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CORNELL UNIVERSITY.
THE
Roswell P. Flower Library
THE GIFT OF
ROSWELL P. FLOWER
FOR THE USE OF
THEN. Y. STATE VETERINARY COLLEGE
1897
Cornell University Library
A
MANUAL OF ZOOLOGY
BY
RICHARD HERTWIG
Professor of Zoology in the University at Munich
FROM THE FIFTH GERMAN EDITION
TRANSLATED AND EDITED BY
J. S. KINGSLEY
Professor of Zoology in Tufts College
Av P. £7 >
Jf \ v rd
LIBRARY.
3 s
Py AS x
NEW YORK
HENRY HOLT AND COMPANY
1902
Copyright, 1902,
BY
HENRY HOLT & CO.
~~
ROBERT DRUMMOND, PRINTER, NEW YORK.
PREFACE.
On account of its clearness and breadth of view, its comparatively
simple character and moderate size, Professor Richard Hertwig’s
‘Lehrbuch der Zoologie’ has for ten years held the foremost place
in German schools. The first or general part of the work was
translated in 1896 by Dr. George W. Field, and the cordial recep-
tion which this has had in America has led to the present reproduc-
tion of the whole.
This American edition is not an exact translation. With the
consent of the author the whole text has been edited and modified
in places to accord with American usage. For these changes the
translator alone can be held responsible. Some of the alterations
are slight, but others are very considerable. Thus the group of
Vermes of the original has been broken up and its members dis-
tributed among several phyla; the account of the Arthropoda has
been largely rewritten and the classification materially altered ; while
the Tunicata and the Enteropneusti have been removed from their
position as appendices to the Vermes and united with the Verte-
brata to form the phylum Chordata. Other changes, like those in
the classification of the Reptilia and the nephridial system of the
vertebrates, are of less importance.
A large number of illustrations have been added, either to make
clearer points of structure or to aid in the identification of American
forms. Except in the Protozoa, American genera have in most
cases been indicated by an asterisk. Numerous genera have been
mentioned so that the student may see the relationships of forms
described in morphological literature.
In the translation the word Anlage, meaning the embryonic
material from which an organ or a part is developed, has been
transferred directly. As our language is Germanic in its genius,
there can be no valid objection to the adoption of the word.
As this work is intended for beginners, no bibliography has been
given. A list of literature to be of much value would have been so
large as to materially increase the size of the volume. Experience
iii
iv PREFACE.
has shown that beginners rarely go to the original sources. This
omission is the less important since in all schools where the book is
likely to be used other works containing good bibliographies are
accessible. Reference might here be made to those in the Anat-
omies of Lang and Wiedersheim, the Embryologies of Balfour,
Korschelt and Heider, Minot, and Hertwig, aud Wilson’s work on
The Cell.
The editor must here return his thanks to Dr. George W. Field
for his kindness in allowing the use of his translation of the first
part of the book as the basis of the present edition.
J.S. KINGSLEY.
‘Turrs COLLEGE, Mass., Sept. 19, 1902.
TABLE OF CONTENTS,
GENERAT GANATOMY his cant tauaa s adglins VagNs. 2 Amataeetvea ae mice ede ous
The Morphological Units of the Animal Body....................
The Tissues of the Animal Body........... 0.0.0. cece eee ee cues
Fopitheltall Missiies: paeete on atennate we leusts cesreas Peel aune ate Gueciaeee eo
CONNECTIVE TISSUES! Stee see deren eu eh eee aasthe lo eeencce austen ocelere
Muscular Tissues in casas ts ee. cise oe See Ale rae digs eaun es anes
Nervous: Tissues: sos padkcecs Saiaa- duane emnae Gen eens Duele ate cs
UMMA Y sanded thles s Ha eas qa Tame U fait Munem. ara ace ates
The Combination of Tissues into Organs..........0..0000 000 cca ee
Vegetative Organs). ow cities casas pede das ee ee yee ee TA oe
Organsiof Assimilationy, acsccieae deoudds ca dale ie aean ee ease’
Digestive: Tract. sy sa cowtusaki gran ac aera aan cee pene aie neste ag gneaees
Respiratory OTgans. si sai 0a 6 ek sais Weal awceiece dale oath a Beles
Circulatony-Apparatuss cb ce cued sane ciwedh wa naaisiay eee dons
Excretory OffatiSine esc cans otede eal dad nv ewe one aes
Sexital (Organs dio a ae Na en Wale ale au Mateh Maulana ia ntecany
AnimalCOnansy.. ic.ucicn Vas Maageins qosagalaulied vase ae ey bee
Organsor LOcomotlons
weg c\aneenscdanasinewerelusa'e teases Gels 365
Order Il. Heteroconchige® 4. ose c.d4 ance oe bau loy vane da bles 367
Glass TIT) Seaphopodaw sao. cot be eg ac eh awe Sad Awe) 369
ClassLVy Gasteropoda ns ah 'ncyae mies Gtiow. De eens bees Oe cae tale 369
Order. L.. Prosobranchiata’ , 64 24 sawy sie cis eae HS RRO 378
Order II. Opisthobranchiata ....................0..0.... 381
Order ITD) sPulmiona tay sie 2 ike tad eaty actu tea eaten ace mana en wee 383
Class'V. Cephalopoda. soca. casas pn acuau icin ens ane wes alee ane 384
Order: Vetrabranchias is 544.444.n12acpanesn Gs oo aay rleasc scales 394
Order Il, Dibranthiain..c siete eee mana ae morn Ode oe des 394
NO UIMIMAIR Vio. 2ios ic Satu ene ANS e ai Sue IN NO Aon en BUS rec dive) akin ae lal ae 395
Phyl TX ARTHROPOD EB 32.345. ci: scaipdans- Sale hee RAMON AA eY Bm an eater alee 398
Class Tt Su St aCe a0 8 as ne ae ste can ores eather Aaah gate oe eee 408
Sub Classi, Urilobites co.) sodas Soc el awison ehh ee be ars 414
Sub Class II. Phyllopodas isc ccc ccc cede seen ned we bees 415
Orderl, -Braniehiopoda yy. pat ys tee cans 5 oceania nw hemes 436
Order Tl Cladocera i. cesses aye ate RaW A see eres ca ee Lala a 417
Sub:Class TIT “Copepoda. 0a. Sons og he pee ee ea Sande ae: 417
Order l: Hueco pe pod an. ese tie thie ha want hos Be Se Oe oe 421
Order ll. Siphonostomatay 2. fo suc Gana eis betas evew os 422
SubClass iV. Ostracodays, «h 284 on antag Geen puaeme es oe oe week 422
SubClass:Vi" Cirripediaic | dai. cg masts oakten oe ees eae he ee 423
Orders epadidae tA cere rd ce rn nace ae eae eet ee aioe 425
Order Thy Balamidaes tion. o.4 nen sie he Rach Rawiaee oeeereaeeas cored ee 425
OrderIIl.“Rhizoce pha aveeac3 ca sact ss.cep dee ee mean ve ees 426
Sub Class Vi; ‘Malacostraca sone sige coos, cee bad ow ad cnen 426
Tepionl, eptostiaca, sss ois sees. gate cstld hades et Re oon nee 427
Legion I. Thoracostraca.... 6.046 ccc ee Sane scenes cacsuavs 427
Orderly Schizopodai a, a2 Gic gs ud ontogueracaies oe wut hoen c 428
Order II; Stomatopoday.< ou cc. soe c4nh eh Cavan eee teicen 429
TABLE OF CONTENTS.
PAGE
Orderdil Decapoda: 2 satan, cae tie ds Manes woe eas as 429
OrderiIVi Cuma cla: eno: oe ote 22.8 aan eee aR aoe 437
Legion Ll. Arthrocostracds 23 44 ye ane tanec ned amines oho e 438
Order tT Am plnpodasi.s io 6 keen ei vac aloe eee ae 438
Ordér Te [sopoday oc 55 eo a yaar araecetneuaeanonn ane sheets et 440
Class TE ACerata vas sata mo un moe ceca aes eae ne eee aie 442
Stib: Class I. Gigantostrata ae ti.- Go adel ic de tke ee yak been vous 443
Order]. Xaphosura ss acdc. t esac s ates ee eu Oe Ee he 444
Ordetll. Hury pterdacc..« occstees ender ee ne Hea ae eas eed 444
SiibClassdl; ArachmMid ads «ac samckieiaba aadel scene ¢ Rew aided) ee 444
ecion |; Arthrogastridas scented G Galaaioe sanael waveeuaur: 447
Order. vSCOrpionsd ais cre ese ahece ie vaee « oeso. a eet Led Agee tenor 447
Orderdh. Phrynoideas.. 5. eat ee ies o beadh eieccioam se get 448
Order III. Microthelyphorida ......................... 448
Ordér LV... Solpigida. 54502 ccc4i4 erean gs ntyerus peewee ed 449
Order V2 Pseudoscorpil. whee as ds pane wea ndr esas enaet 450
Order Vi; Phalangiday. «2.007 <4 04 sek Bees eee y o8 450
Lesion Ll. -Spherorastriday, ocxccia sos Mea named SGaN ane 451
Order Amaneing: oi. cc sci oann aenvae dae ae aia eamen Am ames 451
Order Ti; Acarifia: 25.5.cc0hs 25 uk abe yes eeteae BH Ae tins 453
Order lil: Waingaatulida. <..:03 des yang eed ye hw bad See bees 454
Tardigrada:<. 24 anaseehnanach 4 menses Hodes Rance oe 455
PY ENOCOMG AL co Apmis a tee es RUE A hehe een eae 456
Class eELL Malacopoday ts. set stuvnpusni einen Aone ne eek a ayaa 456
Gla SoHU Ve CITI SEC tas coat ices fess Crean ences stats asa ease eR lah Site ma eA eee 458
SubClass Le Chilopodacc syed anenac eon weues Mateus hen enmtess 460
Siib Classi sHexapodar acc) asain omen watah tl oeite aeedae s 461
Order. Apteryeole: oun ick te she tee OR te eee sad 477
Order 11s. Axchipterars) - pat onde ns es eace ue vas ee Re ee 477
Order IIb, :Orthoptera., a5 ce sek ne Shea See odes ve ee ss 480
Order TV. sNeuropterdn wis comes cae acai, axe eects oe ee oe 481
Order Ve iStrepsipteray ion tise oie gaat aa athe a aautvea ww Raed 4g 483
Order Vis ‘Coleoptera: fn ciai.cus stanqeaians sve wae svn hos g-heen s 483
Order VIL) “Hymenoptera: ai. 00 tee a Milas oe eee eee 485
Order VILI2RDynChOla cs aah eA hee cis aoe cscs ha iceabhs asc avente lars 489
Order Tes Di pierre, c..a cena inds arecareas we. ceee am einen: awieainnlert 491
Order X, Aphaniptetan cccrsns acd cage ee daaed ea een ow hae 493
Order Xl. Lepidoptera... obec ot ag sone ee een tan Gaoe ae 494
Class'Ve MDiplopodar ys 2it. 54 cagiemeendt.sqeiws ay ae ea ee Goes 496
SUMMARY =..2cvies Sm ae nist WERE RANGE hdile HATS eb tenes sng RE aA 497
Phylum X CHORDATA. 9% Mani ia aa bese ulead ha Aetna HE a mes 501
Sub Phylum, Leptocandi.. 05s... 5 sarees cA ieee ete eee 502
Sub Phylum-iL, Dumeatas sc. o06acrsaceqeca deo aent Sas teneae eee aan OS
Order tl. - Copelatan,) anc duducpeac menos be MANERA ROA ACNE S 506
Orderly ‘Tethyoideayn ss 26:8 ae avs aad ae Aes meee 508
Order TITS Pialiaceas ctracieseccconend, aathscee eo ene t Mn eh 510
Sub Phylum III. Enteropneusta ......... 0... eee eee eee 512
Sub: Phylum lV. Vertebrata scods-itied ere eae Agia us aaree ho stung BRO ao 514
Series Tenth y ops dans cea dase stars cea: sits secede: a ditebst aay bioas Waste Sopa AR 555
Class I. Cyclostomata
TABLE OF CONTENTS.
PAGE
Sub: Class... Myzontesii3.2 oes gn-ce Mae Se oes ohare Ses 556
Sub: Class Il... Petromyzontes 3.4 sitio tw series Saree es dole 557
Class IT. Pisces jae gate satan seven nears Mt ie eee ee Mee oe Be ES 557
Sub Class I. Elasmobranchit 2. i. jaccg04 4. saceds va vate eas aas 569
Order I. Selachits.se .yesdwa hey Sock Hed aa Seer AY oe oe Ne 570
Order Ib. Holocephalit.+.27 os spice dbs ehh e Sh ato ooo de, oe 572
Sub Classll <:Ganoider winccretee eu et He bas ad Maeee Morne iS er 572
Order; Crossoptery ei sig jens renee aw eine sm ens wa alee 573
Order HT Chord rosten cic cah occas vine saide ne nesintssece eleacde es alesse) Bee 573
Order Tilo Holostomi: shoe eau iviesuuicaoscsun senses ale caer iier os 573
SubClass: LIL: “Leleostets 10 een acq-ahersianey won biguaea aati pouch bee ardes Bw 574
Order: (PhysSostOmiss ad sau gtepacm amas ee weak A oe Boe 575
Order Th: Pharyngopnathi . 222.5214 is tated eda eas te 576
Order III. Acanthopteri ........... 000.00. ee eee ee eee 577
Order IV; Anacanthinis. cao dse acces cease ahaa bo 577
Order Vi Lophobranehit .incccsdcanneg anaes ae eRe RaUNes 578
Order Vil; Plectognathion aco suins ay mere awe aa thse en ns 578
Sub: Class IV. DipnOk s4.sncuin ec oe che A LRA E EAR eRe Re as 579
Classi IL Amiphibiay occ kesdas samdosvoniae aaaloneaan nein se telaeee 580
Order. Stegocephalty oo scp paid wap sues ccausde Peuel bin nee ee 586
Order IL. Gymnophiona, cavenca cavicte ste ag aca cieuieag scons 587
Order LL. UrOd ela. ssscs soe. ee ane ek ae, Secadosn Boe sesohvaea enna Sore 587
Order TV." Amutatccaly cucte seo ag sabe aw Sane Sie aoeiaaee Skane ze 588
Series: Lie -AimmiO tars sees oe ns who eek ERS See Ee Se Se ERA ad 588
Class? In Reptilia: s.vsars ewiineceias ance nave n cide: ions oe Saw eens eset 588
Order], “Cheromorphay wicccwndeeas eee nak ak ne Het aaeares 594
Order li... Plesiosauriaos acc casrslew.cke Seowee we oak, wane wens 594
OrderIll, Tehthyosatitia: ...25 2 cass nak ne vols du ain en ae ome 594
Order IV. Cnelonia, space feng Gace ek ae ain as AIS cele eile ddepsako ee 594
Order V. Rhynchocephalia ..........0.0.00.0000 00000 cee ee 595
Order Vil2 DimGsatna: a'o 66:5 bacon naar ara ovation kaa ek 595
Order Vil Sa uiamatac ois 2s ena haat aude eed ces te oe 596
Order VATE CrOCOGMa x on). Scot cla dient Roe ae ae oe Gas ees 601
Order Ix: Pterodactyllayss.uc a uus bade v da og sawule oesekacese: 602
Classi A ViCStin eet nar ant eee Sid aie dacce yaa. et anenieeay gee ape neacs & 603,
Orderly Saul nae ear aie ae iachmasecanlani lita cis ators lacs eilalh me 612
OrderTT. “Odontornithes: . ¢ en cece ds Sach ach oe ous 612
Order Le sRatitaes an seta nec aay ee ies Matte ows e cea cae 612
Orderly Carinae. aos saci tailon Sinnen tet Cee ee he tena 613
Glass Tis Matirmalliace eth natct Ae eee eens ete ne eh 617
Sub Class I. Monotremata, 224.005 564 05 een bensn decane. . 631
Sub; Class: Ti. “Marsipialtas sy <2. iced vag. dunes Re eetep eke bed 632
Order J. ‘Roly protodontar, . «ces Carak pause aeawas cay eee see 633
Order ILL Diprotodonta win.c.2-va es ou abies she ewan oclen each 633
Sub Class TIT; Placentalia 4:94. 64.04. sune duartinare utavaaie copie anes 634
Order ls Rdentata un ae scu cers tacks noeince cenan sa tees 635
Onderdls Tmsectivoral afc vate surges Gon tere css eed lee oe es 636
OrderlM.- Giirop tera es creeds restos hetietueen se eauiels ake ae 637
Order LV,“ Rodentilae sirens ch acstascunnlewcraien oan Meals ueetan 638
Order V_ Ungulata
TABLE OF CONTENTS. X1
PAGE
Order Vi, Proboscidia: ni cocci cacacsiouses MR ee 643
Order: VIE. Piyracoideas «sna tae et oe ad Ge ey cme as 644
Order VAULT, Sikenla oh Bas a4 wasted aun thea. ab a aes arenas allen 644
Orden IES, = Ceta Ged: mien aiataaen soonmiinia Saint ts a ee Bleue enn aendeh 645
Orden2k;= Car niviordscrn care wae SecA ey Memes eens eh eae 646
Order Del, PEOSIII TES ai dre isie.-giicus ss ss Soho hee Who dst aaa bse Weare aN 648
Order MIT: Primatess.c);o%e-acb sc aete seas a aeanneu aa eave ace: 649
GENERAL PRINCIPLES OF ZOOLOGY.
INTRODUCTION.
Man’s Relation to Other Animals.—The man who has learned
to observe nature in a disinterested manner sees himself in the
midst of a manifold variety of organisms, which in their structure,
and even more in their vital phenomena, disclose to him a simi-
larity to his own being. This similarity, with many of the
mammals, especially the anthropoid apes, has the sharpness of a
caricature. In the invertebrate animals it is softened; yet even
in the lowest organisms, for our knowledge of which we are
indebted to the microscope, it is still to be found: although here
the vital processes which have reached such an astonishing com-
plexity and perfection in ourselves can only be recognized in their
simplest outlines. Man is part of a great whole, the Animal
Kingdom, one form among the many thousand forms in which
animal organization has found expression.
Purpose of Zoological Study.—If we would, therefore, fully
understand the structure of man, we must, as it were, look at it
upon the background which is formed by the conditions of
organization of the other animals, and for this purpose we must
investigate these conditions. To such endeavors the scientific
knowledge of animal life, or Zoology, owes its origin and continued
advancement. But meanwhile the subject of zoology has widened;
for, apart from its relations to man, zoology has to explain the
organization of animals and their relations to one another. This
is a rich field for scientific activity; its enormous range is a conse-
quence, on the one hand, of the well-nigh exhaustless variety of
animal organization, and, on the other hand, of the different
points of view from which the zoologist enters upon the solution
of his problem.
2 GENERAL PRINCIPLES OF ZOOLOGY.
In the first half of the last century the conception, which is
still held by the public at large, was prevalent, if not quite uni-
versal, in scientific circles, that the aim of zoology is to furnish
every animal with a name, to characterize it according to some
easily recognizable features, and to classify it in a way to facilitate
quick identification. By Natural History was understood the
classification of animals, that is to say, only one part of zoology,
indeed a part of minor importance, which can pretend to scientific
value only when it is brought into relation with other problems
(geographical distribution, evolution). This conception has
during the past five decades become more and more subordinated.
The ambition to describe the largest possible number of new forms
and to shine by means of an extensive knowledge of species belongs
to the past. In fact there is a tendency to undue neglect of
classification. Morphology and Physiology to-day dominate the
sphere of the zoologist’s work.
Morphology, or the study of form, begins with the appearances
of animals, and has first to describe all which can be seen exter-
nally, as size, color, proportion of parts. But since the external
appearance of an animal cannot be understood without knowledge
of the internal organs which condition the external form, the
morphologist must make these accessible by the aid of dissection,
of Anatomy, and likewise describe their forms and methods of
combination. In his investigation he only stops when he has
arrived at the morphological elements of the animal body, the
cells. Everywhere the morphologist has to do with conditions of
form: the only difference lies in the instruments by means of
which he obtains his insight, according to whether he gathers his
knowledge through immediate observation, or after a previous
dissection with scalpel and scissors, or by use of the micro-
scope. Therefore we cannot contrast Morphology and Anatomy,
and ascribe to the former the description of only the external, and
to the latter of only the internal parts. The distinction is not
logically correct, since the kind of knowledge and the mental
processes are the same in both cases. The distinction, too, is
unnatural, since in many instances organs which in some cases lie
in the interior of the body, and must be dissected out, belong in
other cases to the surface of the body, and are accessible for direct
description. Further, on account of their transparency the in-
ternal parts of many animals can be studied without dissection.
Comparative Anatomy.—For morphology, as for every science,
the proposition is true that the mere accumulation of facts is not
INTRODUCTION. 3
sufficient to give the subject the character of a science; an addi-
tional mental elaboration of this material is necessary. Such a
result is reached by comparison. The morphologist compares
animals with each other according to their structure, in order to
ascertain what parts of the organization recur everywhere, what
only within narrow limits, possibly restricted to the representatives
of a single species. He thus gains a double advantage: (1) an
insight into the relationships of animals, and hence the foundation
for a Natural System; (2) the evidence of the laws which govern
organisms. Any organism is not a structure which has arisen
independently and which is hence intelligible by itself: it stands
rather in a regular dependent relation to the other members of the
animal kingdom. We can only understand its structure when we
compare it with the closely and the more distantly related animals,
e.g., when we compare man with the other vertebrates and with
many lower invertebrate forms. Here we have to consider one of
the most mysterious phenomena of the organic world, the path to
the full explanation of which was first broken by the Theory of
Evolution, as will be shown in another chapter.
Ontogeny.—To morphology belongs, as an important integral
part, Ontogeny or Embryology. Only a few animals are com-
pletely formed in all their parts at the beginning of their individual
existence; most of them arise from the egg, a relatively simple
body, and then step by step attain their permanent form by com-
plicated changes. The morphologist must, with the completest
possible series, determine by observation the different stages, com-
pare them with the mature animals, and with the structure and
developmental stages of other animals. Here is revealed to him
the same conformity to law which dominates the mature animals,
and a knowledge of this conformity is of fundamental importance
as well for classification as for the causal explanation of the animal
form. The developmental stages of man show definite regular
agreements, not only with the structure of the adult human being,
which in and of itself would be intelligible, but also with the
structure of lower vertebrates, like the fishes, and even with many
of the still lower animals of the invertebrate groups.
Physiology.—In the same way as the morphologist studies the
structure, the physiologist studies the vital phenomena of animals
and the functions of their organs. Formerly life was regarded as
the expression of a special vital force peculiar to organisms, and
any attempt at a logical explanation of the vital processes was
thereby renounced. Modern physiology has abandoned this theory
4 GENERAL PRINCIPLES OF ZOOLOGY.
of vital force; it has begun the attempt to explain life as the
summation of extremely complicated chemico-physical processes,
and thus to apply to the organic world those explanatory princi-
ples which prevail in the inorganic realm. The results obtained
show that it is the correct method.
Since each organic form is the product of its development,
since, further, the development represents to us the summation of
most complicated vital processes, the explanation of the organic
bodily form is, therefore, in ultimate analysis a physiological
problem; though of course a problem whose solution les still in
the indefinitely distant future. What has been actually accom-
plished in this direction is only the smallest beginning, even in
comparison with that which many falsely regard as already attained.
Biology.— According as the relations of each organism to the
external world are brought about through its vital phenomena,
there belongs to physiology, or at least 1s connected with it, the
study of the conditions of animal existence, (Ecology or Biology.
This branch of the science has of late attained a very considerable
importance. How animals are distributed over the globe, how
climate and conditions influence their distribution, how by known
factors the structure and the mode of life become changed, are
questions which are to-day discussed more than ever before.
Paleontology.—Finally in the realm of zoology belongs also
Paleozoology or Paleontology, the study of the extinct animals.
For between the extinct and the living animals there exists a
genetic relationship: the former are the precursors of the latter,
and their fossil remains are the most trustworthy records of the
history of the race, or Phylogeny. As in human affairs the
present conditions can only be completely understood by the aid
of history, so in many cases the zoologist must draw upon the
results of paleontology for an explanation of the living animal
world.
The science of zoology would be subdivided in the aboye-men-
tioned manner if we wished to proceed entirely on a scientific basis.
Yet practical considerations have made many modifications neces-
sary. On account of their paramount importance to the medical
profession human anatomy and embryology have been raised to
independent branches of science. In comparative physiology only
the most general foundations have been laid; a more special
physiology exists only for man and the higher vertebrates: this,
too, for the above-named reasons has been made a special branch
of science. Paleontology also has, in addition to its specific
INTRODUCTION. 5
zoological tasks, attained importance as a scientific aid to geology,
since it furnishes the materials for characterizing and fixing the
various geological ages and the earth’s history during those ages.
When, therefore, at the present day we speak of zoology, we
usually refer to morphology and classification of living animals
with consideration of their general vital phenomena.
The views here given of the character of zoology have not been
the same in all time. Like every science zoology has developed
gradually; it has varied with each epoch and tendency, according
as the systematic or the morphological or the physiological point
of view was the prevailing one. It will now be interesting to take
a hasty glance at the most important phases in the development
of zoology. The reader will better understand the questions which
now dominate zoological inquiry, if he know how these have arisen
historically.
HISTORY OF ZOOLOGY.
Methods of Zoological Study.—In the history of zoology we
can distinguish two great currents, which have been united in a
few men, but which on the whole have developed independently,
nay, more often in pronounced opposition to each other; these are
on the one side the systematic, on the other the morphologico-
physiological mode of studying animals. In this brief historical
summary they will be kept distinct from one another, although in
the commencement of zoological investigation there was no oppo-
sition between the two points of view, and even later this has in
many instances disappeared.
Aristotle, the great Greek philosopher, has been distinguished
as the Father of Natural History, which means that his predeces-
sors’ fragmentary knowledge of zoology could not be compared
with the well-arranged order in which Aristotle had brought
together his own and the previously existing knowledge of the
nature of animals. In Aristotle favorable external conditions were
united with more ‘favorable mental ability. Equipped with the
literary aid of an extensive library, and the pecuniary means then
more indispensable than now for natural-history investigation, he
pursued the inductive method, the only one which is capable of
furnishing secure foundations in the realm of natural science. It
is a matter for great regret that there have been preserved only
parts of his three most important zoological works, ‘ Historia
animalium,” ‘*De partibus,” and ‘De generatione,” works in
which zoology is founded as a universal science, since anatomy and
embryology, physiology and classification find equal consideration.
Ilow far Aristotle, notwithstanding many errors, attained to a
correct knowledge of the structure and embryology of animals, is
shown by the fact that many of his discoveries have been confirmed
only within a century. Thus it was known to Aristotle, though
only lately rediscovered by Johannes Miiller, that many sharks are
not only viviparous, but that also in their case the embryo becomes
fixed to the maternal uterus and there is formed a contrivance for
ve
8 GENERAL PRINCIPLES OF ZOOLOGY.
nutrition resembling the mammalian and even the human pla-
centa; he knew the difference between male and female cephalo-
pods, and that the young cuttlefish has a preoral yolk-sac.
The position which Aristotle took in reference to the classifica-
tion of animals is of great interest; he mentions in his writings the
very considerable number of about five hundred species. Since he
does not mention very well-known forms, like the badger, dragon-
fly, etc., we can assume that he knew many more, but did not
regard it necessary to give a catalogue of all the forms known to
him, and that he mentioned them only if it was necessary to refer
to certain physiological or morphological conditions found in them.
This neglect of the systematic side is further shown in the fact
that the great philosopher is satisfied with two sy stematic cate-
gories, with Eidos, species or kind, and yeévos or group. His
eight yevn ueyzora would about correspond with the Classes of
modern zoology; they have been the starting-point for all later
attempts at classification, and may therefore be enumerated here:
H
Mammals (C@oroKobvta év avrois).
2. Birds (opriées).
3. Oviparous quadrupeds (retpamoda @OTOKOOYTA).
4, Fishes (iy@ves).
5. Molluses (uadakia).
6. Crustaceans (uadaxooTpaka).
7. Insects (Evropa).
8. Animals with shells (oorpaxodéppata).
Aristotle also noticed the close connexion of the first four
groups, since he, without indeed actually carrying out the divi-
sion, has contrasted the animals with blood, evarpa (better,
ental: with red blood), with the bloodless, avaiya (better,
animals with colorless blood or with no blood at all).
DEVELOPMENT OF SYSTEMATIC ZOOLOGY.
Pliny.—It is a remarkable fact that after the writings of
Aristotle, in which classification is much subordinated and only
serves to express the anatomical relationships in animals, an
exclusively systematic direction should have been taken. This is
explicable only when we consider that the mental continuity of
investigation was completely broken on the one hand by the
decline. and ultimate complete collapse of ancient classic civiliza-
tion, and on the other by the triumphant advance of Christianity.
HISTORY OF ZOOLOGY. 9
The decay of zoological investigation, that had only just begun to
bloom, begins in the writings of Pliny. Although this Roman
general and scholar was long lauded as the foremost zoologist of
antiquity, he is now given the place of a not even fortunate com-
piler, who collected from the writings of others the accurate and
the fabulous indiscriminately, and replaced the natural classifica-
tion of animals according to structure by the unnatural, purely
arbitrary division according to their place of abode (flying animals,
land animals, water animals).
Zoology of the Middle Ages.—The rise of Christianity resulted
in the complete annihilation of natural science and investigation.
The world-renouncing character, which originally was peculiar to
the Christian conception, led naturally to a disposition hostile to
any mental occupation with natural things. Then came a time
when answers to questions capable of solution by the simplest
observation were sought by painstaking learned rummaging of the
works of standard authors. How many teeth the horse has, was
debated in many polemics, which would have led to bloodshed if
one of the authors had not taken occasion to look into a horse’s
mouth. Significant of this mental bias which prevailed through-
out the entire Middle Ages is the ‘ Physiologus’ or ‘ Bestiarius,’ a
book from which the zoological authors of the Middle Ages drew
much material. The book in its various editions names about
seventy animals, among them many creatures of fable: the dragon,
the unicorn, the phoenix, etc. Most of the accounts given of
various animals are fables, intended to illustrate religions or
ethical teachings. In a similar way the religious element played
an important réle in the many-volumed Natural History of the
Dominican Albertus Magnus, and Vincentius Bellovacensis, and
of the Augustine Thomas Cantimpratensis, although these used
as a foundation for their expositions the Latin translation of
Aristotle, the works of Pliny and other authors of antiquity.
Wotton.—Under such conditions we must regard it as an im-
portant advance that at the close of the Middle Ages, when the
interest in scientific investigation awoke anew, Aristotle’s concep-
tions were taken up and elaborated from a scientific standpoint.
In this sense we can call the Englishman Wotton the successor of
Aristotle. In 1552 he published his work ‘‘De differentiis
animalium,” in which he essentially copied the system of Aristotle,
except that he admitted the new group of plant-animals or
zoophytes. However, the title, ‘On the Distinguishing Characters
of Animals,’ shows that of the rich treasury of Aristotelian knowl-
10 GENERAL PRINCIPLES OF ZOOLOGY.
edge the systematic results obtained the chief recognition, and
thus Wotton’s work inaugurated the period of systematic zoology,
which in the Englishman Ray, but even more in Linneus, has
found its most brilliant exponents.
Linnzus, the descendant of a Swedish clergyman, whose
family name Ingemarsson had been changed after a linden-tree
near the parsonage, to Lindelius, was born in Rashult in 1707.
Pronounced by his teachers to be good for nothing at study, he
was saved from the fate of learning the cobbler’s trade through
the influence of a physician, who recognized the fine abilities of
the boy, and won him for medical studies. He studied at Lund
and Upsala; at the age of twenty-eight he made extended tours on
the Continent, and at that time gained recognition from the fore-
most men in his profession. In 1741 he became professor of medi-
cine in Upsala, some years later professor of natural history. He
died in 1778.
Improvement of Zoological Nomenclature by Linneus.—
Linneus’s most important work is his ‘‘ Systema Nature,” which,
first appearing in 1735, up to 1766-68 passed through twelve
editions; after his death there came out a thirteenth, edited by
Gmelin. This has become the foundation for systematic zoology,
since it introduces for the first time (1) a sharper division into the
system, (2) a definite scientific terminology, the binomial nomen-
clature, and (3) brief, comprehensive, clear diagnoses. In classi-
fication Linneeus employed four categories; he divided the entire
Animal Kingdom into Classes, the Classes into Orders, these into
Genera, the Genera finally into Species. The term Family was
not employed in the ‘‘Systema Nature.” Still more important
was the binomial nomenclature. Hitherto the common names were
in use in the scientific world, and led to much confusion: the same
animals had different names, and different animals had the same
names; in the naming of newly discovered animals there prevailed
no generally accepted principle. This inconvenience was entirely
obviated by Linneus in the tenth edition of his Systema by the
introduction of a scientific nomenclature. The first word, a noun,
designates the genus to which the animal belongs, the following
word, usually an adjective, the species within the genus. The
names Canis familiaris, Canis lupus, Canis vulpes, indicate that
the dog, wolf, and fox are related to one another, since they belong
to the same genus, the genus of doglike animals, of which they are
different species. Linnus’s method of naming was particularly
valuable in the description of new species, inasmuch as it at the
HISTORY OF ZOOLOGY. 11
outset informed the reader to what position of relationship the new
species was to be assigned.
In his characterization of the various systematic groups Linneus
broke completely with the hitherto-prevailing custom. His
predecessors (as Gessner, Aldrovandus) in their Natural Histories
had given a verbose and detailed description of each animal, from
which the beginner was scarcely able to see what was specially
characteristic for that animal, a matter which should have been
emphasized in the definition. Linneus, on the other hand, intro-
duced brief diagnoses, which in a few words, never in sentence
form, gave only what was necessary for recognition. Thus a way
was found which insured comprehensibility in the enormously
increasing number of known animals.
Influence of the Linnean System.—But in the great superiority
of the Linnean System lay at the same time the germ of the one-
sided development which zoology came to take under his influence.
The logical perfecting of the system, which undoubtedly had
become necessary, gave that a brilliant aspect, and hid the fact
that classification is not the ultimate purpose of investigation, but
only an important and indispensable aid to it. In the zeal for
naming and classifying animals, the higher goal of investigation,
knowledge of the nature of animals, was lost sight of, and the
interest in anatomy, physiology, and embryology flagged.
From these reproaches we can scarcely spare Linnzus himself,
the father of this tendency. For while in his ‘* Systema Nature”
he treated of a much larger number of animals than any earlier
zoologist, he brought about no deepening of our knowledge. The
manner in which he divided the animal kingdom, in comparison
with the Aristotelian system, is rather a retrogression than an
advance. Linneus divided the animal kingdom into six classes:
Mammalia, Aves, Amphibia, Pisces, Insecta, Vermes. The first
four classes correspond to Aristotle’s four groups of animals with
blood. In the division of the invertebrated animals into Insecta
and Vermes Linneus stands undoubtedly behind Aristotle, who
attempted, and in part successfully, to set up a larger number of
groups.
But in his successors, even more than in Linneus himself, we
see the damage wrought by the systematic method. The diagnoses
of Linneus were for the most part models, which, mutatis
mutandis, could be employed for new species with little trouble.
There was needed only some exchanging of adjectives to express
the differences. With the hundreds of thousands of different
12 GENERAL PRINCIPLES OF ZOOLOGY.
species of animals there was no lack of material, and so the arena
was opened for that spiritless zoology of species-making which in
the first half of the last century brought zoology into such discredit.
Zoology would have been in danger of growing into a Tower of
Babel of species-describing had not a counterpoise been created in
the strengthening of the physiologico-anatomical side.
DEVELOPMENT OF MORPHOLOGY.
Anatomists of Classic Antiquity.—Comparative anatomy—for
this chiefly concerns us here—for a long time owed its development
to the students of human anatomy; this is due to the fact that even
up to a recent date comparative anatomy was assigned to the
medical faculty, while zoology belonged to the philosophical
faculty, as if it were an entirely separate study. The disciples of
Hippocrates had previously studied animal anatomy for the pur-
pose of obtaining an idea of human organization, from the struc-
ture of other mammals, and thus to gain a secure foundation for
the diagnosis of human diseases. The work of classical antiquity
most prominent in this respect, the celebrated Human Anatomy
by Claudius Galenus (131-201 a.p.), is based chiefly upon obser-
vations upon dogs, monkeys, etc.; for in ancient times, and even
in the Middle Ages, men showed considerable repugnance to
making the human cadaver a subject of scientific investigation.
Middle Ages.—The first thousand years in which Christianity
formed the ruling power in the mental life of the people was quite
fruitless for anatomy; in the main men held to the writings of
Galen and the works of his commentators, and seldom took occa-
sion to prove their correctness by their own observations. With
the ending of the Middle Ages the interest in independent scien-
tific research first broke its bounds.
Vesal (1514-1564), the creator of modern anatomy, had the
courage carefully to investigate the human cadaver and to point
out numerous errors in Galen’s writings which had arisen through
the unwarranted application to human anatomy of the discoveries
made upon other animals. By his corrections of Galen, Vesal was
drawn into a violent controversy with his teacher, Sylvius, an
energetic defender of Galen’s authority, and with his renowned
contemporary Eustachius, which did much for the development of
comparative anatomy. At first animals were dissected only for
the purpose of disclosing the cause of Galen’s mistakes, but later
through a zeal and love for facts. It was natural that first of all
HISTORY OF ZOOLOGY. 13
vertebrates found consideration, since they stand next to man in
structure. Thus there appeared in the same century with Vesal’s
Human Anatomy drawings of skeletons of vertebrates by the
Nuremberg physician Coiter; the anatomical writings of Fabricius
ab Aquapendente, etc.
Beginning of Zootomy.—But later attention was turned also to
insects and molluscs, indeed even to the marine echinoderms,
celenterates, and Protozoa. Here, above all, three men who lived
at the end of the seventeenth century deserve mention, the Italian
Malpighi and the Dutchmen Swammerdam and Leeuwenhoek.
The former’s ‘‘ Dissertatio de bombyce ” was the pioneer for insect:
anatomy, since by the discovery of the vasa Malpighii, the heart,
the nervous system, the tracheex, etc., an extraordinary extension
of our knowledge was brought about. Of Swammerdam’s writings
attention should be called particularly to “The Bible of Nature,”
a work to which no other of that time is comparable, since it con-
tains discoveries of great accuracy on the structure of bees, May-
flies, snails, etc. Leeuwenhoek, finally, was a most fortunate
discoverer in the field of microscopic research, by him introduced
into science. Besides other things he studied especially the
minute inhabitants of the fresh waters, the ‘ infusion-animalcules,’
a more careful investigation of which has led to a complete reversal
of our conception of the essentials of animal organization.
The Dawn of Independent Observation.—The great service of
the men named above consists chiefly in that they broke away from
the thraldom of book-learning and, relying alone upon their own
eyes and their own judgment, regained what had been lost, the
blessing of independent and-unbiassed observation. They spread
the interest in observation of nature over a wide circle so that in
the eighteenth century the number of independent natural-history
writings had increased enormously. There were busy with the
study of insect structure and development, de Geer in Sweden,
Réaumur in France, Lyonet in Belgium, Résel von Rosenhof in
Germany; the latter besides wrote a monograph on the indigenous
batrachia, which is still worth reading. The investigation of the
infusoria formed a favorite occupation for the learned and the laity,
as Wrisberg, von Gleichen-Russwurm, Schaffer, Eichhorn, and
O. F. Miller. In most of the writings the religious character of
the contemplations of nature are extraordinarily emphasized, and
since we find that among these writers numerous clergymen
(Eichhorn in Danzig, Gaze in Quedlinburg, Schiffer in Regens-
burg) attained distinction, we have a sign that a reconciliation
14 GENERAL PRINCIPLES OF ZOOLOGY.
had taken place between Christianity and natural science. As a
criterion of the progress made in comparison with the earlier
centuries, a mere glance at the illustrations is sufficient. Any one
will at the first glance recognize the difference between the shabby
drawings of an Aldrovandus and the masterly figures of a Lyonet
or a Résel von Rosenhof.
Period of Comparative Anatomy.—Thus through the zeal of
numerous men filled with a love of nature a store of anatomical
facts was collected, which needed only a mental reworking; and
this mental reworking was brought about, or at least entered upon,
by the great comparative anatomists who lived at the end of the
eighteenth and the beginning of the nineteenth century. Among
these the French zoologists Lamarck, Savigny, Geoffroy St. Hilaire,
Cuvier, and the Germans Meckel and Goethe are especially to be
named.
Correlation of Parts.—When the various animals were com-
pared with one another with reference to their structure there was
obtained a series of important fundamental laws, particularly the
law of the Correlation of Parts and the law of the Homology of
Organs. The former established the fact that there exists a
dependent relation between the organs of the same animal, that
local changes in one single organ also lead to corresponding
changes at some distant part of the body, and that therefore from
the constitution of certain parts an inference can be drawn as to
the constitution of another part of the body. Cuvier particularly
made use of this principle in reconstructing the form of extinct
amimals.
Homology and Analogy.—Still more important was the theory
of the Homology of Organs. In the organs of animals a distinction
was drawn between an anatomical and a physiological character;
the anatomical character is the sum of all the anatomical features,
as found in form, structure, position, and mode of connection of
organs; the physiological character is their function. Anatomically
similar organs in closely related animals will usually have the same
functions, as, for example, the liver of all vertebrates has the
function of producing gall; here anatomical and physiological
characteristics go hand in hand. But this need not necessarily be
the case; very often it may happen that one and the same function
is possessed by organs anatomically different; as, for example, the
respiration of vertebrates is carried on in fishes by gills, in mammals
by lungs. Conversely, anatomically similar organs may haye
different functions, as the lungs of mammals and the swim-bladdey
HISTORY OF ZOOLOGY. 15
of fishes; similar organs may also undergo a change of function
from one group to another; the hydrostatic apparatus of fishes has
come to be the seat of respiration in the mammals. Organs with
like functions — physiologically equivalent organs—are called
‘analogous’; organs of like anatomical constitution—anatomically
equivalent organs—are called ‘homologous.’ It is the task of
comparative anatomy to discover in the various parts of animals
those which are homologous, i.e. those anatomically equivalent,
and to follow the changes in them conditioned by a change of
function.
Cuvier.—The foremost representative of comparative anatomy
was Georges Dagobert Cuvier. He was born in 1769 in the town
of Mompelgardt (Montbeillard), then belonging to Wirtemberg,
and obtained his early training in the Karlschule at Stuttgart,
where, through the influence of his teacher Kielmeyer, he was led
to the study of comparative anatomy. The opportunity of going
to the seashore which was offered to him as private instructor to
Count d’Héricy he employed for his epoch-making investigations
upon the structure of molluscs. In 1794, upon the persuasion
largely of the man who afterwards became his great opponent,
Geoffroy St. Hilaire, he moved to Paris, where he was made at
first Professor of Natural History in the central school and in the
College of France, later Professor of Comparative Anatomy in the
Jardin des Plantes. As a sign of the great regard in which Cuvier
was held, it should be noticed that he was repeatedly intrusted with
high educational positions and was made a French peer. As such
he died in 1832.
Type Theory.—Cuvier’s investigations, apart from the mol-
luses, extended to the celenterates, arthropods, and vertebrates,
living and fossil. Tle collected his extensive observations into his
two chief works ‘‘ Le régne animal distribué d’aprés son organiza-
tion’ and ‘* Lecons d’anatomie comparée.” Of quite epoch-making
importance was his little pamphlet ‘Sur un rapprochement a ¢tablir
entre les différentes classes des animaux,” in which he founded his
celebrated type theory, and which in 1812 introduced a complete
reform of classification. The Cuvicrian division, which has become
the starting-point for all later classifications, differed, broadly
speaking, from all the earlier systems in this, that the classes of
mammals, birds, reptiles, and fishes were brought together into a
higher grade under the name, introduced by Lamarck, of ‘ verte-
brate animals’; that further the so-called ‘invertebrate animals’
were divided into three similar grades, each equal to that of the
16 GENERAL PRINCIPLES OF ZOOLOGY.
vertebrate animals, viz., Mollusca, Articulata, and Radiata. Cuvier
called these grades standing above the classes, provinces or chief
branches (embranchements), for which later the name Types was
introduced by Blainville. But still more important are the differ-
ences which appear in the structural basis of the system. Instead
of, like the earlier systematists, using a few external character-
istics for the division, Cuvier built upon the totality of internal
organization, as expressed in the relative positions of the most
important organs, especially the position of the nervous system,
as determining the arrangement of the other organs. <‘‘The
type is the relative position of parts” (von Baer). Thus for the
first time comparative anatomy was employed in the formation of
a natural system of animals.
Lastly the type theory established an entirely new conception
of the arrangement of animals. Cuvier found prevalent the theory
that all animals formed a single connected series ascending from
the lowest infusorian to man; within this series the position of
each animal was definitely determined by the degree of its organi-
zation. On the other hand Cuvier taught that the animal kingdom
consisted of several co-ordinated unities, the types, which exist
quite independently side by side, within which again there are
higher and lower forms. The position of an animal is determined
by two factors: first, by its conformity to a type, by the structural
plan which it represents; second, by its degree of organization, by
the stage to which it attains within its type.
Comparative Embryology.—Evolution vs, Epigenesis.—The
same results which Cuvier reached by the way of comparative
anatomy were attained two decades later by C. E. von Baer by the
aid of embryology. Embryology is the youngest branch of
zoology. What Aristotle really knew, what was written by
Fabricius ab Aquapendente and Malpighi upon the embryology of
the chick, did not rise above the range of aphorisms, and were not
of sufficient value to make a science. The difficulties of observa-
tion, due to the delicacy and the minuteness of the developmental
stages, were lessened by the invention of the microscope and
microscopical technique. Further, the prevailing philosophical
conceptions placed hindrances in the way; there was no belief in
Embryology in the present sense of the word; each organism was
thought to be laid down from the first complete in all its parts,
and only needed growth. to unfold its organs (evolutio*); eithe the
* This original meaning of ‘evolution’ is different from that prevailing
at present.
HISTORY OF ZOOLOGY. 17
spermatozoon must be the young creature which found favorable
conditions for growth in the store of food in the egg, or the egg
represents the individual and was stimulated to the ‘evolutio’ by
the spermatozoon. This theory led to the doctrine of inclusion,
which taught that in the ovary of Eve were included the germs of
all human beings who have lived or ever will live.
Caspar Friedrich Wolff combated this idea with his ‘‘ Theoria
generationis” (1759); he sought to prove by observation that the
hen’s egg at the beginning is without any organization, and that
gradually the various organs appear in it. In the embryo there is
a new formation of all parts, an Hpigenesis. This first assault
upon the evolutionist school was entirely without result, chiefly
because Albrecht von Haller, the most celebrated physiologist of
the eighteenth century, by his influence suppressed the idea of
epigenesis. Wolff was not able to establish himself in scientific
circles in Germany, and was obliged to emigrate to Russia. Only
after his death did his writings find, through Oken and Meckel,
proper recognition.
Von Baer.—Thus it remained for Carl Ernst von Baer in his
classic work, ‘‘ Die Entwicklung des Hihnchens, Beobachtung
und Reflexion” (1832), to establish embryology as an independent
study. Baer confirmed Wolff’s doctrine of the appearance of
layerlike Anlagen, from which the organs arose; and on account
of the accuracy with which he proved this he is considered the
founder of the germ-layer theory. Further, he came to the con-
clusion that each type had not only its peculiar structural plan,
but also its peculiar course of development; that for vertebrates
an evolutio bigemina was characteristic, for the articulates the
evolutio gemina, for the molluscs the evolutio contorta, and for
the radiates the evolutio radiata. Here we meet for the first time
the idea that for the correct solution of the questions of relation-
ship of animals, and therefore a basis for a natural classification,
comparative embryology is indispensable; an idea which in recent
years has proved exceedingly fruitful.
Cell Theory.—Of fundamental importance for the further
growth of comparative anatomy and embryology was the proof
that all organisms, as well as their embryonic forms, were com-
posed of the same elements, the cells. This knowledge is the
quintessence of the cell theory, which during the third decade of
the last century was advanced by Schleiden and Schwann, and
which two decades later was completely remodelled by the proto-
plasm theory of Max Schultze. In the cell theory a simple prin-
18 GHNERAL PRINCIPLES OF ZOOLOGY.
ciple of organization was found for all living creatures, for highly
and for lowly organized plants and animals, and the wide realm of
histology was laid open for scientific treatment.
REFORM OF THE SYSTEM.
Foundation of Modern Zoology.—With the establishment of
comparative anatomy and embryology and the application of these
to classification, and with the development of the cell theory and
of histology, which is connected with it, we may say that the
foundation of zoology was laid. Wonderful advances were made
in vertebrate anatomy by the classic researches of Owen, Johannes
Miller, Rathke, Gegenbaur, and others; our conceptions of organ-
ization have been completely altered by the work of Dujardin,
Max Schultze, Haeckel, and others, who have proved the unicellu-
larity of the lowest animals. The germ-layer theory was further
elaborated by Remak and Kélliker; and applied to the invertebrate
animals by Kowalewsky, Haeckel, and Huxley. It is beyond the
limits of this brief historical summary to go into what has been
accomplished in regard to the other branches of the animal king-
dom; it must here be sufficient to mention the most important
changes which the Cuvierian system has undergone under the
influence of increasing knowledge.
The Division of the Radiata.—Of the four types of Cuvier the
branch Radiata was undoubtedly the one of whose representatives
he had the least knowledge; it was therefore the least natural,
since it comprised, besides the radially symmetrical celenterates
and echinoderms, other forms, which, like the worms, were
bilaterally symmetrical, or, like many infusorians, were asym-
metrical. Thus it came about that most reforms have here found
their point of attack.
C. Th. von Siebold was the originator of the first important
reform. He limited the type Radiata, or, as he termed them, the
Zoophytes, to those animals with radially symmetrical structure
(Echinoderms and the Plant-animals); separating all the others,
he formed of the unicellular organisms the branch of * primitive
animals’ or Protozoa; the higher organized animals he grouped
together as worms or Vermes; at the same time he transferred a
part of the Articulata, the annelids, to the worm group, and pro-
posed for the other articulates, crabs, millipedes, spiders, and
insects, the term Arthropoda.
Leuckart, about the same time (1848), divided the branch
HISTORY OF ZOOLOGY. 19
Radiata into two branches differing greatly in structure. The
lower forms, in which no special body-cavity is present, the
interior of the body consisting only of a system of cavities serving
for digestion, the alimentary canal, he called the Cclentera
(essentially the Zoophyta of the older zoologists); to the rest, in
which the alimentary canal and the body-cavity occur as two
separate cavities, he gave the name Echinoderma.
The Present System.—Thus there resulted seven classes:
Protozoa, Celentera, Echinoderma, Vermes, Arthropoda, Mol-
lusca, and Vertebrata. Still this arrangement does not meet the
requirements of a natural system and hence is ‘more or less unsat-
isfactory. Some zoologists are returning to the Cuvierian classifi-
cation to the extent of uniting the segmented worms with the
arthropods in a group Articulata. Upon the ground of important
anatomical and embryological characters the Brachiopoda, the
Bryozoa, and the Tunicata have been separated from the Mollusca;
they form the subject of diverse opinions. The relationships of
the first two groups have not yet been settled: of the Tunicata we
know indeed that they are related to the Vertebrata, but the
differences are such that they cannot be included in that group.
The only way out of the difficulty is to unite vertebrates, tunicates,
and some other forms in a larger division, Chordata. The Vermes,
too, must be divided, as will appear in the second part of this
volume.
HISTORY OF THE THEORY OF EVOLUTION.
Importance of the Subject.—Before closing the historical intro-
duction we must consider the historical development of a question
whose importance might, on a superficial examination, be under-
rated, but which from a small beginning has grown into a problem —
completely dominating zoological research, and has oceupied not
only zoologists, but all interested in science generally. This is the
question of the logical value of the systematic conceptions species,
genus, family, etc.
The Nature of Species.—In nature we find only separate
animals: how comes it that we classify them into larger and smaller
groups? Are the single species, genera, and the other divisions
which the systematist distinguishes, fixed quantities, as it were
fundamental conceptions of nature, or a Creator’s thoughts, which
find expression in the single forms? Or are they abstractions
which man has brought into nature for the purpose of making it
20 GENERAL PRINCIPLES OF ZOOLOGY.
comprehensible to his mental capabilities? Are the specific and
generic names only expressions which have become necessary, from
the nature of our mental capacity, for the gradation of relation-
ship in nature, which in and for themselves are not immutable,
and hence can undergo a gradual change? Practically speaking,
the problem reads: are species constant or changeable? What is
true for species must necessarily be true for all other categories of
the system, all of which in the ultimate analysis rest upon the
conception of species.
Ray’s Conception of Species.—One of the first to consider the
conception of species was Linneus’s predecessor, the Englishman
John Ray. In the attempt to define what should be understood
as a species he encountered difficulties. In practice, animals which
differ little in structure and appearance from one another are
ascribed to the same species; this practical procedure cannot be
carried out theoretically; for there are males and females within
the same species which differ more from one another than do the
representatives of different species. Thus John Ray reached the
genetic definition when he said: for plants there is no more
certain criterion of specific unity than their origin from the seeds
of specifically or individually like plants; that is to say, generalized
for all organisms: to one and the same species belong individuals
which spring from similar ancestors.
The ‘Cataclysm Theory.’—With Ray’s definition an entirely
uncontrollable element was brought into the conception of species,
since no systematist usually knew anything, nor indeed could he
know anything, as to whether the representatives of the species
placed before him sprang from similar parents. It was therefore
only natural that the conception of species put on a religious garb,
since by resting upon theological ideas it found a firmer support.
Linneus said: ‘* Tot sunt species quot ab initio creavit infinitum
Ens”; with this he built up a conception of species upon the
tradition of the Mosaic history of creation, a procedure quite
unjustified upon grounds of natural science, since it drew one of
its fundamental ideas from transcendental conceptions, not from
the experience of natural science. Linneus’s definition showed
itself untenable, as soon as paleontology began to make accessible
a vast quantity of extinct animals deposited as fossils. With an
odd fancy, the fossils, being inconvenient for study, were for a
long time regarded as outside the pale of scientific research. They
might be sports of nature, it was said, or remains of the Flood, or
of the influence of the stars upon the earth, or products of an aura
HISTORY OF ZOOLOGY. 21
semtnalis, a fertilizing breath, which, if it fell upon organic bodies,
led to the formation of animals and plants, but if it strayed upon
inorganic materials gave rise to fossils. The foundation of
scientific paleontology by Cuvier put an end to such empty specu-
lations. Cuvier proved beyond a doubt that these fossils were the
remains of animals of a previous time. Just as the formation of
the earth’s crust by successive overlying layers made possible the
recognition of different periods in the earth’s history, so paleon-
tology taught how to recognize also the different periods in the
vegetable and animal world of life on our globe. Each geological
age was characterized by a special world of animals quite peculiar
to it; and these animal worlds differed the more from the present,
the older the period of the earth to which they belonged. All
these generalizations led Cuvier to his cataclysm theory, that a
great revolution brought each period of the earth’s history to an
end, destroying all life, and that upon the newly formed virgin
earth a new organic world of immutable species sprang up.
Objections to the Cataclysm Theory.—By the supposition of
numerous acts of creation the Linnean conception of species
seemed to be rescued, though, to be sure, by summoning to its aid
hypotheses which had neither foundation in science nor justifica-
tion in theology. The logical results of Cuvier’s cataclysm theory
were conceptions of a Creator who built up an animal world only
for the purpose of destroying it after a time as a troublesome toy;
it has therefore at no time found warm supporters, at least among
geologists, for whom it was intended. Of the prominent zoologists
there is only to be mentioned Louis Agassiz, who till the end of
his life remained faithful to this theory.
Under these conditions it is readily understood how thinking
naturalists, who felt the necessity of explaining the character of
organic nature simply and by a natural law capable of general
application, began to doubt the fixity of species, and were led to
the theory of change of form, the Theory of Descent, or Evolution.
Darwin’s Predecessors.— Even in Cuvier’s time there prevailed
a strong current in favor of this theory. It found expression in
England in the writings of Erasmus Darwin (grandfather of the
renowned Charles Darwin); in Germany in the works of Goethe,
Oken, and the disciples of the ‘natural philosophical’ school; in
France the genealogical theory was developed particularly by
Buffon, Geoffroy St. Hilaire, and Lamarck. Its completest ex-
pression was found in Lamarck’s ‘* Philosophie zoologique ” (1809);
its arguments will be considered in the following paragraphs.
22 GENERAL PRINCIPLES OF ZOOLOGY.
Lamarck (Jean Baptiste de Monet, Chevalier de Lamarck,
born in Picardy, 1744, died, Professor at the Jardin des Plantes,
1829) taught that on the earth at first organisms of the simplest
structure arose in the natural way through spontaneous generation
from non-living matter. From these simplest living creatures have
developed, by gradual changes in the course of an immeasurably
vast space of time, the present species of plants and animals,
without any break in the continuity of life upon our globe; the
terminal point of this series is man; the other animals are the
descendants of those forms from which man has developed.
Lamarck, in accordance with the then prevailing conceptions,
regarded the animal kingdom as a single series grading from the
lowest primitive animal upto man. Among the causes which may
influence the change and perfecting of organisms, Lamarck
emphasized particularly wse and disuse; the giraffe has obtained a
long neck because by a special condition of life he was compelled
to stretch, in order to browse the leaves on high trees; conversely,
the eyes of animals which live in the dark have degenerated from
lack of use into functionless structures. The direct influence of
the external world must be unimportant; the changes in the sur-
roundings (Geoffroy St. Hilaire’s le monde ambient) must for the
most part act indirectly upon animals by altering the conditions
for the use of organs.
Evolution vs. Creation.—Lamarck’s ingenious work remained
almost unnoticed by his contemporaries. On the other hand there
arose a violent controversy between the defenders and the
opponents of the evolution theory when [1830] Geoffroy St. Hilaire
in a debate in the Academy at Paris defended against Cuvier the
thesis of a near relationship of the vertebrates and the insects, and
set up the proposition that the latter were ‘* vertebrates running
on their backs.” The conflict ended in the complete overthrow ot
the theory of evolution; the defeat was so complete that the
problem vanished for a long time from scientific discussion, and
the theory of the fixity of species again became dominant. This
error was occasioned by many causes. Above all, the theory of
Geoffroy St. Hilaire and Lamarck was rather a clever conception
than founded on abundant facts; besides, it had in it as a funda-
mental error the doctrine of the serial arrangement of the animal
world. Opposed to this stood Cuvier’s great authority and his
extensive knowledge, the latter making it easy for him to show
that the animal kingdom was made up of separate co-ordinated
groups, the types.
HISTORY OF ZOOLOGY. 23
Lyell.—In the same year in which Cuvier obtained his victory
over Geoffroy St. Hilaire, his theory of the succession of numerous
animal worlds upon the globe received its first destructive blow.
Cuvier’s cataclysm theory had two sides, a geological anda
biological. Cuvier denied the continuity of the various terrestrial
periods, as well as the continuity of the fauna and flora belonging
to them. In 1830-32 appeared the ‘ Principles of Geology” by
Lyell, an epoch-making work, which, in the realm of geology,
completely set aside the cataclysm theory. Lyell proved that the
supposition of violent revolutions on the earth was not necessary
in order to explain the changes of the earth’s surface and the
superposition of its strata; that rather the constantly acting
forces, elevations and depressions, the erosive action of water, be
it as ebb and flow of the tide, as rain, snow, or ice, or as the flow
of rivers and brooks rushing as torrents towards the sea, are suffi-
cient to furnish a complete explanation. Very gradually in the
course of a vast space of time the earth’s surface has changed, and
passed from one period into the next, and still at the present day
the constant process of change is going on. The continuity in the
geological history of the earth, here postulated for the first time,
has since then become one of the fundamental axioms of Geology;
on the other hand the discontinuity of living creatures, although
the geological support of this was frail, was for a long time
regarded as correct.
Darwin.—lIt is the great merit of Charles Darwin that he took
up the theory of descent anew after it had rested a decade, and
later brought it into general recognition. With this began the
most important period in the history of zoology, a period in which
the science not only made such an advance as never before, but
also began to obtain a permanent influence upon the general views
of men.
Charles Darwin was born at Shrewsbury, Eng., in 1800. After
studying at the universities of Edinburgh and Cambridge, he joined
as naturalist the English war-ship ‘‘ Beagle.” In its voyage from
1831 to 36 around the globe, Darwin recognized the peculiar
character of island faunas, particularly of the Galapagos Islands,
and the remarkable geological succession of edentates in South
America; these facts formed for him the germ of his epoch-making
theory. Further results of this journey were his beautiful mono-
graph on the Cirripedia, and the classic investigation of coral-reefs.
After his return to England Darwin lived, entirely devoted to
scientific work, chiefly in the hamlet of Down, county Kent, up
24 GHNEHRAL PRINCIPLES OF ZOOLOGY.
to the time of his death in 1882. He was incessantly busy in
developing his conception of the origin of species, and in collect-
ing for this a constantly increasing array of facts. The first
written notes, the fundamental ideas of which he communicated
to friends, particularly the geologist Lyell and the botanist
Hooker, were made in 1844, but the author was not persuaded to
give them publicity. Not until 1858 did Darwin decide to make
his first contribution to science. In this year he received an essay
sent by the traveller Wallace, which in its most important points
coincided with his own views. At the same time with Wallace’s
manuscript an abstract of Darwin’s theory was published. In the
next year (1859) appeared the most important of his writings,
“On the Origin of Species by means of Natural Selection,” and
in rapid succession a splendid series of works, the fruit of many
years of preparatory labors. For the history of the theory the
most important of these are: (1) ‘‘ Upon the Variation of Plants
and Animals under Domestication,” two volumes, which chiefly
contain a collection of material for proofs; (2) on ‘‘The Descent
of Man,” a work which gives the application of the theory to man.
No scientific work of this century has attracted so much atten-
tion in the zoological, we may even say in the whole educated
world, as ‘“‘The Origin of Species.” It was generally received as
something entirely new, so completely had the scientific tradition
been lost. In professional circles it was stoutly combated by one
faction, with another it found well-wishing but hesitating accept-
ance. Only a few men placed themselves from the beginning in
a decided manner on the side of the great British investigator.
There was a lively scientific battle, which ended in a brilliant vic-
tory for the theory of evolution. At the present time all our
scientific thoughts are so permeated with the idea of evolution that
we can scarcely speak of any considerable opposition to it.
Post-Darwinian Writers.—Among the men who have most
influenced this rapid advance is to be mentioned, besides A. R.
Wallace, the co-founder of Darwinism, above all Ernst Haeckel,
who in his ‘“‘ General Morphology” and his ‘‘ Natural History of
Creation” has done much towards the extension of the theory.
Among other energetic defenders of the theory in Germany should
be mentioned Fritz Miller, Carl Vogt, Weismann, Moritz Wagner,
and Nageli, even if they have taken special standpoints in refer-
ence to the causes conditioning the changes of form. Among the
Hinglish naturalists are to be named particularly Huxley, Hooker,
and Lyell. In America Gray, Cope, and IHyatt were early sup-
HISTORY OF ZOOLOGY. 25
porters. Darwinism was long in obtaining an entrance into
France.
DARWIN'S THEORY OF THE ORIGIN OF SPECIES.
Before Darwin wrote the idea of fixity of species prevailed
among systematists. It was recognized that all the individuals of
a species are not alike, and that more or less pronounced variability
occurs, so that it was possible to distinguish races and varieties
within the species, but it was beleved that the variations never
transcended specific bounds.
The Problem Stated.—Darwin begins with a criticism of the
term species. Is the conception of species on the one side and
that of race and variety on the other something entirely different ?
Are there special criteria for determining beyond the possibility
of a doubt whether in a definite case we have to do with a variety
of a species or with a different species? or do the conceptions in
nature pass into one another? Are species varieties which have
become constant, and precisely in the same manner are varieties
species in the process of formation ?
Morphological Characters.— A. Distinction between Species and
Variety. —For the settlement of this fundamental question morpho-
logical and physiological characters can be considered. In the
practice of the systematists usually the morphological characters
prevail exclusively; for that reason they will be here considered
first. If, among a great number of forms similar to one another,
two groups can be adduced which differ considerably from one
another, if the difference between them be obliterated by no inter-
mediate forms, and if in several successive generations they
remain constant, then the systematist speaks of a ‘ good species’ ;
on the other hand he speaks of varieties of the same species when
the differences are slight and inconstant, and when they lose their
importance through the existence of intermediate forms. A
definite application of this rule discloses great incongruities, many
animal and vegetable groups being regarded by one set of systema-
tists as good species, by another only as ‘sports,’ ie., as varieties
of the same species. The differences between the ‘sports’ of our
domestic animals are in many instances so considerable that
formerly they were regarded not only as sufficient for the founda-
tion of good species, but even of genera and families. In the
fantail pigeon the number of tail-feathers, formerly only 12-14,
has increased to 30-42 (fig. 1c); among the other races of pigeons
26 GENERAL PRINCIPLES OF ZOOLOGY.
enormous variations are found in the size of the beak and feet in
comparison with the rest of the body (figs. 1a, 1p); even the
skeleton itself varticipates in this variation, as is shown by the fact
Fig. 1A.—English carrier-pigeon. (After Darwin.)
Fic. 1B.—English tumbler-pigeon. (After Darwin.)
that the total number of vertebre varies from 38 (in the carrier-
pigeon) to 43 (in the pouter), the number of sacral vertebra from
14 to 11.
B. Variation within the Species.—Now in respect to the occur-
rence of transitional forms and the constancy of differences, there
is within one and the same ‘ good species’ the greatest conceivable
difference. In many very variable species the extremes are united
HISTORY OF ZOOLOGY. 27
by many transitions; in other cases sharply circumscribed groups
of forms, or races, can be distinguished within the same species.
In the race, the peculiar characteristics are inherited from genera-
Fia. 1c.—English fantail pigeon. (After Darwin.)
tion to generation with the same constancy as in good species.
This is shown in the human races, and in many pure, cultivated
races of domesticated animals.
Physiological Characters.—A. Crossing of Species and Varie-
ties.—A critical examination leads to the conclusion that Mor-
phology is indeed useful for grouping animals into species and
varieties, but that it leaves us completely in the lurch when it is
called upon to show the distinctions between what should be called
a species and what a variety. Therefore there remains open to
the systematist only one resource, i.e., to summon Physiology
to his aid. This has been done, and it has disclosed considerable
distinctions in reproduction. We should expect a prior/ that the
individuals of different species would not reproduce with each
other; on the other hand under normal conditions the individuals
of one and the same species, even though they are of different
varieties or races, should be entirely fertile. One must beware of
arguing in a circle in proof of these two propositions; it would be
an argument in a circle if an experimenter should regard two
animals as representatives of one species only because they proved
to be fertile together, while under their former relations he
assigned them to different species. Rather the question for him
must read: does physiological experiment lead to the same
28 GENERAL PRINCIPLES OF ZOOLOGY.
systematic distinctions as does the common systematic experience,
viz., the depreciation of constancy and the divergence of distin-
euishing characters ?
iB. Lhe Intercrossing of Species. —This field is as yet far from
being sufficiently investigated experimentally; yet some general
propositions can be set up: (1) that not a few so-called * good
species’ can be crossed with one another; (2) that in general the
difficulty of crossing increases, the more distant the systematic
relationship of the species used; (3) that these difficulties are by
no means directly proportional to the systematic divergence of the
species. The most favorable material for research is furnished by
those animals in which artificial fertilization can be carried out,
i.e., of which one can take the eggs and spermatozoa and mix
them independently of the will of the animals. Thus hybrids
have been obtained from species which belong to quite different
genera, while very often nearly-related species will not cross.
Among fishes we know hybrids of Adramis brama and Blicca
bjirkna, of Trutta salar (salmon) and Trutta fario (trout); among
sea-urchins the spermatozoa of Strongylocentrotus lividus fertilize
with great readiness the eggs of Lchinus microtuberculatus, but
only rarely the eggs of Spherechinus granularis, which is nearer
in the system. It also happens that crossing in one direction
(male of A and female of B) is easily accomplished, but in the
other direction (male of B and female of A) it completely fails; as,
for example, the sperm of Strongylocentrotus lividus fertilizes well
the eggs of Hchinus microtuberculatus, but, conversely, the sperm
of £. microtuberculatus does not fertilize the eggs of S. lividus.
Even better known is the fact that salmon eggs are fertilized by
trout sperm but not trout eggs by salmon sperm. Eggs have been
fertilized by sperm belonging to different families, orders, and
possibly classes. Eggs of Plewronectes platessa and Labrus rupestris
by sperm of the cod (Gadus morrhua), frogs’ eggs (Rana arvalis)
by sperm of two species of Triton, eggs of a starfish (Asferias
Forbesi) by milt from a sea-urchin, Arbacia pustulosa (??). In
these extreme cases, it is true, the hybrids die during or at the
close of segmentation, before the embryo is outlined.
In the case of animals where copulation is necessary the difti-
culties of experimentation increase, since here often between males
and females of different species there exists an aversion which
prevents any union of the sexes. Yet in this case we know crosses
of different species; among the vertebrates crossing takes place,
e.g., between the horse and the ass; our domestic cattle and the
HISTORY OF ZOOLOGY. 20
zebu; ibex (or wild buck) and she-goat; sheep and goats; dog and
jackal; dog and wolf; hare and rabbit (Lepus darwini); American
bison and domestic cattle; etc.; among birds, between different
species of finches and of grouse; mallard (Anas boschas) and the
pintail duck (Dajfila acuta); the European goose and the Chinese
goose (Anser ferus and A. cygnoides). Among the insects,
especially the Lepidoptera, the cases are many, but the resulting
eggs produce larve of slight vital force only in the case of Actias
luna and A. tsabelle.
C. Fertility of Hybrids and Mongrels.—Since many hybrids,
as the mule, have been known for thousands of years, the criterion
is, as it were, pushed back one stage; if the infertility in cases of
crosses in many species is not immediately noticeable, yet it may
be apparent in the products of the cross. While the products of
the crossing of varieties, the ‘mongrels,’ always have a normal,
often an increased, fertility, the products of the crossing of
species, the hybrids, should always be sterile. But even this is
a rule, not a law. The mule (which only very rarely reproduces)
and many other hybrids are indeed sterile, but there are not a few
exceptions, although the number of experiments in reference to
this point is very small. Hybrids of hares and rabbits have con-
tinued fruitful for generations; the same is true of hybrids
obtained from the wild buck and the domesticated she-goat; from
Anser cygnoides and A. domesticus; from Salmo salvelinus and
S. fontinalis; Cyprinus carpio and Carassius vulgaris; Bombyx
cynthia and B. arrindia.
D. Inbreeding.—Even the second of the above statements, that
individuals of a species, provided they are sound, always reproduce
with one another, needs limitation. Breeders of animals have long
known the disastrous consequences of inbreeding—that the repro-
ductive power is reduced even to sterility if, for breeding,
descendants of a single pair be continually chosen. Darwin has
collected not a few cases where undoubted members of the same
species have been completely sterile with one another; as certain
forms of primrose and other di- and tri-morphic species. Exam-
ples of the sterility of mongrels are known only in botany (certain
varieties of maize and mullein).
Conditions Governing Fertility in Sexual Reproduction.—
When we look over these facts it would seem as if continued
fertility in sexual reproduction were guaranteed by a not too con-
siderable difference in the sexual products. Too great similarities,
as these exist in inbreeding, and too great differences, as in the
30 GENERAL PRINCIPLES OF ZOOLOGY.
hybridization of different species, are injurious and are abhorred
by Nature. Sexual reproduction possesses an optimum; if this be
departed from in either direction, diminution gradually follows.
But for that reason it has already been said that here gradual and
not primary differences exist, and therefore this character cannot
be employed asa primary distinction between species and varieties.
Difficulties in Classification.—The final result of all this dis-
cussion may be summed up as follows: up to the present time,
neither by physiological nor by morphological evidence has there
been successfully fixed in a clear and generally applicable way a
criterion which can guide the systematist in deciding whether
certain series of forms are to be regarded as good species or as
varieties of a species. Zoologists are guided rather in practice by
a certain tact for classification, which, however, in difficult cases
leaves them in the lurch, and thus the opinions of various investi-
gators vary.
Change of Varieties into Species.—The conditions above dis-
cussed find their natural explanation in the assumption that sharp
distinctions between species and variety do not exist; that species
are varieties which have become constant, and varieties are incipient
species. The meaning of the above can be made clear by explana-
tion of a concrete case. Individuals of a species begin to vary,
i.e., compared with one another they attain a greater or less
difference in character. So Jong as the extreme differences are
bridged by transitional forms we speak of varieties of a species;
if, on the other hand, the intermediate transitions have died out.
and the differences have in the course of a long space of time
become fixed, and so very much intensified that a sexual union of
the extreme forms results either in complete sterility or at least in
a marked tendency towards sterility, then we speak of different
species.
Species may be Related to each other in Unequal Degrees.—
In favor of this view, that varieties will in longer time become
species, is the great agreement which in the large majority of
cases exists between the two. In genera which comprise a remark-
able number of species, the species usually show also many
varieties; the species are then usually grouped in sub-genera, i.e.,
they are related to each other in unequal degrees, since they form
small groups arranged around certain species. In regard to the
varieties also the case is similar. In such genera the formation of
species is in active progress; but each species formation presup-
poses a high degree of variability.
HISTORY OF ZOOLOGY. 31
Phylogeny.—It is now clear that what has here been worked
out in the case of the species must also apply to the other cate-
gories of the system. Just as by divergent development varieties
become species, so must species by continued divergence become
so far removed from one another that we distinguish them as
genera. It will be only a question of time when these differences
will become still greater, and cause the establishment of orders,
classes, and branches, just as the tender shoots of the young
plantlet become in the strong tree the chief branches from which
spring lateral branches and twigs. If we pursue this train of
thought to its ultimate consequences, we reach the conception that
all the animals and plants living at present have arisen by means
of variation from a few primitive organisms. Inasmuch as at least
many thousands of years are required for the formation of several
new species through the variability of one, there must then have
been necessary for this historical development of the animal and
vegetable kingdoms a space of time greater than our mental
capacity can grasp. Since for the idea of the individual develop-
ment (embryology) of an animal the term Ontogeny has been
chosen, it has also proved convenient to apply to the historical
development of animals—though this has not been observed, but
only inferred—the term History of the Race or Phylogeny.
Spontaneous Generation.—If we attempt to derive all living
animals from a common primitive form, we are compelled to
assume that this was of extremely simple organization, that it was
unicellular; for the simpler, the less specialized, the organization,
so much the greater is its capacity for variation. Only from
simple organisms can the lower unicellular organisms, the
Protozoa, be derived. Finally, for the simple organisms alone
can we conceive a natural origin. Since there was undoubtedly a
time upon our earth when temperatures prevailed which made life
impossible, life must at some time have arisen, either through an
act of creation or in a natural way through spontaneous generation.
If, in agreement with the spirit of natural science, we invoke for
the explanation of natural facts only the forces of nature, we are
driven to the hypothesis of spontaneous generation, namely, that
by a peculiar combination of materials without life, the compli-
cated mechanism which we call ‘life’ has arisen. This hypothesis
also supposes that the first organisms possessed the simplest con-
ceivable structure.
Variability not proven to be a Universal Principle.—Starting
from a basis of facts, by generalization we reach a simple concep-
32 GHNHRAL PRINCIPLES OF ZOOLOGY.
tion of the origin of the animal kingdom, but we have in equal
measure departed from the results of direct observation. Observa-
tions only show us that species are capable of changes and can
from themselves produce new species. That this capacity for
variation is a universal principle, a principle which explains to us
the origin of the animal world, needs further demonstration.
Proofs of Phylogeny.—The rise of the existing animal world
is a process which has taken place in the thousands of years long
past, but is no longer accessible for direct observation, and there-
fore it can never be proved in the sense that we explain the indi-
vidual development of an organism. In regard to the conception
of the simple evolution of animals we can merely prove the
probability; yet it is shown that all our observations of accessible
facts not only agree with this conception, but find in it their only
simple explanation. Such facts are furnished to us by the classi-
fication of animals, paleontology, geographical distribution, com-
parative anatomy, and comparative embryology.
(1) Proofs from Classification.—For a long time it has been
recognized, and in recent times finds ever-increasing confirmation,
that if we wish to express graphically the relationships of animals,
theiz classes, orders, genera, and species, simple co-ordination and
subordination are not sufficient, but one must select a treelike
arrangement, in which the principal divisions, more closely or dis-
tantly related to one another,—the branches, phyla, or types,—
represent the main limbs, while the smaller branches and twigs
correspond to the several classes, orders, etc. This is, in fact,
the arrangement to which the theory of evolution, as seen above,
necessarily leads.
(2) Paleontological Demonstration approaches nearest to what
one might call direct proof; for paleontology gives us the only
traces of existence which the predecessors of the present animal
world have left. Even here a hypothetical element has crept into
the demonstration. We can only observe that various grades of
forms of an animal group are found in successive strata: if we
unite these into a developmental series, and regard the younger
as derived from the older by variation, we depart, strictly speak-
ing, from the basis of fact. But the value of paleontological
evidence is weakened much more by its extreme incompleteness.
In fossils only the hard parts are generally preserved; the soft
parts, on the other hand, which alone are present, or at least make
up the most important part of many animals, are almost always
lost. Only rarely are the soft parts (muscle of fishes, ink-bag of
HISTORY OF ZOOLOGY. 33
cephalopods, outlines of medusew) preserved in the rocks. Even
the hard parts remain connected only under exceptionally favor-
able conditions. If further we take into consideration the fact
that these treasures are buried in the bosom of the earth, and are
usually obtained only by accident, in quarrying and road-build-
ing, and besides only extremely seldom excavated with scientific
care, it becomes sufficiently clear how little is to be expected from
the past and indeed future material of paleontology.
Examples of Paleontological Proof.—Yet paleontology has
already furnished many important proofs of the theory of descent.
Fria. 2.—Archeopteryx lithographica. (After Zittel.) cl, clavicle; co, coracoid; h,
humerus; r, radius; u, ulna; ec, carpus; I-IV, digits; sc, scapula.
It has shown that the lower forms appeared first, and the more
highly organized later, Among animals in general the latest to
appear were the vertebrates, and of these the mammals; among
the mammals man. For smaller groups genealogical material has
3 GENERAL PRINCIPLES OF ZOOLOGY.
fortunately been found. Transitional forms connect the single-
toed horse of the present with the four-toed Mohippos of the
eocene; for all the hoofed animals a common starting-point or
ancestral form has been found in the Condylarthra. Transitional
forms have also been found between the greater divisions, as, ¢.g.,
between reptiles and birds, the remarkable toothed birds, und the
Archeopteryx (fig. 2), a bird with a long, feathered, lizard-like
tail.
(3) Morphological Proofs. —When we employ comparative
anatomy and embryology in support of evolution, we find that the
two studies have so many points in common that they can best be
treated together.
Cuvier and von Baer taught that the separate types of the
animal kingdom are units, each with a special structure and plan
of development peculiar to it; farther, that there are no similari-
ties in structure and in the development forming a bridge from
type to type. The first of these two propositions is still regarded
as correct, but the second, which alone is important for the theory
of evolution, has become quite untenable. All animals have a
common organic basis in the cell and are thereby brought close
to one another; all multicellular animals agree in the principal
points during the first stages of their development, during the
fertilization, cleavage of the egg, and the formation of the first
two germ-layers, and vary from one another only in such differ-
ences as may occur within one and the same type. Also the
peculharities which distinguish each type in structure and in the
mode of development are not without intermediate phases.
Hspecially from the branch of the worms there lead cff transitional
forms to the other branches: Balanoglossus to both echinoderms
and chordates, the annelids and Peripatus to the arthropods, the
tunicates and Amphiowus to the vertebrates. In some representa-
tives of each type the structure and the mode of development are
simpler, thereby approaching to the conditions which obtain in
the other types. The existence of such transitional forms is one
of the most important proofs in favor of the theory of evolution,
and speaks against the assumption of a rigid unvarying type in
Cuvier’s sense.
Fundamental Law of Biogenesis.—A fact that weighs heavily
in the balance in favor of the theory of evolution is the fact that
the structure and mode of development of animals is ruled by a
law which at present can only be explained by the assumption of
common ancestry. Hach animal during its development passes
30
HISTORY OF ZOOLOGY.
through esseutially the stages which remain permanent in the case
of lower or at least more primitive animals of the same branch, as
43 2 1
|__
Fia. 3.—Human Embryo, about third or fourth week. 1-4, visceral arches with gill-
slits between them: 1, mandibular arch; 2, hyoid arch; 3 and 4, first and second
gill-arches. «a, eye; 1, nasal pit: h, cardiac region; ¢ [and e IJ, fore and hind
extremities; m, mesodermal somites.
Fic. 4.—Tadpoles of Rana temporaria. m, mouth; g, upper jaw; z, lower jaw; s,
sucking-disc ; kh, external gills; ik, region of the internal gills; m, nose; a, eye;
0, auditory vesicle ; h, cardiac region ; d, operculum.
the three following examples will show: (1) In the early stages of
development the human embryo (fig. 3) possesses remarkable
36 GHNERAL PRINCIPLES OF ZOOLOGY.
resemblances to the lowest vertebrates, the fishes. Like these it
hag gill-shts, the same arrangement of the heart and of the arterial
vessels, certain fundamental features in the development of the
skeleton, etc. (2) Frogs in their tadpole stage have an organiza-
tion similar to that which remains permanent in the case of
certain Amphibia, the Perennibranchiata (fig. 5), which stand
Fia. 5.—Siredon pisciformis (arva of Amblystoma tigrinum). (After Duméril and
Bibron.)
lower in the system; they have a swimming tail and tuft-like gills,
which are lacking in the adult frog. (3) There are certain para-
sitic crustacea, which live upon the gills of fishes, and seem not
Fra. 6.—Achtheres percarwumn. a, nauplius-, b, cyclops-stage ; ¢, adult female.
(After Claus.)
at all like their relatives. They are shapeless masses which were
formerly regarded as parasitic worms. Their systematic position
ras only determined by their embryology (fig. 6). Tlere it is
HISTORY OF ZOOLOGY. BT
eo
shown that they pass through a nauplius-stage (fig. 6a), charac-
teristic of most crustacea, and that they then assume the shape of
small crustacea (fig. 6, 4), like Cyclops (fig. 7, A), so widely dis-
Fia. 7.—Cyclops coronatus (A) and also the nauplius in lateral (B) and in ventral view
(GC). T, head; T/-V, the five thoracic, and behind these the five abdominal seg-
ments; F’, furca; 1, the first, 2, the second, antenne; 3, mandibles; 4, maxille;
5, maxillipeds ; (9, the first four pairs of biramous feet, while the rudimentary
fifth pair are hidden; au, eye; 0, upper lip; e, egg-sacs; d, gut; m, muscle.
tributed in fresh waters. Very often the males inake a halt in the
cyclops-stage while the female develops farther and assumes a
shapeless form, so that there arises a very remarkable sexual
dimorphism (fig. 8). All these examples, which can be multiplied
by hundreds, can be explained in the same way. The higher forms
35 GENERAL PRINCIPLES OF ZOOLOGY.
pass through the stage of organization of the lower, because
they spring from ancestors which were
Ss" 4. more or less similar to the latter. Man
\ in his embryological development passes
through the fish stage, the frog the per-
, ennibranchiate stage, the parasitic crus-
7 \ tacean first the nauplius- and then the
cyclops-stage, because their ancestors
were once fish-like, perennibranchiate-
like, nauplius- and cyclops-like. Here
is expressed a general phenomenon
which Haeckel has stated in a general
proposition under the name of ‘the
Fundamenta)] Law of Biogenesis.’ ‘* The
y-\ development history (ontogeny) of an
A \ individual animal briefly recapitulates
the history of the race (phylogeny);
ee “ fe i.e., the most important stages of organi-
(ater Bergeoel, 18: zation which its ancestors have passed
through appear again, even if somewhat modified, in the develop-
ment of individual animals.”
Examples of the Application of this Law.—TZhe Nervous
System.—This law applies as well to single organs as to entire
animals. The central nervous system of the lower animals
(echinoderms, celenterates, many worms) forms part of the skin;
in its first appearance it belongs to the surface of the body, because
it has to mediate the relations with the external world. In the
case of higher animals, e.g., the vertebrates, the brain and spinal
cord lie deeply embedded in the interior of the body; but in the
embryo it is laid down likewise as a part of the skin (medullary
plate) and which gradually through infolding and cutting off from
this comes to lie internally. One can demonstrate this change of
position by cross-sections through the dorsal region of embryos of
different ages of any vertebrate (fig. ‘)). ;
The Skeletal System.—The skeleton of vertebrates is a further
example. In the lowest chordates, amphiorus and the cyclostomes,
the vertebre are lacking, and in their place we find a cylindrical
cord of tissue, the chorda dorsalis (notochord). In the fishes and
Amphibia the notochord usually persists; but it is partially
reduced and constricted by the vertebre, which in the lower forms
consist of cartilage, and in the higher of bone or a combination of
bone and cartilage. Mature birds and mammals finally have a
HISTORY OF ZOOLOGY. 39
MIG. 9.—Cross-sections through the dorsal region of Triton embryos at different aoe
(from O. Hertwig). In [the medullary plate (anlage of spinal cord) Me eels
off from the skin (epidermis, ep) by the medullary folds (mf). In IT t he 1 et
lary plate by inrolling of the medullary folds is converted into a groove. ae
the groove has closed into a tube (7), the spinal cord, which has Separates rT mn
the rest of the ectoderm (epidermis). C, body cavity (c@lom);_ ch, His .
cp, cavity of primitive somite (myotome); dz, yolk-cells; ik, entoderm; (Ig, lume
of gut; mk}, mk*, somatic and splanchnic layers of mesoderm.
40 GENERAL PRINCIPLES OF ZOOLOGY.
completely ossified vertebral column; their embryos, on the other
hand, have in the early stages only the notochord (amphioxus
stage); later this notochord becomes constricted by the vertebra
(fish-aiphibian stage) and finally entirely replaced; the vertebral
column is in the beginning cartilaginous, only later becoming ossi-
fied. Comparative anatomy and embryology thus give the same
developmental stage of the axial skeleton: (1) notochord, (2)
notochord and vertebral column, the latter at first formed of
cartilage, then of bone.
We have here spoken of a parallelism between the facts of
comparative anatomy and those of embryology. But in reality we
should expect a threefold parallelism: for according to the theory
of evolution the systematic arrangement of animals is conditioned
by a third factor—the historical development of the animal world,
or phylogeny. The mile-stones of phylogenesis, the fossils, should
give the same progressive series in the successive geological strata
as the stages of forms found by comparative anatomy and embry-
ology. We actually know instances of such threefold parallelisms.
Comparative anatomy teaches that the lowest developed form of a
fish’s tail is the diphycercal (fig. 10, A); that from this the
heterocercal (/4), and from the heterocercal the homocercal form
of tail-fin (C, 2) can be derived. Embryologically the most highly
developed fishes are first diphycercal, later heterocercal, and
finally become homocercal. Last of all, paleontologically the
oldest fishes are diphycercal or heterocercal, and only later do
homocercal forms appear.
What has here been referred to is only a small fraction of the
weighty proofs which morphology offers in favor of evolution; it
can only serve to show how morphological observations can be
employed. For the reflecting naturalist the facts of morphology
are a single great inductive proof in favor of the theory of evolu-
tion.
Distribution of Animals.—From Animal Geography we learn
that the present distribution of animals is the product of past
hundreds and thousands of years. It will therefore be possible
from this to figure out many of the earlier conditions of things,
hy proceeding with the utmost caution and overcoming extreme
difficulties.
If we assume that from the beginning all animal species were
constituted as they now are, they would then have been placed by
the purposeful Creator in the regions best suited to their organiza-
tion; their distribution would therefore have been determined by
HISTORY OF ZOOLOGY. 41
favorable or unfavorable conditions of life prevailing in the various
regions, as the climate, food-supply, ete. If, on the other hand,
we assume that the animal species have arisen from one another
through variation, then there must have been, as an influence
determining the manner of distribution, besides the conditions
of existence, still a second factor, which we will call the geo-
Fia. 10.—Tail-fins of various fishes. (From Zittel.) A, Diphycercal fin of Polypterus
bichir. (Vertebral column and notochord divide the tail into symmetrical dorsal
and ventral portions.) B, Heterocercal tail of the sturgeon. (Asa result of an
upward bending of the notochord and vertebral column the fin has become
asymmetrical, the ventral portion much larger than the dorsal.) C, D, Homo-
cercal fins, C, of Amia calva; D, of Truttu salar. (By a still greater upward bend-
ing of the notochord and vertebral column the dorsal portion has almost en-
tirely disappeared and the ventral portion almost alone forms the fin, externally
apparently symmetrical, but in its internal structure very asymmetrical.) ch,
chorda; a, b, ¢, cover-plates.
logical. We know that the configuration of the earth’s surface
has changed in many respects in the course of the enormous space
of time of the geological periods; that land areas, which earlier
were united, have become separated by the encroachments of the
sea; that by the upheaval of mountains important barriers to the
distribution of animals were also formed. From the fact that
42 GENERAL PRINCIPLES OF ZOOLOGY.
these two changes—the changes in the earth’s surface and in the
animal world established upon it—have gone on hand in hand
there follows necessarily the consequence that greater differences
in the faunal character of two lands must result the longer they
have developed independently of one another, without interchange
of their animal populations, and the Jonger the inhabitants have
been separated by impassable barriers. For the various groups
the character of the barriers is different; terrestrial animals, which
cannot fly, are hindered in their distribution by arms of the sea;
marine forms by land barriers; for terrestrial molluscs high moun-
tain ranges, which are dry and barren, or constantly snow-capped,
are effectual.
Instances of Proofs.—Since attention has been called to these
conditions, many geographical facts favorable to the theory of
evolution have been ascertained: (1) Of the various continents
Australia has faunally an independent character; when discovered
it contained almost none of the higher (placental) mammals,
except such as can fly (Chiroptera), or marine forms (Cetacea), or
such as are easily transported by floating wood (small rodents), or
such as could be introduced by man (dingo, the Australian dog);
instead, it had remarkable birdlike animals (with beak and cloaca),
and, the marsupials, which have become extinct in the Old World
and the opossums excepted, in America as well. The phenomenon
is explained by the geological fact that in the earth’s history
Australia, with its surrounding islands, was certainly the earliest
to lose its connexion with the other continents. While’ in the
other four parts of the earth the higher vertebrates, which were
developed from the marsupials and their lower contemporaries,
came, by way of the lands connecting the various continents, to
have a wide or even a cosmopolitan distribution, in isolated
Australia this process of evolution did not go on, and its ancient
faunal character was preserved. (2) As Wallace has shown, the
Malay Archipelago is divided faunally into an eastern and a western
half; within each group there are islands which, in spite of a
different climate, have a very similar fauna. On the other hand,
the faunal boundary (‘ Wallace’s line’) passes between the two
islands Bali and Lombok, which have the same climate and
geographically are very close together. But the depth of the strait
in this region shows that here runs a boundary of extraordinarily
long geological duration, and that in the earth’s history Bali has
developed in connexion with the western, Lombok with the eastern
chain of islands. More recent studies make it probable that there
HISTORY OF ZOOLOGY. 45
is an island zone between the two in which a mixture of faunas
occurs. Celebes especially belongs here. (3) A long time before
Darwin, the renowned geologist Leopold von Buch, from the dis-
tribution of plants on the Canary Islands, had come to the conclu-
sion of a change of species into new species; viz., on islands
peculiar species develop in secluded valleys, because high mountain-
chains isolate plants more effectually than do wide areas of water.
M. Wagner has collected many instances which prove that locali-
ties inhabited by certain species of beetles and snails have been
sharply divided by wide rivers or by mountain-chains, while in
neighboring regions related so-called ‘ vicarious species’ are found.
Causal Foundation of the Theory of Evolution.—The Dar-
winian theory, so far as the above exposition shows, is fundamen-
tally like the theories of descent advocated at the beginning of this
century by Lamarck and other zoologists; it is distinguished from
these only by its much more extensive foundation of facts, and
further in that it abandoned the successional arrangement over-
thrown by the type theory, and replaced it by the branched, tree-
like mode of arrangement,—the genealogical tree. But still more
important are those advances of Darwinism which relate to the
causal foundation of the descent theory. The doctrine of causes
which has brought about the change of species forms the nucleus
of the Darwinian theory, by which it is especially distinguished from
Lamarckism. In order to substantiate causally the change of
species, Darwin proposed his highly. important principle of
‘Natural Selection by means of the Struggle for Existence.’
Artificial Selection.—_In the development of this principle
Darwin started from the limited and hence easily comprehended
subject of Domestication, the artificial breeding of our races of
domesticated animals. Many of these undoubtedly sprang from a
single wild living species; others arose from several species, but
now have the appearance of a single species. Ilow have arisen
such extraordinarily different races of pigeons—the fantail, the
pouter, long- and short-billed pigeons, etc., the long- and short-
horned cattle, the heavy, slow Percherons and the slenderly-built,
fleet-footed Arabian horses? Undoubtedly through that same
more or less conscious influence of man, which is still employed
by the skilful animal-breeder. If he wish to obtain a particular
form, he chooses from his stock suitable animals, which he pairs
together if they in ever so slight a manner approach nearer than
the others to the desired ideal. By repetition of this selection
according to plan, the breeder attains a slow but sure approx-
44 GENERAL PRINCIPLES OF ZOOLOGY.
imation to the goal, since he uses for breeding only the suitable
individuals from each new generation. If he wish, for example,
to breed fantail pigeons, he selects from his stock animals with
the most numerous and strongest tail-feathers. In the course of
generations, then, characteristics cumulate; the number of pigeons
having an increased number of tail-feathers becomes greater, and
thus material is obtained which is adapted to a further increase in
the number of feathers.
Factors of Evolution in Breeding.—The remarkable results of
breeding which are well known to every observer of our domesti-
cated animals depend mainly upon three factors: (1) Vartability;
the descendants of one pair of parents have the capability of
developing new characteristics, thereby differimg in appearance
from their parents. (2) Hereditability of newly-acquired charac-
ters. This consists in the tendency of the daughter-generation to
transmit the newly-developed characteristic to the succeeding
generation. (3) Artificial selection; man selects for breeding pur-
poses suitable individuals, and prevents a new character which has
arisen through variation from disappearing through crossing again
with animals of the opposite variational tendencies.
Factors of Evolution in Nature.—If we compare with the facts
of domestication the conditions of animals living in the state of
nature, we find again variability and heredity, as efficient forces,
inherent in all organisms, though the former is not everywhere of
the same intensity. There are many species which vary only
shghtly or not at all, and therefore have remained unchanged for
thousands of years. But contrasted with these conservative species
are in every group progressive species, active species, which are
in the process of rapid change, and these alone are of importance
in causing the appearance of new species. Since heredity is
present in all organisms, there is only lacking a factor correspond-
ing to artificial selection, and this Darwin discovered in the
so-called ‘natural selection.’
Natural Selection: Struggle for Existence.— Natural selection
finds its basis in the enormous number of descendants which every
animal produces, There are animals (e.g., most fishes) which
produce many thousands of young in the course of their lives: not
to mention parasites, whose eges are numbered by millions. For
the development of this animal throng there is ‘no room on the
earth; for even if we compute upon the basis of a slowly-multiply-
ing animal, like the elephant, and assume that all the progeny live
and reproduce normally, it would only be a few centuries before
HISTORY OF ZOOLOGY. +5
the entire earth would be occupied by herds of elephants. In
order to preserve the equilibrium in nature great numbers of
unfertilized and fertilized eggs, as well as young animals and many
that are mature but have not yet attained their physiological
destiny, must perish. Many individuals will undoubtedly be
blotted out by purely accidental causes; yet on the whole those
individuals which are best protected will best withstand adverse
conditions. Slight superiority in structure will be of importance
in this struggle for existence, and the possessors of this will gain
an advantage over their companions of the same species, just as in
domestication each character which is or is fancied to be useful to
mun insures advantage to the possessor. Among the numerous
varieties that appear the fittest will survive, and in the course of
many generations the fortunate variations will increase by sum-
mation, while destruction overtakes the unsuitable varieties. Thus
will arise new forms, which owe their existence to ‘natural selec-
tion in the struggle for existence.’
The ‘Struggle for Existence.’—The expression ‘struggle for
existence’ is figurative, for only in rare cases does an active con-
scious struggle decide the question of an animal’s existence; for
example, in the case of the beasts of prey, that one which by
means of his bodily strength is best able to struggle with his com-
petitors for his prey is best provided in times of limited food-
supply. Much more common is the unconscious struggle: each
man who attains a more favorable position by special intelligence
and energy, limits to an equal degree the conditions of life for
many of his fellow men, however much he may interest himself in
humanity. The prey which by special craft or swiftness escapes
the pursuer turns the enemy upon, the less favored of its com-
panions. It is noticeable that in severe epidemics certain men do
not fall victims to the disease, because their organization better
withstands infection. Here the term ‘survival of the fittest,’
which Spencer has adopted in preference to ‘struggle for exist-
ence,’ is better.
Instances of the Struggle for Existence.—Although the fore-
going general considerations suffice to show that the struggle for
existence plays a very prominent réle in the organic world, yet on
account of the importance of this feature it will be illustrated by
a few concrete examples. The migratory rat (Mus decumanis),
which swarmed out from Asia at the beginning of the eighteenth
century, has since then almost completely exterminated the house-
rat (Mus rattus) in Europe, and has made existence impossible for
46 GHNERAL PRINCIPLES OF ZOOLOGY.
it in other parts of the world. Several European species of thistle
have increased so enormously in the La Plata states that they have
in places completely crowded out the native plants. Another
European plant (Hypocheris radicata) has become a weed, over-
running everything in New Zealand. Certain races of men, like
the Dravidian and Indian, die off to the same degree that other
races of men, like the Caucasian, Mongolian, and Negro, spread.
The more one attempts to explain that endlessly complicated web
of the relations of animals to one another, the relations of animals
to plants and to climatic conditions, as Darwin has done, so much
the more does he learn to appreciate the methods and results of
the struggle for existence. He will become conversant with many
interesting phenomena, formerly unintelligible, which immediately
find an explanation through this doctrine. Islands lying in the
midst of the ocean have a disproportionately large number of
species of wingless insects, because the flying forms are easily
carried out to sea. For example, on the Kerguelen Islands,
remarkably exposed to storms, the insects are wingless; among
them one species of butterfly, several flies, and numerous beetles.
Sympathetic Coloration.—Very often, in regions which have a
permanent or prevailing uniform color, the coat of the animals is
distinguished by the same or at least by a similar hue; this
phenomenon is called sympathetic coloration. Inhabitants of
regions of snow are white, desert animals have the pale yellow
color of the desert, animals which live at the surface of the sea are
transparent; representatives of the most diverse animal branches
show the same phenomenon. The advantages connected therewith
scarcely need an explanation. Every animal may have occasion
to conceal himself from his pursuers; or it may be his lot to
approach his prey by stealth: he is much better adapted for this
the closer he resembles his surroundings. Natural selection fixes
every advantage in either of these directions, and in the course of
many generations these advantages increase.
Mimicry is referable to the same principle, except that the
imitation is not here limited to the color, but also influences form
and marking. Frequently parts of plants are imitated, sometimes
leaves, sometimes stems. Certain butterflies with the upper sur-
faces of the wings beautifully colored escape their pursuers by the
rapidity of their flight; if they alight to rest, they are protected by
their great similarity to the leaves of the plants around which
they chiefly fly. When the wings are folded over the back, the
dark coloring of the under sides comes into sight and the color on
HISTORY OF ZOOLOGY. 47
the upper side is concealed. The parts are so arranged that the
whole takes on a leaf-like form, and certain markings heighten
the imitation of the neuration of the leat (fig. 11). Among the
numerous species of leaf-butterflies there are different grades of
completeness of mimicry; in many even the depredations of insects
Fia. ce butterflies. .4, Kallima paralecta, flying; a, at.rest. (After Wallace.)
B, Siderone strigosus, flying; b, at rest. (After C. Sterne.)
is imitated; in others the form and marking are still incompletely
leaf-like, the marking being the first to come into existence.
Among the grasshoppers also there are imitations of leaves, like
the ‘walking-leaf,’ Phylliwm siccifolium, P. scythe, while other
nearly related forms more or less completely approach the appear-
ance of dried, sometimes of thorny twigs (fig. 12, a and 4).
Examples of Mimicry.—Very often insects are copied by other
animals. Certain butterflies, Heliconia, fly in large swarms,
48 GENERAL PRINCIPLES OF ZOOLOGY.
seythe 2.
Fig. 18.—Methona psidii, a bad-tasting Heliconiid, copied by the Pierid, Leptatis orise.
(After Wallace.)
HISTORY OF ZOOLOGY. 49
clumsy and yet unmolested by birds, because they contain bad-
tasting fat bodies. Another species of butterfly accompanies them
(Pieridze), which does not taste bad, and yet are not eaten, because
in flight, in cut, and marking of the wings they imitate the
Heliconie so closely that even a systematist might easily be
confused (fig. 13). In a similar way bees and wasps, feared on
account of their sting, are imitated by other insects. In Borneo
there is a large black wasp, whose wings have a broad white spot
F1a. 14.--a, Mygnimia aviculus, a wasp imitated by a beetle; b, Coloborhombus fascia-
tipennis. (After Wallace.) } nat. size.
near the tip (Mygnimia aviculus). Its imitator is a heteromerous
beetle (Coloborhombus fasciatipennis), which, contrary to the habit
of beetles, keeps its hinder wings extended, showing the white spot
at their tips, while the wing-covers have become small oval scales
(fig. 14).
Sexual Selection is a special phase of natural selection, chiefly
observed in birds and hoofed animals. For the fulfilment of his
sexual instincts the male seeks to drive his competitors from the
field, either in battle or by impressing the female by his special
50 GENERAL PRINCIPLES OF ZOOLOGY.
excellences. With strong wings and with spurs the cock main-
tains possession of his flock, the stag by means of his antlers, the
bull with his horns. The birds of paradise by means of beautiful
coloring win the favor of the females, most singing-birds by means
of song; many species of the fowl by peculiar love-dances. Since
a
Fic. 15a.—Paradisea apoda, male. (After Levaillant.)
all these characters belong chiefly to the male, and since it is
only exceptionally that they are inherited by the female (and even
then are less pronounced), it is almost certain that in a great
measure they have been acquired by the males through the struggle
for the female. In the case of birds a second factor has un-
doubtedly co-operated to impress distinctly the often enormous
difference between the feathers of the male and of the female—as
HISTORY OF ZOOLOGY. 51
is shown, for example, in the case of the birds of paradise
(fig. 15); for the nesting female inconspicuous colors and a close-
lying coat of feathers are necessary in order that, undisturbed by
enemies, she may devote herself to incubation.
On the Efficiency of Natural Selection.—In the course of the
last decade there has been much controversy as to how far natural
selection alone is a species-forming factor. A number of objectors
dispute the possibility of fortuitous variations being utilized in the
struggle for existence. It is not easy to see how many characters,
=
Fia. 158.—Paradisea apoda, female. (After Levaillant.)
especially such as are used in classification, can be of use to their
owners. It can only be said that they have developed in correla-
tion, that is in necessary organic connexion, with other important
characters. But useful characters must be considerable in order
to be seized upon by natural selection. Fortuitous variations with
which Darwinism deals are too inconsiderable to be utilized by the
organism and so to be of value in the struggle for existence. In
most cases, too, alteration in one organ alone is not enough to be
of value; usually a whole series of accessory structures must be
modified. In short, there must exist a harmonious co-operation
of parts, which presupposes a progressive and well-regulated
development extending through a long space of time during which
the struggle for existence could have exerted no directing influ-
ence. Thus, for example, the wing of a bird in order to be used
for flight must have already reached a considerable size; the
muscles for moving it, the supporting skeletal parts, the nerves
running to it must have a definite formation and arrangement.
Then there are difficulties in that most animals are bilaterally or
52 GENERAL PRINCIPLES OF ZOOLOGY.
radially symmetrical, many in addition segmented. In all these
cases the same organ is repeated two or more times. Organs which
are repeated symmetrically and usually those which are segmental
agree in general in structure. One must therefore admit that the
alterations of chance must have occurred at at least two points
simultaneously and in exactly the same way.
A further objection is that the action of natural selection would
under ordinary conditions be negatived by unhindered crossing of
the varying forms. If, for example, we do not isolate fantails from
other pigeons, they will cross with these, and their descendants
will soon resume the character of common pigeons. Finally, it
has been claimed that for the formation of new species a simple
variation of forms is not sufficient; it must reach still farther: (1)
a variation in different directions, a divergent development of the
individual members of a species; (2) the disappearance of the
transitional forms which unite the divergent forms.
The objection that the struggle for existence cannot bring
about the divergent development of individuals necessary for
improvement is of least importance. It need only be added that
of the many variations appearing at the same time in a species two
or more may be equally useful; that then one set of individuals
will seize upon one, another set upon the other advantage, and that
in consequence of this both sets will develop in different directions.
Consequently the intermediate forms which are not pronounced in
the one or the other direction will be in an unfavorable position,
and must carry on the struggle for existence with both groups of
partially differentiated companions of their species, and, being less
completely adapted, must fall.
More important are the first two objections; they have led to
theories which originally seemed destined to complete the Dar-
winian theory, but in the course of discussion they have more and
more raised the claim of entirely supplanting it. In the following
paragraphs will be found an outline of these theories, but it is to
be taken into consideration that, at the present time, we are still
in the midst of the reform movement, and it cannot yet be said
whether they will be able to stand beside the theory of the struggle
for existence or will supplant it.
Migration Theory.—T'o explain how characters newly formed
by variation become fixed, and do not disappear again through
crossing with differently modified individuals, M. Wagner has pro-
posed the Theory of Geographical Isolation, or the Migration
Theory. New species may arise if a part of the individuals of one
HISTORY OF ZOOLOGY. 53
species should take to wandering, or should be transplanted, and
thus come to a new place, in which crossing with the companions
of their species who were left behind is not possible. The same
might occur, if the region inhabited by a species should by
geological changes be divided into two parts, between which inter-
change of forms would be no longer possible. The animals
remaining under the old conditions would retain the original
characteristics; the wanderers, on the other hand, would change
into a new species. Direct observations support this theory. A
litter of rabbits placed at the beginning of the fifteenth century
on the island of Porto Santo has in the mean time increased
enormously and the descendants have taken on the characteristics
of a new species. The animals have become smaller and fiercer,
have acquired a uniformly reddish color, and no longer pair with
the European rabbit. A further proof in favor of the theory of
geographical isolation is the peculiar faunal character of regions
separated from adjacent lands by impassable barriers, broad rivers
or straits, or high mountains (comp. p. 42); especially instructive
in this regard is the peculiar faunal character of almost every
island. The fauna of an island resembles in general the fauna of
the mainland from which the island has become separated by
geological changes; it usually has not only these but also so-called
‘vicarious species,’ i.e., species which in certain characteristics
closely resemble the species of the mainland. Such vicarious
species have plainly arisen from the fact that isolated groups of
individuals, scattered over the islands, have taken on a develop-
ment divergent from the form from which they started. With all
due recognition of the migration theory, it will never be possible
by it alone to explain the multiformity of the organic world. In
addition, it must be assumed that formerly the earth’s surface
possessed an enormous capacity for change; but the more recent
investigations make it probable that the distribution of land and
water has not varied to the degree that was formerly believed.
The experience of botanists, too, teaches that several varieties can
arise in the same locality and become constant.
Lamarckism.—While the migration theory agrees with Dar-
winism in this, that the new characters appearing through varia-
tion are to be regarded as the products of chance, yet it is just
this part of the theory which has been subjected to searching
criticism. Many zoologists have again adopted the causal founda-
tion of the descent theory proposed by Lamarck and believe that
the cause of species formation is to be found in part in the
54 GENERAL PRINCIPLES OF ZOOLOGY.
immediate influence of ‘changing environment, in part in the
varying use and disuse of organs, brought about by alterations in
the conditions of life. Both principles, they say, are sufficient,
even without the help of the struggle for existence, to explain the
phylogenesis of organisms.
Influence of Environment.—T'o what extent can the environ-
ment bring about a permanent change in the structure of plants
and animals? To decide this is no simple problem, on account of
the complexity of the factors entering into the question.
In cases where the food-supply is altered, organisms change in
a very remarkable manner and within a short time; but these
changes (Niigeli’s ‘ Modifications through Nutrition’) seem to have
no permanence. Plants which, found in nature in poor soil, are
transplanted into rich soil, or vice versa, soon acquire quite a
different appearance, and preserve this through the following
generations, so long as they remain in the rich soil; but the plant
quickly returns to its former appearance when replaced in its
previous surroundings.
In general, a change seems to be the more permanent the more
slowly it has developed. In researches upon the influence of
environment, we can, therefore, rely soonest upon results if we
experiment with slowly-working factors, such as light and heat,
dry or moist air, different intensities of gravitation, of stimuli,
etc., which can be excluded from the environment of the organism.
Use and Disuse.—Regarding the efficiency of use and disuse,
there is no doubt that the shape of an animal is influenced to a
great extent by the manner in which the organs are used. The
organs which are much used will become especially strong and vice
versa those which are not used will become weak. The only ques-
tion is whether these, in the strict sense of the word, newly-acquired
characteristics are transmitted to the offspring, or whether the
descendants, in order to attain to the same stage, must not repeat
in the same way use and disuse. In the latter case the cumulation
of characteristics, and with it the possibility that these may
become permanent, is excluded. It is to be regretted that accurate
results are still lacking on a point so well adapted for experimental
treatment. At this time rudimentary organs strongly favor the
Lamarckian principle; for we see that cave animals, which for
many generations have lived in darkness, are blind, either having
no eyes, or only vestiges of them, incapable of function. This
seems to justify the view that this condition is attributable to lack
of use, since it has brought about a functional and anatomical
HISTORY OF ZOOLOGY. 5d
incapacity, which has increased from generation to generation.
Now we must believe that what is true for disuse must express
itself in the reverse sense in the case of use.
Nageli’s Principle of Progression.—In conclusion, there is still
to be considered the change of species from internal causes, to
which von Baer gave the poorly adapted because easily misleading
term ‘‘ Zielstrebigkeit ” (the striving toward an ideal), and which
Niigeli has termed the ‘ perfecting principle,’ or the ‘ principle of
progression.’ It cannot, indeed, be denied that each species is
compelled, by some peculiar internal cause, to develop into new
forms, independently of the environment, and up to a certain
degree, independently of the struggle for existence. In all animal
branches we see the progress from lower to higher going on, very
often ina quite similar way, in spite of the fact that the animals
live under very different conditions of development. We see how
the nervous system lying near the surface in the lower animals
becomes in the higher animals concealed in the depths of the body;
how the eye, at first a simple pigment-spot, becomes in worms,
arthropods, molluses, and vertebrates, provided with accessory
apparatus, as lens, vitreous body, iris, choroid, etc. Here we see
an energy for perfection which, since it occurs everywhere, must
be independent of the individual conditions of life, and must have
its special explanation in the character of the living substance.
It is by no means justifiable to call an assumption, as here
expressed, teleological, and to reject it as unscientific; rather the
organism seems to be just as mechanically conditioned as a billiard-
ball, whose course is determined not only by contact with the
cushions of the billiard-table, but also in a large measure by its
indwelling force, imparted to it by the stroke of the eve. An
organism, too, is a store of energy which must necessarily from
itself develop more, but it is of more extraordinary complexity,
and to an equal degree also is independent of the external world.
A complete independence is naturally never present, and Nigeli
has not so maintained. Along with it rather goes always an
‘action’ of the external world, a modifying influence which is
carried on by the external conditions of existence, either directly
or by the mediation of use and disuse.
This exposition of evolution has been given in a rather detailed
way, because in the history of zoology it is undoubtedly the most
important feature. No other theory in the course of the develop-
ment of zoological investigation has gained such a hold, none has
propounded so many new problems and opened so many new fields
56 GENERAL PRINCIPLES OF ZOOLOGY.
for research. There is no other zoological theory which compares
with it in value as a working hypothesis. To the many objections
which have been made that the theory is insufficiently grounded,
it can only be replied that in the present state of our knowledge it
is the only theory which agrees with our experiences and explains
these in a simple way and on a scientific basis. In this sentence
is given the merit of the theory, but at the same time also a limita-
tion of its applicability. For on the one side the statement attrib-
utes the merit in the applicability of the system to the necessity
of the human mind for simple explanations of the facts of natural
science, and on the other hand it makes the degree of correctness
dependent upon the state, whatever it may be, of our knowledge.
On both sides no constant quantities are involved. Many investi-
gators see no necessity of reconciling paleontology and our knowl-
edge of plants and animals. To such, therefore, the Darwinian
theory proves just as little as any opposing theory. Meanwhile
thoughtful naturalists will keep in mind that our knowledge of
nature is making considerable advances, and is visibly becoming
wider and deeper. It is possible, even probable, that these
advances will lead to many modifications of the theory. For
instance, the theory of the causes which condition the formation
of new species will undergo numerous changes. On the other
hand, we can affirm with great certainty that the principle of
descent, which first obtained credence through Darwinism, will be
a permanent landmark of zoological investigation.
GENERAL MORPHOLOGY AND PHYSIOLOGY
General Zoology: Animal Morphology.—lIn the vital phenom-
ena of animals a certain degree of similarity can be followed
through the entire animal kingdom; the way in which animals are
nourished and reproduce their kind, how they move, and how they
gain experience, is essentially the same in great groups, and even
widely separated forms show many agreements. Corresponding
to this, the apparatus which is concerned with the above-men-
tioned functions, the organs of nutrition and reproduction, of
motion and sensation in their grosser and finer structure, and in
their ontogeny, must be similar to one another and show evidence
of some fundamental characters which always or frequently recur.
All this needs a general explanation before we can go into a
description of the separate branches of animals. This explanation
is the subject of general zoology, specially of general anatomy and
embryology, or animal morphology.
cology or Biology.—If by means of anatomy and embryology
we have learned the general character of the animal organism, we
must yet farther study its relations to the environment. For this
study of the conditions of animal life, ecology or biology, we have
to consider the geographical range of animals, their distribution
over the surface of the earth and in the different depths of the
sea; further, the reciprocal relations of animals and plants, and
of beast to beast, as these find special expression in colony-build-
ing, symbiosis, parasitism, etc.
General Anatomy.—In the case of General Anatomy, with
which we shall begin, the fundamental proposition will be, /fow is
an organism formed from its constituent parts ? We shall thus in
spirit follow the opposite course from that which anatomy actually
takes, for this resolves the animal body into its elementary parts,
its organs, tissues, and cells. Instead of analytical we will pursue
synthetic anatomy.
The synthesis of an organism, of which by general anatomy we
can only gain an idea, actually takes place in nature during the
ny
o4
58 GENERAL PRINCIPLES OF ZOOLOGY.
development of every animal. Embryologically every organism
is at some time a simple element, a cell; this divides and gives rise
to tissues; from the tissues are formed organs, and from the
organs the regularly membered whole of the animal body is com-
bined. If the general ontogeny proceeds synthetically, it then
agrees in its manifestations with the processes which go on in
nature and which are accessible to direct observation.
GENERAL ANATOMY.
The Morphological Units.—The expression ‘constituent parts
of the animal body’ can be used in a double sense. We can speak
of the chemical units, the chemical combinations, which form the
tissues; these are the subject of animal chemistry, and may there-
fore be passed over here. But we may also speak of the constituent
units (morphological units) of the animal body; these are the cells.
These and their transformation into tissues, organs, and entire
animals are for us of vastly greater importance.
I. THe MorpnonocicaL UNITS oF THE ANIMAL Bopy.
The Cell.— The study of the morphological units of the organic
body first found a firm foundation in the cell theory. Every
scientific study of the anatomy of plants and animals must there-
fore take the cell as its starting-point.
History of the Cell Theory.—The conception of the cell of animals and
plants has in the course of time undergone many changes, which must
be known to some extent in order to understand completely the name and
the conception. When, in the seventeenth century, Hooker, Marcello
Malpighi, and Nehemia Grew introduced the term into vegetable anatomy,
they meant small chambers surrounded by firm walls and filled with air
or fluid contents. When, also, early in the nineteenth century, it was cor-
rectly recognized that the cell is the anatomical and physiological vegetable
unit from which all the other parts of the plant are formed, and when the
English botanist Brown discovered in the interior of the cell that small
body previously overlooked, the kernel or mzclews, the old conception
remained, and as such was accepted by Schleiden in his cell theory.
Schleiden added as new a completely erroneous view of the origin of cells:
that in a sort of matrix (the ‘cytoblast’) first a granule, the nuclear
body, was formed, then around this granule a membrane, the nuclear mem-
brane, arose by precipitation, and around the thus completed nucleus a
larger membrane (the cell membrane) was deposited. Hence for the
formation of the cell the nucleus would be of most importance,
The Schleiden-Schwann Cell Theory.—Since it is the nuclei which are
most easily seen in the animal body, and even now are particularly useful
GENERAL ANATOMY. 59
in deciding questions concerning the presence of cells, it is readily under-
stood how Schleiden’s theory, which placed the nucleus so much in the
foreground, should have led Schwann to apply the cell theory to the
animal kingdom, and thus raise it toa principle of general application.
We usually, therefore, speak of the Schleiden-Schwann cell theory.
As a result of this theory the walls, the cell membrane, were regarded
as most important for the function of the cell; through the cell mem-
brane diffusion-currents must pass between the surrounding medium
and the contents of the cell; the character of the membrane and of the
cell-sap must determine the condition of the diffusion-currents, and
hence the functional character of the cell; the different appearance of
tissues depends chiefly upon the fact that the cells, spherical in the
beginning, change their form; in the case of fibrillar connective tis-
sue, for example, they increase enormously in length and become fine
fibrillee. Since the life of an organism is nothing else than the co-operative
work of all its cells, they flattered themselves that through the cell theory
and the discovery, brought about by it, of the physical unity of the animal
and vegetable body they had made an important advance in the great
problem of the physical explanation of the phenomena of life. Cell gene-
sis also seemed, according to the theory, to be just as satisfactorily
explained on a mechanical basis as the formation of a crystal. In the
‘eytoblast’ the nuclear bodies, nuclear membrane, and cell membrane
must be formed by deposition just as in the process of crystallization.
Reform Movements,—Since that time our conception of the nature of
cells has completely changed. The cell does not, after the manner of a
erystal, arise as a new formation in a matrix, but it presupposes the
existence of a living mother-cell, from which it arises by division or bud-
ding. Just so also the cell is not a physical unit, but is itself an organism
which shows to us all the enigmas of life, the physical basis of which our
investigations must ever keep in view as a goal, though it be still indiscern-
ibly distant. The membrane and cell-sap are of quite subordinate impor-
tance for the existence of the cell; rather the most important thing in it is
the previously disregarded substance, for which von Mohl introduced the
name protoplasm. According to the newer conception the cell is practically
asmall mass of protoplasm, usually, probably always, provided with one
or more nuclei. This newer conception of the cell has developed so gradu-
ally, and has so slowly supplanted the Schleiden-Schwann view, that the
old name has been retained, although it no longer at all fits the new con-
ception. We have indeed become so thoroughly accustomed to the name
that we no longer notice the contradiction of terms when we call a solid
lump without a membrane a ‘ cell.’
Discovery of Protoplasm.—The reformation of the cell theory was begun
by discoveries which were made in very different regions and only lately
have been brought to a focus.
1. At about the beginning of the nineteenth century, Bonaventura
Corti and Treviranus had seen that the chlorophyl bodies, which cause the
green color of plants, in many species stream around in a lively manner in
the interior of the cell, but Moll was the first to find out that this motion
60 GENERAL PRINCIPLES OF ZOOLOGY.
was not active, but rather that they are moved by a homogeneous sub-
stance in which they are embedded. This substance, which Mohl, in
order to bring it into prominence, named protoplasma, became by other
studies still more important. In the reproduction of the simplest alge, it
was found that the protoplasm, together with the chlorophyl bodies, col-
lected itself into an oval mass, and that this body left the cell membrane
aud swam freely in the water. Since the cell-wall no longer showed signs
of life, while on the other hand the protoplasmic body came to rest and
formed a new plant, it was shown beyond doubt that this was the most
important constituent part of the cell (comp. fig. 115).
2. In the study of animal tissues the importance of the peculiar cell-
substance, the protoplasm, was still more plainly brought out. Here, in
spite of the long-prevailing preconceived idea, unbiassed observation led to
the discovery that most animal cells had no cell-membrane.
3. Very important, finally, was the study of the lowest organisms, the
Protozoa. Dujardin sought by extremely careful observations to prove
that these animals had no organs, but consisted of a uniform granular sub-
stance, the sarcode. The sarcode alone could produce all the vital phe-
nomena, such as movement, sensation, assimilation, previously ascribed to
many organs. Dujardin’s theory was stoutly contested by Ehrenberg and
his school, but finally attained general acceptance through the epoch-
making work of Max Schultze and Haeckel.
Schultze’s Protoplasm Theory.—On the basis of these three series of
observations, Max Schultze finally established the reformation of the cell
theory briefly sketched above, when by accurate study of the appearance
and the vital phenomena, and by means of numerous experiments, he
proved that the cell-substance of animals, the sarcode of Protozoa, and
the protoplasm of plants are identical, and that to this substance, for
which he retained the name protoplasm, all the vital phenomena of animals
and plants are referable in the ultimate analysis. The second important
modification concerns the changes of cells into tissues. These follow not
so much through changes of form and modification of the cells into the
tissue elements, as Schwann thought, but rather by means of chemical
changes. By means of its formative potentiality the protoplasm gives rise
to non-protoplasmic structural parts, as, for example, connective-tissue
fibrils, muscle fibrils, nerve fibres, etc. These give the various tissues their
specific character and perform their functions. The tissues also retain as
the source of life and formation the unemployed remnants of cells, the
connective-tissue corpuscles, muscle corpuscles, ete. We will now trace
out farther these two fundamental ideas in Max Schultze’s ‘protoplasm
theory,’ and thereby briefly sketch the elements of the modern theory of
tissues.
Nature of the Cell.—The size of the animal cell varies to a
considerable degree; the smallest elements are the male sexual
cells, the spermatozoa, whose bodies, particularly in case of the
mammals, are even less than 0.003 mm.; the largest, on the other
hand, with the exception of the giant plasmodia of some
GENERAL ANATOMY. 61
Mycetozoa, are the egg cells. The yolk of the bird’s egg, which
alone forms the egg in the narrower sense, apart from its coverings,
has for a time the morphological value of a cell, and in the case of
the ostrich egg may reach a diameter of several inches. The form
of the cell is hkewise variable. Free cells, whose form is not
determined by the environment, are usually spherical or oval in
the resting condition, as the egg cell shows; united into tissues,
the cells, on the contrary, may be pressed together into polygonal
or prismatic bodies, or may send out spindle- or star-shaped
branching processes.
Protoplasm.—So there is left to characterize the cell only the
constitution of its substance: the cell is a mass of protoplasm with
one or more nuclei. It is not known whether protoplasm is a
definite chemical body, which from its constitution is capable of
infinite variation, or whether it is a varying mixture of different
chemical substances. So, also, we are by no means certain whether
or not these substances (as one is inclined to believe) belong to
those other enigmatical substances, the proteids. We can only
say that the constitution of protoplasm must, with a certain degree
of homogeneity, have a very extraordinary diversity. For if we
see that from the egg of a dog there comes always and only a dog,
and indeed an animal with all his individual peculiarities, that a
sea-urchin’s egg, placed under the most diverse conditions, pro-
duces always a sea-urchin, that a species of amceba always performs
only the movements characteristic of that species, we must assume
that the functioning constituent part of this cell, the protoplasm,
has in each case its peculiarities. We are driven to the assumption
of an almost unlimited diversity of protoplasm, even if we concede
an impertant share in the prominent differences to the nucleus, of
which we shall speak later.
General Properties of Protoplasm.—The similarity of proto-
plasm, still recognizable through all its variations, expresses itself
in its appearance and in its vital phenomena. Under slight mag-
nification, protoplasm appears as a faintly-gray substance, some-
times colored yellowish, reddish, etc., by pigments taken up by
imbibition, in which numerous strongly-refracting granules are
embedded. The vital characteristics of this substance are move-
ment, irritability, power of assimilation and of reproduction.
By using higher powers a finer structure can be seen in the
protoplasmic substance, the ‘homogeneous protoplasm ’ of earher
writers. The nature of this is as yet in question: a fine-meshed
framework (filar substance, spongioplasm, cell reticulum) the
62 GENERAL PRINCIPLES OF ZOOLOGY.
interstices of which are filled with other material (interfilar sub-
stance, enchylema, ground substance). The dispute les especially
around the question whether this framework is formed of threads
and trabeculae or whether the appearance is not formed by small
chambers, bounded by fine partition-walls (foam structure of
protoplasm).
Movement of Protoplasm.—Movement expresses itself first in
changes of form of the whole body—ameboid movement—and
secondly in the change of position
of the small granules in the interior
of the protoplasm—streaming of
granules. Examples of amceboid
movement (fig. 16) are chiefly the
movements of many Protozoa, and
of the colorless blood-cells (leuco-
cytes) of multicellular animals;
here the protoplasmic body sends
out coarser and finer processes,
which may be again withdrawn,
serving for locomotion and hence
called pseudopodia or false feet.
The streaming of granules can be
observed in the interior of the cell-
body, as well as in the pseudopodia
Fic. 16.— Ameba proteus. (After extending from this. The pseudo-
Bees cis eer ee podia may even be so fine as to be
ice at the limits of visibility with our
strongest magnifications (fig. 17), yet in them it can still be
observed that the granules wander hither and thither like people
on a promenade, simultaneously centripetally and centrifugally,
some with greater, others with Jess speed. And yet the granules
are only passively moved by the protoplasm, for if we feed the
creature with some pigment granules, like tinely-pulverized car-
mine, these granules show the same remarkable streaming. Indeed
nothing better illustrates the great complexity in the structure
of protoplasm than these extremely complicated phenomena of
motion in such narrow limits as pseudopodia in general.
Irritability of Protoplasm.—That ammboid movements and
streaming of granules can be induced, brought to a standstill, and
modified by mechanical, chemical, and thermal stimuli, is a sure
proof of the irritability of protoplasm. Most important are the
thermal stimuli; if the surrounding medium rise above the
GENERAL ANATOMY.
Fia. 17.—Gromia oviformis.
(From Lang, after M. Schultze.)
64 GENERAL PRINCIPLES OF ZOOLOGY.
ordinary temperature, the movements at first become more rapid
up to a maximum: from that point begins a slowing, finally coming
to a standstill.—eat-rigor. If the high temperature continue
much longer, or if it rise still higher, death results. The fatal
temperature is found for most animals between 40° and 50° C.
(104°-122° F.); its influence explains a part of the injurious
effects which high-fever temperatures have upon the human
organism. Like the heat-rigor, there is also a cold-rigor, induced
by a sharp sinking of the temperature below the normal. This
is accompanied by a gradual diminution of mobility; it results
in death by freezing, which is, however, not so easily produced as
death by heat. It is a remarkable fact that many animals, conse-
quently their cells, may be frozen; and in this condition can
endure still severer cold without dying. (For example: goldfish,
a temperature of — 8° to —15° C.; frogs, to — 28°; newts, to
= 28°),
Nutrition and Reproduction.—Irritability and power of motion
are the prerequisites of assimilation, the change of food-substance
into protoplasm. Most animal cells, for example almost all tissues
cells, are not suitable for studying assimilation, because they live
upon liquid nourishment. But certain cells of higher animals,
the colorless blood-cells, and most unicellular animals can be fed
also with solid substances; they take the food-particles into the
midst of the protoplasm by flowing around them with the pseudo-
podia. They extract all the assimilable and reject the indigestible
portions (fig. 16).
In the case of assimilation it is to be noted not only that the
cells use the food which they have taken for their own growth and
for replacing worn-out parts, but also that most of them have the
power of producing substances other than protoplasm; for
example, many Protozoa form organic shells or skeletons which
are hardened with silica or lime. This formative power, the
building of ‘plasmic products,’ is, as we shall shortly see, the
starting-point for tissue-formation.
Cell Nucleus.—The reproduction of protoplasmic bodies is
synonymous with the division of the cell: but to understand this
we must first consider the second important constituent, the
nucleus. This is a body enclosed in the protoplasm, whose form,
though definite for each kind of cell, shows in general wide varia-
tions. Usually it is a spherical or oval vesicle; but it may be
elongated or club-shaped, bent into a horseshoe, with constrictions
like a rosary, or even be branched, treelike (fig. 18); in many cells
GENERAL ANATOMY. 65
it is disproportionally large, so that the protoplasm surrounds it
only with a thin layer, in others again it is so small that it can
scarcely be found in the protoplasm among the other substances.
Formerly, on this account, it was in very many cases overlooked,
and even now it can often be demonstrated only by great care,
Fig. 18.—Various forms of nuclei. a, horseshoe-shaped nucleus of an Acinete; I»
branching nucleus from the Malpighian vessel of a Sphinyid larva; c, rosary-
shaped nucleus of Stentor cwruleus.
and by employment of a special technique based upon the micro-
chemical reaction of the nuclear substance.
The Nuclear Substance,—The nuclear substance is distin-
guished from protoplasm, among other ways, by its greater
coagulability in certain acids, e.g., acetic and chromic, which
therefore are often used for demonstrating the nucleus. If ina
living cell the nucleus be invisible on account of the similarity of
its refraction to that of the protoplasm, the addition of 2% acetic
acid will often bring it into sharp contour.
Structure of the Nucleus.—In its minute structure the nucleus
affords a wonderful variety of pictures varying according to the
objects chosen, but which are not sufficiently understood to permit
of a single description accepted by all. According to their reac-
tions to stains two substances in particular are distinguished:
chromatin or nuclein (fig. 19, ch), which is easily stained by certain
staining-fluids (carmine, hematoxylon, saffranin), and the achroma-
tin or linin, which stains not at all or only under special conditions.
The achromatin forms a network or reticulum (according to
another view a honeycomb structure) filled with a nuclear fluid,
66 GENERAL PRINCIPLES OF ZOOLOGY.
bounded externally by a nuclear membrane, easily isolated in large
nuclei. If little nuclear fluid be present, and the reticulum con-
sequently be coarse-meshed, the nucleus seems compact. If the
fluid be abundant, the nucleus appears vesicular. This is especially
ab
chp
6
4
Fic. 19.—Vesicular nuclei with achromatic reticulum and different arrangements of
the chromatin and nucleolar substance. p, plastin (nucleolar substance); ch,
chromatin; chp, chromatin plus plastin. land 2, nuclei of Actinospharium; 3,
of Ceratium hirundella (after Lauterborn); 4, germinal vesicle of Unio (after
Flemming); 5, nucleus with many chromatin nucleoli.
the case when the lines of the framework are separated by con-
siderable amounts of nuclear fluid (fig. 19, 4).
The chromatin enters into close relations with a less stainable
substance, the plastin or paranuclein (also sharply distinct from
achromatin). In the nuclei of Protozoa plastin and chromatin
are usually intimately united, the first forming a substratum in
which the latter is embedded (chp). The united substances
are most frequently closely and regularly distributed as fine gran-
wes on the reticulum, so that the entire nucleus appears uni-
formly chromatic (fig. 18). More rarely the mixture collects into
one or more special bodies, the chromatic nucleoli (1, 2). The
nucleolus is ordinarily a rounded body, more rarely branched
(fig. 19, 7).
In the nuclei of the Metazoa there may occur the same intimate
mixture of plastin and chromatin (6). As a rule, however, the
plastin (apparently not the whole, but a surplus) is separate from
the chromatin. Thus there occur in the nuclei of many eggs
GENERAL ANATOMY. 6T
nuclei which contain, the one chromatin, the other exclusively
plastin (4). In tissue cells only the plastin has the form of
nucieoli (true or chromatin-free nucleoli, 5), while the chromatin
is distributed on the nuclear reticulum (chromatin reticulum).
Somewhat the same may occur in the Protozoa (fig. 19, 2).
Significance of the Cell Nucleus.—For a long time the func-
tional significance of the nucleus in the cell was shrouded in
complete darkness, so that it began to be regarded, in comparison
with the protoplasm, as a thing of little importance. The evidence
that the nucleus plays the most prominent role in fertilization has
altered this conception. Then arose the view that the nucleus
determines the character of the cell; that the potentiality of the
protoplasm is influenced by the nucleus. If from the egg a definite
kind of animal develop, if a cell in the animal’s body assume a
definite histological character, we are, at the present time, inclined
to ascribe this to the nucleus. From this, then, it follows farther
that the nucleus is also the bearer of heredity; for the transmission
of the parental characteristics to the children (a fact shown to us
by our daily experience) can only be accomplished through the
sexual cells of the parents, the egg and sperm cells. Again, since
the character of the sexual cells is determined by the nucleus, the
transmission in its ultimate analysis is carried on by the nucleus.
This idea has a further support in experiments on Protozoa. If
one of these unicellular animals be cut into nucleate and anucleate
halves, the latter sooner or later degenerates, the former persists
and regenerates the lost parts. Within the nucleus it is probably
the chromatin which controls the functions of the protoplasm and
is accordingly (as observations on fertilization also seem to show)
the bearer of heredity, while the achromatin is the seat of contrac-
tility, and as such plays a part in cell multiplication.
The Centrosome.—Besides the nucleus there frequently occurs
a special body in the protoplasm, the centrosome, which on
account of its small size and a behavior similar to achromatin
with reference to staining-fluids was long overlooked, and even
now its demonstration is difficult. It is apparently well distributed
among the Metazoa, but is absent from most Protozoa. In many
it appears only at certain times and then disappears. What is
known of it makes it probable that it is a derivative of the nucleus,
a part of the achromatin which has left the nucleus; in other cases
possibly a second nucleus which by degeneration has lost the
chromatin and retained only the active nuclear substance, the
achromatin. In its function the centrosome is a specific organ of
68 GENERAL PRINCIPLES OF ZOOLOGY.
cell division which controls both the division of the nucleus and
that of the cell itself.
Multiplication of Cells.—Increase in cells occurs exclusively
by division or by budding (gemmaticn). Most common is binary
division in which a circular furrow appears on the surface of the
cell, deepens and cuts the cell into two equal parts. Multiple
division is more rare and can only occur in multinucleate cells,
Here the cell divides simultaneously into as many (sometimes
hundreds) daughter-cells ag
there were nuclei present. In
all forms of division the simi-
larity of the products is char-
acteristic, while in budding the
resulting parts are unequal.
In budding one or more
smaller daughter-cells, the
buds, are constructed from a
a large mother-cell (fig. 20).
i Direct Cell Division. —
Every cell division is accom-
panied by nuclear division or
Fic. 20.—Cell budding. _Podophrya gemmi- at least presumes that nuclear
Bare een padeie) Waiga scuurstenod form division. has previously oé-
curred. Direct and indirect
division are recognized. Direct division is most common in
Protozoa, and especially in nuclei with abundant chromatin (fig.
20, 145). The nucleus is elongated and is divided by constriction,
in the same way that the cell itself constricts. Since the proto-
plasm has no special arrangement with regard to the dividing
nucleus (the latter besides protected by its membrane), we must
conclude that the nucleus divides itself and is not passively
divided. The dividing force resides in the achromatic framework,
which correspondingly often exhibits a certain arrangement, a
fibrous structure in the direction of the elongating nucleus.
Indirect Cell Division, Karyokinesis.—Indirect cell division,
karyokinesis or mitosis, is most beautifully shown in cells, poor in
chromatin, which possess a centrosome. The process is introduced
by a division of the centrosome (fig. 21). The daughter centro-
somes migrate to two opposite poles of the nucleus, which now
loses its membrane and becomes the nuclear spindle. The
characteristics of the spindle are that it is drawn out into points
at two poles which are indicated by the position of the centro-
GENERAL ANATOMY. 69
somes, while from these poles fine threads, the spindle-fibres,
run to the centre or equator of the
nucleus. These fibres are in many
cases certainly derived from the
achromatic nuclear reticulum, while
in others a greater or less part in
their formation is taken by the
protoplasm.
ete tremely rare in man.
III, ANNELIDA. 305
Class III. Annelida.
The segmentation of the body, which was shown ina slight de-
gree in the Chetognathi, reaches its highest development in the
Fic, 272.—Diagram of annelid somites (orig.). am, acicular muscles; c, ccelom: cm,
circular muscles ; cv, circular blood-vessels; d, dorsal blood-vessel; i, intestine ;
dm, longitudinal muscles; m, mesentery ; x, nerve cord; na, nephridium; ne, nv,
neuro- and notopodia, forming parapodium; s, septum; so, somatopleure; sp,
splanchnopleure; t, typhlosole.
Annelids, where it appears both in the outer ringing of the body
and in the arrangement of the most important systems of organs—
metameric arrangement of excretory organs, nervous system, blood-
WS |i
(tn
a
ath
Fig. 273.—Trochophore (Loven’s larva) of Polygordius. (From Hatschek.) 4, anus;
dLM, dorsal muscles; £D, hind gut: J, stomach; J,, imtestine: Msfr, mesoderma
band; 1, nerves; Neph, protonephridia; O, mouth; Oe, esophagus; oe LM, cesopha-
geal muscle; SP, apical plate; vLM, ventral muscle; vLN, lateral nerve; Wkr, wkr,
pre- and post-oral zones of cilia; WS, apical cilia; wz, adoral cilia.
vessels—internal segmentation. To this is added an extraordinary
increase in number of body segments (somites, metameres), which
can far exceed a hundred. We can thus define the Annelids as
306 C@LUHELMINTHES.
worms with colom and with external and internal segmentation.
In the development there frequently occurs a type of larva, the
trochophore, which must be referred to here, since it is of great
morphological significance, resembling in structure the rotifers
and recalling the larva of the molluscs and to a less extent that of
the echinoderms. It is (fig. 273) a gelatinous body traversed by an
alimentary canal with fore-, mid-, and hind-gut regions. At first
it is everywhere ciliated, but with advance of development the
cilia become restricted to certain thickened tracts of epithelium,
the ciliated bands. One of these, the preoral band, is especially
constant. It runs circularly (W4r) around the body, surrounding
a circular prestomial area, in the centre of which is the anlage of
the cerebral ganglia, a thickened patch (apical plate) of ectoderm,
often bearing a tuft of cilia. Other ciliated bands (post-oral,
perianal) often occur. Of internal organs, besides numerous
muscle fibres, the most noticeable are the excretory organs, true
protonephridia, which open to the exterior either side below. The
trochophore in some respects resembles the larve of some Turbel-
laria (fig. 231) and Nemertines (fig. 256), showing that the annelids
are related to these groups.
The above account applies most closely to the Chetopoda and
the closely related Archianellida. In other forms one or more
features may be lacking—in the Gephyraa segmentation of the
body; in the Hirudinei most of the celom and the trochophore.
Yet these are so closely related that they must be included under
the common head; the missing
characters have been lost during
evolution.
Sub Class I. Chetopoda.
These, like the Nematoda, are
cylindrical worms, but are at once
distinguished by the segmenta-
tion. Deep circular constrictions
(fig. 274) bound the somites ex-
ternally. Internally the cclom
is divided by the septa—delicate
double membranes which extend
ae pita in er end from the ectoderm to the alimen-
and Jung.) 1, first segment with MATE Ae = ae ‘ r ahe J
mouth and prostomium; 15, male tary canal—into as many cham
sexual opening; 33-37, clitellum. bers as there are metameres, while
a longitudinal mesentery, also double, separates the coelomic
TI, ANNELIDA: CHA?TOPODA. 807
pouches of the right side from those of the left (figs. 275 and
272). The alimentary canal also shows distinctions; for while it
differs greatly in the various species, it has constantly a terminal
anus, while the mouth is ventral and is overhung by the preoral
segment, the prostomium.
Nervous system, blood-vessels, and excretory organs are influ-
enced by the segmentation. The nervous system is built on the
ladder plan. It begins with asupracsophageal ganglion (‘ brain ’)
lying in the prostomium, from which the esophageal commissures
pass around the esophagus to form the ventral chain, which con-
sists of as many pairs of ganglia, united by longitudinal commis-
Cal Ss / ani
WI PHTT
Fic. 275.—Anterior end of Nais elinguis. h, cerebrum, connected by commissure with
ventral chain, n; dy, contractile dorsal, vg, ventral blood-vessel; m, muscular
aA of skin; db, vb, dorsal and ventral chet; d, septa; k, prostomium; 0,
mouth.
sures, as there are somites present. These ganglia of the ventral
chain are closely similar, since the segmentation of the body is
homonymous. ‘There is but the slightest division of labor among
the somites, and hence they differ but slightly among themselves.
The prostomium always bears tactile organs and frequently eyes,
which in many marine forms are highly developed, with lens,
vitreous body, and retina. Otocysts are rare, but occur in diverse
species. Ciliated pits (olfactory) occur on the head, goblet organs
(taste) on head and trunk, and, lastly, lateral organs, sensory struc-
tures of unknown function, may be metamerically arranged.
The blood-vessels are most frequently represented by two main
trunks which frequently (as in earthworms) contain blood colored
red by hemoglobin. One trunk runs dorsal, the other ventral,
to the intestine, the two being connected by vessels (figs. 272, 276)
in each segment. The blood passes forward in the dorsal vessel,
backwards in the ventral. It is propelled by contractile portions
of the vessels; usually the dorsal vessel pulsates, but, as in the earth-
worms, certain of the circular vessels in the anterior part of the
308 CQ@LHELMINTHES.
body may function as hearts (fig. 276, c). Rarely, as in the
Capitellidx, circulatory organs may be lacking.
The excretory organs (nephridia) were formerly known as
‘segmental organs,’ since they occur in pairs in each segment.
These supplant the embryonic protonephridia; each consists of an
internal ciliated funnel, the nephrostome, a more or less conyo-
¢ oe dg ly a pr st ae
Fic. 276.—Anterior end of Pontodrilus marionis. (After Perrier.) a, vascular arches;
b, ventral nerve chain; ¢, ‘hearts’; co, esophageal commissure; dg, dorsal blood-
vessel; ds, septa; gc, cerebrum; l, retractors of pharynx; lg, lateral blood-vessel:
0, Ovary; oe, @sophagus; Dp, receptacula seminis; ph, pharynx; pt, ciliated
funnels of vas deferens; s, nephridia; vd, vas deferens,
= ae sec sees
/
m up bm
Fie. 277.—Schematic cross-section of an annelid. (After Lang.) ac, aciculum; ),
cheetee ; bm, ventral nerve cord; dc, dorsal cirrus: dp, notopodium, ky, gill: Im,
longitudinal muscles; md, digestive tract; mp, nephridium,; ov, ovary; rm, circu-
lar muscles; tm, transverse muscles; tr, nephrostome; vc, ventral cirrus; vd, vv,
dorsal and ventral blood-vessels; vp, neuropodium.
ov
luted tube, and the external opening (fig. 69). In many instances
(Oligochetes, some Polychvtes) the nephrostome is in one somite,
the external opening in the succeeding. The nephridia also usually
serve as genital ducts, carrying away the reproductive cells, which
III, ANNELIDA: CHA#TOPODA. 309
always arise from the ccelomic epithelium. In the Oligocheta
(fig. 286), besides the nephridia in the genital segments special
oviducts and vasa deferentia occur which are usually regarded as
modified nephridia.
Of the many modifications of nephridia only a few can be noticed here.
Occasionally there may be more than one pair in a somite, or they may
have more than one nephrostome. Again, they may be lacking from more
or fewer somites. In many Oligochztes they may empty into the anterior
or posterior ends of the digestive tract. In many (@lycera, Hesione,
Nephthys, Gontada) the internal ends of the nephridia are branched,
the branches being closed by ‘ solenocytes,’ tubular cells bearing an internal
bundle of cilia.
In many marine annelids there occurs a metamorphosis in which
pelagic larvee occur. These, in spite of their many modifications,
are comparable with the ‘ Loven’s larva,’ the trochophore already
mes
mes a
Pra. 278.—4, larva of Polygordius; B, same changing to segmented worm. (After
Hatschek.) a, anus; kn, excretory organ; mes, segmented mesoderm.
described (p. 306). The differences largely consist of modifica-
tions of the ciliary apparatus; the number of bands may be
increased (polytroche larva), or a single band may occur at the
middle (mesotroche) or at the end (telotroche) of the body. The
larva becomes a segmented worm by the hinder end of the larva
growing out and dividing into segments (fig. 278). In this
310 COLUELMINTHES.
jointed portion the celom arises as a new formation, divided from
the first into separate chambers. The nephridia also arise de
novo, independent of the protonephridial system, which is often
called head kidney because the chief part of the trochophore forms
the head of the adult.
The fresh-water annelids develop directly, but the embryos pos-
sess a reminiscence of an earlier larval life in that the head lobes are
very apparent and contain protonephridia, which leads to the con-
clusion that these animals earlier hada metamorphosis. From the
resemblance of the trochophore to the Rotifera the farther conclu-
sion is drawn that the annelids have descended from Rotifer-lke
ancestors, the body cavity, nephridia, blood-vessels, and veutral
nerve chain being new formations.
Besides sexual reproduction many fresh-water and marine
species may reproduce asexually, this being rendered possible by
the great homonymy of the segmentation. By rapid growth at
the hinder end as well as at a more anterior budding zone numer-
ous somites are formed which separate in groups from the parent
to form young worms. In some cases the formation of new somites
Fig. 279.—Budding in Myrianida. (After Milne-Edwards.) The sequence of letters
shows the ages of the individuals.
may take place more rapidly than the separation, the result being
chains of worms (fig. 279).
By a combination of sexual and asexual reproduction a typical alter-
nation of generations occurs, the origin of which receives light from the
following facts: In many polychwtes which reproduce exclusively by the
sexual process the sexless slowly-moving young (atoke) at sexual ma-
turity becomes so altered in appearance as to have been described under
another name. It becomes very active in its movements, and the hinder
WI. ANNELIDA: CHA#TOPODA. 311
somites, which contain the sexual organs, develop special bristles and para-
podia (fig. 284, 4). Thus many species of Wereis pass into the ‘ Hetero-
nereis’ stage. In other Polychzetes the sexual part (epitoke) separates
from the sexless atoke portion and swims freely, while the atoke produces
new epitokes. In the Samoan Islands Hunice viridis reproduces in this
way, the epitokes coming to the surface at certain times in incredible
numbers, forming the ‘palolo worm,’ a delicacy in the Samoan diet. In
still other species the epitoke regenerates the head and thus becomes an
independent generation. Syllis and Heterosyllis ave thus related. The
Autolytide furnish the most complication. Here the atoke, by budding
as in Myrianida, fig. 279) forms chains of dimorphic individuals which
later separate. The individuals of male chains were formerly described as
‘ Polybostrichus,’ the females as ‘ Sacconereis.’ This same homonymy ex-
plains the regenerative powers of many worms. Thus if certain earth-
worms be cut in two, they will continue to live and will reproduce the lost
parts.
Another important character of the Chetopoda is the posses-
sion of bristles or chet. These arise in special follicles, singly
or several in a bunch, of which usually there are four—right and
left, dorsal or lateral and ventral—in each somite. Each follicle
(fig. 280) is a sac of epithelhum opening on the surface and having
at the base a special cell for the development of each bristle. The
developed bristles project from the follicle and, moved by appro-
Fig. 280.—Arrangement of a bristle in an Oligochete. (After Vejdowski.) ¢, epithe-
lium; rm, Un, circular and longitudinal muscles; m, muscle of the follicle; by,,
cheta follicle, its cheta in function; b2, follicle for replacement, the formative
cell at its base.
priate muscles, form small levers of use in locomotion. Their
numbers, shape, and support are of much systematic importance.
Order I. Polychete.
The Polychxtz owe their name to the fact that each group of
bristles contains many chete; but more important is that the
312 CALUELMINTIES.
bristles of each side are supported by a fleshy outgrowth of the
somite, the parapodium, in which two portions corresponding to the
bunches of bristles—dorsal, notopodium; ventral, neuropodium
—may be recognized (fig. 281). This is the first appearance of
Fig. 281—A, head with protruded pharynx; 8, parapodium of Nereis versipedata.
(After Ehlers.) ¢, cirri; k, jaws; l, head with eyes; p, palpi; t, tentacles.
true appendages, but they differ from those of Arthropoda in
that they are not jointed to the body nor jointed in themselves.
On the dorsal surface may occur diverse outgrowths, known, accord-
Fig, 282.—aAmphitrite ornata.* (From Verrill.)
ing to position or function, as cirri, elytra, gills, ete.; onthe head,
palpi and tentacles. The cirri are long processes on the parapodia,
and like palpi are tactile (fig. 281). Elytra are thin lamelle
which cover the back like shingles and thus protect the body.
Ill, ANNELIDA: CHATOPODA. 3138
Nearly all Polycheetes are dicecious and undergo a more or less pro-
nounced metamorphosis; with few exceptions (Manyunkia*-~from the
Schuylkill, Weveis * in California) they are marine. They are usually
divided according to their habits into fixed (Sedentaria) and free forms
(Errantia), but this classification lacks a morphological basis. The Seden-
taria feed on vegetable matter, usually form tubes of leathery organic
substances, in which foreign matter may be incorporated or which may
be calcified. The worms project their anterior segments from the tubes.
The Errantia often secrete gelatinous tubes in which they can hide, but
they never lose their powers of locomotion, and from time to time leave their
retreats and swim about preying on other animals. Correlated with habits
are differences in structure. In the Errantia the head and trunk are not
very different ; the anterior part of the alimentary tract can be protruded
ag a proboscis, and, corresponding to their predaceous habits, is often
armed with strong jaws (fig. 281, A). In the Sedentaria there are no such
pharyngeal teeth, but, on the other hand, there is a greater difference
between anterior and posterior somites. In the
latter the parapodia are weakly developed, and
this part resembles the Oligochetes in ap-
pearance. The head and beginning of the
trunk (thorax) are richly provided with gills
and tentacles for respiration and the taking of
food (fig. 282).
Sub Order I. ERRANTIA. Predaceous
annelids with strongly armed pharynx. The
EUNICID£, mostly represented on our shores by
small species, contains some species a yard in
length. Diopatra,* Nothria.* The ALCIOPIDE
are transparent pelagic forms with large, highly
Fie. 283.—Head of Polynoe
developed eyes. The SyLuIp# usually have © spinifera (After Ehlers.)
three long tentacles ; Avtolytus,* Myrianida* Faek entirely covered win
(p. 210). The PoLynoib& * (Lepidonotus,* Poly- projecting at the sides.
noe* (fig. 284), Aphrodite aculeata,* the sea
mouse, 6 inches long) are bottom forms with elytra covering the back.
NEREIDE ; Nereis virens,* the clam worm of all northern seas.
Sub Order II. SEDENTARIA (Tubicola, Cryptocephala). These forms
cannot wander about at will, but live in their tubes. In the SABELLI-
D& the tube is membranous and there is a crown of gills ; Myxicola,*
Chone,* Manyunkia.* In the SERPULID& the tube is calcified and is
closed by an operculum on one of the gills. Hydroides;* Spirorbis,*
forming coiled tubes on seaweed ; Protula.* The ARENICOLIDA,* which
burrow in sand, have gills on the sides of the body. The MaALDANID#®
(Clymene,* Azxiothea*) have extremely long segments and build tubes
of sand. The TEREBELLID (Terebella,* Amphitrite (fig. 282), Thelepus *)
have numerous filiform tentacles and branched gills on the anterior end.
The ARCHIANELLID.E, which lack bristles and parapodia,
must be placed near the Polychxte and are usually regarded as very
3814 C@ZLHELMINTHES.
primitive forms which in structure and development (fig. 250) are
of importance in the phvlogenesis of the Annelids. Polygordius.*
Fia. 284.—New England Annelids. A, male Autolytus ; B, Sternaspis fossor ; C, Cise
tenides gouldii; D, Clymene torquata. (From Emerton and Verrill.)
Order II. Oligochete.
The Oligochetes are almost as preeminently fresh-water and
terrestrial forms as the Polychetes are marine. They are in
most respects simpler than their marine relatives, apparently the
result of degeneration, which has followed from the more simple
conditions of existence. Eyes are rudimentary or lacking, they
have no palpi, cirri, or tentacles; gills are rare, but most striking
is the absence of parapodia, the bristles projecting directly from
the body wall (fig. 280). The chet may be regularly distributed
around each somite (Pericheta) or gathered on the sides (Jfegas-
cole) or arranged in four bunches so that in the animal four
longitudinal rows occur. The animals are hermaphroditic, testes
and ovaries lying in different somites. Usually the integument in
the neighborhood of the sexual openings is thickened by the
ITI, ANNELIDA: CILHTOPODA. 315
presence of numerous glands, forming a clitellum (fig. 274), which
secretes the egg cocoons. The clitellum also functions in copula-
tion, secreting bands which hold the animals together so that
the sperm from one passes into the receptaculum seminis of the
other. After impregation the eggs are usually enclosed in
cocoons.
Sub Order I. LIMICOLA (Microdrili.) Mostly fresh-water forms.
The TUBIFICIDA, in consequence of the red blood, when present in large
numbers color the bottom red. They quickly retract into the tubes which
FIG. 285.—Aulophorus vagus, in tube of Pectinatella statoblasts. (After Leidy.)
they form in the mud. Z'ubifea,* Petoscolex ; Clitellio inoratus * common
on our seashores. The NaIpID# are transparent forms living on water
plants which reproduce asexually throughout nearly the whole year.
i ee
Ot ow br dito a
Fia. 286.—Sexual organs of Lumbricus herculeus (after Vogt et Jung); the seminal
vesicles of the right side cut away. bm, ventral nerve cord; bl, bv, lateral and ven-
tral rows of sete; di, septa; hy, he, testes, enclosed in sperm reservoir; 0, ovaries ;
ov, oviducts; sbu, sperm reservoir ; sh,, 4 3, Sperm sacs (seminal vesicles) : sty
seminal receptacles; t,, te, seminal *funne
to, funnels of oviducts; vd, vas deferens.
Dero* and Aulophorus* have gills around the anus. ENCHYTR#IDA;
Distichopus, Pachydrilus. The DiscopRitips& (Bdellodrilus, Myzobdella)
are parasitic.
Sub Order II. TERRICOLA (Macrodrili). Here belong the terrestrial
forms, the earthworms, our species of moderate size, in the tropics large
s connected with the vasa deferentia ;
316 COLHELMINTHES.
species (Megascolex australis four feet long). Our species belong to
Lumbricus,* Allobophora*; Pericheta* has been introduced from the
tropics; Diplocardia * with double dorsal blood-vessel. Most species agree
in habits; they burrow through the earth, swallowing the humus and
casting the indigestible portions on the surface. They loosen the soil and
are continually bringing the deeper parts to the surface, and thus do great
good. Contrary to oft-repeated statements, earthworms occurred in our
prairies and plains when first broken up by the plow. Details of the
reproductive organs of one species are shown in fig. 286. These vary
greatly and are largely used in classification.
Sub Class II. Gephyrea.
The exclusively marine Gephyrea are distinguished at the first
glance from the Chetopoda by the
entire absence of segmentation. The
body is oval or spindle-shaped, circular
in section. The mouth, at the ex-
treme anterior end, is either surround-
ed by a circle of tentacles (fig. 287)
and is retracted together with the
anterior end of the body by internal
retractor muscles, or is overhung by a
Fig, 287. Fig, 288.
Fie. 287.—Anatomy of Phascalosoma gouldi (orig.). a, anus; a, anterior retractors;
d, digestive tract; g, gonads; m, mouth; 7, nephridia; nce, ventral nerve cord;
pr, posterior retractors.
Fie. 288.—Larva (trocophore) of Echiurus. (After Hatschek.) a, anus; d, intestine;
hw, postoral cilia; kn, protonephridia; m, mouth; mes, mesoderm bands with indi-
cation of segments; 7, ventral nerve cord; sc, esophageal commissure; sp, apical
plate; vw, preoral ciliated band.
III. ANNELIDA: GEPHYRAA. 317
dorsal spatulate preoral lobe or ‘ proboscis’ which may be several
times the length of the body and forked at its tip (fig. 289).
Internal segmentation is also lost, septa being entirely lacking.
The nephridia are also reduced in number, at most but three pairs
being present, and in some but a single unpaired organ. They
are sexual ducts, and in the Chetiferi there are special excretory
organs (fig. 289, g) covered with branching canals opening to the
body cavity by nephrostomes and emptying into the intestine. These
resemble somewhat the branchial trees of the holothurians (infra),
and hence these animals were formerly supposed to bridge the gap
between holothurians and annelids, whence the name (ye@upa,
bridge) Gephyrea. The vascular and nervous systems are more
like those of other annelids. The vascular system consists of a
dorsal and usually a ventral longitudinal trunk; the nervous
system of a brain, esophageal collar, and ventral cord, the latter
without division into ganglia. The relations of the Gephyrea
to the Chetopoda are shown by the development. In some
(Chetiferi) there is a trochophore (fig. 288) from which the worm
arises, as in the Chetopoda, by growth at the hinder end; this at
first has a segmented ceelom and nervous system, the metamerism
being lost later.
Order I. Chetiferi (Armata, Echiuroidea).
With spatulate preoral lobe, often forked at the tip; at least a pair of
ventral sete; a trochophore in development. Hchiurus pallasti* in our
northern waters, Thalassema* farther south. In Bonellia there is a
marked sexual dimorphism (fig. 389). The female is 2 to 3 inches long and
has a proboscis 8 to 12 inches long. The male is only 1 mm. long, totally
different in appearance, and lives parasitically in the cesophagus of the
female (fig. 289, B).
Order II. Inermes (Achzta, Sipunculoidea).
Distinguished by lack of chzete, the mouth surrounded by tentacles,
and the dorsal and anterior position of the anus. The larva is a modified
trochophore without preoral ciliated band and without segmentation; only
two, sometimes but one, nephridia. Phascalosoma* common on our
shores. Phascolion strombi* builds tubes in deserted snail shells. Sipun-
culus.*
Order III. Priapuloidea.
No tentacles, mouth with chitinous teeth, terminal anus, no nephridia;
two protonephridia united with the sexual organs and opening either side
of the vent. Development unknown. Priapulus, Halicryptus.
318 COG@LHELMINTHES.
Sub Class III. Hirudinet (Discophort).
Three points of external structure clearly distinguish the leeches
from the chetopods. First, the absence of bristles (except in
Acanthobdella) and the presence of two suckers, one of which occurs
on the posterior ventral surface and is used only for attachment
and locomotion, the other, sometimes scarcely differentiated,
Fia. 289.—Bonellia viridis. A, female (after Huxley); B, male (after Spengel). cc,
cloaca; d, rudimentary intestine; g, excretory organ; 7, intestine; m, muscles sup-
porting intestine; s, balls of spermatozoa in B, in A, proboscis (preoral lobe); u,
single segmented organ, functioning as oviduct; vd, nephridium with ciliated
funnel serving as vas deferens.
surrounds the mouth and is used in sucking the food. In locomo-
tion anterior and posterior suckers are alternately attached, the
body being looped up and extended like that of a ‘span worm.’
The animals can also swim well by a snake-like motion of the
whole body.
A second point is the fine ringing of the body, there being
usually many more rings than somites, the primitive segment rings
being divided by secondary constructions, there being three, five,
or even eleven rings toasegment. The middle or one of the
anterior rings is often distinguished by bearing strongly developed
sense organs. Asinthe earthworms, certain of the somites at the
time of reproduction may develop into a clitellum which secretes the
egg cocoons.
III, ANNELIDA: HIRUDINEI. 319
A third character is the marked flattening of the body in the
dorsoventral direction (except in Ichthyobdellide and a few other
forms), the animals thus recalling the flatworms. This may be
the result of the very slight development of the celom. In most
leeches there is a body parenchyma, traversed by longi-
tudinal transverse and dorsoventral muscles in which
the organs are immediately imbedded (fig. 290).
The alimentary tract is provided with paired
diverticula (fig. 291), varying in number in different
species. Between the last and largest pair of these
sacs is the intestine, which opens dorsal to the pos-
terior sucker. The jawed and jawless leeches show
considerable differences in the pharyngeal region.
Fie. 290.
Fic. 290.—Transverse section of Hirudo medicinalis. (From Lang.) dm, lm, rm, dorso-
ventral, longitudinal, and circular muscles; vl, vd, vv, lateral, dorsal, and ventral
blood-vessels, the latter surrounding the ventral nerve cord, ni; h, testes; vd, vas
deferens; md, midgut; np, nephridial tubule; enp, urinary bladder.
Fig. 21.—Digestive tract of Hirudo medicinalis. (From Lang.) a, oesophagus; b, in-
testine; d,, dy, gastric diverticula.
In the first there are three jaws in the phaynx, semicircular chitin-
ous plates, the free edge of each armed with numerous calcified
teeth (fig. 292). To these are attached two muscles, one to retract
them, when not in use, into pockets, while the other exserts them
and rotates them, causing a triradiate wound from which the blood
flows. This bleeding is difficult to staunch, since glands on the
lips and between the jaws produce a secretion which hinders the
coagulation of the blood, In the jawless leeches a sharp conical
process arising from the pharynx can be protruded from the
mouth, and serves for wounding and sucking. The vascular
system usually contains red blood, and consists, in the Gnatho-
bdellidw, of four longitudinal trunks, a dorsal, two lateral, and a
320 C@LHELMINTHES.
ventral, the latter surrounding the ventral nerve cord. These are
connected by a complicated system of capillaries.
The nervous system consists of brain and ventral cord, the Jat-
ter containing frequently twenty-three ganglia (the first of five
fused, the last of seven). Nerves from the brain go to the eyes.
Right and left of the ventral cord are the hermaphroditic sexual
organs. In Hirudo medicinalis (fig. 293) there are nine pairs of
Fie. 292. Fic. 293.
Fie. 292.—Hirudo medicinalis, medicinal leech. (After Leuckart.) a, anterior end
with three jaws (x); 6b, a single jaw with its muscles.
Fig. 293.—Nervous system, blood-vessels, sexual organs, and nephridia of a leech,
ventral view. h, testes; hb, urinary bladder; /g, lateral blood-vessel; n, ventral
nerve cord; nh, epididymis; 0v, ovary; p, penis; sc, nephridia; uw, uterus and
vagina; vd, vas deferens; vg, ventral blood-vessel.
testes (2), the ducts of which unite to form a vas deferens on
either side (vd). These pass forward, form by coiling a so-called
epididymis (xk) and empty into the median unpaired penis (p).
In the space between the epididymis and the first pair of testes
are the ovaries (ov) and oviducts and an unpaired vagina (wz). The
nephridia (17 pairs in this species) are complicated and are pro-
vided with bladder-like expansions.
That the Hirudinei are true annelids and not segmented Plathelminthes
is based upon the view that their ccelom is reduced by ingrowth of paren-
chyma and altered to canals connected with the vascular system. At
any rate the ventral and lateral vessels are to be regarded as remnants of
acelom. In Clepsine there are the dorsal and ventral blood-vessels of
the Cheetopoda and besides four longitudinal ccelomic sinuses connected
by transverse anastomoses. The dorsal sinus encloses the dorsal blood-
vessel, the ventral many of the viscera, among them the ventral nerve
cord, This is also to be regarded as ewlomic, since the nephrostomes con-
nect with it. In most Hirudinei a canal system filled with blood has
arisen from the calom and blood-vessel, and in Nephelis is in part lacunar
in character. Further relations are shown by Acanthobdella peledina,
parasitic on fishes. This has both blood-vessels of the Oligocheetes, a
IV. POLYZOA. 321
body cavity divided by septa and chete. On the other hand it is leech-
like in other features; two suckers and sexual apparatus on the Hiru-
dinean pattern.
Order I, Gnathobdellide.
The jawed leeches include Htrudo medicinalis, once extensively used
for blood-letting but now little employed. Ha@madipsa includes land
leeches, one of the terrors of travelers in the tropics. In Wephelis* the
jaws are soft. Dacrobdella * includes our largest native species.
Order II. Rhynchobdellide.
Without jaws. The CLEpPsINIDH mostly feed on snails and fishes.
Clepsine* in our waters. Hementaria officinalis of Mexico is used for
blood-letting ; H. ghiliant of South America is poisonous. The IcHTHyo-
BDELLIDA, cylindrical, occur in salt and fresh water, parasitic on fishes.
Ichthyobdella,* Pontobdella,* marine ; Piscicola, fresh water,
Class IV. Polyzoa (Bryozoa).
In external appearance the Polyzoa closely resemble the
hydroids, so that the inexperienced have difficulty in distinguishing
them. Like them by budding they form colonies which are either
gelatinous or calcareous incrusting sheets or assume a more bush-
like character. Further they have a crown of ciliated tentacles
which can be spread out or quickly retracted. In internal charac-
ters the two groups are greatly different. The Polyzoa have a
complete alimentary canal, with its three divisions, which is bent
upon itself so that the anus lies near the mouth. The central nerv-
ous system lies between mouth and anus, and the single pair of
nephridia empty by a common opening. Beyond this it is diffi-
cult to go, since the two groups of Polyzoa—Endoprocta and Kcto-
procta—differ so widely that one may doubt whether they belong
together. The Entoprocta have no celom, and resemble in this
respect the Rotifera; the Ectoprocta are true Celhelminthes and
by way of Phoronis show resemblances to the Sipunculoida and so
to the Annelida.
Sub Class I. Entoprocta.
The single individuals of the Entoprocta (fig. 294) are shaped
like a wine-glass and are placed on stalks which rise from (usually)
creeping stolons. The circle of tentacles, arising from the edge
of the cup, enclose the peristomial area in which are both mouth
and anus, and between these the excretory and reproductive organs
322 CQLHELMINTHES.
open. The Bpaee between the horseshoe-shaped intestine and the
body surface is completely filled by a pa-
repchyma containing muscle cells, and
correspondingly the excretory organs are
protonephridia. In our fresh-water Urna-
tilla gracilis * these organs are branched
and begin with flame alls Pedicellina*
and Loxosoma, marine.
Sub Class II. Ectoprocta.
In the Ectoprocta there is a spacious,
often ciliated, celom between the alimen-
tary canal and skin, so that these are
separated and have a certain amount of
independence (fig. 295). On this account
Tee Sone Sigee ee has arisen a peculiar method (wholly
years is pee ae indefensible morphologically) of treating
J, intestine; 7, tentacles: Vs them as two individuals, polypid, the in-
testine and tentacles; cystid the rest, especially the body wall and
skeleton.
Fig. 295.—Flustra membranacea (after Nitsche), asingle animal. a,anus; ek, ectocyst;
en, entocyst; f/, funiculus; g, ganglion; hk, collar which permits complete retrac-
tion; m, stomach, aiso dermal muscular sac; 0, esophagus. A, avicularium; B,
vibracularium of Bugula. (After Clapardde.)
The cystid is cup-shaped or saceular. It consists of an endo-
cyst—the body wall—and an ectocyst—a cuticular skeleton, usually
strongly calcified, secreted by the ectoderm. The surface of the
IV. POLYZOA: ECTOPROCTA. 328
entocyst is always covered by the ectocyst only on the basis and side
walls: the peripheral end remains soft and forms a sort of collar
into which the tentacles and adjacent parts of the cystid can
be retracted. In the ectocyst there is, as will be seen, a larger or
smaller opening which in many species (Chilostomata) can be
closed by a lid (operculum). The circle of tentacles surrounds
the mouth alone, while the anus is outside near the collar. The
strongly bent alimentary canal extends into the cystid and is bound
at its hinder end by a cord, the funiculus, to the base of the cystid.
Ganglion and nephridia lie between the mouth and anus. The
gonads arise from the epithelium of the celom, the testes usually
on the funiculus, the ovaries on the wall of the cystid.
Hundreds and thousands of individuals form colonies (fig. 297)
in which cystid abuts against cystid. The ceelom of adjacent cystids
may be distinct or a wide communication may exist. The colonies
grow by budding; in the Gymnolemata a part of a cystid becomes
eut off as a daughter cystid in which the polypid—alimentary tract
and tentacles—arises by new formation; or (Phylactolemata) the bud
anlage of the polypid arises before the first appearance of the cystid.
Division of labor or polymorphism is common. Besides the
animals already described, which are primarily for nourishment,
three other individuals may occur, ovicells, vibracularia, and avic-
ularia. All three are cystids which have lost the polypid. The
ovicells are round capsules which serve as receptacles for the
fertilized eggs. The vibracularia (fig. 295, B) are long tactile
threads; the avicularia (fig. 295, A) are grasping structures of
uncertain function. They have been seen to seize small animals
and hold them until decay set in. It is possible that the fragments
serve as food for the polypids. The avicularia have the form of a
bird’s head, the movable lower jaw being a modified operculum.
Under unfavorable conditions a polypid in a cystid may break down
and be lacking for some time until better relations cause its new forma-
tion. Besides in the depopulated cystids there may appear statoblasts,
lens-shaped many-celled internal buds enveloped in a firm envelope which
form a resting stage for the preservation and distribution of the species.
Each statoblast is surrounded by a girdle of chambers which by drying
become filled with air, causing the statoblast to float when it again comes
into water. From the statoblast a smaller polyzoon escapes which de-
velops a new colony. The statoblasts are adaptations to the conditions
of fresh-water life and occur only in the Phylactoleemata.
Order I. Gymnolemata (Stelmatapoda).
The tentacles in a ring around the mouth. The numerous species are
almost exclusively marine and are abundant on every coast. In the
3824 CQ@LHELMINTHES.
CHILOSTOMATA the cystids can be closed by an operculum: Gemmel-
laria,* Cellularia,* Bugula,* Flustra* (fig. 295), Eschara.* The
CYCLOSTOMATA have tubular cystids without an operculum. Crisia,*
Fie. 296.—American gymnolematous Polyzoa. (After Busk, Hincks, Norman, and
Verrill.) A, Uubulipora flabellaris, young; B, Flustrella hispida; C, Eucratea
chelata ; D, Gemellaria loricata ; E, Kinetoskias smitti; F, Membranipora spini-
Sera; G, Porella levis ; H, Lepralia americana: I, Cribillina puncturata.
Tubulipora,* Hornera.* In the CTENOSTOMATA the cystid is more cal-
careous and the opening is closed by a folded membrane. Alcyonidiwim,*
Vesicularia, Valkeria,* Paludicella * (fresh-water).
Order II. Phylactolemata (Lophopoda).
Tentacles borne on a horseshoe-shaped lophophore extending on either
Fie. 297.—Small colony of Lophopus crystallinus (after Kraepelin), with young and
old, some extended, others more or less retracted. 0, statoblasts.
side of the mouth, the tentacles on its margins. All are fresh-water
species. Pectinatella,* Lophopus (fig. 227), Plumatella*
V. PHORONIDEA. VI. BRACHIOPODA. 825
Class V. Phoronidea.
The single genus Phoronis* occurs on our eastern shores.
The animal was first placed among the Chetopoda on account of
its worm-like body situated in a chitinous tube like many sedentary
annelids. Then it was placed in the Polyzoa, with which it is
more nearly related. The young, described as Actinotrocha, is a
modified trochophore with the mouth overhung by a large hood
and the postoral band of cilia drawn out into a series of fingers
which become the tentacles of the adult; the anus is terminal.
At the time of metamorphosis this larva becomes doubled on itself
by a complicated process, so that the alimentary canal is U-shaped
and the anus is near the mouth, while the tentacles are borne on a
horseshoe-shaped basis around the mouth.
Class VI. Brachiopoda.
On account of the bivalve calcareous shells the Brachiopoda
were long regarded as molluscs, but later the fact that the valves
are not paired as in the lamellibranchs, but are dorsal and ventral,
that the nervous system, the excretory and reproductive organs,
the body cavity, and the development resemble those of the annelids
rather than those of the molluscs, led to their recognition as a dis-
tinct class allied to the former group.
The body has a greatly shortened long axis (fig. 298) and in
consequence a transversely oval visceral sac. In most a stalk (st) for
Fic. 298.—Anatomy of Rhynchonellu psittucea. (After Hancock.) a’, left, a%, right
arm; a, opening into the cavity of the arm: d, intestine; ¢, blind end of the intes-
tine; g, stomach with liver; m, adductors and divaricators of shell; 0, esophagus;
p’, p®, dorsal and ventral mantle lobes; st, stalk; 1, 2, first and second septum, on
the second a nephrostome.
attachment arises from the posterior end. From the anterior side
two folds, the mantle lobes, extend forwards (jp), one ventral, the
326 CQ@LUELMINTHES.
other dorsal, their free edges bearing bristles. Each mantle
secretes a shell largely composed of carbonate and phosphate of
lime. In a few the dorsal and ventral shells are similar, but
usually the ventral valve (in Crania attached directly without the
Fig. 299.—Waldheimia flavescens. (From Zittel.) Shell with arms and muscles. a, arm
with fringed border (h): c, c’, divaricators; d, adductors; D, hinge process (the
vertical line shows position of hinge).
intervention of a stalk) is more strongly arched and has an opening
at the posterior end for the passage of the stalk (figs. 299, 300).
The flatter dorsal valve frequently bears a characteristic feature in
the skeleton of the arms (fig. 300) which, when present, has greatly
Fie. 300.—Waldheimia flavescens. (From Zittel.) .A, dorsal, B, ventral valve; a, b,c,
impressions of muscular insertions; a, adductors; 0’’, adjustors (stalk muscles):
c,c’, divaricators; s, hinge groove of upper valve in which the tooth (t) of the
lower valve passes ; /, support of arms; d, deltidium; /, foramen for stalk.
different expression. Its basis consists of two calcareous rods
which, bilaterally symmetrical, project downwards from the dorsal
valve. These may be connected by a curved transverse band, and
from their ends a spiral process may extend on either side. This
apparatus supports the spiral arms. When closed the valves com-
pletely enclose the body. When they open the gape is anterior,
VI, BRACIIOPODA. BOT.
the posterior parts remaining in contact. At this part, except in
the Ecardines, a hinge is developed just in front of the posterior
margin, consisting of projections (teeth) in the ventral valve which
fit into corresponding grooves in the dorsal. Opening and closing
the valves are, contrary to what occurs in Lamellibranchs, active
processes, accomplished by appropriate divaricator and adductor
muscles (fig. 299). These produce scars on the shell, important in
the study of fossil forms.
The usually spirally coiled arms, which lie right and left of the
mouth and which give the name to the class, fill most of the shell.
On the outer side of the spiral axis runs a longitudinal groove
which reaches to the tip of the arms and is bounded by a row of
small tentacles. By means of cilia on tentacles and groove food is
brought to the mouth, These arms strongly resemble the lopho-
phore of a phylactolemate Polyzoan, which only needs extension
and coiling to produce this condition. In development the arms
of the Brachiopod pass through a lophophore stage.
In the body there is a body cavity which extends into both
mantle folds. It encloses alimentary tract, gonads, and liver, and
is divided into right and left halves by a dorsal mesentery support-
ing the intestine. Each half in turn is divided by incomplete
septa into anterior, middle, and posterior divisions recalling those
of Sagitta (p. 296). If the arrangement of the septa is not so
clear ag in that form, it is to be explained by the shortening of the
long axis and the twisting of the alimentary tract. This latter
consists of esophagus, stomach, which receives the liver ducts, and
intestine, which in some species terminates blindly.
The gonads are chiefly in the mantle lobes. The sexual cells
pass outwards through the nephridia, which begin in one cwlomic
pouch with a wide nephrostome, perforate the septum, and open
to the exterior in the next somite. Since usually there are two
septa, two pairs of nephridia may occur, hut one is usually degen-
erate. The nervous system consists of an esophageal ring with
weak dorsal ganglion, which sends nerves into the arms, and a
stronger ventral mass representing the ventral chain. The heart
lies dorsal to the stomach.
In development the brachiopods recall both Sagitta and the Annelida.
They resemble Sugitta in thatin Argiope the colom arises by out-
growths from the archenteron, divided by septa into three pairs of pouches.
They are annelid-like in the form of larva and in the presence of chetz
which are formed in separate follicles. In an earlier period of the earth
brachiopods were so numerous in species and individuals that they are
among the most important fossils in the determination of geologic horizons.
328 C@LHELMINTHES.
Now there are but few species, some inhabitants of the greatest depths of
the sea.
Fie. 301.—Development of brachiopod. (After Kowalevsky.) A, gastrula with early
enteroccelic pouches; B, closure of blastopore; C, celom separated, body annu-
lated; D, cephalic disc and mantle developing, the latter with long seta; E, at-
tached embryo, the mantle lobes folded over cephalic disc (seta omitted). ¢,
ee disc; d, dorsal lobe of mantle; e, enteroccele; m, mantle; v, ventral man-
e lobe.
Order I. Ecardines.
Hinge absent; valves similar when the stalk passes out between them
(Lingula *), or unequal when the ventral is perfo-
vated by the stalk (Disctna) or when the animal is
directly attached by the shell (Crania).
Order II. Testicardines.
Hinge present, valves unequal, the ventral
perforated by the stalk; anusdegenerate. Rhyn-
chonella,* Terebratulina* in our colder waters.
Fic. ie age alee ee tae oe Spirifer, Orthis, Pentamerus, Atryna, important
tentrionculis,* fossil gener.
Summary of Important Facts.
(1) The CELHELMINTHES are characterized by a well-developed body
cavity (ecelom).
(2) The CH&TOGNATHI are hermaphroditic worms with three pairs of
celemic pouches, with fins, and bristle-like jaws.
(8) The NeEmaTopa are mostly di@cious, usually parasitic elongate
worms, with cylindrical unsegmented body, with nerve ring (no ganglia),
paired excretory organs, and tubular gonads.
(4) The most important species parasitic in man are Ascaris lumbri-
coides in the small intestine, Oxvyuris vermicularis in the large intestine,
the blood-sucking AnAylostoma duodenalis, and the notorious Trichina
spiralis, In hot climates occur Filaria sanguinis hominis and Dracun-
culus medinensis,
(5) The GoRDIACEA have mesenteries and splanchnic mesoderm; they
are parasitic in insects.
(6) The ACANTHOCEPHALI lack alimentary tract, have a spiny proboscis
and a very complicated reproductive apparatus. The adults are parasitic
in vertebrates, the young in arthropods,
ECHINODERMA. 329
(7) The CH&TOPOoD ANNELIDS have segmented bodies, the segmentation
showing itself in ringing of the body wall and in the separation of the
ceelcem into a series of pouches by transverse septa and the metameric
arrangements of blood-vessels, ganglia, and excretory organs.
(8) The CHa&TopopDA are distinguished from other annelids by the
cheete (usually four bunches in a somite) arising in special follicles. The
chvetee are few in the hermaphroditic Oligochetz, numerous and borne on
special parapodia in the Polychete.
(9) The GEPHYR#A are closely related to the Cheetopoda. They are
saccular, with a crown of tentacles or well-developed preoral lobe. They
have largely or entirely lost the segmentation. Evidence of segmentation
appears in some cases in development and in the ventral nerve cord and
nephridia,
(10) The HrrupINEI are hermaphroditic Annelida which lack cheete
but have sucking discs. Their flattened bodies, rudimentary ccelom, and
rich body parenchyma give them a certain similarity to the Plathelmin-
thes.
(11) The Hirudinei have either a protrusible pharynx (Rhynchobdella)
or three toothed jaws (Gnathobdella). To the latter belongs the medici-
nal leech (Hirudo medicinalis).
(12) The Potyzoa are like the Hydrozoa in being colonial and having a
cireumoral crown of tentacles. They are distinguished by the complete
alimentary canal, the large celom, and the ganglionic nervous system.
(13) The PHoronIDEA are closely like the Polyzoa.
(14) The Braculopopa have a bivalve shell, the valves being dorsal and
ventral.
(15) The body cavity is divided by two septa into three (paired) cham-
bers, of which one, rarely two, are provided with nephridia.
(16) Most brachiopods are attached by means of a stalk. They are
divided into Ecardines, without a hinge and with anus, and Testicardines,
with a hinge and no anus.
PHYLUM V. ECHINODERMA.
The Echinoderma are separated from most other animals by
their radial symmetry, but recall in this respect the Celenterata,
a fact which led to their inclusion by Cuvier in the group
‘Radiata,’ a view of their relationships which was set aside by
Leuckart on account of their different structure, especially the
presence of ac@lom. In fact the radial symmetry of the echino-
derms has a different value, for while in the Coelenterata the
number four or six (apparently derived from four) is fundamental,
Echinoderma are, with few exceptions, five-radiate. Further, the
radial symmetry of the Cclenterata is primitive, that of the
Echinoderma, as development shows, is derived from the bilateral
330 ECIINODERMA
type. In other words, the echinoderms have descended from
bilateral, possibly worm-lke, ancestors.
The structure of the integument gives these animals a charac-
teristic appearance. In the mesoderm under the
epithelium calcareous plates arise, forming a body
armor or test, and since these are usually pro-
duced into spines, they have given the name
Echinoderma, spine skin, to the group. This
mesodermal skeleton at times becomes degenerate,
as in the Holothurians (it rarely entirely disappears
as in Pelagothuria), but even then shows itself
as spicules and ‘wheels’ of lime. The spheridia
and pedicellaria (fig. 303)—not always present—
PIG. a close eg are characteristic appendages of the integument.
open The first are sense organs; the latter are usually
stalked forceps-like grasping structures with calcareous skeleton.
In life they are active and apparently either clean the skin or are
defensive.
Certain plates possess a morphological interest since they appear early
in many larve, and in the adults of different classes can be recognized in
similar positions. In the neighborhood of the arms are five basalia, inter-
radial in position, farther five radialia (‘apical skeleton’) and five inter-
radial ‘ oralia’ around the mouth.
Not less characteristic than the skeleton is the ambulacral (or
water-vascular) system (fig. 304). This begins usually externally
and then ordinarily by a calcareous
plate, the madreporite, which is
perforated with fine pores and
serves for the entrance of sea water.
The water passes into a canal
which, on account of its calcified
walls in the starfish (fig. 305), is
culled the stone canal and leads
Fa. 304. Fia. 305.
Fig, 304.—Water-vascular system of starfish (orig.). a, ampulle ; ab, ambulacra; ¢,
radial canal; m, madreporite; 7, radial nerve; p, Polian vesicle; 7, ring canal,
beneath it the nerve ring; s, stone canal; t, racemose vesicle.
Fic. 305.—Transverse section of stone canal of Astropecten aurantiacus. (After
Ludwig.)
ECHINODERMA. 351
orally to a ring canal around the mouth. The ring canal bears
usually several (up to five pairs) Polian vesicles, which, with
Tiedemann’s vesicles of the starfishes, are now regarded as appen-
dages which, like lymph glands, produce the leucocytes. From
the ring canal radiate five radial canals which give off right and
left in pairs the ambulacral canals. These in turn connect with
the ambulacra and ampulle, the highly characteristic locomotor
organs of the echinoderms. An ambulacrum is a muscular sac
which can be distended and lengthened by forcing in fluid from
the ambulacral vessels, on the other hand can be retracted and
shortened by its muscles. The ampulla is a sac connected with
the ambulacrum and projecting into the body cavity. In locomo-
tion the animal extends its ambulacra, anchors them by the suck-
ing dise at the tips, and then pulls the body along by contraction of
the ambulacral muscle. In the sessile crinoids and the ophiuroids
(which move by their snake-like arms) the ambulacra are not
locomotor but tactile in function, lacking suckers and ampulle.
So among the holothurians and sea urchins the ambulacra are in
many places replaced by tentacles. Frequently each radial canal
ends in an unpaired tentacle with olfactory functions.
The arrangement of the ambulacral system conditions the
arrangement of other organs. Alongside the stone canal is a
saccular organ formerly called the ‘heart,’ but now regarded as a
lymphoid gland (ovoid gland, paraxon gland). Ring and radial
canals are accompanied by corresponding blood canals, with which
are often associated two vessels to the alimentary canal. There is
a similar nerve ring and radial nerve, frequently in the ectoderm,
to which may be added an enterocelic or apical nervous system,
possibly of peritoneal origin.
The courses of the radial vessels and nerves mark out five chief
lines in the animal, the radii; between them come the secondary
radii or interradii. The stone canal, madreporite, and lymphoid
gland are interradial in position, as are the gonads, usually five
single or five pairs of racemose glands; in some cases but one is
present. The gonads are supported in the spacious cceloin by
special bands, while mesenteries support the alimentary tract and
its derivatives.
Respiratory organs are represented by very various structures: branchie,
or thin-walled outpushings of the ccelom, either around the mouth, as in
Echinoidea, or on the aboral surface, as in the Asteroidea, the burs of the
Ophiuroidea, the branchial trees of the Holothuroidea and the various
parts of the ambulacral system.
332 ECHINODERMA.
The Echinoderma are exclusively marine, occurring in large
numbers even in the deepest seas. Many groups, like the Crinoids,
are largely bathybial, others frequent rocky coasts. At the period
of reproduction the urchins, starfish, and holothurians frequent
the shallow waters, passing their sexual cells into the sea, where
fertilization occurs. In some, however, the young are carried
about in brood cases until the earlier developmental stages are past.
Fie. 306.—Echinoderm larve. (After J. Miller.) a, anus; m, mouth; the black line:
the course of the ciliated bands. J, form common to all: JJ, IJ, developmental
stages of auricularia (Holothurian); IV, V, stages of the Asteroid bipinnaria;
VI, pluteus of a spatangoid; VJJ, larva (Brachiolaria) of Asterias (orig.). m,
mouth; v, vent.
Where there is no brood pouch the young escape from the egg
as larvee which swim at the surface, and are distinguishable from
the adults (fig. 306, 7) by their soft consistency, transparency, and
bilateral symmetry. By the development of lobe-like processes
and slender arms supported by calcareous rods the larve assume
the most different and bizarre shapes (plutei of echinoids and
ophiuroids, brachiolaria and bipinnaria of asteroids, auricularia of
holothurians), all of which can be referred back to a common type
with tri-regional alimentary tract and a ciliated band around the
mouth, strikingly resembling tornaria, the larva of Balanoglossus.
The different appearances of the larve are due to the drawing out
of the ciliated band into lobes and arms, and also to its becuming
broken into parts which unite themselves into complete rings
(fig. 306, V).
The metamorphosis of the bilateral larva into the radial adult is very
complicated, It begins early with the formation of outgrowths from the
archenteron (fig. 307), which become separated and form the anlagen of
I, ASTEROIDEA. 333
the celom and ambulacral system. This becomes divided, and one por-
tion develops itself as a ring around the cesophagus, the future ring
canal, and from this five outgrowths, the radial canals, arise. Since these
canals, as they grow out, carry the body walls before them, the arms in the
starfishes, which show the process most clearly, arise as outgrowths which
recall buds (fig. 308). This has given rise to one view which regards the
arms as individuals, the whole body (and hence that of all echinoderms) as
a colony of five individuals. According to this the development would be
a kind of alternaticn of generations, the larva being the asexual genera-
F1a. 307. Fia. 308.
F 1G. 307.—Formation of the celom in Echinus. (From Korschelt and Heider.) 4,
first anlage of ccelom; B, later stage; C, complete constriction of c@lom (vaso-
peritoneal vesicle) from archenteron
Fra. 308.—Formation of Ophiuran from the pluteus larva. (After Miiller, from Kor-
shelt-Heider.)
tion which by budding produces the colony. Yet this view does not agree
with the actual relations, since it draws an untenable contrast between the
larva and the perfect echinoderm. The most important organs of the
former are carried over into the latter, and the echinoderm brings the anla-
gen to further development. In the insects many features which are lack-
ing or incompletely developed in the larva are developed in the course of
the metamorphosis. There is a metamorphosis in the echinoderms as in
insects. It is a question as to which group of Echinoderma is the most
primitive, but ease of treatment makes it best to begin with the Asteroidea.
Class I. Asteroidea (Starfish).
Two parts can be recognized in the body of a starfish, a
central disc and the arms, usually five in number, which radiate
from it (fig. 316). The relations in which these parts stand to
each other vary between two extremes. In many starfish the
arms play the chief réle and the disc appears as only their united
proximal ends (figs. 309, 310). On the other hand the disc may
334 ECHINODERMA.
increase at the expense of the arms, absorbing these in its growth so
that they form merely the angles of a pentagonal disc (fig. 311).
In both arms and disc two surfaces are recognized, oral and
aboral, which pass into each other, usually without a sharp margin.
In the normal position the oral side is downwards and has in the
¥Fia. 309.—Comet form of Linckia multiflora. (From Korschelt-Heider.) One of the
arms is producing a new animal by budding.
LE RGS:
ah a ell
Fia. 310. Fig. 311.
F1G. 310.—Ophidiaster ehrenbergi. (After Haeckel). Comet form: one of the original
arms shown only in part.
Fic. 311.—Culcita pentanguluris, aboral view. (From Ludwig.) a, madreporite; }, re-
flexed end of ambulacral grooves.
centre the mouth and radiating from it to the tips of the arms the
five ambulacral grooves. On the aboral surface is the anus (when
not degenerate) near the centre, and excentric from it in an inter-
radius is the madreporite (in many armed species two to sixteen
radii may have madreporites).
A line passing through the madreporite and the opposite arm divides the
body into symmetrical halves. This ray is frequently spoken of as anterior,
since in the irregular sea urchins (Spatangoids) the homologous arm is
clearly anterior, while the madreporic interradius is posterior, This plane
of symmetry does not correspond with that of the larva. The two rays on
either side of the madreporite form the bivium, the three others the
trivium.
The skin is everywhere protected by large and small plates
jointed together. These make a dry starfish hard and stiff, but
in life it is extremely flexible, the arms can be bent in any direc-
I. ASTHEROIDEA. 335
tion, and the animal can work its way through narrow openings.
Of the skeletal pieces the ambulacral plates need special mention.
B
Fic. 312.—A, cross-section of starfish arm (orig.). a, adambulacral plates; am, ambu-
lacra; ap, ambulacral plates; 6, branchi; c, ceelom: h, hepatic ceca; ¢, inter-
ambulacral plates; », radial nerve; p, ampulla; 7», radial canal; v, radial blood-
pone B, ambulacral plates, ventral view, showing the ambulacral pores
etween.
These form the roofs of the ambulacral grooves, and between them
are openings, the ambulacral pores, through which connexion is
made between the ambulacra and ampullew. In each arm the pairs
of ambulacral plates meet above the groove like the rafters of a
Fic. 313.—Asteriscus verruculatus, aboral surface removed. (After Gegenbaur.) g
gonads; h, hepatic ceca; /, stomach with anus.
roof. Laterally each ambulacral plate abuts against a small inter-
ambulacral plate, bearing usually movable spines. Beyond these
comes the adambulacral plates, and then those of the aboral sur-
face. Each ambulacral area terminates at the tip of the arm with
an unpaired (ocular) plate.
336 ECHINODERMA.
The organs lie in part in the celom, in part in the ambulacral
grooves. The alimentary tract is in the celom and extends
straight upward from the mouth to the aboral surface, where it
ends with an anus or is entirely closed (figs. 313, 314). By a
Fic. 314.—Section through ray and opposite interradium of a starfish (orig.). B,
branchie ; C, cardiac pouch of stomach; EF, eye spot; G, gonad; 4H, ‘liver’; M,
mouth; N, radial nerve; P, pyloric part of stomach; RC, ring canal; RD, radial
canal of water-vascular system; S, stone canal.
constriction it is divided into a larger, lower cardiac portion and
asmaller, upper pyloric division. From the latter arise five hepatic
ducts which connect with five pairs of hepatic glands lying in the
arms. The cardiac division gives origin to five gastric pouches
which can be protruded from the mouth or retracted by appro-
priate muscles. The gonads are five pairs of racemose glands lying
in the basis of the arms and opening interradially between the
arms. Lastly, the stone canal, extending from the aboral madre-
porite to the ring canal, and the lymphoid gland lie in the celom.
The radial nerve, canal, and blood-vessel lie in the roof of the
ambulacral groove between the ambulacra. The nerve ends at the
F1a. 315.—Longitudinal section of eye of Asterias. (Orig.)
tip of the arm in a compound eye spot colored with red or orange
pigment which experiment shows is sensitive to light. A second
nerve has been described lying in the celom of the arm. The
ambulacral system corresponds with the foregoing description
II. OPHIUROIDEA. 8337
(p. 330), the ampulle as well as the five or more Polian and
Tiedemann’s (racemose) vesicles projecting into the cwlom.
Since the arms contain nearly all important organs, the physiological
independence of these is easily understood. Arms broken off not only
live, but regenerate first the disc and then
new arms which appear at first like small
buds (comet form, figs. 309, 310). This
separation of arms may occur through
accident, or it may be, and not infre-
quently is, produced by the animal itself.
Examples of species with well-devel-
oped arms and ambulacra in four rows
are furnished by the ASTERID#&, repre-
sented on our shores by the five-finger
Asterias* and Leptasterias,* and Heli-
aster * with numerous arms. In the
SOLASTERIDZ the ambulacra are two-
rowed; arms sometimes numerous. Py-
thonaster (fig. 316). In the ASTERINIDE&
the arms are short or the body is pentag-
onal, no large plates on the margins of
the arms. Asteriscus (fig. 313). In other
forms (Culcita,* fig. 317, Hippasteria,*
Ctenodiscus*) the body is more or less Fic. 316. — Pythonaster murrayi.
: (After Sladen.) Aboral view
pentangular, the margin being covered showing ambulacral grooves.
with large plates.
Class II. Ophiuroidea (Brittle Stars).
In these the animal consists, as in the Asteroidea, of disc and
arms, the latter sometimes branched, but the internal anatomy is
different. The ambulacral plates
have been drawn inside the arm
and each pair fused to a large
‘vertebra’ (fig. 317). As a result
the celom of the arms is greatly
reduced, the hepatic ceca are lack-
ing, and the alimentary canal, which
lacks an anus, is confined to the
disc. By the ingrowth of ven-
Fic. 317.—Section of Ophiuroid arm tral plates the ambulacral grooves
. a@,ambulacrum; b, blood ves- z
; ¢, celom; m, muscles of arm; 7, are converted into tubes, and the
eye MT Ree eA cisaral Glses ambulacra, which lack sucking
discs, are tactile organs, locomotion being effected by the snake-
like motion of the arms. The madreporite is on the ventral sur-
SS
SSN
338 ECHINODERMA.
face. Also on the ventral surface are five slits which connect
with as many burs, thin-walled respiratory sacs into which the
sexual organs open.
In many brittle stars (Ophioenida, Ophiothelia, Ophiocoma), especially
in young specimens, there is a kind of asexual generation (schizogony), the
animal dividing through the disc, the halves regenerating the missing parts.
The classification is based largely on small details. In the majority the
arms are unbranched (Ophiopholis * (fig. 318), Ophioglypha,* Amphiura *),
but in the EuRYALID&, or basket fish, the arms are branched (Astrophyton,*
fig. 319), but not, as usually stated, dichotomously.
Fig. 318.
FG. 318.—Ophiopholis aculeata.* (From Morse.)
Fig. 319.—Astrophyton arborescens, basket fish. (From Ludwig.)
Class III. Crinoidea (Pelmatozoa).
The crinoids or sea lilies are on the road to extinction. In
early times, especially in the paleozoic, they were very abundant,
but to-day there are but few genera and species, these mostly
restricted to the greater depths of the ocean, only the Comatulide
occurring near the shore. The crinoids are attached to the sea
bottom by a long stalk which contains a central canal (fig. 320).
This stalk is composed of cylindrical dises and often bears five rows
of outgrowths, the cirri. In the Comatulide (fig. 821) the adult
is not thus attached, swimming about in the water with the arms
or moving about on the tang. In these earlier stages these animals
have a stalk (fig. 322), passing through a Pentacrinus stage, a
proof that the fixed condition was the primitive one. In these
forms, when the separation takes place, one joint of the stalk with
its cirri remains attached to the animal, as the centrodorsal united
with the lowest cup plates, the infrabasals (fig. 321).
On the upper joint of the stalk is a cup-shaped body (theca)
the edges of which bear five or ten (usually branched) arms. The
III. CRINOIDEA. 339
S AAS a7
Sen)
YS ;
Oa
ix : i
D ‘
‘i
e P +
B
Ahead
S
zy
Z ay Sy
ow : oy.
4 =
<5 =
i’
bt ti z
Bay gas
BER He
NES
y) 4
7h
‘zl BY
~) ij Yard
AN
7 ;
¥Fia. 320.—Pentacrinus macleayanus,
(After Wyville Thompson.)
Fia. 321.
Fic, 321.—Adult of Antedon macronema, (After Carpenter.) eae
Fic. 322.—Different Pentacrinus stages (a, ), ¢) of Antedon rosacea, 1, arms; 7, cirri;
3, stalk.
FG, 322.
340 ECHINODERMA.
walls of the theca are covered with polygonal calcareous plates.
Usually the stalk bears five plates, the basalia, and then come five
radialia, alternating in order with the basalia (fig. 323). In some
Fie. 323.—Hyocrinus bethleyanus. A, upper end of stalk with cup, and the bases of
the arms; b, basalia; br, brachialia; r, radialia. B, oral surface of cup with
mouth, five oralia, and the bases of the arms.
there is a circle of infrabasalia in a line with the radialia. Fre-
quently the elements of the arm, the brachialia, are directly attached
to the radials (fig. 823). But often the arm branches once or
several times dichotomously, and the first branching takes place at
the base, so that the arms seem to spring from the theca. In
these cases the first brachialia are considered as part of the theca
and are called radialia distichalia (figs. 320, 321). From the arms
arise, right and left, a row of pinnule, lancet-shaped processes
supported by calcareous bodies in which the sexual products ripen
until freed by dehiscence (fig. 325).
The mouth opening, in the middle of the oral dise which closes
the theca, is frequently surrounded by five radial plates, the oralia.
The mouth, which in contrast to other echinoderms is directed
upwards, connects with a spacious digestive tract in which wsopha-
gus, stomach, and intestine can be distinguished. The anus is
interradial and near the mouth (fig. 324). Five ambulacral
grooves begin at the mouth and extend out on the arms, branching
with them and extending to the tips of the pinnule. These are
III, CRINOIDEA. 341
ciliated and serve as conduits to bring food to the mouth. Nervous,
ambulacral, and blood systems begin with a circumoral ring. They
follow the ambulacral grooves as in the asteroids, but the ambulacra
Fia. 324. Fia. 325.
Fre. 324.—Oral area of crinoid (Antedon), showing by dotted lines the course of the in-
testine from the mouth (m) to the anus («); gy, ciliated grooves leading from the
arms to the mouth (orig.).
Fig. 325 —Cross-section of pinnula of Antedon. (After Ludwig.) a, axial nerve
cord; c, ciliated cups; c,c, coeliac canal; g, gonad; s, sacculi; sc, subtentacular
canal; ¢, tentacles.
here have no suckers nor ampulle and are merely tactile tentacles.
A typical stone canal is also lacking; in its place are five or several
hundred tubules leading from the ring canal to the cwlom. Oppo-
site their celomic mouths are fine pores in the oral disc through
which water enters to pass through the tubules into the ambulacral
system. The ambulacral nervous system is weakly developed.
The enterocele system, on the other hand, is well developed and
forms the axial cord running through the brachialia and radialia
to unite ina ring in the centrodorsal. A problematical organ,
the so-called dorsal organ, also begins in the centrodorsal and
extends up through the axis of the theca to the oral disc. It is
possibly a lymphoid gland, possibly a structure for the transfer of
nutriment, and is apparently homologous with the ‘heart’ of the
starfish.
349 ECHINODERMA.
Sub Class I. Fucrinoidea.,
The foregoing account applies entirely to the Encrinoidea, which may
be divided into two groups :
Order I. TESSELLATA (Paleocrinoidea), Theca with its side walls
composed of immovably united thin plates ; the ambulacral grooves usu-
ally completely covered by calcareous plates. Exclusively paleozoic.
Order II. ARTICULATA (Neocrinoidea). Ambulacral grooves open,
theca with compact, in part movably articulated, plates. This order left
the other in the mesozoic age, and some families have persisted until now.
Rhizocrinus * (fig. 323) and Pentacrinus (fig. 320), deep seas ; the Coma-
TULID# are fixed in the young, free in the adult. Antedon* (fig. 321).
Sub Class II. Edrioasteroidea (Agelacrinotdea).
Theca of irregular plates ; arms unbranched and lying on the theca.
Possibly the ancestors of the noncrinoid echinoderms. Paleozoic. Ageda-
crinus.
Sub Class III. Cystidea.
Exclusively paleozoic ; body spherical, composed of polygonal plates.
Stalk and arm structures rudimentary, sometimes lacking. The AmPHo-
RIDA are by some regarded as ancestral of all echinoderms. Holocystites,
Echinospherites (fig. 326).
Fie. 326. Fie. 327.
Fia. 326.—Kchinospheerites aurantium. (From Zittel )
Fia. 327.—Pentremites florealis. (From Zittel). Lateral, oral, and aboral views.
Sub Class IV. Blastotdea.
Arms lacking ; the mouth surrounded by five petal-like ambulaeral
areas. The group appears at end of Silurian and dies out with the earbon-
iferous. Pentremites (fig. 327).
IV. HCHINOIDEA. 348
Class IV. Echinoidea (Sea Urchins).
The structure of the sea urchins is best understood in the
spherical forms (figs. 328, 330).
Mouth and anus lie at opposite
poles of the main axis, each open-
ing immediately surrounded by
areas covered by calcareous plates,
the arrangement of which varies
with the family. Around the anus
is the periproct, around the mouth
the peristome, the latter bearing
spheridia and in the Echinoids five
pairs of interambulacral gills. Be-
tween peristome and periproct the ie see Ceara aanidanisetaaies
body wall is composed of calcareous Agassiz.) Aboral ‘view, the spines
: : ay tte removed to show the ambulacral (a)
plates, which, except in the Echino- and () interambulacral areas, end-
oe . fi ing respectively in the ocular and
thuride, are immovably united. genital plates: in the centre the four
Aside from the extinct Palechei- Ptes of the periproct.
noidea the plates are arranged in twenty meridional rows, or, more
accurately, in ten double rows, two rows being always intimately
associated together. Five of these double rows are ambulacral,
Fia. 329. Fia. 330.
Fig. 329.—Clypeaster subdepressus. (After Agassiz.) Aboral view, showing the peta-
loid ends of the ambulacral areas. i ;
Fria, 330.—Diagrammatic longitudinal section through a sea urchin.
the alternating five interambulacral. Both bear small hemispheri-
cal articular surfaces on which are situated the spines, either long
and pointed or swollen to spherical plates. These spines are ex-
tremely mobile and are moved by muscles so that they serve both as
protecting and locomotor structures. The ambulacral plates are
344 HCHINODERMA.
distinguished from the interambulacral by the ambulacral pores
by which the ambulacra on the surface are connected with the
internal ampulle. In most sea urchins the paired grouping of
the pores results from the fact that a double canal extends from
ampulla to ambulacrum.
In the arrangement of the ambulacra two modifications, the band form
and the petaloid, occur. In the first the ambulacra are equally developed
from peristome to periproct (fig. 328). In the second oral and aboral
regions may be distinguished (fig. 329). In the oral region alone are loco-
motor feet always present, but these are so irregularly arranged that no
striking figure results. In the aboral area the ambulacra are tentacular
in character and are regularly arranged, their pores bounding five petal-
like figures around the periproct, very distinct after removal of the spines.
In the Echinoids, the Cidarids excepted, the interambulacral plates around
the peristome show five pairs of notches for the gills, five pairs of thin-
walled branching extensions of the body cavity.
Ambulacral and interambulacral fields both end at the periproct
with an unpaired plate, the five ambulacral plates (radialia of mor-
phology) being called ocular plates, since they often bear pigment
spots formerly regarded as eyes. They are perforated by the end
of the radial canal and nerve, the latter here uniting with the epi-
thelium of the skin. The five interambulacral plates are called
genital plates, since they usually contain the openings of the genital
ducts: One is often madreporite as well.
The interior of the body is occupied by a spacious celom, to
I 1] A st
Fra. 331.—Sea urchin opened around the equator. 4, ambulacral area; J, interam-
bulacral area; L, lantern; d, intestine; ed, anal end of intestine; g, gonads; nd,
siphon; oe, esophagus; p, p’, ring canal and Polian vesicles; st, stone canal.
the walls of which the thin-walled alimentary tract is fastened by
a mesentery. In the Clypeastroids this tract forms a simple spiral,
IV. ECHINOIDEA. 3845
but elsewhere it is a double spiral. It ascends from the mouth,
turning once, and then, bending on itself, coils
in the reverse direction to the anus (fig. 331).
Usually the first portion of the canal is accom-
panied by a siphon, an accessory tube opening
into the main tube at either end. Except in
the Spatangoids the mouth is surrounded by hy
five sharp-pointed calcareous plates, the teeth, ric. 332. — Aristotle’s
which in the Echinoids are supported by a [anfern of imongine
centratus lividus. (Af-
complicated system of levers, fulcra, and mus- ee ee eee ne
cles, the ‘lantern of Aristotle’ (fig. 332). cr
The ring canal and the ring of the blood system lie on the
lantern, the stone canal and ovoid gland (‘heart’) extending
upwards from them (fig. 330). The blood-vascular ring gives off
two blood-vessels which run along the alimentary canal, while from
the ring canal arise five ambulacral or radial canais which run on
Fie. 333.—Oral (A) and aboral (B) surfaces of the sand dollar, Echinarachnius parma.
a, anus; g, genital pores; 7, ambulacral areas ; m, madreporite ; 0, mouth.
the inner side of the test accompanied by nerves which radiate
from a nerve ring. The gonads are five (rarely four or two)
unpaired organs in the aboral half of the test, opening through the
genital plates, that is, interradially as in the starfish.
Order I. Palwechinoidea.
Paleozoic forms with five ambulacral areas, the interambulacral areas
containing more than two rows of plates. J/elonites.
Order II. Cidaridea (Regulares).
Ambulacral areas band-like, body more or less spherical, mouth and
anus polar. Here belong the common urchins, represented on our coasts
by Toxopneustes,* Strongylocentrotus,* Arbacia,* Calopleurus* (fig. 328).
B46 ECHINODERMA.
Order III. Clypeastroidea.
Trregular flattened echinoids with central mouth and teeth; anus out-
side the periproct in the posterior interradius, sometimes marginal ; five
petaloid ambulacral areas. Clypeaster (tropical), Echinarachnius* (sand
dollar, fig. 833), Ifellita,* with holes through the test.
Order IV. Spatangoidea.
Bilateral flattened forms more or less
heart-shaped ; mouth and anus excentric,
no teeth; usually five petaloid ambulacral
areas and four genital plates. From the
forward position of the mouth it follows
that only two ambulacral areas (bivium,
p. _) are upon the lower surface. Warmer
seas. Spatangus,* Echinocardium, Brissus.
Class V. Holothuroidea.
The sea cucumbers are most re-
Fic. 334. Young Spatangus pur- "4 : 0 ic
ieee ee moved of any group from the typical
removed, oral surface. In front, echinoderm appearance. At the first
the slit-like mouth ; behind, the :
anus. The bivium without tu- glance the skin appears naked and the
ercies, . . 7
characteristic plates absent. Yet these
are imbedded in the skin in the shape of plates, wheels, and anchors
(fig. 835). The integment is tough, leathery, and muscular with
POR
SA2DOOS
AVIIWNY
Fig. 335.—Dermal plates of Holothurians. 4, Myriotrochus rinkii. (After Daniels-
sen.) B, Thyone briareus; C, Synapta giraraii (orig.).
longitudinal and circular fibres. The saccular body gives these
forms a worm-like appearance, strengthened by its elongation in
the main axis, and with the mouth and anus at the poles. Unlike
other echinoderms these move with the main axis parallel to the
ground, a condition which, to a greater or less extent, leads to a
replacement of radial by bilateral symmetry. One surface (trivium)
becomes ventral, the bivium dorsal, and in many the trivial ambu-
V. HOLOTHUROIDEA. 347
Fia. 336.—Anatomy of Caudina arenata. (After Kingsley.) a, anastomoses of dorsal
blood-vessel ; b, branchial tree; d, dorsal blood-vessel; f. mesenterial filaments;
g, genital opening; /, alimentary canal; /, longitudinal muscles; m, mouth: 0,
genital duct; p, pharyngeal ring; 7, gonads, cut away on right side; f, ampulle
348 ECHINODERMA.
lacra alone are locomotor, those of the bivium being tactile or
wholly absent.
In the body cavity (fig. 336) lies the alimentary canal, which
(except in Synapta) is coiled in a uniform manner, although many
minor convolutions may obscure this. It passes backwards in the
median dorsal interradius foi ward in the left ventral interradins, and
then back in the right dorsal interradius to the anus. It is held in
position by mesenteries (fig. 337), and
ed near the anus by numerous muscular
filaments. Into the terminal portion
one or two branchial trees may empty.
These are tubular sacs with small
branched outgrewths which are filled
with water. The similarity of these to
an = the excretory organs of some Gephy-
“Int 7? yea (p. 317%) was one ground for re-
Fic. 337.—Transverse section of garding those forms as intermediate
Holothuria tubulosa. (After Lud- :
wig.) d, digestive tract; db,dor- between worms and echinoderms.
sal blood-vessel; g, gonad duct; ,,, E 2
h, skin; Im, longitudinal muscles: They are to be regarded as respiratory,
betiegse See eitdiccralconmpiex since they are periodically filled with
or ancium fambulacra} vessel) fresh water. In many species ‘ Cuvie-
ee rian organs’ occur; these are morpho-
logically specially modified portions of the branchial tree and are
either connected with them or separately with the cloaca. Many
zoologists regard them as defensive structures because of their
sticky nature and because they can be cast out through the anus.
The esophagus is usually surrounded by a ring of five radial
and five interradial plates which serve as points of attachment for
the longitudinal muscles. Just behind it He the ring canal, ring
nerve, and the ring of the blood system, each giving off a radial
branch which here runs inside the muscular sac of the body. From
the beginning of the radial canals (rarely, as in Synapta, from the
ring canal) tubes extend outward to form the extremely sensitive
retractile tentacles which surround the mouth, and which either
branch (Dendrochirotw) or bear frilled shield-shaped extremities
(Aspidochirotw). A single Polian vesicle is usually present, and
the stone canal (except in the Elasipoda) connects with the
celom. Blood-vessels going from the vascular ring form rich
anastomoses on the alimentary canal. Only a single gonad (or é
pair of united gonads) occurs. This consists of numerous tubules
which open usually interradially near the mouth.
V. HOLOTHUROIDEA. 349
The regenerative powers of these animals are of interest. In unfavor-
able conditions (hence in preserving the animals in alcohol without nar-
cotization with chloral) they void the whole viscera and yet may live and
reproduce the lost parts. In certain species are found a few parasites.
One or two harbor a small fish (Fierasfer) in their cloaca and branchial
trees. A parasitic snail, Hntoconcha mirabilis, lives in one species of
Synapta, and a mussel, Entovalva mirabilis, in another.
Order I. Actinopoda.
Radial canals present, sending branches to the tentacles and am-
bulacra when present. Divided into Pedata, with ambulacra, and Apoda,
without. The PEDATA include the Holothuride with peltate tentacles.
Fia, 338.—Cucumaria frondosa, sea cucumber. (From Emerton.)
Holothuria* in warmer waters, one species furnishing the trepang of
Chinese markets. The CucuMArim. represented in our waters by Cucu-
maria* (Pentacta) with regular rows of ambulacra, Thyone* with them
scattered, and Psolus,* scaly with a creeping disc. The deep-sea Eva-
sIPODA belong to the Pedata. The APODA are represented by Caudina *
(fig. 836) and Molpadia.*
Order II. Paractinopoda.
No radial canals nor ambulacra. Tentacular canals arising from ring
canal, Myriotrochus,* Synapta,* Oligotrochus* (fig. 339).
350 ECHINODERMA.
Summary of Important Facts.
1. The ECHINODERMA share the radiate structure with the
Coelenterata, but differ from them (a) in the numerical] basis of
the symmetry (five); (4) in that, as embryology shows, they have
descended from bilateral forms.
2. Farther characters are the existence of a ccelom, the
ambulacral system, and the mesodermal spiny skeleton, which has
given the name to the phylum.
3. Theambulacral system is locomotor and occurs nowhere else.
It consists of a sieve-like plate, the madreporite (not always pres-
ent), which passes water to the stone canal, and from this to the
Fie 339.—Oligotrochus vitreus.* (After Danielssen and Koren.)
ring canal and the radial canals to fill the ampulle and ambulacra.
Lateral branches supply the tentacles and cause their extension.
4. Blood-vessels and nerve cords run in the same radii as the
radial canals of the ambulacral system; stone canal, madreporite,
ovoid gland, and genital ducts are interradial.
5. The Echinoderma are divided into five classes: (1) Aster-
oidea, (2) Ophiuroidea, (3) Crinoidea, (4) Echinoidea, and (5)
Holothuroidea.
6. The AsTEROIDEA have a disc and (usually) five arms into
which the gastric pouches and hepatic ceca extend. The ambu-
lacral groove open.
%. The Opnivrormea also have dise and arms, but the ambu-
lacral groove is closed and the hepatic ceca absent.
8. The CRrINOIDEA have a cup-shaped body bearing arms,
usually branching, with pinnule, and a stalk, usually with cirri.
They are either temporarily or permanently attached. The
Crinoidea are subdivided into Eucrinoidea, Edrioasteroidea,
Cystidea, and Blastoidea.
9. The EcHINOIDEA are usually spherical or oval, armored with
calcareous plates which extend as meridional bands from peristome
to periproct, five pairs of ambulacral and five of interambulacral.
10. The ambulacral plates end at the periproct with a single
ocular plate; the interambulacral with a similar genital plate.
The madreporite is fused with one of the genital plates. :
MOLLUSCA. 351
11. The regular sea urchins have the anus in the centre of the
periproct, the mouth in the peristome; the ambulacral areas are
band-like.
12. The Clypeastroidea have a central mouth, the anus outside
the periproct in the posterior interradius; the ambulacral areas
petaloid.
13. The Spatangoidea are markedly bilateral, the mouth an-
terior, the anus posterior; ambulacral areas petaloid.
14. The Honornurormea are elongate and worm-like; the
skeletal system greatly reduced; they are more or less bilaterally
symmetrical and have usually a single gonad and two branchial
trees. They are divided into Actinopoda, with radial canals, and
Paractinopoda, without.
PHYLUM VI. MOLLUSCA.
At the first glance the molluscs, like the flatworms and leeches,
give the impression of parenchymatous animals. A spacious celom
is absent; what was formerly regarded as a body cavity is a system
of sinuses surrounding the viscera and connected with the blood
system, and is especially developed in the Acephala. More recently
the view has gained ground that the molluscs have descended
from calomate animals, and from forms in which, by encroach-
ments of a connective tissue and muscular parenchyma, the eelom
has been reduced to the inconspicuous remnants of the pericardium
and the lumen of the gonads.
Where the molluscan organization is well developed, as in the
snails, four parts may be recognized in the body (fig. 340). The
‘visceral sac forms the chief mass of the body; it is less rich in
muscles than the rest because it is reduced to a thin peripheral
layer by the alimentary canal, liver, nephridia, and gonads. In
front it is continuous with the head, which, according to the group,
is more or less marked off by a neck, and bears, besides the mouth,
the tentacles and eyes, the most important sense organs. Below,
the visceral sac passes into a muscular mass, usually used for loco-
motion, the foot. From the back extends the pallium or mantle,
a dermal fold which envelops a goodly part of the body. The
Acephala (fig. 340, C’) have a double mantle, right and left, both
halves springing from the dorsal line and extending down over the
visceral sac and foot. The cephalopods (fig. 340, 1) and the snails
(fig. 340, 2), on the other hand, have an unpaired mantle which
arises from about the central part of the back and either extends
352 MOLLUSCA.
down on all sides or, like a cowl, covers either the anterior or
posterior parts of the body. The mantle is of importance in two
ways: its outer surface is covered with epithelium which, like that
of the adjacent surface, has the power of secreting shell, a thick
cuticular layer of organic matter (conchiolin) largely impregnated
with calcic carbonate. The inner surface of the mantle, together
Fic. 340.—Diagrams of three molluscan classes. 4.a cephalopod (Sepia); B, a gas-
teropod (Helix); C, an acephal (Anodonta), a, anus; c, cerebral ganglion; fu, foot;
m, mantle chamber; sch, shell; ¢, siphon; v, visceral ganglion. Visceral sac
dotted; mantle lined, shell black.
with the outer surface of the body, bounds a space, the mantle cavity,
which, from its most important function, is also called the dran-
chial chamber. Since most molluscs are aquatic, special vascular
processes of the body, the gills or branchix, le in this space; in
the terrestrial forms its walls serve as lungs and thus are respiratory.
From the foregoing it will be seen that the character of the
mantle must exert an influence on the shape of the shell and on
the respiratory organs. Paired mantle folds necessitate two valves,
right and left, to the shell; a right and Jeft branchial chamber,
and right and left gills. With an unpaired mantle the shell is
MOLLUSCA. 353
always unpaired, while the gills may retain their primitive paired
condition,
The gills in the mantle cavity are called ctenitdia, from their resem-
blauce to combs with two rows of teeth. Each consists of an axial portion
(back of the comb), containing the chief blood-vessels and two rows of
branchial leaves. The whole is united to the wall of the branchial cavity
by the axis (fig. 885). In many aquatic forms the ctenidia are lacking,
and then the respiration is either diffuse by the skin or by accessory gills
which by structure (usually outside the mantle cavity) are distinguished
from the ctenidia.
Those parts of the surface of the molluse which are not covered
by the shell have a columnar epithelium which is frequently ciliated
and which contains unicellular mucus glands, especially abundant
on the edge of the mantle. These give these animals the soft slip-
pery skin which is implied in the name Mollusca (mollis, soft).
Many-celled glands, like the byssus gland of the Acephala, the
pedal gland of many snails, occur.
Although the existence of head, foot, and mantle is very char-
acteristic of the molluscs, they are not always present. In the:
Acephala there is no distinct head region; many gasteropods lack
the mantle and hence the shell; in the Cephalopoda the foot is
converted into other appendages, the siphon and arms. These
modifications are to be explained by degeneration and evolution.
In the nervous system are also some highly characteristic features.
As a rule it consists of three pairs of ganglia associated with
important sense organs and connected by nerve cords. One pair
lies dorsal to the esophagus and corresponds to the suprawesophageal
ganglion of the worms; it is the brain (cerebrum) and supplies
RNG pl. . _ & a LUE re
* ; ae > Seemmeree ‘ \ par
&
pe. a re. at pe
A B Cc
¥ 19. 341.—Nervous systems of Molluscs. A, most gasteropods; B, acephals; C, cepha-
lopods and pulmonates. c, cerebral; pu, parietal, pe, pedal, p/, pleural, and v,
visceral ganglia.
the tentacles and eyes. A second pair lies ventral to the alimentary
tract on the front part of the muscle mass of the foot; these are
the pedal gangha which are connected with the otocysts. The
third pair, the visceral ganglia, are also ventral, and near them
are the third sense organs, which are widely distributeed through
the Mollusea, and which from position and structure are regarded
354 MOLLUSCA.
as organs of smell (osphradia). They are thickened patches of
ciliated epithelia extending into the mantle cavity. Pedal and
visceral ganglia are united to the cerebrum by nerve cords, the
cerebropedal and cerebrovisceral connectives respectively. Accord-
ingly as these connectives are long or short the ganglia are wide
apart or united into a nerve mass around the wsophagus.
Primitive Mollusca (Amphineura) have a simpler condition. The
cerebral ganglia lie dorsal to the cesophagus and are united by a cord
around the cesophagus (fig. 844). From it are given off two pairs of lat-
eral nerve tracts, the ventral or pedal cords, and lateral or pleural cords,
the latter united by a loop dorsal to the anus. By a concentration of
ganglion cells the pedal cords give rise to the pedal ganglia, and similarly
the pleural cords form three pairs of ganglia, the pleural and the parietal,
as well as the visceral already mentioned, of the cerebrovisceral cord (fig.
341, 4). Thepleural ganglia are connected with the pedal by nerve cords;
the parietal innervates the osphradium. When farther concentration takes
place the pleural may unite with the cerebral, and the parietal with the
visceral (fig. 841, B), or both may fuse with the visceral (C),. In the latter
case the visceral ganglion (in the wider sense) is associated with the pedal
by the pleuropedal connective ; while in the other the connective is appa-
rently absent because fused with the cerebropedal. Although the otocyst
receives its name from the pedal ganglion, the centre of innervation lies in
the cerebrum.
The heart, which lies dorsally, consists of auricles and ven-
tricles. The ventricle is always unpaired, but there are two auricles
where two gills exist from which the blood flows to the heart, but
with the loss of one gill one auricle may disappear. Distinct arteries
and veins occur; capillaries are found only in the Cephalopoda,
while in the lower molluscs, and especially in the Acephala, the
smaller arteries open into lacunar spaces which were formerly
regarded as the body cavity. A completely closed vascular system
does not exist even in the Cephalopoda.
The heart is enclosed in a spacious sac or pericardium, which,
with few exceptions, is connected with the nephridia by a ciliated
canal, and in many molluses (Cephalopoda and some Acephala) is
also related to the gonads. These facts support the view, already
mentioned, that the pericardium and the lumen of the gonads are
the remnants of the calom; for here, as in the annelids, the
nephridia open by ciliated nephrostomes into the ccelom, and the
sexual cells arise either from the celomic walls or from sacs cut
off from them. Even more important for this view would be
confirmation of the disputed statement that in Paludina vivipara
the celom (enterocawle) arises as diverticula from the archenteron.
MOLLUSCA. 355
Nephridia and sexual organs are primitively paired, but fre-
quently are single by the degeneration of the structures of one side.
The animals are either hermaphroditic or dicecious, but the gonads
are always very large. Even more room in the visceral sac is
demanded by the digestive tract in which wsophagus, stomach, a
coiled intestine, a voluminous liver, and frequently salivary glands
may be recognized. The radula or lingual ribbon is also a char-
acteristic organ, and its absence from the Acephala is probably to
be explained by degeneration. It is a plate or band armed with
teeth which lies on the floor of the pharynx on a ventral ridge,
the tongue, and is used for the communication of food (figs. 366,
367).
Reproduction is exclusively sexual; budding, fission, or parthen-
ogenesis have not yet been observed. The eggs, united in large
numbers, are usually enveloped in jelly and are either rich in
deutoplasm or are enveloped in a nourishing albumen. A few
molluses (¢.9., Paludina vivipara) are viviparous. A metamor-
Sp oS
wS~
Ce
, branchial siphon ; f, foot;
k’, outer, 4’’, inner gill lamella; m, mantle; s, shell.
of these is the branchial opening by which fresh water passes into
the mantle (branchial) chamber; it flows out after passing over
the gills, along with the feces, through the upper or cloacal open-
ing. In many bivalves the free edges of the mantle grow together,
Ee
Fig. 350.—Section of shell of Anodonta. c, cuticula; p, prismatic layer; 7, nacreous
layer.
leaving three openings, one for the protrusion of the foot, the
others the two just described, which are now called the incurrent
(branchial) and excurrent (cloacal) siphons (fig. 349, 2B). By
further development the margins of these openings are drawn out
It, ACEPHALA. 361
into two long conjoined tubes (fig. 349, 4), which for their retrac-
tion need special muscles, which are attached to the valves and
thus cause the pallial sinus referred to above (fig. 348).
In the shell three layers may be distinguished (fig. 350): on the outside
a thin organic cuticula and below two layers largely of calcie carbonate.
In many these two layers are distinguished as the prismatic layer and the
nacreous layer, the first consisting of closely packed prisms; the nacreous
layer of thin lamelle generally parallel to the surface. These by their free
edges produce diffraction spectra and so the iridescent appearance of the
shell; the finer the lines thus formed the more beautiful the play of colors.
This is especially noticeable in the mother-of-pearl shells Meleagrina and
Margaritina margaritifera. When foreign substances get between mantle
and shell they stimulate a greater secretion of nacreous substance and
become surrounded by layers of it. In this way pearls are formed.
2
K* Ks K' ml gf
Fig. 351.—Anatomy of Anodonta, the mantle, gill, and liver of the right side
removed, the pericardium opened. 1, ?, anterior and posterior adductors; I, II,
IIT, cerebral, pedal, and visceral ganglia; a, anus; 1}, b?, upper and lower limbs
of organs of Bojanus,; br, branchial siphon; «, intestine; ¢, nephridial opening ;
fu, foot; g, gonad; hj, h®, ventricle and auricle of heart; k’, insertion of both
lamelle of right gill; k%, k4, inner and outer lamelle of left gill; 1, left liver 2 I a
its opening in m, stomach; ml, pallial line; 7, anterior, r?, posterior retractor
muscle: sp, nephrostome ; v, labial palpus. The arrows show the planes of sec-
tion of fig. 352.
The gills lie between the mantle and the body and from their
lamellar character have given rise to the name Lamellibranchiata
(figs. 851, 352). Two gill-leaves occur on either side, although
occasionally the outer or both may degenerate. Frequently the
gills of the two sides unite behind the body and produce a parti-
tion which separates the mantle cavity into a small dorsal cloacal
362 MOLLUSCA.
chamber and the larger lower respiratory cavity. Into the cloaca
empty the anus and the water which has passed over the gills; it
opens to the exterior through the excurrent siphon. The incurrent
siphon leads into the branchial chamber. In front of the gills
are two more pairs of leaf-like lobes, the labial palpi, between
which is the mouth.
The gills are variously developed. The Nuculide—the most primitive
of living Acephala—have true ctenidia consisting of an axis grown to
the body and an inner and an outer row of gill leaves (fig. 355). From
this the filibranch type is easily derived. The gill leaves grow out into
Fie. 352.—Projection of sections shown by the arrows in fig. 351. b1, b2, upper and
lower limbs of nephridium (organ of Bojanus); @, intestine; e, nephridiopore;
fu, foot; g, gonad; 4}, ventricle surrounding the intestine ; 22, auricle: k!, k2,inner
and outer gill lamelle; /, hinge ligament ; m, mantle; n, cerebro-visceral com-
missure; sp, hephrostome; v, venous sinus.
long filaments, each bent on itself so that it presents two limbs, a descend-
ing and an ascending. These branchial threads are so matted together
that they give the impression of a continuous leaf. In the true lamellar
gill the threads of the filibranch grow together at intervals, leaving open-
ings, the gill slits. Since there is an ascending anda descending limb, it
follows that each gill consists of an inner and an outer leaf (fig. 852), leav-
ing a space between into which the gill slits open. This internal space in
some serves to contain the young.
The complete enclosure of the body in the mantle folds has
led to a degeneration of the head and its normal appendages
IT, ACEPHALA. 38638
(Acephala). Hence there are only two divisions in the body,
dorsally the visceral sac and ventrally the foot. The foot, degener-
ate in many, has a broad sole only in Pectunculus and the Nuculi-
de; usually it is hatchet-shaped (Pelecypoda), that is, compressed
with a rounded ventral margin. It may be enormously expanded
and contracted again. This expansion is often explained by the
taking of water into the blood, but now it is generally accepted
that it is accomplished hy forcing blood from other regions into
it. While the foot by this extensibility can serve as a locomotor
organ, it also functions in many as an organ of attachment.
Inside is a large byssus gland which can secrete silky threads, the
byssus (fig. 853), one end of which is fastened to foreign objects by
Fig. 353.—Mytilus edulis. (After Blanchard.) a, edge of mantle; b, spinning finger of
foot; c, byssus; d,e, retractors of foot; f, mouth; gy, labial palpi; h, mantle ; i, j,
inner and outer gills.
means of a finger-like process of the foot, while the other end
remains in connection with the foot. Molluscs which have a byssal
gland are found anchored by a thick bunch of byssal threads to
stones, etc.
The heart, surrounded by a pericardium, usually occupies the
most dorsal part of the visceral sac. It consists of a ventricle and
a pair of auricles (figs. 351, 352, A’, 4°). The auricles receive the
blood direct from the gills; the ventricle forces it out through
anterior and posterior aorte (fig. 351), the latter lacking in many
species.
The excretory organs (organs of Bojanus) lie immediately
below the pericardium. The organs of the two sides touch in the
364 MOLLUSOA.
middle line. Each consists of a dorsal smooth-walled chamber
and a lower portion traversed by threads, both connected behind
but separated elsewhere by a thin partition. The lower chamber
is connected in front with the pericardium by a ciliated canal, the
nephrostome, while the upper opens to the outside by a short
canal, the ureter, the external opening being in the region of the
inner cavity of the inner gill. In this way a connexion is estab-
lished from the pericardium to the exterior, the apparatus being
apparently a true nephridium. In many it serves also as genital
duct, but usually the genital and reproductive ducts are separate.
The animals are usually diwcious, the gonads being acinose
glands.
The digestive tract (fig. 351) begins with a short oesophagus,
widens out to alarge stomach from which a slender intestine leads,
with many convolutions, to the anus. In the majority of Acephals
the terminal portion enters the pericardium in front and below,
passes through the ventricle and out through the upper posterior
wall of the pericardium. In its course the alimentary tract is
enveloped by the gonads and the voluminous liver, the secretion
of the latter emptying by two ducts into the stomach. Usually
the stomach has a blind sac, in which lies the ‘ crystalline style,’ a
rod-like structure of uncertain significance.
The three typical molluscan ganglia (p. 355) are uncommonly
wide apart. The two brain ganglia (cerebropleural ganglia) lie
either side of the mouth at the base of the labial palpi and ventral
to the anterior adductor. They are very small, since cephalic
sense organs are lacking, and are united by a transverse supra-
cesophageal commissure. The posterior ganglia, composed of the
united parietal and pedal ganglia, he near the anus ventral to the
posterior adductor. The pedal ganglia, rather far forward in
the muscles of the foot, are closely approximate. Of the higher
sense organs only the otocysts near the foot are constant. The
labial palpi are also highly sensory, while two small osphradia occur
at the basis of the gills. When eyes occur they are, as in the seal-
lops (Pectinide), arranged in a row like pearls on the margin of
the mantle. Small tentacles with sensory powers may oceur both
on the margin of the mantle and on the tip of the siphon.
Veligers (fig. 842) are very common in development. When this stage
is lacking the history may ecntain a metamorphosis as in the fresh-water
Anodonta, The young which grow in the maternal gills are known as
Glochidia, which are distinguished from the adult by a byssus thread,
by only a single adductor, and by a hook or tooth on the free margin of
IT, ACEPHALA: PROTOCHONCHLE. 365
the shell (fig. 354). After escape from the gills they swim about by
opening and closing the shells, and by
means of the hooks attach themselves
to passing fish. They produce an ulcer
in the skin of the fish in which they,
grow, and by renewal of the shell and “
the adductor muscles attain the de-
finitive condition. After this metamor-
phosis they fall to the bottom, to live
henceforth half buried in the mud.
Structure of gills, hinge, edge of Fic. Pee ae Resa:
mantle, and adductor muscles have been (From Balfour.) ad, adductor ; by,
used as basis of classification, the usual DySsHeG © Sense Mains ah Shell,
divisions being founded on characters derived from only one of these
organs.
Order I. Protochonchie.
The primitive character of these forms is shown by the struc-
ture of the gills, which are either ctenidia (Protobranchiata) or
ct Ob P
Fic. 355.—Anatomy of Nucula. (After Drew.) aa, anterior adductor; b, byssal gland;
c, cerebral ganglion; ct, ctenidium; f, foot; , heart; 1, labial palpus; 0, otocyst;
p, pedal ganglion; pa, posterior adductor; s, stomach ; t, appendage of palpus;
v, Visceral ganglion.
filamentary (Filibranchiata), yet here and there, as in the scal-
lops and oysters (Pseudolamellibranchiata), the fusion of gill fila-
ments is already begun. ITlinge and ligament are symmetrical
with regard to the umbo, or vary little from symmetry. The hinge
may be Jacking, and the ligament is wholly or in part internal.
The mantle edges are free, and rarely is there the first trace of fusion.
366 | MOLLUSCA.
Fic. 356.—Yoldia limatula.* (From Binney-Gould.)
Fie. 357.—A, Modiola plicutula*; RB. Pecten irradians*; C, Mytilus edulis.* (From
Binney-Gould.)
Il, ACEPHALA: HETEROCONCHLE. 367
Sub Order I. DIMYARIA. Two equally developed adductors. The
taxodont NucuLID# have ctenidia, a broad foot, pleural and cerebral gan-
glia separate, and gonads emptying through the nephridia, all points which
show them extremely primitive. Mecwla,* Leda,* Yoldia.* The ARcID#
are also taxadont, but filibranch. Scapharea,* Argina.* SOLEMYIDA,
Sub Order I]. ANISOMYARIA. Anterior adductor rudimentary
(Heteromyaria) or wanting (Monomyaria). With the exception of the
isodont SPONDYLID#, all the families lack a hinge (dysodont). To the
Heteromyaria belong the MYLILIDA, or mussels, with strong byssus and
shells pointed anteriorly. Modiola,* Pinna,* Mytilus edulis, abundant on
our mud flats; eaten in Europe, but occasionally poisonous. Dreissenia
polymorpha, a brackish and fresh-water species, has spread from the
Caspian through central Europe. JZthodomus* bores into stone. The
AVICULIDA of warm seas have wing-like projections either side of the umbo,
The pearl oysters of the East and West Indies (Meleagrina) belong here.
The OsTR#ID& and the PECTINIDZ are monomyarian, The Ostreide, or
oysters, usually become attached by the right valve. Our American
Ostrea virginiana differs from the European species in having the sexes
separate. The Pectinidee, or scallops, are free-swimming and are well
known for their highly developed green eyes on the edge of the mantle.
Order II. Heteroconchie.
Gills always lamellar, their outer surface frequently plaited.
Hinge—in rare cases (Anodonta) lost by degeneration—is hetero-
dont or modified from a heterodont condition. The mantle edges
but rarely free in their whole extent; siphons usually present, but
in some so small (Integripalliata) as to cause no sinus in the pallial
line; in others (Sinupalliata) large, the pallial line having a marked
sinus. Anterior and posterior adductors equally developed.
Sub Order I. INTEGRIPALLIATA. The UNIONID# (Naiade) include
the fresh-water mussels, of which hundreds of species occur in the Missis-
sippi basin, some of which are markedly iridescent and afford material
for pearl buttons. In some pearls of value are occasionally found, Unio,
Anodonta. The tropical TripaentpZ, with small siphons, includes the
Fic. 358.—A, Saricara arctica; B, Astarte suleata; C. Siliqua costata. (From Binney-
Gould.)
largest Acephala, Tridacna gigas, the shell of which may be four feet
long and weigh three hundred pounds. The heart shells (CARDIIDE—
368
MOLLUSCA.
Cardium*, Serripes*) and ASTARTIDA, marine, and the fresh-water CycLa-
DIDE (Cyclas, Pisidium*), about the size of peas, belong here, as probably
do the extinct RupIstTip& of the cretaceous.
Sub Order II]. SINUPALLIATA. The VENERID# with swollen shells,
represented by the quahog, Venus mercenaria on our east coast and by
Fig. 359.— Te-
redo neavatis,
ship worm in
its tube, the
siphons (a,
anal; b, bran-
chial) drawn
out of the
tube (r); ky,
shell. B,teeth
of the shell
enlarged.
brightly colored species in the tropics; the MACTRIDZ or
hen clams, and the flattened delicate TELLINIDE (Tellina*,
Macoma*), have short siphons. In others the siphons are
so large that they cannot be entirely retracted within the
shell. This is the case in the MyIp#, represented in all
northern seas by the long clam, Mya arenaria, and in the
razor clams (SOLENIDE; Solen Ensatella*). The allied Sax1-
cAVIDz have burrowing species. These forms connect
with others in which the united siphons far exceed the rest
of the body in length, giving the animal a worm-like ap-
pearance (fig. 359). Since the valves do not
cover the whole shell, they are supplemented
by accessory shells, or the worm-like body
secretes a tube in which the rudimentary
valves are imbedded (fig. 360). The PHo-
LADIDH, some of which are phosphorescent,
burrow in wood, clay, or stone. The shell is
well developed. In the ship worms (TERE-
pIp#) the shells, on the other hand, are small,
while in some species the burrows made by
these animals in wood are lined by calcareous
deposits. The several species of Teredo* by
their boring habits do much damage to wood
jn the sea, especially in the tropics. The
GASTROCH/NID also form tubular shells, the
valves being imbedded in the tube (fig. 360);
at the smaller anterior end the tube is open,
but the other end is closed by a perforated
plate, giving these animals the name of
‘sprinkling-pot’ shells.
; Lastly, there should be mentioned the
me 3 eran. little-known Septibranchiata, in which the
ee gills have the shape of a septum perforated
Ludwig-Leu- by gill slits separating the branchial and clo-
nis.) a, shell. ah ‘ % .
acal chambers. Silenta, Cuspidaria.
III, SCAPHOPODA, IV. GASTEROPODA., 369
Class III. Scaphopoda (Solenoconche),.
The tooth shells are primitive forms which have some resem-
blances to the Acephala in the paired
liver and nephridia and in structure of the
nervous system (with the exception that a
buccal ganglion is present and the pleural
ganglia are distinct from the cerebral).
In some points they are primitive (persis-
tence of jaws and radula), but in others
they are considerably modified. They lack
gills, have unpaired diwcious gonads,
rudimentary heart (no auricle), and have
two bunches of thread-like tentacles either
side of the mouth. The mantle lobes,
which are paired in the larva, unite below,
forming a sac open at either end, and this pig 361 —pentarium Seana
secretes a shell shaped like the tusk of an — fim. tooth shell; left the
elephant, from the larger end of which eae eee
protrudes the long three-lobed foot used
for boring in the sand. Dentalium (fig. 361), £ntalis *.
Class IV. Gasteropoda.
Although more highly organized than the Acephala, the snails
are in some respects more primitive. The regions of the body—
foot, visceral sac, head, and mantle—occur in all orders, although
in each one or more forms may occur in which one or another part
is lost.
As a rule the foot is flattened ventrally to a creeping sole. In
it may be distinguished anterior and posterior processes, the pro-
podium and metapodium, a sharp lateral margin, the parapodium,
and, above these, appendages or ridges, the epipodia. Inside the
foot is usually a pedal gland.
The head bears (1) the tentacles, a pair of muscular lobes or
hollow retractile processes; (2) a pair of primitive vesicular eyes,
which usually lie at the basis of the tentacles, but may rise even
to their tips. In many snails the eyes are on special stalks which,
as in the stylommatophorous Pulmonata, form a second pair of
tentacles. The protrusion of the tentacles is caused by an inflow
of blood, their retraction by muscles attached to the tip which
draw them in like a finger of a glove.
370 MOLLUSCA.
The mantle begins on the back and extends thence forward
over the body to near the beginning of the head. It covers the
mantle cavity, a spacious chamber, which in the water-breathing
Prosobranchiata, etc., contains the gills (ctenidia) and opens
outward by a large aperture under the margin of the mantle.
The edge of the mantle may be produced into a long groove-like
siphon, conveying water to and from the branchial chamber,
which is of importance in determining the shape of the shell.
When, by degeneration of the gill, the animals become air-breath-
ing, the mantle cavity becomes a lung, and the opening, by growth
of the mantle edges to the body, becomes a small spiraculum,
closed by muscles.
The visceral sac, by the great development of the gonads and
liver, becomes very large. Since growth downwards is prevented
by the muscular foot, the organs press towards the back, carrying
before them the dorsal wall at the origin of the mantle folds, the
line of least resistance. Some organs, like nephridia and heart,
may be pressed into the mantle cavity. When the visceral sac, as
often occurs, becomes enormous, it does not stand directly upwards,
but coils from left to right ina spiral. The older the animal
the more the spiral coils and the larger the last or body whorl.
The visceral spiral therefore begins at the tip with narrow whorls
which increase in size with approach to the rest of the body.
From the foregoing the shape of the shell is easily understood.
As a secretion of the mantle it takes the form which the mantle
assumes under the influence of the visceral sac. With slight devel-
opment of the visceral sac it forms a flattened cone (fig. 362, 1),
or is slightly coiled at the apex, as in the abalone (B). When the
visceral sac is greatly elongate the shell is correspondingly an
elongate cone. It is rarely irregularly coiled (Vermetida, fig.
362, C). It is usually coiled like a watch spring in one plane, or
like a spiral staircase; in the latter case the shell is more or less
conical (fig. 362, D, #) and one can speak of its apex and base.
In the middle of the base is usually a depression, the umbilicus.
Sometimes the coils are loose and do not touch in the axis con-
necting umbilicus and apex, so that one can look into the space,
but usually the coils fuse together into a calcareous pillar, the
columella, around which the whorls pass (fig. 362, 2, c).
The shell increases to a certain size by additions from the mantle
edge; and since this determines the aperture, the shell is marked
with parallel lines of growth. The pigment is elaborated on the
edge of the mantle, and in the formation of the shell passes into
IV. GASTEROPODA. 871
it, causing its color pattern. When the siphon is present the
shell shows a corresponding process. Thus are distinguished
holostomate shells with smooth mouths (fig. 362, D) and siphono-
FG. 362. Various forms of shells. (After Schmarda, Bronn, and Clessin.) A, Patella
costata ; B, Haliotis tuberculata ; C, Vermetus dentiferus ; D, Lithoglyphus naticoides ;
E, shell of Murex opened to show c, columella; 0, siphon.
stome shells, in which the anterior margin is drawn out in a
groove (fig. 362, £).
A simple conical shell without further evidence is not proof of primi-
tive structure. It may arise from the spiral form by degeneration, if the
visceral sac be reduced. Thus the shells of Fisswrella and Patella are to be
explained, for the viscera here show the results of an earlier spiral twist.
In most places the union between shell and soft parts is not very firm,
but the connexion at the aperture is more intimate, while a muscle is at-
tached to the columella (musculus columellaris) at about the middle point
of its height, the other end being inserted in the foot. It is for the retrac-
tion of the animal within the shell, first the anterior part with the head
and then the rest with the metapodium. In this the metapodium is folded
so that its dorsal surface lies towards the aperture. Hence in many species
this surface secretes a door, or operculum, Which closes the aperture when
the body retracts. Since the aperture increases in size with growth, the
operculum must also enlarge, which is accomplished in a spiral manner
(fig, 362, D), the process sometimes showing in a spiral line on the out-
side. So-called eye stones are the opercula of small Trochidee and Tur-
binide. Land snails are usually without opercula, but at certain times,
872 MOLLUSCA,
as in hibernation, they can close the shell by a calcareous plate, the epi-
phragm. In the spring this separates from the shell and is lost.
In most gasteropods the shell is coiled to the right, but in some species
(fig. 8363) the whorls are constantly turned to the left,
while reversed specimens occasionally occur in many
species which are normally dextral.
In the shell there are at most two layers, an inner
lamellar layer (not always present), which sometimes
is highly iridescent, and an outer porcellanous layer,
FIG. 363.—Sinistral shell “hich is opaque and contains the pigment. In rare
of Lanistes carinatus. cases the mantle and consequently the shell are lack-
(From Ludwig-Leu-. ‘ : a é
nis.) ing, or the mantle is present but the shell is rudi-
mentary and not visible externally because the mantle folds have grown
over it. In these cases the visceral sac is not prominent. Since the shell-
less forms possess a mantle and shell in the young, the adult conditions
are explained by degeneration.
Only a few gasteropods are like the Amphineura and Acephala
in being bilaterally symmetrical. Usually the spiral twist of the
visceral sac has resulted in a torsion of other parts from left to
right, in which alimentary tract, nephridia, gills, heart, and nerv-
ous system take part. The intestine is bent in this way, the anus
opening into the mantle chamber on the right side, or the twisting
may be continued so far as to double the intestine on itself, the
anus being in the middle line in front, near the head. Nephridia,
Fig. 3€4.—Three diagrams illustrating the torsion of the body and the twisting of the
nervous system in gasteropods. (After Lang.) 4, bilateral, B, asymmetrical,
C, streptoneurous condition. The reference letters are placed upon the organs
ot the primitive left side. a,anus: c, cerebral ganglion: g, ctenidium; 7, auri-
cle; m, mouth; 2, nephridial opening; o, osphradium; pa, parietal ganglion;
pe, pedal ganglion; p/, pleural ganglion; v, ventricle. :
gills (with them the osphradia), and heart wander in company, so
that the organs primitively belonging on the left side may be trans-
%)
IV. GASTEROPODA. 373
e
ferred to the right and vice versa. With this there is a tendency
to asymmetry and the loss of the organs (usually of the primitively
left side). When the nervous system takes part in the twisting
a notable crossing of the cerebrovisceral commissures takes place,
known as streptoneury or chiastoneury.
The alimentary canal begins with a muscular region which in
some groups is developed into a large protrusible proboscis (fig.
365). The pharynx, which follows, contains the tongue, a ventral
ridge supported by one or more cartilages and covered by a cutic-
ular layer, the radula or lingual ribbon (odondophore). The upper
surface of the radula is armed
with sharp, backwardly di-
rected teeth (fig. 366) which
are usually arranged in trans-
verse and longitudinal rows,
but which vary so in num-
ber, form, size, and arrange-
ment that they are of value in
classification. Although the
radula covers the tongue, it is
Fie, 365. Fie, 366.
Fia. 365.—Pyrula tuba, male. (After Souleyet.) The mantle has been cut on the
right side and turned to the left, reversing the pallial organs. a, anus; ¢, ctenid-
jium:em, columellar muscle; f, foot; h, heart in pericardium ; t, intestine ; 1,
liver; m, mantle; mf, floor of mantle cavity; 1", nephridium, ns, Opening ot
nephridium; 09, osphradium ; p, proboscis; pe, penis; t, testes; v, vas deferens
cut in two. : : : ;
Fig. 366.—Pharyngeal region of Helix pomatia. A, side view ; B, section. m, muscle;
oc, @sophagus ; 1’, radula; 7s, radula sac ; sp, salivary duct; 2, lingual cartilage.
formed in the radula sac, which lies behind the tongue. From
this it grows forward like a nail over its bed as fast as it is worn
out in front. It is opposed in eating by a single median or a
pair of lateral jaws (lacking in carnivorous forms).
The rest of the alimentary canal is convoluted, the anus being
374 MOLLUSCA.
usually on the right side in front, in or beside the mantle chamber,
tarely it empties in the middle line behind (figs. 365, 370, 371).
(Esophagus, stomach, and intestine are slightly marked off
from each other. The convolutions of the intestine are enveloped
Fic. 367.—Row of teeth from the radula of Trochus cinerarius. (After Schmarda.)
by the liver, which by its large size forms the chief part of the
visceral suc. rey myocele; ws, primary
form of the primitive somites (fig. 967), Ti gele GceTenE Gh Fos
formerly called protovertebre. vomer:
Another important point in the musculature lies in the fact
that it is dorsal in origin and therefore in fishes is largely dorsal
552 CHORDATA.
in position throughout life. The muscles which are ventral have
largely been transferred from the back, and the cause of the migra-
tion is to be recognized to a large extent in the progressive devel-
opment of the appendages. The dorsal position of the muscles is
only a part of a general fact, that the skeletal axis divides the body
into a dorsal zone, containing only animal organs, and a ventral
zone, chiefly vegetal in character. Besides, the muscles, the cen
tral nervous system, and the most important sense organs—eyes,
nose, ears—belong to the dorsal zone.
The central nervous system of vertebrates consists of brain and
spinal cord. Like that of all chordates it is distinguished from
that of other segmented animals—annelids, arthropods, in which
there is a dorsal brain and a ventral nerve chain—in its purely
dorsal position. It is further distinguished from that of all non-
chordates by its tubular character, that is, by the presence of a
central canal in the axis of the elongate central system (fig. 76),
lined by a special epithelium, the ependyma, and containing a
fluid, the liquor cerebrospinalis. This central canal is the result
of the mode of development, the nervous system arising by an in-
rolling of the ectoderm and not by a splitting from it as in the
invertebrates (fig. 9). Besides the neurenteric canal already
referred to (p. 502), there long persists at the anterior end an
opening to the exterior, the neuropore. In all vertebrates, in con-
tradistinction to the lower chordates, the brain is large and sharply
marked off from the spinal cord.
The spinal cord is a cylindrical structure (flattened in Cyclo-
stomes, fig. 555) which, in the middle line above and below, is
marked by two longitudinal grooves, the dorsal and ventral fissures
of the cord (sp, sa, fig. 76). The central canal (Cc) has its lumen
greatly narrowed by the growth of the nervous tissue, in which,
as in the ganglia of the invertebrates, two layers are distinguished,
one containing almost solely nerve fibres, the other both fibres and
nerve or ganglion cells. The arrangement of these layers is con-
trasted with that of the invertebrates in that the ganglion-cell
layer—the gray matter—lies in the centre, the fibrous layer—white
matter (JI’)—on the periphery, a reversed position consequent
upon the development by infolding. The distinction in color
indicated in the names depends upon the fact that white medullated
fibres run in the cortex, while in the gray matter gray non-medullated
fibres are present between the nerve cells. The color distinctions
fail in the cyclostomes (and Ainphiorus), which have no medullated
fibres, although the same general structure occurs.
IV. VERTEBRATA. 538
The gray matter surrounds the central canal, but extends on
either side dorsally and ventrally into the white matter, so that
in section it resembles somewhat the letter H, with its dorsal
(fig. 76, HH) and ventralhorns (VH). By means of these horns
and the dorsal and ventral nerve roots arising from them, the white
matter on either side is divided into three tracts, the dorsal (/7),
ventral (s), and lateral (YX) columns of the cord.
Corresponding to each muscle segment two nerve roots arise
from the cord, a dorsal root, with a ganglion (spinal ganglion) at
some distance from the cord, and a ventral root, without a ganglion.
The dorsal root contains only sensory fibres—i.e., those carrying
nervous impulses to the cord—and is afferent, while the ventral
roots are efferent and contain only motor elements (Bell’s Law).
These roots unite into a mixed root, which then divides.into dorsal
and ventral branches.
The brain of vertebrates in general corresponds in its funda-
mental plan (fig. 568), best seen in development, with the brain of
man. At an early stage it consists of three
vesicles, one after the other, a fore brain
(prosencephalon), a mid brain (mesencepha-
lon), and a hind brain (metencephalon).
Usually this stage is reached hefore the
closure of the medullary folds. Formerly it
was stated that a condition with five vesicles
Fria, 568. Fic. 560.
Fig. 568.—Diagram of a vertebrate brain. (From Wiedersheim.) Aq, aqueduet ; Ce,
central canal; FM, foramen of Monro (connexion of lateral ventricles with each
other and with the third); HH, cerebellum ; MH, corpora bigemina, (optic lobes) ;
NH, medulla oblongata; A, spinal cord; SV, lateral ventricles e VH, cerebrum;
ZH, optic thalami (twixt prain); J, IV, third and fourth ventricles.
Fre. 569. Scheme of brain in sagittal section. ¢, cerebrum; cb, cerebellum; cc, canal
of spinal cord; ch, notochord ; ¢s, corpus striatum; , hypophysis; ¢, infundibu-
lum; m, medullary region ; 0, optic chiasma ; of, olfactory lobe ; ol, optic lobes ;
Pp, pinealis.
followed upon this with three, the mid brain remaining undivided,
while the hind brain divides into cerebellum (cd) and medulla
oblongata (7); the fore brain into cerebrum and ’twixt brain.
This is unnatural so far as the hind brain is concerned, for cere-
584 CHORDATA.
bellum and medulla are related to one another as roof and floor of
one and the same cavity (fig. 569). The distinction between the
first and second vesicles is problematical. The fore brain becomes
divided into three parts by an inpushing at its anterior end: an
unpaired middle portion, and in front a right and a left diver-
ticulum. These paired portions, increasing in size, form the cere-
bral hemispheres, and together with a small connecting part
represent the first cerebral vesicle, while the unpaired portion
forms a second vesicle, the ’twixt brain.
Introducing the terms of human anatomy for the separate parts
of the brain, the first vesicle consists of the two cerebral hemi-
spheres whose dorsal and lateral walls are usually thick and are
called the pallium, while in the floor of each hemisphere is an
enlargement, the corpus striatum (cs). The spaces in the hemi-
spheres are the first and second ventricles (sv). From the front
portion of each hemisphere arises a distinct region, the olfactory
lobe (of), which gives origin to the olfactory nerve. Since the
organ of smell is frequently at some distance from the brain, the
olfactory nerve must be elongate, as in the Amphibia (fig. 614), or
the olfactory lobe must lengthen, as in many Elasmobranchs (fig
592). In the latter case the swollen end of the lobe is close to
the olfactory epithelium and is connected with the brain by a long
stalk, the tractus, while the swelling is called the bulbus olfacto-
rius. Both, as parts of the brain, must be distinguished from the
olfactory nerve.
In the region of the second vesicle only the lateral walls become
thickened, producing the optic thalami, directly adjoining the
corpora striata; the roof of this vesicle develops no nervous sub-
stance, but remains a thin layer of epithelium closing in the third
ventricle above (f//). The floor is also thin-walled between the
thalami and is pushed downwards, forming a funnel-like pocket,
the infundibulum (7). The third vesicle, as a rule, is divided by
a deep longitudinal dorsal groove, dividing the cavity into a right
and left ventricle, while the two halves of the roof are known as
the optic lobes or corpora bigemini. In the mammals alone (in
which there is also a transverse groove dividing the optic lobes
into the corpora quadrigemini) the cavity of this mid brain is re-
duced, by thickening of the walls, to a narrow canal, the iter or
aqueduct of Sylvius, with the result that the term fourth ventricle
is transferred to the cavity of the hind brain.
This last region is called the medulla oblongata; it isa prolonga-
tion of the spinal cord, and in many respects shows a similar struc-
IV. VERTEBRATA. 530
ture. It is distinguished from the cord externally in that it
gradually increases in size in front, while its roof is reduced to a
thin epithelium, often torn away in dissection, leaving an opening,
the fossa rhomboidalis, into the ventricle. In front of this fossa is
the cerebellum, often a thin transverse nervous lamella, but usually
is a considerable part of the brain, composed of a median ‘ vermis’
and two lateral cerebellar hemispheres.
Although these five parts are present in all vertebrates, the
appearance of the brain in the various classes is very different,
because the relative size and form of the parts undergo great
yariations. In the lower vertebrates optic lobes and medulla
oblongata are disproportionately large, while the cerebrum, and
often the cerebellum, are insignificant in size; in the cerebrum,
again, the hemispheres may be smaller than the corpora striata
and the olfactory lobes. In the higher vertebrates, on the other
hand, the cerebrum and cerebellum far surpass the other parts,
the increase in size of the cerebrum being proportional to the in-
crease in intelligence. The cerebral hemispheres grow backwards,
in man and the apes covering the other parts, while in front the
olfactory lobes are carried by a similar overgrowth to the lower
surface. Since the capacity of the skull is limited, the cortex of
the cerebrum, the seat of intelligence, is increased in amount by
the development of folds, gyri, separated by sulci. Somewhat
similar conditions exist in the cerebellum, which in mammals and
birds is, next to the cerebrum, the largest part of the brain.
Connected with the ’twixt brain are two problematical organs, one, the
epiphysis (pinealis), being dorsal; the other, the hypophysis (pituitary
body), ventral. The hypophysis arises like a gland by an outgrowth from
the embryonic mouth. This hypophysial pocket cuts off from its source,
inereases by budding, and fuses with parts derived from the end of the
infundibulum to a single two-lobed body. It has been compared with the
subneural gland of the Tunicata (p. 509), The epiphysis is an outgrowth
from the roof of the brain, from which develops in many vertebrates the
parietal organ. In many reptiles this has the structure of an eye (pineal
eye), and in these, separated from the brain, but connected with it by a
nerve, it lies in a special cavity in the parietal bone, which occurs not only
in recent but in fossil forms. Above the eye the skin may be transparent.
The nerves which come from the brain mostly arise from the
region between the mid brain and the spinal cord, especially from
the medulla oblongata. The olfactory and optic nerves are an
exception, the one arising from the cerebrum, the other from the
*twixt brain, but both, and especially the optic, differ so much from
the peripheral nerves that they can hardly be classed with them.
536 CHORDATA.
Development shows that the optic nerve is a part of the brain.
Following custom, however, and including these two, the pairs of
cranial nerves may be enumerated in the terms of human anatomy
as follows: I, N. olfactorius; H, N. opticus; III, N. oculomotorius;
IV, N. trochlearis (patheticus); V, N. trigeminus; VI, N. abducens;
VII, N. facialis; VIII, N. acusticus; IX, N. glossopharyngeus;
Fia. 570.—Diagram of cranial nerves (shark). a, alveolaris; h, buccalis ; c, cere-
brum ; cb, cerebellum; ct, chorda tympani; e, ear; er, external rectus muscle;
f, inferior rectus muscle; g, Gasserian ganglion; h, hyoid cartilage; hm, hyoman-
dibular; 7, internal rectus muscle; io, inferior oblique muscle; j, Jacobson's
commissure ; J, lateralis of vagus ; m, mouth; mc, Meckel's cartilage: md, mandi-
bularis ; mx, maxillaris superior; n, nose ; 0, optic lobes; op, ophthalmicus profun-
dus; os, ophthalmicus superficialis; p, pinealis; pl, palatine; po, posttrematic
branches; pr, pretrematic branches; pn, pneumogastric (intestinal) of vagus;
ptq, pterygoquadrate; s, spiracle; so, superior oblique muscle; sr, superior rectus
muscle; ¢, ’twixt brain; J-X, cranial nerves: 1-5, gill clefts.
X, N. vagus (pneumogastricus), XI, N. accessorius; XII, N.
hypoglossus. The accessorius in fishes and amphibia is a part of
the vagus; the hypoglossus, strictly speaking, belongs to the spinal
nerves and only secondarily is associated with the cranial nerves,
which explains its course, outside the skull, in eyclostomes and
amphibia.
Since the head undoubtedly consists of several coalesced body seg-
ments (at least as many as there are visceral arches, and apparently
more), the question arises whether the cranial nerves are as evidently seg-
mental as are those of the trunk. To this is allied the further question
whether Bell’s Law that a mixed nerve consists of dorsal sensory, and
ventral motor components is appleable here. Both problems have been
much discussed in recent years, but as yet the final answers have not been
given. It is probable that the present cranial nerves, the optic and olfac-
tory excepted, have arisen by manifold rearrangements of segmental
nerves. On the other hand it seems impossible to accept Bell’s Law here
without considerable modification, since many cranial nerves (facialis,
trigemenus, ete.) contain motor fibres, although they are formed like
dorsal roots.
IV. VERTEBRATA. 537
Besides the nervous system of the body already outlined, the vertebrates
have a special nervous system supplying the viscera,—the sympathetic
system,—and in this a special central organ consisting of right and left
cords beneath the vertebral column, in which ganglia are incorporated,
The last of these ganglia lies at the base of the caudal vertebree, the most
anterior at the beginning of the neck. From the latter nerve cords
extend into the head and are connected with ganglia (otic, sphenopalatine).
This system sends out nerves in the form of delicate networks (plexus
sympathetici) which usually accompany the blood-vessels to the vegeta-
tive organs (intestine, sexual apparatus, etc.). It is also connected with
the spinal nerves.
Regarding the sense organs of the vertebrates we stand on
firmer ground than with the invertebrates, since their great simi-
larity to those of man supports the ideas of their functions derived
from studies of their structure. The tactile organs make an ex-
ception, since only in land animals, and not in fishes, do they
resemble those of man. These organs, in all forms above fishes,
have the peculiarity that the nerves do not end in epithelial cells,
but in special tactile cells of the derma, which either lie isolated in
the connective tissue (Amphibia, reptiles), or, grouped together,
produce tactile corpuscles (birds, mammals,
fig. 571). These are oval bodies and are im-
bedded in special papille of the derma. In
form and position they are much like the
Vater-Pacinian corpuscles, which are distin- 4%
guished by their histological structure (fig. 78)
and, since they also occur in internal organs
(mesentery of cat), are of problematic function.
Besides these mesodermal nerve endings there Fie. 571.—Tactile cor-
< Shoe eet ye puscle from bird’s
are present in all vertebrates intraepithelial fongue. #7, outer en-
nerve branchings which are best seen in the ang ee
cornea of the eye and in animals, like pigs and — Partitions.
moles, with sensitive snouts. Even here the finest nerve twigs do
not end in epithelial cells, but in small knobs between them.
Fishes lack tactile cells, tactile corpuscles, and end bulbs;
hence the skin is provided with sense organs in which a sensory
epithelium occurs. The dermal nerves pass into the epidermis
and end in oval corpuscles, which, while imbedded in a stratified
epithelium, consist of a single layer of sense cells. According to
structure, nerve hillocks and nerve-end buds are distinguished,
The first are the specific organs of the lateral line, to be men-
tioned later, of fishes and branchiate amphibians and amphibian
larve, and therefore appear to subserve special and important sensa-
538 CHORDATA.
tions connected with aquatic life; hence the idea of a ‘sixth
sense,’ lacking to man (c/. p. 125). The end buds are especially
collected in the neighborhood of the mouth, on the lips and bar-
bels. Since they also occur in the mucous membrane of the mouth,
especially in the palatal regions, they connect with the taste organs.
The taste buds have the same structure as the end buds of fishes.
They occur in all classes of vertebrates, and are most abundant in
man in the walls of the circumvallate papille at the base of the
tongue; in rodents on the large foliate papille, etc.
The end buds also lead to the olfactory organs. The olfactory
epithelium of many fishes and amphibia is a stratified epithelium
with closely arranged end buds (fig. 572). By disappearance of
Fia. 572.—Section of olfactory epithelium of a fish (Belone). (From O. Hertwig, after
Blaue.) e, epithelium: k, olfactory buds; n, nerves.
the isolating parts of the ordinary epithelium the end buds form
a continuous sensory epithelium, which is the rule in most ver-
tebrates.
The olfactory organ, the nose, lined with its sensory epithelium,
acquires a special interest both from its grade of development and
from the important systematic distinctions it affords. Except the
cyclostomes, which have an unpaired nasal sac, all vertebrates have
paired olfactory organs. In adult fishes and in the embryos of
higher forms are two pits which he in front of or dorsal to the
mouth; they are either distinct from it or only connected with
it by an oronasal groove in the skin (fig. 599). If the animal
be terrestrial and replace branchial by pulmonary respiration, a
respiratory canal is developed in connexion with the nose. The
eronasal groove closes to a tube which begins with an opening
(nostril) on the surface and ends with a second opening (choana)
in the mouth cavity. The olfactory sac proper is included in the
wall of this tube, usually on its dorsal surface (fig. 573). In Am-
IV. VERTEBRATA. 539
phibia, lizards, snakes, and birds the choana is far forward, behind
the upper jaw; in alligators, turtles, and mammals it is carried far
back, in crocodiles and some mammals
(edentates) nearly to the vertebral col-
umn. ‘This position is brought about by
the development of the hard palate, a
parting wall which divides the primitive yj
mouth cavity into two portions, a lower, ‘ = \
the persistent or secondary mouth cavity, oh Ch
and an upper, which, as secondary nasal Pig. 573. Diagram of nose of
Z ; . izard. (After Wiedersheim.)
cavity, contributes to the air passages. 4N, outer nasal cavity; €,
olfactory sac: Ca, canal from
The bones of the maxillary and palatine Jacobson’s organ to mouth:
3 é ; Ch, choana; IN, inner nasal
series contribute to the hard palate, since cavity; MS, roof of mouth;
premaxillaries, maxillaries, and rarely the Gonen emres Tea cane
pterygoids send out horizontal processes "°*
which meet in the middle line. In the mammals this partition
is continued back by the muscular soft palate. In crocodiles there
is a fibrous palate.
In the olfactory organ of the chordates two constituents must be
recognized, an unpaired and two paired portions. The unpaired portion
alone occurs in Amphioxus, this being supplied by the lobus olfactorius
impar ; in all vertebrates there are paired sacs with paired olfactory lobes.
The unpaired sac of the cyclostomes has apparently arisen from a union of
paired and unpaired parts, hence the double olfactorius.
A further increase in the nasal cavity is brought about by complicated
folds in the walls supported by special skeletal parts, the turbinal bones,
and also by the outgrowth of chambers, lined with mucous membrane
which extends into the neighboring bones. Thus are formed the sinus
frontalis in the frontal bone ; behind, the sphenoid sinus in the sphenoid,
and the antrum of Highmore in the maxillary. Again, a part of the primi-
tive chamber lined with olfactory epithelium can be cut off from the rest
and form an accessory nose, Jacobson’s organ, which opens into the
mouth behind the premaxillaries by ‘Stenson’s duct’ (fig. 578, P). This
organ is best developed in lizards, monotremes and ungulates, but often
occurs in a reduced condition in other terrestrial vertebrates.
In all vertebrates with the exception of Myzine and a few forms
living in the dark the eyes are composed of all the principal con-
stituents which occur in the human eye and which have already
been briefly described (p. 131, fig. 83). In most vertebrates it is ¢
nearly spherical body with the optic nerve entering it from behind,
with its interior occupied by transparent, refractive substances
(lens, vitreous body), and its walls of three concentric layers.
The outer of these is the tough protecting sclera (sclerotic), a
540 CHORDATA,
usually fibrous, but in many fishes a cartilaginous, layer, which in
front becomes transparent and strongly curved, forming the cornea.
The second layer, the choroid coat, is richly vascular and pig-
mented; at the boundary between sclerotic and cornea it is changed
to the iris. The inner layer is the retina, the structure and
arrangement of which are characteristic of the vertebrates.
From the developmental standpoint the retina (fig. 82) con-
sists of two parts, the retina proper and the tapetum nigrum
(pigmented epithelium), formerly regarded as part of the choroid.
In the retina the following layers are distinguished: (1) the limi-
tans interna; (2) nerve-fibre layer; (3) ganglionic layer; (4) inner
molecular layer; (5) inner granular layer; (6) outer molecular
layer; (7) outer granular layer; (8) limitans externa; and (9) layer
of rods and cones. The limitans externa is the bounding mem-
brane of the embryonic retina, which is later penetrated by the
rods and cones. Between the two limiting membranes Miiller’s
fibres (m) extend, large supporting cells occurring in other sensory
epithelia, the nuclei of which lie in the inner granular layer, and
which are aided in their supporting function by the fine horny
framework of both molecular layers. The nervous elements
which are imbedded in this support are best understood by begin-
ning with the optic nerve. This spreads out in the nerve-fibre
layer, and on its way to the end apparatus comes twice into relation
with ganglion cells; first in the ganglionic layer, second in the
inner granular layer. Thus a great part of the retina (layers 1 to
6) are to be considered as an optic ganglion, such as occurs in
molluses and arthropods, but which there lies outside the sensory
apparatus. The sensory epithelium (the retina in the sense this
term is used in invertebrates) consists of but two layers, the
outer granular layer and the rods and cones. ‘The outer granules
are the nuclei of the extremely slender epithelial cells which bear
the rhabdomes (rods and cones) on their peripheral ends. Pigment
cells are lacking between these visual cells, but the pigment so
necessary for the visual function is supplied by the tapetum
nigrum already mentioned. This isa layer of hexagonal epithelial
cells which lies on the tips of the rhabdomes and sends pseudopodia-
like processes between them, and since the tapetum is rich in
black pigment granules, the rods and cones are enveloped ina
close pigment mantle.
Although in this relation of pigment and in the union of the
optic ganglon with the sensory cells important differences are to
be noted from the eyes of the invertebrates, even of the closely
IV. VERTEBRATA. 541
similar cephalopod eye (p. 385), the most striking difference re-
mains to be mentioned. The retina, with its limitans interna and
nerve-fibre layer, abuts against the vitreous body; with its rhab-
domes and tapetum against the choroid. Hence the incoming
light must traverse the optic ganglion and pass through the layer of
sense cells before reaching the end organs, the rhabdomes. In
nearly all invertebrates, for example the Cephalopoda (fig. 353),
the light falls directly on the peripheral end of the rhabdome.
The rhabdomes in cephalopods, as in most invertebrates, are
turned towards the light, in the vertebrates away from it.
This peculiar and functionally purposeless inversion of the vertebrate
retina is explained by the development of the eye. This can be divided,
according to origin, into two parts, a cerebral (optic nerve, retina, tape-
tum) and a peripheral (all other parts). As the eye in tunicates and Am-
phioxus is permanently a part of the brain, so is the retina of vertebrates
genetically, and of the first cerebral vesicle. An outgrowth occurs on
either side (fig. 574, B) of the ’twixt brain and becomes expanded distally
Fic. 574.—Diagram showing the inversion of layers in the formation of the retina
(orig.). The nuclei are placed in the (morphologically) deeper ends of the cells.
In 4 the brain ()) has been closed in; in B the optic vesicle (v) has reached the
iens (J) and on the right is being converted into the double-walled optic cup
with, as shown in C, an outer tapetal (¢) and an inner retinal layer (").
to an optic vesicle which is connected with the brain by an optic stalk.
The vesicle extends out to the periphery and, coincidently with the de-
velopment of the lens, is folded into a double-walled optic cup with outer
or tapetal, inner or retinal layers. If the position of the epithelial cells
be followed, it will be seen that the peripheral ends rest upon the tapetum,
and when these ends develop the rhabdomes, these must grow into the
tapetal layer.
In contrast to the retina, the lens develops as an invagination from
the epithelium of the body (fig. 574) ; sclera, cornea and vitreous body from
connective tissue. Thus the important part of the eye arises from the
brain and is later provided with accessory apparatus which arise from
peripheral parts. The invertebrate eye, on the other hand, with all its
parts arises from the skin.
The vertebrate eye is furnished with secondary structures: with mus-
cles which move it, with lids which protect the cornea from injury and
drying. The lids are dermal folds which extend over the eyeball from
above and below. To these a third lid, the nictitating membrane, may
542 CHORDATA,
be added. It arises from the inner angle of the eye, and can extend over
the cornea beneath the upper and lower lids. A special lachrymal gland,
which occurs at the outer angle of the eye, provides the fluid to moisten
the cornea, while a second or Harder’s gland occurs at the inner angle
when a nictitating membrane is present. Both are lacking in the An-
amnia,
The ear, at the level of the medulla oblongata, rivals the eye in
its complication of structure. In development it has one point in
common with the invertebrate
otocyst—it arises as an ecto-
dermal pit which is usually
completely cut off from its par-
ent layer, and only in elasmo-
branchs remains connected with
the exterior by a tube, the
elsewhere closed endolymphatic
duct. In the cyclostomes it con-
sists of a single vesicle with a
single macula acustica; from
the fishes upwards the vesicle
becomes divided by a constric-
tion into an upper utriculus
and a lower sacculus (fig. 575),
the connecting utriculosaccular
Fic. 575.—Diagram of membranous laby- duct being narrow in the mam-
rinth of a fish. (From Wiedersheim.) i :
au, de, ap, anterior, external, and poste- mals. Both utriculus and sac-
rir mpulle; am, superiet MeneUAT onlns receive a part of the
ne oe canalss usuttl: macula acustica. Diverticula
dolemppaticus; & legends zee Teesseus from the vesicle occur, giving
snipe necatiGg Eo en the whole the name of labyrinth.
doly mph duet: From the utriculus arise three
semicircular canals, connected at either end with this cavity, each
swollen at one end to an ampulla, containing a special nerve
termination, the crista acustica. These canals stand at right
angles to each other in the three dimensions of space and with-
out doubt subserve the sensation of equilibration (p. 128). They
are an outer horizontal, an anterior vertical (nearly sagittal), and
a posterior vertical (nearly transverse). The non-ampullar ends
of the two vertical canals unite, a condition which is understood
when it is recalled that in cyclostomes these canals alone are
present, and in Myzxine form a single canal with two ampulle.
A later formation is a diverticulum from the saeculus, which
IV. VERTEBRATA. 543
appears even in the fishes as a small pocket, the lagena, containing
a part of the macula acustica; in the reptiles and birds the lagena
becomes much larger, and in the mammals is spirally coiled and is
known asthe cochlea. A part of the macula acustica of the lagena
develops into a special nerve-end apparatus, the organ of Corti.
The membranous labyrinth described above is partially or en-
tirely enclosed in the side wall of the skull in the otic capsule,
which may ossify to the otic or petrosal bones. In the birds and
mammals the enclosure is such that the structure is duplicated in
bone, so that the membranous labyrinth lies in a bony labyrinth,
We
ILO (Be
Ve ea 4
rot
Fig. 576.—Diagram of human ear. (From Wiedersheim.) a, , vertical semicircular
canals ; ¢, their upper connexion; Co, the connexion in bony labyrinth; Con,
ductus cochlearis; Con’, cochlea; Cr, canalis_reuniens; Ct, tympanic cavity
(left), cupula terminales (right); d, perilymph; De ductus endolymphaticus; Dp,
Dp’, ductus perilymphaticus ; Kl, AU, bony labyrinth surrounding the mem-
branous labyrinth, the perilymph space black; MW, conch of ear (left), membrane
closing fenestra rotunda (right); Mae, external auditory meatus; Mt, tympanic
membrane; NS, sacculus; SAp, ear bones (represented as a rod); Ne, sacculus en-
dolymphaticus; St, Sv, scale tympaniand vestibuli; Th, Th’, Eustachian tube and
its entrance into pharynx; *, connexion between scale tympani and vestibuli;
+, insertion of ear bones in fenestra ovalis; 2, utriculus.
the two being separated by lymph spaces (fig. 576). These spaces
are developed in the cochlea into two tubes, the scala tynipani and
scala vestibuli, the two connecting only at the tip, being separated
elsewhere in part by the membranous cochlea (the ductus cochlearis
or scala media). The spaces of the bony labyrinth are filled by
two different fluids: inside the membranous labvrinth an en-
dolymph, and between this and the walls of the bony labyrinth a
perilymph.
o44 CHORDATA.
Accessory structures may be added to this auditory apparatus
proper, their purpose being to bring sound waves to it. Such
structures are but occasionally present in fishes (it is not certain
that they hear), since the sound waves are easily carried by the
water to the tissues and thence directly to the ears. On the other
hand, with the change to terrestrial life such a sound-conducting
apparatus is necessary on account of the differing densities of the
air and the tissues. So we find from Amphibia onwards a vibrat-
ing membrane—the tympanic membrane—which receives the sound
vibrations from the air and carries them to a chain of ear bones
(ossicula auditus), which in turn transmits them to the inner ear
orlabyrinth. These structures are not always functional (cetacea),
and they may be wholly or in part rudimentary (urodeles, snakes,
Amphisbenids).
To understand this apparatus it must be recalled that the ear
lies between the hyoidand mandibular arches in the neighborhood
of a canal which leads from the surface to the pharynx. In
the fishes this canal is the spiracle, a reduced gill cleft. In the
Anura and amniotes it consists of an air chamber closed exter-
nally by the tympanic membrane, stretched on a tympanic an-
nulus, while the opening to the pharynx is retained. The
part next the membrane becomes expanded into the tympanic
cavity, this with the membrane forming the tympanum or drum.
The part connecting with the pharynx is usually narrowed and is
called the Eustachian tube. The membranous labyrinth lies in
the wall of the tympanic cavity and touches it at one or two points
where the bony auditory capsule is interrupted, the always present
fenestra ovalis, and the fenestra rotunda, lacking in Amphibia.
When it is recalled that the mandibular arch lies just in front
of the spiracle, and the hyoid close behind it, it is readily under-
stood how parts of these arches can
enter the tympanum and produce the
ear bones. In Anura, reptiles, and birds
a columella has one end attached to
the stapedial plate, which lies in the
fenestra ovalis, while the other is in-
serted in the drum membrane, the
whole conveying the waves across the
tympanum to the labyrinth. In the
F1a. 517.—Ear bones ofman. mammals the structure is different, since
ae bel ageene pom 4 the columella is replaced by two bones,
nS the malleus, which is attached to the
drum membrane, and the ineus, which articulates with the
IV. VERTEBRATA. 545:
stapes. Most students believe incus and malleus to be parts
(quadrate and articulare) of the mandibular arch—a view which
has its opponents, who believe these to be a divided columella
(fig. 577).
The tympanic membrane is usually flush with the surrounding
skin or only slightly below its level. In the mammals it is pro-
tected by being placed at the bottom of a deep tube, the external
auditory meatus. The ear conch, a fold of skin supported by
cartilage, is also confined to the mammals.
The more important vegetative organs of the body are enclosed
in a large body cavity or celom beneath the vertebral column.
This is, as development shows, an outgrowth from the primi-
tive digestive tract, an enterocwle (pp. 109 and 158), lined
with epithelium. Since it arises, as in other colomate animals,
by paired outgrowths from the archenteron, it follows that
at first the two cavities must be separated by a_ partition
Wh
Rox WS
As
Fia. 578.—Section of vertebrate in abdominal region. (From Kingsley.) a, dorsal
aorta; ¢, ccelom; y, gonad; gl, glomerulus; i, digestive tract; l, iver; m, mesen-
tery; mu, muscular part of myotomes; my, its caelom (myoccele); 0, omentum;
s, Spinal cord; so, sp, somatic and splanchnic epithelia; t, nephridial tubule; vm,
sy alc atic
ventral mesentery ; w, Wolffian duct.
< = a) RO ml :
which also encloses the intestinal tract (fig. 578). These
walls furnish the mesentery which supports the intestine in its
546 CHORDATA.
whole length from the vertebral column, but ventral of the diges-
tive tract (as the mediastinum, omentum minus, and suspensory
ligament of the liver of human anatomy) only reaches as far back
as the liver, so that right and left coloms unite behind. Some
other organs are also suspended in the body cavity by membranes:
the testes by the mesorchium, the ovary by the mesovarium.
The body cavity is frequently called the pleuroperitoneal cavity,
since in mammals it is divided by a partition, the diaphragm, into
an anterior or pleural and a posterior or peritoneal (abdominal)
cavity. The lining membranes of these cavities are called pleura
and peritoneum respectively. The pericardial cavity is also a de-
rivative of the celom, and the lining, the pericardium, but a part
of the pleuroperitoneal membrane. Hence it is that in many
fishes (sharks, sturgeon) a communication persists between the
pericardial and the other celom. In most fishes and in many rep-
tiles there is a direct connexion of the celom with the exterior by
one or two pori abdominales, beside or behind the anus.
The alimentary tract possesses the greatest systematic interest
of the vegetative organs, for it not only is concerned with diges-
tion, but furnishes, as in all chordates, the respiratory organs (gills
and lungs) as well, these arising in the non-chordates from the
ectoderm. It begins with the anterior ventral mouth and ends
ventrally with the anus, some distance in front of the tip of the
tail; it is almost wholly entodermal in origin, there being but
slight ectodermal portions at either end.
The first division is spacious and consists of the ectodermal
mouth cavity and the entodermal pharynx, two spaces which, in
most vertebrates, are not sharply marked off, but in alligators and
mammals are separated by the soft palate. Then begins the
narrow cesophagus, wideuing behind to the stomach. From the
hinder or pyloric end of the stomach begins the small intestine,
which enlarges into the large intestine, separated from the small
intestine in the higher vertebrates by a valve and one or two ecxea.
The terminal portion in most vertebrates is called the cloaca be-
cause it receives the urogenital ducts. The liver is the only gland
constantly present; it isa large compact brown organ, generally
provided with a gall bladder. Usually a smaller gland, the
pancreas, occurs. The ducts of the liver (bile duct, ductus
choledochus) and pancreas empty into the small intestine near the
pylorus. . The mouth cavity may have salivary glands connected
with it, while the rectal region occasionally has blind sacs and
glands.
47
or
IV. VERTEBRATA. :
A striking vertebrate characteristic occurs in the dentition.
In the cyclostomes there are horny teeth—strongly cornified epi-
thelial products seated on connective-tissue papille; in the higher
groups occur true teeth of dentine and enamel, enclosing a richly
vascular pulp. They occur in places where the underlying skele-
ton affords them a firm support, especially on the upper or lower
jaws, but they may occur on other bones of the mouth and
pharyngeal cavities (roof of the mouth, gill arches). They have
apparently arisen from a diffuse dentition, recalling the scales of
the skin, since many elasmobranchs possess, besides the ordinary
teeth, rudimentary teeth in mouth and pharynx. Where teeth are
lacking (birds, turtles, baleen whales) they have been lost.
The respiratory organs arise from the pharynx. In the fishes
and some Amphibia its walls, right and left, are perforated by
gill clefts, each of which lies between two successive visceral arches
(fig. 570). These are canals which open internally into the
pharynx, while the outer gill openings are on the outer surface.
The anterior and posterior walls of the clefts bear delicate vascular
folds of mucous membrane, the gill filaments. These are the in-
ternal gills, in contrast to the external gills of Amphibian larve,
which are dendritic external ectodermal growths arising above and
between the gill slits (figs. 4,5). Itis important for the phylogeny
of the vertebrates to note that reptiles, birds, and mammals, which
never breathe by gills, have gill clefts outlined and later lost with
the exception of the Eustachian cleft.
Two problematical organs, the thymus and the lateral lobes of the thy-
roid gland, develop from the epithelium of the gill clefts. The middle
unpaired part of the thyroid has been regarded as a modification of the
endostyle of the Tunicata (p. 506). The thyroid, which produces iodine
compounds, is doubtless very important; disease or extirpation of it
causes serious nervous disturbances.
The lungs also arise from the pharynx as two sacs (one oc-
casionally remaining rudimentary), which grow downwards and
backwards. They retain their opening into it either directly or
by means of a trachea or windpipe, which just before its entrance
into the lungs usually divides into two bronchi (figs. 579, 620).
At the opening into the pharynx (glottis) the supporting cartilages
(remnants of the visceral skeleton, p. 524) are strong and form
the larynx, which in mammals may be closed from the pharynx
by a valve, the epiglottis. The lungs and trachea have their
counterparts in the fishes in the swim bladder, a hydrostatic
apparatus, and its duct.
548 CHORDATA.
The swim bladder of fishes and the lungs of most amphibia are smooth-
walled sacs, but in some have greater respiratory surface since folds ex-
tend into the central space. This peripheral folding increases in the rep-
tiles at the expense of the central chamber, this in some being completely
divided by the partitions, which extend inwards from the walls to the
bronchus. In the mammals a central chamber is lacking; the bronchi
extend into the lungs, branching again and again to the fine bronchioles
which give off alveolar ducts lined with minute air cells or alveoli.
The circulatory apparatus is easily derived from that of annelids,
and, like it, is completely closed. In the annelids (p. 307, figs.
272, 275, 276) above and below the
digestive tract is a longitudinal blood-
vessel, these being connected in each
somite by loops which pass around
the intestine. The vertebrate scheme
varies in the development of a heart
in the ventral trunk (the dorsal of
the annelid). In the lower verte-
brates, the fishes (figs. 65, 597), the
heart lies close behind the gills and
sends to them the blood which it
receives from the body. Hence, like
y 3” the whole ventral trunk, it carries
Fie. 579.—Lungs of man, ventral view. Venous blood. Since the anterior
(From Wiedersheim,) S, sulcus for loops, the gill arteries, pass through
subclavian artery; Tr, trachea di-
viding below into the two bronchi; the gills, the dorsal trunk, which
Z, position of diaphragm; 1, 2, 3, 2a,
3a, lobes of right and left lungs. — gg}lects from these, must contain
oxygenated blood, which is sent by the carotids to the head, and
by the dorsal aorta and the vascular loops to the body. It thus
becomes venous and flows back into the ventral trunk.
This scheme of circulation in fishes needs further description.
The heart, a strong muscular organ enclosed in a pericardium, con-
sists of two parts, auricle and ventricle, separated by valves. The
trunk (ventral aorta) arising from the auricle is arterial and cor-
responds to the ascending aorta and pulmonary artery of man.
The arterial arches of the gill region which arise from it pass di-
rectly into the dorsal vessel only in young fishes (fig. 597); later
they furnish the branchial circulation of gill arteries, gill capillaries,
and gill veins (fig. 65). The dorsal trunk is the dorsal aorta
(aorta descendens); the ventral trunk, which only oceurs in the
embryo, is the subintestinal vein, from which the portal vein arises.
To this are added a system of paired veins, consisting of Cuvierian
IV. VERTEBRATA, f40
ducts and jugular and cardinal veins, the latter with growth en-
croaching more and more into the territory of the subintestinal
vein.
The circulation of the fish type undergoes a great modification
with the loss of gillsand the appearance of pulmonary respiration.
Gills and gill capillaries disappear, and the branchial circulation is
reduced to arterial arches leading direct from the ventral to the
dorsal aorta. The swim bladder received its blood from the body
(systemic) circulation, but with the functioning of the lungs pul-
monary arteries and veins come into existence, while the arterial
arches in part disappear, in part are divided between the pulmonary
III LIV
Fig. 580.—Diagram of modification of arterial arches in various vertebrate classes. White,
vessels which degenerate; cross-lined, vessels containing arterial blood; black, vessels
containing venous blood. J, Dipnoi; J/, Urodeles witb pulmonary respiration; JJJ,
Reptiles; JV, Birds (in mammals the left instead of the right aortic arch persists). ao},
venous aorta of reptiles; co?, arterial aorta; ast, arterial trunk; a, bh, arches which
usually disappear; ad, dorsal aorta; «¢.8. ductus Botalli; k, gill capillaries; pu, pul-
monary artery; 1-4, persistent arterial arches.
and systemic circulations (fig. 580). Of the six arches which
usually appear in the embryo, the first and second, and the fifth
in animals with lungs, usually disappear. The last arch (4), which
even in the Dipnoi supplies the swim bladder, becomes a pulmonary
artery, the other arches (7 and 2) furnish the systemic portions,
the dorsal aorta (2) and the carotids supplying the head (2).
Since special pulmonary veins, distinct from the systemic circula-
tion, carry the blood from the lungs to the heart, the heart be-
comes divided by a septum which separates it into right and left
halves. The right half retains the venous character of the fish
heart; since the right auricle receives the systemic veins, the right
ventricle gives off the pulmonary artery. The left half is purely
arterial, receiving arterial blood by the left auricle from the lungs
and sending it out through the aorta ascendens to the body. A
complete separation of pulmonary and systemic circulation, and a
corresponding division of the heart, occurs only in birds and mam-
550 CHORDATA.
mals. Reptiles and amphibia show how the modification has been
accomplished. In these the separation begins in the venous sys-
tem and extends to the auricle, in the reptiles the septum arises in
the ventricle. In the arterial system remnants may persist, such
as a connexion (ductus Botalli) of the puJmonalis with the aorta
(1/, d.B), or an aortic arch may arise with the pulmonalis from
the right side of the heart (J/J, ao).
Besides blood-vessels, lymph vessels occur in the vertebrates as com-
plements of the venous system. The fluids which collect in the spaces of
the connective tissue are taken by them and carried into the large venous
trunks. Usually the action of the heart and the movements of the body
are sufficient to cause the flow of this lymph, but special lymph hearts
may occur. The lymph vessels distributed to the digestive tract play an
important role, since they serve in the resorbtion of digested food. They
are called chyle ducts because their contents, the chyle, rendered white
by oil globules at the time of digestion, distinguishes them from other
lymphatics. The most important features of lymph and blood have
already been noticed (p. 88). In special places small bodies, the lymph
glands, are inserted in the course of the lymph vessels, in which lymph
corpuscles arise. Among these from its structure is to be enumerated the
spleen, colored bright red by its rich blood supply.
The sexual and excretory organs are so closely associated that
they are generally united as the urogenital system. The sexual
products are formed in the embryo from a special region of the
peritoneal epithelium on either side of the vertebral column.
These primordial cells early leave their primitive position, and sink
into the underlying connective tissue (fig. 33), forming in the
male glandular tubes, in the female cords which break up into
numbers of round follicles, each containing a single larger cell, the
ovum. In the male the gonads thus formed are compact and fre-
quently oval, the testes; in the female they are looser and follic-
ular ovaries.
The deposition of the sexual cells occurs in many fishes by way
of the body cavity and the abdominal pores, and in this case a part
of the celom may be cut off as a special vas deferens or oviduct.
In most vertebrates the ducts are formed from a part of the
nephridial system. Embryology shows that there are three kinds
of nephridia in vertebrates: (1) the pronephros, or head kidney;
(2) mesonephros, or Wolflian body; (3) metanephros, or kidney
proper, with the corresponding pronephric, mesonephric (Wolf-
fian), and metanephric (ureter) ducts. The first two of these
ducts are genetically connected, since the development of the
elasmobranchs shows that the pronephric duct, by splitting, gives
Or
ay
IV. VERTEBRATA. 5)
rise to two canals, the Wolffian (mesonephric), and the Millerian
ducts, the latter retaining its relation to the pronephros.
The pronephros is usually functional only in embryonic life
and then only in early stages, possibly in some cases not at all.
Its relations to the other parts are yet in question. In most
Fie. 581.—Scheme of urodele urogenital system based on Triton. (From Wieders-
heim, after Spengel.) A, male; B, female. a, excretory ducts; yn, sexual part
of mesonephros; Ho, testis; lg, Leydig’s duct (ureter); mg, Millerian duct
(oviduct); mg’, its vestigial end in male: NW, functional part of mesonephros;
Ov, ovary; Ot, ostium tube; Ve, vasa efferentia; *, collecting duct of vasa effer-
entia (rudimentary in B).
teleosts the mesonephros is equally developed in nearly the whole
length of the body cavity, but in the Amphibia (fig. 581) and
many elasmobranchs its anterior part is smaller than the rest, a
condition which has its explanation in its relations to the sexual
apparatus.
552 CHORDATA.
In the males (excepting many fishes) the testes become con-
nected with the anterior end of the Wolffian body (fig. 581, A),
so that the urinary tubules of the latter come to be seminal ducts,
while the hinder portion remains excretory, this condition being
permanent in the Amphibia. In the amniotes the anterior meso-
nephros retains its connexion with the testes, forming the vasa
efferentia, while the Wolffian duct forms the vas deferens, a por-
tion of it greatly coiled being the epididymis. The remainder
of the Wolffian body degenerates, a portion only persisting as the
paradidymis,
In the females (fig. 581, 4) the mesonephros is smaller in front,
as in the males, but the connexion of this with the ovary does not
exist, so here the Wolffian duct is solely excretory, and not, as in
the males, excretory and seminal duct. In the female amniotes
the Wolffian body almost entirely disappears, for in both sexes of
the reptiles, birds, and mammals the metanephros or kidney proper
is a new formation, growing forwards from the posterior end of
the Wolffian duct. In the females of elasmobranchs, Amphibia,
and Amniotes the Miillerian duct serves as an oviduct, its anterior
end opening by the ostium tube into the abdominal cavity and
receiving the eggs as they escape from the ovary. In the male the
Millerian duct disappears early.
The union of sexual and excretory organs to a urogenital system arises
from the same relations as in the annelids; both organs arise from the
celomic epithelium and have temporary or permanent connexion with the
body cavity. This has already been described for the gonads. The
urinary tubules of both pro- and mesonephros are derivatives of the celomic
epithelium and possess an arrangement recalling that of the annelids in
a striking manner. As is shown (fig. 70) in the scheme of the embryo
selachian, the nephridial system consists of numerous canals, segmentally
arranged, connected by funnels (nephrostomes) with the body cavity;
and differs from the segmental organs of the annelids in that they do not
open singly to the exterior, but by a common duct. They also differ in
their further development by increasing greatly in number and forming
a compact organ, and, finally, by the formation in a certain part of a
network of blood-vessels, the glomerulus, which pushes into the lumen of
the tube.
The ducts of the urogenital system open behind the anus in
most fishes on a urogenital papilla; in the elasmobranchs, amphib-
ians, birds, and most reptiles dorsally into the hinder part of
the digestive tract, which thus becomes a cloaca. In turtles and
mammals the urogenital canal opens into the urinary bladder, a
ventral diverticulum of the rectum which first appears in the
IV. VERTEBRATA, 553
Amphibia. Urinary and sexual ducts then either open into the
urogenital sinus, the lowest part of the bladder leading to the
cloaca (turtles, monotremes), or this part receives only the geni-
tal ducts, while the ureters enter the base of the bladder. The
urogenital sinus remains in connexion with the cloaca in the
turtles and monotremes; in the other mammals a cloaca occurs
only in embryonic life. Later, by formation of the perineum, the
cloaca is divided into a hinder digestive and an anterior urogenital
canal. Step by step the stages may be followed from urogenital
ducts opening behind to those opening in front of the anus.
Asexual and parthenogenetic reproduction are unknown in the
vertebrates. The impregnation of the eggs in the lower groups
is usually external and occurs during oviposition; in the higher
internal copulation is effected by opposition of the genital ori-
fices or by the development of an intromittent organ, the penis.
The fertilized egg can undergo a part or the whole of its devel-
opment in specialized parts of the oviduct (uterus). Accordingly
viviparous and oviparous forms are distinguished, and between
these extremes those that are ovoviviparous (cf. p. 161). Most
elasmobranchs are viviparous, but many are oviparous. In the
teleosts oviparous forms predominate, but there are viviparous
exceptions. So, too, among the reptiles and Amphibia there are
some viviparous species among the egg-laying majority. The
birds and mammals are most constant, the first being exclusively
ovoviviparous, while all the mammals bring forth living young
with the exception of the ovoviviparous monotremes,
Three embryonal appendages may occur in the development,
the yolk sac, the amnion, and the allantois. The yolk sac is small
in those vertebrates which have some yolk, but not enough to
cause meroblastic segmentation (Amphibia), yet it is everywhere
present and is best developed in those groups (fishes, fig. 582,
reptiles and birds) with discoidal segmentation, and is the result
of the accumulation of food material in the digestive tract, which
forces out its ventral wall like a hernia. Its presence in the mam-
mals, which have small eggs lacking in yolk, is an indication that
these have descended from large-yolked forms, such as the mono-
tremes yet are. The embryo either lies directly on the yolk or is
connected with it by a yolk stalk.
While the yolk sac is widely distributed, the amnion and allan-
tois are restricted to reptiles, birds, and mammals, which are con-
sequently spoken of as Amniota or Allantoidea, in contrast to the
fishesand Amphibia, which are frequently called Anamnia or Anal-
554 CHORDATA.
lantoidea, from the absence of these structures. The amnion isa
sac which envelops the whole embryo and is connected with the
rest only at the umbilicus, that is, the point where the yolk sac
projects from the ventral wall. In this sac is an albuminous
Fie. 582. Fig. 583.
Fie. 582.-Shark embryo. (From Boas.) y, part of yolk sac; yg, external gills in front
of pectoral fins. ;
Fig. 583.—Embryonic envelopes of amammal. (Diagram after KGlliker.) ah, amni-
otic cavity ; al, allantois; am, amnion; dg, yolk stalk ; ds, yolk sac; e, embryo:
hh, ventral wall of embryo; r, extra-embryonic ccelom; sh, serosa; sz, serosal
villi.
amniotic fluid. The amnion is genetically a part of the ventral
surface; it develops ventrally as folds—lateral, anterior, and pos-
terior—which grow up over the back on all sides and unite above
the embryo.
The allantois is an enlargement of the urinary bladder. This
grows out from the body cavity at the umbilicus and extends be-
tween yolk sac and amnion and then grows in all directions until
its folds meet above the back. The part of the allantois which re-
ceives the urine may be enlarged or not. The rest of the out-
growth consists of blood-vessels and connective tissue. The blood-
vessels are the most important, for the allantois forms the respira-
tory apparatus of the embryo, and in the mammals it develops the
placenta, by which nourishment as well is conveyed to the young.
Yolk sac, amnion, and allantois are enveloped in a common coat,
the serosa.
Aristotle and his followers recognized four divisions of vertebrates, and
these were retained by Linné and Cuvier under the names Pisces, Reptilia
or Amphibia, Aves, and Mammalia. Blainville (1818) divided the second
of these into two classes, retaining the name Reptilia for the one, Amphibia
for the other. Milne Edwards showed that this division corresponded
IV. VERTEBRATA: CYCLOSTOMATA. 555
with one between the higher and lower groups, the amniote and the anam-
niote divisions. Later Haeckel divided the fishes, separating the Cyclo-
stomes from the others as a distinct class, while Huxley pointed out the
close resemblances between the reptiles and birds, grouping them as
Sauropsida, Another division of convenience but not of much systematic
importance contrasts the fishes with all other forms, the Tetrapoda, so
called from the possession of legs rather than fins.
Series J. IcuTHyopsipa (ANAMNIA, ANALLANTOIDA).
Vertebrates respiring for a time or throughout life by means
of gills ; neither amnion nor allantois present in the embryo.
Class I. Cyclostomata (Marsipobranchii, Agnatha).
The class of Cyclostomes contains but few species, among
which the lamprey eels and the slime or hag fishes are best known.
In shape they are eel-like. They are distinctly vertebrate in the
possession of large liver and nephridia; of a muscular heart with
auricle and ventricle, lying in a pericardium; olfactory lobes,
epiphysis and hypophysis, and the higher sense organs. In the
brain, cerebrum and cerebellum are not so prominent as are the
optic lobes and medulla. The inner ear is not divided into utric-
ulus and sacculus, and it has but one or two semicircular canals,
but always two ampulle. The skin (fig. 26) consists of derma
and a stratified epidermis.
The cyclostomes are distinguished from the true fishes by the
lack of a vertebral column. The axial skeleton of the trunk consists
either of the notochord alone or of it and small neural arches. A
cranium and a basket-like gill skeleton are present, but so different
are these from those of other vertebrates that homologies are dif-
ficult. The absence of paired fins isimportant. Since the median
fins are supported by horny threads alone, the cartilaginous appen-
dicular skeleton—alone of importance—is entirely wanting. Then
the skin lacks scales, and the mouth true dentine teeth, for the
pointed brown teeth arranged in circles in the mouth of the lam-
prey (fig. 584), and the fewer teeth of the myxinoids, are purely
epidermal products and cannot be compared with the teeth of
other vertebrates. Other important differences have given rise to
names applied to the group.
The name Cyclostomata refers to the circular mouth, an ex-
ternal feature, which, however, rests on the important fact that
the jaws are absent or extremely rudimentary, and do not close on
each other as do the jaws of other vertebrates. This cyclostome
condition is of value to the animals, as it aids them in sucking on
556 CHORDATA.
to other animals. At the base of the dome-like mouth cavity is
the so-called tongue, which is the sucking apparatus, since it can be
drawn backwards like a piston (fig. 584).
The name Marsipobranchs has been given on account of the
form of the gills, which are usually six or seven in number, but in
Bdellostoma may be twelve or fourteen on either side. Lach gill
cleft consists of three parts, the gill sac (marsupium), which alone
contains gills, and the afferent and efferent ducts (fig. 585). These
canals arise separately, and may continue so (Bdellostoma), but in
Petromyzon the afferent ducts unite to a single tube which opens
ventrally in the pharynx. In Myzine (fig. 585) the conditions are
reversed, the efferent canals uniting to empty through a single
external opening.
A third name, Monorhina, has been given, since these forms,
in contrast to all other vertebrates, have an unpaired olfactory
organ. The single nostril, lying in the mid line of the head,
Fia. 584. Fig. 585.
Fig. 584.—Mouth of Petromyzon marinus with horny teeth and tongue. (From Gegenbaur.)
Fie. 585.—Gill apparatus of Mywxine gluitinosa. (After J. Miller.) a, atrium; «bd, gill artery
and gill arch: br, gill sac (the lines show the gills): hr’, efferent canal; c, cesophaged-
cutaneus duct; d, skin turned away; ‘, afferent gill canal; 0, esophagus; s, mouth
of atrium; v, ventricle of heart. ;
opens into a nasal sac, from the bottom of which a canal descends
towards the roof of the mouth, ending blindly in Petromyzontes
(Hyperoartia), or penetrating it in the Myzontes (Hyperotretia),
so that an inner nasal opening (choana) occurs. A paired olfac-
tory nerve supplhes the organ.
Sub Class I. Myzontes (Hyperotretia).
Semiparasitic eyclostomes with cirri around the mouth, very primitive
nephridia, right and left rows of slime saes, eyes rudimentary (lens, sclera,
IV. VERTEBRATA: PISCES. 55T
and choroid lacking). From the large amount of mucus they are known
as slime eels. They bore into fishes and eat the flesh. Myaine* on the
east coast, Bdellostoma* (Polistotrema) on the west.
Sub Class If, Petromyzontes (Hyperoartia).
Several American species of lampreys, all belonging to Petromyzon*
(with sub genera), have well-developed dorsal fins, and seven branchial
openings. They occur in salt and fresh water, some marine species
Fic. 586.—Petromyzon marinus,* sea lamprey. (After Goode.)
ascending streams to lay their eggs. The young pass through a larval
(Ammocceetes) stage with rudimentary eyes and slit-like mouth. Many of
the species live on the mucus and blood which they rasp from fishes.
Here may be mentioned a group of fossils, the OSTRACODERMI, of
uncertain position. They have fish-like bodies, but no skeleton or jaws are
known. They flourished in paleozoic seas. Preraspis, Cephalaspis,
Pterichthys.
Class II. Pisces (Fishes).
The term fish is used in a wider and a narrower sense. In the
first it includes any aquatic vertebrate swimming by means of fins
and breathing by gills; in the more strict sense, as used here, it
means aquatic branchiate forms with vertebral column, cranium,
and well-developed visceral skeleton; with paired as well as
unpaired fins, these supported by a cartilaginous or bony skeleton
in addition to horny rays; with double nasal pits; with a skin and
oral mucous membrane which can produce ossifications, the scales
and teeth. The cyclostomes are thus excluded. The fishes are
the best adapted of all vertebrates for an aquatic life, and their
whole organization must therefore be considered from this stand-
point.
The epidermis consists of numerous layers of protoplasmic cells
with an extremely thin external cuticle. Cornifications of this
epidermis are lacking under ordinary conditions, with the excep-
tion of a thin portion of the external subcuticular layer. At the
time of sexual maturity cornifications increase greatly in most
Cyprinoids and many Salmonids, producing hard bodies in the skin,
5S CHORDATA.
the ‘pearl organs.’ Enormous numbers of large slime cells give
the fishes their well-known slippery skins. Since the epidermis
contributes nothing to the firmness of the body walls, all protective
structures arise from the derma, which is composed of many layers
of dense connective tissue and furnishes the characteristic dermal
skeleton, the scales. These lie at the boundary of epidermis and
derma, commonly imbedded in pockets of the latter, and are, on
account of their different structure, of systematic value, although
the classification based entirely upon them is no longer retained.
The placoid scales (fig. 554, 587, 4) have already been men-
tioned, because they form the
starting point for dermal ossifica-
tious and teeth (p. 515). They
are rhombic bony plates, usually
close together like a mosaic, but
not overlapping. In the centre
of each is a spine, directed back-
yards, In which is a pulp cavity,
while the tip of the spine is coy-
ered witha cap of hard substance,
variously called enamel or vitro-
dentine.
The ganoid scales (fig. 587,
3) are usually rhomboid and
Fig. 587.—Scales of fishes. 1, cycloid; 2, arranged like parquetry. In the
EG BY NOTE Ae MNS early stages they may bear teeth,
but these are lost in the adult. The outer surface is always covered
with a thick layer of ‘ganoin,’ which gives, even in fossils, an
iridescent effect, a most characteristic feature. The ganoin is no
longer regarded as enamel, but the most superficial layer of dentine
(vitrodentine).
Cycloid and ctenoid scales are closely related. They are always
more loosely placed in the pockets, from which they are easily with-
drawn as in ‘scaling’ a fish. They are arranged in oblique, trans-
verse, and Jongitudinal rows, and overlap like shingles, one scale
covering the parts of two scales behind. The cyeloid scales (fig.
587, 1) are approximately circular with a middle point, surrounded
by concentric lines, from which go radiating lines. The ctenoid
seale (2) has the radial and concentric lines of the eycloid, but has
the hinder edge truncate and the free portion bearing small spines
or teeth, processes of the concentric ridges.
Besides these types of scales many fishes bear considerable
IV. VERTEBRATA: PISCES. 559
spines (strongly developed single scales) and larger bony plates,
these last usually resulting from the fusion of numerous scales.
The coloration of fishes is threefold in origin. The silvery lustre is
due to crystals of guanin which occur not only in the skin but in the peri-
toneum and pericardial walls. In some fishes from their iridescence
(Alburnus lucidus) these crystals become of commercial value. They are
freed from the skin by boiling with ammonia and, suspended in the fluid,
form the important part of essence of pearl (essence d’orient) which is
used in making artificial pearls, being either applied to the outside of ala-
baster balls (Roman pearls) or as a coating to the inside of glass beads
(Paris pearls). The other colors of fishes are due in part to the numerous
strongly pigmented fat cells, in part to ‘chromatophores’ in the derma,
which, under control of the nervous system, can alter their form and
extent and thus produce color changes in the fish. It is by means of
these chromatophores that fishes adapt themselves to their surroundings.
It is of interest to note that destruction of the eyes results in loss of power
to change color.
The axial skeleton shows many conditions which are unknown
outside the class, and varies in character from group to group, the
most important differences consisting in its cartilaginous or bony
character. The vertebre are nearly always amphicelous, the
notochord persisting in the cavities between the successive centra
(fig. 557). Neural and hemal arches occur, these having as key-
stones the unpaired spinous processes. The neural arches extend
throughout the columns; the hemal are complete only in the tail;
in the trunk the hemal spines are absent and the hemal processes,
divided into basal processes and ribs, surround the viscera. A
sternum is everywhere lacking. When ossification is lacking or
is incomplete, two pairs of arches may occur in each segment, the
anterior being the stronger and alone persisting in fishes with ossi-
fied vertebre; the second is much smaller, so that its elements are
not called arches, but intercalaria (figs. 556, 588).
The great number of visceral arches, and their independence
from the cranium, are characteristic of fishes. After removal of
these the cranium in all cartilaginous fishes is very simple (fig.
588), but in the teleosts, with the appearance of ossification, be-
comes very complicated, since the bones are very numerous and
are not, as in mammals, in purt fused to larger bones. There are
also great differences between the different families of fishes, some
having bones which are lacking in others (figs. 560, 589). The
large membrane bones of the cranial roof (parietals, p, frontals, /,
and nasals, na) and the large ventral parasphenoid (ps) are
especially constant. The vomer in front of the parasphenoid is
dO0 CHORDATA,
unpaired, while in all other vertebrates it is paired. Most con-
stant of the cartilage bones are the ethmoids (the paired ecteth-
moids, ce, and the sometimes paired mesethmoid), and the four
occipitals. On the other hand the otic and optic regions vary
considerably; the otic region, from its great size, has several bones,
usually (fig. 589) five in number: pterotic, pto, often called
Fig. 588.—Cranium, visceral arches, and part of vertebral columnof Mustelus vulgaris. aor
antorbital process; co, copula; gp, foramen for glossopbaryngeal; H, otic capsule and
hyoid; Hm, hyomandibular; ic, intercalare; Md, mandible (Meckel’s cartilage) ;
N, nasal capsule; 0, optic foramen; ob, neural arcb; po, postorbital process; Py.
pterygoquadrate; ps, spinous process; R, rostrum; r, ribs; tr, trigeminus foramen ;
v, vagus foramen; J-‘, visceral arches: 1, labial; 2, mandibular; 3, byoid; 4-8. gill
arches.
squamosal; sphenotic, spo, frequently called postfrontal; epiotic,
epo; prootic, pro; and opisthotic, oo, the last sometimes lacking.
In the region of the eye the cartilaginous sphenoids are rarely
well developed, the large parasphenoid taking their place. The
same is true of the ali- and orbitosphenoids, these sometimes form-
ing an interorbital septum (fig. 560) or a more or less wide in-
terorbital fenestra (fig. 589).
The character of the visceral skeleton is related to the aquatic
life. All fishes have numerons gill arches (five to seven, mostly
five), which, since their function—gill supporting
similar in structure. So far as they are not degenerate they con-
sist each of four parts and are connected by unpaired copule, these
often being fused. The upper ends are frequently toothed and,
in chewing, are opposed by the rudimentary last arch, on which
account these are spoken of as the superior and inferior pharyngeal
bones. The anterior visceral arches are greatly different in car-
tilaginous and bony fishes. In the former (fig. 588) the pterygo-
quadrate (py) and the Meckelian cartilage bear teeth and oppose
ach other in biting. Tn the bony fishes (fig. 589) the teeth of
ts
is similar, are
IV. VERTEBRATA: PISCES. dL
the lower jaw oppose the tooth-bearing elements, premaxillary
and maxillary, of the maxillary series, while the pterygoquadrate
clements—the palatine and the series of pterygoids—are the an-
tagonists of the hyoid.
A second characteristic of the bony fishes is already outlined in
the Be aa sae the hea Cneion of the o pyomandibulyy, to
Fa. 589. —Skull of haddock. Infraorbital ring and operculum outlined in red. a,
; ar, articulare ; as, alisphenoid; de, dentary; ee, ectethmoid; ekt,
ory goid; eng, O8 entoglossum ; ent, entopterygoid; epo, epiotic; fr, frontal;
h'-h3, hyoid elements: hm, hy mandibular; ih, interhyal; ma, maxilla: me,
mesethmoid; mt, metapterygoi na, nasal; och, ocl, ocs, basi-, ex-, and supra-
occipital 3 00, opisthotic 3 p, parietal; pa, palatine; prm, premaxillary ; pro, pro-
otic ; ps, parasphenoid ; pto, pterotic; qu, quadrate; rbr, branchiostegals ;_ spho,
sphenotic: sy, symplectic; vo, vomer; w, vertebra. Bones outlined in red: inf,
infraorbital ; io, interoperculum ; 0, operculum ; pro, preoperculum ; so, suboper-
culum ; 1,/, 3, axes of ,abial, mandibular, and hyoid arches.
a suspensor of the jaws. In the elasmobranchs (especially the
skates) the parallelism of hyoid and mandibular arches is lost, the
hyomandibular separating from the hyoid and attaching itself to
the hinge of the jaws. In the teleosts the hyomandibular is thus
brought in connexion with the quadrate, and lies between it and
the cranium, the joint being thus indirectly supported from the
cranium, a bone, the symplectic (known only in fishes) helping
out the suspensor, while another bone, the interhyal, connects this
with the hyoid, which itself divides into two, so that the hyoid
arch, like a gill arch, consists of four clements.
562 CHORDATA.
The opercular apparatus does not occur in all fishes. It isa
number of bony plates and processes which arise from the hyoid
arch and extend backwards over the gills, protecting them. It
arises in part (opercular bones—O, Pro, So, Jo, fig. 589) from
the hyomandibular, in part (branchostegal rays) from the hyoid
bone. The significance of this apparatus will be spoken of in con-
nexion with the gills; it gives the fish head a definite character,
but covers its structure, on which account it, like the infraorbital
ring, is shown in red in the figure 589.
The appendages are also influenced by the aquatic life. In
contrast to the cyclostomes, they have two pairs of paired fins, the
thoracic or pectoral, and the pelvic, ventral, or abdominal fins; in
contrast with Amphibia, reptiles, and mammals, which occasionally
have fin-like structures, the fishes have three unpaired fins, dorsal,
caudal, and anal fins. Only rarely, as in the eels, the ventral fins
are lacking; more rarely (Murenidz) the pectorals are lost. The
function of the fins in swimming and in balancing makes it neces-
sary that they be broad and well-supported plates. Hence it is
that numerous skeletal parts are present; besides those preformed
in cartilage, numerous horny or bony rays; further, that all parts
should be similar and closely, even if flexibly, bound to each other.
Joints are unnecessary except at the base where the fins join the
supports and move upon the body. The supports of the paired
fins are the girdles, arched skeletal parts, which in the sharks are
held only by muscles, a statement which is true for the pelvic
girdle of all fishes. This is why the ventral fins so readily change
their place. Their primitive position is at the hinder end of the
body cavity (Pisces abdominales, figs. 598, 601). From this point
they can move forward to beneath the pectorals (Pisces thoracici,
fig. 602), or may even come to lie in front of them (Pisces jugu-
lares) in the throat region (fig. 606). The pectoral arch is united
to the vertebral column in the skates; to the skull by a series of
bones in the teleosts. =
The dorsal and anal fins are supported by clements preformed
in cartilages which rest upon the neural or hemal spines and in
turn support the fin rays. In the caudal fin the rays rest directly
upon the spinous processes. Three types of caudal fin are rec-
ognized—diphycereal, heterocercal, and homocercal (fig. 10).
distinctions of great importance. The primitive type is the diphy-
cereal, in which the vertebral column extends directly into the
middle of the fin, dividing it into symmetrical halves. In the
heterocercal type the vertebral axis binds slightly upwards at the
IV. VERTEBRATA: PISCES, 563
base of the fin, so that the dorsal part is reduced, the ventral greatly
enlarged, the result being extremely asymmetrical, as seen from
the exterior. The homocercal fin is symmetrical externally, but
in reality is extremely asymmetrical. The end of the vertebral
column, the unossified notochord, is bent abruptly upwards, and
hence the fin is almost entirely formed of the ventral portion,
which is usually divided by a terminal notch into upper and lower
halves. The homocercal fin begins with a diphycercal and passes
through a heterocercal stage in development.
In correspondence with the simple motions the musculature is simple
and consists largely of longitudinal muscles divided into myotomes, which
are conical with the apex in front, and are ly ;
so inserted in each other that a cross-sec- iv op eN sibe([ ae
tion gives concentric circles. In a section t \ OLA EZ A
there are at least two such systems, the
muscles being divided by a lateral in-
cision into dorsal and ventral halves.
There are also smaller groups of muscles
related to fins, gill arches, jaws, ete., but
of much smaller size, derivatives from the
larger mass. Electric and pseudelectric
organs, which occur in different fishes,
sometimes in the trunk, at others in the
tail, are formed by the modification of mus- qr i
cles. Each organ consists of numerous if
closely packed vertical or horizontal col- ~ of electrical apparatus. (From
Wiedersheim.) The arrow points
umns, each column, like a Voltaic pile,
consisting of layers of gelatinous plates
(equivalents of muscle bundles) in which
the nerves, with special end plates, termi-
nate. The discharge is electronegative.
dorsally or anteriorly. BG, con-
nectiv issue framework; FP,
electrical plates; G, gelatin-
ous tissue; WV, nerves entering
through the septa; NN, nerve
terminations.
The brain shows the low position of the class in the slight development
of the cerebrum (fig. 591). This is especially true of the teleosts, in which,
in place of a cortex, there is only a thin epithelial layer (Pall), what was
formerly called cerebrum being only the corpora striata. The independent
olfactory lobes lie either close to the cerebrum (most teleosts, ol) or are
separated from it by an olfactory tract (fig. 592, 2). The optic thalami
are small (@), but below them are enlargements characteristic of fishes, the
lobi inferiores, and between them the sacculus vasculosus. Both optic
lobes and cerebellum are greatly developed.
The nose consists of two preoral pits, the opening being divided by
a bridge of skin into anterior and posterior nostrils. In many selachians
the nostrils are connected with the mouth by a groove covered by a fold of
skin, and in the Dipnoi there is a choana. The eye has several peculiari-
ties. The lens is very convex, almost conical, due to the slight refraction
caused by the passage of light from the water into the cornea. Further,
564 CHORDATA.
the eye is very short-sighted because light is so absorbed by water that
objects forty feet away are invisible. With this is connected the cam-
panula Halleri. The processus falciformis, a sickle-shaped outgrowth of
the choroid, extends from the entrance of the optic nerve into the vitreous
body as far as the lens, swelling out into the campanula; this contains a
muscle which draws back the lens and so is an apparatus of accommoda-
tion. Near the entrance of the optic nerve is a problematic organ, the
Tie. 591. Fie, 592.
Fia. 591.—Brain of trout. (After Wiedersheim.) BG, corpus striatum; GP, pine-
alis; HH, cerebellum; Lol, olfactory lobes; MH, optic lobes; NH, Medulla
oblongata; Pali, pallium, in part cut away; VH, cerebrum; XJ, nerves.
(See p. 536.)
Fig. 592.—Brain and nasal capsules of Scyllium catulus. (From Gegenbaur.) a, me-
dulla; 6, cerebellum ; c, optic lobes; d, *twixt brain; g, cerebruin; /i, bulbus and
tractus olfactorius ; 0, nasal capsules.
choroid gland, consisting largely of blood-vessels (rete mirabile). Chon-
drifications and ossifications of the sclera are common. Lids are weakly
developed or absent, and only some elasmobranchs have a nictitating mem-
brane.
The ear has a relative size found in no other vertebrates, the labyrinth
corresponding well with fig. 575. The labyrinth contains in many teleosts
two otoliths, the asteriscus and sagitta, the first being especially large.
Experiments show that the ears are primarily for balance, and hearing is
doubtful. Strychninized fish do not respond to sound, if in its production
mechanical vibrations are avoided.
Of all sense organs the most noticeable are those of the skin, especially
those of the lateral line, which are nowhere else so well developed and
which occur elsewhere only in cyclostomes and aquatic amphibia. In
fishes a line on either side usually begins at the tail and extends to the
head, where it divides into several curved lines (fig. 602, SZ). Its position
is marked by a groove or a canal in the seales which opens to the exterior
by numerous canals through the scales. Branches of trigeminus, facialis,
glossopharyngcus, and especially the lateral branch of the vagus (fig. 570) go
to these organs, the latter extending back to the tail. These supply special
IV. VERTEBRATA: PISCES. 565
sense organs, which may be grouped in several lines or occur in pits (am-
pulle) in the skin in other places. Their function is obscure, since noth-
ing of the sort occurs in man or mammals. They are specific organs of
aquatic vertebrates and possibly have to do with the perception of water
pressure.
The alimentary tract is spacious only in the oropharyngeal
region. Then it narrows to a tube in which the various regions
are not sharply marked off from each other. Mouth and pharynx
frequently bear teeth. In the teleosts the bones of the floor of
the cranium and those of the visceral arches may be covered with
coalesced heckel-like teeth. In the elasmobranchs the teeth are
mostly confined to the lower jaw and the pterygoquadrate, but are
in rows, the anterior row alone being functional; but as these are
loosely held they are easily torn out, when they are replaced by
the row behind. Liver and spleen are always present; pancreas
and gall bladder usually occur. In many fishes blind sacs, the
pyloric ceca, occur at the junction of stomach and intestine (fig.
593, B); others have a spiral valve (4), a fold of mucous mem-
brane, which extends like a spiral stairway into the lumen of the
intestine, increasing the digestive surface. Czeca and spiral valve
rarely occur in the same fish.
Fia. 593.—Digestive tracts of (A) Squatina vulyaris (partly opened) and (B) _Tra-
chinus radiatus. (From Gegenbaur.) ap, pyloric ceca; c,rectuin; d, bile duct:
dp, duct of air bladder; i, intestine; 0e, esophagus; », pylorus; 1, stomach; vs,
spiral gland ; x, rectal gland.
Gills of two types occur (fig. 594, 4 and BL). In both the gill
clefts, which le between successive branchial arches, begin by
openings in the pharynx, but differ in their external openings.
In the elasmobranch type (.1) the external openings are a series of
slits separated by broad dermal bridges which cover the gills and
gill clefts (fig. 598). The gills are vascular folds of mucous mem-
66 CHORDATA,
brane with secondary folds which extend on anterior and posterior
sides of the cleft. Hach arch except the last, as the sections (fig.
594, A, und 595) show, bears two rows of gill folds (demi-
A, B
Ley am {3
7 \ 7
. Yo deaf WAN ONNS pis are
Fic. 594.—Pharynges of (4A) Elasmobranch (Zygena) and (B) Teleost (Gadus), the
skull removed and on the left the gill slits cut across. a,attachment of upper
jaw to cranium, cs, outer gill slit; b, gill arch; bU, DI2, anterior and posterior
gills (demibranchs); h, dermal projection ; lon, hyomandibular ; is, inner gill
cleft; m, mouth ; ma, maxillare ; 0, esophagus; op, operculum ; ops, opercular
opening; pe, palatine; phi, inferior pharyngeal bones; pq, pterygoquadrate;
prim, premaxilla; s, shoulder girdle ; wk, lower jaw; 2, tongue.
branchs) which belong to different clefts and are separated from
each other by tissue containing the cartilaginous gill rays.
In the second type (2), which occurs in all other fishes, the
dermal bridges are lacking, and the tissue between the demi-
branchs has more or less completely disappeared, so that the
demibranchs of one arch become connected, their free ends pro-
jecting into the water like the teeth of a double comb. Here,
on account of their very delicate structure, they would be ex-
posed to serious injury were they not protected by the opereulum
or gill cover, This is a fold of skin arising from the hyoid arch
and extending back over the gillregion. It is supported by two
groups of bones, the opereular bones proper (fig. 589, O, Se, Jo,
Pro), attached to the hyomandibular, and the branchiostegals
1V. VERTEBRATA: PISCES. 567
(ror) from the hyoid, these latter supporting the branchiostegal
membrane. Between the free edge of the
operculum and the branchiostegal mem-
brane and the skin of the body behind is
the opercular cleft (fig. 594, ops), which is
obviously not identical with a gill cleft. but
leads into an atrium into which the gill
clefts empty. In many elasmobranchs and
ganoids there is a rudimentary cleft, the
spiracle, between the pterygoquadrate and
hyomandibular, in which a rudimentary gill,
or pseudobranch, may occur, this often per-
sisting when the spiracle is closed.
Besides gills, fishes, with the exception
of elasmobranchs and some teleosts, have a
Fig. 595.—Sections of gill
arches of Cradus (left)
swim bladder which is usually regarded as aehidg uolareen ae
the homologue of the lungs. It is often — jnteny.s A ey
shaped like an hour glass, filled with air, and My er ee ona
may open into the esophagus bya pneumatic — veins 2, tooth.
duct (Physostomi), or this, appearing in development, may be lost
in the adult (Physoclisti). The air Dladder serves for respiration
in the Dipnoi and possibly in some ganoids (Lepidosteus aud
Amia), but is usually a hydrostatic apparatus, its enlargement or
compression altering the specific gravity of the fish. In fishes
brought up from great depths the expansion of air in the swim
bladder frequently forces the viscera out through the mouth.
The heart, enclosed in the pericardium, lies immediately
behind the gill region, and is protected by the shoulder girdle.
It always consists of auricle and yentricle (fig. 506), separated by
a pair of valves to prevent back-flow of the blood; it sends the
blood to the gills by the arterial trunk (vertral aorta), and receives
it from the body through a thin-walled sac, the venous sinus, in
which the hepatic veins and the Cuvierian ducts (formed by union
of jugular and cardinal veins) enipty (figs. 65, 597).
The most important differences lie in the development of conus
and bulbus arteriosus. These are muscular accessory organs, the
first arising from the heart, the other from the arterial trunk; and
correspondingly the conus has striped, the bulbus smooth musele
fibres. The anterior end of the heart contains ‘semilunar’ valves,
which, like the auriculo-ventricular valves, prevent the back-flow
of the blood. When, by increase in the number of valves, this part
becomes clongute, a conus arteriosus (fig. 596, uf) is formed. The
568 CHORDATA.
bulbus ((’) is a muscular swelling in front of the conus, in the
arterial trunk.
The connexion of ventral and dorsal aortw is effected in young
fishes (fig. 597) by the gill urteries directly; later by means of the
complicated loops of the gill circulation. When these are de-
Fic. 596.—Forms of hearts of fishes in schematic long section. (After Boas.) 4. sela-
chian and most ganoids: B, Amid; C, Teleost. a, auricle; b, bulbus arteriosus: ¢. conus
arteriosus; /, valves; s, sinus venosus; ft, truncus aorte; v, ventricle.
ad
Secon F :
>
——
Fie. 597.—Head of embryo teleost. (Diagram from Gegenbaur.) «a, auricle: abr, ventral
aorta with arterial arches; ad, dorsal aorta: c, carotid ; dc. Cuvierian duct, formed by
union of jugular and posterior cardival veins ; 1, nostril; s, gill clefts; sv. sinus veno-
sus; ¢, ventricle.
veloped, afferent branchial arteries, gill capillaries, and efferent
arteries cun be recognized, the latter uniting to form the dorsal
vorta and also giving off the arteries (carotids), which go to the
head.
The nephridia are a pair of large reddish-brown organs lying
outside the body cavity to the right and left of the vertebral
column, usually extending from heart to anus. Their ducts empty
behind the anus or in the dorsal wall of the intestine and are often
provided with enlargements called, from their functions, urinary
bladders, although totally different morphologically from the
urinary bladder of the higher vertebrates. The gonads, suspended
IV. VERTEBRATA: PISCES. 569
by mesorchia or mesovaria, are large and project into the body
cavity. They are rarely unpaired. In the elasmobranchs and
most ganoids these products pass out by the urogenital system (p.
552), in other forms by the pori abdominales or by special ducts.
Cuvier divided the fishes into cartilaginous and bony groups, an im-
portant step so far as the extremes (elasmobranchs and teleosts) were
concerned, Agassiz recognized a middle group which he named Ganoidei,
from the character of the scales, but his account was modified and made
more accurate by Johannes Miiller, who also included the Dipnoi among
the fishes. At present the group of ganoids is retained largely as a matter
of convenience. Its members are more closely related with the teleosts
than with the elasmobranchs, and in America Ganoids and Teleosts are
united under the head Teleostomi, the name alluding to the presence of
a true upper jaw comparable to that found in higher vertebrates,
Sub Class 1. Hlasmobranchit (Plagiostomt, Chondropterygit).
The elasmobranchs, the shark-like fishes, are almost exclu-
sively marine, varying in length from a foot and a half to sixty
feet, living almost exclusively on other vertebrates, and noted for
their voracity. Sometimes slender and cylindrical, as in the sharks
(fig. 598), sometimes flattened dorsoventrally, as in the skates (fig.
Fia. 598.—Acanthias vulgaris,* dogfish. (From Claus.)7 2B, ventral fin; Br, pectoral fin;
Ks, gill clefts ; 1, nostril ; R’, R?, dorsal fins ; S, heterocercal caudal fin ; Spl, spiracle.
599), they agree in form in that the head is prolonged into a snout,
which is usually supported by a cartilaginous prolongation of the
cranium, the rostrum (fig. 588, #). The mouth lies ventrally, at
more or less distance from the anterior end, and is transverse,
whence the name Plagiostomi—transverse mouth. This position
makes it necessary that a shark approaching its prey from below
must turn on its back before biting. The tail is heterocercal or is
drawn out in a long filament. The skin is covered with placoid
scales, usually close together, these being so small in some cases
that the skin—shagreen—is used instead of sandpaper for polish-
ing. More rarely the scales are larger, and the spines, which
project from the skin, justify in size and form the term dermal
teeth. Such strong spines occur especially at the front of the
570 CHORDATA.
dorsal fins (ichthyodolurites of paleontologists). The skeleton is
cartilaginous, frequently calcified on the outside. The calcifica-
tion can also extend into the vertebra, producing star-like figures.
Since bone is lacking, the sharks have no upper jaws, but bite with
the pterygoquadrate. The amphicelous vertebre (lacking in the
Holocephali and the extinct Cladoselachii, Ichthyotomi, and
Acanthodide), have neural arches, small ribs, and intercalaria.
The number of gill arches and clefts varies between five and seven,
the first cleft lying between the hyoid and the first branchial arch.
Besides, most elasmobranchs have a spiracle and pseudobranch
(fig. 598, Spl). Except in the Nolocephali the gill clefts open sep-
arately, the hyoid arch being without an operculum.
In the visceral anatomy these points are of importance as dis-
tinguishing elasmobranchs from Teleostomes. (1) The heart hasa
large conus, with several rows of valves (fig. 596, A), but lacks a
bulbus. (2) The alimentary tract (fig. 593, 4) has a spiral valve,
but lacks swim bladder and pyloric ceca. (3) The sexual products
are carried to the exterior by the urogenital ducts. The eggs
escape from the follicles of the ovary (occasionally unpaired) by
dehiscence into the body cavity, and from thence by the unpaired
ostium tube and the paired Millerian ducts to the exterior. The
spermatozoa traverse the anterior part of the Wolffian body (‘kid-
ney’). Sexual and reproductive ducts open dorsally into the
cloaca.
Male elasmobranchs are distinguished by the presence of a copu-
latory structure (mixipterygium) developed by enlargement of some radii
of the ventral fin (fig. 599, c). The large eggs, rich in yolk, are fertilized
in the oviducts and usually develop in uterine enlargements of the ducts.
The embryos (fig. 582), with long gill filaments protruding from the gill
slits, are nourished by the yolk in the yolk sac. In Mustelus aud Car-
charias, as Aristotle knew, there is the formation of a placenta, which
differs from that of the mammals in that the embryonic blood supply
arises from the blood-vessels of the yolk sac and are not allantoic. There
are oviparous elasmobranchs, and in these the egg is surrounded by
albumen and a shell, but these eggs differ from those of birds in that the
skull is horny and is usually drawn out at the four corners, sometimes
with threads for attaching the egg to plants, ete.
Order I. Selachii.
With the notochord more or less completely replaced by verte-
bral centra; no dermal bones.
Sub Order I, DIPLOSPONDYLI. Gill slits lateral, six or seven in
number, a single dorsal fin. Ch/amydoselachus with terminal mouth.
Hexanchus,* mouth normal, six gill slits; Heptanchus, seven gill slits.
IV. VERTEBRATA: PISCES, SELACHIL. 571
Sub Order II. SQUALI (Euselachii). Normal sharks, with cylindrical
bodies, free thoracic fins, heterocercal tail, lateral gill slits. Most of them
are fast swimmers and are rapacious, the teeth being usually pointed, with
sharp or toothed edges, but in some the teeth are pavement-like and are
used for crushing shell fish. The numerous families are distinguished by
vertebral characters, number of dorsal fins, presence of nictitating mem-
brane, ete. In the GaALEIDA, in which the nictitating membrane is
present, belong, besides the dog-sharks (Jfustelus * and Galeus*), the largest
of all sharks, Carcharinus,* some of which have man-eating reputations.
The hammer heads (Zygeena*) are closely allied. The mackerel sharks
Lamna*) and the great white‘ man-eater,’ Carcharodon,* lack nictitating
membranes. All of the foregoing have star-shaped figures in the verte-
bree (p. 570). In the dog-fishes, represented by Acanthias vulgaris * (or
Squalus acanthias, fig. 598), there is a spine in front of each dorsal fin.
Sub Order IIT. RAIA. In the skates the body is flattened horizontally
fig. 599), and the pectoral fins, also flattened, are united to the sides of
Fic. 599.—Raia batis, male, ventral view. (After Mébius and Heincke.) B. ventral,
Br, pectoral fin; R, rostrum, a, anus: ¢ copulatory part of ventral; ks, gill clefts:
m, mouth; n, nostril; between them the oronasal groove. ?
the body, the union usually extending clear to the tip of the snout, and
frequently back to the pelvis, giving the body a rhombic appearance from
above. The animals swim by undulating motions of these fins. They
mostly lie quiet on the bottom, and hence the lower surface is white,
the upper colored. The union of the fins to the side has resulted in trans-
572 CHORDATA.
fer of the gill slits to the lower surface, the spiracles to the upper. The
teeth are usually pavement-like. ‘The PRisTiDa, or sawfishes, are the most
shark-like, but are readily recognized as belonging here by the position of
the gill slits. The common name is due to the fact that the snout is pro-
longed into a paddle-shaped blade, the edges armed with teeth. Pristis.*
Raup#; the typical members of the group; Raia.* Closely allied are the
TRYGONIDA, or sting rays, with whip-like tail with one or two spines, the
‘stings’ at the base; Dasyatis.* The torpedos (TORPEDINIDZ) have
smooth skins, and have electrical organs, kidney-shaped bodies, on either
side between gill arches and pectoral skeleton. Torpedo.*
Order II. Holocephali.
These forms, which have no common English names, differ from
the selachii in having the pterygoquadrate arch, which bears a few
large chisel teeth, fused with the cranium without a suspensor; in
Fia. 600.—Chimera monstrosa. (From Kingsley.)
having a dermal fold constituting an operculum, which covers the
gill slits; and corresponding with this, the gills more on the teleost
type (p. 566). Lastly, the vertebral centra are not developed.
Chimera.* Fossils appear in the Devonian.
The CLADOSELACHI (Cladoselache), ICHTHYOTOMI (Pleuracanthus), and
ACANTHODID are paleozoic forms in which vertebral centra were lacking.
In Cladoselache the skeleton of the paired fin consisted of numerous simi-
lar radii and was more primitive than the archipterygium; Plewracanthus
was diphycercal, and the head, as in Acanthodes, bore dermal bones.
Sub Class II, Ganoidei.
The ganoids form a transition group in which elasmobranch
and teleost characters are mingled ina notable manner. They
have the spiral valve of the sharks, the swim bladder of the telosts;
the heart with the conus is selachian, the respiratory structures—
the comb-like gills and the operculum—are as distinctly teleostean.
The hyoid arch, with the development of the operculum, has not
entirely lost its respiratory function, since in garpike and sturgeon
it bears an opercular gill, and often there is a pseudobranch in
the spiracle. The skeleton is always ossified in certain parts; large
IV. VERTEBRATA: PISCES, GANOIDET. 5738
membrane bones lie on the shoulder girdle, on the roof and floor
of the skull (parasphenoid); the horny threads of the fins are bony
rays. In general the skeleton ranges between two extremes—an
extremely primitive cartilaginous condition with persistent noto-
chord, and one with a more than ordinary degree of ossification.
It is important for the systematist to find characters in all ganoids
which occur only in the group. The ganoid scales, used by Agassiz,
are not sufficient, since the sturgeon has bony plates free from
ganoin, while the paddle bill (Polyodon*) has almost no dermal
skeleton, and Amia has cycloid scales. Most recent and fossil
forms possess fulcra, bony plates with forked ends lying shingle-
like in front of the fins (fig. 10, B), but these are not universal,
and are absent, ¢.g., in Amia and Polypterus (fig. 10, 4 and C).
The group is largely American. The few recent ganoids fall into
three distinct groups.
Order I. Crossopterygii.
These are largely extinct, but two genera persisting to-day. The tails
are diphycercal or heterocercal; the pectoral fins have the basal portion
scaled; broad gular plates beneath the jaws in place of branchiostegals; the
skeleton well ossified. Polypterus and Calamoichthys from Africa. The
order was probably ancestral to the Amphibia.
Order II. Chondrostei.
These forms resemble the sharks externally in the heterocercal tail,
spiracle, ventral position of the mouth; internally in the cartilaginous
skull and (except Polyodon) in the pterygoquadrate serving as upper jaw.
In the vertebral column they are more primitive than most selachians,
since centra are lacking, the neural and hmal arches and the intercalaria
resting direct on the notochordal sheath (fig. 556). ACIPENSERID, with
Kia, 601.—Acipenser sturio,* common sturgeon. (After Goode.)
large bony dermal plates. Acipenser,* sturgeon. The swim bladder
furnishes isinglass, the ovaries make ecaviare. POLYODONTID&, with
naked skin and long paddle-like snout, toothed maxillaries present.
Polyodon,* paddle fish.
Order III. Holostei.
In these the skull is ossified as in teleosts; maxillary and premaxillary
bones are present, the pterygoquadrates reduced and not meeting in front,
and the mouth terminal. The body may be covered either with ganoid or
574 CHORDATA.
eycloid scales. The living forms (the group appears in the trias) have ossi-
fied opisthocelous vertebree and diphy- or homocercal tails.
Lepiposteip%. Scales rhomboid, branchiostegal rays present, a pseudo-
branch, but no spiracle. Lepidosteus,* garpike. AMIIDa&, distinctly teleos-
tean in appearance with cycloid scales, amphiccelous vertebre, and heart
with reduced conus (fig. 596, B). Améa,* bow fin.
Sub Class IL, Teleoster.
The teleosts owe their name to the extensive ossification of
the skeleton, which consists, in the trunk, of amphicclous vertebra,
and in front a skull with numerous primary and secondary bones,
already enumerated (p. 560, fig. 589). Maxillaries and premaxil-
laries are present, but these are frequently without teeth, since
other bones of the mouth (vomers, palatines, hyoid, gill arches,
superior pharyngeals—the latter alone in Cyprinoids) may bear
teeth. Frequently there are present small bones, usually forked,
lying in the intermuscular septa above the ribs, which are not pre-
formed in cartilage. These are the epipleurals, and are distinct
from the ribs. In the fins both cartilage and dermal rays are ossi-
fied, the former remaining small, the rays forming most of the
support. These rays may either be soft and flexible (Malacopteri)
or hard and spine-like (Acanthopteri), a matter of classificatory
value. In the first case they consist of numerous small threads
Fic. 602.—Perea fluviatilis. (From Ludwig-Leunis.) 4, anal fin; B, ventral fin; Br,
pectoral fin, K, operculum; NV, nostrils; R;, Ry, spinous and soft dorsal fins; S,
caudal fin ; Si, lateral line.
(fig. 602, Br, A, B, R,), in the other the parts of a ray are fused
to a spine which, sometimes provided with poison glands (Scorpena,
Amphacanthe, etc.), become good defensive weapons. The tail is
usually homocercal; the diphycercy of eels and other fishes is sec-
ondary. The dermal skeleton consists of ctenoid or cycloid scales,
sometimes of spines or body plates. In rare instances the skin is
naked.
IV. VERTEBRATA: PISCES, TELEOSTEI. 575
The hyoid arch always bears an operculum and _ branchiostegal
membrane, but there is no opercular gill. The gills of the
comb-like type, are confined to the four anterior gill, arches, but
they may be reduced to even two and one-half pairs of demi-
branchs. Instead of a conus (present in Butrinws), the bulbus
arteriosus is well developed; a spiral valve is lacking, but pyloric
appendages are common. A swim bladder is usually present, but
its duct is frequently closed.
The teleosts are distinguished from all vertebrates except the cyclo-
stomes and perhaps some ganoids in that the nephridial system does not
form part of the sexual ducts. The eggs and milt are deposited through
the abdominal pores or by special canals developed from the body cavity.
Copulation occurs in only a few viviparous forms (Embiotocide, Gambu-
sta, etc.). The rule isthat males and females deposit their reproductive
products in the water at the same time, and this leads to the enormous
schools of herring and other fishes which occur yearly at certain times.
This also explains the ease with which artificial impregnation in fish
culture is performed,
In rare instances the males care for the young, as in the case of the
sticklebacks; more noticeable are the conditions in the lophobranchs (sea
horses and pipe fish), where the males receive the eggs in a brood pouch on
the ventral surface. A metamorphosis is known only in the eel-like fishes,
the larvee of which—originally described as distinct under the name Lepto-
cephalus—are flat, transparent forms with colorless blood, enormous tails,
and extremely small trunk, These larvee normally occur in the sea at the
depth of some hundred fathoms. The fresh-water eels go to the ocean
for propagation. On the other hand many salt-water fish go to fresh
water for reproduction.
The classification of the fishes is yet in an unsettled state, partly owing
to the large number of forms, partly to the fact that the groups intergrade.
Most European writers recognize six divisions, Physostomi, Anacanthini,
Pharyngognathi, Acanthopteri, Cheetognathi, and Lophobranchii. Our
authorities separate the Ostariophysi from the Physostomi, the Pediculati
and Hemibranchii from the Acanthopteri, and unite the Anacanthini and
some of the Pharyngognathi with the Acanthopteri and make a distinct
group, Synentognathi, of the others. The characters on which these divi-
sions are based are less convenient for the tyro than those adopted here,
Order I. Physostomi.
The character to which this name refers is not readily seen
without dissection, the persistence of the duct of the swim bladder.
This is, however, correlated with the soft character of the fin
rays (few exceptions) and the abdominal position of the ventral
fins. The Ostariophysi are remarkable in having a chain of bones
connecting the swim bladder with the ear. More than a third of
the food fishes and nearly all of the fresh-water fishes belong here.
576 CHORDATA,
The Ostariophysial families are the Sirurip# (1000 species),
or cat-fish, with barbles about the mouth, of which Malapterurus,
Fra, 603.—Salmo salar,* Atlantic salmon. (After Goode.)
the electric cat of Africa, is most noteworthy. The CyPRINIDZ,
or carp (1000 species), and the suckers, Carostomip&®, have little
food value. The electric eel of South America belongs to the
Gymnonoti. The other families are true Physostomes. The Sa.-
MONID# are easily recognized by the ‘adipose dorsal,’ a fin formed
of a fold of skin without fin rays. The trout and salmon (Sa/mo *)
belong here and are among the most important food fishes.
Osmerus,* smelt; Coregonus,* white fish; CLUPEID£, herring,
shad; ANGUILLID#, eels, the breeding habits referred to above.
Esocipm, pike and pickerel. AxmBLyopsip., blind fish of Mam-
moth Cave.
Order II. Paryngognathi.
In many fishes the inferior pharyngeal bones (7.e., the last
rudimentary gill arch) fuse to form a single bone, and these forms
are called Pharyngognathi. Some have spiny fins, among the
Lasrip.x, including Ctenolabrus,* the cunners, and Tautoga,* the
Fria. 604.—Clenolabrus crruleus,* cunner. (After Goode.)
tautog. These are placed among the Acanthopteri by American
authors, Others have only soft fin rays. These are the Synento-
gnathi and include the Exocarrp.x, or some of the flying fishes,
in which the pectoral fins are very large, acting as parachutes
when the fish leap from the water. Lvocetaus.*
LV. VERTEBRATA: PISCES, TELEOSTEI. BIT
Order III. Acanthopteri (Acanthopterygii).
This is the largest group of fishes, its members usually having
the ventral fins thoracic in position and more than three rays spiny
in dorsal, anal, and ventral fins. The sticklebacks (GAsTERO-
STELD.£) and some other forms have the pharyngeal bones reduced,
the ventral fins farther back, and form the group Hemibranchii.
Gasterosteus.* The perch of fresh water (PERcID®), Perca*
and Micropterus* (black bass), and the marine SERRANIDZ, some
of which are hermaphroditic, have ctenoid scales. The Scompripx,
with Scomber,* the mackerel, and Thynnas,* the horse mackerel, and
Fic. 605.—Scomber scombrus, mackerel.
the AXIPHIID#, or sword fishes, in which the snout is prolonged into
along sword, are the most important edible fishes of the group. The
Lonrivatr, including the sculpins (Coltus,* Hemitripterus,* fre-
quently have the body armored with bony plates. The Emprotocips,
or surf perches of the Pacific, are viviparous. The suck fishes,
Remora,* Echeneis,* have the first dorsal modified into a sucker on
the top of the head.
Order IV. Anacanthini.
These are soft-finned fishes in which the ventral fins lie in
Fic. 606.—Gadus morrhua,* cod. (After Storer.)
front of the pectorals. Structure goes to show that these have
descended from qn]
clavinns v's 0% right and left ven- nects with most of the remaining
tricles (the arrows show the direc- yptapjg] areheag 5 fare hie veg
tion of the blood flow): 1, 2, 4, arterial arches, the first, which gives
arches comparable with those of P ANYot) ‘ ee olf
amphibia. Notice the origin of the off the carotid, and the right halt
vessels from the heart, and the (aortic arch ad) of the second
connexion (foramen Panizza) be- - an :
tween the arterial trunk and the greh. The third vessel connects on
left aortic arch, just in front of A ate
the heart. the one hand with the remaining
(left, second) arch and on the other with the right or venous half
of the heart. The foramen Panizze occurs between this and the
right aortic arch.
IV. VERTEBRATA: REPTILIA. 593
The venous character of the left aortic arch and the incomplete
ventricular septum (or presence of foramen Panizze) prevent a
complete separation of systemic and pulmonary circulations. In
the turtles a third element enters, the persistence of a ductus
Botalli (as in Urodeles, fig. 580, //, dB).
To the foregoing adaptations to a terrestrial life may be added
indications of higher development. The brain shows two advances.
The cerebellum, especially in turtles and alligators, has be-
come large, and the cerebrum grows dorsally and backwards over
the *twixt brain and forms the temporal lobes of the hemispheres.
The parietal organ is developed as nowhere clse. In many lizards
it forms an unpaired dorsal eye lying beneath the skin in the
parietal foramen. The paired eyes possess ds (usually upper and
lower as well as a nictitating membrane), and frequently (turtles,
lizards, and many fossils) a ring of bony plates (sclerotic bones)
in the sclera. A new opening in the petrosal, the fenestra rotunda,
places the tympanic cavity and the labyrinth in close relations.
In the excretory system amniote characters prevail. The
Wolffian body with its ductis functional in the embryo. Later
there arises behind it the permanent kidney (metanephros) with
the ureter, while the embryonic structures disappear with the ex-
ception of those retained as accessory to the genital apparatus.
Thus in the male the vas deferens and epididymis are formed
from the Wolffian duct; in the female the Millerian duct (early
lost in the male) becomes the oviduct. Usually the urogenital
canals open dorsally in the cloaca, rarely in an elongation of the
urinary bladder (Chelonia). This latter is lacking in snakes and
crocodiles.
Almost all reptiles lay eggs; only in the Squamata (some snakes
and lizards) are viviparous or ovoviviparous forms present. The
eggs much resemble those of birds, in that the large yolk is sur-
rounded with a layer of albumen and enclosed in a fibrous, often
calcified shell. To open the egg the embryo has an egg tooth on
the tip of the snout; this consists, of dentine in the Squamata, but
elsewhere, as in birds, is horny. From these relations it follows
that internal impregnation must occur; the eggs undergo a discoidal
(meroblastic) segmentation. Copulatory organs to accomplish
this internal fertilization occur, and these are of classificatory im-
portance, since they differ in character in the Squamata on the one
hand, the turtles and crocodiles on the other. These differences
are correlated with differences in the form of cloacal opening and
in structure of skull and skin, so that all living species may be
594 CHORDATA.
divided into two groups, the Lepidosauria, containing the lizards,
snakes and Sphenodon, and the Hydrosauria with turtles and croc-
odiles. This, however, ignores the fossil forms. When these are
taken into consideration another grouping must be adopted.
Order I. Theromorpha.
Extinct reptiles from the Permian and triassic which are closely re-
lated to the stegocephalous amphibia; with amphicclous vertebre, im-
movable quadrate, and from two to six sacral vertebre, The ANOMODON-
TIA, with partial or complete loss of teeth, stand near the turtles, while
the THERIODONTA, in which a heterodont dentition is developed, resemble
in this and some other respects the mammals, which, by many, are sup-
posed to have descended from them.
Order II. Plesiosauria.
Extinet aquatic forms from the triassic to the cretaceous, some forty
feet in length. They had long necks, and the limbs were modified into
swimming paddles recalling the flippers of the whales. The quadrate was
immovable, and the jaws, with numerous teeth in sockets, were long.
Order III. Ichthyosauria.
These forms resembled the Plesiosaurs in skin, swimming feet, elongate
jaws, and quadrate, but had the teeth (sometimes absent) in grooves rather
Fia. 622.—Restoration of Plesiosaur. (After Dames.)
than in sockets, and short necks. Some species at least were viviparous.
Their range in time was like that of the preceding order.
Order IV. Chelonia (Testudinata).
The turtles form in external appearance a sharply circumscribed
group, with the short and compact body enclosed in a bony case,
from which only head, tail, and legs protrude (fig. 623). The
case consists of a convex dorsal portion, the carapace and a flat-
tened ventral plastron, the two being united in most forms at the
margins. Hach consists of bony plates, the positions and names
of which may be learned from the adjacent eut. It only needs
mention that the neural plates are united with the spinous pro-
cesses, the costals with the ribs, and that the entoplastron is re-
IV. VERTEBRATA: REPTILIA, CHELONIA. 595
garded as an episternum. It is not connected with the internal
skeleton, since the sternum is lacking. The pelvis is only rarely
fused with the plastron. This bony case is usually covered with
horny shields, their number and arrangement usually agreeing
with the plates of the case, although without their contours exactly
coinciding.
More important are the great firmness of the skull and the
immovable condition of the quadrate, the lack of an os transver-
sum and of any but basisphenoid of the sphenoidal bones, and by
Fic. 623.—Carapace (A) and Plastron (B) of Testudo greeca. (From Wiedersheim.) C,
costal plates; HE, entoplastron ; Kp. epiplastron: H, posterior; Hp, hypoplastron;
Hy, hyoplastron ; M, marginal plates ; N, neural plates; Np, nuchal plate ; Py,
pygal plate; R, ribs; V, anterior ; Xi, xiphisternum.
growth forward, and backwards by which the girdles are brought
inside the ribs. The teeth are entirely lost, and, as in birds, the
jaws are enclosed in sharp horny beaks, in many cases efficient
weapons against larger vertebrates. The cloacal opening is oval,
its major axis corresponding to that of the body, and in its anterior
end is an unpaired erectile penis used in copulation. Turtles
appeared in the Permian, and the group has persisted until now.
Characters of armor and legs serve to contrast sharply the land and
sea turtles; the first with well-developed legs, five-toed in front, four-
toed behind, the toes with claws; the carapace arched, into which legs,
head, and tail may be retracted. In the sea turtles the feet are flipper-
like (fig. 624), claws mostly absent, and the carapace weakly united to or
free from the plastron, flat and incapable of covering head or appendages.
The fresh-water species are intermediate in position.
Sub Order I. ATHECA. Carapace of numerous mosaic scales and not
connected with ribs and vertebra; skin leathery. Dermochelys (Sphargis)
coriacea,* the leather-back tortoise of warmer seas, reaches a weight of
1500 pounds.
596 CHORDATA,
Sub Order II. TRIONYCHIA. Fresh-water forms with poorly ossified
carapace, but ribs and vertebrae connected with it. Our leather turtles
(Amyda*) and soft-shelled turties (Aspidonectes *) of savage habits belong
here.
Sub Order III. CRYPTODIRA. Carapace well developed and united
with ribs and vertebree, but the pelvic arch free. The species are numer-
ous, including terrestrial, fresh-water, and marine forms. CHELYDRIDA,
fresh water, taillong. Chelydra serpentina,* snapping turtle; Jachrochelys
Fig. 624.—Chelone imbricata, tortoise-shell turtle. (From Hajek.)
lacertina,* alligator turtle. CHELONID®, marine, paddle-like feet. Tha-
lassochelys caretta,* loggerhead; Chelone mydas,* green turtle, the favorite
of epicures; Hretmochelys imbricata, whose horny shields furnish tortoise
shell. TESTUDINID, terrestrial, including Xerobates,* the ‘gopher turtle’
of the South, the giant Testudo of the Galapagos Islands, and the enormous
fossil Colossochelys atlas of India, 18-20 feet long, 8 feet high. Other
families contain our mud turtles (Kinosternon *), box turtles (Cistudo*),
and terrapins (Malaclemmys *).
Sub Order IV. PLEURODIRA. Pelvis united to carapace and plastron.
All belong to the southern hemisphere.
Order V. Rhynchocephalia.
These resemble the lizards not only in body form (four five-
toed feet) and in scaly skin, but in certain anatomical matters as
well: lack of hard palate, presence of epipterygoid, transverse
cloacal opening, and heart, lungs, and brain. On the other hand
they recall the crocodiles in having two postorbital arches and
immovable quadrate. The large abdominal sternum and abdominal
ribs are noticeable as well as the uncinate processes of the true
ribs. The notochord is but incompletely replaced. The group
appears in the Permian and is thus one of the oldest of reptilian
types, and is usually regarded as ancestral to all the orders yet to
be mentioned. The only living species, Sphenodon (Hatteria)
punctata, belongs to the New Zealand region.
Order VI. Dinosauria.
This order included some of the largest land animals which have ever
existed. Some of them were from forty to one hundred feet long and
twelve to twenty feet high (Amphicwlias, Camarasaurus). In some there
IV. VERTEBRATA: REPTILIA, SQUAMATA. 597
was an exoskeleton, some of the plates of which in the stegosaurs measured
a yard across. Among the characters of the group are the fixed quadrate,
jugal and postorbital arches, three to ten sacral vertebra, and ilium
elongate in front of and behind the acetabulum. Some of these forms
(Orthopoda) in pneumaticity of bones, in having the pubic bones directed
backwards, and in the formation of an intratarsal joint, resembled the
birds, and have been regarded as the ancestors of that group. The Dino-
saurs were confined to mesozoic time.
Order VII. Squamata (Lepidosauria, Plagiotremata).
One of the characters which unite lizards and snakes and which
has given the name Plagiotremata is the transverse form of the
cloacal opening (fig. 625), behind which, in the male, are the
ae jer Sr ji
Fia. 625, Fia. 626.
Fie. 625.—Hinder trunk and hind limbs of a lizard. (From Ludwig-Leunis.) a,
cloacal slit ; b, femoral pores ; sca, anal shield.
Fia. 626.—Skull of Ameiva vulgaris. an, angulare ; ar, articulare ; co, epipterygoid ;
er, coronoid ; d, dentary ; fr, frontal; j, jugal ; la, lachrymal; m, maxillary ; na,
nasal; p, postorbital, above and behind it the parietal; pf, prefrontal; pr, pre-
maxilla; pt, pterygoid ; y, quadrate ; gj, quadratojugal; sy, squamosal ; tr, trans-
versum.
paired copulatory organs, each lying in a sac from which they can
be everted like the finger of a glove. The names Squamata and
Lepidosauria refer to the scaly condition of the skin. These
scales are horny structures and somewhat distinct from the bony
scales of fishes. The derma forms flattened papillae which resemble
the scales of fishes in that in many species they contain bony
plates. These papille determine the character of the epidermis.
Since the stratum corneum is especially thick on the top of the
papille and thinner between them, rhomboid and oval plates occur,
which either lie flush with each other (shields) or overlap like
shingles (scales). The rule is that the head is covered with reeu-
larly arranged shields, each with its name, the trunk with scales
in longitudinal, transverse, and oblique lines. Outside these is a
layer of cornified cells, the pseudocuticula, and outside of all an
inconspicuous true cuticle. Since all cornified cells are dead and
598 CHORDATA.
require periodic removal, the horny layers are cast yearly and re-
placed by new. During this periodic molting, which recalls that
of arthropods, the animals are sickly and apt to die in captivity.
All Squamata are characterized by the slenderness of the
cranial bones (fig. 619, 626, 627), which, especially in the Lacertilia,
incompletely close in the cranium. The quadrate is movable,
and the squamosal is intercalated between it and the cranium. A
hard palate is lacking, and the choanz, as in the amphibia, lie far
forward (fig. 619, Ch). There is a wide gap in the partition
between the two ventricles of the heart.
Sub Order I. LACERTILIA (Saurii). The lizards are usually distin-
guished from the snakes by the possession of limbs, but a few forms,
undoubted lizards, like the glass snakes and Amphisbenew, lack limbs.
These are distinguished by the existence of the scapula and the iliac bone
united to the vertebra, and especially by the presence of a sternum, which
never occurs in snakes. In the skull is a peculiar bone (lacking only
in Chameleons and Amphishzene), found nowhere else, the epipterygoid
(fig. 626, co); it reaches from the pterygoid to the parietal, and from its
WLS Fr Tf Pa Sq Os
Fia. 627.—Skull of rattlesnake. (From Boas.) Fr, frontal; h, hyomandibular (colu-
mella); Ma, maxillary; N, nasal; Os, supraoccipital; Pa, parietal; Pul, palatine;
Pf, postfrontal; Prf, prefrontal; Pt, pterygoid; Px, premaxilla; Q, quadrate; Sq,
squamosal; 7, transversum; 1, dentary; 3, articulare.
slender shape is sometimes called columella, but is not to be confounded
with the true columella of the ear. The bones of the jaws are firmly united,
so that the mouth has no special capacity for opening widely. The jugal-
quadratojugal arch is present.
In external appearance the presence of eyelids, nictitating membrane,
tympanic membrane, and Eustachian tube are noticeable, these being
absent only in the Amphisbeenee. In the Ascalabotee, as in snakes, the lids
grow together, forming a transparent covering over the eyes. Fossil
lizards are rare, but the group dates back to the eretaceous.
Section I. AscaLaBor.z (geckos). Skeleton incompletely ossified, noto-
chord persistent, amphiceele vertebrie; skin granular rather than scaly,
usually adhesive dises on the toes by which they climb vertical surfaces or
ean walk upon ceilings. Two hundred species. Phyllodactylus.*
IV. VERTEBRATA: REPTILIA, SQUAMATA. 599
Section II. Crassmninauta. Tongue thick, fleshy, not protrusible from
the mouth, or only slightly so. IGuaNIDa; American, often a comb of
spines on the back, teeth pleurodont, @.e., firmly united to the inner side
of the jaw. Three hundred species. Anolis,* Sceleporus,* Phrynosoma,*
‘horned toads.’ AGAMID&; Old World, teeth acrodont, @.e., seated on the
angle of the jaw bones. One hundred and fifty species. Chlamydosaurus,
Draco volans, with ribs greatly elongate and supporting a dermal fold
which acts as a parachute.
Section III. Fissruincura. Tongue long and thin, divided at the tip,
and capable of wide protrusion from the mouth, and in Varanus retractile
into a sheath. Trsipa#; American, teeth acrodont; Cnemidophorus,*
Tejus. NHELODERMATID&, pleurodont; Heloderma,* the ‘ Gila monsters,’ are
the only poisonous lizards. Lacertinipas (Lacerta) and VARANID& (Vara-
nus, the monitors) are Old World forms, Lacerta vivipara bringing forth
living young.
Section IV. Brevitineura. Tongue short, slightly notched at the tip,
slightly protrusible. Four hundred species. Scrincip@®, with tendency to
reduction of the limbs. Humeces,* Oligosoma.* In Anguts and Typhline
the legs are absent. ZONURIDa&, with a finely scaled groove along the side;
all Old World except our Ophisaurus ventralis,* the glass snake, a limb-
less form with brittle tail.
Section V. ANNULATA. In many respects snake-like; legs and epi-
pterygoid, tympanum, and movable eyelids lacking and usually girdles ;
tropical or subtropical. In Chirotes sternum and reduced fore legs
retained. Amphisbana.
Section VI. VERMILINGUIA; includes the Old World chameleons (our
_
ellie,
BE Manze
Fia. 628.—Head of chameleon with tongue extended.
‘chameleon’ is Anolis,—supra) with long fleshy tongue, lying rolled up in
the mouth, but protrusible and used for catching insects, its end being
covered with a sticky mucus. Other characteristics are the ring-like eye-
lids functioning as an iris, the climbing feet in which the toes are united
into two opposable groups; epipterygoids, clavicle, sternum, and tympanic
membrane lacking. The chameleons are best known from their changes
of color, produced by rapid alterations in the size and shapes of the
chromatophores. Color changes occur in other lizards, but not to such an
extent as here.
600 CHORDATA.
Sub Order II. PYTHONOMORPHA. Large, extinct, extremely elon-
gate reptiles with four flipper-like limbs and strong swimming tail.
Flourished in the cretaceous. Mosasaurus, Clidastes.
Sub Order III. OPHIDIA. The snakes are distinguished from most
lizards by the absence of limbs, and connected with this the similar verte-
bree in which only trunk and caudals can be distinguished. The caudals
lack ribs, but these are present and long in the trunk region, serving for
locomotion and supporting the body on their distal ends. Since there are
legless lizards, it is further necessary to say that in the Ophidia the girdles
and sternum are lost, only the Peropoda having remnants of the hinder
appendages and pelvis, but these not connected with the vertebral column.
Further distinctions exist in sense organs and jaws. The columella is
indeed present, but tympanum and Eustachian tube are lacking. The eye-
lids also seem to be wanting, but examination shows, in front of the cornea
and separated from it by a lachrymal sac, a transparent membrane, com-
posed of the fused eyelids (outer cornea). The apparatus of the jaws (figs.
619, 627) is remarkable for its great extensibility, which enables snakes to
swallow animals larger than themselves, after coiling around them and
crushing them. This extensibility is in part due to the fact that the bones
of the lower jaw are bound together at the symphysis by elastic ligaments,
in part to the freedom of motion of the bones of the upper jaw (excepting
the small premaxillaries) and the palate. Further, the squamosal (Sg),
quadrate (Q), and transversum (77) are elongate and slender, the quadrate
being widely separated by the squamosal from the skull, while the zygo-
matic arch is entirely absent. The food is forced down the throat by
hook-shaped bones on palatines and pterygoids. A wide distension of
the stomach is rendered possible by the elasticity of its walls and the great
mobility of the ribs, which are not united ventrally by a sternum.
In the non-poisonous snakes the dentition is similar on jaws and
palate bones (fig. 619). The vomer and, usually, the premaxilla are tooth-
less. In the poisonous serpents poison
fangs appear on the maxilla (fig. 627)
and are distinguished from the other
teeth by their greater size and connex-
ion with a large poison gland. The
duct of the gland opens at the base of
the tooth; the poison which is pressed
out by the pressure of the jaw muscles
is led to the tip of the tooth either by a
groove (proteroglyphie tooth, fig. 629, A)
or, when the groove is closed to a canal
(solenoglyphie tooth, B), through this
Pia. 629.—Poison fangs. 4, -4,, pro- canal which opens at base and tip of
teroglyphic (grooved) tooth of co-
bra, and section of same; B, 2,, so- the tooth.
lenoglyphic tooth (tubular) of rattle- The asymmetrical character of the
snake; g, poison canal; p, pulp eee i
cavity. lungs is interesting. In the Peropoda
one lung (apparently the left) is much
smaller than the other; in the poison snakes and some others it is rudi-
IV. VERTEBRATA: REPTILIA, CROUVODILIA. 6OL
mentary oreven absent. In the Typhlophide, on the other hand, the right
appears to be degenerate. The urinary bladder is always absent. The
excreta, chiefly urie acid, accumulate as a solid mass in the cloaca and
form the chief part of the excrement; the faces, on account of the
extraordinary digestive powers, being small in amount.
Section I. OporeroponTa (Angiostoma). Burrowing blind tropical
snakes with the mouth incapable of distension, the animals living on
small insects. Typhlops.
Section II. PeEropopa. These large snakes have paired lungs and rudi-
ments of hind extremities ; lack poison fangs, and kill their prey by mus-
cular power. Python, Africa; Boa and Hunectes (anaconda), South
America.
Section III. CoLUBRIFORMIA. Ordinary snakes (over 500 species) with
numerous teeth in the upper jaw, but with appendages entirely absent.
Some are poisonous, some not, but no structural lines can be drawn be-
tween them. The AGLYPHA have no grooved teeth. Tropidonotus,* water
snakes ; Bascanion,* black snakes; Hutainia,* garter snakes. The Pro-
TEROGLYPHA, with grooved teeth, perma-
nently erect, are poisonous. Most are
brightly colored. Evaps,* the coral snake;
Naja tripudians, the cobra of India; NV.
haje, Cleopatra’s asp. Here belong the
pelagic sea snakes of the Indo-Pacific,
which are viviparous.
Section TV. SOLENOGLYPHA. With the
maxilla reduced and serving as a socket
for the single large tubular tooth with
one or more reserve teeth (fig. 627).
VIPERID&, Old World, no pit between
nostril andeye. CroTaLip#, New World
and Asia, with a pit between nose and
eye. Crotalus,* with the tail ending in
arattle formed by remnants of cast skins,
is common throughout the United States.
Agkistrodon contortrix,* copperhead,
and A. piscivorus, moccasin, lack the
rattle. Bothrops lanceolatus of the An-
tilles, possibly the most poisonous snake.
Ph, |i
Wi A NII
—
Order VIII. Crocodilia (Loricata).
The crocodiles, alligators, etc.,
: q Fia, 630.—Ventral surface of skull
agree with some of the forms already “of crocodile. (from \Wiedersheim.)
: : Coce, occipital condyle; Ch, cho-
mentioned in the oval cloacal open- ay parE ML maxillary: Oh,
. a8 Vera hae elite aes basioccipital ; Orb, orbit ; Qi, quad-
ing with single copulatory organ, patojueal: Qu, quadrate § Pl, pala-
1 . ‘ Albay) 6a tine ; Pmx, premaxilla; Pt, ptery-
immovable quadrate, and the bony ,oia! 7s. transversum.
plates in theskin. Inshape they are
lizard-like, but in structure they differ from all other living reptiles
GOL CHORDATA.
and approach most nearly to the Theromorphs. The maxillaries,
palatines, and pterygoids have united in the lving species in the
middle line, forming a hard palate and forcing the vomers upwards
into the nasal region. This same process has carried the choana
(fig. 630, Ch) to the back of the skull. Some of the ribs have
two heads; the ears and nostrils are provided with valves. A
sternum is present and, farther back, abdominal ribs and an ab-
dominal sternum. ‘The jaws are extended into a long snout, and
the teeth, which oceur only on the margins, are placed in sockets
(alveoli). The four-chambered heart has already been described
(p. 592). The animals move slowly on land, but in the water,
thanks to their strong, keeled tail, they are very active. They have
a strong smell, owing to musk glands in the cloaca and on the
under jaw. The group appeared in the trias, and of the three sub
orders two, the Pseudosuchia and Parasuchia, are extinct.
Sub Order EUSUCHIA. External nostrils united, choana posterior;
five toes in front, four behind. Gav?alis, India, snout long and slender.
Alligator luctus,* alligator ; Crocodilus,* most species Old World, one, C.
americanus,* occurring in our southern waters.
Order IX. Pterodactylia (Pterosauria).
Extinet reptiles of the Jurassic and cretaceous, adapted for flight.
The bones were hollow and the wings were broad membranes, supported,
like those of a bat, by the body and the greatly elongated fifth digit of the
—_
Fra. 631.—Dimorphodon, a pterodactyle. (After Woodward.)
fore limbs. Some were sparrow-like in size and some, Pferanodon, had a
wing expanse of twenty feet. Yet one of these large forms from Kansas
had its pelvic opening so small that its eggs could not have been more
than half an inch in diameter.
IV. VERTEBRATA: AVES. 603
Class II. Aves.
While structurally the birds stand very near the reptiles, yet by
the development of wings and the feathering of the body the group
is one strictly circumscribed. The skin is in some places, as the
lower part of the legs, covered with horny scales and shields, on
the toes are claws, but as a rule the fingers are feathered. On
most places the skin is soft and thin, since the derma and stratum
corneum are poorly developed. Periodic molts of the integument
do not oceur, since the horny layer, as in mammals, undergoes a
constant renewal. These peculiarities of the skin are correlated
with the appearance of the protecting plumage.
The feather, like the hair of mammals, is exclusively epithelial
in character, but of a much more complicated structure. The cor-
nified epithelium forms a firm axis, the scape, from which, right
and left, arise branches, or barbs. The scape is solid as far as the
barbs extend (rachis, or shaft), while below it is hollow (quill, or
calamus). The quill is inserted deep in the derma, in a follicle,
and is provided with muscles for its movement. Its hollow in
most fully developed feathers is empty save for the ‘ pith,’ a small
amount of dried tissue. In young growing feathers it is occupied
by a richly vascular connective tissue, the feather papilla, which,
for purposes of nourishment, extends inwards from the derma.
The feather may therefore be regarded as a cornified outgrowth
from the skin which has arisen on a papilla of the derma, a view
which corresponds well with its development and shows its
homology with the scales. In many birds (cassowaries) two well-
developed feathers arise from the same follicle—a fact which
explains the existence of a rudimentary feather, the hyporachis,
or after-shaft, attached to the scape below.
In contour feathers the barbs are, to a great extent, united into a
vane. Right and left of the shaft they lie close together and parallel,
each repeating in miniature the entire feather, the barb having branches
or barbules, which, overlapping the barbules of adjacent barbs, give the
vane its close texture. The vane is held together by minute hooks on the
barbules of one barb interlocking with those of the next. Down feathers
(plumes) differ from contour feathers in the absence of hooks and the
loose arrangement of the barbs. Since feathers consist of cornified epithe-
lium and these cells are held firmly (only in powder down is there a
gradual loss), they, like the scaly coat of the snakes and lizards, must be
molted yearly and replaced by new.
Young birds or embryos have only down feathers. Later the contour
feathers arise in regular order in the feather tracts, or pteryle, between
604 CHORDATA.
which are apteria in which no contour feathers appear (fig. 682). Since
the contour feathers overlap like shingles, they form a firm coat of
plumage beneath which the down and semiplumes form a warm coat.
Fia. 632. Fic. 633.
Fia. 632.—Feather tracts and apteria of pigeon, dorsal view. (From Ludwig-Leunis.)
Fic. 633.— Regions and feathers of Falco lanarius. (From Schmarda.) As, secondaries ;
Ba, belly; Br, breast; Bz,rump; D’-D’’, wing coverts; Di, gonys of bill; EF,
alula ; F,culmen of bill; H, occiput; HS, primaries; K, throat ; L, legs; N, neck;
Sch, crown; SF, parapterium; St, forehead, lower tail coverts; Sz, rectrices; W,
cheek; WH, cere with nostril; Zh, toes.
Besides these covering feathers (coverts, or tectrices, fig. 633, D) there are
the longer feathers of the wing, the remiges, and the tail feathers, or
rectrices (Sz). The larger remiges form the chief part of the wing; they
spring from the part of the limb corresponding to the hand (carpus,
metacarpus, phalanges) and are known as primaries (HS), while the
secondaries (As), arising from the forearm, are shorter. These are over-
lapped at the base by the coverts (D, D’, D’) and by the parapterium (SF)
ye
eee
Fic. 634.—Wing skeleton of stork. (From Gegenbaur.) ¢, c’, carpalia of first row:
h, humerus; m, fused metacarpals and carpals of second row; p-p’’, phalanges of
first three fingers ; 7, radius; u, ulna.
springing from the shoulder. A few feathers arising from the first finger
remain distinct from the remiges and form the alula (ZF). In the water
birds especially the feathers are oiled by the secretion of a pair of glands
at the base of the tail above the coecyx.
Since the feathers are not only for protection, but give to most
birds the power of prolonged flight, they predicate a special mode
IV. VERTEBRATA: AVES. 605
of life, under the influence of which all of the other organs exist.
The character of the skeleton, the respiratory organs, and in part
the sense organs and brain, are connected with the powers of flight.
As the feathers of the wings, like the fins, form what may
be called a paddle working as a whole, the skeleton of these limbs is
simplified (fig. 634), first, by the reduction of the fingers, of which
only three with a small number of phalanges persist (p, p’, p’’);
second, by fusion of the corresponding metacarpals (m) with each
other and with the adjacent carpal
bones. On the other hand, in order
that there may be the necessary en-
ergy and the most complete transfer
of the same to the body, the con-
nexion with the skeletal axis is
strengthened by special development
of the parts. In the shoulder girdle
(fig. 635) all three elements are firm,
asword-shaped scapula (s), a colum-
nar coracoid (c), and clavicles which
are usually united to a ‘wish-bone,’
or furcula (f). Clavicles and furcula
are united directly or by ligaments
to the broad sternum, the anterior
face of which is developed into a
strong keel, the carina, or crista
sterni, in order to give the largest
, 2 . Fia. 635.—Trunk skeleton of stork.
surface for attachment of the large “(From Gegenbaur.) as, sternal
muscles of flight. Usually the greater PR fehtP ‘iced cuurinieal mp.
the powers of flight the more devel- [wsed,spinons processes of thoracie
oped the carina, yet in somé cases Jette irel pare oe ee eae
(albatross) the weak carina is com- IR ase ne a Nana ee
pensated for by the enormous width !™-
of the sternal plate. In running birds (ostriches, etc.) the
carina is entirely gone. The thoracic framework is rendered
more firm by the development of uncinate processes from the ver-
tebral parts of the ribs (w) which overlap the succeeding ribs.
Since the fore limbs are no longer used for walking, the sup-
port of the body depends upon the hinder extremities, and this
has brought about two striking characteristics—the broad union of
the pelvis with the vertebral column, and the simplification of the
leg skeleton. In the embryo the ilium (fig. 635, 7) is connected
only with the two sacral vertebre present in most reptiles, but
606 CHORDATA.
later it extends forward and back, uniting with at least nine ver-
tebre and sometimes with as many as twenty-three; while the iliac
bones of the two sides meet dorsal to the vertebral column. This
extensive union of pelvis and axial skeleton is understood when
we recall that in walking or at rest the vertebral column is not
vertical as in man, but isinclined. Ischium and pubis are peculiar
in that they extend backwards, parallel to each other, from the
acetabulum, and that only exceptionally (ostrich) are the bones
of the two sides united by a symphysis.
In the hind limbs occur conditions similar to those which will
B Cc
Fic. 636.—A, leg of Buteo vulgaris. a, femur; b, tibio-tarsus; b’, remains of fibula; c,
tarso-metatarsus; c’, same, front view; d!-/3, toes. B, lower leg of bird embryo;
C, of lizard. f, femur; t, tibia; p, fibula; ts, tarsales of first row (talus); ti, tar-
sales of second row; between these intertarsal joint; J-V, digits. (From
Gegenbaur.)
be repeated in the ungulates. The weight of the body makes it
necessary that the simplification found in the wing should be re-
peated in the lower leg and foot, and that the numerous bones
usually occurring in these regions be replaced by one which shall
support the pressure (fig. 636). Therefore the fibula, well de-
veloped in the embryo (2), becomes reduced to an inconspicuous
rudiment; the metatarsals, distinct in the embryo (B), fuse toa
IV. VERTEBRATA: AVES, 60T
single tarso-metatarsus (4, ¢), which has below as many articular
surfaces as there are toes (since the fifth toe only appears in the
embryo, at most four, in some three or even two, d-d’’’). At the
same time the tarsals disappear by fusion with adjacent parts.
Even in reptiles (C’) a part of the tarsals unite with the bones of
the shank, and the remainder with the metatarsals; in the birds
the union is completed, the proximal series fusing with the lower
end of the tibia to form a tibio-tarsus, the distal with the metacar-
pus to form the tarso-metatarsus, in this way producing the inter-
tarsal joint so characteristic of birds.
In respect to the vertebral column, it only needs mention that
the vertebra articulate with each other by a so-called saddle-joint,
that (in living birds) only a few caudal vertebre persist behind the
pelvis, that these are partially fused to a single bone, the pygo-
style, which supports the tail feathers, and that, corresponding to
the well-developed neck, there are many cervical vertebre, among
them an atlas and an axis, all except the last two fused with the
corresponding cervical ribs.
The skull (fig. 637) resembles closely that of the lizards in the
presence of a single occipital condyle, in the movable condition
of the quadrate upon the cranium, and in the presence of a slender
columella. On the other hand an os transversum is lacking. The
cranium, as a result of the increase in size of the brain, is more
spacious; the bones of its walls fusing early so that the sutures
Fie. 637.—Skull of young bustard. (From Claus.) Als, alisphenoid ; Ang, angulare ;
Art, articulare; D, dentary ; Et, mesethmoid; Fr, frontal; Jmr, premaxillary ;
J, jugal; L, lachrymal; Mx, maxillary; N, nasal; Ol, exoccipital; Os, supra-
occipital; Pa, parietal ; Pal, palatine; Pt, pterygoid; Q, quadrate; Qj, quadrato-
jugal; Sm, interorbital septum ; Spb, basi- and presphenoid.
between them are obliterated. The occipital condyle is on the
under surface, so that the skull is carried at nearly right angles
to the axis of the vertebral column. Teeth are lacking in living
birds, although they occurred in some fossil forms. In their place
608 CHORDATA.
are hard horny sheaths covering the jaws which are frequently car-
ried back on the outside into a softer cere (fig. 634, WA).
The cranium consists of four occipitals, a basi- and a presphenoid; above,
the parietals and frontals; and on the sides prootics, alisphenoids and
orbitosphenoids, while the broad squamosals also enter its wall. The large
mesethmoid lies in the interorbital septum ; the nasal cavity is roofed by
the nasals, and beside them are the lachrymals. The quadrate articulates
with the squamosal, and from it extend forward internally the pterygoid,
palatine, and vomer; externally a zygomatic arch of quadratojugal and
jugal to the maxillaries and premaxillaries. The maxillaries are hinged
in the ethmoidal region, so that in opening the mouth there is besides the
depression of the lower jaw an upward motion of the upper jaw.
The pneumaticity of the bones is an important feature of the
skeleton. In place of marrow and bony tissue, the inside of the
bones in strong flying birds is more or less completely occupied by
wir spaces, around which, as a sheath, is the compact bone. This
gives the greatest possible lightness and strength to the skeleton.
In Buceros and Palamedea all of the bones are pneumatic; in
others (Pelecanus, Sula, Tachypetes, etc.) only the phalanges of
the toes contain marrow, while in the penguin and Apteryz, as in
mammals, air spaces occur only in some of the cranial bones.
The air spaces of the bones are in part (skull) connected with
the nose and tympanum, but most of them, by means of the air
sacs, communicate with the lungs. The long trachea forks at its
lower end into two bronchi. At its upper end is a larynx, as in
other vertebrates, but this is not vocal; the notes of birds are pro-
duced by the syrinx, which les at the division of trachea into
bronchi. It is usually formed of both trachea and bronchi, but
more rarely of either trachea or bronchi alone. Its vocal cords
are regulated by special muscles, which in the singing birds have
a complicated arrangement. The relatively
‘small lungs send out from their surface air
sacs, especially well seen in embryos (fig. 638,
I-65). These later become large, thin-walled
spaces, easily torn away in dissection, leaving
large openings on the surface of the lungs
(fig. 639, 1-3). Usually five pairs of these
ae he ee air sacs are present, largely in the cclom,
ee Saag ea but extending in between the muscles (breast
trachea; 1-", lung sacs. and axillary region), and also into the bones.
The spongy lungs lie on either side of the vertebral column and are
IV. VERTEBRATA: AVES. 609
united to the ribs. On entrance to the lung the bronchus (fig. 639, b7”) loses
its cartilage supports and enlarges into a
vestibule (v) and extends thence asa
mesobronchus (0m) backwards, termi-
nating in the abdominal air sac (4). A
side branch supplies the hinder sub-
costal sac (4). Secondary bronchi arise
from the vestibule and mesobronchus;
of these there are three to five ento-
bronchi (J-JV ) supplying the remaining
air-sacs and six or more ectobronchi.
Arising from the mesobronchi and
secondary bronchi are tertiary bronchi,
or air pipes, running parallel to each
other and anastomosing frequently.
Each air pipe has a thick spongy wall eR ecco gees
(tig. 640) composed of numerous thin- shows a mesobronchus with its
: branches. a, artery; bm, meso-
walled sacs, the lung vesicles, closely bronchus, arising from the vesti-
enveloped by capillaries, and connected Duie: pr, bronchus swelling to
with the central air-conducting tube, lung pipes; I-IV, mesobronchi;
. 1-5, ducts of lung sacs.
the lumen of the pipe.
Inspiration is effected by raising the framework of the chest, this
causing a straightening of the hinged ribs and an increase of the sterno-
vertebral diameter ; expiration by the reverse motion. By this the lungs,
attached to the ribs, are alternately enlarged and contracted in spite of
their slight elasticity. This is also true of the lung sacs, which, on account
of their poor blood supply, are not respiratory but serve as accessory air
Fia. 640.—Section of lung pipe. (After Schulze.)
pumps. It is probable that in flight this air-pump action occurs espe-
cially with the subpectoral and axillary air sacs, drawing air through the
lungs and rendering other respiratory motions superfluous, thus enabling
the thorax to remain quiet, an important matter. If the trachea be
closed and the air canal in the humerus opened. the bird can breathe
through the latter.
610 CHORDATA.
The circulation in the birds has arisen from that of the reptiles
by complete separation of systemic and pulmonary systems. Of
the three great arterial tranks present there (fig. 621), the pul-
monary artery and the right aortic arch, arising from the left ven-
tricle, are retained, the left venous arch being lost. The septum
between the ventricles is complete. The striking features of the
alimentary canal (fig. 60) are the crop (not always present), a
glandular stomach or proventriculus (¢), and a muscular chewing
stomach or gizzard (d), as well as two long, rarely rudimentary,
ceca (k) at the junction of small and large intestine. Liver and
gall bladder (¢, f), pancreas (g), and spleen are present. A blind
sac (the bursa Fabricii), the paired ureters (), and the sexual
ducts (2) open into the cloaca. The latter show the peculiarity
that the right oviduct and ovary are degenerate, while those of the
left side are correspondingly larger. Since copulation occurs the
large eggs (the ‘ yolk’) are fertilized in the oviduct (fig. 99). As
they pass slowly through the duct, they become enveloped first
with a thick layer of albumen, ‘white’ (w), then with a double
egg membrane (ism, sm,) the two parts being separate and enclos-
ing an air chamber at the larger end of the egg. Lastly comes the
shell. All of these accessory structures are secreted by the gland-
war walls of the enlarged oviducts. During the passage down the
oviduct the first phenomena of development (segmentation, gastru-
lation) occur, and after oviposition the development stops and again
starts when the necessary warmth is supplied.
The care for the young, the sexual life connected with copula-
a tion, and the complicated conditions of ex-
Zo istence connected with flight have resulted in
an intelligence far superior to that of the
—_-vx reptiles, which finds its expression in the bet-
Z ter development of sense organs and brain.
--tm In the brain (fig. 641) the cerebellum, which
ee wa isthe central organ for the coordination of the
action of parts, is strikingly developed. Cor-
9/4 respondingly large are the cerebral hemi-
Fie. 641.—Brain_ of pig- Spheres, the frontal lobes of which begin to
as Ae ee cover the olfactory lobes, the temporal lobes
Cee etl ens ot in like manner extending back over the *twixt
Loe ees TT Ontis brain and optic lobes. Corresponding to the
ee ee iT care: Vocal apparatus, the ear is highly organized,
brum ; Z, pinealis. the lagena of the labyrinth being greatly en-
larged and the sound-conducting apparatus (columella, tympanum,
IV. VERTEBRATA: AVES. 611
etc.) well developed. The beginnings of an external ear are seen in
the deeper position of the drum membrane. Since the power of
flight necessitates vision at great distances, most birds have exceed-
ingly sharp sight, and the eye itself (fig. 642) is in general con-
Co
Fig. 642.—Eye of owl. (From Wiedersheim.) Ch, choroid; CJf, ciliary muscle; Co,
cornea; Cv, vitreous body; Jr, iris; L, lens; Op, optic nerve; OS, sheath of nerve;
P, pecten; Rt, retina; Sc, sclera; VK, anterior chamber; +, sclerotic bones.
structed for distance. Peculiarities of the bird’s eye, already
weakly developed in the reptiles, are the pecten (P), a comb-
shaped growth of the choroid into the vitreous body, and the
scleral ring, a circle of bones developed in the sclera and support-
ing the outer part of the eye.
Among birds there is spirited rivalry for the females, especially
among polygamous species. At the time of mating the males seek to win
the favor of the females either through striking motions (dances), by
singing, or by beauty of plumage. All of these peculiarities are confined
to the male and frequently lead to a marked sexual dimorphism. The dis-
tinction in plumage is commonly strengthened at this time, the male
receiving the brilliant wedding dress. Thus we speak of the spring molt,
although there is only a color change and only exceptionally a renewal of
the feathers. The return to every-day clothes only occurs with a molt, and
this comes at the close of the reproductive season.
The reason for the dull plumage of the female is due to the fact that
she usually sets on the nest, at which time inconspicuous colors protect her
from destruction by enemies. In only a few instances is the heat neces-
sary for incubation produced by other causes, such as the heat of the sun
upon the sand in which the eggs are buried, or the increase of temperature
caused by fermentation in decaying vegetation (Megapodes). The rule is
612 CHORDATA.
that both sexes build the nest, which with the weaver birds is most skil-
fully constructed; occasionally among social species the nests are placed
under a common roof, When the clutch of eggs is complete the female
(rarely the male) begins the incubation, at this time in some instances
losing the feathers from certain regions the better to warm the eggs.
Many birds, like hens and ducks, are so far advanced when they leave the
nest that they can follow the mother and feed themselves. Such birds
are called Preecoces—in contrast to the Altrices, which hatch with incomplete
coat of feathers and therefore need the warmth of the nest and the pro-
tection and care of the parents.
The migrations of birds possess great interest. We distinguish among
birds permanent residents and others which, in order to obtain food, take
long journeys, the migratory species. At the approach of cold weather
these seek the south, following regular paths in their travels. They can-
not, like reptiles and amphibians, hibernate at the period when insects
and fruit are scarce, because their greater intelligence and their more ener-
getic vital processes demand a more rapid metabolism and a continuous
food supply. Hence the birds, like the mammals, in contrast to the
‘cold-blooded’ reptiles, amphibia, and fishes, maintain, under all extremes
of external temperature, a body heat of 38-40° (44° ?) C. (100-104° F.).
The classification of birds is in a state of change. The older system
based upon adaptive characters is not in harmony with the results of care-
ful anatomical study, which would divide the whole class into many small
groups. For this reason it has been thought best to retain the older sys-
tem of larger, easily recognized divisions, and to call attention, where
necessary, to the contradictions with later results.
Order I. Saurure.
The view that birds are closely related to reptiles has received
considerable support by the discovery of fossil birds with teeth.
The most reptilian of these occur in the Jurassic of Bavaria, and
only two specimens have been found. In these (Archeopteryx
lithographica) the carpals and metacarpals have not fused, the
three fingers are well developed and clawed, and the caudal verte-
bra, although bearing feathers, form a long slender tail like that
of a lizard (fig. 2).
Order II. Odontornithes.
These forms, from the cretaceous of Kansas and Colorado, also
had teeth. In the Opontorm.© (/chihyornis) there was a keeled
sternum and normal pygostyle. In the OponToHoLc.® (Hesper-
ornis) the wings were reduced (only the humerus persisting), the
sternum was without a keel, and the caudal vertebrae formed a
broad paddle.
Order III. Ratite.
Here are included several families, very different in structure,
which agree in having the feathers not arranged in feather tracts;
IV. VERTEBRATA: AVES, CARINATA. 613
and in that, together with the lack of flight, many structures
normally connected with it are absent. The bones are but slightly
pneumatic, the sternum has no keel, and a furcula is not formed,
the clavicles being rudimentary (Vromeus) or not present as dis-
tinct bones. The wings are small and lack primaries and seconda-
ries adapted for flight, for typical contour feathers with close
vanes, as well as typical down feathers, are absent.
Since several structures apparently adapted for flight occur here
(fusion of hand bones and often of caudal vertebre; arrangement
of wing muscles), it is probable that the Ratites have descended
from carinate forms by loss of power of flight. The anatomical
distinctions between the various families lead one to believe that
they have arisen from different groups of carinates and hence do
not form a natural assemblage.
Section I. STRUTHIONES, with long humerus, long legs and neck.
STRUTHIONID#, two-toed ostriches of Africa, Struthio camelus. RHEIDA,
South American three-toed ostriches, Rhea americana, nandu. Section
II. CASUARINA ; three toes, humerus short. Dromeus, emus; Casu-
arius, cassowaries. Section III. APTERYGES, bill long, nostrils near the
tip, rudimentary wing skeleton; four toes. Apteryz, kiwi, of New Zealand.
The DINORNITHID#, three toes, wing skeleton absent ; giant birds (thirteen
feet high) of New Zealand; now extinct, but apparently contemporaneous
with man. The Zpiornis, a gigantic bird of Madagascar, possibly belonged
near these. Skeletons and eggs holding two gallons found in alluvium.
Order IV. Carinate.
The name refers to the presence of the keel to the sternum,
which is correlated with the powers of flight possessed by most
species. Other characters of the class are the presence of rectrices
and remiges on tail and wings, and the fusion of clavicles to a
furcula. There are strong fliers, like the raptores and albatrosses,
which have but a small carina; in many poor fliers the carina may
be entirely absent. The furcula is not always present, the clavicles
not uniting (many parrots and toucans) or being absent (JMesi/es).
The remiges are also degenerate in some carinates, as in the pen-
guins (which are flightless, although they have a strong carina),
where they take the shape of small scales. Thus the distinctions
between ratite and carinate birds vanish in places.
Sub Order I. GALLINACEA. The hen-like birds are praecoces with
compact bodies and well-developed wings and legs, so that they run and
fly well without excelling in either direction. The feet have three toes in
front, usually connected by a membrane at the base (fig. 648, c); the fourth
toe is behind and at a higher level. Above this in the male is usually the
614 OHORDATA.
spur, a process of the tarso-metatarsus, covered with horn. The margins
of the upper jaw overlap the lower; the beak is bent downward at the tip
and is about as long as the head. Naked, richly vascular lobes form comb
and wattles which are specially large in the more elegantly plumaged
males.
The PHASIANIDZ are polygamous; Phasianus, with many species of
pheasants; Gallus bankiva of the Sunda Islands, the ancestors of domestic
Fia.643.—Foot forms. (From Schmarda.) a,semi-palmate, wading of Ciconia ; b,perch-
ing of Turdus;c, rasorial of Phasianus; d, raptorial of Falco; e, adherent of
Cypselus; f, cursorial of Struthio ; g, zygodactyl (scansorial) of Picus ; h, lobate of
Podiceps ; i, lobate and scalloped of Fulica ; k, palmate of Anas ; 1, totipalmate of
Lz. Phaethon.
fowl. Meleagris,* the turkeys. The TETRAONIDE are partly polygamous,
partly monogamous. Coturnix,* quail; Perdix,* partridge; Bonasa,*
grouse. The incubation of the Megapodes has been referred to (p. 611).
Sub Order II. COLUMBIN.Z. The pigeons are distinguished from
the Gallinaceze by the more slender bodies, shorter legs, the toes free, and
the longer wings capable of prolonged flight. They are altrical; the crop
produces a milky secretion used in feeding the young. The CoLUMBID&
are the most widely distributed and are represented in the tropics by
numerous beautifully colored species. Colwmba.* According to Darwin
the domestic pigeons come from C. livia, the blue rock pigeon ; Eetopistes
migratorius,* passenger pigeon, practically exterminated. Allied was the
dodo, Didus ineptus, of Madagascar, exterminated in the eighteenth
century,
Sub Order III. NATATORES.