Xtverpool fIDaiinc Btolooyi (Totntntttec,
L.M.B.C. MEMOIRS

ON Typical British Marine Plants s- Animals

EDITED BY IV. A. HERDMAN. D.Sc, F.R.S.

XIV.

LIGIA

C. GORDON HEWITT, B.Sc,

Defnonstrator in Zoology. U)iivcrsitv of jManchestet .

(With 4 Plates)

Price Two Shillings

LONDON

Williams & Norgate
January, 1907

HARVARD UNIVERSITY.

I. I B K A R Y

OP THE

MUSEUM OF COMPARATIVE ZOOLOGY.

a\j<.xyv\}^

^ 5j(1A/vvOW^ ^ \^V0

L.M.B.C. MEMOIRS.

XIV.

L I G I A .

NOTICE.

The Committee desire to intimate that no copies of these
Memoirs will be presented or exchanged, as the prices
have been tixed on such a scale that most of the copies will
have to be sold to meet the cost of production.

The Memoirs may be obtained at the nett prices stated,
from Messrs. Williams and Norgate, 14, Henrietta Street,
Covent Garden, London.

Memoir I. Ascidia— published in October, IS!)!), (JO pp.
and hve plates, price 2s.

,, II. Cardium — published in December, 1899,
92 pp., six plates and a map, price 2s. 6d.

,, III. Echinus -published in Febrviary, 1900.

36 pp. and five plates, price 2s.
„ IV. Codiuni — j)ublished in April, 1900, 26 pp.

and three plates, price Is. 6d.

,, \\ Alcj-onium published in -January, 1901,
'!0 pp. antl three plates, price Is. 6d.

„ YI. Lepeoj)htheirus and Lermea — published in
March, li)Ol, 62 pp. and five plates,
price 2s.

,, VII. Linens — published in April, 1901, 40 pp. and

four plates, price 2s.
,, VIII. Pleuronectes — published in December, 1901,

260 pp. and eleven plates, price 7s.

,, IX. Chondrus — published in July, 1902, 50 pp.
and seven plates, price 2s. 6d.

,, X. Patella — puljlished in May, lOO-'J, 84 pp. and
four plates, price 2s. 6d.

,, XI. Arenicola — published in March, 1904, 126
pp. and eight plates, price 4s. 6d.

,, XII. Gammarus — ])ul)lislied in July. 1904. 5") pp.
and four plates, price 2s.

„ XIII. Anurida- published in October, 1906, 105 pp.
and seven plates, price 4s.

„ XIV. Ligia — published in January, 1907, 45 ])p.
and four phites, price 2s.

•livcrpool ni>afiiic BiolOtjy doinmittec.
L.M.B.C. MEMOIRS

OX Typical British Marine flaxes s^ Aximals

EDITED 7!Y W . A. HeRDMAN. D.Sc, F.R.S.

XIV.

LIGIA

BY

C. GORDON HEWITT, B.Sc,

Dt-iiionstiator in /.oolosw Univrrsitv of Manchester.

(With 4 Plates)
Price Two Shillings

^ LOXDON

Wni.IAMS X XOROATE

jANiMn-. 1907

EDITOR'S PREFACE.

The Liverpool Marine liiolo<i'y Coinmittee was constituted
in 1885, Avitli the object of investip-atino: tlie Fanna and
Flora of the Irish Sea.

The dredg^ing^, traAvling'jand other collecting- expeditions
org-anised bv the Committee have been carried on inter-
niittentlv since that time, and a considerable amount
of material, both published and unpublished, has been
accumulated. Nineteen Annual Reports of the Committee
and five volumes dealings with the " Fauna and Flora '"
have been issued. At an early stagfe of the investigations
it became evident that a Biolog^ical Station or Laboratory
on the sea-shore nearer the usual collecting' grounds than
Liverpool would be a material assistance in the work.
Consequently the Committee, in 188T, established the
Puffin Island Biological Station on the North Coast of
Anglesey, and latei- on, in 1892, moved to the more
commodious and accessible Station at Port Erin in the
centre of the rich collecting grounds of the south end of
the Isle of Man. A new and larger Biological Station and
Fish Hatchery, on a more convenient site, has noAv been
erected, and was opened for work in -Tuly, 1902.

In these nineteen years' experience of a Biological
Station (five years at Puffin Island and fourteen at Port
Erin), where College students and young amateurs form a
larg(^ proportion of the workers, the want has been fre-
(jiiently felt of a series of detailed descriptions of the
structure of certain common topical animals and planis,
chosen as representatives of their groups, and dealt with ])y
specialists. The same want has jirobably been felt in other
similar instituiions and in many College laboratories.

VI.

The objects of the Committee and of the workers at the
Biological Station were at first chiefly faunistic and
speciographic. The work must necessarily be so Avhen
opening np a new district. Some of the workers have
published papers on morphological points, or on embry-
ology and observations on life-histories and habits ; but
the majority of the papers in the voUunes on the '' Fauna
and Flora of Liverpool Bay "' have been, as was intended
from the first, occupied with the names and characteristics
and distribution of the many different kinds of marine
plants and animals in our district. And this faunistic
work will still go on. It is far from finished, and the
Committee hope in the future to add still further to the
records of the Fauna and Flora. But the papers in the
present series, started in 1899, are quite distinct from these
previous publications in name, in treatment, and in pur-
pose. They are called " L.M.B.C. Memoirs," each treats
of one type, and they are issued separately as they are
ready, and will be obtainable Memoir by Memoir as they
appear, or later bound up in convenient volumes. It is
hoped that such a series of special studies, written by
those who are thoroughly familiar with the forms of which
they treat, will be found of value by students of liiology
in laboratories and in Marine Stations, and will be
welcomed by many others working privately at Marine
Natural History.

The forms selected are, as far as possible, common
L.M.B.C. (Irish Sea) animals and plants of whicli no
adequate account already exists in the text-l)ooks.
Probably most of the specialists who have taken part in
the L.M.B.C. Avork in the past will prepare accounts of one
or more representatives of their groups. The following
list shows those who have either performed or promised.

Memoirs fioiii I. 1o XIY. have now liccn iiulilished.

Vll.

Autedoii, by Mr. Chadwick ; Cancel', by Mr. Pearson ; and
Doris, by Sir C. Eliot, are now far advanced and ought to
be out early in 1907. It is hoped that Cycloporus, Pecten,
and the Oyster will follow soon.

Memoir I. Ascidia, A\'. A. Herdman, 60 pp., 5 Pis., 2s.
,, II. Cardilm, J. Johnstone, 92 pp., 7 Pis., 2s. 6d.
,, III. EcHixus, H. C. Chadwick, 36 pp., -3 Pis., 2s.
„ IV. CoiJiUM, 11. J. II. Gibson and Helen Auld,

26 pp., 3 Pis., Is. 6d.
„ V. Alcyoxium, S. J. Hickson, 30 pp., 3 Pis., Is.Bd,
,, XL. Lkpeoi'iitiieirus axu Lern.ea, Andrew Scott,

62 pp., 5 Pis., 2s.
„ YII. LiNEus, P. C. Punnett, 40 pp., 4 Pis., 2s.
„ YIII. Plaice, F. J. Cole and J. Johnstone, 260 pp.,

11 Pis., 7s.
„ IX. CnoNDRUs, O. \. Darbishire, 50 pp., 7 Pis.,

2s. 6d.
„ X. Patella, J. 11. A. Davis and 11. -1. Eleure,

84 pp., 4 Pis., 2s. 6d.
„ XL Arexicola, J. II. Ashworth, 126 pp., 8 Pis.,

4s. 6d.
,. XII. tjAMMARus, M. Cussans, 55 pp., 4 Pis., 2s.
,. XIII. Anlrida, a. D. Imms, 107 pp., 8 Pis., 4s.
,, XIV. LicuA, C. Cr. Hewitt, 45 pp., 4 Pis., 2s.
Caxcer, J. Pearson.
Antedox, 11. C. Chadwick.
Doris, Sir Charles Eli(jt.
Cycloporus, F. F. LaidUuv.
Oyster, W. A. Herdman and J. T. .Jenkins.
Pecten, W, J. Dakin.
OsTRACou fCYTiiERE), Andrew Scott.
IJuccixuM, A\'. B. Pandles.
Bugula, Laura P. Thoniely.

ZosTEKA, Iv. -J. Liaivev (jribsou.
IliMAXTiiALiA, r. J. Lewis.
Diatoms, F. E. Weiss.
Eucus, J. B. Farmer.
lioTRYLLOiDES, W. A. Herdiiiau.
Cuttle-Fish (Eleuoxej, W. E. Hoyle.
AcTiMA, -J. A. Ulubb.
Hydkoid, E. T. Browne.
IIalichoxdria and SvtoN, A. Deiuly.

lu addition to tliese, other Memoirs will be arranged
for, on suitable types, sueli as Pat/iirus, Sdcjitta, Pontoh-
(/(//ti, a ( 'estode and a Pyoiiogonid.

As anuounced in the preface to Ascidia, a donation
from Mr. F. H. (iossage, of Wooltoii, met the expense
of preparing the plates in illustration of the tirst few
Memoirs, and so enabled the Committee to commence
the publication of the series sooner than would otherwise
have been jjossible. Other donations received since from
Mr. Gossage, Mrs. Holt, Sir -lohn Brunner, and others,
are regarded by the Committee as a welcome encourage-
ment, and have been a great help in carrying on the work.

W. A. Herdmax.

University of Liverpooi,

December. l'J06.

L.M.B.C. MEMOIRS

No. XIV. LIGIA.

C. GORDON HEWITT, B.Sc,

DeinouHtidtor in Zunlufij/, Univeraitji of Maiivltesfer.

CONTENTS.

PAGE

PAOK

Inlroduction

• 2

Vascular System .

18

Biology

3

Nervous System .

23

External Chaiacters

5

Sensory Organs .

'24

Appendages .

. 8

pjxcretory System

•26

Body Wall, Muscuhir h)

ystem

Reproductive Organs .

28

and Body Cavity

. 11

Development

29

JMgcstive Svstoni

. 13

Literature .

32

InTKODUCTIOiN".

The Isopod Ligia oceanica (Liuu.j has been selected as
the type for this Memoir on account of its comparatively
large size, being the largest British Isopod, and also
because it is one of the most interesting of the group,
being mid- way between the aquatic and terrestrial forms
The Isopoda, with the Amphipoda, form the sub-order
Arthrostraca, and are characterised by being Malacostracu
with seven distinct thoracic segments, each bearing a
pair of limbs (except the Grnathiidae) and possessing
sessile eyes ; on account of the last character, they are
usually classed together in the sub-order Edriophthalmia.
The Isopoda possess a dorsoventrally depressed body.
The thoracic limbs do not bear branchial appendages, as
in the Amphipoda, but respiration is carried on by means
of the abdominal appendages, which are modified for that
purpose, the modification ^varying in the different tribes.
The terrestrial Isopods, the Oniscoidea, are the only
members of the group which exhibit such a uniformity
in the character of the thoracic ap})endages as to justify
the name.

The following classification of the Isopoda is that
given by Sars (1896), each tribe being defined by three
characters — those of the first pair of legs, the uropoda,
and the pleopoda or abdominal appendages : —

I. First pair of legs cheliform ; Uropoda terminal;
Pleopoda, when distinctly developed, exclu-
sively natatory - - - 1. Cheliferae.
II. First pair of legs not cheliform. (1) L'ropoda
lateral, (i) Pleopoda foi- the most part nata-
tory, forming a ca\ulal f;ni with the terminal
segment of the metasome - 2. Flahellifcrae.

3

(iij Pleupuda to a great extent branchial; the
Uropoda valve like, infiexed, arching- over the
Pleopoda - - - - ;;. Wdvifcnie.

(2) l-ropoda terminal. (i) Pleopoda exclusively
branchial, generally covered by a thin ojier-
cular plate (the modihed 1st pair) -4. Asellota.
fii) Pleopoda for air breathing - o. Oniseoiiha.
(iii) Pleopoda when present, excdusively bran-
chial in the adult animal and not covered by
an opcrcuhim - - - (i. I'j picdrida.

L'ujia ocediilcd belongs to the tribe Oniscoidea, which
are characterised by being terrestrial. This tribe includes
all the so-called *' wood-lice." Their abdominal append-
ages are htted for air breathing, but in Ligia there is a
very near approach to branchial respiration, as moisture
is necessary. The body is oval in shape, and the seven
])airs of thoracic appendages are similar in character.
Lujia ocean icii was first described in lT(j7 by Linnaeus
as Oniscus ocean iciis. Later, in 1798, the genus Ligia
was created by Fabricius to include the Oniscus oceanicus
of Linnaeus.

Biology.

Li(/ia oceanica (PI. I.) has a wide distribution, and is
recorded from the coasts of the British Isles, Faroe Islands,
Norway, Denmark, Germany, Belgium, France, Spain,
Morocco and America. At Plymouth* I have found
Ligia most numerous, and of the maximum size, on
Drake's Island. At Port Erin they occur in the clift' near
the old biological station.

* I wish to express my thanks to the Council of the Marine
Biological Association of Great Britain for the use of a table at the
Plymouth Laboratory, during the Easter vacation, 1906. Other
material for this memoir was obtained at the IMarino Biological
Station, Port Erin, during the Easter vacations, 1903-4.

They are terrestrial, but require a certain amount of
moisture. On the other hand, they are unahle io
withstand prolonged immersion in sea-water, and still
less in fresh Avater.

They are found just above hig-h-water mark in a
7Ame of varying- width. The heig-ht of their habitat above
hig-h-water mark seems to depend on the nature of their
surroundings, which is varied. The g-reatest number are
found in deep narrow crevices in the rocks immediately
above high water. Here they can l)e found in large numbers
packed closely together. They also abound in crevices
on the side of a qii^j, hence their name, ' quay-louse ' or
' quay-lowders.' They are also known as ' sea-carpenters,'
' carpenter ' being a local name of the wood-louse. They
can be found between the wooden piles of a pier or under
the loose stones and rubbish cast up by the tide, which have
accumulated in small dark holes. The highest level is
attained by those specimens which live in the loose clay
and shale forming the cliffs on many parts of our
coast, but the specimens living in these conditions do not
attain the size of those living lower down in the rock
crevices, and are generally of a darker colour.

In St. Kilda, I have found them in the crevices of the
boulders on the top of a hill over 450 feet above sea-level.
This high altitude may be explained by the fact that the
sea spray often reaches that height. It is extremely
improbable, however, that the animals go doAvn to sea
level to feed. Contrary to the usual rule, the majoritv of
individuals found at this high level were females. I
found large numbers of young individuals under rocks
betAveen tide marks, and none at the high level, these
females probably go do\A-n to the sea level to liberate the*
yonng from their brood-ponches.

The colour vai-ies from a dark ffrevish yreen to a

5

light dirty brown. In young specimens tAvo light-coloured
patches occur on the median line of the dorsal side. The
colouration has generally a mottled appearance, the dark
portions being due to presence of closely packed chro-
matophores. In injected specimens a close connection is
observed between the terminations of the fine capillaries
and the chromatophores. In moulting, the cuticle of the
posterior half of the body is shed first, and a short time
elapses before the anterior half is shed, so that individuals
are often found with the posterior half of the body lighter
in colour than the anterior half.

Their food consists chiefly of decaying animal and
vegetable substances, and from a study of the contents of
their guts, the latter appear to form a large proportion of
their diet. In captivity they prefer the weaker members
of their own species, but there is not much evidence that
this is a natural habit.

They are able to run Avith great rapidity, coming out
from their dark retreats after sunset to feed, at which
time they may be caught with the aid of a lantern. The
best instrument for capturing them during the day is a
fairly long wire, having the last half-inch bent at right
angles ; by means of this they can be extracted from their
narrow crevices.

External Characters.

The body is oval in shape, broadest across the fourth
thoracic segment, and gradually decreasing in size towards
the posterior end. It is almost twice as long as it is
broad. The males are larger than the females, the reverse
being usually the case in Arthropods. They may attain
a length of 32-34 mm., the width of the thorax reaching
18 mm. The females are more regularly oval in shape

and may reach a size of 20 mm., their extreme width
being 11 mm. The (hjrsal face of the body is moderately
convex, and its surface is granulated. The body can be con-
veniently divided into four regions — 1, the head segment
or cephalou ; 2, the mesosome or thorax, consisting of
seven segments; 3, the metasome or abdomen, consisting
of five distinct abdominal segments, together with a
terminal segment, which is, 4, the telson.

The cephalon is sunk into a depression formed by
the forward growth of the epimeral portions of the first
thoracic segment. It is evenly convex in front ; the
posterior border is depressed, the edge being marked by
a ridge. The dorsal surface is slightly curved trans-
versely. A pair of large compound sessile eyes are
situated laterally, each having a slight reniform appear-
ance from a dorsal view. On the anterior face, which is
almost vertical, the minute pair of first antennae are
situated, one on each side the median line. To the
Dutside of these are the large and robust second antennae.
When moving, the animal holds the large antennae in a
forward position, and constantly tests the nature of the
surface over which it is proceeding with the very sensitive
fiagella, appearing to trust for guidance more by this
means than by means of its sight. When at rest, they are
folded back along the sides of the mesosome.

( )u the ventral side of the head the mouth-parts form
a prominent projection. The mouth is bounded in front
by a large transversely-hinged labrum. The sides ar(^
composed of the powerful mandibles and two pairs of
maxillae. Tlie posterior border is formed by the two
maxillipedes, which are imperfectly fused together,
forming an apparent lower lip; there is, however, internal
to these, a lingua-like bilobed chitinous plate, deeply
incised in the middle and having a small median plate;

this forms a true posterior lip. The maxillary excretory
organ opeus at the base of the second maxilla.

The seven segments of the thorax form the greater part
of the animal's body. They are convex on their anterior
margins and concave behind. The lateral portions form
large epimeral plates, obtusely acuminate, and directed
backwards. The segments slightly overlap, and the
uniting membrane, which is not impregnated with
calcareous salts, sinks into the hypodermal tissues. On
the ventral sides, where the epimera join the body, the
walking legs or pereiopods arise. The first three pairs
of pereiopods are approximately the same size, the last
four pairs gradually increase in size. The ventral wall
of the thorax is thin and transparent, and strengthened
by slightly curved transverse bars. On the ventral side
of the thorax of the female, the brood-pouch full of ova
is very conspicuous in the breeding season. It is formed
by lamellae which arise inside the origin of the five
anterior pairs of thoracic appendages and grow ventrally,
overlapping the adjacent lamellae distally and laterally.
The paired female genital apertures are situated on the
inside of the fifth pair of pereiopods. The male genital
products are ejected through a pair of styliform
appendages on the posterior border of the last thoracic
segment, immediately in front of the branchiae.

The metasome or abdomen consists of five segments
and the telson, and is about a third of the entire length
of the animal. The two anterior segments are narrow,
and do not reach the margins, but are lodged in the
concave posterior border of the seventh thoracic segment.
The three posterior segments have their lateral margins
produced into tooth-like backwardly projecting processes.
Five pairs of uropoda, which are of the nature of
branchiae, are borne on the ventral side of the abdomen.

8

Eaeh of these consists of an outer ' opercular ' lobe and
an inner lobe. In the male the inner niar^-ins of the
outer lobes of the first pair may, or may not, be prodiiced
into short spinous processes; the inner margins of the
second pair are produced into two long slender styles for
copulatory purposes.

The terminal segment or telson is composed of the
last two segments of the metasonie, which are fused to
form a rather broad terminal segment. The posterior
edge is evenly rounded ; the lateral portions are acuminate
and subtend a sinus in Avliich the terminal pair of uropoda
arise. Each of the terminal uropoda consists of a faiiiy
stout basal portion, from which the styliform appendages
arise. The anus is a longitudinal slit on the ventral side
of the terminal segment.

The external apertures are — the mouth, the openings
of the maxillary excretory organs on the protopodites of
the second maxillae, the openings of the oviducts at the
base of the fifth pair of pereiopods, or of the vasa
deferentia at the posterior border of the ventral side of the
seventh thoracic segment, and the anus.

Appendages.

The first pair of antennae (PI. IL, Fig. 1) may be truly
called antennules, as they are extremely small, measuring
a little over 1 mm. in length. They are situated internal
to, and at the base of, the second antennae. They are
triarticulate, the terminal joint being quite rudimentary
and bearing two terminal groups of small setae.

The second pair of antennae (PL II. , Fig. i) are long,
attaining a length of 25 mm. in the male and 14 mm. in the
female; when folded back they extend to the posterior
border of the fourth thoracic segment. The protopodite
is composed of five joints, the fourth and fifth being the

larfiS-est : the fifth joint equals in length the proximal five
joints of the flagellum. The flagellum may have as many
as 1-) or 14 joints, which are setose, the setae being- of two
sizes.

The mandibles (IM. II., tigs. 3, 4) are very powerful and
consist of a single protopodite. The point of attachment
is D shaped, the curve being internal. From the anterior
end of the external border, a powerful tooth-bearing
process curves inwards, forming a quadrant-shaped
anterior face. The curve of the quadrant is external ;
the lower side is partially attached and the vertical side
liears the mandilmlar processes. Internally, on this
vertical side, there are — a stout molar process, the fiat
extremity of which is covered with minute closely-set
teeth ; a small palpiform structure bearing setose bristles,
and two stout mandibular teeth separated by a sinus, each
being sub-divided into three smaller teeth. The ventral
edge of the mandible is produced into a rounded keel,
which gradually diminishes in depth towards the posterior
edge.

The first pair of maxillae (Tl. 11., tig. 5). Each maxilla
is composed of two lobes. The outer lobe is calcified and
much stronger than the inner, which is more flexible.
The inner lobe is terminated by three deflexed setose
bristles ; the outer lobe has a terminal group of short
thick spines.

The second pair of maxillae (PI. II., tig. 6) are much
uiodihed, being thin and flexible. Each consists of a long
protopodite, terminated by an oblique setose joint
having internally at its base two setose bristles. The
maxillary excretory organ o]iens at the base of the
protopodite.

The maxillipedes (PI. 11., tig. 7) are closely approxi-
mated on their inner sides and joined at the base, so thai

10

they form an oiiter lower lij). Each consists of an inner
rectangular plate spinose at its anterior margin, and an
(tuter five-jointed palp. The joints f)f the palp are spinose,
each of the four distal joints having on its inner side a
setose pad. At the base there is a small lamella.

The pereiopods (PI. II., tigs. 8, 9) or ambulatory
appendages. Each pereiopod ccmsists of a basal pmtopodite
and an endopodite, which is composed of five spinose joints
(ischiopodite, meropodite, carpopodite, propodite and
dactylopodite). In the posterior pairs of pereiopods, the
carpopodite and ])ropodite are long in comparison witli
the other joints. The dactyh)])odite bears two strong re-
curved claws.

In the female each of the anterior hve pairs of
pereiopods (PL II., tig. 12) subtends a thin foliaceous
lamella which curves downwards and inwards, overlapping
the opposite and adjacent lamellae, and thus they form a
brood ])oiich in which the eggs are carried. These lamellae
are not out-growths of the limbs, but of the sterna.

Abdominal appendages. There are five pairs of
abdominal a])pendages or pleopoda, and a terminal i)air of
uropoda. Each pleopod (PI. II., tigs. 11, l-\) consists of a
pear-shaped superior lobe covering a small inferior lamella.
lu the terrestrial isopods, the outer lobe is termed
opercular and the inner branchial. In Ligia both are
branchial, as will be shown later.

At the base of the outer edge of the superior lobe,
there is a small lamella. The edge of the superior lobe is
fringed with a border of setose bristles. The second pair
of pleopods are modified in the male by having a two-
jointed style arising at the base of the inner margin of
the superior lobe (PL II., fig. 10). The style is long and
grooved, and reaches to the fourth ])leopod ; its distal
extremity is slightly swollen and finely pointed. It is

11

for fopulatorv purposes. The superior lobe of the first
pair of pleopods also may be modified slij^-htly foi' eopuhi-
torv purposes. The tliird, fourth, and fifth pairs of
pleopods are very similar in eliaraeter, the third pair being-
the largest.

The uropoda (,P1. U., tig. 14) are situated at the posterior
edge of the sixth abdominal segment or telson, the pleural
regions of which are produced posteriorly, thus forming a
small sinus in which the uropods can bend laterallj-.
Each consists of a stout basal joint which is
widest in the middle and truncated distally. The middle
region is thick, narrowing olf sharply to the
outer edge, and slightly to the inner edge, which is
adjacent to that of its fellow. Distally, two setose styli-
form processes arise. These are about twice the length
of the basal portion. During life they are carried in a
diverged position, being separated vertically by a wide
angle. The inner style has a well-developed terminal
spine which is trailed over the ground, and is probably
of a sensory nature, as it has similar nervous connections
to the sensory bristles of the antennae.

lioDY Wall, ]\lrscrL.\R System and Body r'AViTv.

The body wall consists of three layers, the outer
cuticular layer, the hy})odermis an<l the connective tissue.
The cuticular layer is composed of a thin cuticula resting
on a thicker layer of chitin, in whicii two distinct layers
can generally be observed (PI. IV., hg. o). The chitin is
impregnated with salts of calcium, which cause it to have
a fairly resistant and brittle texture, lietween the segments
the chitinous layer is thin. In the middle region of the
dorsal side the intersegmental membrane does not dip
deeply into the tissues, as it does at the sides. The
cuticula bears spines and setae in many regions of the

12

body. The liypodermal layer is composed of a single
layer of cubical cells, rnderiieath the hypodermis the
chroniatophores are found. The rest of the body wall is
)nade up of connectiye tissue, in which groups of large
adipose tissue cells occur. These last cells also occur in
large numbers on the walls of the alimentary canal, and
dorsal to the heart. (PI. II., tig. IG, ad. tis.)

The body cavity (h.c.) is a haemocoel, the alimentary
canal and other organs being in contact with the blood.
A horizontal septum (sep.) divides the body cavity into a
small dorsal pericardial cavity enclosing the heart, and a
large sinus enclosing the other organs.

The muscular system, excluding the muscles of the
wall of the gut, heart, &c., consists of three sets of muscles
— 1, those in connection with the gastric mill; 2, the
muscles moving the segments of the body; 8, the muscles
moving the appendages.

The muscles are composed of striated muscle fibres.
The muscles controlling the gastric mill occupy almost
the whole of the cavity of the cephalon and are very
conspicuous on opening this. They are attached to the
dorsal side of the cephalon, and most of them are inserted
into the large lateral cardiac teeth. The muscles moving
the segments of the body are also segmented, but the
muscle segments alternate with the body segments. On
each side of the dorsal lino there is a series of longitudinal
miiscles (d.l.m.). The anterior end of each bundle
of muscle fibres of this series is inserted immediately
behind the anterior end of one segment, and the posterior
end is inserted at the anterior border of the succeeding
segment. In the lateral regions, where the cuticle dips
into the body, a number of oblique muscle bands, which •
form a series of muscles on each side, have their posterior
ends attached to the anterior wall of the cuticular

13

invaginatiou, and their anterior ends attached io the
hypoderniis of the middle of the preceding segment. In
the hiteritl, and also tlie epinieral regi(ms, the body cavity
is almost entirely filled up with the nuiscles moving the
appendages; these mnscles are attached to the dorsal
side of the animal in these regions (/ev. m.). The
longitudinal muscles of the ventral side (vJ.in.) have
their attachments similar to those of the dorsal side.
The joints of the thoracic appendages, like the appendages
themselves, are provided with extensor and flexor muscles.
The mandibles are provided with a powerful set of
muscles, attached to the dorsal side. The proximal ends
of the remaining mouth appendages have thickened
skeletal rods, forming an internal framework, to which the
muscles moving them are attached.

The Digestive System.

The digestive system consists of the alimentary canal
and its glands — the salivary glands, and the hepato-
pancreas or digestive gland.

The alimentaiy canal can be divided into four parts —
the oesophagus, the stomach, mid-gut and rectum.

The oesophagus (PI. II., fig. 15 ocs.) opens l)y a slit-like
ajjerture surrounded by the mouth appendages. Its course
is almost vertical, and it opens into the anterior end of the
stomach on the ventral side. The oesophagus receives the
secretions of the salivary glands.

The stomach forms an efficient mill for triturating
the miscellaneous substances upon which the animal feeds.
It lies in the cephalic and first thoracic segments. The
wall of the stomach, wlii'ch is composed of columnar cells,
is lined with chitin, and is folded in a complicated
manner ; thus a number of chitinous lamellae are formed,

14

\\ hifli prdjtM't into the cavity of tlif stomach. The dorsal
Avail of the stomach is almost flal. and is continuons
l)osteriorly with tlic mid-g-ut. The anterior end of
the stomach is sligditly oblitjue, and when seen
from the upper surface is semi-eircuhir. The plates
and " teeth " wliich foiin the gastric mill are
arranged in the fcdlowing manner. ( >n each of
the lateral margins cd' the anterior end of the stomach
a hilobcMl ampulliform tritniating ' tooth ' (Fig. 1-3, Let.)
arises, and meets its fellow of the opposite side above the
opening of the oesophag-us. These lateral cardiac teeth
are the (diief masticatory agents of the gastric mill.
Between these, on the anterior wall of the stomach, three
teeth till up the space, a small median anterior tooth
(m.a.t.) situated between two antero-lateral teeth (a.l.t.).
Posteriorly, the closure of the entrance to the stomach is
effected by a ventral transverse setiferous ridge, the
ventral cardiac tooth (v.c.f.). In the ])receding descri})tion,
the word ' tooth ' has been used to designate a chitinous
})rotuberance of the wall of the stomach, which is covered
with short, closely-set, re-curved setae. (h\ the ventral
side (»f the stomach, in the middle region, three tooth-
shaped processes arise, their apices directed backwards;
they are the median, veutial and ventro-lateral teeth
(r.Lt.). On each side of the cardiac region of the stomach,
a narrow lamella, the lateral cardiac lamella (l.c.L) runs
in an oblique direction from the antero-dorsal region to
the ventral side, and terminates near the ventro-lateral
tooth. In the pyloric region of the stonmch. a deep
invagination of the dorsal surface forms a broad dorsal
lamella (d.h), which extends across the dorsal side and
half-way down the lateral sides. Between the lateral
portions of the dorsal lamella and the wall of the stomach,
two large lamellae (rJ.p.f.) have their lateral limits; these

15

are the veiitro-lateial pyloiic lamellae. Tliey arise on the
ventral side immediately behind the ventro-lateral teeth.
Their ventral edges almost meet along their whole h-ngth ;
their lateral edges extend in an oblique direction from
behind the dorsal lamella to the ventral side at the anterior
end of the mid-gut, where each ventro-lateial lamella
terminates in a fine point.

The mid-gut (PI. II., tig. 16 iiiid.fj.) extends in a straight
line from the posterior end of the stomach to tlie rectum
in the posterior region of the abdomen. It is of uniform
width throughout, except at the posterior end, where it
narrows considerably, and is surrounded by a sphincter
muscle. Three regions can be roughly made out, the
arrangement of the epithelial cells of the gut being the
means of demarcation. The wall is composed of three
layers, an outer muscular layer, a median basement
membrane, and internally the epithelium, which is
covered by a chitiuous intima. This intinia is perforated
and is shed Avhen the animal moults. The muscular layer
is composed of two sets of muscles, an outer longitudinal
and an inner circular layer, but this only applies strictly
to the anterior end of the mid-gut ; further back the
mus(de hbres become separated by the bulging out of the
epithelial cells. The epithelial cells of the gut are very
large and contain correspondingly large nu(dei. They
form a syncytium, as they do not possess complete cell
walls, but are se})arated by inter-cellular hbres, extending
fi-om the basement membrane to the intima, and probably
of cytoplasmic origin, 'ilie arrangement of the epithelial
cells varies in different regions of the mid-gut. In the
anterior region, which is almost half the entire length of
the mid-gut, the cells are irregularly arranged. On the
lateral sides they extend in longitudinal lows; the two
median ventral rows of cells extend from the anterior end

IB

of the luitl-g-ut to the po.sterioi' ciul. On the dorsal side of
the anterioi' region, in the median line, a typhlosole {t.y.)
is foi lued hy tlie floor of a groove beiug" re-invagiuated ;
posteriorly, the sides of the groove widen out into an
elongate spoon-shaped structure. The function of the
ty])hlosole is ])rohably not, as is usual, to assist in the
absorption of food, but to provide a channel along whi(di the
secretion of the hepatopancreas is able to flow to the middle
region of the intestine, hi tlie middle region of the mid-
gut the epithelial cells exhibit a very regular arrangement.
They are arranged in double roAvs, which run out in an
oblique direction from the median line. The rows of cells
pi-oject into the body cavity, so that grooves are formed
between the double rows. In these grooves the muscle
fibres are lodged, underneath the blood-vessels from the
intestinal arteries. The posterior region is marked by the
presence of the sphincter muscle, which separates the mid-
gut from the rectum. In the sphinctal region the faeoal
pellets are formed.

The rectum is a short uniform tube o])ening by the
longitudinal slit-like anus.

The salivary glands. There are two pairs of salivary
glands situated in the cephalon, on each side of, and
opening into, the oesophagus. Each is made up of
a large number of rosette-like masses of gland cells, Avhich
are very similar to the mucous glands described by Allen
(1892) in Ft/laemonefes. In section, they have the
appearance shown in the figure (PI. II., tig. 17). Each
acinus is made u]) of a number of concentric
cells, in which two regions can be recognised — a peripheral
cytoplasmic region containing the nucleus, and a central
glandular region. Each of the cells lias at' its internal
apex an intracellular duct (ic.d.), whicli opens into a duct
common to the mass of cells [cd.). This duct is probably

17

formed by a single cell, the nucleus of whicli can be seen
near the centre of the gland [ii.c.d.).

The hepatopancreas. This is also known as the liver
and the digestive gland ; the last name describes its true
function. In Liqiu it consists of three pairs of tubules,
which extend from the pyloric region of the stomach to
the posterior end of the abdomen, where they gradually
taper oif, and are generally doubled back for a short
distance. The three pairs are situated in relation to the
intestine, dorso-lateral, ventro-lateral and ventral (PI. II.,
fig. 16, V. he'p.,vl. hej).). The muscles of the walls of the
distal two-thirds of the tubules are so arranged, that a
spiral appearance is produced. The spiral arrangement
of the muscles no doubt aids their peristaltic con-
tractions. The tubules of each side open into the
pyloric region of the stomach by a single aperture,
behind and below the ventro-lateral teeth. The two
ventrally placed tubules of each side fuse and then open
into the stomach. Anterior to the opening the
dorso-lateral tubules curve ventralwards, and fuse with
the anterior end of the ventro-lateral tubules. A small
tube is given off from the front of the common hepatopan-
creatic duct, which runs forward for a shoii; distance and
ends blindly. The epithelial cells of the hepatopancreas
are of two kinds — large secreting cells containing large
nuclei, and smaller cells which may be either young
secreting cells, or cells of an excretory nature.

The physiology of the digestive system of terrestrial
Isopods has been studied by Murlin (1902). He finds
that the secretion of the hepatopancreas, which may be
liberated by the dissolution of the cell, fragmentation of
the cell, or evacuation from the cell, contains ferments,
which are able to act upon proteids, carbohydrates and
fats,
c

IS

Vasculae System.

Delag'e (1881) has described the vascular system of
Ligia oceanica in his Memoir ou the circiilatiou of the
Edriophthalmia, and, except in a few details, my results
confirm his account.

The heart (PL III., tig. 1, ht) is a fairly wide tubuhir
structure, extending from the fifth abdominal segment to
the anterior end of the fourth thoracic segment. This
posterior position of the heart is correlated with a posterior
position of the organs of respiration. Its walls are
muscular, and are perforated by two ostia (asf.), which are
oblique slit-like orifices provided with muscles and two
small inwardly projecting flaps. They are situated in the
anterior and posterior regions on the right and left sides
respectively.

The pericardium (PI. II., tig". 16, p.c.) extends from the
anterior end of the heart to beyond the posterior end.
It receives the efferent vessels from the branchiae, and is
continuous with the venous lacunae in the anterior
regions of the body. It is separated from the body cavity
by a horizontal septum upon which the heart rests.

The heart is continued anteriorly as the median
aorta. On each side four arterial thoracic trunks arise.
The first pair may be termed the lateral arteries; the
remaining three pairs are the fifth, sixth and seventh
thoracic arteries, and they arise in the anterior half of the
heart.

The median aorta (PI. III., fig. 1, mcd.ao.) runs
forward along the dorsal wall of the gut to the
cephalic region. In the anterior region of the
second thoracic segment two arteries arise from the
dorsal side, and run a sinuous course in the
hypodermal tissues towards the epimera. In the first
thoracic segment a pair of large arteries arise laterally

li)

and run outwards at right angles. Each gives off a large

branch which supplies the walls of the stomach, a branch

running to the hepatic tubules, a few small arteries to the

soft parts, and, after giving oif another branch which

runs into the epimeron {ep. art.), it unites with an artery

(i) which is the anterior prolongation of the lateral artery.

Immediately on entering the cephalic segment, a small

median unpaired artery arises on the dorsal side, and

bifurcating, runs in the hypodermis. In front of this the

aorta gives off a pair of ophthalmic arteries (op. a.) which

run outwards to the eyes, giving off many small branches

to the soft parts. The aorta now bends down in front of

the stomach, where it dilates somewhat, the dilation lying

in a cavity on the anterior face of the stomach. This

dilation serves as a kind of cephalic ' heart,' as it has

on each side muscles connected with a pair of chitinous

rods from the anterior face of the stomach. These muscles

will aid in the contraction and dilation of the cephalic

' heart,' and so help to pump the blood into the rest of

the vessels of the median dorsal aorta ; the blood, on

account of the posterior position of the heart, would not

be driven into these vessels so effectively, if it were not

assisted by the action of the cephalic heart.* At the point

where the aorta bends, it gives oif dorsally a small median

artery, and lower down two median unpaired arteries,

each of which bifurcates, the superior one supplying the

posterior side of the cerebral ganglion {cef. g.), and the

inferior artery the anterior side of the ganglion. The aorta

then bifurcates. Each branch, besides giving off numerous

small arteries, which can be better understood by reference

to the figure (PL III., fig. 2), gives oif a large antennary

artery {ant. art.), and is then continued as the facial

* Contractile vascular saos occur in the heads of certain insects.
Pawlowa (1895) has described them in the heads of certain Orthoptera,
and, according to Selvatico, they occur in certain Lepidoptera.

20

artery [fac. art.), which supplies the mandibles and the
lateral regions of the face. Neither the injections nor the
serial sections showed any oesophageal ring of the nature
described by Delage. Several small arteries are given off
from the posterior border of the antenno-facial arteries
which supply the oesophagus {oes. art.) and neighbouring
soft parts, as will be seen from the figure. The fact that
in many cases these small arteries dilate to an exaggerated
extent when injected, may account for the mistake.

The lateral arteries (PI. III., fig. 1, lat. art.) run
forward and outward from the anterior end of the
heart, and in the first thoracic segment each anasto-
moses with the transverse artery from the dorsal
aorta. On the external side of each lateral

artery, four thoracic arteries arise (i., ii., iii., iv.j,
supplying the first, second, third and fourth thoracic
segments. On the internal side of the thoracic artery a
number of branches are given off which ramify on the
walls of the gut {^int. art.) and hepatic tubules [hep. art.).
Close to the origin of the fourth thoracic artery a large
branch {f/en. art.) is given off, which supplies the terminal
portion of the vas deferens. A number of arteries arise
from the dorsal side of each lateral artery, and ramify
in the hypodermal tissues.

The thoracic arteries (i., ii., iii., iv., v., vi., vii.). — The
course of each of the thoracic arteries, with the exception
of the sixth and seventh, is somewhat the same. Each
runs directly outwards, and, when dorsal to the hepatic
tubules, gives oft' a ventral branch which supplies these.
Following the curvature of the dorsal surface the arteiy
curves ventralh' ; a small artery arises which runs into
the dorsal longitudinal muscles. When it reaches the
insertion of the limb it bifurcates, the inner branch runs
inwards and supplies the ventral surface, the outer branch

21

soou bifurcates again, the dorsal branch supplying the
epimeral (e^^. art.) region, and the ventral branch is the
crural artery supplying the leg (cr. art.). The inner
branches of the first thoracic artery supplying the
ventral surface of the first thoracic segment unite
in the mid-ventral line at the base of the maxilli-
pedes, forming a median artery (PI. III., fig. 2)
which runs forwards and gives o& paired arteries to the
maxillipedes {mxi).), second [mx") and first maxillae (rnx'),
and terminates in the lingua-like lower lip.

The sixth thoracic artery soon after its origin gives
off a branch which runs ventrally, and unites with its
fellow of the opposite side in the mid-ventral line of the
intestine ; from the point of junction a median arteiy
runs forwards and backwards, forming a sub-intestinal
artery. From the sides of the sub-intestinal artery paired
transverse branches arise in a very regular manner, and
run on the walls of the intestine in the oblique grooves
which have been described above.

The seventh thoracic artery, after running obliquely
backwards for a short distance, gives ofl: an artery which
bifurcates and supplies the lateral regions of the intestine.
It soon gives ofi: from its posterior side a large artery, the
abdominal artery which runs posteriorly; the rest of its
course is similar to that of the other thoracic arteries.

The abdominal artery (PI. III., fig. 1, ah. art.)
of each side runs in an undulating manner, midway
between the lateral margins and the median line ;
from it arise small arteries supplying the intestine,
muscles and other tissues. In the third abdominal
segment it gives off' a ventral branch which supplies
the three anterior branchiae and the body-wall. The
fourth and fifth pairs of abdominal appendages are
supplied by an artery wlich arises from the abdominal

•2-2

artery in the fourtli abdominal segment. In the last
sesrment an artery is o-iven oit' internally to the intestine,
on the ventral side of which it anastomoses Avith its fellow
and the sub-intestinal artery. The abdominal artery
finally terminates in the uropoda.

The venous system is lacunar. A large thoracic
sinus runs into the abdominal sternal sinus, from which
five afferent branchial vessels arise; each of these
bifurcates at the base of the branchial appendages,
supplying the superior and inferior lobes of the branchiae.

The vascular system of the branchiae. The branchiae
are supplied by venous vessels from the abdominal sinus.
The efferent branchial vessels open into the pericardium
by way of the branchio-pericardial canals (PI. II., fig. 16,
hr.p.c). The circulation in the superior and inferior lobes
of the abdominal appendages, both of which are respira-
tory, is different. The interior of the inferior lobe of
the branchiae [inf. lam.) is fenestrated by an irregular
system of lacunae, those of the outer side containing
venous blood and those of the inner side arterial. On the
other hand, the vascular system of the superior lobe [stip.
lam.) is very definite and uniform throughout the five
pairs. It consists of a venous portion (PL II., fig. 13, a.h.v.),
which is ventral (looking at the gill from the anterior face)
to the arterial system of vessels (e.h.v.). The individual
arteries and veins interdigitate in a very complete manner,
and the vascular supply is veiy rich, as will be seen by
reference to the figure (PL III., fig. 3). On this account,
the superior gills cannot be looked upon as beng merely
opercular in function in this animal, but are certainly
respiratory appendages of a very perfect nature.

The blood is colourless and contains nucleated
corpuscles which vary in size. As in most anthropods, it
is Yery coagulable.

23

Nervous System.

Tlie nervous system (PL III., fig. 4) is composed of a
series of paired ganglia, the o-anglia of each pair being
closely apposed ; the ganglia are connected by distinct
commissures.

The supra-oesophageal or cerebral ganglion [cer. g.)
extends across the space between the eyes, anterior and
dorsal to the gut. The ganglion cells have large deeply-
staining nuclei, and the fibres arising from them decussate
and connect the ganglia. In the supra-oesophageal
ganglia several lobes can be distinguished. On the dorsal
side there is a large pair of lobes, from the sides of which
the optic stalks arise. Each of these optic stalks consists
of a proximal lobe, connected by closely apposed parallel
fibres with a distal lobe, from which the optic fibres arise
and run direct to the retinulae. On the ventral sides of
the superior lobes a small pair of median lobes is
situated ; these are anterior to, and connected with, a larger
pair of ventral lobes, the olfactory lobes, from which the
the large antennal nerves (ant. n.) arise. The supra-
oesophageal ganglion is connected with the sub-
oesophageal ganglion by a pair of peri-oesophageal com-
missures.

The sub-oesophageal ganglionic mass is perforated
near the anterior end by a vertical muscle band. The
mouth-parts are innervated by two pairs of nerves (m.y^.n.),
the first of which arises lateral to the perforation, and the
second pair posterior to this, and latero-ventral. A pair
of nerves {g-n.) arise posterior to thc&e and run ventrally
to the stomach.

The sub-oesophageal ganglion is connected with the
ganglia of the first thoracic segment by a pair of cords,
from the middle of each of which a bifurcating nerve
arises supplying the muscles of the body.

24

There are seven pairs of thoracic ganglia {th. g.), the
ganglia of each pair being closely connected. The pairs
of ganglia are connected by commissures, those between
the sixth and seventh pairs of ganglia being very
short. Each pair of thoracic ganglia gives off a pair of
stout nerves, which split into several parts, and supply the
appendages. From the middle of the length of the
commissures connectinsr the ffanfflia, nerv'^es arise which
innervate the muscles of the body.

In Ligia the abdominal ganglia are all fused into a
single ganglionic mass [(ib. g.) situated in the anterior
region of the abdomen. In the Isopoda all stages are
found, from the original separate condition of the
abdominal ganglia to the fused condition occurring in
Ligia. From the abdominal ganglion nerves arise, which
supply the appendages and muscles of the abdomen ; a
large pair of nerves run from the posterior end of the
ganglion to supply the uropoda.

A small median nerve runs between the commissures
connecting the thoracic ganglia from the sub-oesophageal
ganglion to the seventh pair of thoracic ganglia. It has
been termed the ' sympathetic ' nei^^^e, but there is no
evidence that it is of such a nature.

Sensory Organs.

The eyes. — As the eyes of the Ligia oceanica are
different from the eyes of other Isopods, which have been
described by Parker, Beddard and others, their structure
will be given in detail.

They are compound and sessile, occupying almost the
vrhole of the lateral region of the head. In the mature
animal each eye consists of upwards of 500 ommatidia.
The corneal cuticula is facetted. The corneal facets of the

25

central ommatidia are plano-convex, with the flat side
internal ; those in the peripheral regions have the inner
side slightly convex also.

In a single ommatidium (PI. lY., fig. 1) the following
parts can be recognised. The internal face of the corneal
cuticular facet {corn, cut.) is covered with two thin cells,
the subcoTneal hypodermal cells (.*;.r. hyp.) The nuclei of
these cells can be seen in the figure. Internal to these are
the nuclei of the two cone cells, [nuc. con.). Each of the
cone cells secretes a hemispherical transparent mass (con.),
the two segments with tlieir fiat surfaces apposed form the
cone. The cone cells surround the cone segments, and on
the proximal side form two sub-cylindrical, transparent
accessory cones (ace. con.), which is the most interesting
and exceptional feature of this eye. The cone cells
are surrounded by two pigment cells (jyg. c.) which
completely invest the upper half of each ommatidium.
The retinula consists of six retinulae cells, and not seven,
as stated by Beddard (1888). In this it agrees with
Idotea inovata, which also has six retinulae cells (Parker,
1891). The retinulae cells [ret.) have fibrillar axes which
are continuous with those of the nerve fibres. The six
nuclei of the retinulae cells are situated at their proximal
ends (nuc. ret.). The rhabdom consists of six individual
rhabdomeres, each rhabdomere (rh.) remaining attached to
the retinula cell which forms it, and separate throughout
its length from the other rhabdomeres. There is a dense
mass of pigment (jjg.) between each of the rhabdomeres
and its retinula cell. This may have been formed by the
retinula cell, which also contains a large amount of
pigment, or it may have resulted from an intrusion of a
process from one of the pigment cells. The latter view
is probably the correct one. The nerve fibrils of the
retinulae pierce the basement membrane (h.m.); those

2(i

of a single ommatidiuni fusing on the proximal side to
form a single nerve fibre (op.n.f.), which runs direct to
the distal portion of the optic lobe.

Sensory bristles. — On the flagellae of the large antennae
there are a number of sensory bristles on each segment.
These have been figured before by Nemec (1895). Each
bristle (PI. IV., fig. 3, s.b.) is enclosed by a sheath (sA),
which is continuous with the rest of the cuticula (ctla.). The
thick inner layer of chitin is pierced by a canal, the lumen
of which is continuous with that of the bristle. From the
bristle, by way of the canal, a number of delicate fibres
{n. f.) run and communicate with a number of nerve fibres
lying beneath the hypodermis (hyp.). These sensory
bristles are probably the most important organs of sense
which the animal possesses, as the antennae are con-
tinually in use. Besides their undoubted tactile functio}i,
they may take the place of auditory organs.

The inner of the two styles of the uropods, as
described previously, are probably of a sensory nature.

Excretory System.

The excretory system may be studied in two waj' s — by
feedincr animals on food mixed with ammonium carminate
or indigo-carmine, and by injecting aqueous solutions of
these substances into the body cavity. The latter method
is the most satisfactory, but should be supplemented by
the first. The injections are made with a hypodermic
syringe (or a pipette drawn out to a fine point). The
animal is injected on the ventral side, at the base of one
of the ajjpendages, and may be killed from 3 to 48 hours
after the injection and fixed in absolute alcohol or
Flemming's solution.

The excretory organs of Isojxxls have been studied

27

bv Bruiitz (1904), witli whose results my observations
on Lii/ui oceanica are in agreement. There are four
kinds of excretory organs, two of which are nephrocytes,
either grouped or scattered ; the third is a definite
nephridium, or ' kidney,' and the fourth, certain cells in
the hepatopancreas.

The maxillary kidneys, or nephridia, occur in the
basal portion of the second pair of maxillae. They
consist of two parts — the saccule and labyrinth. The
saccule is a slightly convoluted tube, which Vejdowsky
considers is a remnant of the obliterated coelom. It is
blind at one end, and opens at the other intothe labyrinth.
The cells forming the wall of the saccule are large and of
excretory nature. The labyrinth communicates with the
exterior by an aperture at the base of the second maxilla.

The cephalic nephrocytes are situated at the bases of
the first antennae. They occur along the ventral and
lateral sides of the levator muscles of these appendages.
These cells are fairly large ; the protoplasm is homo-
geneous and contains a number of granules.

The branchial nephrocytes occur in the abdominal
region, dorsal to the attachment of the branchiae. There
are five pairs of groups of branchial nephrocytes. They
are situated in two lateral lines, each line running above
the points of attachment of the abdominal appendages,
and their outer edges reach tlie bases of the epimeral
plates. Each group borders on two segments, the first
group bordering on the last thoracic and first abdominal
segment. The nephrocytes lie on the sides of the branchio-
pericardial canal (PL II., fig. Ki, hr. nepJi.). They are
large cells, and the cytoplasm, which is vacuolated,
contains many granules. Nemec considers the branchial
nephrocytes to be a syncytium, but the cell boundaries
are very distinct, as Bruntz also noticed.

28

Briintz found that certain small cells of the
epithelium of the hepatopancreas, called ' Fermentzellen '
by Weber, pass coloured solutions such as acid fuchsin
from the body cavity into the lumen of the duct and are
excretory in nature.

Reproductive Organs.

The reproductive organs of Ligia oceanica are simple
in structure. In the male (PL IV., fig. 4) there are three
pairs of elongate fusiform testes (t.), each being prolonged
into a fine filament. They are situated dorsal to the
intestine in the second and third thoracic segments. The
three testes of each siide are placed in series, and open into
a vas deferens (v. d.) of uniform width throughout the
greater part of its length. The vasa deferentia are usually
white and extended. They are situated on the dorso-
lateral sides of the intestine, and extend in a horizontal
direction to the seventh thoracic segment. In this
segment they narrow abruptly to form two narrow ducts,
which curve ventrally round the hepatic tubules; each
opens at the base of a styliform appendage {st. ap.)
situated on the ventral side of the seventh thoracic
segment, on one side the median line. The testes are
divided by slight constrictions which indicate different
regions of spermatogenesis in the interior. In the
process of spermatogenesis the spermatids unite in varying
numbers to form colonies, their cell walls disappearing.
The nuclei elongate considerably, and very fine fibres are
formed which may be attached to the nuclei, but on
account of their extreme tenuity the writer is unable to
be certain on this point. Miss Nichols found the same
difficulty in the spermatogenesis of Oniscus asellus. The
whole sperm colony, as it may be termed, together with
the cytoplasmic fibres, is surrounded by a protoplasmic

29

sheath. The anterior end of this is flageUate, and by
contractions of the slightly muscular walls of the testis it
is forced into the vas deferens. Here the sperm colonies
are found bound together in masses fPl. TV., fig. 5). The
substance which causes this cohesion is probably secreted
by a number of large cells which are situated in the
anterior end of the vas deferens near the openings of the
testes.

In the female the ovaries are very conspicuous in the
breeding season, entirely filling up the dorsal part of the
body cavity. They lie at each side of, and beneath, the
heart, and extend from the first thoracic segment to about
the fourth abdominal segment. They are usually filled
with eggs of approximately the same size. A short
distance behind the middle of the ovary a thin walled
oviduct is given off. This opens to the exterior by a small
longitudinal slit at the base of the fifth pair of pereiopods,
immediately at the base of the brood pouch lamellae, these
being the last pair of brood pouch lamellae.

The ova are large, oval in shape, and contain a large
amount of yolk. In copulating, the male walks on to the
back of the female and grasps the anterior thoracic
segments with the first three pairs of pereiopods ;
copulation may last one or more days. After the eggs
are extruded, they are carried about by the female in the
brood pouch, where they develop ; the young remain for
a short time in the brood pouch.

Development.

The development of Ligia oceanica has been studied by
Nusbaum. According to Nusbaum, the early cleavage
is discoidal, although McMurrich has found superficial
or centrolecithal segmentation in the Isopods which he
has investigated. The first cleavage cell becomes separated

80

from the rest of the yolk and lies on the periphery, where
it continues to divide, and so forms a cap of blastoderm
cells, no cleavage cells remaining in the food yolk. The
starting point coiTesponds to the point where invagination
takes place later, that being at the posterior end of the
ventral side of the embryo.

After the formation of the blastoderm a thickening
is formed, corresponding to the future ventral side. This
thickening is the germ disc. Shortly afterwards, three
divisit)ns of the germ disc make their appearance. Two
anterior paired portions (PI. lY., fig, 6, a.m.) represent the
formative region of the mesoderm; a median thickening
[end.) situated posterior to, and between these, represents
the fundament of the endoderm. The germ band is next
formed by a probable forward growth of the mesoblast
rudiments below the ectoderm, the ectoderm increasing in
thickness. Three pairs of buds arise ; these are the
rudiments of the limbs, and this stage (PL lY., fig. 7)
corresponds to the Xauplius stage. Behind these rudi-
ments, and in front of the anal aperture, is a mesoblastic
area, termed the formative area (/.-.), from which the
remaining segments of the body will develop. The
arrangement of the mesoderm cells in the formative area
is extremely regular.

At the beginning of the formation of the mid-gut a
number of cells (vitellophags) leave the endoderm and
wander inwards ; they do not take any part in the forma-
tion of the mid-gut, but assist in the disintegration of the
yolk. The mid-gut is formed from two layers of cells
which arise from the endoderm rudiment ; these lie beloAv
the germ band, and gradually grow round the yolk, each
being concave on its inner surface. By means of a ventral
median piece they unite in the anterior region, and tinally
enclose the yolk by growing round to the dorsal side, so

ai

that the yolk becomes suirouiuled by mid-gut epithelium.
Two flask-shaped vesicles are coiLstricted off on each side
of the anterior end of the mid-gut. These are the
rudiments of the hepatic tubules, which are formed by
their backward growth, and a longitudinal constriction
and division of each rudiment into three parts.

The rudiments of the thoracic limbs are biramous
(PI. I v., fig. 8), a fact which is used in support of the
theory that the C'rustacea have descended from a schizo-
podo'us ancestor. In Liyia the inner limb (endopodite)
alone develops, the exopodite being suppressed.

The nervous system arises as a continuous whole from
the ventral thickening of the ectoderm between the limb
rudiments. The thoracic ganglionic rudiments are paired,
but those of the abdominal segments are unpaired. Three
pairs of ganglia form the supra-oesophageal ganglion,
namely the optic, first and second antenna!. The sub-
oesophageal ganglion is formed by the fusion of four pairs
of ganglia — the mandibular, first and second maxillar and
the maxillipedal. There are rudiments of seven ganglia
in the abdomen ; rudiments also of seven pairs of
abdominal appendages are originall}' formed.

The heart is formed by the fusion of two dorso-lateral
layers of cells, crescentic in section, and lying dorsal to the
gut. The limbs develop successively from before back-
wards. In the earlier stages of development the embryo
has a dorsal curvature, but later it becomes ventral. On
hatching, the young isopod possesses six pairs only of
thoracic appendages, which are imperfectlj' segmented,
and not setose ; the cephalic region of the young auimal is
large in proportion to the rest of the animal. It leaves
the brood pouch of the female, and after several moults
attains the adult form.

32

Literature.

Allen, E. J., 1S92. On the Minute Structure of the Gills of
Palaemonetes varians. Quart. Joiirn. Micrs. Sci., Vol. XXXIV,
pp. 75-84; 1 pi.

Beddard, p. E., 1888. On the Minute Structure of the Eyes in
certain Cyniothoidae. Proc. Roy. Soc. Edin., Vol. XXXIII, pp. 443-
452 ; 1 pi.

BmiNTZ, L., 1904. Contribution a I'etude de rExcretion chez
les Arthropodes. Arch. Biol., Vol. XX, pp. 217-422 ; 3 pis.

Delage, Y., 1881. Contribution a I'etude de I'appareil circula-
toire des Crustac^s Edriophthalmes marius, 175 pp. ; 12 pis.

Fabricius, J. C, 1798. Entomologia systematica, SuppL, p. 301.

LiNNEUS, C, 1767. Systema naturae. 12th ed., ii, p. 1061.

McMuREiCH, J. P., 1895. The Development of Isopods. Journ.
Morph., Vol. XI, pp. 63-155 ; 5 pis.

MuRLiN, J. E., 1902. Absorption and Secretion in the Digestive
System of the Laud Isopods. Proc. Acad. Naf. Sci. PJiilad., Vol.
LIV, pp. 284-360; 23 figs., 1 pi.

Nemec, B., 1895. Studie o Isopodech. Vestnik KrdlovsJce
Ceske Spolecnosti Nank. (Prag.), pp. 1-46 ; 4 pis.

Nichols, M. L., 1902. The Spermatogenesis of Otiisciis asellns,
■with especial reference to the history of the Chromatin. Proc. Avier.
Phil. Soc, Vol. XLI, pp. 77-112 ; 8 pis.

NuSBAUM, J., 1891. Beitrjige zur Embvyologie der Isopodeu.
Biol. Centrlh., Vol. XI, pp. 42-49 ; 0 figs.

1893. Materyaly do Embryogeuii i Histogenii Rownonogow

(Isopoda). Ahh. der Krakauer Akad. der Wissenschaften math, nat..
Vol. XXV, 99 pp.; 6 pis. Abstract in Biol. Ccntrlb., Vol. XIII,
pp. 429-485,

Parker, G. H., 1891. The Compound Eyes of Crustaceans.
Bull. Mils. Comp. Zool. Harvard., Vol. XXI, pp. 45-142 ; 10 pis.

Pavvlowa, Mary, 1895. Ueber ampullenartige Blutcirculations-
organe im Kopfe verschiedener Orthopteren. Zool. Anzeiger.,
Vol. XVIII, pp. 7-13; Ifig.

Sars, G. 0., 1896. An Account of the Crustacea of Norway.
Isopoda. Vol. II, 270 pp. ; 100 pis.

38

Explanation of Plates.

Plate I.

Tjifiia ocenuica (Linn.), dorsal view, x 4.

Plate II.

Fip'. 1. First antenna seen from the side: much

enl argued.
Fig'. 2. Second antenna of left side seen from ahove.
Fig. 3. Left mandible seen from below.
Fig. 4. Teeth and molar process of mandible from the

posterior side.
Fiff. 5. First maxilla of left side.
Fig. 6. Second maxilla of left side.
Fig. 7. Left maxillipede.
Fig. 8. First pereiopod of male.
Fig. 9. Seventh pereif)pod of male.
Fig. 10. Second abdominal appendage (or pleopod) of

left side of male, showing copulatory style;

anterior view.
Fig. 11. Fonrth abdominal appendage (pleopod) of male,

])osterior view, showing the inner lamella.
Fig. VI. '^riiird pereiopod of female, attached to the

epimeral plate, showing one of the brood ponch

lamellae arising from the sternnm internal to

the appendage.
Fig. 1-"). Fonrth abdominal a])pendage of female,

anterior view, 'lire inner lamella can be seen

by transparency ; also the afferent (a.h.v.) and

efferent (e.h.r.) branchial vessels.
Fig. 14. Uropod, showing sensory process at the tip of

the inner style.

34

Ficr. 10. Interior of the right side of the stomach. The
stomach has been opened by a vertical section,
a little to the right (if tlie median line so that
the median ventral tooth has been removed.
The greater part of the cardiac region of the
stomach is surrounded by a layer of connective
tissue. (aJ.f.) ant(>ro-laieral tooth; (d.I.) dorsal
lamella; (hep.) hepatopancreatic tubule ; (l.cJ.)
lateral cardiac lamella; {J.c.i.) lateral cardiac
tooth: (m.ti.f.) median anterior tooth; (oes.)
oesophagus; (r.i.f.) ventro-lateral tooth;
{v-Lj).!.) ventro-lateral ])yloric. lamella ; (v.c.f.)
ventral cardiac tooth.

Fig. 10. Transverse section through the abdominal
region. The abdominal appendage of the left
side is omitted, (nd. tiss.) adipose tissue cells;
(h.c.) bod}' cavity (haemocoel); {hr.iieph.)
branchial nephrocytes; (hr.p.c.) branchio-
pericardial canal; [d.l.m.) dorsal longitudinal
muscles; (epim.) epimeron ; (hi.) heart; (???/•
l/iin.) inferior lamella of gill; (lev. in) levator
muscle of appendage ; (m/d. f/.) nud-gxit \ (p.c.)
pericardium; (■•<('p.) septum forming floor of
pericardium; (.■<i(p. I(tiu.) superior lamella of
gill; (fi/.) typhlosole; (vl.Jiep.) ventro-lateral
hepatopancreatic tu1)ule; (r./iep.) ventral
hepatopancreatic tubule; (f.Lm.) ventral
longitudinal muscles.

Fig. 17. Transverse section of two of tlie rosette-like
salivary glands. The left section shows the
common duct (r.d.) of the glandular cells,
formed by the uniting of the intracellular
ducts (/<;'. r/.); n.c.d. is tlie nucdeus of the cell
which probably forms the common duct.

36

Plate III.

Fig. 1. Dissection of the arterial system from the dorsal
side. The main arterial trunks are drawn
somewhat larger than they are naturally. For
the sake of clearness, details of the anatomy
have been omitted. {ah. art.) abdominal
artery; (rev. (j.) cerebral gangdion ; (cr.arf.)
crural arteiy; (cji.ai-t.) epinun-al arteries;
[gen. art.) g-cnital artery; (hep. art.) hepatic
arteries; (///.) heart; (int. art.) intestinal
artery; (/at. art.) lateral artery; (med.ao.)
median aorta; (op. a.) ophthalmic artery;
i^ost.) ostium; (vent, art.) ventral artery; (i-vii)
thoracic arteries.

Fig. 2. Arteries of the left side of ventral surface of
head and first thoracic segments, [ant. art.)
antennal artery; (fae. art.) facial artery;
(^mand. art.) mandibular artery; (inx.) artery
of first maxilla; (in.v''.) arterj' of second
maxilla ; [tn.vp.) artery of m.axillipede ; (^oes.
art.) oesophageal artery. Other references as
in the preceding hgure.

Fig. o. Afferent vessels of the superior lamella of one
of the abdominal a[)pendages injected with
indigo-carmine from the sternal sinus.

Fig. 4. The nervous system, seen from above after the
removal of the muscles and viscera, (ah. (/.)
abdominal ganglion; (ant. n.) antennaiy
nerve; (f/.n-) nerve to stomach; (med.n.)
median nerve; (^m.p.n.) nerves of mouth
appendages; (op. I.) optic lobes; [snh-oos. g.)
sub-oesophageal ganglion; (Tli'.g.) tirst
thoracic a'anpdion.

o('.

Platk IV.

Fig. 1. Longitudinal section of a single omniatidiuni.
(Drawn with the camera liicida.) (ace. con.)
accessory cone; (J). in.) basement membrane;
{con.) cone; {corn, cut.) corneal cuticula ; {nuc.
con.) nucleus of cone cell; {nuc. ret.) nucleus of
the retinular cell ; (oj). n. f .) optic nerve fibre ;
[pfl.) pigment; {[-></. c.) pigment cell; {ret.)
retinular cell; ('■/;.) rhabdomere ; (s-cJii/p.)
sub-corneal hypodermal cell.

Fig. 2. Transverse section of ommatidium showing the
six retinulae cells and their rhabdomeres.

Fig. 3. Longitudinal section through the cuticle of a
segment of the fiagella of the second pair of
antennae to show a sensory bristle and its
nerve supply. (ch.) thick layer of (diitin ;
(cfla.) cuticula; i/iyp.) hypodermal layer; (n-f.)
nerve fibrils; {.^i.h.) sensory bristle; (.sVk) sheath
of sensory bristle.

Fig. 4. (ienerative organs of the male. (t.) testes ;
opening into {j-'.d.) the vas deferens, which
opens externally by the styliform a})pendages
{st. a p.).

Fig. 5. A collection of ' sperm-colonies ' as they are
found in the vas deferens, in the cohesive
substance secreted by the latter.

Fig. 0. Early stage in the development of the q^\i,
showing the cleavage of the germ disc into the
two antero-lateral mesoderm fuiuhunents [(i.m.)
and the median jiosteiior endoderm fundament

{end.).

37

Fig-. 7. The ' uaupliiis ' sta^e of the (leveh)})ineiit of
Ligia, showincr the three naiipliar appendag^es.
(l.c/.) and (2.^/.) the hrst and second pairs of
antennae, and the mandibles (maiul.); (end.)
endoderni fnnchinient ; (/.r. ) formative zone ;
(op.) optic lobe.

Fi^. (S. A later .sta^'i' in the development, showing' the
eloiig'ation of llie formative and the formation
of appendao-es, also the rudiments of the
g'antiflia. (((n.) anus; (1. and 2. ni,v.) hrst and
secoud pairs of maxillae; (iiuvp.) maxillipedes.

The last three fifjures are after Nusbaum.

C. Tinling& Co., Ijtd., 53, Victoria Street, Liverpool.

L.M. B. C. Memoir XIV.

Plate I.

ft-

L 1 G i A

JlTarla-nt IKralime.LiLh Edii

L.M. B. C. Memoir XIV.

Plate 11.

LIGIA

M*Fa.rla.no S-ErakincLttli Edir

L.M. B. C. Memoir XIV.

Plate III.

Fig. 3

C. e. H. del.

LIGIA

M^F.irUue i Erakioe Lull Edm^

L.M. B. C. Memoir XIV.

Plate IV

L I G 1 A.

MtFa-rlanc iErskine. LiLh.Eflin'

\

u

MA^S isei

DatJ^Due

SI