated 
than  those  which  take  place  in  Holothurians.  For  an  accurate 
knowledge  of  them  we  are  largely  indebted  to  Alex.  Agassiz 
(No.  543).  The  development  of  the  starfish  takes  place  entirely 
at  the  posterior  end  of  the  larva  close  to  the  stomach. 

On  the  right  and  dorsal  side  of  the  stomach,  and  externally 
to  the  rig/it  peritoneal  space,  are  formed  five  radially  situated 
calcareous  rods  arranged  in  the  form  of  a  somewhat  irregular 
pentagon.  The  surface  on  which  they  are  deposited  has  a 
spiral  form,  and  constitutes  together  with  its  calcareous  rods,  the 


ECHINODERMATA.  559 


abactinal  or  dorsal  surface  of  the  future  starfish.  Close  to  its 
dorsal,  i.e.  embryonic  dorsal,  edge  lies  the  dorsal  pore  of  the 
water-vascular  system  (madreporic  canal),  and  close  to  its  ventral 
edge  the  anus.  On  the  left  and  ventral  side  of  the  stomach  is 
placed  the  water-vascular  rosette,  the  development  of  which  was 
described  on  p.  549.  It  is  situated  on  the  actinal  or  ventral  surface 
of  the  future  starfish,  and  is  related  to  the  left  peritoneal  vesicle. 

Metschnikoff  (No.  560)  and  Agassiz  (No.  543)  differ  slightly  as  to  the 
constitution  of  the  water- vascular  rosette.  The  former  describes  and  figures 
it  as  a  completely  closed  rosette,  the  latter  states  that  '  it  does  not  form  a 
completely  closed  curve  but  is  always  open,  forming  a  sort  of  twisted 
crescent-shaped  arc.' 

The  water-vascular  rosette  is  provided  with  five  lobes,  corre- 
sponding to  which  are  folds  in  the  larval  skin,  and  each  lobe 
corresponds  to  one  of  the  calcareous  plates  developed  on  the 
abactinal  disc.  The  plane  of  the  actinal  surface  at  first  meets 
that  of  the  abactinal  at  an  acute  or  nearly  right  angle.  The  two 
surfaces  are  separated  by  the  whole  width  of  the  stomach.  The 
general  appearance  of  the  larva  from  the  ventral  surface  after 
the  development  of  the  water-vascular  rosette  (i)  and  abactinal 
disc  (A)  is  shewn  in  fig.  260. 

As  development  proceeds  the  abactinal  surface  becomes  a 
firm  and  definite  disc,  owing  to  the  growth  of  the  original 
calcareous  spicules  into  more  or  less  definite  plates,  and  to  the 
development  of  five  fresh  plates  nearer  the  centre  of  the  disc  and 
interradial  in  position.  Still  later  a  central  calcareous  plate 
appears  on  the  abactinal  surface,  which  is  thus  formed  of  a 
central  plate,  surrounded  by  a  ring  of  five  interradial  plates,  and 
then  again  by  a  ring  of  five  radial  plates.  The  abactinal  disc 
now  also  grows  out  into  five  short  processes,  separated  by  five 
shallow  notches.  These  processes  are  the  rudiments  of  the  five 
arms,  and  each  of  them  corresponds  to  one  of  the  lobes  of  the 
water-vascular  rosette.  A  calcareous  deposit  is  formed  round 
the  opening  of  the  water-vascular  canal,  which  becomes  the 
madreporic  tubercle1.  At  about  this  stage  the  absorption  of  the 
larval  appendages  takes  place.  The  whole  anterior  part  of  the 

1  The  exact  position  of  the  madreporic  tubercle  in  relation  to  the  abactinal  plates 
does  not  seem  to  have  been  made  out.  It  might  have  been  anticipated  that  it  would 
be  placed  in  one  of  the  primary  interradial  plates,  but  this  does  not  seem  to  be  the 
case.  The  position  of  the  anus  is  also  obscure. 


56o 


BIPINNARIA. 


larva  with  the  great  prae-oral  lobe  has  hitherto  remained 
unchanged,  but  now  it  contracts  and  undergoes  absorption,  and 
becomes  completely  withdrawn  into  the  disc  of  the  future  starfish. 
The  larval  mouth  is  transported  into 
the  centre  of  the  actinal  disc.  In  the 
larvae  observed  by  Agassiz  and  Met- 
schnikoff  nothing  was  cast  off,  but  the 
whole  absorbed. 

According  to  M  tiller  and  Koren  and 
Danielssen  this  is  not  the  case  in  the  larva 
observed  by  them,  but  part  of  the  larva  is 
thrown  off,  and  lives  for  some  time  indepen- 
dently. 

After  the  absorption  of  the  larval 
appendages  the  actinal  and  abactinal 
surfaces  of  the  young  starfish  approach 
each  other,  owing  to  the  flattening  of 
the  stomach ;  at  the  same  time  they 
lose  their  spiral  form,  and  become  flat 
discs,  which  fit  each  other.  Each  of 
the  lobes  of  the  rosette  of  the  water- 
vascular  system  becomes  one  of  the 
radial  water-vascular  canals.  It  first 
becomes  five-lobed,  each  lobe  forming 
a  rudimentary  tube  foot,  and  on  each  ^dctinal  disc  of  youn£  Aste' 
side  of  the  middle  lobe  two  fresh  ones 

next  spring  out,  and  so  on  in  succession.  The  terminal  median 
lobe  forms  the  tentacle  at  the  end  of  the  arm,  and  the  eye  is 
developed  at  its  base.  The  growth  of  the  water-vascular  canals 
keeps  pace  with  that  of  the  arms,  and  the  tube  feet  become 
supported  at  their  base  by  an  ingrowth  of  calcareous  matter. 
The  whole  of  the  calcareous  skeleton  of  the  larva  passes  directly 
into  that  of  the  adult,  and  spines  are  very  soon  formed  on  the 
plates  of  the  abactinal  surface.  The  original  radial  plates, 
together  with  the  spines  which  they  have,  are  gradually  pushed 
outwards  with  the  growth  of  the  arms  by  the  continual  addition 
of  fresh  rows  of  spines  between  the  terminal  plate  and  the  plate 
next  to  it.  It  thus  comes  about  that  the  original  radial  plates 
persist  at  the  end  of  the  arms,  in  connection  with  the  unpaired 


FIG.  260.  BIPINNARIA 
LARVA  OF  AN  ASTEROID.  (From 
Gegenbaur ;  after  Miiller.) 

b.  mouth ;  a.  anus  ;  h.  ma- 
dreporic  canal ;  t.  ambulacral 
rosette  ;  c .  stomach ;  d.  g.  e. 
etc.  arms  of  Bipinnaria ;  A. 


ECHINODERMATA.  561 


tentacles   which    form   the   apex   of   the   radial   water-vascular 
tubes. 

It  has  already  been  mentioned  that  according  to  Metschnikoff  (No.  560) 
a  new  oesophagus  is  formed  which  perforates  the  water-vascular  ring,  and 
connects  the  original  stomach  with  the  original  mouth.  Agassiz  (No.  543) 
maintains  that  the  water-vascular  ring  grows  round  the  primitive  oesophagus. 
He  says — "  During  the  shrinking  of  the  larva  the  long  oesophagus  becomes 
"  shortened  and  contracted,  bringing  the  opening  of  the  mouth  of  the  larva 
"  to  the  level  of  the  opening  of  the  oesophagus,  which  eventually  becomes 
"the  true  mouth  of  the  starfish."  The  primitive  anus  is  believed  by 
Metschnikoff  to  disappear,  but  by  Agassiz  to  remain.  This  discrepancy 
very  possibly  depends  upon  these  investigators  having  worked  at  different 
species. 

There  is  no  doubt  that  the  whole  of  the  larval  organs,  with 
the  possible  exception  of  the  oesophagus,  and  anus  (where  absent 
in  the  adult),  pass  directly  into  the  corresponding  organs  of  the 
starfish — and  that  the  prae-oral  part  of  the  body  and  arms  of  the 
larva  are  absorbed  and  not  cast  off. 

In  addition  to  the  Bipinnarian  type  of  Asteroid  larva  a  series  of  other 
forms  has  been  described  by  Miiller  (No.  561),  Sars,  Keren,  and  Danielssen 
(No.  554)  and  other  investigators,  which  are  however  very  imperfectly 
known.  The  best-known  form  is  one  first  of  all  discovered  by  Sars  in 
Echinaster  Sarsii,  and  the  more  or  less  similar  larvae  subsequently  investi- 
gated by  Agassiz,  Busch,  Miiller,  Wyville  Thomson,  etc.  of  another  species 
of  Echinaster  and  of  Asteracanthion.  These  larvae  on  leaving  the  egg  have 
an  oval  form,  and  are  uniformly  covered  by  cilia.  Four  processes  (or  in 
Agassiz'  type  one  process)  grow  out  from  the  body ;  by  these  the  larvae  fix 
themselves.  In  the  case  of  Echinaster  the  larvae  are  fixed  in  the  ventral 
concavity  of  the  disc  of  the  mother,  between  the  five  arms,  where  a  tempo- 
rary brood-pouch  is  established.  The  main  part  of  the  body  is  converted 
directly  into  the  disc  of  the  young  starfish,  while  the  four  processes  come  to 
spring  from  the  ventral  surface,  and  are  attached  to  the  water- vascular  ring. 
Eventually  they  atrophy  completely.  Of  the  internal  structure  but  little  is 
known  ;  till  the  permanent  mouth  is  formed,  after  the  development  of  the 
young  starfish  is  pretty  well  advanced,  the  stomach  has  no  communication 
with  the  exterior. 

A  second  abnormal  type  of  development  is  presented  by  the  embryo  of 
Pteraster  miliaris,  as  described  by  Koren  and  Danielssen1.  The  larvae  to 
the  number  of  eight  to  twenty  develop  in  a  peculiar  pouch  on  the  dorsal 
surface  of  the  body.  The  early  stages  are  not  known,  but  in  the  later  ones 
the  whole  body  assumes  a  pentagonal  appearance  with  a  mouth  at  one  edge 

1  The  following  statements  are  taken  from  the  abstract  in  Bronn's  Thierreichs. 
B.  II.  36 


562 


OPHIUROID   PLUTEUS. 


of  the  disc.  At  a  later  stage  the  anus  is  formed  on  the  dorsal  side  of  an  arm 
opposite  the  mouth.  The  stomach  is  surrounded  by  a  water-vascular  ring, 
from  which  the  madreporic  canal  passes  to  the  dorsal  surface,  but  does  not 
open.  At  a  later  stage  the  embryonic  mouth  and  anus  vanish,  to  be  replaced 
by  a  permanent  mouth  and  anus  in  the  normal  positions. 

A  third,  and  in  some  respects  very  curious,  form  is  a  worm  like  larva  of 
Miiller,  which  is  without  bands  of  cilia.  The  dorsal  surface  of  the  youngest 
larva  is  divided  by  transverse  constrictions  into  five  segments.  On  the 
under  side  of  the  first  of  these  is  a  five-lobed  disc,  each  lobe  being  provided 
with  a  pair  of  tube  feet. 

At  a  later  period  only  three  segments  are  visible  on  the  dorsal  surface, 
but  the  ventral  surface  has  assumed  a  pentagonal  aspect.  The  later  stages 
are  not  known. 

Ophiuroidea.  The  full-grown  larva  of  the  Ophiuroids  is 
known  as  a  Pluteus.  It  commences  with  the  usual  more  or  less 
spherical  form ;  from  this  it  passes  to  a  form  closely  resembling 


FIG.  261.  DIAGRAMMATIC  FIGURES  SHEWING  THE  EVOLUTION  OK  AN  OPHIU- 
ROID PLUTEUS  FROM  A  SIMPLE  ECHINODERM  LARVA.  (Copied  from  Miiller.)  The 
calcareous  skeleton  is  not  represented. 

///.  mouth;  an.  anus;  d.  anterior  arms;  d'.  lateral  arms;  e'.  posterior  arms;  tf. 
anterolateral  arms. 

that  of  Auricularia  with  a  rounded  dorsal  surface,  and  a  flattened 
ventral  one.  Soon  however  it  becomes  distinguished  by  the 
growth  of  a  post-anal  lobe  and  the  absence  of  a  prae-oral  lobe 
(fig.  261  B).  The  post-anal  lobe  forms  the  somewhat  rounded 
apex  of  the  body.  In  front  of  the  mouth,  and  between  the 
mouth  and  anus,  arise  the  anal  and  oral  ciliated  ridges,  which 
soon  become  continued  into  a  single  longitudinal  ciliated  ring. 
At  the  same  time  the  body  becomes  prolonged  into  a  series  of 


ECHINODERMATA. 


563 


processes  along  the  ciliated  band,  which  is  continued  to  the 
extremity  of  each.  The  primitive  ciliated  ring  never  becomes 
broken  up  into  two  or  more  rings.  A  ciliated  crown  is  usually 
developed  at  the  extremity  of  the  post-anal  lobe.  The  arms  are 
arranged  in  the  form  of  a  ring  surrounding  the  mouth,  and  are 
all  directed  forwards. 

The  first  arms  to  appear  are  two  lateral  ones,  which  usually  remain  the 
most  conspicuous  (fig.  261  B  and  C,  cf\  Next  arises  a  pair  on  the  sides  of 
the  mouth,  which  may  be  called  the  mouth  or  anterior  arms  (C,  d}.  A  pair 
ventral  to  and  behind  the  lateral  arms  is  then  formed,  constituting  the 
posterior  arms  (D,  e'\  and  finally  a  pair  between  the  lateral  arms  and  the 
anterior,  constituting  the  anterolateral  arms  (D,^). 

The  concave  area  between  the  arms  forms  the  greater  part  of 
the  ventral  surface  of  the  body.  Even  before  the  appearance  of 
any  of  the  arms,  and  before  the  formation  of  the  mouth,  two 
calcareous  rods  are  formed,  which  meet  behind  at  the  apex  of 
the  post-anal  lobe,  and  are  continued  as  a  central  support  into 
each  of  the  arms  as  they  are  successively  formed.  These  rods 
are  shewn  at  their  full  development  in  fig.  262.  The  important 
points  which  distinguish  a  Pluteus 
larva  from  the  Auricularia  or 
Bipinnaria  are  the  following  : 

(i)  The  presence  of  the  post- 
anal  lobe  at  the  hind  end  of  the 
body.  (2)  The  slight  develop- 
ment of  a  prae-oral  lobe.  (3)  The 
provisional  calcareous  skeleton  in 
the  larval  arms. 

Great  variations  are  presented 
in  the  development  of  the  arms 
and  provisional  skeleton.  The 
presence  of  lateral  arms  is  however 
a  distinctive  characteristic  of  the 
Ophiuroid  Pluteus.  The  other 
arms  may  be  quite  absent,  but 
the  lateral  arms  never. 

The  formation  of  the  perma- 
nent Ophiuroid  takes  place  in 
much  the  same  way  as  in  the  Asteroidea. 

36-2 


FIG.  262. 
OPHIUROID. 
after  Miiller.) 


PLUTEUS  LARVA  OF  AN 
(From     Gegenbaur ; 


A.  rudiment  of  young  Ophiuroid  ; 
(?.  lateral  arms;  d.  anterior  arms; 
e  .  posterior  arms. 


564 


OPHIUROID   PLUTEUS. 


There  is  formed  (fig.  262)  on  the  right  and  dorsal  side  of  stomach  the 
abactinal  disc  supported  by  calcareous  plates,  at  first  only  five  in  number 
and  radial  in  position1.  The  disc  is  at  first  not  symmetrical,  but  becomes  so 
at  the  time  of  the  resorption  of  the  larval  arms.  It  grows  out  into  five 
processes — the  five  future  rays.  The  original  five  radial  plates  remain  as  the 
terminal  segments  of  the  adult  rays,  and  new  plates  are  always  added 
between  the  ultimate  and  penultimate  plate  (Mu'ller),  though  it  is  probable 
that  in  the  later  stages  fresh  plates  are  added  in  the  disc. 

The  ventral  surface  of  the  permanent  Ophiuroid  is  formed  by  the  concave 
surface  between  the  mouth  and  anus.  Between  this  and  the  stomach  is 


FIG.  263.  DIAGRAMMATIC  FIGURES  SHEWING  THE  EVOLUTION  OF  ECHINOID 
PLUTEI.  (Copied  from  Miiller.)  The  calcareous  skeleton  is  not  represented.  E. 
Pluteus  of  Spatangus. 

m.  mouth;  an.  anus;  d.  anterior  arms;  d' .  point  where  lateral  arms  arise  in  the 
Ophiuroid  Pluteus;  e.  anterointernal  arms;  e.  posterior  arms;  g'.  anterolateral  arms; 
g.  anteroexternal  arms. 

situated  the  water-vascular  ring.  It  is  at  first  not  closed,  but  is  horseshoe- 
shaped,  with  five  blind  appendages  (fig.  262).  It  eventually  grows  round 
the  cesophagus,  which,  together  with  the  larval  mouth,  is  retained  in  the 
adult.  The  five  blind  appendages  become  themselves  lobed  in  the  same 
way  as  in  Asterias,  and  grow  out  along  the  five  arms  of  the  disc  and  become 
the  radial  canals  and  tentacles.  All  these  parts  of  the  water-vascular  system 
are  of  course  covered  by  skin,  and  probably  also  surrounded  by  mesoblast 
cells,  in  which  at  a  later  period  the  calcareous  plates  which  lie  ventral  to  the 
radial  canal  are  formed.  The  larval  anus  disappears.  As  long  as  the  larval 
appendages  are  not  absorbed  the  ventral  and  dorsal  discs  of  the  permanent 
Ophiuroid  fit  as  little  as  in  the  case  of  the  Brachiolaria,  but  at  a  certain 
period  the  appendages  are  absorbed.  The  calcareous  rods  of  the  larval  arms 

1  Whether  interradial  plates  are  developed  as  in  Asterias  is  not  clear.  They  seem 
to  be  found  in  Ophiopholis  bellis,  Agassiz,  but  have  not  been  recognised  in  other 
forms  (vide  Carpenter,  No.  548,  p.  369). 


ECHINODERMATA.  565 


break  up,  the  arms  and  anal  lobe  become  absorbed,  and  the  dorsal  and 
ventral  discs,  with  the  intervening  stomach  and  other  organs,  are  alone  left. 
After  this  the  discs  fit  together,  and  there  is  thus  formed  a  complete  young 
Ophiuroid. 

The  whole  of  the  internal  organs  of  the  larva  (except  the  anus),  including 
the  mouth,  cesophagus,  the  body  cavity,  etc.  are  carried  on  directly  into  the 
adult. 

The  larval  skeleton  is,  as  above  stated,  absorbed. 

The  viviparous  larva  of  Amphiura  squamata  does  not  differ  very  greatly 
from  the  larvae  with  very  imperfect  arms.  It  does  not  develop  a  distinct 
ciliated  band,  and  the  provisional  skeleton  is  very  imperfect.  The  absence 
of  these  parts,  as  well  as  of  the  anus,  mentioned  on  p.  549,  may  probably  be 
correlated  with  the  viviparous  habits  of  the  larva.  With  reference  to  the 
passage  of  this  larva  into  the  adult  there  is  practically  nothing  to  add  to 
what  has  just  been  stated.  When  the  development  of  the  adult  is  fairly 
advanced  the  part  of  the  body  with  the  provisional  skeleton  forms  an 
elongated  rod-like  process  attached  to  the  developing  disc.  It  becomes 
eventually  absorbed. 

Echinoidea.     The  Echinus  larva    (fig.   263}   has  a  Pluteus 
form  like  that  of  the  Ophiuroids,  and  in  most  points,  such  as  the 


FIG.  264.     Two  LARV/E  OF  STRONGYLOCENTRUS.    (From  Agassiz.) 
m.  mouth;  a.  anus;  o.  cesophagus;  d.  stomach;  c.  intestine;  »'.  and  v.  ciliated 
ridges;  iv.  water- vascular  tube;  r.  calcareous  rods. 

presence  of  the  anal  lobe,  the  ciliated  band,  the  provisional 
skeleton,  etc.,  develops  in  the  same  manner.  The  chief  difference 
between  the  two  Pluteus  forms  concerns  the  development  of  the 
lateral  arms.  These,  which  form  the  most  prominent  arms  in 
the  Ophiuroid  Pluteus,  are  entirely  absent  in  the  Echinoid 


566 


ECHINOID  PLUTEUS. 


Pluteus,  which  accordingly  has,  as  a  rule,  a  much  narrower  form 
than  the  Ophiuroid  Pluteus. 

A  pair  of  ciliated  epaulettes  on  each  side  of  and  behind  the 
ciliated  ring  is  very  characteristic  of  some  Echinoid  larvae. 
They  are  originally  developed  from  the  ciliated  ring  (fig.  266  A 


FIG.  265.     LATERAL  AND  VENTRAL  VIEW  OF  A  LARVA  OF  STRONGYLOCENTRUS. 

(From  Agassiz.)     General  references  as  in  fig.  264. 

b.  dorsal  opening  of  madreporic  canal;  e '.  posterior  arms ;  e'".  anterior  arms; 
flV.  anterointernal  arms. 

and  B,  z>").  The  presence  of  three  processes  from  the  anal  lobe 
supported  by  calcareous  rods  is  characteristic  of  the  Spatangoid 
Pluteus  (fig.  263  E). 

The  first  two  pairs  of  arms  to  develop,  employing  the  same  names  as  in 
Ophiuroids,  are  the  anterior  attached  to  the  oral  process  (fig.  263  C,  d]  and 
the  posterior  pair  (*?')•  A  pair  of  anterolateral  arms  next  becomes  developed 
(j^).  A  fourth  pair  (not  represented  in  Ophiuroids)  appears  on  the  inner 
side  of  the  anterior  pair  forming  an  anterointernal  pair  (e},  and  in  the 
Spatangoid  Pluteus  a  fifth  pair  may  be  added  on  the  external  side  of  the 
anterior  pair  forming  an  anteroexternal  pair  (g). 

Each  of  the  first-formed  paired  calcareous  rods  is  composed  of  three 
processes,  two  of  which  extend  into  the  anterior  and  posterior  arms  ;  and  the 
third  and  strongest  passes  into  the  anal  lobe,  and  there  meets  its  fellow 
(fig.  265).  A  transverse  bar  in  front  of  the  arms  joins  the  rods  of  the  two 
sides  meeting  them  at  the  point  where  the  three  processes  diverge.  The 
process  in  the  anterolateral  arm  (fig.  266  B)  is  at  first  independent  of  this 
system  of  rods,  but  eventually  unites  with  it.  Although  our  knowledge  of 


ECHINODERMATA.  567 


the  Pluteus  types  in  the  different  groups  is  not  sufficient  to  generalise  with 
great  confidence,  a  few  points  seem  to  have  been  fairly  determined1.  The 
Plutei  of  Strongylocentrus  (figs.  266  and  267)  and  Echinus  have  eight  arms 
and  four  ciliated  epaulettes.  The  only  Cidaris-like  form,  the  Pluteus  of 
which  is  known,  is  Arbacia  :  it  presents  certain  peculiarities.  The  anal  lobe 
develops  a  pair  of  posterior  (auricular)  appendages,  and  the  ciliated  ring, 
besides  growing  out  into  the  normal  eight  appendages,  has  a  pair  of  short 
blunt  anterior  and  posterior  lobes.  An  extra  pair  of  non-ciliated  accessory 
mouth  arms  appears  also  to  be  developed.  Ciliated  epaulettes  are  not 
present.  So  far  as  is  known  the  Clypeastroid  larva  is  chiefly  characterized 
by  the  round  form  of  the  anal  lobe.  The  calcareous  rods  are  latticed.  In  the 
Pluteus  of  Spatangoids  there  are  (fig.  263)  five  pairs  of  arms  around  the 
mouth  pointing  forwards,  and  three  arms  developed  from  the  anal  lobe 
pointing  backwards.  One  of  these  is  unpaired,  and  starts  from  the  apex  of 
the  anal  lobe.  All  the  arms  have  calcareous  rods  which,  in  the  case  of  the 
posterior  pair,  the  anterolateral  pair,  and  the  unpaired  arm  of  the  anal  lobe, 
are  latticed.  Ciliated  epaulettes  are  not  developed. 

Viviparous  larvae  of  Echinoids  have  been  described  by  Agassiz2. 

The  development  of  the  permanent  Echinus  has  been  chiefly  worked  out 
by  Agassiz  and  Metschnikoff. 

In  the  Pluteus  of  Echinus  lividus  the  first  indication  of  the  adult  arises, 
when  three  pairs  of  arms  are  already  developed,  as  an  invagination  of  the 
skin  on  the  left  side,  between  the  posterior  and  anterolateral  arms,  the 
bottom  of  which  is  placed  close  to  the  water-vascular  vesicle  (fig.  266  B,  u/\ 
The  base  of  this  invagination  becomes  very  thick,  and  forms  the  ventral  disc 
of  the  future  Echinus.  The  parts  connecting  this  disc  with  the  external 
skin  become  however  thin,  and,  on  the  narrowing  of  the  external  aperture  of 
invagination  and  the  growth  of  the  thickened  disc,  constitute  a  covering  for 
the  disc,  called  by  Metschnikoff  the  amnion.  The  water- vascular  vesicle 
adjoining  this  disc  grows  out  into  five  processes,  forming  as  many  tube  feet, 
which  cause  the  surface  of  the  involuted  disc  to  be  produced  into  the  same 
number  of  processes.  The  external  opening  of  the  invagination  of  the  disc 
never  closes,  and  after  the  development  of  the  tube  feet  begins  to  widen 
again,  and  the  amnion  to  atrophy.  Through  the  opening  of  the  invagination 
the  tube  feet  now  project.  The  dorsal  and  right  surface  of  the  Pluteus, 
which  extends  so  as  to  embrace  the  opening  of  the  madreporic  canal  and 
the  anus,  forms  the  abactinal  or  dorsal  surface  of  the  future  Echinus 
(fig.  267,  a).  This  disc  fits  on  to  the  actinal  invaginated  surface  which  arises 
on  the  left  side  of  the  Pluteus.  On  the  right  surface  of  the  larva  (dorsal  of 
permanent  Echinus)  two  pedicellariae  appear,  and  at  a  later  period  spines 
are  formed,  which  are  at  first  arranged  in  a  ring-like  form  round  the  edge  of 
the  primitively  flat  test.  While  these  changes  are  taking  place,  and  the  two 
surfaces  of  the  future  Echinus  are  gradually  moulding  themselves  so  as  to 

1  Vide  especially  Muller,  Agassiz,  and  Metschnikoff. 

2  For  viviparous  Echini  vide  Agassiz,  Proc.  Amer.  Acad.  1876. 


568 


ECHINOID    PLUTEUS. 


form  what  is  obviously  a  young  Echinus,  the  arms  of  the  Pluteus  with  their 
contained  skeleton  have  been  gradually  undergoing  atrophy.  They  become 
irregular  in  form,  their  contained  skeleton  breaks  up  into  small  pieces,  and 
they  are  gradually  absorbed. 

The  water-vascular  ring  is   from   the    first    complete,   so    that,   as    in 
Asterias,  it  is  perforated  through  the  centre  by  a  new  oesophagus.    According 


FIG.  266.    SIDE  AND  DORSAL  VIEW  OF  A  LARVA  OF  STRONGYLOCENTRUS. 

(From  Agassiz.)     General  reference  letters  as  in  figs.  264  and  265. 
e" .     anterolateral  arms;  v" '.  ciliated  epaulettes;  ?&'.  invagination  to  form  the  disc 
of  Echinus. 

to  Agassiz  the  first  five  tentacles  or  tube  feet  grow  into  the  radial  canals, 
and  form  the  odd  terminal  tentacles  exactly  as  in  Asterias1.  Spatangus 
only  differs  in  development  from  Echinus  in  the  fact  that  the  opening  of  the 
invagination  to  form  the  ventral  disc  becomes  completely  closed,  and  that 
the  tube  feet  have  eventually  to  force  their  way  through  the  larval  epidermis 
of  the  amnion,  which  is  ruptured  in  the  process  and  eventually  thrown 
off. 

Crinoidea.     The   larva   of  Antedon,  while  still  within  the 
egg-shell,  assumes  an  oval  form  and  uniform  ciliation.     Before  it 

1  Gotte  (No.  549)  supported  by  Muller's  and  Krohn's  older,  and  in  some  points 
extremely  erroneous  observations,  has  enunciated  the  view  that  the  radial  canals  in 
Echinoids  and  Holothuroids  have  a  different  nature  from  those  in  Asteroids  and 
Ophiuroids. 


ECHINODERMATA. 


569 


becomes  hatched  the  uniform  layer  of  cilia  is  replaced  by  four 
transverse  bands  of  cilia,  and  a  tuft  of  cilia  at  the  posterior 
extremity.  In  this  condition  it  escapes  from  the  egg-shell 


FIG.  267.     FULL-GROWN  LARVA  OF  STRONGYLOCENTRUS.    (From  Agassiz.) 
The  figure  shews   the   largely-developed   abactinal  disc  of  the  young  Echinus 
enclosing  the  larval  stomach.     Reference  letters  as  in  previous  figs. 

(fig.  268  A),  and  becomes  bilateral,  owing  to  a  flattening  of  the 
ventral    surface.     On   the   flattened   surface  appears  a  ciliated 


570 


CRINOID   LARVA. 


depression  corresponding  in  position  with  the  now  closed  blas- 
topore  (vide  p.  550).  The  third  ciliated  band  bends  forward 
to  pass  in  front  of  this  (fig.  269).  Behind  the  last  ciliated  band 
there  is  present  a  small  depression  of  unknown  function,  also 


FIG.  768.     THREB  STAGES  IN  THE  DEVELOPMENT  OF  ANTEDON  (COMATULA.) 

(From  Lubbock;  after  Thomson.) 

A.  larva  just  hatched;  B.  larva  with  rudiment  of  the  calcareous  plates;  C.  Penta- 
crinoid  larva. 


ECHINODERMATA. 


571 


situated  on  the  ventral  surface.  The  posterior  extremity  of  the 
embryo  elongates  to  form  the  rudiment  of  the  future  stem,  and 
a  fresh  depression,  marking  the  position  of  the  future  mouth, 
makes  its  appearance  on  the  anterior  and  ventral  part. 

While  the  ciliated  bands  are  still  at  their  full  development, 
the  calcareous  skeleton  of  the  future  calyx  makes  its  appearance 
in  the  form  of  two  rows,  each  of  five  plates,  formed  of  a  network 
of  spicula  (figs.  268  B  and  269).  The  plates  of  the  anterior  ring 
are  known  as  the  orals,  those  of  the  posterior  as  the  basals. 
The  former  surround  the  left,  i.e.  anterior 
peritoneal  sack ;  the  latter  the  right,  i.e. 
posterior  peritoneal  sack.  The  two  rows 
of  plates  are  at  first  not  quite  transverse, 
but  form  two  oblique  circles,  the  dorsal 
end  being  in  advance  of  the  ventral. 
The  rows  soon  become  transverse,  while 
the  originally  somewhat  ventral  oral 
surface  is  carried  into  the  centre  of  the 
area  enclosed  by  the  oral  plates. 

By  the  change  in  position  of  the 
original  ventral  surface  relatively  to  the 
axis  of  the  body,  the  bilateral  symmetry 
of  the  larva  passes  into  a  radial  sym- 
metry. While  the  first  skeletal  elements 
of  the  calyx  are  being  formed,  the 
skeleton  of  the  stem  is  also  established. 
The  terminal  plate  is  first  of  all  esta- 
blished, then  the  joints,  eight  at  first,  of 
the  stem.  The  centro-dorsal  plate  is 
stated  by  Thomson  to  be  formed  as  the 
uppermost  joint  of  the  stem1.  The  larva,  after  the  completion 
of  the  above  changes,  is  shewn  in  fig.  268  B,  and  somewhat  more 
diagrammatically  in  fig.  269. 

After  the  above  elements  of  the  skeleton  have  become  es- 
tablished the  ciliated  bands  undergo  atrophy,  and  shortly  after- 

1  Gotte  (No.  549)  on  the  other  hand  holds  that  the  centro-dorsal  plate  is  developed 
by  the  coalescence  of  a  series  of  at  first  independent  rods,  which  originate  simul- 
taneously with,  and  close  to,  the  lower  edges  of  the  basals,  and  that  it  is  therefore 
similar  in  its  origin  to  the  basals. 


FIG.  269.  LARVA  OF 
ANTEDON  WITH  RUDIMENTS 
OF  CALCAREOUS  SKELETON. 
(From  Carpenter;  after 
Thomson.) 

i.  Terminal  plate  at  the 
end  of  the  stem  ;  3.  basals  ; 
or.  orals  ;  bl.  position  of  blas- 
topore. 


572 


CRINOID   LARVA. 


wards  the  larva  becomes  attached  by  the  terminal  plate  of  its 
stem.  It  then  passes  into  the  Pentacrinoid  stage!  The  larva  in 
this  stage  is  shewn  in  fig.  268  C  and  fig.  270.  New  joints  are 
added  at  the  upper  end  of  the  stem  next  the  calyx,  and  a  new 
element — the  radials — makes  its  appearance  as  a  ring  of  five 
small  plates,  placed  in  the  space  between  the  basals  and  orals, 
and  in  the  intervals  alternating  with  them 
(fig.  270,  4).  The  roof  of  the  oral  vesti- 
bule (vide  fig.  253  and  p.  551)  has  in 
the  meantime  become  ruptured ;  and 
the  external  opening  of  the  mouth  thus 
becomes  established.  Surrounding  the 
mouth  are  five  petal-like  lobes,  each  of 
them  supported  by  an  oral  plate  (fig. 
268  C).  In  the  intervals  between  them 
five  branched  and  highly  contractile  ten- 
tacles, which  were  previously  enclosed 
within  the  vestibule,  now  sprout  out : 
they  mark  the  position  of  the  future 
radial  canals,  and  are  outgrowths  of  the 
water-vascular  ring.  At  the  base  of  each 
of  them  a  pair  of  additional  tentacles  is 
soon  formed.  Each  primary  tentacle  cor- 
responds to  one  of  the  radials.  These 
latter  are  therefore,  as  their  name  implies, 
radial  in  position;  while  the  basals  and 
orals  are  interradial.  In  addition  to  the 
contractile  radial  tentacles  ten  non-con- 
tractile tentacles,  also  diverticula  of  the 
water- vascular  ring,  are  soon  formed,  two 
for  each  interradius. 

In  the  course  of  the  further  develop- 
ment  the  equatorial  space   between   the         FlG-  27<>.  YOUNG  PEN- 

.  TACRINOID  LARVA  OF  AN 

orals  and  the  basals  enlarges,  and  gives  TEDON.  (From  Carpenter ; 
rise  to  a  wide  oral  disc,  the  sides  of  which  after  w>'ville  Thoms°"-) 

-  ,    ,         .  ...  .  i.  terminal  plate  of  stem; 

are  formed  by  the  radials  resting  on  the  cd.  centro-donal  plate;  3. 
basals;  while  in  the  centre  of  it  are  bftsalsJ  4-  radials;  or.  orals. 
placed  the  five  orals,  each  with  its  special  lobe. 

The  anus,  which  is  formed  on  the  ventral  side  in  the  position 


ECHINODERMATA.  573 


of  the  blastopore  (p.  551),  becomes  surrounded  by  an  anal  plate, 
which  is  interradial  in  position,  and  lies  on  the  surface  of  the 
oral  disc  between  the  orals  and  radials.  On  the  oral  plate  in 
the  next  interradius  is  placed  the  opening  of  a  single  funnel 
leading  into  the  body  cavity,  which  Ludwig  regards  as  equiva- 
lent to  the  opening  of  the  madreporic  canal  (vide  p.  55 1)1. 

From  the  edge  of  the  vestibule  the  arms  grow  out,  carrying 
with  them  the  tentacular  prolongation  of  the  water-vascular  ring. 
Two  additional  rows  of  radials  are  soon  added. 

The  stalked  Pentacrinoid  larva  becomes  converted,  on  the 
absorption  of  the  stalk,  into  the  adult  Antedon.  The  stalk  is 
functionally  replaced  by  a  number  of  short  cirri  springing  from 
the  centro-dorsal  plate.  The  five  basals  coalesce  into  a  single 
plate,  known  as  the  rosette,  and  the  five  orals  disappear,  though 
the  lobes  on  which  they  were  placed  persist.  In  some  stalked 
forms,  e.g.  Rhizocrinus  Hyocrinus,  the  orals  are  permanently 
retained.  The  arms  bifurcate  at  the  end  of  the  third  radial,  and 
the  first  radial  becomes  in  Antedon  rosacea  (though  not  in  all 
species  of  Antedon)  concealed  from  the  surface  by  the  growth  of 
the  centro-dorsal  plate.  An  immense  number  of  funnels,  leading 
into  the  body  cavity,  are  formed  in  addition  to  the  single  one 
present  in  the  young  larva.  These  are  regarded  by  Ludwig  as 
equivalent  to  so  many  openings  of  the  madreporic  canal ;  and 
there  are  developed,  in  correspondence  with  them,  diverticula  of 
the  water-vascular  ring. 

Comparison  of  Echinoderm  Larvce  and  General  Conclusions. 

In  any  comparison  of  the  various  types  of  Echinoderm  larvae 
it  is  necessary  to  distinguish  between  the  free-swimming  forms, 
and  the  viviparous  or  fixed  forms.  A  very  superficial  examina- 
tion suffices  to  shew  that  the  free-swimming  forms  agree  very 
much  more  closely  amongst  themselves  than  the  viviparous 

1  I  have  made  no  attempt  to  discuss  the  homologies  of  the  plates  of  the  larval 
Echinodermata  because  the  criteria  for  such  a  discussion  are  still  in  dispute.  The 
suggestive  memoirs  of  P.  H.  Carpenter  (No.  548)  on  this  subject  may  be  consulted  by 
the  reader.  Carpenter  attempts  to  found  his  homologies  on  the  relation  of  the  plates 
to  the  primitive  peritoneal  vesicles,  and  I  am  inclined  to  believe  that  this  method  of 
dealing  with  these  homologies  is  the  right  one.  Ludwig  (No.  559)  by  regarding  the 
opening  of  the  madreporic  canal  as  a  fixed  point  has  arrived  at  very  different  results. 


574 


COMPARISON   OF   ECHINODERM    LARV.-E. 


forms.  We  are  therefore  justified  in  concluding  that  in  the 
viviparous  forms  the  development  is  abbreviated  and  modified. 

All  the  free  forms  are  nearly  alike  in  their  earliest  stage  after 
the  formation  of  the  archenteron.  The  surface  between  the 
anus  and  the  future  mouth  becomes  flattened,  and  (except  in 
Antedon,  Cucumaria,  Psolinus,  etc.  which  practically  have  an 
abbreviated  development  like  that  of  the  viviparous  forms)  a 
ridge  of  cilia  becomes  established  in  front  of  the  mouth,  and  a 
second  ridge  between  the  mouth  and  the  anus.  This  larval 
form,  which  is  shewn  in  fig.  264  A,  is  the  type  from  which  the 
various  forms  of  Echinoderm  larvae  start. 

In  all  cases,  except  in  Bipinnaria,  the  two  ciliated  ridges 
soon  become  united,  and  constitute  a  single  longitudinal  post- 
oral  ciliated  ring. 

The  larvae  in  their  further  growth  undergo  various  changes, 
and  in  the  later  stages  they  may  be  divided  into  two  groups  : 

(1)  The  Pluteus  larva  of  Echinoids  and  Ophiuroids. 

(2)  The  Auricularia  (Holothuroids)  and  Bipinnaria  (Aster- 
oids) type. 

The  first  group  is  characterized  by  the  growth  of  a  number 
of  arms  more  or  less  surrounding  the  mouth,  and  supported 
by  calcareous  rods.  The  ciliated  band  retains  its  primitive 
condition  as  a  simple  longitudinal  band  throughout  larval  life. 
There  is  a  very  small  prae-oral  lobe,  while  an  anal  lobe  is  very 
largely  developed. 

The  Auricularia  and   Bi-  A.  B 

pinnaria  resemble  each  other 
in  shape,  in  the  development 
of  a  large  prae-oral  lobe,  and 
in  the  absence  of  provisional 
calcareous  rods  ;  but  differ  in 
the  fact  that  the  ciliated  band 
is  single  in  Auricularia  (fig 
271  A),  and  is  double  in  Bi- 
pinnaria (fig.  271  B). 

TheBipinnarialarvashews 

THUROID.    B.    THE  LARVA  OF  AN  ASTE- 
a  great  tendency  to  develop     RIAS. 

soft  arms;   while  in  the  Auri-          .  »'•  mouth;    st.   stomach;    a.  anus;    I.e. 
,      .     ,_,      ,          •*_   ••      1-1-  primitive   longitudinal  ciliated   band;  pr.c. 

cularia  the  longitudinal  ciliat-     pr3e-oral  ciliated  band. 


FlG- 


THE  LARVA  OF  A 


ECHINODERMATA.  575 


ed  band  breaks  up  into  a  number  of  transverse  ciliated  bands. 
This  condition  is  in  .some  instances  reached  directly,  and  such 
larvae  undoubtedly  approximate  to  the  larvae  of  Antedon,  in 
which  the  uniformly  ciliated  condition  is  succeeded  by  one  with 
four  transverse  bands,  of  which  one  is  prae-oral. 

All  or  nearly  all  Echinoderm  larvae  are  bilaterally  symmetrical, 
and  since  all  Echinodermata  eventually  attain  a  radial  sym- 
metry, a  change  necessarily  takes  place  from  the  bilateral  to  the 
radial  type. 

In  the  case  of  the  Holothurians  and  Antedon,  and  generally 
in  the  viviparous  types,  this  change  is  more  or  less  completely 
effected  in  the  embryonic  condition  ;  but  in  the  Bipinnaria  and 
Pluteus  types  a  radial  symmetry  does  not  become  apparent  till 
after  the  absorption  of  the  larval  appendages.  It  is  a  re- 
markable fact,  which  seems  to  hold  for  the  Asteroids,  Ophiur- 
oids,  Echinoids,  and  Crinoids,  that  the  dorsal  side  of  the  larva  is 
not  directly  converted  into  the  dorsal  disc  of  the  adult;  but 
the  dorsal  and  right  side  becomes  the  adult  dorsal  or  abactinal 
surface,  while  the  ventral  and  left  becomes  the  actinal  or  ventral 
surface. 

It  is  interesting  to  note  with  reference  to  the  larvae  of  the 
Echinodermata  that  the  various  existing  types  of  larvae  must 
have  been  formed  after  the  differentiation  of  the  existing  groups 
of  the  Echinodermata ;  otherwise  it  would  be  necessary  to  adopt 
the  impossible  position  that  the  different  groups  of  Echinoder- 
mata were  severally  descended  from  the  different  types  of  larvae. 
The  various  special  appendages,  etc.  of  the  different  larvae  have 
therefore  a  purely  secondary  significance;  and  their  atrophy 
at  the  time  of  the  passage  of  the  larva  into  the  adult,  which 
is  nothing  else  but  a  complicated  metamorphosis,  is  easily  ex- 
plained. 

Originally,  no  doubt,  the  transition  from  the  larva  to  the 
adult  was  very  simple,  as  it  is  at  present  in  most  Holothurians ; 
but  as  the  larvae  developed  various  provisional  appendages,  it 
became  necessary  that  these  should  be  absorbed  in  the  passage 
to  the  adult  state. 

It  would  obviously  be  advantageous  that  their  absorption 
should  be  as  rapid  as  possible,  since  the  larva  in  a  state  of 
transition  to  the  adult  would  be  in  a  very  disadvantageous 


576  COMPARISON   OF  ECHINODERM 

position.  The  rapid  metamorphosis,  which  we  find  in  Asteroids, 
Ophiuroids,  and  Echinoids  in  the  passage  from  the  larval  to  the 
adult  state,  has  no  doubt  arisen  for  this  reason. 

In  spite  of  the  varying  provisional  appendages  possessed  by 
Echinoderm  larvae  it  is  possible,  as  stated  above  (p.  574),  to 
recognise  a  type  of  larva,  of  which  all  the  existing  Echinoderm 
larval  forms  are  modifications.  This  type  does  not  appear  to 
me  to  be  closely  related  to  that  of  the  larvae  of  any  group 
described  in  the  preceding  pages.  It  has  no  doubt  certain 
resemblances  to  the  trochosphere  larva  of  Chaetopoda,  Mollusca, 
etc.,  but  the  differences  between  the  two  types  are  more  striking 
than  the  resemblances.  It  firstly  differs  from  the  trochosphere 
larva  in  the  character  of  the  ciliation.  Both  larvae  start  from  the 
uniformly  ciliated  condition,  but  while  the  prae-oral  ring  is  almost 
invariable,  and  a  peri-anal  ring  very  common  in  the  trochosphere; 
in  the  Echinoderm  larva  such  rings  are  rarely  found  ;  and  even 
when  present,  i.e.  the  prae-oral  ring  of  Bipinnaria  and  the  terminal 
though  hardly  peri-anal  patch  of  Antedon,  do  not  resemble 
closely  the  more  or  less  similar  structures  of  the  trochosphere. 
The  two  ciliated  ridges  (fig.  264  A)  common  to  all  the  Echino- 
derm larvae,  and  subsequently  continued  into  a  longitudinal  ring, 
have  not  yet  been  found  in  any  trochosphere.  The  transverse 
ciliated  rings  of  the  Holothurian  and  Crinoid  larvae  are  of  no 
importance  in  the  comparison  between  the  trochosphere  larvae 
and  the  larvae  of  Echinodermata,  since  such  rings  are  frequently 
secondarily  developed.  Cf.  Pneumodermon  and  Dentalium  a- 
mongst  Mollusca. 

In  the  character  of  the  prae-oral  lobe  the  two  types  again 
differ.  Though  the  prae-oral  lobe  is  often  found  in  Echinoderm 
larvae  it  is  never  the  seat  of  an  important  (supra-oesophageal) 
ganglion  and  organs  of  special  sense,  as  it  invariably  is  in  the 
trochosphere. 

Nothing  like  the  vaso-peritoneal  vesicles  of  the  Echinoderm 
larvae  has  been  found  in  the  trochosphere ;  nor  have  the  charac- 
teristic trochosphere  excretory  organs  been  found  in  the  Echino- 
derm larvae. 

The  larva  which  most  nearly  approaches  those  of  the  Echino- 
dermata is  the  larva  of  Balanoglossus  described  in  the  next 
chapter. 


ECHINODERMATA.  577 


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(542)  Alex.  Agassiz.     Revision  of  the  Echini.    Cambridge,  U.S.  1872— 74. 

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(544)  J.  Barrois.     "  Embryogenie  de  1'Asteriscus  verruculatus  "    Journal  dc 
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(545)  A.   Baur.     Beitrdge  zur  Naturgeschichte  d.  Synapta  digitata.     Dresden, 
1864. 

(546)  H.  G.   Bronn.     Klassen  u.  Ordnungen  etc.  Strahlenthiere,  Vol.  II.  1860. 

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(550)  R.  Greeff.     "Ueber  die  Entwicklung  des  Asteracanthion  rubens  vom  Ei 
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(551)  R.  Greeff.     "Ueber  den  Bau  u.  die  Entwicklung  d.  Echinodermen."   Sitz. 
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(554)  Koren  and  Danielssen.    "Observations  on  the  development  of  the  Star- 
fishes."   Ann.  and  Mag.  of  Nat.  Hist.,  Vol.  XX.     1857. 

(555)  A.  Kowalevsky.    " Entwicklungsgeschichte  d.  Holothurien. "   Mhn.Ac. 
Petersbourg,  Ser.  VII.,  Tom.  XL,  No.  6. 

(556)  A.   Krohn.      "Beobacht.  a.  d.    Entwick.   d.    Holothurien   u.    Seeigel." 
Miillers  Archiv,  1851. 

(557)  A.  Krohn.     "Ueb.  d.  Entwick.  d.  Seesterne  u.  Holothurien."     Miillcr's 
Archiv,  1853. 

(558)  A.  Krohn.      "Beobacht.  lib.    Echinodermenlarven."      Mailer's  Archiv, 
1854. 

(559)  H.  Ludwig.     "Ueb.  d.  primar.  Steinkanal  d.  Crinoideen,  nebst  vergl. 
anat.  Bemerk.  lib.  d.  Echinodermen."     Zeit.f.  wiss.  ZooL,  Vol.  xxxiv.     1880. 

(560)  E.    Metschnikoff.      "Studien   iib.   d.   Entwick.   d.   Echinodermen   u. 
Nemertinen."     Mem.  Ac.  Petersboiirg,  Series  vii.,  Tom.  xiv.,  No.  8.     1869. 

(561)1  Joh.  Miiller.  "Ueb.  d.  Larven  u.  d.  Metamorphosed.  Echinodermen." 
Abhandlungen  d.  Berlin.  Akad.  (Five  Memoirs),  1848,  49,  50,  52  (two  Memoirs). 

(562)  Joh.  Mtiller.  "Allgemeiner  Plan  d.  Entwicklung  d.  Echinodermen." 
Abhandl.  d.  Berlin.  Akad.,  1853. 

1  The  dates  in  this  reference  are  the  dates  of  publication. 
B.  II.  37 


578  BIBLIOGRAPHY. 


(563)  E.  Selenka.     "Zur  Entwicklung  d.   Holothurien."     Zeit.  f.  wiss.  Zool., 
Bd.  xxvii.     1876. 

(564)  E.  Selenka.     "Keimblatter  u.  Organanlage  bei  Echiniden."   Zeit.f.-wiss. 
Zool.,  Vol.  xxxin.     1879. 

(565)  Sir  Wyville  Thomson.     "On  the  Embryology  of  the  Echinodermata." 
Natural  History  Review,  1 864. 

(566)  Sir  Wyville  Thomson.     "On  the  Embryogeny  of  Antedon  rosaceus." 
Phil.  Trans.  1865. 


CHAPTER   XXI. 


ENTEROPNEUSTA. 


THE  larva  of  Balanoglossus  is  known  as  Tornaria.  The  prse- 
larval  development  is  not  known,  and  the  youngest  stage  (fig. 
272)  so  far  described  (Gotte,  No.  569)  has 
many  remarkable  points  of  resemblance  to 
a  young  Bipinnaria. 

A  mouth  (m\  situated  on  the  ventral 
surface,  leads  into  an  alimentary  canal  with 
a  terminal  anus  (an).  A  prae-oral  lobe  is 
well  developed,  as  in  Bipinnaria,  but  there 
is  no  post-anal  lobe.  The  bands  of  cilia 
have  the  same  general  form  as  in  Bipin- 
naria. There  is  a  prae-oral  band,  and  a 
longitudinal  post-oral  band ;  and  the  two 
bands  nearly  meet  at  the  apex  of  the  prae- 
oral  lobe  (fig.  273).  A  contractile  band 


an 

FIG.  272.  EARLY 
STAGE  IN  THE  DEVELOP- 
MENT OF  TORNARIA. 
(After  Gotte.) 

W.  so-called  water- 
vascular  vesicle  develop- 
ing as  an  outgrowth 
of  the  mesenteron;  m. 

passes  from  the  oesophagus  to  the  apex  of    mouth;  an.  anus, 
the  prae-oral  lobe,  and  a  diverticulum  (fig.  272,    W)  from  the 
alimentary  tract,  directed  towards  the  dorsal  surface,  is  present. 
Contractile  cells  are  scattered  in  the  space  between  the  body 
wall  and  the  gut. 

In  the  following  stage  (fig.  274  A)  a  conspicuous  transverse 
post-oral  band  of  a  single  row  of  long  cilia  is  formed,  and  the 
original  bands  become  more  sinuous.  The  alimentary  diverti- 
culum of  the  last  stage  becomes  an  independent  vesicle  opening 
by  a  pore  on  the  dorsal  surface  (fig.  274  A,  w).  The  contractile 
cord  is  now  inserted  on  this  vesicle.  Where  this  cord  joins  the 
apex  of  the  prae-oral  lobe  between  the  two  anterior  bands  of 
cilia  a  thickening  of  the  epiblast  (?  a  ganglion)  has  become 

37—2 


580 


ENTEROPNEUSTA. 


C.C. 


an. 

FIG.  273.    YOUNG  TORNARIA. 

(After  Miiller.) 

m.  mouth ;  an.  anus ;  w.  water- 
vascular  vesicle  ;  oc.  eye-spots  ;  c.c. 
contractile  cord. 


established,  and  on  it  are  placed 
two  eye-spots  (fig.  273  oc,  and 
fig.  274  A).  A  deep  bay  is 
formed  on  the  ventral  surface  of 
the  larva. 

As  the  larva  grows  older  the 
original   bands  of  cilia  become 
more    sinuous,    and    a    second 
transverse  band  with  small  cilia 
is  formed  (in  the  Mediterranean 
larva)     between     the     previous 
transverse   band  and  the  anus. 
The    water-vascular    vesicle    is 
prolonged    into  two  spurs,  one 
on  each   side   of  the   stomach. 
A  pulsating  vesicle  or  heart  is 
also  formed   (fig.  274  B,  ht),  and  arises,  according  to  Spcngel 
(No.  572),  as  a  thicken- 
ing   of  the   epidermis. 
It     subsequently     be- 
comes enveloped  in  a 
pericardium,      and      is 
placed  in  a  depression 
in    the   water-vascular 
vesicle.     Two  pairs  of 
diverticula,  one  behind 
the    other,    grow    out 
(Agassiz,  No.  568)  from 
the    gastric    region   of 
the    alimentary   canal. 
The  two  parts  of  each 
pair      form      flattened 
compartments,     which 
together  give  rise  to  a 
complete  investment  of 
the  adjoining  parts  of 
the    alimentary    tract. 
The  two  parts  of  each 
coalesce,  and  thus  form 


FlG.   274.      TWO  STAGKS  IN  THK   1  >KY  KI.<  >I'M  KN  I 

OF  TORNARIA.    (After  Metschnikoff.) 

The  black  lines  represent  the  ciliated  hands. 
m.   mouth;  an.   anus;  br.    branchial    cleft;  ///. 

heart ;    c.    Ixxly    cavity    between    splanchnic    and 

somatic  mesoblast  layers;  7.-'.  watcr-vascvilar  vesicle: 

v.  circular  blood-vessel. 


ENTEROPNEUSTA. 


58l 


a  double-walled  cylinder  round  the  alimentary  tract,  but  their 
cavities  remain  separated  by  a  dorsal  and  ventral  septum. 

Eventually  (Spengel)  the  cavity  of  the  anterior  cylinder 
forms  the  section  of  the  body  cavity  in  the  collar  of  the  adult, 
and  that  of  the  posterior  (fig.  274  B,  c)  the  remainder  of  the 
body  cavity.  The  septa,  separating  the  two  halves  of  each, 
remain  as  dorsal  and  ventral  mesenteries. 

The  conversion  of  Tornaria  (fig.  274  A)  into  Balanoglossus 
(fig.  274  B)  is  effected  in  a  few  hours,  and  consists  mainly  in 
certain  changes  in  configuration,  and  in  the  disappearance  of 
the  longitudinal  ciliated  band. 

The  body  of  the  young  Balanoglossus  (fig.  274  B)  is  divided 
into  three  regions  (i)  the  proboscidian  region,  (2)  the  collar, 
(3)  the  trunk  proper.  The  proboscidian  region  is  formed  by  the 
elongation  of  the  prae-oral  lobe  into  an  oval  body  with  the  eye- 
spots  at  its  extremity,  and  provided  with  strong  longitudinal 
muscles.  The  heart  (hi)  and  water-vascular  vesicle  lie  near  its 
base,  but  the  contractile  cord  con- 
nected with  the  latter  is  no  longer 
present.  The  mouth  is  placed  on 
the  ventral  side  at  the  base  of  the 
prae-oral  lobe,  and  immediately  be- 
hind it  is  the  collar.  The  remainder 
of  the  body  is  more  or  less  conical, 
and  is  still  girt  with  the  larval 
transverse  ciliated  band,  which  lies 
in  the  middle  of  the  gastric  region 
in  the  Mediterranean  species,  but 
in  the  cesophageal  region  in  the 
American  one. 

The  whole  of  the  body,  including 
the  proboscis,  becomes  richly  cili- 
ated. 

One  of  the  most  important  cha-  Sus  WITH  FOUR  BRANCHIAL 
racters  of  the  adult  Balanoglossus  CLEFTS*  (After  Alex.  Agossiz.) 

r  .  m.  mouth  ;  an.  anus  ;  br.  bran- 

consists  in  the  presence  of  respira-     chial  cleft .  hL  heart ;  IV.  water- 
tory  structures  comparable  with  the     vascular  vesicle, 
vertebrate    gill   slits.     The    earliest  traces  of  these  structures 
are   distinctly  formed  while  the  larva  is  still  in   the   Tornaria 


FIG.  275.    LATE  STAGE  IN  THE 
DEVELOPMENT  OF   BALANOGLOS- 


582  I'N  I'KUOl'NKUSTA. 


condition,  as  one  pair  of  pouches  from  the  oesophagus  in  the 
Mediterranean  species,  and  four  pairs  in  the  American  one 
(fig.  275,  br). 

In  the  Mediterranean  Tornaria  the  two  pouches  meet  the 
skin  dorsally,  and  in  the  young  Balanoglossus  (fig.  274  B,  br) 
acquire  an  external  opening  on  the  dorsal  side.  In  the  American 
species  the  first  four  pouches  are  without  external  openings 
till  additional  pouches  have  been  formed.  Fresh  gill  pouches 
continue  to  be  formed  both  in  the  American  and  probably 
the  Mediterranean  species,  but  the  conversion  of  the  simple 
pouches  into  the  complicated  gill  structure  of  the  adult 
has  only  been  studied  by  Agassiz  (No.  568)  in  the  American 
species.  It  would  seem  in  the  first  place  that  the  structure  of 
the  adult  gill  slits  is  much  less  complicated  in  the  American  than 
in  the  Mediterranean  species.  The  simple  pouches  of  the  young 
become  fairly  numerous.  They  are  at  first  circular ;  they  then 
become  elliptical,  and  the  dorsal  wall  of  each  slit  becomes  folded ; 
subsequently  fresh  folds  are  formed  which  greatly  increase  the 
complexity  of  the  gills.  The  external  openings  are  not  acquired 
till  comparatively  late. 

Our  knowledge  of  the  development  of  the  internal  organs,  mainly 
derived  from  Agassiz,  is  still  imperfect.  The  vascular  system  appears  early 
in  the  form  of  a  dorsal  and  a  ventral  vessel,  both  pointed,  and  apparently 
ending  blindly  at  their  two  extremities.  The  two  spurs  of  the  water-vascular 
vesicle,  which  in  the  Tornaria  stage  rested  upon  the  stomach,  now  grow 
round  the  oesophagus,  and  form  an  anterior  vascular  ring,  which  Agassiz 
describes  as  becoming  connected  with  the  heart,  though  it  still  communicates 
with  the  exterior  by  the  dorsal  pore  and  seems  to  become  connected  with  the 
remainder  of  the  vascular  system.  According  to  Spengel  (No.  572)  the 
dorsal  vessel  becomes  connected  with  the  heart,  which  remains  through  life 
in  the  proboscis :  the  cavity  of  the  water-vascular  vesicle  forms  the  cavity  of 
the  proboscis  in  the  adult,  and  its  pore  remains  as  a  dorsal  (not,  as  usually 
stated,  ventral)  pore  leading  to  the  exterior. 

The  eye-spots  disappear. 

Tornaria  is  a  very  interesting  larval  form,  since  it  is  inter- 
mediate in  structure  between  the  larva  of  an  Echinoderm  and 
trochosphere  type  common  to  the  Mollusca,  Chxtopoda,  etc. 
The  shape  of  the  body  especially  the  form  of  the  ventral 
depression,  the  character  of  the  longitudinal  ciliated  band,  the 
structure  and  derivation  of  the  water-vascular  vesicle,  and  the 


ENTEROPNEUSTA.  583 


formation  of  the  walls  of  the  body  cavity  as  gastric  diverticula, 
are  all  characters  which  point  to  a  connection  with  Echinodcrm 
larvae. 

On  the  other  hand  the  eye-spots  at  the  end  of  the  prae-oral 
lobe1,  the  contractile  band  passing  from  the  oesophagus  to  the 
eye-spots  (fig.  273),  the  two  posterior  bands  of  cilia,  and  the 
terminal  anus  are  all  trochosphere  characters. 

The  persistence  of  the  prae-oral  lobe  as  the  proboscis  is 
interesting,  as  tending  to  shew  that  Balanoglossus  is  the  sur- 
viving representative  of  a  primitive  group. 

* 

BIBLIOGRAPHY. 

(567)  A.  Agassiz.     "Tornaria."     Ann.  Lyceum  Nat.  Hist.\u\.     New  York, 
1866. 

(568)  A.  Agassiz.      "The   History  of  Balanoglossus   and   Tornaria."     Mem. 
Amer.  Acad.  of  Arts  and  Stien.,  Vol.  IX.     1873. 

(569)  A.  Gotte.     "  Entwicklangsgeschichte  d.  Comatula  Mediterranea."   Archiv 
fur  mikr.  Anat.,  Bd.  xii.,  1876,  p.  641. 

(570)  E.  Metschnikoff.     " Untersuchungen  iib  d.  Metamorphose,  etc.  (Tor- 
naria)."    Zeit.fiir  wiss.  ZooL,  Bd.  xx.     1870. 

(571)  J.  M  tiller.     "  Ueb.  d.  Larven  u.  Metamor.  d.  Echinodermen."     Berlin 
Akad.,  1849  and  1850. 

(572)  J.  W.   Spengel.     "Ban  u.   Entwicklung  von  Balanoglossus.      Tagebl.  d. 
Naturf.  Vers.  Miinchen,  1877. 

1  It  would  be  interesting  to  have  further  information  about  the  fate  of  the  thicken- 
ing of  epiblast  in  the  vicinity  of  the  eye-spots.  The  thickening  should  by  rights  be  the 
supra-oesophageal  ganglion,  and  it  does  not  seem  absolutely  impossible  that  it  may  give 
rise  to  the  dorso-median  cord  in  the  region  of  the  collar,  which  constitutes,  according 
to  Spengel,  the  main  ganglion  of  the  adult. 


INDEX. 


Abdominalia,  459,  493,  499 

Acanthocephala,  379 

Acanthosoma,  473,  474,  475 

Acarina,  444,  454 

Accipenser,  102 

Achaeta,  319 

Achelia,  538 

Achtheres  percarum,  490 

Acineta,  7,  8 

Acraspeda,  152,  165,  167,  178,  179,  182, 

185,  186 

Actinia,  169,  171,  179 
Actinophrys,  9 

Actinotrocha,  315,  318,  363,  364 
Actinozoa,  26,   102,   152,  j66,  170,  171, 

172,  176,  178,  179,  181,  182,  186 
Actinula,  155 
Aculeata,  421 
^Egineta  flavescens,  158 
yEginidae,  156,  158 
^Eginopsis  Mediterranea,  158 
/Equorea  Mitrocoma,  182 
Agalma,  163 
Agelena,  436,  450 
Agelena  labyrinthica,  119,  438 
Alciope,  74 
Alcippidae,  499 
Alcyonaria,  152 
Alcyonidse,  167,  168 
Alcyonidium  mytili,  297,  300,  302 
Alcyonium  palmatum,  119,  148,  167,  182 
Alima,  484,  486 
Amoeba,  19,  20 
Amphibia,  22,  54,  56,  59,  60,  63,  66,  74, 

83,  102 

Amphilina,  218 

Amphioxus,  54,  56,  59,  61,  66,  93,  426 
Amphipoda,  518 
Amphiporus  lactifloreus,  202 
Amphistomum,  31 

,,  subclavatum,  205 

Amphitrochae,  330 
Amphiura  squamata,  565 


Anchorella,  108,  492,  520 

Anelasma  squalicola,  499 

Anguillulidse,  371 

Annelida,  14,  25,  98,  503,  525 

Anodon,  37,  38,  39,  100,  107,  259,  260, 

265,  266,  268 
Anopla,  189,  202 
Anura,  5 

Antedon,  568,  573,  574 
Aphides,  15,  16,  76,  79,  116,  428,  429 
Aphrodite,  42 

Apis,  402,  407,  408,  412,  413 
Aplysia,  99,  226,  238,  252,  253 
Aplysinidaa,  146 
Apoda,  459,  493 
Aptera,  395,  420 
Apus,  1 6,  79,  460,  463 
Arachnida,  22,  114,  119,  413,  4.51,  435, 

444,  454,  455,  458,  537,  539 
Arachnitis,  171 
Araneina,  50,  51,  436 
Arbacia,  567 
Area,  38 
Archigetes,  218 
Archizosea  gigas,  494 
Arenicola,  42 

Argiope,  311,  312,  315,  317 
Argonauta,  247,  248 
Argulus,  492 
Armata,  355 
Arthropoda,  12,  16,  18,  22,  75,  77,79,  83, 

108,  no,  221,  382,  383,  434,  448,503, 

525>  534»  54',  542 
Articulata,  311,  313,  316,  317 
Ascaridiae,  371 
Ascaris  nigrovenosa,  16,  82 

,,      lumbricoides,  375 
Ascetta,  144 
Ascidia  canina,  53 
Ascidians,  74,  102,  208,  426 
Asellus  aquaticus,  112,120,  516 
Astacus,   66,    465,   477,  511,   512,  513, 

525 


586 


INDKX. 


Asteracanthion,  69,  70,  561 

Asterias,  20,  68,  69,  71,  78,  80,  84,  549, 

564 
Asteroidea,  35,  36,  544,  549,  557,  563, 

576 
Astnea,  169 

Astroides,  169 

Atax  Bonzi,  445 

Atlanta,  231,  240 

Atrochae,  330 

Aurelia,  167 

Auricularia,  553,  554,  562,  574 

Autolytus  cornutus,  319,  343 

Aves,  56,  59,  61,  64,  107.  109 

Axolotl,  1 6 

Balanoglossus,  576,  579,  581 

Balanus  balanoides,  75,  493 

Belemnites,  252,  253 

Bipinnaria,  557,  563,  574,  576,  579 

Blatta,  374,  395 

Bojanus,  organ  of,  264,  282 

Bonellia,  20,  43,  44,  98,  324,   355,  358, 

359 
Bothriocephalus  salmonis,  211 

,,  proboscideus,  212 

Brachiella,  492 
Brachiolaria,  558,  564 
Brachiopoda,  311,  317,  318 
Brachyura,  466,  480,  483 
Branchiobdella,  42,  43,  346 
Branchiogasteropoda,  272 
Branchiopoda,  79,  459,  523,  524 
Branchipus,  463,  524 
Branchiura,  459,  492 
Branchionus  urceolaris,  221 
Braula,  396 
Uuccinum,  237,  280 
Bulimus  citrinus,  229 
Bunodes,  169,  171 
Buthus,  431 

Calcispongiae,  138,  148 

Calopteryx,  402 

Calycophoridce,  152,  159 

Calyptoblastic  Hydroids,  184,  185 

Calyptraea,  223,  280 

Campanularidse,  183,  184 

Capitclla,  330,  332 

Carabidae,  476 

Carcinus  Mcenas,  481,  483 

Cardium,  260,  262 

"        pygmaeum,  262 

Carinaria,  240 

Caryophyllium,  168,  171 
•pea,  165,  167 

Cecidomyia,  15,  79,  416,  417,  429 

Cephalopoda,  20,  40,  41,  102,  108,  109, 
135.  "5.  240,  242,  244,  250,  252,  253, 
270,  271,  272,  274,  279,  282,  287 

Cephalothrix  galatheae,  202 

Ceratosponguc,  146 


Cercariae,  207,  208,  209 

Cerianthus,  168,  171 

Cestodes,  14,  29,  31,  32,  33,  189,  210, 
212,  218,  313,  425,  541 

Chsetogaster,  342 

Chaetopoda,  5,  18,  23,  41,  43,  44,  54, 
67,  209,  215,  270,  275,  307,  312,  317, 
318,  319,  320,  326,  334,  33<S,  342,  346, 
349,  350,  351,  364.  369.  3«3,  3«6,  408, 
448,  457,  458,  521,  576,582 

ChiXitopteridte,  333 

Cha^tosomoidea,  371 

Chelifer,  434,  436,  442,  446,  454 

Chermes,  15,  429 

Chilognatha,   113,  387,   389,    391,    393, 

395 

Chilopoda,  387,  392,  394 
Chilostomata,  292,  297,  298,  304,  305 
Chironomus,  15,  378,  401,  402,  415,  416, 

429 

Chiton,  254,  256,  257,  273 
Chordata,  5 
Chrysaora,  165 
Chthonius,  436 
Cicada,  395 

Cirripedia,  459,  492,  496,503,  509,  520 
Cladocera,  459,  464,  519 
Clausilia,  239 
Clavella,  520 
Clavularia  crassa,  167 
Cleodora,  241 
Clepsine,   73,   346,   347,  349,  351,  352, 

353,  354 
Clio,  242,  278 
Clubione,  436 
Clupeidae,  64 
Cobitis  barbatula,  378 
Coccida;,  429 
Coccus,  50 
Ccelebogyne,  79 
Coelenterata,  3,  5,  13,  18,  26,  27,  2«S,  35, 

74,  93,  94,  126,  148,  170,  178,179,  1 80, 

181,  191,  342 

Ccenurus  cerebralis,  213,  214 
Coleochaete,  1 1 

Coleoptera,  396,  402,  409,  412,  420,  421, 
^5 

Collembola,  395,  426 
Comatula,  5,  552,  553 
Condracanthus,  ill,  120,  520 
Conochilus  volvox,  22 1 
Convoluta,  32 
Copepoda,   109,  120,  459,  460,  487,  489, 

493,  496,  503,  509,  519 
Corallium  rubrum,  168,  182 
Corethra,  422,  423,  424 
Crangoninoe,  476 
Crnniiuhv,  311 
Craniata,  5,  6,  19,  20,  54,  56,  59,  6l,  62, 

64,  74,  102 

Crinoidea,  35,  36,  544,  550,  568,  576 
Criodilus,  321,  324,  328,  341 


INDEX. 


Crisia,  304 
Crocodilia,  63 

Crustacea,  5,  6,  18,  51,  66,  102,  109,  120, 
458,  465>  487*  5<>2,  521,  524,  537,  541 
Cryptophialus,  499,  509 
Crystalloides,  163 
Ctenophora,  26,  93,   102,  152,  173,  175, 

177,  178,  179,  180,  181,  182 
Ctenostomata,  292,  297,  298,  304,  305 
Cucullarms  elegans,  46,  75,  82,  371,  376 
Cucumaria,  546,  556,  574 
Cumaceae,  459,  465,  486,  506 
Curculio,  421 

Cyclas,  259,  260,  261,  265 
Cyclops,  376,  377,  418,  489,  503 
Cyclostomata,  102,  -292,  304 
Cymbulia,  241,  242 

Cymothoa,  516,  517,  519,  520,524,  528 
Cynipidae,  15,  421,  428 
Cyphonautes,  297,  301,  304,  306,  308 
Cypridina,  500,  502 
Cysticercus  cellulosce,  214,  217 

,,          fasciolaris,  216 

,,          limacis,  213 

Daphnia,  79,  464 

Dasychone,  331,  336 

Decapoda,  66,  248,  459,  465,  469,  504, 

511 

Dendroccela,  32,  33,  189,  195,  196 
Dentalium,  258,  576 
Desmacidon,  147 
Desor,  type  of,   196,  197,  201,  202,  204, 

212,  424 
Diastopora,  304 
Dibranchiata,  225,  253 
Dicyema,  9,  131,  134,  135,  136 
Dimya,  225 
Diphyes,  159 
Diplozoon,  11,  209,  210 
Diporpa,  210 
Diptera,  49,  194,  204,  396,  401,402,407, 

409,  412,  416,  420,  429 
Discina  radiata,  317 
Discinidse,  311 

Discophora,  18,  42,  165,  346,  383 
Distomese,  189,  205,  425 
Distomum,  31 

,,         cygnoides,  209 

,,         globiparum,  207 

,,         lanceolatum,  205 
Dochmius  duodenale,  375 

,,         trigonocephalus,  375 
Donacia,  401 
Dracunculus,  376,  377 

Echinaster  fallax,  23 

,,         Sarsii,  102,  561 
Echinodermata,   5,   18,  24,  35,    74,  102, 

325,  424,  544,  573,  574,  576>  582 
Echinoidea,  35,  36,  544,  549,  565,  576 
Echinorhyncus,  379,  380 


Echinus  lividus,  83,  84,  88 

Echiurus,  44,  357,  358 

Ectoprocta,  297,  306 

Edriophthalmata,  459,  465 

Elaphocaris,  473 

Elasmobranchii,   23,   56,   59,  61,  62,  64, 

67,  105,  106.  107,  108,  109 
Enopla,  189,  202 
Entoconcha  mirabilis,  237 
Entomophaga,  421 

Entoprocta,  292,  298,  300,  302,  304,  306 
Epeira,  436 

Ephemera,  395,  409,  420,  422 
Ephyra,  186 

Epibulia  auranliaca,  159,  165 
Erichthus,  484,  507 
Errantia,  319,  336 
Esperia,  147 
Estheria,  463,  464 
Euaxes,  lol,  322,  324,  341,  346,349 
Eucharis,  178     • 

,,         multicornis,  178 

Eucopepoda,  459 
Eucope  polystyla,  23,  154 

Eunice  sanguinea,  319 

Eupagurus  prideauxii,  112,  113,  115,  511, 
520 

Euphausia,  465,  468,  504,  505,  518,  523 

Eurostomata,  176 

Eurylepta  auriculata,  192 

Eurynome,  483 

Euspongia,  146,  147 

Filaria,  377 
Filaridae,  371 
Firoloidea,  240 
Flagellata,  7,  8 
Flustrella,  301,   303 
Formica,  396 
Fungia,  182,  186 
Fusus,  275,  280,  284,  288 

Gammarus,  122,  518 

,,          fluviatilis,  117 
,,         locusta,  no,  112 
Ganoids,  54,  102 
Gasteropoda,  39,  41,  98,  225,  226,  229, 

230,  232,  233,  240,  258,  260,  261,  270, 

272,  275,  279,  283,  324 
Gasterosteus,  64,  210 
Gastrotricha,  370 
Gasterotrochce,  330,  333 
Gecarcinus,  465 
Geophilus,  392,  393 
Gephyrea,   5,   18,   24,   44,   54,  67,    102, 

318,  320,  325,  355,  357,  361,  364 
Germogen,  134 
Geryonia  hastata,  156 
Geryonidse,  156 
Glochidia,  267,  268 
Gnathobdellidas,  346,  349 
Gordiacea,  94 


588 


INDEX. 


Cimlioidca,  371,  374,  378 

;»nia,  168 
Gorgonidce,  181 
Gorgoninrc,  181 
Gregarinidae,  8 
Gryllotalpa,  401,  412,  413 
Gunnnineiv,  147,  148 
Gymnoblastic  Hydroids,  184,  185 
Gymnoloemata,  292 

Gymnosomata,  225,  240,  241,  242,  270 
Gyrodactylus,  210 

Halichondria,  147 

Ilalisarca,  22,  66,  145 

Halistemma,  165 

Helicidce,  238 

Helioporidae,  182 

Helix,  67,  229 

Hemiptera,  395,  402,  403,  409,  420,  421 

Hessia,  108,  492 

Heterakis  vermicularis,  .374 

Heteronereis,  343 

Heteropoda,  71,  72,  225,  226,  231,  278 

Hexacoralla,  152,  179,  182 

Hippopodius  gleba,  27,  159 

Hirudinea,  74,  84 

Hirudo,  350,  351,  352,  353,  354 

Holometabola,  420,  422 

Holostomum,  205 

Holothuria,  19,  25,  35,  549,  558,  576 

Holothuroidea,  35,  544,  553,  556 

Homarus,  477 

Hyaleacea,  273,  275 

Hyaleidce,  241 

Hydra,  21,  22,  26,  28,  29,  34,  152,   154, 

155.  179,  183 
Hydractinia,  539 
Hydrocoralla,  152,  181,  185 
Hydroidea,  152 
Hydromedusae,  152,  179,   182,   183,  184, 

185,  186,  187 
Hydrophilus,   374,    396,   400,   401,  402, 

404,  408,  409 
Hydrozoa,   14,   19,  26,  27,  67,  102,  152, 

155.  165.  179,  1 80,  181,  182,  539 
Ilymenoptera,  396,  401,  402,  412,  420, 

421,  425 

Ichneumon,  396 

Inarticulata,  311,  316 

Incrmi 

Infusoria,  7,  8 

Insecta,  5,  15,  18,  19,  25,  46,  395,  396, 

4^5,  455.  45« 
Intoshia  gigas,  136 
Isidimc,  181 
Ixxlyctia,  147 
Isopoda,  109,  515,  519,  521,  523,  527 

Julus  Moneletei,  387,  388,  389 
Kochlorine,  499 


Lacertilia,  64 
Lacinularia,  221,  223 

„         socialis,  75 
Lamellibranchiata,  23,  25,    37,    39,    98, 

225,  241,  257,  258,  259,  269,  270,  271, 

273,  274,  288 
Lepadkue,  498 

Lepas  fascicularis,  224,  493,  494,  495 
Lepidoptera,  79,  396,  402,  407,  408,  412, 

413,  415,  417,  420,  421,  423,  415,  426. 

455 

Leptodora,  16,  51 
Leptoplana,  74,  189,  192,  193 
Lernseopoda,  490,  492,  520 
Leucifer,  507 

Libellulidae,  402,  403,  409,  420 
Limax,  229,  232,  239,  278,  280 
Limnadia,  79,  524 
Limulus,  534 
Lina,  402 

Lingulidae,  311,  316 
Lithobius,  393 
Lobatse,  178 

Loligo,  242,  243,  244,  247,  253 
Loricata,  507,  514 
Lota,  105 

Loxosoma,  292,  294,  296,  306,  307 
Lucernaria,  185 
Lumbricus,  341,  368 

,,          agricola,  321 

,,          rubellus,  324 

„          trapczoides,  13,  321,  323 
Lumbriconereis,  334 
Lymnseus,    82,  98,  226,  227,  232,   238, 

281 
Lycosa,  436 

Macrostomum,  32,  34 

Macrura,  476 

Malacobdella,  203 

Malacodermata,  171 

Malacostraca,    66,    459,    462,   465,   504, 

505,  506,  511,  523 
Mammalia,  56,  58,  59,  64,  66 
Marsipobranchii,  59 
Mastigopus,  473,  474 
Medusoe,  27,  154,  157,  i.^s,  16;,  164,  176, 

178,  181,  182,  183,  184,  185,  186 
Megalopa,  482,  483,  484 
Melolontha,  402,  421 
Membranipora,  297,  303 
Mermithido;,  371 
Mesotrochoe,  330 
Metachoetoe,  335 
Metazoa,  Q,  10,  12,67,  86,  125,  135,  14^, 

ISO,  179 

Millepora,  152,  181 

Mitraria,  308,  337 

Molgula,  102 

Mollusca,  5,  18,  24,  66,  74,  84,  99,  225, 
247,  248,  251,  256,  257,  262,  271,  285, 
288,  307,  325,  333,  352,  576,  582 


INDEX. 


589 


Monomya,  -225 
Monostomum  capitellum,  205 

,,  mutabile,  205,  206 

Monotrochse,  330 
Montacuta,  260,  262 
Musca,  396 
Muscidae,  420,  423 
Myobia,  444,  445 
Myrianida,  343 
Myriapoda,  22,  iir,  113,  387,  394,  395, 

4i.3»  458 
Mynothela,  155 
Myrmeleon,  396 

Mysis,  120,  469,  472,  486,  504,  509,  525 
Mytilus,  260,  261 
Myxinoids,  5 
Myxispongise,  145 
Myzostomea,  369 

Nais,  342 

Nassa  mutabilis,  101,  226,  227,  233,  262, 

278,  279,  288,  3^4 
Natantia,  487 
Natica,  237,  283 
Nauplius,  5,  16,  460,  461,  463,  465,  466, 

469,  473,  490,  491,  493,  497 
Nautilus  pompilius,  253,  276 
Nebaliadse,  459,  465,  486 
Nematoda,  45,  46,  50,  66,  74,  75,  371, 

373.  374>  376 
Nematogens,  131 
Nematoidea,  18,  84,  94,  371,  374 
Nematus  ventricosus,  13,  427 
Nemertea,  94,  189,  196,  202,  204 
Nemertines,   30,   31,    33,   93,    136,   195, 

202,  328,  333,  424 
Nephelis,  82,  346,  349,  350,   351,   352, 

354 

Nereis,  343 

,,      diversicolor,  319 

,,       Dumerilii,  343 
Neritina,  229,  237 
Neuroptera,  396,  401,  420,  421 
Neuroterus  ventricularis,  428 
Notonecta,  395 
Nototrochse,  330,  353 
Nudibranchiata,  229,  241 

Ocellata,  184 

Octocoralla,  152,  179 

Octopus,  248 

Odontophora,  225,  257,  271 

Odontosyllis,  333 

Oedogonium,  1 1 

Oligochseta,  42,  319,  321,  325,  330,  338, 

346,  352 
Olynthus,  144 
Oniscus   murarius,    107,    108,    109,    120, 

516,  520,  528 
Opercula,  31 
Ophiothryx,  36,  549 
Ophidia,  64 


Ophiuroidea,    136,   544,  553,  562,  565, 

576 

Ophryotrochoe  puerilis,  333 
Opisthobranchiata,  225,  232,  237 
Ornithodelphia,  109 
Orthonectidae,  136 

Orthoptera,  395,  414,  420,  421,  425,  426 
Ostracoda,  459,  500,  510 
Ostrea,  259,  260,  262 
Oxyuridse,  46,  373,  374 
Oxyurus  ambigua,  374 
,,        vermicularis,  375 

PcEcilopoda,  534 
Paguridse,  477 
Pakemon,  no 
Palaemonetes,  476 
Pakemoninre,  476,  511,  512 
Palinurus,  478,  480 

Paludina,  66,  227,  229,  235,   270,  278, 
280 

,,        costata,  229 

,,         vivipara,  226 
Pandorina,  n 
Parasita,  489 
Pedalion,  221 

Pedicellina,  98,  292,  296,  299,  307 
Pelagia,  167,  185 
Penseinse,  476 
Penaeus,  no,   113,  465,  469,   473,  474, 

504,  518 

Pennatulidae,  181 
Pentacrinus,  5 
Pentastomida,  539,  540 
Pentastomum  denticulatum,  540,  54! 

tsenoicles,  539,  540,  541 
Percidae,  64 
PerennichaetcE,  335 
Peripatus,  5,  386,  411,  412,  413,  542 
Petromyzon,  61,  63,  64,  74,  83 
Phalangella,  304 
Phalangidse,  436 
Phallusia,  83 

Phascolosoma,  44,  355,  356,  361 
Pholcus,  436,  442 
Phoronis,  315,  355,  363,  364 
Phoxinus  laevis,  378 
Phryganea,  396,  401,  409 
Phylactokemata,  292,  294,  297,  305,  306 
Phyllobothrium,  218 
Phyllodoce,  329 
Phyllopoda,  16,  459,  461,  505 
Phyllosoma,  479,  480 
Phylloxera,  429 
Physophoridoe,  152,  16-2,  164 
Pilidium,  type  of,  196,  200,  201,  202,  704, 

424 

Pisces,  5 
Piscicola,  20,  43 
Pisidium,  259,  260,  262,  264 
Planaria  Neapolitana,  193 
Planorbis,  273,  281,  325 


590 


INDEX. 


Platyelminthes,  18,  20,  24,  221,  424 
Platygaster,  396,  416,  417,  418,  419 
Pleurohrachia,  176,  177,  238 
Pneumodermon,  242,  576 
Podostomata,  292 
Poduridce,  401,  405 
Polychaeta,  42,  319,  325,  338 
Polydesmus  complanatus,  387,  388 
Polygordius,   319,    325,    326,    327,   328, 

332,  357»  386 
Polynoe,  42,  331 
Polyophthalmus,  328 
Polyplacophora,  225,  254,  270,  271,  288 
Polystomeas,  189,  205,  209 
Polystomum,  209 

,,  integerrimum,  30,  31,  210 

Polytrochne,  330,  333 
Polyxenia  leucostyla,  158 
Polyxenus  lagurus,  387 
Polyzoa,  98,  303,  305,  306.  308>  3!5.  3^ 
Porcellana,  483 
Porifera,  102,  138,  148 
Porthesia,  115 
Prorhyncus,  32,  34 
Prosobranchiata,  225,  237,  281 
Prostomum,  32,  34,  38,  196 
Protozoa,  8,  9,  lo,  n,  86,  135,  149 
Protozoaea,  471 
Protula  Dysteri,  342 
Pseudoneuroptera,  426 
Pseudoscorpionid;e,  434 
Psolinus,  556,  574 
Psychidae,  16 
Pteraster  miliaris,  561 
Pteropoda,  98,  225,  226,  229,  230,  232, 

240,  258,  270,  272,  279,  283 
Pterotrachcea,  71,  229,  240 
Pulex,  396 

Pulmonata,  39,  225,  232,  238,  281,  282 
I'urpura  lapillus,  78 
Pycnogonida,  538 
Pyrosoma,  13,  53,  109 

Rana  temporaria,  210 
Kaspailia,  147 
Rcdia,  206,  207,  208,  209 
Reniera,  147 

Kcptilia,  56,  59,  60,  61,  62,  64,  109 
Rhabditis  dolichura,  82 
Khabdoccela,  32,  33,  iSy,  ic/> 
Khnbdopleura,  294,  306 
Rhi/occphala,  459,  493,  499,  500 
Klii/.ocrinus,  5 
klii/.ostoma,  167 
Rhomlx>gens,  131,  134 
Khynchoncllidaj,  311 
Rhyncdbddlkbe,  346 
Rotifera,  5,   12,    18,   75,   76,   77,  79,  83, 
102,  221,  308,  325 

Saccocirrus,  328,  329,  332,  340 
Sacculina,  500 


Sagartia,  169,  171 

Sagitta,  33,  74,  94,  130,  366,  367,  368 

Salmonidrc,  64 

Salpa,  102 

Sarcia,  164 

Seaphopoda,  225,  257,  270,  271 

Schistocephalus,  2 1 1 

Schizopoda,  459,  465,  466 

Scolopendra,  392 

Scorpio,  120,  43  r,  446,  454,  455,  457 

Scrobicularia,  38,  39 

Scyllarus,  477 

Scyphistoma,  179,  185,  186 

Sedentaria,  319,  336 

Sepia,  20,  40,  41,  242,   243,   244,   245, 

247>  249>  253 
Sergestidce,  473,  507 
Serpula,  319.  325,  331 
Sertularia,  152,  183,  184 
Silicispongia.',  147 
Simulia,  401,  415 
Siphonophora,    13,   77,    152,    159,    163, 

165,  179,  1 80,  182,  185 
Sipunculida,  24 
Sipunculus,  44 
Sirex,  396 
Sitaris,  42! 

Spathegaster  baccarum,  428 
Spjo,  42>  332>  333 
Spiroptera  obtusa,  376 
Spirorbis  Pagenstecheri,  319 

„          spirillum,  319,  336 
Spirula,  252 
Spirulirostra,  252 
Spongelia,  147 
Spongida,  138,  144,  148,  149 
Spongilla,  147,  150 
Sporocysts,  206,  207,  208,  209 
Squilla,  66,  504,  507 
Stephanomia  pictum,  162,  165 
Stomalopoda,  459,  465,  4X4 
Stomodoeum,  413 
Strongylidrc,  371,  375 
Strongylocentrus,  567 
Strongysoloma  Guerinii,  3<S7,  388,  390 
Stylasterictae,  152,  r8r 
Styliolidic,  24! 
Stylochopsis  ponticus,  193 
Sycandra,  93,  138,  144,  145,  147,  150 

,,         raphanus,  i^S,  174 
Syllis,  343 

„      vivipara,  319 
Sympodium  coralloidcs,  168 

Taeniatoe,  178 
Tardigrada,  541 
Teoenaria,  436 

'I'clcDsti'i,  IS,  25,   5^),  59,  C>4.   107,   io<) 
I'r].)troch;i.-,  330 
Tcndra,  300 
'I '(.'nth reds,  396 
Tcrcbdla  concliilcga,  332,  333,  337 


INDEX. 


591 


Terebella  nebulosa,  332,  333 
Terebratula,  311,  315 
Terebratulina,  311,  315,  316 

,,  septentrionalis,  315,  316 

Teredo,  larva  of,  262 
Tergipes,  232,  238 

,,        Edwardsii,  238 
,,       lacinulatus,  238 
Tethya,  147 
Tetrabranchiata,  225 
Tetranychus  telarius,  116 
Tetrastemma  varicolor,  203 
Thalassema,  44,  355,  357 
Thalassinidae,  477 
Thallophytes,  n 
Thecidium,  311,  312,  315,  316 
Thecosomata,  225 
Thoracica,  459,  493,  499,  500 
Thysanozoon,  192,  193 
Thysanura,  395,  408,  425,  458 
Tichogonia,  39 
Tipula,  396 
Tipulidae,  420,  421 
Toenia  cosnurus,  214 

,,       echinococcus,  215,  217 

„       solium,  217 
Tornaria,  579,  581 
Toxopneustes,  22,  24,  35,  85,  88,  89 
Tracheata,  385,  426,  432,  448,  455,  457, 

458,  538,  54i 

Trachymedusae,  152,  156,  179,  185 
Trematodes,   14,   16,  29,  30,  31,  32,  33, 

46,  94,  189,  205,  208,  210,  212,  216 


Trichina,  377,  378 

Trichinidse,  371 

Trichocepha'lus  affinis,  374 

Trochosphsera  aequatorialis,  221 

TubiporidcE,  182 

Tubularia,  34,  38,  152,  154,  158 

Tubularidse,  29,  179,  183 

Tunicata,  5,  I4,  53 

Turbellaria,  5,  30,  31,  33,  74,  98,  102, 

136,  179,  189,  193,  333 
Tyroglyphus,  445 

Unio,  37,  38,  39,  100,  101,  259,  260,265, 
266,  445 

Vaginulus  luzonicus,  229 

Vermes,  5,  74,  102,  223,  324,  352 

Verongia  rosea,  146 

Vertebrata,   14,   18,   19,  24,  59,  64,  83, 

272,  349.  397'  4^6 
Vesiculata,  184 
Vitrina,  229 
Vorticella,  8,  9,  10 

Wilsia,  164 
Xiphoteuthis,  252 

Zoantharia,  152,  168,  169 
Zooea,  465,  468,  471,  474,  482,  483,  484, 
486,  504 


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1879. 

(11)  S.  MaxSchultze.     Beitrdge  z.  Naturgeschichte  d.  Turbellarien.  Greifs- 
wald,  1851. 

(12)  C.  Th.  von  Siebold.     '  '  Helminthologische  Beitrage."    Miiller's  Archiv, 
1836. 

(13)  C.  Th.  von  Siebold.     Lehrbuch  d.  vergleich.  Anat.d.  wirbellosen  Thiere. 
Berlin,  1848. 

(14)  E.  Zeller.     "  Weitere   Beitrage  z.   Kenntniss  d.    Polystomen."     Zeit.  f. 
wiss.  ZooL,  Bd.  xxvu.  1876. 

[Vide  also  Ed.  van  Beneden  (No.  i).] 

THE   OVUM  OF  ECHINODERMATA. 

(15)  C.  K.   Hoffmann.     "  Zur  Anatomic  d.  Echiniden  u.  Spatangen."    Nieder- 
llindisch.  Archiv  f.  Zoologie,  Vol.  I.  1871. 

(16)  C.  K.  Hoffmann.    "  Zur  Anatomic  d.  Asteriden.   Niederldndisch.  Ardiiv 
/.  Zoologie,  Vol.  n.  1873. 

(17)  H.   Ludwig.     "Beitrage  zur  Anat.  d.  Crinoiden."    Zeil.  f.  wiss.  Zool., 
Vol.  xxvin.  1877. 

(18)  Job.  Miiller.     "Ueber  d.  Canal  in  d.  Eiern  d.  Holothurien."      Miiller's 
Archiv,  1854. 

(19)  C.  Semper.     Holothurien.     Leipzig,  1868. 

(20)  E.   Selenka.     Befruchtung  d.  Eies  v.  Toxopneustes  variegalits,  1878. 

[Vide  also  Ludwig  (No.  4),  etc.] 

1  A  very  complete  and  critical  account  of  the  literature  is  contained  in  this  paper. 
B.  II.  a 


BIBLIOGRAPHY. 


THE  OVUM  OF  MOLLUSC  A. 
Lamellibranchiata. 

(21)  II.    Lacaze-Duthiers.      "  Organes    genitaux    des   Acephales   Lamelli- 
branches."     Ann.  Set.  Nat.,  4mc  serie,  Vol.  1 1.  1854. 

(22)  W.  F lemming.    "  Ueb.  d.  er.  Entwick.  am  Ei  d.  Teichmuschel."    Archiv 
f.  mikr.  Anat.,  Vol.  x.  1874. 

(23)  W.  Flamming.    "Studien  lib.  d.  Entwick.  d.  Najaden."    Sitz.  d.  t:  Akad. 
Wiss.  men,  Vol.  LXXI.  1875. 

(24)  Th.  von  Hassling.     "  Einige  Bemerkungen,  etc."     Zeit.  f.  wiss.  ZooL, 
Bd.  v.  1854. 

(25)  H.  von  Jhering.     "Zur  Kenntniss  d.  Eibildung  bei  d.  Muscheln."    Zeit. 
f.  wiss.  ZooL,  Vol.  xxix.  1877. 

(26)  Keber.     De  Introihi  Spermatozoorum  in  ovula,  etc.     Konigsberg,  1853. 

(27)  Fr.  Leydig.     "  Kleinere  Mittheilung  etc."     Miiller's  Archiv,  1854. 

Gasteropoda. 

(28)  C.  Semper.     "Beitrage  z.  Anat.  u.   Physiol.   d.  Pulmonaten."     Zeit.  f. 
wiss.  ZooL,  Vol.  vni.  1857. 

(29)  H.  Eisig.     "  Beitrage  z.  Anat.  u.  Entwick.  d.  Pulmonaten."    Zeit.f.  wiss. 
ZooL,  Vol.  xix.  1869. 

(30)  Fr.  Leydig.    "  Ueb.  Paludina  vivipara."    Zeit.f.  wiss.  ZooL,  Vol.  u.  1850. 

Cephalopoda. 

(31)  Al.  Kolliker.     Entwicklungsgeschichte  d.  Cephalopoden.    Zurich,  1844. 

(32)  E.  R.  Lankester.     "On   the  Developmental  History  of  the  Mollusca." 
Phil.  Trans.,  1875. 

THE  OVUM  OF  THE   CHJETOPODA. 

(33)  Ed.  Claparede.      "  Les  Annelides  Chaetopodes   d.  Golfe   de  Naples." 
Mem.d.  1.  Soctit.  phys.  eld1  hist.  nat.  de  Geneve,  1868 — 9  and  1870. 

(34)  E.  Ehlers.     Die  Borstcnwiirmer  nach  system,  und  anat.  Untersuchungen. 
Leipzig,  1864—68. 

(35)  E.  Selenka.     "  Das  Gefass-System  d.  Aphrodite  aculeata."     Niedcrldndi- 
sches  Archiv  f.  ZooL,  Vol.  n.  1873. 

THE   OVUM  OF  DISCOPHORA. 

(36)  H.  Dorner.     "  Ueber  d.  Gattung  Branchiobdella."     Zeit.f.  wiss.  ZooL, 
Vol.  xv.  1865. 

(37)  R.  Leuckart.     Die  menschlichen  Parasiten. 

(38)  Fr.  Leydig.     "Zur  Anatomie  v.  Piscicola  eeometrica,  etc."     Zeit.  f.  wiss. 
ZooL,  Vol.  I.  1849. 

(30)     C.    O.    Whitman.      "Embryology  of  Clepsine."     Quart.    7.    of  Alter. 
Sci.,  Vol.  xvin.  1878. 

THE  OVUM  OF  GEPHYREA. 

(40)  Keferstein  u.  Ehlers.     Zoologische  Beitrage.     Leipzig,  1861. 

(41)  C.   Semper.    Holothurien,  1868,  p.  145. 

(42)  J.  W.  Spengel.      "  Beitrage  z.  Kenntniss  d   Gephyreen."    Beitriigc  a.  d. 
zool.  Stationz.  Neapcl,  Vol.  I.  1879. 

(43)  J.  W.  Spengel.     "  Anatomische  Mittheilungen  lib.  Gephyreen."     Tagcbl. 
d.  Naturf.  Vers.     Munchen,  1877. 

THE  OVUM  OF  NEMATODA. 

(44)  Ed.   Claparede.     De  la  formation  ct  de  la  fccondaiiou  dcs-  n-uf.\  chcz  Ics 
I'crs  Ntmatodcs.     (ienevc,  1859. 

(J-r))      K.    I. (.-nek art.      Hif  nirnsf/i lichen  Paras! ten. 


BIBLIOGRAPHY.  jjj 


d.Nematoden." 

^'  Nels0n*      "On  the  reproduction  of  Ascaris   mystax,  etc."       Phil. 
(48)     A.Schneider.     Monographie  d.'  Nematoden.     Berlin,  1866. 
THE  OVUM  OF  INSECT  A. 


£Sm     £  '  T?  run  d  V     Ueb,?'*a5  Ei  u'  seine  Bildungsstdtte.    Leipzig,  1  878. 
(50)     T.  H.  Huxley.    "  On  the  agamic  reproduction  and  morphology  of  Aphis. 
Ltnnean  Trans.,  Vol.  xxn.  1858.    Vide  also  Manual  of  Invertebrate*  Animals,  1877. 

1  *  ^  ^  ^  * 


(51) 
bei  den  *,++,*„»„ 

/-a\    ?r',key<MS'    Der Eierstock  u.  die  Samentasche  d.  Insecten.    Dresden,  1866. 
tSl     ~ub. bock-     "  The  ova  and  pseudova  of  Insects."     Phil.  Trans.  1850. 
(o4)     Stem.    Die  weiblichen  Geschlcchtsorgane d.  Ktifer.     Berlin,  1847. 
[Conf.  also  Glaus,  Landois,  Weismann,  Ludwig  (No.  4).] 

THE   OVUM  OF  ARANEINA. 

(55)  Victor  Cams.     "  Ueb.  d.  Entwick.  d.  Spinneneies."     Zeit.  f.  wiss.  Zool.t 
Vol.  ii.  1850. 

(56)  v.   Wittich.     "Die   Entstehung    d.   Arachnideneies  im  Eierstock,    etc." 
Miiller  s  Archiv,  1849. 

[Conf.  Leydig,  Balbiani,  Ludwig  (No.  4),  etc.] 

THE   OVUM  OF  CRUSTACEA. 

(57)  Aug.   Weismann.     "Ueb.   d.   Bildung  von  Wintereiern  bei  Leptodora 
hyalina."     Zeit.f.  wiss.ZooL,  Vol.  xxvn.  1876. 

[For  general  literature  vide  Ludwig,  No.  4,  and  Ed.  van  Beneden,  No.  i.] 

THE   OVUM  OF  CHORD  ATA. 

Urochorda  (Tunicata). 

(58)  A.  Kowalevsky.     "  Weitere  Studien  ii.  d.   Entwicklung  d.  Ascidien." 
Archiv  f.  micr.  Anat.,  Vol.  VII.  1871. 

(59)  A.    Kowalevsky.      "Ueber    Entwicklungsgeschichte    d.     Pyrosoma." 
Arch.f.  micr.  Anat.,  Vol.  xi.  1875. 

(60)  Kupffer.    "  Stammverwandtschaft  zwischen  Ascidien  u.  Wirbelthieren." 
Arch.  f.  micr.  Anat.,  Vol.  VI.  1870. 

(61)  Giard.     "  Etudes  critiques  des  travaux,  etc. "     Archives Zool.  experiment., 
Vol.  I.  1872. 

(62)  C.  Semper.     "  Ueber  die   Entstehung,   etc."     Arbeiten   a.  d.  zool.-zoot. 
Institut  Wiirzburg,  Bd.  II.  1875. 

Cephalochorda. 

(63)  P.  Langerhans.    "Z.  Anatomic  d.  Amphioxus  lanceolatus,"  pp.  330 — 3. 
Archiv  f.  mikr.  Anat.,  Vol.  xil.  1876. 

Craniata. 

(64)  F.  M.  Balfour.       "On  the  structure  and  development  of  the  Vertebrate 
Ovary."     Quart.  J.  of  Micr.  Science,  Vol.  xvm.  1878. 

(65)  Th.    Eimer.      "  Untersuchungen   ii.   d.   Eier  d.   Reptilien."     Arckiv  f. 
mikr.  Anat.,  Vol.  vni.  1872. 

(66)  Pfliiger.     Die  Eierstbcke  d.  Sdugethiere  u.  d.  Menschen.     Leipzig,  1863. 

(67)  J.  Foulis.     "  On  the  development  of  the  ova  and  structure  of  the  ovary  in 
Man  and  other  Mammalia."     Quart.  J.  of  Micr.  Science,  Vol.  XVI.  1876. 

(68)  J.  Foulis.     "  The  development  of  the  ova,  etc."    Journal  of  Anat.  and 
Phys.,  Vol.  xni.  1878—9. 

a  2 


IV  BIBLIOGRAPHY. 


(69)  C.  Gegenbaur.     "  Ueb.  d.  Bau  u.  d.  Entwicklung  d.  Wirbelthiereier  mit 
partieller  Dottertheilung."    Muller's  Archiv,  1861. 

(70)  Alex.  Gotte.    Entwicklungsgeschichte  d.  Unke.     Leipzig,  1875. 

(71)  W.  His.     Untersuchungen  iib.  d.  Ei u.  d.  Eientwicklung  bei Knochenfischcn. 
Leipzig,  1873. 

(72)  A.    Kolliker.      Entwicklungsgeschichte  d.  Menschen   u.  hoherer   Thicre, 
Leipzig,  1878. 

(73)  J.  Miiller.     "  Ueber  d.  zahlreichen  Porenkanale  in  d.  Eikapsel  d.  Fische." 
Muller's  Archiv,  1854. 

(74)  W.  H.  Ransom.    "  On  the  impregnation  of  the  ovum  in  the  Stickleback." 
Pro.  K.  Society,  Vol.  vn.  1854. 

(75)  C.  Semper.      "  Das  Urogenitalsystem  d.  Plagiostomen  etc."     Arbeiten  a. 
d.  zool.-zoot.  Instit.  Wiirzburg,  Vol.  II.  1875. 

[Cf.  Ludwig,  No.  4,  Ed.  van  Beneden,  No.  i,  Waldeyer,  No.  6,  etc.] 


MATURATION  AND  IMPREGNATION  OF  THE  OVUM. 

(76)  Auerbach.      Organologische  Studien,  Heft  2.     Breslau,  1874. 

(77)  Bambeke.      "  Recherches  s.    Embryologie   des   Batraciens."      Bull,    de 
royale  de  Belgique,  2me  ser.,  T.  LXI.     1876. 

(78)  E.  van  Beneden.      "  La  Maturation  de  1'CEufdes  Mammiferes."     Bull, 
de  fAcad.  royale  de  Belgique,  2me  ser.,  T.  XL.  No.  12,  1875. 

(79)  Id  em.    "  Contributions  a  1'Histoire  de  la  Vesicule  Germinative,  &c."    Bull, 
de  fAcad.  royale  de  Belgique,  sme  ser.,  T.  XLI.  No.  i,  1876. 

(80)  O.  Biitschli.      Eizelle,   Zelltheilung,    und   Conjugation    der    Infusorien. 
Frankfurt,  1876. 

(81)  F.  M.  Balfour.     "  On  the  Phenomena  accompanying  the  Maturation  and 
Impregnation  of  the  Ovum."     Quart.  J.  of  Micros.  Science,  Vol.  xvm.  1878. 

(82)  Calberla.      "  Befruchtungsvorgang  beim   Ei  von   Petromyzon    Planeri.*' 
Zeit.  f.  iviss.  Zool.,  Vol.  xxx. 

(83)  W.  Flemming.     "Studien  in  d.  Entwickelungsgeschichte  der  Najaden." 
Sitz.  d.  k.  Akad.  Wiett,  B.  LXXI.     1875. 

(84)  H.    Fol.      "Die  erste    Entwickelung    des   Geryonideneies. "     Jenaische 
Zeitschrift,  Vol.  vn.  1873. 

(85)  Idem.    "  Sur  le   Developpement  des  Pte"ropodes."    Archives  de  Zoologic 
Experimental  et  Gtnerale,  Vol.  iv.  and  v.     1875 — 6. 

(86)  Idem.     "  Sur  le  Commencement  de  1'Henog^nie."     Archives  des  Sciences 
Physiques  et  Naturelles.     Geneve,  1877. 

(87)  Idem.  Recherches  s.  I.  Ftcondation  etl.  comrnen.  d.  rHcnogcnic.  Geneve,  1879. 

(88)  R.  Greeff.     "  Ueb.  d.  Bau  u.  d.  Entwickelung  d.  Echinodermen."  Sitzun. 
der  Gesellschaft  z.  Befonlerung  d.  gesammten  Naturwiss.  z.  Marburg,  No.  5,  1876. 

(89)  Oscar  Hertwig.     " Beit.  z.  Kenntniss  d.   Bildung,  &c.,  d.  thier.   Eies." 
Morphologisches  Jahrbuch,  Vol.  I.  1876. 

(90)  Idem.     Ibid.     Morphologisches  Jahrlntch,  Vol.  ill.  Heft  i,  1877. 

(91)  Idem.    "  Weitere  Beitrage,  &c."  Morphologisches  Jahrbuch,  Vol.  in.  1877. 
Heft  3. 

(92)  Idem.     "Beit.  z.  Kenntniss,  &c."    Morphologisches  Jahrbuch,  Vol.  iv. 
Heft  i  and  2.     1878. 

(93)  N.  Kleinenberg.     Hydra.    Leipzig,  1872. 

(94)  C.   Kupffer  u.  B.  Benecke.     Der  Vorgang  d.  llcfnichtinig  am  Eie  d. 
Neunaugen.     Konigsberg,  1878. 

(95)  J.  Oellacher.     "Beitrage  zur  Geschichte  des  Keimblaschens  im  Wirbel- 
thicreie."    Archiv  f.  micr.  Anat.,  Bd.  VIII.     1872. 

(%)     W.    Salensky.      "  Befruchtung   u.    P^urchung   d.    Sterlets-Eies."     Zoolo- 
gischer  Anzeigcr,  No.  11,   1878. 

(97)     E.  Selenka.    Befruchtung  des  Eies  von  Toxopncustcs  variegatus.    Leipzig, 
1878. 

fl     Strasburger.      Ucber  Zclllnldu n- n.  /.clltln •////;/;••.     Ji-na,  1876. 

Idem.     Utber  Befrvehtung  u.  Zdlthdhing.    Jena,  1X78. 

(HiO)     C.  ().  \V  hi  tin.in.      "Tlic-  Kniliryology  of  Clepsine."     Quart.  J.  of  A/i<r. 
Science,  Vol.  xvm.     1878. 


BIBLIOGRAPHY. 


DIVISION  OF  NUCLEUS. 

(101)  W.  Flamming.      "Beitrage  z.  Kenntniss  d.  Xclle  u.  ihrcr  Lcbun.scrschci- 
nungen."     Archiv  f.  mikr.  Anat.,  Vol.  xvi.     1878. 

(102)  E.   Klein.     "Observations  on  the  glandular  epithelium  and  division  of 
nuclei  in  the  skin  of  the  Newt."     Quart,  y.  of  Micr.  Science,  VoL  XIX.     1879. 

(103)  Peremeschko.     "Ueber  d.  Theilung  d.  thierischen  Zellen."    Archiv  f. 
mikr.  Anat.,  Vol.  xvi.      1878. 

(104)  E.  Strasburger.     "Ueber  ein  z.  Demonstration  geeignetes  Zclltheilungs- 
Object."     Sitz.  d.  Jenaischen  Gesell.f.  Med.  u.  Naturwiss.,  July  18,  1879. 

SEGMENTATION. 

(105)  E.  Haeckel.     "Die  Gastrula  u.  Eifurchung."    Jenaische  Ztitschrift,  Vol. 
ix.     1877. 

(106)  Fr.  Leydig.     "Die  Dotterfurchung  nach  ihrem  Vorkommen  in  d.  Thier- 
welt  u.  n.  ihrer  Bedeutung."     Oken.     /sis,  1848. 

GENERAL  WORKS  ON  EMBRYOLOGY. 

(107)  K.  E.  von  Baer.     " Ueb.  Entwicklungsgeschichte  d.  Thiere."      Konigs- 
berg,  1828—37. 

(108)  C.  Glaus.     Grundziige  d.  Zoologie.     Marburg  und  Leipzig,  1879. 

(109)  C.  Gegenbaur.     Grundriss  d.  vergleichenden  Anatomie.     Leipzig,  1878. 
Vide  also  Translation.     Elements  of  Comparative  Anatomy.      Macmillan  and  Co., 
1878. 

(110)  E.   Haeckel.     Studien  z.  Gastrcea-Theorie.     Jena,  1877,  and  also  Jena- 
ische Zeitschrift,  Vols.  vni.  and  ix. 

(111)  E.  Haeckel.     Schopfungsgeschichte.     Leipzig.      Vide  also   Translation. 
The  History  of  Creation.     King  and  Co.,  London,  1876. 

(112)  E.  Haeckel.    Anthropogenie.    Leipzig.     Vide  also  Translation.     Anthro- 
pogeny  (Translation).     Kegan  Paul  and  Co.,  London,  1878. 

(113)  Th.   H.Huxley.      The  Anatomy  of Invertebrated  Animals.      Churchill, 
1877. 

(114)  E.  R.  Lankester.     "Notes  on  Embryology  and  Classification."     Quart. 
J.  of  Micr.  Science,  Vol.  xvi  I.  1877. 

(115)  A.  S.  P.  Packard.     Life  Histories  of  Animals,  including  Man,  or  Outlines 
of  Comparative  Embryology.     Holt  and  Co.,  New  York,  1876. 

(116)  H.  Rathke.     Abhandlungen  z.  Bildung-  und  Entwicklungsgesch.  d.  Men- 
schen  u.  d.  Thiere.    Leipzig,  1833. 

DICYEMID.E. 

(117)  E    van  Beneden.     "Recherches  sur  les  Dicyemides."     Bull.  d.  FAca- 
dtmie  roy.  de  Belgique,  f  ser.   T.  XLI.  No.  6  and  T.  XLII.  No.  7,  1876.     Vide  this 
paper  for  a  full  account  of  the  literature. 

(118)  A.  K 611  ike r.     Ueber  Dicyema  paradoxum  den  Schmarotzer  der  Venenan- 
hiinge  der  Cephalopoden.  , 

(119)  Aug.    Krohn.      "Ueb.   d.    Vorkommen  von  Entozoen,   etc.       Fronep 

Notizen,  vn.  1839. 

ORTHONECTID^E. 

(120)  A  If.  Giard.      "Les  Orthonectida  classe  nouv.  d.  Phylum  des   Vers." 
journal  de  tAnat.  et  de  la  Physiol.,  Vol.  XV.  1879. 

(121)  El.  Metschnikoff.     "Zur  Naturgeschichte  d.  OrthonecUdae."    Zoologi- 

scher  Anzeiger,  No.  40— 43»  l879- 

PORIFERA. 
'(122)     C    Barrois.     " Embryologie  de  quelques  eponges  de  la  Manche. "     An- 

""$)  &£&™ZS^£^«'<£t*>*  SP°"6es."    A~*  ^  M«g.  cf 
Nat.  Hist.,  4th  series,  Vol.  xiv.  1874. 


vi  BIBLIOGRAPHY. 


(124)  Ganin1.     "  Zur  Entwicklung  d.  Spongilla  fluviatilis."    Zoologischer  Anzei- 
gtr,  Vol.  i.  No.  9,  1878. 

(125)  Robert  Grant.     "Observations  and  Experiments  on  the  Structure  and 
Functions  of  the  Sponge."     Edinburgh  Phil.  jf.,  Vol.  xm.  and  XIV.,  1825,  1816. 

(126)  E.  Haeckel.     Die  Kalkschwamme,  1872. 

(127)  E.  Haeckel.    Studien  zur  Gastraa-  Theorie.    Jena,  1877. 

(128)  C.  Keller.     Unterstichungen  iiber  Anatomic  und  Entwicklungsgeschichte 
einiger  Spongien.     Basel,  1876. 

(129)  C.  Keller.     "Studien  lib.  Organisation  u.  Entwick.  d.  Chalineen."    Zeit. 
f.  wiss.  Zoo/.,  Bd.  xxvin.  1879. 

(130)  LieberkUhn.      "Beitr.  z.  Entwick.   d.   Spongillen."     Muller's  Archiv, 
1856. 

(131)  LieberkUhn.     "Neue  Beitrage  zur  Anatomie  der  Spongien."     Miiller's 
Archiv,  1859. 

(132)  El.  Metschnikoff.     " Zur  Entwicklungsgeschichte  der  Kalkschwamme. " 
Zeit.f.  wiss.  Zool.,  Bd.  xxiv.  1874. 

(133)  El.  Metschnikoff.     "Beitrage  zur  Morphologic  der  Spongien."     Zeit. 
f.  wiss.  Zool.,  Bd.  xxvii.  1876. 

(134)  El.  Metschnikoff.     " Spongeologische  Studien."     Zeit.  f.  wiss.  Zool., 
Bd.  xxxn.  1879. 

(135)  Miklucho  Maklay.     "Beitrage  zur  Kenntniss  der  Spongien."  Jenaische 
Zeitschrift,  Bd.  iv.  1868. 

(136)  O.  Schmidt.     "Zur  Orientirung  iiber  die  Entwicklung  der  Schwamme." 
Zeit.f.  wiss.  Zool.,  Bd.  xxv.  1875. 

(137)  O.  Schmidt.     "Nochmals  die  Gastrula  der  Kalkschwamme."     Archiv 
fur  mikrosk.  Anat.,  Bd.  XII.  1876. 

(138)  O.   Schmidt.     "Das  Larvenstadium  von  Ascetta  primordialis  und  Asc. 
clathrus."     Archiv  fur  mikrosk.  Anatomie,  Bd.  xiv.  1877. 

(139)  F.   E.   Schulze.     "Ueber  den  Bau  und  die  Entwicklung  von  Sycandra 
raphanus."     Zeit.f.  wiss.  Zool.,  Bd.  xxv.  1875. 

(140)  F.  E.  Schulze.     "Zur  Entwicklungsgeschichte  von  Sycandra."     Zeit.  f. 
wiss.  Zool.,  Bd.  XXVII.  1876. 

(141)  F.  E.  S  chulze.    "  Untersuchung  Ub.  d.  Bau,  etc.     Die  Gattung  Halisarca." 
Zeit.f.  wiss.  Zoo/.,  Bd.  xxvin.  1877. 

(142)  F.  E.  Schulze.     "Untersuchungen  iib.  d.  Bau,  etc.     Die  Metamorphose 
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(143)  F.  E.  Schulze.     "Untersuchungen  u.  d.   Bau,  etc.     Die  Familie  Aply- 
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(144)  F.  E.  Schulze.     "Untersuchungen  u.  d.  Bau,  etc.     Die  Gattung  Sponge- 
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CCELENTERATA. 
General. 

(145)  Alex.   Agassi  z.      Illustrated  Catalogue  of  the  Museum  of  Comparative 
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(140)     O.  and   R.   Hertwig.     Der  Organismus  d.  Medusa:  u,  seine  Stellung  z. 
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(147)  A.    Kowalevsky.     "Untersuchungen  lib.  d.   Entwicklung  d.   Coelente- 
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logie  u.  Ethnographie.     Moskau,  1873.     (Russian).     For  abstract  vide  Jahresberichtc 
d.  Anat.  u.  Phys.  (Hoffman  u.  Schwalbe),  1873. 

Hydrozoa. 

(148)  L.  A  gas  si  z.     Contributions  to  the  Natural  History  of  the  United  States  of 
America.     Boston,  1862.     Vol.  IV. 

(149)  G.  J.   Allman.      A  Monograph  of  the  Gymnoblastic  or  Tubularian  Hy- 
drotds.     Ray  Society,  1871-2. 

1  There  is  a  Russian  paper  by  the  same  author,  containing  a  full  account,  with 
clear  illustrations,  of  his  observations. 


BIBLIOGRAPHY,  vii 


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(179)  J.  Price.      "Embryology  of  Ciliogrades."     Proceed,  of  British  Assoc., 
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(182)  Dalyell.     "Powers  of  the  Creator." 

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(185)  P.  Halle  z.     Contributions  a  Thistoire  naturelle  des  Turbellarits.     Thesis  a 
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(263)  E.   R.  Lankester.     "On  the  coincidence  of  the  blastopore  and  anus  in 
Paludina  vivipara."     Quart.  J.  of  Micr.  Scie.,  Vol.  XVI.     1876. 

(264)  F.  Leydig.     "Ueber  Paludina  vivipara."     Zeitschr.  f.  w.  Zool.,  Vol.  11. 
1850. 

(265)  J.  MUller.     Ueber  Synapta  dig.  u.  iib.  d.  Erzeug.  v.  Schnecken  in  Holoth., 
1852. 

(266)  J.  Miiller.     "Bemerk.   aus  d.  Entwickl.  der  Pteropoden."    Monatsber. 
Berl.  Akad.,  1857. 

(267)  C.   Rabl.     "Die  Ontogenie  d.  Siisswasser-Pulmonaten."    Jenaische  Zeit- 
schrift,  Vol.  IX.     1875. 

(268)  C.  Rabl.     "Ueb.  d.  Entwick.  d.  Tellerschnecke  (Planorbis)."    Morph. 
Jahrbuch,  Vol.  v.     1879. 

(269)  W.  Salensky.     " Beitr.  zur  Entwickl.  d.  Prosobr."    Zeitschr.  f.  iv.  Zool. , 
Vol.  xxii.    1872. 

(270)  O.   Schmidt.     "Ueb.  Entwick.  von  Limax  agrestis."     Miillcr's  Archiv, 
1851. 

(271)  Max    S.    Schultze.     "Ueber  d.   Entwick.   des  Tergipes   lacinulatus." 
Arch.  f.  Naturg.,  Jahrg.  XV.     1849. 

(272)  E.  Selenka.    "Entwick.  von  Tergipes  claviger."    Niederl.  Arch.f.  Zool., 
Vol.  I.    1871. 

(273)  E.   Selenka.     "Die  Anlage  d.  Keimbl.  bei  Purpura  lapillus."     Niederl. 
Arch.f.  Zool.,  Vol.  i.    1872. 

(274)  C.  Semper.     "Entwickl.  der  Ampullaria  polita,  etc."     Natuurk.   Ver- 
handl.  Utrechts  Genootsch.,  1862. 

(275)  An.    Stecker.      "Furchung    u.    Keimblatterbildung    bei    Calyptraa." 
Morphol.  Jahrbuch,  Vol.  n.    1876. 

(276)  A.Stuart.    "  Ueb.  d.  Entwickl.  einiger  Opisthobr."    Zeitschr.  f.  w.  Zool., 
Vol.  XV.    1865. 

(277)  N.   A.   Warneck.     "Ueber  d.  Bild.  u.  Entwick.  d.  Embryos  bei  Gas- 
terop."     Bullet.  Soc.  natural,  de  Moscou,  T.  xxm.    1850. 

Cephalopoda. 

(278)  P.   J.  van  Beneden.     "  Recherches  sur  1'Embryogenie  des  Sepioles." 
Nouv.  Mem.  Acad.  Roy.  de  Bruxelles,  Vol.  xiv.     1841. 

(279)  N.   Bobretzky.     Observation  on  the  Development  of  the  Cephalopoda 
(Russian).    Nachrichten  d.  kaiserlichen  Gesell.  d.  Freunde  der  Naturwiss.  Anthropolog. 
Rthnogr.  bei  d.  Universitdt  Moskau. 

(280)  H.  Grenacher.     "  Zur  Entwicklungsgeschichte  d.  Cephalopoden."    Zeit. 
f.  wiss.  Zool.,  Bd.  xxiv.     1874. 

(281)  A.  K6 Hiker.     Entwicklungsgeschichte  d.  Cephalopoden.     Zurich,  1844. 

(282)  E.  R.  Lankester.     "Observations  on  the  development  of  the  Cephalo- 
poda."    Quart.  J.  of  Micr.  Science,  Vol.  xv.     1875. 

(283)  E.  Metschnikoff.     "  Le  developpement  des  Sepioles."     Archives  d.  Sc. 
phys.  et  nat.,  Vol.  xxx.     Geneve,  1867. 

Polyplacophora. 

(284)  A.    Kowalevsky.       "Ueb.    d.    Entwick.   d.    Chitonen."      Zoologischer 
Anzeiger,  No.  37.     1879. 

(285)  S     L.    Loven.      "  Om    utvecklingen  hos    sliigtet    Chiton."      Stockholm 
ofversigt,  xn.     1855.     [Vide  also  Ann.  and  Mag.  of  Nat.  Hist.,  Vol.  xvii.  1856,  ami 
Archivf.  Naturgeschichte,  1856.] 

Scaphopoda. 

(286)  H.  Lacaze-Duthiers.     "Developpement  du  Dentale."    Ann.  d.  Sci. 

Nat.,  Series  iv.  Vol.  VII.     1857. 

Lamellibranchiata. 

(287)  M.  Braun.      "  Postembryonale   Entwicklung    d.    Susswasser-Muscheln." 

Zoologischer  Garten. 


xii  BIBLIOGRAPHY. 


(288)  C.  G.     Carus.     "  Neue  Untersuch.   lib.  d.   Entvvickl.  unscrer  Flussmu- 
Vcrh.  Leop.-Car.  Akad.,  Vol.  xvi.     1832. 

(289)  W.  Flemming.     "  Studien  in  d.  Entwicklungsgeschichte  der  Najadcn." 
Sit*,  d.  k.  Akad.  Wiss.  Wien,  Vol.  LXXI.     1875. 

(290)  F.  Ley  dig.     "  Ueber  Cyclas  Cornea."     Miiller's  Archiv,  1855. 

(2111)     S.   L.   Loven.      "  Bidrag  til  Kanned.  om    Utveckl.  af   Moll.    Acephala 
I^amellibr."     Vetensk.  Akad.  Handl.,  1848.     [FiVfcalso  Arch.  f.  Naturg.,  1849.] 

(292)  C.   Rabl.      "Ueber  d.  Entwicklungsgeschichte  d.  Malermuschel."     Je- 
naische  Zeitschrift,  Vol.  X.     1876. 

(293)  W.  Salensky.    "  Bemerkungen  uber  Haeckels  Gastraea-Theorie  (Ostrea)." 
Arch.  f.  Naturg.,  1874. 

(294)  O.  Schmidt.     "  Ueb.   d.   Entwick.   von   Cyclas  calyculata."     Muller's 
Arch.,  1854. 

(295)  O.  Schmidt.       "Zur    Entwickl.    der    Najaden."       Wien.    Sitzungsber. 
math.-nat.  C!.,  Vol.  xix.     1856. 

(296)  P.  Stepanoff.     "  Ueber  die  Geschlechtsorgane  u.  die  Entwicklung  von 
Cyclas."    Archivf.  Naturgeschichte,  1865. 

(297)  H.  Lacaze-Duthiers.      "  Ueveloppement   d.  branchies  d.   Mollusques 
Acephales."     An.  Sc.  Nat.,  Ser.  iv.  Vol.  v.     1856. 

POLYZOA. 
General. 

(298)  J.  Barrois.     Recherches  sur  Cembi yologie  des  Bryozoaires.     Lille,  1877. 

Entoprocta. 

(299)  B.  Hatschek.      " Embryonalentwicklung   u.    Knospung   d.    Pedicellina 
echinata."     Zeitschrift  fiir  wiss.  Zool.,  Bd.  xxix.     1877. 

(300)  M.  Salensky.     "  Etudes  sur  les  Bryozoaires  entoproctes."     Ann.  Scien. 
Nat.,  Ser.  vi.  Tom.  v.     1877. 

(301)  O.   Schmidt.     "Die  Gattung  Loxosoma."      Archivf.  mik.  Anat.,Rd. 
xii.     1876. 

(302)  C.  Vogt.     "Sur  le  Loxosome  des   Phascolosomes."    Archives  de  Zool. 
cxptr.  et  gtnfr.,  To.n.  v.     1876. 

(303)  C.  Vogt.     "Bemerkungen  zu  Dr  Hatschek's  Aufsatz  lib.  Embryonalent- 
wicklung u.  Knospung  von  Pedicellina   echinata."     Zeit.  f.  wiss.  Zool.,  Bd.  XXX. 
1878. 

Ectoprocta. 

(304)  G.  J.  A 11  man.     Monograph  of  fresh  water  Polyzoa.     Ray  Society. 

(305)  G.  J.  Allman.     "  On  the  structure  of  Cyphonautes."     Quart.  J.  of  Micr. 
Scie.,  Vol.  xii.     1872. 

(306)  G.  J.  Allman.     "On  the  structure  and  development  of  the  Phylactola> 
matous  Polyzoa."     Journal  of  the  Linnean  Society,  Vol.  xiv.  No.  77.     1878. 

(307)  J.   Barrois.     "  Le  developpement  d.  Bryozoaires  Chilostomes."     Comptes 
rendus,  Sept.  23,  1878. 

(308)  E.  Claparede.     "  Beitrage  zur  Anatomic  u.  Entwicklungsgeschichte  d. 
Seebryozoen."     Zeit.  fiir  wiss.  Zool.,  Bd.  xxi.     1871. 

(309)  E.  Claparede.     "Cyphonautes."    Anat.   u.  Entwick.  wirbell.    Thiere. 
Leipzig,  1864. 

(310)  R.  E.  Grant.     "Observations  on  the  structure  and  nature  of  Flustrae." 
Edinburgh  New  Philosoph.  Journal,  1827. 

(311)  B.  Hatschek.      "Embryonalentwicklung  u.    Knospung   d.    Pedicellina 
echinata"  (Description  of  Cyphonautes).     Zeit.  f.  wiss.  Zool.,  Bd.  xxix.     1877. 

(312)  T.  II.  Huxley.     "Note  on  the  reproductive  organs  of  the  Cheilostome 
Polyzoa."     Quart.  Jour,  of  Micr.  Science,  Vol.  IV.     1856. 

(313)  L.  Joliet.      "Contributions  a   1'histoire   naturelle  des   Bryozoaires   des 
cotes  de  France."     Archives  ie  Zoologic  Experimental,  Vol.  VI.     1877. 

(314)  E.  Metschnikoff.     "  Ueber  d.  Metamorphose  einiger  Seethiere."     Got- 
tingische  Nachrichten,  1869. 


BIBLIOGRAPHY.  xiii 


(315)  E.  Metschnikoff.     Bull.  deTAcad.  de  St  Pttersbourg,  XV.  1871,  p.  507. 

(316)  H.   Nitsche.      "  Beitrage  zur   Kenntniss   d.   Bryozoen."     Zrit.  f.  wiss. 

Zool.,  Bd.  xx.     1870. 

(317)  W.  Repiachoff.     "Zur  Naturgeschichte  d.  chilostomen  Seebryozoen." 
Zeit.f.  wiss.  Zool.,  Bd.  xxvi.     1876. 

(318)  W.  Repiachoff.    "  Ueber  die  ersten  Entwicklungsvorgange  bei  Tendra 
zostericola.''     Zeit.  f.  wiss.  Zoo!.,  Bd.  xxx.     1878.     Supplement. 

(319)  W.   Repiachoff.      "Zur  Kenntniss  der   Bryozoen."      Zoologischer  An- 
zeiger,  No.  10,  Vol.  i.     1878. 

(320)  W.   Repiachoff.     "  Bemerkungen  lib.  Cyphonautes. "     Zoologischer  An- 
zeiger,  Vol.  n.     1879. 

(321)  M.  Salensky.     "  Untersuchung  an  Seebryozoen."     Zeit.  fur  wiss.  Zool.. 
Bd.  xxiv.     1874. 

(322)  A.   Schneider.      "Die  Entwicklung  u.  syst.   Stellung  d.  Bryozoen  u. 
Gephyreen."     Archiv f.  mikr.  Anaf.,  Vol.  v.     1869. 

(323)  Smitt.     "  Om  Hafsbryozoernas  utveckling  och  fettkroppar. "    Aftryck  ur 
ofvers.  of  Kong.  Vet.  Akad.  Fork.     Stockholm,  1865. 

(324)  T.  Hincks.     British  Marine  Polyzoa.     Van  Voorst,  1880. 
[Conf.  also  works  by  Farre,  Hincks,  Van  Beneden,  Dalyell,  Nordmann.] 

BRACHIOPODA. 

(325)  W.  K.  Brooks.      "  Development   of  Lingula."      Chesapeake  Zoological 
Laboratory,  Scientific  Results  of  the  Session  of  1878.     Baltimore,  J.  Murphy  and  Co. 

(326)  A.  Kowalevsky.     "Development  of  the  Brachiopoda."     Protocol  of  the 
First  Session  of  the  United  Sections  of  Anatomy,  Physiology,  and  Comparative  Ana- 
tomy at  the  Meeting  of  Russian  Naturalists  in  Kasan,  1873.     (Russian.) 

(327)  H.  Lacaze-Duthiers.     "  Histoire  de  la  Thecidie."    Ann.  Scien.  Nat. 
etc.  Ser.  4,  Vol.  xv.      1861. 

(328)  Morse.     "  On  the  Early  Stages  of  Terebratulina  septentrionalis."    Mem. 
Boston  Soc.  Nat.  History,  Vol.  n.   1869,  also  Ann.  &>  Mag.  of  Nat.  Hist.  Series  4, 
Vol.  vm.     1871. 

(329)  Morse.      "On  the  Embryology  of  Terebratulina."      Mem.  Boston  Soc. 
Nat.  History,  Vol.  ill.     1873. 

(330)  Morse.     "  On  the  Systematic  Position  of  the  Brachiopoda."     Proceedings 
of  the  Boston  Soc.  of  Nat.  Hist.,  1873. 

(331)  Fritz  Miiller.      "  Beschreibung  einer   Brachiopoden-Larve."     Miiller's 
Archiv,  1860. 

CKLETOPODA. 

(332)  Alex.  Agassiz.     "On  the  young  stages  of  a  few  Annelids."    Annals 
Lyceum  Nat.  Hist,  of  New  York,  Vol.  vm.     1866. 

(333)  Alex.  Agassiz.      "  On  the  embryology  of  Autolytus  cornutus  and  alter- 
nations of  generations,  etc."     Boston  Journal  of  Nat.  History,  Vol.  VH.     1859-63. 

(334)  W.  Busch.     Beobachtungen  it.  Anaf.  u.  Entwick.  einiger  wirbelloser  See- 
thiere,  1851. 

(335)  Ed.  Claparede.     Beobachtungen  u.  Anat.  u.  Entwick.  'wirbelloser  Thiert 
an  d.  Kiiste  von  Normandie.     Leipzig,  1 863. 

(336)  Ed.   Claparede  u.  E.   Metschnikoff.      "Beitrage  z.   Kenntniss  lib. 
Entwicklungsgeschichte  d.  Chsetopoden."     Zeit.f.  wiss.  Zool.,  Vol.  xix.     1869. 

(337)  E.  Grube.     Untersuchungen  ub.  Entivicklung  d.  Anneliden.     Komgsberg, 

4(338)     B.  Hatschek.     "  Beitrage  z.  Entwick.  u.  Morphol.  d  Anneliden."    Si/*. 
d.  k.  Akad.  Wiss.  Wien,  Vol.  LXXIV.     1876. 

(339)  B.  Hatschek.     "Studien  liber  Entwicklungsgeschichte  der  Anneliden. 
Arbeiten  aus  d.  zoologischcn  Institute  d.  Universitiit  Wien.      Von  C.  Claus.     Heft  in. 

OwQ 

(340)  Th.  H.  Huxley.      "On  hermaphrodite  and  fissiparous  species  of  tubi- 
colar  Annelidse  (Protula)."     Edinburgh  New  Phil.  Journal,  Vol.  I.     1855. 

(341)  N.  Kleinenberg.      "The  development  of   the  earthworm   Lumbncus 
trapezoides."     Quart.  J,  of  Micr.  Science,  Vol.  xix.     1879      Sullo  ariAtfff  del  tn»< 

bricus  trapezoides.     Napoli,  1878. 


XIV  BIBLIOGRAPHY. 


(342)  A.  Kowalevsky.      "  Embryologische  Studien  an  Wiirmern  u.  Arthro- 
poden."    Mem.  Acad.  Pttersbourg,  Series  VH.  Vol.  xvi.     1871. 

(343)  A.  Krohn.     "  Ueber  die  Erscheinungen  bei  d.  Fortpfianzung  von  Syllis 
prolifera  u.  Autolytus  prolifer."    Archiv  f.  Naturgesch.     1852. 

(344)  R.  Leuckart.     "  Ueb.  d.  Jugendzustande  ein.  Anneliden,  etc."     Archiv 
f.  Naturgesch.     1855. 

(345)  S.  Love"n.      "  Beobachtungen   ii.   die   Metamorphose  von   Anneliden." 
Wiegmann's  Archiv,  1842. 

(346)  E.  Metschnikoff.      "Ueber  die  Metamorphose  einiger  Seethiere  (Mi- 
traria)."     Zeit.f.  tuiss.  Zool.,  Vol.  XXI.     1871. 

(347)  M.  Milne- Ed\vards.      "  Recherches    zoologiques,   etc."      Ann.   Scie. 
Natur.  HI.  Se"rie,  Vol.  ill.     1845. 

(348)  J.  Miiller.     "Ueb.  d.  Jugendzustande  einiger  Seethiere."     Monats.  d. 
k.Akad.  Wiss.     Berlin,  1851. 

(349)  Max  Miiller.     "Ueber  d.  weit.  Entwick.  von  Mesotrocha  sexoculata." 
Miiller's  Archiv,  1855. 

(350)  Quatrefages.     "  Memoire  s.  1'embryogenie  des  Annelides."     Ann.  Scie. 
Natur.  HI.  Serie,  Vol.  x.     1848. 

(351)  M.  Sars.     "  Zur  Entwicklung  d.  Anneliden."     Archiv  f.  Naturgeschichte, 
Vol.  xi.     1845. 

(352)  A.  Schneider.    "  Ueber  Bau  u.  Entwicklung  von  Polygordius."   Miiller's 
Archiv,  1868. 

(353)  A.  Schneider.     "Entwicklung  u.  system.  Stell.  d.  Bryozoen  u.  Gephy- 
reen  (Mitraria)."     Archiv  f.  mikr.  Anat.  Vol.  v.     1869. 

(354)  M.  Schultze.     Ueb.  die  Entwicklung  von  Arcnicola  piscatorum  u.  anderer 
Kiemenwurmer.     Halle,  1856. 

(355)  C.  Semper.     "Die  Verwandschaftbeziehungen  d.  gegliederten  Thiere." 
Arbeiten  a.  d.  zool.-zoot.  Instit.  Wiirzburg,  Vol.  in.     1876-7. 

(356)  C.  Semper.      "Beitrage  z.  Biologie  d.  Oligochjeten."      Arbeiten  a.  d. 
zool.-zoot.  Instit.  Wiirzburg,  Vol.  IV.     1877-8. 

(357)  M.   Stossich.     " Beitrage  zur  Entwicklung  d.  Chaetopoden."    Sitz.  d.  k. 
k.  Akad.  Wiss.  Wien,  B.  LXXVII.     1878. 

(358)  R.  v.  Willemoes-Suhm.       "  Biologische    Beobachtungen    ii.   niedrige 
Meeresthiere."    Zeit.f.  wiss.  Zool.  Bd.  xxi.     1871. 

DlSCOPHORA. 

(359)  O.  BUtschli.     "  Entwicklungsgeschichtliche  Beitrage  (Nephelis)."    Zeit. 
f.  wiss.  Zool.  Vol.  xxix.  1877. 

(360)  E.  Grube.     Untersuchungen  ub.  d.  Entwicklung  d.  Anneliden.     Konigs- 
berg,  1844. 

(361)  C.  K.  Hoffmann.     "Zur  Entwicklungsgeschichte  d.  Clepsineen."    NIC- 
derland.  Archiv  f.  Zool.  Vol.  IV.  1877. 

(362)  R.  Leuckart.      Die  menschlichen   Parasiten  (Hirudo},   Vol.  I.  p.   686, 
et  seq. 

(363)  H.  Rathke.     Beit.  z.  Entwicklungsgesch.  d.  Hirudineen.     Leipzig,  1862. 

(364)  Ch.  Robin.     Mem.  snr  le  Developpement  embryogenique  des  Ilirndiih'c*. 
Paris,  1875. 

(365)  C.  O.  Whitman.     "Embryology  of  Clepsine."      Quart.   J.  of  Micro. 
Science,  Vol.  xvm.  1878. 

[  Vide  also  C.  Semper  (No.  355)  and  Kowalevsky  (No.  342)  for  isolated  observa- 
tions.] 

GEPHYREA. 
GepJiyrea  nuda. 

(366)  A.  Kowalevsky.     Sitz.   d.   zool.  Abth.  d.  Iff.    Versam.   rtiss.   Naturf 
(Thalassema).     Zeit.f.  wiss.  Zool.  Vol.  xxn.  1872,  p.  284. 

(367)  A.  Krohn.     "Ueb.  d.  Larve  d.  Sipunculus  nudus  ncbst    I'.c  UK  iknii^cn,  ' 
etc.     Miiller's  Archiv,  1857. 


BIBLIOGRAPHY.  xv 


(368)  M.  Salensky.    "  Ueber  die  Metamorphose  d.  Echiurus."   Morfhohgisches 
Jahrbuch,  Bd.  11. 

(369)  E.  Selenka.     "Eifurchung  u.  Larvenbilflung  von  Phascolosoma  elonga- 
tum."     Zeit.f.  wiss.  Zool.  1875,  Bd-  xxv.  p.  i. 

(370)  J.  W.   Spengel.      "  Beitrage   z.    Kenntniss  d.   Gephyreen   (lionellia)." 
MitlheiL  a.  d.  zool.  Station  z.  Neapel,  Vol.  i.  1879. 

Gephyrea  tubicola  (Actinotroc/ia). 

(371)  A.   Krohn.     "  Ueb.  Pilidium  u.  Actinotrocha."      Muller's  Archiv,  1858. 

(372)  A.  Kowalevsky.     "  On  anatomy  and  development  of  Phoronis,"  Peters- 
burg, 1867.     i  PI.     Russian.     Vide  Leuckart's  Bcricht,  1866-7. 

(373)  E.  Metschnikoff.     "  Ueber  d.  Metamorphose  einiger  Seethiere  (Actino- 
trocha)."    Zeit.f.  wiss.  Zool.  Bd.  XXI.  1871. 

(374)  J.  Miiller.    "  Bericht  ub.  ein.  Thierformen  d.  Nordsee."    Muller's  Archiv, 
1846. 

(375)  An.  Schneider.     "Ueb.  d.  Metamorphose  d.  Actinotrocha  branchiata." 
Muller's  Arch.  1862. 

CH^TOGNATHA. 

(376)  O.  Butschli.     "  Zur  Entwicklungsgeschichte  der  Sagitta."    Zeilschrifl  f. 
wiss.  Zool.,  Vol.  xxin.  1873. 

(377)  C.  Gegenbaur.      "Ueber  die  Entwicklung  der  Sagitta."     Abhand.  d. 
natiirforschenden  Gesellschaft  in  Halle,  1857. 

(378)  A.  Kowalevsky.     "  Embryologische  Studien  an  Wiirmern  u.  Arthropo- 
den."     Mem.  Acad.  Petersbourg,  vn.  ser.,  Tom.  xvi.,  No.  12.     1871. 

MYZOSTOMEA. 

(379)  L.Graff.     Das  Genus  Myzostoma.     Leipzig,  1877. 

(380)  E.   Metschnikoff.      "Zur    Entwicklungsgeschichte    d.    Myzostomum." 
Zeit.f.  wiss.  Zool.,  Vol.  xvi.     1866. 

(381)  C.  Semper.     "Z.  Anat.  u.   Entwick.  d.   Gat.   Myzostomum."     Zeit.f. 
wiss.  Zool.,  Vol.  IX.  1858. 

GASTROTRICHA. 

(382)  H.  Ludwig.     "  Ueber  die  Ordnung  Gastrotricha  Metschn."     Zeit.f.  wiss. 
Zool.,  Vol.  xxvi.     1876. 

NEMATELMINTHES. 

(383)  O.  Butschli.     "Entwicklungsgeschichte  d.  Cucullanus  elegans."    Zeit. 
f.  wiss.  Zool.,  B.  xxvi.     1876. 

(384)  T.  S.  Cobbold.     Entozoa.     Groombridge  and  Son,  1864. 

(385)  T.  S.  Cobbold.     Parasites;    a    Treatise  on  the  Entozoa  of  Man  and 
Animals.     Churchill,  1879. 

(386)  O.    Caleb.      "Organisation    et    developpement    des    Oxyurides,      etc. 
Archives  de  Zool.  exper.  et  gener.,  Vol.  VII.     1878. 

(387)  R.  Leuckart.     Untersuchungeniib.  Trichina  spiralis.     2nd  ed.     Leipzig, 
1866. 

(388)  R.  Leuckart.     Die  menschlichen  Parasiten,  Bd.  u.  1876. 

(389)  H.   A.    Pagenstecher.      Die    Trichinen    nach    Versuchen    dargestellt. 

(390)  A.Schneider.     Monographie  d.  Nematoden.     Berlin,  1866. 

(391)  A.  Villot.     "  Monographie  des  Dragoneaux  "  (Gordioidea).     Archives  de 
Zool.  exptr.  et  gtner.,  Vol.  in.  1874. 

ACANTHOCEPHALA. 

(392)  R.   Greeff.     "  Untersuchungen  u.    d.  Ban  u.    Entwicklung  des   Echin. 
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xvi  BIBLIOGRAPHY. 


(394)  An.  Schneider.     "  Ueb.  d.  Bau  d.  Acanthocephalen."    Archiv  f.  Anat. 
w.  Phys.     1868. 

(395)  G.  R.  Wagener.     Beitrdge  z.  Entrvicklungsgeschichte  d.  Eingeweidewiir- 
mer.     Haarlem,  1865. 

TRACHEATA. 

PRO  TO  TRA  CHE  A  TA . 

(396)  H.  N.  Moseley.      "On  the  Structure  and   Development   of  Peripatus 
capensis."    Phil.  Trans.  Vol.  164,  1874. 

MYRIAPODA. 

(397)  G.  Newport.     "On  the  Organs  of  Reproduction  and  Development  of  the 
Myriapoda."    Philosophical  Transactions,  1841. 

(398)  E.    Metschnikoff.       "  Embryologie    der    doppeltfiissigen    Myriapoclen 
(Chilognatha)."     Zeit.f.  wiss.  Zool.,  Vol.  xxiv.     1874. 

(399)  E.  Metschnikoff.     "  Embryologisches  iiber  Geophilus."     Zeit.  f.  wiss. 
'Zool.,  Vol.  xxv.     1875. 

(400)  Anton  Stecker.      "Die  Anlage  d.   Keimblatter  bei  den  Diplopoden." 
Archiv  f.  mik.  Anatomic,  Bd.  xiv.     1877. 

INSECTA. 

(401)  M   Balbiani.    "  Observations  s.  la  reproduction  d.  Phylloxera  du  Chene." 
An.  Sc.  Nat.  Ser.  v.  Vol.  xix.     1874. 

(402)  E.  Bessels.     "Studien  u.  d.  Entwicklung  d.  Sexualdrtisen  bei  den  Lepi- 
doptera."    Zeit.f.  miss.  Zool.  Bd.  xvn.     1867. 

(403)  Alex.  Brandt.     " Beitrage  zur  Entwicklungsgeschichte  d.  Libellulida  u. 
Hemiptera,  mit  besonderer  Beriicksichtigung  d.  Embryonalhiillen  derselben."     Mem. 
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(404)  Alex.  Brandt.     Ueber  das  Ei  u.  seine  Bildungsstdtte.     Leipzig,  1878. 

(405)  O.    Butschli.     "  Zur  Entwicklungsgeschichte  d.  Biene."     Zeit.  f.  wiss. 
Zool.  Bd.  xx.     1870. 

(406)  H.  Dewitz.     "Bau  u.  Entwicklung  d.  Stachels,  etc."     Zeit.f.  wiss.  Zool. 
Vols.  xxv.  and  xxvin.     1875  and  1877. 

(407)  H.  Dewitz.     "Beitrage  zur  Kenntniss  d.  Postembryonalentwicklung  d. 
Gliedmassen  bei  den  Insecten."     Zeit.  /.  wiss.  Zool.  xxx.     Supplement.      1878. 

(408)  A.  Dohrn.      "Notizen   zur    Kenntniss   d.   Insectenentwicklung."     Zeit- 
schriftf.  wiss.  Zool.  Bd.  xxvi.     1876. 

(409)  M.  Fabre.     "  L'hypermetamorphose  et  les  mceurs  des  Meloides."    An. 
Sci.  Nat.  Series  iv.  Vol.  vn.     1857. 

(410)  Ganin.      "Beitrage  zur   Erkenntniss  d.  Entwicklungsgeschichte   d.  In- 
secten."    Zeit.  f.  wiss.  Zool.  Bd.  xix.      1869. 

(411)  V.  Graber.     Die  Insecten.     Miinchen,  1877. 

(412)  V.  Graber.      "  Vorlauf.   Ergeb.  lib.   vergl.    Embryologie  d.    Insecten." 
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(413)  O.v.Grimm.     "  Ungeschlechtliche  Fortpflanzung  einer  Chironomus-Art 
u.  deren  Entwicklung  aus  dem  unbefruchteten  Ei."     Mhn.  Acad.  Pttcrsbourg.     1X70. 

(414)  B.  Ilatschek.     "  Beitrage  zur  Entwicklung  d.  Lepidopteren."     Jenaischc 
Zeitschrift,  Bd.  xi. 

(415)  A.  Kollikcr.     "  Observationes  de  prima  insectorum  gcnese,  etc. "     Ann. 
Sc.  Nat.  Vol.  xx.     1843. 

(11(5)     A.  Kowalevsky.     "  Embryologische  Studien  an  Wiirmern  u.  Arthropo- 
den."     Mtm.  Ac.  imp.  J\'(,-rstn>nr«,  Ser.  VII.  Vol.  XVI.     iSji. 

(417)  C.   Kraepelin.       "  Untersuchungen    lib.    d.    Bau,    Mechanismus    u.     d. 
Entwick.  des  Stachels  d.  l.icnartigai  Tliicrc."     Zeit.f.  wiss.  Zool.  Vol.  xxm.     1X7.5. 

(418)  C.  Kupffcr.      "  Faltcnblatt  nn  d.  Embryoncn  d.   Gattung  Chirononnis." 
Arch.f.  mikr.  Anat.  Vol.  n.      iS66. 

(419)  R.  Leuckart.      Zur  Kemituiss  d.  Gi'ncratiomxi'ffhscls  it.  d.  /'/; -Ihetii^ •,  < 
b.  d.  Insecten.     Frankfurt,  iH.nS. 


BIBLIOGRAPHY.  xvii 


(420)  Lubbock.     Origin  and  Metamorphosis  of  Insi-cts.     1874. 

(421)  Lubbock.      Monograph  on   Collembola  ami   Thysanura.      Ray   Society, 
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(422)  Melnikow.      "  Beitrage  z.  Embryonalentwicklung  d.  Insecten."     Archiv 
f.  Naturgeschichte,  Bd.  XXXV.     1869. 

(423)  E.  Metschnikoff.      ' '  Embryologische  Studien  an   Insecten."      '/.fit.  /. 
wiss.  Zool  Bd.  xvi.     1866. 

(424)  P.  Meyer.     "  Ontogenie  und  Phylogenie  d.    Insecten."     Jcnaischt  Zfit- 
schrift,  Vol.  X.     1876. 

(425)  Fritz  Miiller.     "  Beitrage  z.  Kenntniss  d.  Termiten."     Jcnaische  Zeit- 
schrift,  Vol.  IX.      1875. 

(426)  A.  S.  Packard.     "  Embryological  Studies  on  Diplex,  Perithemis,  and  the 
Thysanurous  genus  Isotoma."     Mem.  Pea  body  Acad.  Science,  \.  2.     1871. 

(427)  Suckow.     " Geschlechtsorgane  d.  Insecten."     Heusinger's  Zeitschrift f. 
organ.  Physik,  Bd.  II.     1828. 

(428)  Tichomiroff.     "  Ueber  die  Entwicklungsgeschichte  des  Seidenwiirms." 
Zoologischer  Anzeigcr,  n.  Jahr.  No.  20  (Preliminary  Notice). 

(429)  Aug.  Weismann.      "Zur  Embryologie  d.  Insecten."    Arehiv  f.  Anat. 
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(430)  Aug.  Weismann.      "  Entwicklung  d.  Dipteren."      Zeit.  f.  wiss.  Zool. 
Vols.  xni.  and  xiv.     Leipzig,  1863 — 4. 

(431)  Aug.  Weismann.     "  Die  Metamorphose  d.  Corethra  plumicornis."    Ztit. 
f.  unss.  Zool.  Vol.  xvi.     1866. 

(432)  N.  Wagner.     "  Beitrag  z.    Lehre  d.    Fortpflanzung  d.   Insectenlarven." 
Zeit.f.  wiss.  Zool.  Vol.  Xlll.     1860. 

(433)  Zaddach.      Untersuchnngen  tib.  d.  Bau  u.  d.  Enhuicklung  d.  Gliederlhifre. 
Berlin,  1854. 

ARACHNID  A. 
Scorpionidce. 

(434)  El.  Metschnikoff.     "  Embryologie  des  Scorpions."     Zeit.f.  unss.  Zool. 
Bd.  xxi.     1870. 

(435)  H.  Rathke.     Reisebemerknngen  aus  Taurien  (Scorpio),  Leipzig,  1837. 

Pseudoscorpionidce. 

(436)  El.  Metschnikoff.    "  Entwicklungsgeschichte  d.  Chelifer."   Zeit.f.  unss. 
Zool.  Bd.  xxi.     1870. 

(437)  A.  Stecker.     "Entwicklung  der  Chthonius-Eier  im  Mutterleibe  und  c 
Bildung  des  Blastoderms."     Sitzung.  konigl.  bohmisch.  Gesellschaft  Wissensch.t  1876, 
3.  Heft,  and  Annal.  and  Mag.  Nat.  History,  1876,  xvill.  197. 

Phalangida. 

(438)  M.  Balbiani.     "  Memoire  sur  le  developpement  des  Phalangides."    Ann. 
Scien.  Nat.  Series  v.  Vol.  xvi.     1872. 

Araneina. 

(439)  M.  Balbiani.     "Memoire  sur  le  developpement  des  Araneides."     Ann. 
Scien.  Nat.  Series  v.  Vol.  xvn.     1873.  •»/->./ 

(440)  F.  M.  Balfour.     "Notes  on  the  development  of  the  Arane 

Journ.  of  Micr.  Science,  Vol.  XX.      1880.  „     v  , 

(441)  J.  Barrois.     "  Recherches  s.  1.  developpement  des  Araign^es. 

de  I' Anat.  et  de  la  Physiol.     1878.  ,  ,        fiA 

(442)  E.  Claparede.     Recherches  s.  revolution  des  Aratgnees.      Jtrecht,  1862. 

(443)  Her  old.     De  generation  Araneorum  in  Ovo.     Marburg,  1824. 

(444)  H.  Ludwig.     "Ueber  die  Bildung  des  Blastoderms  bei  den 
Zeit.f.  wiss.  Zool.  Vol.  xxvi.     1876. 

B.  II.  b 


xviii  BIBLIOGRAPHY. 


Acariua. 

(445)  P.  van  Beneden.     "  Developpement  de  1'Atax  ypsilophora."  Acad.  Bru- 
xelles,  t.  xxiv. 

(446)  Ed.  Claparede.     "Studien  uber  Acarinen."    Zeit.  /.  wits.  Zoo/.,  Bd. 
xvin.  1868. 

CRUSTACEA. 
General  Works. 

(447)  C.  Spence  Bate.     "  Report  on  the  present  state  of  our  knowledge  of  the 
Crustacea."     Report  of  the  British  Association  for  1878. 

(448)  C.  Claus.     Untersuchungen  zur  Erforschung der  genealogischen  Grundlage 
des  Crustaceen -Systems.     Wien,  1876. 

(449)  A.    Dohrn.     "Geschichte   des   Krebsstammes. "     Jenaische    Zeitschrift, 
Vol.  vi.  1871. 

(450)  A.  Gerstaecker.     Bronris  Thierreich,  Bd.  v.     Arthropoda,  1866. 

(451)  Th.    II.    Huxley.      The  Anatomy  of  Invertebrated  Animals.     London, 
1877. 

(452)  Fritz   Miiller.     Fiir  Darwin,    1864.     Translation,  Facts  for  Darwin. 
London,  1869. 

Branchiopoda. 

(453)  Brauer.     "Vorlaufige  Mittheilung  iiber  die  Entwicklung  u.  Lebensweise 
des  Lepidurus  (Apus)  productus."     Sitz.  der  Ak.  d.  Wiss.  Wien,  Vol.  LXIX.,  1874. 

(454)  C.  Claus.     '•  Zur  Kenntniss  d.  Baues  u.  d.  Entwicklung  von  Branchipus 
stagnalisu.  Apuscancriformis."  Abh.  d.  konig.  Gesell.  der  Wiss.  Gbttingen,  Vol.  xviii. 

l8'3-_ 

(455)  C.    Grobben.     "Zur    Entwicklungsgeschichte    d.    Moina    rectirostris." 
Arbeit,  a.  d.  zoologisch.  Institute  Wier.,  Vol.  II.,  1879. 

(456)  E.  Grube.     " Bemerkungen  iiber  die  Phyllopoden  nebst  einer  Uebersicht 
etc."     Archivf.  Naturgcschichte,  Vol.  xix.,  1853. 

(457)  N.  Joly.    "  Histoire  d'un  petit  Crustace  (Artemia  salina,  Leach)  etc."    An- 
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(458)  N.  Joly.    "  Recherches  zoologiques  anatomiques   et   physiologiques  sur 
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ser.,  Vol.  xvii.,  1842. 

(459)  Lereboullet.    "  Observations  sur  la  generation  et  le  de veloppement  de  la 
Limnadia  de  Hermann."    Annales  d.  Sciences  Nattir.,  ^th  ser.,  Vol.  v.,  1866. 

(460)  F.  Ley  dig.     "  Ueber  Artemia  salina  u.  Branchipus  stagnalis."     Zeit.  f. 
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(461)  G.  O.  Sars.     "Om  en  dimorph   Udvikling  samt   Generationsvexel  hos 
I^eptodora."     Vidensk.  Selskab.  For  hand,  1873. 

(462)  G.  Zaddach.     De  apodis  cancrefortnis  Schaeff.  anatome  ct  historia  evolu- 
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Nebaliadce. 

(463)  C.  Claus.     "  Ueber  den  Bau  u.  die  systematische  Stellung  von  Nebalia." 
Zeit.f.  wiss.  Zool.,  15d.  xxn.  1872. 

(464)  E.Metschnikoff.     Development  of  Nebalia  ( Russian),  1 868. 

Schizopoda. 

(465)  E.  van  Beneden.     "Recherches  sur  1'Embryogenie  des  Crustace's.     u. 
Developpement  des  Mysis."     liullet.  de  rAcadc!mie  roy.  de  Belgique,  second  series, 
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(46H)     C.  Claus.     "  Ueber  einige  Schizopoden  u.  niedere  Malakostraken."    Zeit. 
/.  -t'tss.  Zoologie,  Bd.  XIII.,  1863. 


BIBLIOGRAPHY.  xix 


(467)  A.  Dohrn.    "  Untersuchungen  iib.  Bau  u.  Entwicklung  d.  Arthropoden." 
Zeit.  f.  wiss.  Zool.,  Bd.  xxi.,  1871,  p.  375.     Peneus  zoaea  (larva  of  Euphausia). 

(468)  E.  Metschnikoff.     "  Ueber  ein  Larvenstadium  von  Euphausia."     Zeit. 
fiir  wiss.  Zool.,  Bd.  xix.,  1869. 

(469)  E.  Metschnikoff.    "  Ueber  den  Naupliuszustand  von  Euphausia."    Zeit. 
fiir  wiss.  Zool.,  Bd.  xxi.,  1871. 

Decapoda. 

(470)  Spence   Bate.     "On  the  development  of  Decapod  Crustacea."     Phil. 
Trans.,  1858. 

(471)  Spence  Bate.    "  On  the  development  of  Pagurus."  Ann.  and  Mag.  Nat. 
History,  Series  4,  Vol.  1 1.,  1868. 

(472)  N.    Bobretzky.     Development  of  Astacus  and  Paluemon.     Kiew,  1873. 
(Russian.) 

(473)  C.   Claus.     "Zur  Kenntniss  d.  Malakostrakenlarven."     Wiirzb.  naturw. 
Zeitschrift,  1861. 

(474)  R.  Q.  Couch.     "On  the  Metamorphosis  of  the  Decapod  Crustaceans." 
Report  Cornwall  Polyt.  Society,  1848. 

(475)  Du  Cane.     "On  the  Metamorphosis  of  Crustacea."    Ann.  and  Mag.  of 
Nat.  History,  1839. 

(476)  Walter  Faxon.     "  On  the  development  of  Paloemonetes  vulgaris."    Bull. 
of  the  Mus.  of  Comp.  Anat.  Harvard,  Cambridge,  Mass.,  Vol.  V.,  1879. 

(477)  A.  Dohrn.     "  Untersuchungen  iib.  Bau  u.  Entwicklung  d.  Arthropoden." 
"  Zur   Entwicklungsgeschichte   der    Panzerkrebse.     Scyllarns   Palinurus."     Zeit.   f. 
wiss.  Zool.,  Bd.  xix.,  1870. 

(478)  A.  Dohrn.     "Untersuchungen  iib.  Bau  u.  Entwicklung  d.  Arthropoden. 
Erster  Beitrag  z.   Kenntniss  d.   Malacostrakcn  u.  ilirer  Larven  Amphion  Reynaudi, 
Lophogaster,   Portunus,  Porcellanus,   Elaphocaris. "     Zeit.  f.  ^viss.  Zool.,  Bd.    XX., 
1870. 

(479)  A.   Dohrn.     "Untersuchungen  iib.  Bau  u.  Entwicklung  d.  Arthropoden. 
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(480)  N.  J  oly.     "  Sur  la  Caridina  Desmarestii."    Ann.  Scien.  Nat.,  Tom.  xix., 
1843. 

(481)  Lereboullet.     "  Recherches  d.  1'embryologie  comparee  sur  le  developpe- 
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(482)  P.     Mayer.     "Zur   Entwicklungsgeschichte  d.   Dekapoden."     Jenaische 
Zeitschrift,  Vol.  XI.,  1877. 

(483)  Fritz  Miiller.    '*  Die  Verwandlung  der  Porcellana."    Archivf.  Naturge- 
schichte,  1862. 

(484)  Fritz  Miiller.    "  Die  Verwandlungen  d.  Garneelen."    Archivf.  Natur- 
gesch.,  Tom.  xxix. 

(485)  Fritz  Miiller.     "  Ueber  die  Naupliusbrut  d.  Garneelen."    Zeit.  f.  wiss. 
Zool.,  Bd.  xxx.,  1878. 

(486)  T.  J.  Parker.     "An  account  of  Reichenbach's  researches  on  the  early 
development  of  the  Fresh-water  Crayfish."     Quart.  J.  of  M.  Science,  Vol.  xvui., 
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(487)  H.   Rathke.     Ueber  die  Bildung  u.  Entwicklung  d.  Flusskrebses.     Leip- 
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(488)  H.   Reichenbach.      "  Die  Embryoanlage  u.  erste  Entwicklung  d.  Fluss- 
krebses."     Zeit.  f.  wiss.  Zool.,  Vol.  xxix.,  1877. 

(489)  F.  Richters.     "Ein  Beitrag  zur  Entwicklungsgeschichte  d.  Loricaten." 
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(490)  G.  O.  Sars.     "  Om  Hummers  postembryonale  Udvikling."    Vidcnsk  Selsk. 
Forh.     Christiania,  1874. 

(491)  Sidney  J.  Smith.    "  The  early  stages  of  the  American  Lobster. "    Tratts. 
of  the  Connecticut  Acad.  of  Arts  and  Sciences,  Vol.  II.,  Part  i,  1873. 

(492)  R.  v.  Willemoes  Suhm.    "  Preliminary  note  on  the  development  of  some 
pelagic  Decapoda."     Proc.  of  Royal  Society,  1876. 


XX  BIBLIOGRAPHY. 


Stomatopoda. 

(41KI)  \V.  K.  Brooks.  "  On  the  larval  stages  of  Squilla  empusa.''  Chesapeake 
Zoological  Laboratory^  Scientific  results  of  the  Session  ^1878.  Baltimore,  1879. 

(494)  C.  Claus.  "Die  Metamorphose  der  Squilliden."  Abhand.  dcr  konigl. 
Gesell.  der  IViss.  ztt  Gottingen^  1*7-. 

( 1 '.!">)  Fr.  M  tiller.  4i  Bruchstuck  a.  der  Entwicklungsgeschichte  d.  Maulfiisser  I. 
und  II."  Archivf.  Naturgeschichte,  Vol.  xxvni.,  1862,  and  Vol.  XXIX.,  1863. 

Cumacea* 

( 1%)  A.  Dohrn.  "  Ueber  den  Bau  u.  Entwicklung  d.  Cumaceen."  Jenaische 
Zeitschrift,  Vol.  v.,  1870. 

hopoda. 

(497)  Ed.  van  Beneden.     "  Recherches  sur  1'Embryogenie  des  Crustaces.     i. 
Asellus  aquaticus."     BiuL  de  FAcad.  roy.  Belgique,  2me  serie,  Tom.  XXVIII.,  No.  7, 
1869. 

(498)  N.   Bobretzky.     "Zur  Embryologie  des  Oniscus  murarius."     Ztit.  fur 
wiss.  Zool.,  Bd.  xxiv.,  1874. 

(4119)  J.  F.  Bullar.  "On  the  development  of  the  parasitic  Isopoda."  Phil. 
Trans.,  Part  n.,  1878. 

(500)  A.  Dohrn.     "  Die  embryonale  Entwicklung  des  Asellus  aquaticus."    Zeit. 
f.  wiss.  Zool.,  Vol.  xvii.,  1867. 

(501)  II.  Rathke.     Untersuchungen  iibcr  die  Bildung  und  Entwicklung  der 
VVasser-Assel.     Leipzig,  1832. 

(5u2)  H.  Rathke.  Zur  Morphologic.  Reisebemerkungen  aus  J^aurien.  Riga  u. 
Leipzig,  1837.  (Bopyrus,  Idothea,  Ligia,  lanira.) 

.     A  mphipoda. 

(503)  Ed.  van  Beneden  and  E.  Bessels.  "Memoire  sur  la  formation  du 
blastoderme  chez  les  Amphipodes,  les  Lerneens  et  les  Copepodes."  Classe  des  Sciences 
de  F  Acad.  roy.  de  Belgiqtie,  Vol.  xxxiv.,  1868. 

(004)  De  la  Valette  St  George.  "  Studien  liber  die  Entwicklung  der  Amphi- 
poden."  Abhand.  d.  naturfor.  Gesell.  zu  Halle,  Bd.  v.,  1860. 

Copcpoda. 

(505)  E.  van  Beneden  and  E.  Bessels.  "Memoire  sur  la  formation  du  blas- 
toderme chez  les  Amphipodes,  les  Lerneens  et  Copepodes."  Classe  des  Sciences  dc 
FAcad.  roy.  de  Bel^ique,  Vol.  xxxiv.,  1868. 

(">(Hi)  E.  van  Beneden.  "  Recherches  sur  1'Embryoge'nie  des  Crustaces  I  v.  An- 
chorella,  Lerneopoda,  Branchiella,  Hessia."  Bull,  de  FAcad.  roy.  de  Belgique^  sme 
serie,  T.  xxix.,  1870. 

(507)     C.  Claus.     Zur  Anatomie  u.  Entwicklungsgeschichte  d.  Copepoden. 

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Copepoden."  Witrzburger  nalttnviss.  Zeitschrift,  Bd.  ill.,  1862. 

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I  .">  1  ( i )  C .  ('  1  a  u  s.  Die  frcilcbcnden  Copepoden  mit  bcsonderer  Beritcksichtigiing  der 
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(511)  C.  C  laus.  "  Ueber  d.  Entwicklung,  Organisation  u.  systematische  Stellung 
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(51^)  P.  P.  C.  Hoek.  "Zur  Entwicklungsgeschichte  d.  Entomostracen."  Nie- 
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BIBLIOGRAPHY.  xxi 


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(517)  E.  van  Beneden.     " Developpement  des  Sacculines."     Bull,  de  F  Acad. 
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(518)  C.  Claus.     Die  Cypris-dhnliehe  Larve  der  Cirripedien.     Marburg,  1869. 

(519)  Ch.    Darwin.     A   monograph  of  the  sub-class  Cirripedia,  i  Vols.,  Kay 
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(520)  A.  Dohrn.     "  Untersuchungen  iibcr  Bau  u.  Entwicklung  d.  Arthropoden 
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(521)  P.    P.   C.    Hoek.     "Zur  Entwicklungsgeschichte  der  Entomostraken  I. 
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(522)  R.  Kossmann.    "  Suctoria  u.  Lepadidze.      Arbeiten  a.  d.  zool.-zoot.  Insti- 
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(523)  Aug.  Krohn.     "  Beobachtungen  iiber  die  Entwicklung  der  Cirripedien." 
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(524)  E.  Metschnikoff.    Sitzungsberichte  d.  Versammlung  deutscher  Naturfors- 
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(525)  Fritz    Muller.      "Die    Rhizocephalen."      Archiv  /.    Naturgeschichte, 
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(526)  F.  C.  Noll.    "Kochlorine  hamata,  ein  bohrendes  Cirriped."    Zeit.f.  wiss. 
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(527)  A.  Pagenstecher.     "  Beitrage  zur  Anatomic  und  Entwicklungsgeschichte 
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(52tt)     J.  V.  Thompson.    Zoological  Researches  and  Illustrations,  Vol.  I.,  Part  I. 
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(529)  J.  V.  Thompson.      "  Discovery  of  the  Metamorphosis  in  the  second  type 
of  the  Cirripedes,  viz.  the  Lepades  completing  the  natural  history  of  these  singular 
animals,  and  confirming  their  affinity  with  the  Crustacea."     Phil.  Trans.  1835.     Part 
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(530)  R.  von  Willemoes  Suhm.    "On  the  development  of  Lepas  fascicularis." 
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Ostracoda. 

(531)  C.  Claus.     "  Zur  naheren  Kenntniss  der  Jugendformen  von  Cypris  ovum." 
Zeit.f.  wiss.  Zool.,  Bd.  XV.,  1865. 

(532)  C.  Claus.     "Beitrage   zur   Kenntniss   d.   Ostracoden.     Entwicklungsge- 
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wiss.  zu  Marburg,  Vol.  IX.,  1868. 

PCECILOPODA. 

(533)  A.   Dohrn.     "Untersuch.  lib.  Bau  u.  Entwick.  d.  Arthropoden  (Limulus 
polyphemus)."     Jcnaische  Zeitschrift,  Vol.  VI.,  1871. 

(534)  A.  S.  Packard.     "The  development  of  Limulus  polyphemus."    Mem. 
Boston  Soc.  Nat.  History,  Vol.  II.,  1872. 

PYCNOGONIDA. 

(535)  G.  C  a  van  n  a.     "  Studie  e  ricerche  sui  Picnogonidi."    PiMIicazioni  del  R. 
Instittito  di  Studi  super iori  in  Firenze,  1877. 

(536)  An.  Dohrn.     "  Ueber  Entwicklung  u.  Bau  d.  Pycnogoniden."    Jenaische 
Zeitschrift,  Vol.  v.  1870,  and  "Neue  Untersuchungen  ub.  Pycnogoniden."    Mittheil. 
a.  d.  zoologischen  Station  zu  Ncapel,  Bd.  I.  1878. 

(537)  G.  Hodge.     "  Observations  on  a  species  of  Pycnogon,  etc."    Annal.  and 
Mag.  of  Nat.  Hist.  Vol.  ix.  1862. 

(538)  C.  Semper.     "  Ueber  Pycnogoniden  u.  ihre  in  Hydroiden  schmarotzenden 
Larvenformen.1'     Arbeiten  a.  d.  zool.-zoot.  Instit.   IViiizburg,  Vol.  I.  1874. 


xxii  BIBLIOGRAPHY. 


PENTASTOMIDA. 

(539)  P.  T.  van  Ben e den.  "  Recherches  s.  1'organisation  et  le  developpement  d. 
Linguatules.  Ann.  d.  Scien.  Nat.,  3  Ser.,  Vol.  XI. 

("•I'M  R.  Leuckart.  "  Bau  u.  Entwicklungsgeschichte  d.  Pentastomen."  Leipzig 
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TARDIGRADA. 

(541)  J.  Kaufmann.     "  Ueber  die  Entwicklung  u.  systematische  Stellung  d. 
Tardigraden."    Zeit.f.  iviss.  Zool.,  Bd.  ill.  1851. 

ECHINODERMATA. 

(542)  Alex.  Agassiz.     Revision  of  the  Echini.     Cambridge,  U.S.  1872 — 74. 

(543)  Alex.  Agassiz.    "  North  American  Starfishes."    Memoirs  of  the  Museum 
of  Comparative  Anatomy  and  Zoology  at  Harvard  College,  Vol.   v.,  No.   i.    1877 
(originally  published  in  1864). 

(544)  J.  Barrois.     "  Embryogenie  de  1'Asteriscus  verruculatus  "    Journal  de 
VAnat.  et  Phys.     1879. 

(545)  A.   Baur.     Beitrdge  zur  Naturgeschichte  d.  Synapta  digitata.     Dresden, 
1864. 

(546)  H.  G.   Bronn.     Kiassen  u.  Ordnungen  etc.  Strahlenthiere,  Vol.  II.  1860. 

(547)  W.  B.  Carpenter.     "Researches  on  the  structure,  physiology  and  de- 
velopment of  Antedon."     Phil.  Trans.  CLVI.  1866,  and  Proceedings  of  the  Roy.  Soc., 
No.  166.     1876. 

(548)  P.  H.  Carpenter.    " On  the  oral  and  apical  systems  of  the  Echinoderms." 
Quart.  J.  of  Micr.  Science,  Vol.  xvni.  and  xix.     1878 — 9. 

(549)  A.  Gotte.     " Vergleichende  Entwicklungsgeschichte  d.  Comatula  medi- 
terranea."     Arch.fiir  micr.  Anat.,  Vol.  xn.     1876. 

(550)  R.  Greeff.     "Ueber  die  Entwicklung  des  Asteracanthion  rubens  vom  Ei 
bis  zur  Bipinnaria  u.  Brachiolaria."     Schriftcn  d.  Gesellschaft  zur  Beforderung  d.  ge- 
sarnmlen  Natnrwissenschaften  zu  Marburg,  Bd.  XII.      1876. 

(551)  R.  Greeff.     "Ueber  den  Bau  u.  die  Entwicklung  d.  Echinodermen."   Sitz. 
d.  Gesell.  z.  Beforderung  d.  gesam.  Naturwiss.  zu  Marburg,  No.  4.     1879. 

(552)  T.  H.  Huxley.     "Report  upon  the  researches  of  Mliller  into  the  anat. 
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(553)  Koren  and  Danielssen.     "Observations  sur  la  Bipinnaria  asterigera." 
Ann.  Scien.  Nat.,  Ser.  in.,  Vol.  VII.     1847. 

i-">l)  Koren  and  Uanielssen.  "Observations  on  the  development  of  the  Star- 
fishes." Ann.  and  Mag.  of  Nat.  Hist.,  Vol.  XX.  1857. 

(."..",.",)  A.  Kowalevsky.  "Entwicklungsgeschichte  d.  Holothurien."  Aft m.  Ac. 
Petersburg,  Ser.  VII.,  Tom.  XI.,  No.  6. 

(•"'"><'•)  A.  Krohn.  "Beobacht.  a.  d.  Entwick.  d.  Holothurien  u.  Seeigel." 
M  tiller's  Archiv,  1851. 

(•Vi7)  A.  Krohn.  "  Ueb.  d.  Entwick.  d.  Seesterne  u.  Holothurien."  Muller's 
Ardiir, 

A.   Krohn.      "Beobacht.  lib.    Echinodermenlarven."      Miiller's  Archiv, 
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('>'>'.)}  II.  Ludwig.  "Ueb.  d.  primar.  Steinkanal  d.  Crinoideen,  nebst  vergl. 
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(">r,n)  K.  Metschnikoff.  "Studien  lib.  d.  Entwick.  d.  Echinodermen  u. 
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(501)'  Joh.  Miillcr.  "  Ueb.  d.  Larven  u.  d.  Metamorphose  d.  EchinodcM  men.' 
Alhandlnng,-n  d.  />>/•//'«.  Akad.  (Five  Memoirs),  1848,  49,  50.  52  (two  Mnnoirs). 

Joh.  Miiller.      "  Allgemeincr  Plan  d.  Entwicklung  d.  Echinodermen. 
Abhandl.  d.  Berlin.  Akad.,  1853. 

1  The  dates  in  this  reference  are  the  dates  of  publication. 


BIBLIOGRAPHY.  xxiil 


(563).  E.  Selenka.     "Zur  Entwicklung  d.   Holothurien."    Ztit.  f.  wiss.  Zoo/., 
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(564)  E.  Selenka.     "Keimblatter  u.  Organanlage  bei  Echiniden."   Zeit.f.wiss. 
Zool.t  Vol.  xxxni.     1879. 

(565)  Sir  Wyville  Thomson.     "  On  the  Embryology  of  the  Echinodcrmata." 
Natural  History  Review,  1864. 

(566)  Sir  Wyville  Thomson.     "On  the  Embryogeny  of  Antedon  rosacetw." 
Phil.  Trans.  1865. 

ENTEROPNEUSTA. 

(567)  A.  Agassiz.     "Tornaria."    Ann.  Lyceum  Nat.  Hist.  vin.     New  York, 
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(568)  A.  Agassiz.      "The   History  of  Balanoglossus  and  Tornaria."    Mem. 
Amer.  Acad.  of  Arts  and  Scien.,  Vol.  IX.     1873. 

(569)  A.  Gotte.     " Entwicklungsgeschichte d.  Comatula  Mediterranea. "   Archiv 
fur  mikr.  Anat.,  Bd.  XII.,  1876,  p.  641. 

(570)  E.  Metschnikoff.     "  Untersuchungen  lib.  d.  Metamorphose,  etc.  (Tor- 
naria)."    Zeit.fiir  wiss.  Zool.,  Bd.  XX.     1870. 

(571)  J.  M tiller.     "  Ueb.  d.  Larven  u.  Metamor.  d.  Echinodermen."    Berlin. 
Akad.,  1849  and  1850. 

(572)  J.  W.  Spengel.     "Bau  u.  Entwicklung  von  Balanoglossus."     Tagebl.d. 
Naturf.  Vers.  Mtinchen,  1877. 


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