ivcrpool flDartuc 36ioloov> Coinintttcc^
LM.B.C. MEMOIRS

ON Typical British Marine plants s- Animals

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

VI.

LEPEOPHTHEIRUS

AN])

LERN^A.

BY

ANDREW SCOTT,

Resident Fisheries Assistant at the Piel. Hatchery.

(With 5 Plates)
Price Two Shillings.

LONDON

WlJ.I.IAM.S & NORGA'IE

March, 1901.

HARVARD UNIVERSITY.

I. I B R A R Y

MUSEUM OF COMPARATIVE ZOOLOGY.

Si

\A\\ ■

L.M.B.C. MEMOIRS

VI.

LEPEOPHTHEIRUS

AND

LERNy^A.

NOTICE.

The Committee desire to intimate that no copies of these
Memoirs will be presented or exchanged, as the prices
have been fixed so low that most of the copies will have
to be sold to meet the cost of production.

The Memoirs may be obtained, post free at the nett
prices stated, from Messrs. AVilliams and Xorg-ate, 14,
Henrietta Street, Covent Garden, London.

Memoir I. Ascidia — -published in October, 1S99, HO pp.
and five plates, price 2s.
,, II. Cardinm — published in December, 1899,

92 pp., six plates and a map, price 2s. 6d.
,, III. Echinus — published in Febrimry, 1900,

'■)(] pp. and five plates, price 2s.
,, IV. Codium— published in April, 1900, 20 pp.

and three plates, price Is. 6d.
„ Y. Alcyonium — published in January, 1901,

30 pp. and three plates, price Is. 6d.
„ VI. Lepeophtheirus and Lernsea — now ready
(March, 1901), 62 pages and five plates,
price 2s.
,, VII. Linens — (in April), Avith four plates.

Xiverpool fIDartnc Biolo^^ Committee,
L.M.B.C. MEMOIRS

ON Typical British Marine Plants s^ Animals

EDITED BY W. A. HeRDMAN, D.Sc, F.R.S.

VI.

LEPEOPHTHEIRUS

AND

LERN^A.

BY

ANDREW SCOTT,

Resident Fisheries Assistant at the Piel Hatchery.

(With 5 Plates)
Price Two Shillings.

LONDON

W 1 1 . LI A M S ii No RG ATE

March, iqoi

EDITOR'S PREFACE.

The Liverpool Marine Jiiology Coiumittee was constituted
in 1886, with the object of investigating the Fauna and
Flora of the Irish Sea.

The dredging, trawling, and other collecting expeditions
organised hv the Committee have been carried on inter-
mittently since that time, and a considerable amount
of material, both published and unpublished, has been
accumulated. Fourteen Annual Reports of the Committee
and five volumes dealing with the "Fauna and Flora"
have been issued. At an early stage of the investigations
it became evident that a Biological Station or Laboratory
on the sea-shore nearer the usual collecting grounds than
Liverpool would be a material assistance in the work.
Consequent!}^ the Committee, in 1887, established the
PufKn Island Biological Station on the North Coast of
Anglesey, and later on, in 1892, moved to the more
commodious and convenient Station at Port Erin in the
centre of the rich collecting grounds of the south end of
the Isle of Man.

In these thirteen years" experience of a Biological
Station (five years at Puffin Island and eight at Port Erin),
where College students and young amateurs formed a large
proportion of the workers, the want has been constantly
felt of a series of detailed descriptions of the structure
of certain common typical animals and plants, chosen
as representatives of their groups, and dealt with by
specialists. The same want has probably been felt in other
similar institutions and in many College laboratories.

The objects of the Committee and of the workers at the
Biological Station have hitherto been chiefly faunistic and
speciographic. The work must necessarily be so at first

when opening- up a new distriet. Some of the workers
have published papers on morphological points, or on
embryology and observations on life-histories and habits ;
but the majority of the papers in the volumes 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 greatly to
the records of the Fauna and Flora. But the papers in
the present series are quite distinct from these previous
publications in name, in treatment, and in purpose. They
will be called the " L.M.B.C. Memoirs,"' each will treat
of one type, and they will be 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 w^ith the forms of which
they treat, will be found of value by students of Biology
in laboratories and in Marine Stations, and will be
welcomed l)y many others working privately at Marine
Natural History.

It is proposed that the forms selected should, as far as
possible, be common L.M.B.C. (Irish Sea) animals and
plants of which no adequate account already- exists in the
text-books. Probably most of the specialists who have
taken part in the L.M.B.C. work in the past, will prepare
accounts of one or more representatives of their groups.
The following have already promised their services, and
in many cases the Memoir is already far advanced. The
first Memoir appeared in October and the second in
December, 1899, the third in February, and the fourth in
April, 1900, the fifth in Janviary, 1901, and this sixth one

V]]

ill March, while the seventh will he ready early in April :
others will follow, it is hoped, in rapid succession.
Memoir I. Ascidia, W. A. Herdinan, GO pp., 5 Pis., 2s.
,, II. Cardium, J. Johnstone, 92 pp., 7 Pis., 2s. 6d
,, III. Echinus, H. C. Chadwick, -Ui pp., 5 Pis., 2s.
,, lY. CoDiUM, R. J. H. Gribson and Helen Auld,

26 pp., ;j Pis., Is. Gd.
,, V. Alcyonium, S. J. Hickson, 80 pp.. 3 Pis., Is. 6d.
,, VI. Lepeophtheirus and Lern.ea, Andrew Scott,

62 pp., 5 Pis., 2s.
„ VII. LiNEFs, R. C. Pimnett, 4 Pis.
Dendronotus, J. A. Clubb.
Peridinians, (j. Murray and V. (j. A\'hitting'.
ZosTERA, R. J. Harvey Gibson.
Himanthalia, C. E, Jones.
Diatoms, E. E. Weiss.
Fucrs, J. B. Farmer.
(jriGARTiNA, 0. V. Darbishire.
Plaice, E. J. Cole and J. Johnstone.
BoTRYLLoiDES, W. A. Herdmau.
CuTTLE-FisH (Eledone), W. E. HojIb.
OsTRACoD (Cytiiere), Andrew Scott.
Patella, J. R. A. Davis and H. J. Eleure.
Calanus, I. C. Thompson.
Actinia, J. A. Clubb.
BuGULA, Laura R. Thornely.
Hydroid, E. T. Browne.
Myxine, (j. B. Howes.
BucciNUM, M. F. Woodward.
Calcareous Sponge, R. Hanitsch.
Arenicola, j. H. Ashworth.
Antedon, H. C. Chadwick.
Oyster, AV. A. Herdinan and J. T. -lenkins.
Porpoise, A. M. Paterson.

vni.

In addition to these, other Memoirs will be aiiaiig'ed
for, on suitable types, such as Sagitta (by Mr. Cole), a
Cestode and a Turbellarian (by Mr. Shipley), Careinus.
an Amphipod, and a Pycnogonid (probably by Dr. A. H.
Jackson).

As announced in the preface to Ascidia, a donation
from Mr. F. H. Grossage of Woolton met the expense
of preparing the plates in illustration of llie first feAv
Memoirs, and so enabled the Committee to commence
the publication of the series sooner than would otherwise
have been possible. A second donation received since
from Mr. Gossage, and another recently from Mrs. Holt,
are regarded by the Committee as a welcome encourage-
ment, and will be a great help in carrying on the work.

W. A. Herumax.

University College, Liverpool,
March, 1901.

Note. — The present Jlemoir by Mr. Andrew Scott differs from those
published and contemplated inasmuch as it deals with two
generic types. At one time I thought of issuing Lepeophtheirus
and Leen.e A as separate Memoirs ; but, on consideration, it seemed
clear that these two diverse types of Epizoa supplemented one
another and illustrated well the range in form of the parasitic
Copepoda, and that consequently it would be convenient to the
student to have them under one cover. — W.A.H.

L.M.B.C. MEMOIRS.

No. VI. LBPEOPHTHEIEUB AND LKRN^A.

BY

ANDREW SCOTT.

Introduction.

There are comparatively few fishes that do not, at some
period of their life history, prove on careful examination
to be the host of at least one kind of parasite, either crus-
tacean or woim. The worm parasites are usually found
infesting" the alimentary canal (A'eniatodes and Cestodes),
the gills and skin (Trematodes and Bdellodes), while
Crustacean (Copepod) parasites are almost entirely con-
fined to places in direct communication with the exterior,
such as the skin itself, the fins, the mouth, the branchial
chamber, attached to the gills and operculum, in the
nostrils, and in the mucous canals. They may even be
found attached to the eye, as Lernceeninis spratta' in the
sprat (Clupea aprattus) ; and Leivueopoda elongata in the
(jrreenland Shark* [Acanthorhinus carcharias), causing in
the latter at any rate partial blindness ; or burrowing into
the abdominal cavity, as Penella exocceti in the flying
fisht (E.vucoetus volifun.s), till only the ends of the ovisacs
are visible from the exterior.

The Copepod fish parasites have attracted much atten-
tion from Zoologists for a very long period, since the time
when Aristotle, in his " Historia Animalium," tells us
that the tunny and the sword fish are tormented by a sort
of worm which fastens itself under the fin. Many of
* ]\Ir. R. L. Ascroft, of Lythain, who visited Iceland on a " steam liner,"
tishing for halibut, &c., a year or two ago, says nearly all the sharks caught
on the lines had these parasites in their eyes.

t One was exhibited at a meeting of the Liverpool Biol. Soc. in 1897,
infested by two such parasites, recorded as P. blainvilli, which in turn
were covered with a number of small Cirripedes. — Trans. L'pool Biol. Soc.
vol. xi., p. xii.

the parasites now known to be Copepods Aveie not at
tivst recognised as Crustacea, chietiy because of the difh-
eultv of makings out the true characters and the absence
of knowledge as to the life-histories. There was much
difference of opinion even as to which were really the
anterior and posterior extremities of these animals, due
to the fact that the posteriorly placed ovisacs of the then
known forms are cylindrical tubes which were by some
supposed to be the antennules, and therefore that end
was called the anterior. Hence many of the drawings of
the earlier authors represent the animals standing on their
heads.

Baird's " British Entomostraca," published by the
Ray Society in 1850, marks an important epoch in our
knowledge. This author gives an interesting historical
account of the group, brings together all that was pre-
viously known, and gives a very full account, with
excellent figures, of all the British species known at
that time, and although some of these are inaccurate in
detail, or have been added to by more recent investiga-
tions, still Baird's monograph is indispensable to any one
working at the subject. Since 1850 comparatively little
has been done in this country to increase our knowledge
regarding the distribution or habits of these crustaceans.
Within the past few years, however, the study has revived
and some important papers, mainly speciographic, have
been published.

The latest classification of the Copepod fish parasites
arranges them under seven families, as follows : — Ergasi-

hlDJE, CALlGIDiB, DiCHKLESTIDyE, PhILICHTHVID^E, LBRN^IDiE,

Chondracanthid;e, and LERN^opoDiDiE. With the excep-
tion of the PhilichthyidjE, all these families have
representatives living on iislies lound in the seas around
our coasts.

3

These parasites vary considerably in size, raiig-iug- from
one-thirtieth of an inch to nearh' two inches in leng-th.
They also differ very much in shape. Some have theii'
locomotor organs well developed, and are capable when
necessary of leading a pelagic life for a period. Others
have lost all swimming power, and become mere inert
sacs, securely attached to their host by anchor processes,
embedded in the tissues, and when taken offc' their host
they soon die from want of food and oxygen.

The sexes are separate, the males as a rule being much
smaller than the females. In many cases the males are
practically parasitic on ihe females, especially those of the
C'hondracanthidae and Lernseopodidae. The fact that the
males are found upon egg-bearing females of the above
families is due to their power of locomotion having been
lost when they reached maturity. When once they have
settled down and matured they are unable to change their
position to any extent. I'ertilisation of the female is
effected early in its life history, before the metamorphosis
i'^ completed. One copulation, apparentlj', is all that is
necessary to fertilise the female for life. The resulting
embryos remain attached to the external opening of the
oviducts, either in a single or multiserial column, enclosed
in a sac, until they hatch. The period of incubation
extends over several weeks. The young parasites hatch
out as nauplii, with three pairs of appendages. The
nauplii undergo metamorphosis, which in some forms
after a certain stage is reached is retrogressive, hnally
leading to the adult condition.

The Copepod fish parasites are generally regarded as
being composed of about sixteen somites. Usually, how-
ever, some of these somites are suppressed or fused
together, forming one compound segment, the true
character of which is rendered evident by the appendages

attached to it, eaoli ]iair indicating a somite. At one end
of the series, these ])arasites approach very nearly in
strnctnre and g'eneral appearance to the non -parasitic
Copepods. At the other end they are extremely different,
exhibiting' most remarkahle examples of retrograde
deA-elopment, and withont a complete stndy of their life
history it would be (juite impossible to recognise them
even as Crustacea.

In the following pages an account is given of the
an atom \- and metamorphosis of one member from each of
the two very different families, the Caligidse and the
Lernseidse, the forms chosen being Le-peophtheirus
pectoralis and Lcriued hv<(in-]ii(dis.

The Caligidae is the most extensive family of the Cope-
pod fish parasites, and contains a larger number of genera
and species than any of the others. As it stands at
present, there are 124 species representing 25 genera.
Three-fifths of the known species of CaligidtB belong to
two genera, Caligus and Lepeophtheirus. Some earlier
authors have not recognised the latter genus, and include
the various species belonging to it in Cdligus.. There are,
hoAvever, very important differences between the two
which make their appearance early in life. These
differences are constant, and give good cause for establish-
ing a separ?<le genus. Ca/if/its has two semicircular
suckers on the frontal margin of the cephalic shield, which
are developed before the " chalimus "* stage is completed,
and the biting part of the second maxilhe has only one
tooth. In Lepeop/ifheirus these suckers are entirely absent
ail through life, and the biting part of the second maxillae
has two teeth. The changes that take place between the
" nauplius " stage, when the animal is hatched from the

* The stage at which the animal hist becomes attached to its host.
(see p. 32).

pg'g, and the adult condition are practically the same in
the two genera, and probably also in the other members
of this family. From investigations carried on during
the past two years, it may reasonably be concluded that
LcpeophtheirK.^, throughout the remainder of its life and
under normal conditions remains on the same fish that it
attached itself to at the beginning of the " chalimus "
stage. It is very rarely met with in tow-net collections.
On the other hand, C aligns does not always remain on the
same fish. At the completion of its " chalimus " stage it
frequently leaves its host, and for a time leads a pelagic
life. Tow-net collections often contain immature males
and females, and occasionally mature males of Caligus,
especially of CaJigns rapa.v. Amongst these some mav be
found with a large notch in the middle of the frontal
margin. This is due to the breaking of the chitinous
filament by which they were secured to their host. The
metamorphosis is a progressive one.

I.— LEPEOPHTHEIRUS (Miiller).

This species was first described by 0. F. Miiller* under
the name Lerncra pectoralis.

Mode of Occurrence.

Lepeophtheirus pectoralis is most frequently found upon
the "white fiuke " or flounder {Pleuronectes Jiesus). It
also occurs on the plaice, dab, sole, &c. It does not con-
fine itself to any particular part of the exterior. Males and
immature forms of both sexes are to be found all over the
skin on each side of the fish. Mature egg-bearing
females, however, are usually found under the pectoral,
the pelvic, the ventral and the dorsal fins. With careful

* Prodiom. Zoolog. Dan.. 1776.

examination it is possible to collect a series of these
parasites, from the early " chalimiis " stage to the adnlt
condition, from one fish. Sometimes only a few speci-
mens occur on the fish. At other times large numbers
are to he found. It is by no means rare to find between
twenty and thirty mature females iinder each pectoral
fin alone, as in the case which is illustrated in the cut. One
hundred specimens of another species, L. hvppoglossi, have
been collected from the "white" side of a halibut in the
Aberdeen Fish Market. The average length of a mature

TjCjiciijihtlirintx jwcforalifi, 32 specimens, on the pectoral fin of a Flounder
from a photograph.

earS'-bearins" female is one-fifth of an inch, and of a male
one-ninth of an inch. A mature female measures about
one-tenth of an inch at its greatest breadth, and a male
one-twelfth of an inch. These parasites attach themselves
to the fish by means of their powerful second maxillipedes,
assisted by the antennse, and a decided pull has to be
exerted before they can be torn away. By depressing
the edges of the carapace and applying them closely to
the skin, the parasite can increase its holding power to
such an extent that the posterior end can be torn from the

anterior part without detaching it. The anterior part,
when thus separated from the posterior, retains its vital
powers for at least twenty-four hours. At first it swims
about vigorously, but after some hours begins to get
sluggish in its movements, and then finally dies. The
posterior part does not live long when separated from the
anterior. The parasites can be kept alive in sea water for
u])wards of six weeks after removal from the fish.

External Char.\cters.

The animal is depressed dorso-ventrally, and is of a
more or less oval shape, and distinctly divided into four
parts (Plate I., fig. 1). The foremost one of these parts,^
and usually the largest, is almost circular in outline, and
has all the appendages, with the exception of the fourth
and fifth pairs of feet, attached to it. This part is known
as the cephalo-thorax.

YicAved from above, this region is seen to be slightly
convex and divided into four portions by imperfect
sutures. Two of these sutures are longitudinal, and
separate the lateral parts of the region from the central.
The remaining suture joins the centre of the two longi-
tudinal sutures like the cross line of the letter H, dividing
the centre of the cephalo-thorax into an anterior and a
posterior portion, of which the anterior is the greater.
There is also an apparent suture near the frontal margin.
The frontal margin is indented, the greatest depth being
in the middle line. This indentation to some extent is due
to the scar caused by the breaking away of the filament
for attachment in the " chalimus " stage. In the centre
of the hollow, situated on the ventral surface, is an oval-
shaped opening (h) with a chitinous fringe. This is
evidently a sucker, and represents the remains of a median
sucker which is considerably developed during the " chali-

8

Mius " stag-e, when the maxillipedes are nidiinentary. Pro-
bably in the adult it acts as a first aid in securing the
animal to its host. Passing backwards from this sucker,
but distinctly over it, there is a transparent rod {c, fig. 1,
Plate I.: fig. ■], Plate III.), lying inside a triangular
blood space, which terminates in a gland (c g, fig. 3, Plate
III.). The gland is probably the organ that secretes the
substance for the filament in the " chalimus " stage and
the rod the remains of the filament. The filament and
duct are in actual contact during the early part of the
parasite's life (Plate I., figs 4 to 6, c). The eyes, two in
number, are situated in the middle of the cephalo-thorax.
The frontal and lateral margins are surrounded by a trans-
parent membrane with faint transverse lines. This
membrane is simply an extension of the chitinous exo-
skeleton which covers the whole animal. It has
frequently a serrated edge caused by tearing.

The second part of the body is very small, and repre-
sents the fourth thoracic segment of the pelagic Copepoda.
The fourth feet are attached to the external margins of
this segment.

The third part of the body known as the " genital
segment," is of variable shape, according to the degiee
of maturity of the reproductive organs. In an immature
female (Plate II., fig. 6), it is usually very little larger
than the fourth part, whilst in a mature one it is nearly
as large as the cephalo-thorax. The genital segment of a
mature female is somewhat quadrangular in outline,
slightly wider posteriorly than in front. The same
segment in a mature male (Plate I., fig. 2) is oval in shape
and about one-third wider than the fourth part.

The fourth j^art of the body is short and narrow, being
only one-fourth of the width of the female genital seg-
ment, and corresponds to the abdomen of the pelagic Cope-

poda. There is an incomplete articulation near the
middle- At the apex of the abdomen there are two short
papillae known as the fiirca or caudal stylets, which
usually have four or five ])lumose hairs on their posterior
margins.

There are twelve pairs of appendages* (Plate II., fig. 1)
as follows:— One pair of antennules, one pair of antennae,
one pair of mandibles, two pairs of maxillae, two pairs of
maxillipedes, and five pairs of feet, the first three pairs of
feet only being adapted for swimming.

The eyes appear as a reddish spot in the living animal,
and are situated on the dorsal surface mid-way between
the frontal margin and the transverse line of the cephalo-
thorax. When this spot is examined microscopically
(Plate III., fig. 13) it is found to consist of two lateral
eyes closely approximated, embedded in a mass of reddish-
black pigment, and wholly under the carapace. Each eye
has a simple, spherical, crystalline lens, beneath a thin
cornea. Behind the lens is a row of retinal cells of fairly
large size, lined internally with a tapetiim or pigment
layer. A chitin division lined with deep red pigment
separates the two eyes. The earlier Zoologists had con-
siderable doubt as to the true position of the eyes, some
even believed the animals were blind. Others
mistook the semi-circular suckers on the frontal margin
of CnUgua already referred to for the organs of vision,
giving them the name " Binoculus."

The antennules are placed at the external margin, just
l)ehind the suture on the fiontal plate, and each consists
of two joints. The basal joint is much larger than the
apical, and is clothed on its upper margin with plumose
setae.

* The minute details of the jointing and setsp of the appendages are not
shown in these figures,

10

The other appendages are all on the ventral surface.
The first are the antennse. These consist each of two
joints, a short stout basal one, and an apical one in the
form of a strong prehensile claw. The antennse are used
to assist the second maxillipedes in grasping-.* The apex
of the claw ]H'ojects into a small cup in front of the first
maxillae.

The mandibles are enclosed in the suctorial mouth
(Plate II., fig. 8). They are stylet shaped, and composed
of four joints. The apical joint of each mandible is
flattened, curved inwardly, and serrated on its inner
margin. There is no mandibular palp.

The appendages described here as the first maxillse are
given that name with some doubt. The pelagic Copepoda
have only one pair of maxillse, which correspond to the
second pair in this memoir. The identification of the
appendages now to be described as maxillae is based upon
the fact that they are innervated by a nerve from the sub-
(esophageal ganglion that has its origin just anterior to
the nerve supplying the maxillse proper. The first maxillae
consist of one joint which is considerably swollen at the
base, and tapers to a sharp point at the apex, forming as a
whole a curved claw. Two minute setae are attached to
the basal part, and probably represent the exopodite or
palp. These appendages are situated near the lateral
margins, and slightly posterior to the base of the antennae.
The second maxillae are placed at the sides of the
mouth, and consist of a single joint, robust at the base
and dividing into two slightly curved teeth at the apex,
representing the exopodite. There is a distinct endo-
podite, with two setae at its apex, attached to the base of
the anterior surface of the exopodite. The second maxillae

• Baird (op. cit. p. 263) describes these organs as the first pair of
footjaws.

11

appear to act as a scraping- apparatus for removing tlie skin
of the host.

The first maxillipedes consist of two-jointed appendages
placed mid-way between the apex of the mouth, when it
is at rest, and the lateral margin. Their chief function
is apparently to keep the mouth free from obstruction.

The second maxillipedes situated near the middle line,
mid-way between the first maxillipedes and the first pair
of feet, are composed of two joints. The basal joint is
considerably swollen and the apical is in the form of a
powerful claw, which closes upon the basal joint, forming
a strong grasping apparatus. According to Clans, and
others, the first and second maxillipedes are really only
the exopodite and endopodite of one and the same
appendage.

The first three pairs of feet consist of an endopodite
and an exopodite attached to a two-jointed protopodite.
In the first pair the endopodite is rudimentary, and is
represented bv a single minute joint bearing a few setae
at its apex. The exopodite is two-jointed. In the second
])air both the endopodite and exopodite are three-jointed.
The third pair has the protopodite well developed, form-
ing a lamella. The endopodite and exopodite are very
small, the former being composed of two joints and the
latter of three joints. Each of the first three pairs of feet
is attached to a median sternal plate. The exopodite of
the first, and the endopodite and exopodite of the second
and third pairs, are provided with a number of plumose
setae along the internal margins of the joints. The dorsal
and ventral margins of the protopodites of the second and
third pairs of feet are furnished with movable setose
plates. The sterna of the second and third ieet are
clothed with setae on the posterior margins. The fourth
pair of feet (Plate I., fig. 1) have two-jointed protopodites,

12

and exopodites also two-jointed, Init no trace of endopo-
dites. The external angles of the joints are furnished
with short spines. The fifth pair are rudimentary, are
attached to the posterior end of the genital segment, and
consist of a thin lamella, furnished with three setae along
the posterior margin. Situated on the middle line
between the second maxillipedes is a strong chitinous
plate with a bifid apex pointing posteriorly. This is
known as the sternal fork, Init its function is unknown.*

The external openings are the mouth, the vulvae, the
openings of the oviducts and vasa deferentia and the anus.
The mouth (Plate II., fig. 1 and fig. 8) is situated on the
ventral surface of the cephalo-thorax, and is placed at the
apex of a short, movable, conical tube. This tube is com-
posed of the upper and lower lips fused toijether. The
vulvae (Plate II., fig. 6) are situated on each side of the
middle line near the posterior end of the genital segment,
and communicate with the " receptacula seminis.'' They
are difficult to see in the adult female, but have each
frequently a spermatophore attached which indicates the
position. The openings of the oviducts are in the same
segment, but nearer the lateral margins and just under
the fifth feet. The openings of the vasa deferentia (Plate
II., fig. 5) ai'e situated on the postero-lateral margins of
the genital segment of the male. The anus is situated in
the middle line at the apex of the abdomen. In
addition to these more important openings, there are also
apertures of pore-canals and glands on the anterior
surface of the basal joint of the protopodites of the second
and third pairs of feet, and also on the dorsal surface of
cephalo-thorax and abdomen. The opening in some cases

• Mr. I. C. Thompson suggests that the sternal fork appears to him to
be a support or crvitch, serving to raise the body sufficiently from the host
to enable cither the s\^^mming feet or the moiith organs to be used, but
I have not seen it used in this manner,

13

is at tlie apex of a small pajjilla, and (•omiiiiiuicates with
a sac in the interioi (Plate II., tig. 11).

When resting, Lepeoyhthvivu.'^ lies upon the vential
surface, keeping- the first three pairs of feet moving- with
spasmodic jerks. When irritated, as in attempts to remove
them from their host, the males and immature females
move very rapidly over the skin of the fish. The mature
females make no attempt to escape, only clinging- more
securely. On transferring them to clean sea water they
settle on the sides and bottom of the vessel, and sometimes
adhere to the surface him of the water, remaining quiescent
for long periods. When the water is shaken slighth' they
detach themselves and swim about rapidly on their backs.
They soon tire, however, and return to rest again. Lepeo-
phtheirus makes no attempt to leave the water when kept
in small aquaria. The allied form, C ally us, on the other
hand, crawls out of the water and up the sides of the
glass, where it remains, making no attempt to return, and
soon dies owing to the evaporation of the water from
under the carapace. These parasites are very tenacious
of life, and live for a considerable time after the host has
died if they are not allowed to dry up. In some instances,
although the host had been dead over twelve hours, and
the parasites to all appearance were also dead, they
soon revived when placed in sea water. Increase of
temperature to 1()° ('. and over is fatal to them. They
can, hoAvever, stand very considerable decrease of tem-
perature. On one or two occasions during February,
1900, the small aquaria in the tank room at Piel, some
of which contained parasites under observation, were
frozen, and the temperature of the room itself stood at
— 1° C, but the parasites suffered no harm. They can also
be kept alive in sea Avater for weeks without change if the
aquaria are kept cool.

14

The colour of the living' animal varies with the position
in which it lives. On the dark side of the fish they are
of a deep brown, almost black, colour. On the " white
side and under the fins they are nearly colourless, diie to
the contraction of the pigment cells, which appear as
brown spots under the microscope. The dark coloured
forms soon become almost colourless when exposed to
light.

The Body-wall .\xd JioDY-c.wiTv.

The body-wall consists of (1) the chitinous cuticle or
exoskeleton, which has been described in the external
characters, (2) the cellular hypodermis, and (8) the con-
nective tissue laminae Avhich line the integument, traverse
the body cavity, and support the alimentary canal and
other organs. The onlv cavitv left inside the bodv-wall is
the system of lacunae, in which the colourless blood flows
(see below under blood system, p. 20).

The Alimentary Canal.

The mouth, already described, leads into a short, narrow
curved oesophagus, lined with a thin chitinous coat which
is continuous with the exoskeleton. Xear the anterior
end the chitinous coat is much folded. The oesophagus
(Plate III., figs- -3 and 5) passes through the anterior part
of the nervous system, and in a transverse section of that
region appears as a minute pinhole. After leaving the
nervous system, it courses over the sub-oesophageal gang-
lion, and under a short ctecal projection of the stomach,
finally entering the stomach on its ventral aspect, at the
posterior end of the sub-oesophageal ganglion.

The stomach lies along the ventral surface, and is
lageniform in shape (Plate II., fig. -3) At the anterior
end it is produced into a short caecum which extends over
the posterior end of the oesophagus and it terminates by

15

opening into the intestine in front of the second pair of
feet.

The intestine is the direct continiiatiou of the stomach.
Jt commences in front of the second pair of feet, and
passes through the thoracic and g-enital seg-ments into the
abdomen. It M'idens slightly behind its junction with
the stomach, and then contracts as it passes through the
fourth thoracic segment. It expands again in the genital
segment, and contracts as it enters the abdomen. It
terminates in a short rectum leading into the anus at the
apex of the abdomen. There are no convolutions in this
alimentary canal.

The intestine at its anterior end lies on the ventral
surface of the animal. In the centre where it passes
through the genital segment, it courses along the dorsal
surface. It bends down as it a})proaches the abdomen,
and occupies the centre of that part of the body. In
transverse sections of a mature female the stomach is
triangulai' in shape, with the apex pointing dorsally. The
intestine in the genital segment is also triangular in
transverse section, but the apex is directed ventrally. In
immature females the stomach and intestine are of almost
circular outline when cut transversely, so that the
alimentary canal is considerably compressed when the
reproductive organs arrive at maturity.

The wall of the whole alimentary canal is lined with a
thin layer of cliitin continuous with the exterior. In
many places it is consideiably broken up, giving it the
appearance of fine striation. Vnderneath the chitin is a
layer of nucleated cells, which extends from the jjosterior
portion of the oesophagus to the rectum. There does not
appear to be any marked regional differentiation in the
cells. The lining of the stomach and intestine is thrown
into a number of longitudinal folds (Plate III.,lig. 11), the

16

height of which varies considerably. In the anterior por-
tion of the stomach these folds are very little higher than
the general line, but as they pass posteriorly they increase
considerably, diminishing again in the intestine as they
approach the rectum. The greatest height of the folds
is reached in the portion of the intestine passing through
the genital segment. In the intestine and posterior por-
tion of the stomach there are a number of glandular cells,
usually at the apices of the longitudinal folds, the contents
of which stain deeply with eosin. In many of these the
cell contents have disappeared, leaving a clear space,
only the cell wall remaining.

The wall of the stomach and intestine is marked by a
series of transverse constrictions, giving it a crenate
appearance, which is easily seen in the living animal. In
the living animal an intermittent movement of the intes-
tine and stomach is kept up. I'he action is wave-like,
starting at one end, and passing to the other. After con-
tinuing in one direction for a time, it reverses and passes
the opposite way. There is no Aalve between the stomach
and intestine, and when the peristaltic motion is reversed
the fluid in the intestine is sent back into the stomach
again. The only portions of the alimentary canal that
can be closed are the oesophagus and anal end of the
rectum. The former is controlled by two longitudinal
muscles which compress it, the latter l)y a number of
muscles passing obliquely to the body-wall at the sides of
the abdomen. The fluids contained in the alimentary
canal are usually colourless, but occasionally when taken
direct from the flsh and placed under the microscope, a
reddish tint may be detected at the posterior end of the
(esophagus.

In connection with the alimentary canal there is a
distinct paired digestive gland (Plate II., tig. -J and fig. 9).

17

It cousi.sts (tf three portions, two moderately large
masses on the lateral margins of the cephalo-thorax,
just behind the antennules, and a median, smaller one, in
front of the base of the month. The lateral portions are
connected with the median bv a dnct. The median por-
tion g-ives oif a duct, which passes posteriorly along- the
(esophagus and enters the caecum, at the anterior end of
the stomach. When the parasite is first removed from the
fish the digestive gland is usualh* of a dark brown colour,
but after starving for a few weeks it becomes colourless.
The product of the gland is a pale, yellow iiuid, which
can be seen as it passes along the duct between the lateral
and median portions.

Situated between the first and second pairs of thoracic
feet is a pair of glands visible in the living animal as
blown spots. A minute duct passes downward and then
forward along the stomach. The duct appears to enter
the stomach near the posterior end.

The food of this parasite is said to be mucus, and blood
has not been detected in the stomach."^' This fact gives
."onie cause for the opinion advanced by many Zoologists
that Lepeophtheirus and other allied genera are therefore
not parasites in the strict sense of the term, and may not
be hurtful to their hosts. There is considerable dithculty
in settling the question of their true food. tSpecimens
taken direct from the living fish and placed under the
microscope, rarely show even the faintest trace of red
colouring matter in the alimentary canal. The difference in
structure between the Ualigidae and the obviously blood-
sucking Lernese is very great. This will be pointed out
in the section dealing with Lerncea hranchialis, and may
account for the apparent absence of blood. Mucus at the
best is a poor food, but Lepeophtheirus can live for upwards

* They do not hesitate, however, to eat their comrades when these
become feeble.

18

of six weeks in filtered sea water without visible food of
any kind.

From the large niiiubers of tlouuders examined in the
Piel laboratory, partly in connection with this memoii-,
but chiefly in connection with fisheries work, during the
past year or two, the conclusion has been arrived at, that
Lepeophtheirus jjerforal/s to some extent feeds on blood,
and may be hurtful to the fish, especially when present
in numbers (see figure on p. 6).

The appendages are more suited for a sedentary life
than even a semi-pelagic one. The animals can only
remain swimming for short periods, and their presence in
tow-nettings, therefore, is accidental- They do not, under
normal conditions, and as long as the fish remains in a
healthy state, leave their host. In the fish tanks at Piel
over 150 fiouiiders, all more or less infested M-ith
LepeopJitheirus, are kept during the spawning season.
The waste water from the tanks is carefully filtered for
periods of at least three months in the spring, to collect
the eggs shed by the fish. Yet not even one specimen of
the parasite has been found in the filter. When the
fish are examined and the jiarasites removed, no matter
how carefully, the skin, especially where there are a
number close together, is usually lacerated and bleeding.
The males and immature females on the general surface
of the body do not seem to remain long enough in one
place to cause obvious injuries. Under the fins, however,
and on the fins themselves, where the egg-bearing females
are usually found, and where they lie for weeks in the
same position if not di.sturbed, is the part of the fish
chiefly injured. The pectoral fin in some instances may
be partially destroyed, and pieces of the tissues are fre-
quently found enclosed in the second maxillipedes of the
parasite.

19

The anteniuf and claws of the second niaxillipedes are
])lunged into the tissues of the fish along with the teeth
of the maxilla, lacerating" the skin, and into this wound
the suctorial mouth is directed. The blade-like mandibles
assist in collecting the particles of food niatei'ial. These
are sucked up, pass down the oesophagus into the stomach,
where they are at once acted on by the fluid from the
digestive glands, and the colour of any blood present may
then be discharged. It is usually at the junction of the
(jesophagus with the stomach that any red coloured
particles occui-. The food can then be traced along the
stomach and intestine, and the waste matter is expelled
from the anus in long strings.

On comparing transverse sections of the alimentary
canal of LepeopIdJieirui^ and of Lerncea which happen to
contain food, and have been stained in eosin and hsema-
toxylin, there is seen to be a marked similarity in the
nature of the food in the two cases. Both are finely
granular, and stain red with eosin. Mucus from the
flounder has no such granular appearance.

It is stated by some Zoologists that copepod parasites
are generally found most abundantly on weak and
diseased fishes. It is not so with Lepeopidheirus
pectoralis. Flounders with many parasites in our tanks
were in as good condition as those that had none. They
were never found on flounders which were thin and in
poor condition, as they detach themselves and swim away
when the fish becomes feeble. This was proved by actual
experiments and observations at the Piel Hatchery.

The Bloo]) and Circitlation.

There is no heart in Lepeophtheirus, nor are there any
proper blood vessels.

20

The circulation is wholly lacunar, and simply cousisis
of hioad, ineg-ular streams passing througli the spaces
left among' the internal organs, and between the connec-
tive-tissue l)ands of the body-wall. These streams have iu
general certain definite directions, but they are not
uniform, continuous currents. The fluid advances by
successive jerks, depending" upon the movements of the
alimentary canal and, in part, of the rejirodiictive system.
The blood is a clear fluid, containing numerous colourless
corpuscles. The corpuscles vaiy in size and shape, and
can accommodate themselves in diameter to the spaces
through which fhey jiass.

Plate II., fig. 2, shows the course of the main blood
currents. Starting from behind the eye, there are two
currents passing ^Josferiorly, one flowing to each postero-
lateral angle of the cephalo-thorax, where it turns and
courses forward along the lateral margin of the carapace
till it reaches the group of muscles connected with the
mandibles. It then divides, one portion continuing along
the margin to the base of the antennules, where it splits
up into minute currents, all converging to the base of the
mouth, while the other branch of this cephalo-thoracic
current passes along the muscles of the mandibles and
duct of the digestive gland, and meets the currents of the
former branch at the base of the mouth.

A second main current courses posteriorly through the
cephalo-thorax and the fourth thoracic segment, into the
genital segment. It flows there along the reproductive
organs in a broad stream, and turns round at the end of
the segment. The currents from both sides meet in the
middle line, and flow forward under the alimentary canal.
In the region of the second maxillipede, this median
ventral current breaks up into a complicated series of
smaller currents, some of which pass into the two currents

21

flowing posteriorly, and the others into the currents
passing to the base of the month.

The main ouirents are easily seen by plaeing the living-
animal on its back, in a drop of sea water on a slide then
covering with a thin cover glass and examining with a
}in. objective.

The blood currents described above do not continue to
flow for any length of time in the one direction. At one
period they may l)e flowing as indicated by the arrows in
Plate II., fig. 2. Then they suddenly slacken and reverse,
and stream for a time in exactly the opposite course.
Sometimes the blood corpuscles are seen to simply oscillate
backwards and forwards, making no advance, but at other
times they pass rapidly along in a definite manner.

There are no independent organs of respiration. It has
been suggested by Hartog and others that the blood is
[•robablv aerated from the sea water contained in the thin-
walled alimentary canal by the method of " anal respira-
tion,'' which has been described in Ci/do]).-;, Caligiix,
Argidna, Daphnia, Cypris and other lower Crustacea.

The cuticular exoskeleton over the surface of the body
i^ in most places so thick that the respiratory change of
gases may be supposed to take place much more readily
through the very thin layer of chitin which lines the
rectum. There are dilator muscles attached to the wall
close to the anus, and the peristaltic movements oi the
whole alimentary canal may aid in the production of
inhalent and exhalent currents of water. It appears,
however, to the present author that further precise obser-
vations are required to substantiate this hypothesis.

No organ corresponding to the " shell gland '' described
in various lower criistacea, and shown by Claus, Hartog
■,ind others to be a renal organ, has been found,

22

The Mfsculab System.

The miisoles moving- the appendag'es and seg^nients of
ihe liody can be distinctly seen and traced to their
extremities through the transparent exoskeleton (Plate II.,
fig. 1 and fig. 2).

The frontal portion of the cephalo-thorax is controlled
by two short slender muscles, iiiJf.. (Plate 11. , fig. 2) passmg
postero-laterally from near the lateral edge of the cara-
pace. They act in depressing the margin so as to produce
a close attachment to the host. The posterior region of
the cephalo-thorax is supplied with a number of pairs of
muscles, some passing forward and others laterally, which
contract and expand that part of the body. The lateral
margins are controlled by long muscles passing obliquely
outwards from the anterior end of the lateral suture.
The muscles of the fourth thoracic and genital segments
arise near the median line of the posterior portion of the
cephalo-thorax, and pass backwards. They produce a
lateral motion of the posterior parts of the body, and also
a sort of telescoping contraction Avhich draws the genital
segment into the cephalo-thorax. The muscles of the
abdomen arise near the middle of the genital segment
and pass backwards. They produce a telescoping move-
ment of the abdomen.

The various appendages and other organs are also well
supplied with muscles. The antennules have each a pair,
which elevate and depress the joints. The grasping-
action of the antennse is produced by muscles passing
obliquely to the lateral margins. The movements of the
mouth are controlled by a complicated series of muscles
passing anteriorly, posteriorly and laterally, all of whicli
assist in elevating and depressing it when sucking up
food. The mandibles are provided with mviscles of extra-

28

ordinaiT leng'th and power, which pass obliquely back-
ward to the lateral margins of the carapace nearly opposite
the first pair of feet. The muscles of the first maxillse
are very short and thin. They pass along the posterior
surface of the muscles of the antenna, to the lateral
margins. The second maxilhie are controlled by powerful
muscles passing to the lateral margins. The muscles of
the first maxillipedes pass obliquely forward to the lateral
margins. The second maxillipedes are supplied with short
and powerful muscles which pass forward under the
second maxillfc. The terminal claw is provided with
muscles of great strength. The first three pairs of feet
are supplied with a complicated series of muscles passing
dorsally amongst those controlling the posterior portion
of the cephalo-thorax. The fourth pair of feet are
apparently little used, and consequently are only supplied
with feeble muscles. The alimentary canal is controlled
by longitudinal muscles, and also by muscles passing
transversely, which produce the wave-like peristaltic
motions and crenated appearance. The anus is opened
and shut by muscles passing obliquely, which open and
shut each side alternately or simultaneously according to
the requirements of the animal. The reproductive organs
are also controlled by muscles, which give rise to pulsating
movements, and assist in expelling the ova and
spermatophores.

The Neevous System.

The central nervous system in Lepeoj>htheirus consists
of a cerebral or supra-oesophageal ganglion and a large
sub-oesophageal ganglion placed on the ventral surface,
in the median line, and extending from slightly in front
of the second pair of maxillae to near the articulation of
the second pair of maxillipedes with the bodv. The

24

g-anglia aie connected hv hrnad commissures passing- on
each side of the (psophagus, leaving only a narrow opening
for its passage. The sub-fjesophageal ganglion projects
slightly forward under the supra-oesophageal, giving it
the appearance of being separated from it, when viewed
from the ventral aspect (Plate III., fig. 2). These are
the only ganglia, and they supply the various parts of the
body with nerves.

The supra-oesophageal ganglion is about half the size
of the sub-(esopliageal. It is produced on its dorsal
surface into an optic lobe (Plate III., fig. 5), from which
arises a distinct pair of optic nerves. Horizontal sections
of the optic lobe show that the roots of these nerves cross
each other (Plate III., fig. 12). Each optic nerve, there-
fore, is supplied by fibres from both sides of the brain.

The nerves supplying the antennules arise from neai-
the anterior angles of the ganglion. They pass obliquely
forward to the base of the antennules, and there sub-
divide into a number of branches which pass to the sets?
clothing the anterior surface of the basal joint and apex
oi the second (Plate III., fig. 4). From the manner in
vrhich the antennules are supplied by this nerve it is
evident that they are important sensoiy organs (Plate III.,
fig 4).

The antennae are supplied by nerves arising from the
anterior angles of the ganglion, which pass anteriorly
\inder the nerves of the antennules and enter the base
of the antennse. These are the only appendages supplied
from the supra-(esophageal ganglion.

The sub-opsophageal ganglion is heart-shaped, and fully
twice the size of the supra-cesophageal. It represents the
whole of the thoracic and abdominal ganglia of the higher
Crustacea, and supplies the remainder of the appendages.

25

The nerves passing to the mandibles have their origin
on the anterior margin near the middle line. They
course along the muscles of the fpsophagus, and reach the
mandibles near the base of the mouth.

The next pair of nerves arise at the anterior angle of
the ganglion, course forward, under the nerves of the
antennae and antennules, to the frontal plate which they
enter about midway between the lateral margin and
middle line. They then turn abruptly and pass out to
the lateral margins of the frontal plate, just above the
antennules. The margin at this point is destitute of the
transparent membrane which surrounds the carapace.
The nerves terminate in a shallow cup, evidently a
sensory organ.

Three other pairs of nerves arise from the anterior
angles of this ventral ganglion. The first passes to the
rudimentary first pair of maxillae, the second, a short
nerve, passes to the second pair of maxillae, and the third
to the muscles controlling the lateral margins of the
cephalo-thorax.

The nerves supplying the first pair of maxillipedes arise
from the anterior portion of the lateral margin. They
are large nerves at their origin, but immediately divide
into four branches, passing to the maxillipedes and
muscles. The second pair of maxillipedes are also supplied
by nerves arising from the lateral margins. Like those
of the first maxillipedes they have strong roots, and at
once divide into three branches Avhich pass to the second
maxillipedes and their muscles.

The remaining nerves have their roots in the posterior
end of the ganglion. There are three pairs. These
supply the five pairs of feet and the abdomen. The outer
pair of nerves supply the first pair of feet. Near the
origin a branch is given off which passes to the muscles

26

of the stomach. The next pair .supply the second pair of
feet. TheA- course along the median nerves as far as the
sternal fork and then diverge. Just under the sternal
fork a branch is given off which appears to pass to the
muscles of the posterior region of the cephalo-thorax.

The median pair course close together, and unless
carefully examined are easily mistaken for a single nerve.
There is a distinct division, however, which is apparent
even in the roots. Between the second and third pairs
of feet a strong branch is given off which passes to the
third pair of feet. The nerves then diverge, and just
before entering the fourth thoracic segment give oft' a
branch that passes to the fourth feet. The main trunks
course on through the genital segment, still further
diverging. Shortly after entering the broad part of this
segment a third branch is given off which takes a semi-
oval course along the ventral surface of each half of the
segment, finally passing to the setse of the fifth feet. On
entering the abdomen the main trunks split into two
branches, one passing to the anus and the other to the
setae on the apex of the caudal stylets (Plate III., fig. 2).

Each nerve, after leaving the main trunk, sends out
numerous branches Avhich pass to the various muscles
controlling the appendages innervated by that nerve.
Excepting the nerve passing to the fifth feet, the branches
are not shown in the figure (Plate III., fig. 1). There
is considerable difficulty in tracing the endings of the
branches when they pass amongst the muscles.

The chief sense organs connected Avith the nervous
system are the conspicuous e^^es which are described above
(p. 71). There are also the numerous setse scattered over
the surface of the body and appendages, which are possibl}'
tactile in function. Probably the setae upon the anten-
nules, which are richly supplied with nerves from the

27

supra-cesopliageal ganglion, have a special function, which
may be olfactory.

The Repkoductive Organs.

The reproductive organs are paired, and as already
stated, the sexes are separate.

In the female (Plate II., fig. 4) the ovaries are large
kidney-shaped organs lying on each side of the anterior
portion of the stomach and extending from under the first
pair of feet to the base of the second maxillse, when
fully matured. Each oviduct {od .) arises near the anterior
end of the ventral surface of the ovary, and courses .
j)osteriorly as a narrow tube till it enters the genital
segment. It then expands rapidly, and passes to near the
end of the segment. It then reverses its course, passing
forward to the central portion of the segment, where it
turns again in a ^losterior direction, and passing out to the
centre of each half of the segment, it opens to the exterior
just under the fifth feet. Each oviduct thus forms two
loops in the genital segment. On the ventral aspect of
the loops of each oviduct there is a short, semi-transparent
cjdindrical tube [sg.) with the anterior end closed and
rounded, and the posterior produced into a fine duct,
which communicates with the oviduct near its extremity.
This organ is evidently a cement gland for secreting the
enclosing membrane of the ovisac. Each vulva (fig. 6, vu.)
is situated near the middle line behind the junction of the
genital segment with the abdomen. It appears to consist
of a simple opening leading into the vagina which expands
into a " receptaculum seminis." This is an elongated sac
passing from the median line to the oviduct, which it
enters alongside the duct of the cement gland.

In the male the reproductive organs (Plate II., fig. 5)
consist of a pyriform testis, on each side, situated in a
position corresponding with that of the ovary. It is only

28

oboul one-fourth the size of the ovary. Each vas deferens
courses posteriorly into the oral g-enital segment. It coni-
niunicates with the sac of the sperniatophore on the external
margin near the posterior end. A short cement gland
furnishes a duct which passes in at the anterior end of
the sac. The sperniatophore, an oval body containing
the spermatozoa, is expelled from an opening near the
posterior angle of the segment.

In Lepeophtheirus the fertilisation of the female is
accomplished soon after the " clialimus '' stage is com-
pleted. The genital segment is then very small, about
one-fifth the length of that of a mature female. It is
grasped by the male on the dorsal aspect. The antennae
close round the junction of the genital segment with the
fourth thoracic, and the second maxillipedes seize tlie
segment immediately in front of its junction with the
abdomen. The animals remain in this condition for some
time, and can only be separated with difficulty. The
spermatophores are discharged in pairs. When they are
ready for discharging the male folds the whole of the
posterior portion of its body along the ventral surface of
the female. The openings of the sperniatophore sacs are
thus brought in contact with the vulvae. The spermato-
phores are then discharged, and being in a viscid condi-
tion, at once stick to the female. One end of the cover-
ing, probably the last part that leaves the opening, is
drawn out into a fine thread, which helps to secure the
spermatophore. The spermatophores are not, apparently,
always fortunate in reaching the vulvte. It is by no
means uncommon to find thein planted amongst the
appendages in little clusters like grapes. These have
been mistaken by some of the earlier Zoologists for the
eggs, when the true egg sacs were considered to be
antennules.

29

One copulation apparent!}' fertilises all the eggs pro-
duced by tlie female. It is obvious, Avhen one compares
tLe male with a mature female (Plate I., figs. 1 and 2),
that fertilisation cannot be accomplished Avhen the female
genital segment is fully developed. Hence the need of
it being effected at an early stage.

The exact period at which the eggs are fertilised by the
spermatozoa is unknown. The spermatophores may be
found attached to the body for some time after the female
has begun to produce eggs (Plate II., fig. 4, sp.), but they
are then simply empty sacs. Plate II., fig. 7, shows a
pair of spermatophores that have been detached from an
egg-bearing female. The little opening at d. was in
direct communication with the vulva. These sacs were
empty. In an immature female (Plate II., fig. G), the
vulva leads into a short vagina, passing directly into the
oviduct. The spermatozoa probably remain in the vagina
which becomes a " receptaculum seminis." In transverse
or longitudinal sections through the region of the vagina
of a mature female masses of spermatozoa are frequently
found in the swollen part (Plate II., fig. 4, rep.). The
oviduct in the immature female has no communication
with the exterior except through the vulva.

The ovary of a mature female appears as shown in
Plate II., fig. 10. It consists of a number of tubules
lined with nucleated cells representing a germinal epithe-
lium, which will form the eggs. The interior of the
tubules is filled with a granular substance, staining faintly
blue with hsematoxylin and eosin. When the eggs become
mature the walls of the tubule break down and the eggs
pass out into the oviduct. They are then very small, about
•02 mm. in diameter, and do not fill up the duct. They
are simply nucleated cells. As they pass posteriorly they
increase in size. In the fourth thoracic segment they

BO

measure "06 mm., and appear as oval bodies with a tlii;i
vitelline membrane. The cell contents are finely
granular. The nucleus is a large oval body, with a sharp
outline. A single rounded nucleolus is also present.
After passing into the genital segment the cell contents
increase in amount, causing a great enlargement of the
egg, which finally passes out at the opening between the
vulva and the lateral margin of the segment, already
described. As the eggs pass out they are probably
fertilised by the spermatozoa from the " receptaculum
seminis." They are then enclosed in a thin chitinous
tube, secreted by the cement gland, which gradualh^
extends as more eggs are expelled. The ovisacs are often
longer than the animal. The eggs in this tube are
biscuit-shaped, measuring "-SG mm. in diameter and
"11 mm. in thickness. They are arranged in a single
column. When the animal is irritated the tubes are
frequently detached. When the embryos hatch, the
empty, ruptured tube is left, and remains attached to the
animal for a time. After examining many specimens,
the conclusion has been come to that additional eggs are
not developed in the tubules of the ovary after the first
lot have been expelled. Adult females in which the
ovary is only an empty sac are not uncommon.

Life History.

Lepeophtlieirns has no regular breeding season. Mature
females Avith ovisacs may be found at all times. The
state of development reached by the embryos carried by
various females collected at the same time is frequently
widely different. In some the germinal disc has just
begun to segment, in others the larvae are ready to hatch.

The changes that take place in the developing embryo
have not been worked out by the author. The period of

Bl

incubation was found to extend over several weeks at
least. In one case the ovisacs were kept for six weeks,
and in another eight weeks, before the embryos hatched.
The incubation takes long-er than that, however. In
both cases the embryos were pigmented when placed
under observation. The first appendages that make their
appearance are the antennules, antennae and mandibles.
They are in a rudimentary state, and the embrj^o is now
ready to hatch. During this period the embryo increases
in size as it develops.

The whole of the embryos contained in the tube hatch
practically at once. The enclosing membrane ruptures,
then the membrane of the tube splits, and the nauplii
after freeing themselves from the fragments swim to the
surface. Plate I., fig. 3, represents a newly hatched
nauplius, the natural size of which is '46 mm. It leads
a pelagic life for a time, and grows by successive moult-
ings. It next settles down on some fish, and passes into
a cyclopoid state (Plate I., fig. 5). The young parasite
immediately develops a thin chitinous filament from the
median frontal gland already described, which passes into
the tissues of the host, and it becomes fixed. The median
sucker (&., Plate I., fig. 5), with the help of the rudi-
mentary antennae and second maxillipedes, enables the
animal to bring its mouth into contact with the host.

If young plaice, flounder, cod, &c., one to three inches
in length, be examined very carefully at the end of the
summer, it is practically certain that some recently
attached Lepeophtheirus or Caligtts will be found either
on the fins or some other part of the integument. On
examining fins which have parasites attached, the filament
is seen passing through the skin, under it, and along one
of the fin rays, as shown on Plate I., fig. 5 (natural size
•77 mm.). The filament may have the end bluntly

32

pointed or flattened into a disc (Plate I., tig. 4). This is
the " chalimns " stage referred to on previous pages, so
called hecanse liurmeister, in 18-'-)l,* described it as a new
ceniis under the name " C'lialinins.'" This was afterwards
shown l)y Hesse and others to be only a young stage of the
Caligid?e. The young parasite continues to grow by suc-
cessive moultings, and the various appendages make their
appearance in regular order. The duration of this
attached stage has not been determined. When the
appendages are fully developed, as in Plate I., fig. 6, the
filament separates at its junction with the frontal margin
leaving a notch, the remains of which persist all thi'ongh
the adult life.

The male, at the conclusion of the attached stage, is
practically fully developed. The female remains in an
immature condition until fertilisation is effected and the
ova begin to pass down the oviducts. The genital seg-
ment then increases in size from that shown on Plate II.,
fig. 6, to the mature condition of Plate I., fig. 1.

* Nov. Act. Acad. Natur. Cur. Bonn., vol. xvii., p. 294,

33

II.— LEEN^A.

The Lern.eice, although not so extensive a family in
numbers of genera and species as the Caligid.e, are more
interesting to the specialist. They present some of the
most remarkable instances of retrograde development that
are to be found in the whole group of parasitic Copepoda.
There is great excuse for the difficulty experienced by the
earlier Zoologists in giving certain members of this family
their true place in the animal kingdom. The fact that
these animals were placed first in one grouj) and then in
another by successive workers is not surprising, consider-
ing- that nothing was then known about their life history.
It requires some study even at the present day to show
that Lerncea is a Crustacean, still more to demonstrate
that it is related to Lepenphtheirus.

The genus Lerncea as it now stands contains only five
species. Formerly it was ver^^ extensive, and included
many forms, such as Lepeophtheirus j^^f^toi'alis, that had
not the least apparent resemblance to each other in the
adult state. Careful research, along with a better know-
ledge of the minute striicture, gradually eliminated the
unlike species, which were removed to other genera. An
excellent historical account of our knowledge of the group
will be found in Baird's " Entomostraca."

The species described here is Lernaa branchialis, Linn.

Mode of Occurrence.

The adult female is found on the gills of the Gadidse,
such as cod, haddock and whiting. Immature (cyclops
stage) males, and females with adult males attached, are
found on the apex of the gill filaments of the flounder,
sometimes in large numbers. Full-grown females are
not plentiful on the fishes caught in the vicinity of Piel.

34

Tm'o to four specimens have been found after examining
numerous catches of young cod of one dozen each. The
ratio thus varies from one in three fish to one in six. In
one or two instances two and sometimes three specimens
were found on young cod eight inches long. The length
of a full grown female Lermea is a little over one inch.

The adult female is securely fastened to its host by
strong branched horns, three in number, which are buried
in the tissues of various parts of the gill arches- In
many instances the head was found to have actually
penetrated the ventral aorta. To obtain the specimens
in an entire condition the tissues of the host have to be
carefully dissected. Attempts to remove them by force
always result in the head being left in the fish. The
parasite, when once fixed, remains in the same position
throughout life. When it dies the softer parts decay, but
the head continues for a long time embedded in the tissues
of the host, and is often met with there when dissecting
out living specimens.

External Characters.

The adult female (Plate lY., fig. 1) is cylindrical. It
is unsegmented, but roughly divided into three parts — a
globular head with anchor-like processes, connected by a
narrow neck to a much swollen posterior part.

The globular head corresponds to part of the cephalo-
thorax in Lepeophtheirus. It is furnished with three
more or less branched horns, two lateral and one median
and dorsal. The head is slightly curved downwards, ter-
minating in a conical apex.

The anterior portion of the neck represents the
remainder of the cephalo-thorax and the fourth thoracic
segment. The whole of the neck is marked by fine trans-
verse lines.

35

The remainder of the neck and the g'reater part of the
swollen mass behind corresponds to the genital segment.
The abdomen is represented by the terminal portion of
the swollen part, and gradually tapers to a blunt end.
The swollen region of the genital segment is abruptly
bent into the form of the letter S (Plate TV., fig. 1).

The appendages are rudimentary, the greater number
being entirely absent. Those present are the first pair
of maxillipedes placed at the apex of the head, immediately
under the mouth, and four pairs of swimming feet at the
anterior end of the narrow neck. The swimming feet are
exactly as they exist in the cyclops stage both in size and
structure. The protopodite is two-jointed, the exopodite
of the four pairs is two-jointed. The endopodite of the
first two pairs is also two-jointed. The third and fourth
pairs of feet have no endopodite.

The external openings are, the mouth placed at the
apex of the head, the openings of the oviducts situated
on the ventral aspect of the S-shaped region, and the anus
at the blunt apex of the abdomen (Plate IV., fig. 1, an.).

The colour of the living animal is dark red, due to the
contained blood. When removed from the fish and placed
in sea water the colour changes to white. Lerna'a does
not live long after being taken from the fish. The
longest period observed was about twelve hours. They
are simply inert sacs quite incapable of movement.
Occasionally the parasites are covered with colonies of
hydroids which sometimes entirely obscure them. The
exoskeleton consists of a chitinous cuticle moderately thin
and soft in the region of the swollen part, but thick and
hard on the neck and head.

Immature Lerncea hrcmchialis living on the apex of the
gill filaments of the flounder (Plate IV., figs. 3, 4, and 5)
are cyclopoid in appearance. The animal is oval in trans-

36

verse section. It is composed of five distinct parts — an
oval cephalo-tliorax, three tlioracic, and one terminal
segment, representing" the genital segment and abdomen.
The anterior portion of the genital segment in the female
is indistinctly divided into eleven joints.

The cephalo-thorax attains its greatest width just
behind the eyes ; beyond that point the sides converge
until they reach the first thoracic segment. The cephalo-
thorax is produced anteriorly into a broad blunt rostrum.
In the very early Cyclops stage (Plate lY., fig. 3), the
rostrum is further produced into a short triangular
filament which secures the parasite to its host.
The eyes (Plate V., fig. '3) are situated on the dorsal
surface a short distance behind the rostrum. In the
living animal they appear as a dark red spot with a
crystalline lens projecting slightly at each side. When
examined microscopically the structure is found to be the
same as that described in Lepeophtheiriis. A thin cornea
encloses a spherical crystalline lens. Behind the lens a
row of fairly large retinal cells is lined internally with a
tapetum layer. A chitinous septum lined with deep red
pigment separates the two eyes. The appendages attached
to the cephalo-thorax are as follows : —

The antennules are placed at the posterior angles of the
lateral margins of the rostrum. They are short, and are
composed of four nearly equal joints furnished with fine
setse.

The antennse are composed of two joints. The apical
joint is provided with a strong claw on its external angle.
The antennse usually project beyond the rostrum, and it
is by means of these that the attachment to the host is
maintained when the filament is broken oft.

The mandibles are not enclosed in the suctorial mouth.
Thev are situated at the base of the lateral surfaces of the

37

conical tiil)e of the iiioutli, and consist of two parts. The
basal joint is cylindrical. The second joint is flattened,
and terminates in a hroad blade, which is serrated on the
inner margin.

The single pair of maxillae are placed at the base of the
mandibles. They consist of two lobes, cnie of which is
very small. The larger lobe has two moderately lons"
setse at its apex, the smaller one has one seta.

The first pair of maxillipedes are placed immediately
behind the month. They consist of four joints, the last
joint being furnished with a strong claw. The basal
joint has two short hooks near its apex.

The second pair of maxillipedes in the female are rudi-
mentary, and are represented by a minute knob. In the
male they are com])osed of two joints, the apical one being
in the form of a powerful claw. It is by the aid of the
second niaxillijiedes that the male grasps the female
during copulation.

The first pair of swimming feet consist of a two-jointed
protopodite, an endopodite and an exopodite, both two-
jointed. The apical joints of the endopodite and the
exopodite are furnished with long plumose sette on their
inner margin and apex. This pair of feet is attached to
the posterior end of the cephalo-thorax.

I'he second ])air of swimming feet is attached to the
fir.st free thoracic segment, the third pair to the second
free thojacic, and the fourth pair to the third. These
fiee segments really represent the second, third and
fourth thoracic segments, the first being a part of the
cephalo-thorax. The second pair of swimming feet in
eyery respect resemble the first pair. The protopodites
of the third and fourth pairs are two-jointed ; the endo-
podites in both pairs are absent. The exopodites are
similar to those of the first and second pairs. The fifth

38

pair of feet is )epre.seiited by minute papillae. The caudal
stylets are veiv short, and furnished with three or four
short plumose setse at the apex.

The external openings are the mouth, the YulviP, the
opening's of the vasa deferentia and the anus.

The mouth is situated on the ventral surface of the
cephalo-thorax, at the apex of a conical tube, composed
of the upper and lower lips fused together. In the very
early cvclops stage the lips are not fused. The vulvse and
openings of the vasa deferentia are placed at the posterior
angles of the genital segment. The anus is at the apex of
the abdomen, in the middle line. The vulviB open into
the receptacula seminis, which are in direct coiumuni-
cation with the oviducts.

The whole of the genital segment and abdomen in the
female is marked by fine transverse lines. The colour,
which is arranged in patches, varies from dark violet to
light red.

Alimextary C.\xal.

In the adult the mouth opens into the intestine, which
probably acts as the stomach, the CESophagus and true
stomach having disappeared in the metamorphosis of the
cephalo-thorax. The intestine is at first narrow Avhere
it passes through the neck, then it widens considerably in
the swollen part of the genital segment, contracting
slightly in the abdomen, and finally terminates in a short,
narrow rectum leading to the anus (Plate Y., fig. 4).
The intestine is lined with a single layer of nucleated
cells. Attached to this layer, and in some cases embedded
ill it at irregular intervals, are large cells filled with fine
granular material. In some parts these large cells are
grouped together two and three rows high. In other
l)arts they are quite free (Plate Y., figs. 5 and G). The
layer supporting the nucleated cells appears to be com-

39

posed of fine muscle fibres. There is no cliitiuous inner
lining as in Le peoi)htheirus . Between the basement layer
of the intestinal wall and the integnment there is a net-
work of muscles passing- in various directions. This
tissue represents the body-cavity and body-wall. The
spaces between the muscles are filled with the red blood.
The peristaltic movement of the intestine is similar to that
observed in Lejoeophtheims.

In the Cyclops stage the mouth leads into a short,
narrow oesophagus {ce, Plate V., fig. 2), which 2)asses into
the comparatively Avide stomach on its ventral aspect. The
stomach is lageniform, with the narrow end pointing
posteriorly. On the dorsal aspect, at the anterior end, it
is produced into a short, blunt csecum. The narrow end
of the stomach connects with the intestine, a long
straight narrow tube, greatly compressed over the region
of the receptaculum seminis. The intestine terminates
ill a very short rectum leading to the anus. The cells
both free and attached along the wall of the stomach and
intestine are similar to those in the adult. Sometimes
the stomach is filled with free cells, which are kept con-
stantly travelling backward and forward by the move-
ment of the intestine. At other times few free cells can
be seen. JS^o trace of blood between the alimentary canal
and the integument, as found in the adult, has been
observed in the young.

No trace of a digestive gland could be found in the
adult. In the young it is probably represented by a
series of groups of cells running along the lateral margins
of the cephalo-thorax (Plate V., tigs. 1 and 3, I .v.). A short
duct could be traced leading from these groups into the
stomach, just posterior to its junction with the oesophag-us.

When the alimentary canal of a living parasite is
opened, and the free cells are isolated and examined with

40

a liigli power, they are found to be subspherical, gramilar,
and of various shades of greenish yellow colour. Some
of the cells exhibit faint amoeboid movement. It is pro-
bable, therefore, that the digestion is intracellular.

The food of these parasites is undoubtedly blood which
we find in the alimentary canal, but whether the absence
of digestive glands in the adult accounts for its unchanged
appearance has not been ascertained. In the young,
where there is an apparent digestive gland, the contents
of the alimentary canal are not red.

Circulation and Kespiratioiv.

There is no heart or vascular system, and in the adult
no movement of fluids could be observed which would
indicate a blood circulation. The animal is probably
dependent upon the blood sucked from its host for the
supply of oxygen necessary to maintain life. It is there-
fore possible that the early death after removal from the
fish is due largely to the inability to take up oxygen from
the water. The blood ciiculation could not be satisfac-
torily traced in the cyclops stage.

The Muscular System.

The muscular system in the cyclops stage, although not
so highly developed, is practically similar to that of
Lepeo'phtheirus. In the adult female it is simply a net-
work between the integument and the alimentary canal
forming a supporting medium for the latter.

The Nervous System.

In the cyclops stage the central nervous system is the
same as in the adult Leijeophtlieirus. The nerves supply-
ing the various appendages have also the same origin and
direction as described in that type. The nerves marked
4a, 4b, and 5a in Plate III., fig. 2, could not be traced.

41

The nerve supplying the antennules has a similar branch-
ing at its termination to that of Lepeophtheirus.

In the adult Lerniea no trace of a nervous system could
be made out, and certainly if present at all it is very much
reduced.

The Reproductive Organs.

The reproductive organs of Lemma, like those of
Lepeophtheirus, are bilaterally symmetrical. In the
Cyclops stage of the female (Plate V., fig. 1) the ovaries
(o.) are pjaiform organs lying on each side of the
stomach. They are situated on the ventral surface near
the posterior end of the cephalo-thorax. Each oviduct
[od.) arises near the posterior end, and courses posteriorly
as a narrow tube. When it enters the genital segment it
expands rapidly, ending in a large sac, the receptaculum
seminis (s), communicating with the vulva {vu.). The
oviduct has no distinct loops, and no cement gland is
found.

In the adult male (Plate IV., fig. 5) the testes {t.) occupy
the same positions as the ovaries in the female. The vasa
deferentia are straight, narrow tubes coursing posteriorly
and terminating in the sacs of the spermatophores. A
cement gland is present, as in Lej^eophtheirus.

The ovary in the course of the metamorphosis under-
goes great change of position. It is removed from the
cephalo-thorax into the genital segment. It occupies a
narrow region at the apex of the deep indentation (Plate
v., figs. 4 and 5, o.). The two ovaries have also practically
fused together, no separation is visible in transverse sec-
tion. The united ovaries are produced into horn-like pro-
jections anteriorly and posteriorly (Plate Y., fig. 5). The
oviducts (od.) arise near the apex of the anterior horns,
pass across the segment to its ventral surface, and then

42

course alono- each side of the median line to the external
openings. Each cement gland (Plate V., figs. 4, 5 and 7,
sg.) is a long crystalline organ of nearh' the same length
and breadth as the oviduct, lying ventrally to it. The
anterior part terminates at the base of the neck, in a blunt
end. The posterior end communicates with the oviduct
just inside the opening to the exterior.

The structure of the ovary of Lerncea differs con-
siderably from that of Lepeo'phtheiriis. In the cyclops
stage it consists of a mass of minute nucleated cells. In
the adult condition there are no tubules, and all the eggs
are in close contact. The size of the eggs in the ovary
of the adult varies from '04 to "08 mm. They are of the
same structure and undergo the same changes in their
passage along the oviduct as the egg^ of LepeophtheiruH
when they enter the thoracic ends of the oviducts. The
ovisacs consist of long slender tubes very much twisted.
(Plate IV., fig. 1, OS.). When straightened out each tube is
often found to attain the length of seven or eight inches.
The eggs are arraneed in a single column, and tlie period
of incubation is of the same duration as in Lepeophtheirus.
The death of the parent or detachment of the ovisacs has
no effect on the vitality of the embryos.

Fertilisation of the female is effected during the fixed
period of the cyclops stage. The spermatophores are
attached to the female in a similar manner to that
described foi' Lepeophtheirus. The contents pass into the
receptacula seminis, and the empty sacs fall away. They
are then replaced by others in succession, until the recep-
tacula are filled. Each fully charged receptacuhira repre-
sents the contents of four spermatophores {rep., Plate
IV., fig. 4). At first there is a distinct division between
each lot, but this soon disappears, and the whole becomes
one mass of spermatozoa. From a large number of

48

females sectioned in various directions, the conchision has
been arrived at that the spermatozoa at once pass up the
rudimentaiy oviduct to the ovary and fertilise the eg^gs.
This probably accounts for the difference between the
ovary of an adult Lernwa and Lepeophtheiriis. No trace
of a receptaculum seminis could be made out in the adult.

Life History.

The development of the embryo has not been worked
out by the present author. An excellent Avork by D.
Pedaschenko* contains a full description and figures of
the developing embryo.

The young Lerna'a hatches out as a nauplius, with
three pairs of appendages, representing the rudimentary
antennules, antennae, and mandibles (Plate lY., fig. 2,
nat. size, •45 mm.). It then after a short pelagic life,
settles on the apex of the gill filaments of the flounder,
to which it adheres by a broad chitinous filament, and
passes into a cyclopoid form (Plate lY., fig. 4). The
young Lernaia are occasionally found on the gills of the
plaice and lumpsucker. The parasite, by its attachment
to the gill filament, j^yroduces a marked change in that
organ. The whole of the apex assumes a tumid character,
and the filamentous plates on both sides for some little
distance disappear (Plate lY., figs. 8 and 9). While
attached to the gills the various appendages develop. The
male here reaches maturity (Plate lY., fig. 5), and under-
goes no further change. In the female a considerable
lengthening of the genital segment accompanies the
appearance of the various appendages. Fertilisation next
takes place ; then the young female severs its connection

* Development of the embryo and metamorphoses of Lerncea branchia-
lis. Trav. Soc. Imp. des Naturalistes de St. Petersbourg, vol. xxvi.,
livr. 4, No. 7, Sect, de Zool. et de Physiologie, 1898. (In Russian, with
German resume).

44

with the chitinous filament, and leads a pelagic life (Plate
IV., fig. 4. Nat. size 2*-') mm.). This condition is fre-
quently found in collections of plankton, and unless care
be taken ma}^ readily be confused with immature stages
of allied forms. I. C. Thompson, r.L.8.,t was the first to
recognise certain copepods taken in collections of plank-
ton from Liverpool Bay, &.C., as the young of Lerncea, from
Claus' figures. The presence of the males of Lerncea in
plankton is to some extent accidental, as only the females
lead a pelagic life. The males remain on the gills after
the females have gone. The result of the examination
of the contents of a fine filter, through which the waste
water was passed from the tanks containing flounders in
the Piel Hatchery, showed that females were always more
numerous than males. The ratio, after a number of
trials, was found to be one male to twenty-five females.

At the conclusion of the pelagic life the young Lerncea
again fixes itself to the gills of a fish, and the retrogres-
sive metamorphosis commences. The parasite buries its
cephalo-thorax into the tissues. This region then
develops into horns, which are situated one at each side
and one dorsal. These pass out at right angles to the
body into the tissues of the host. At first they are simple,
but by gradual division in each horn they acquire the
characters found in the adult (Plate Y., fig. 8). The
anterior part of the segment curves over, taking up the
position shown on Plate Y., fig. 4. The eyes, antennules,
antenna?, mandibles and maxillfe disappear, leaving only
the first maxillipedes, which are represented by small
hooks in the adult. The free thoracic segments fuse, but
the feet remain as in the cyclops stage. The genital
segment elongates until fully fifteen times the original

t Eevised Eeport on L.M.B.C. Copepoda. Trans. L'pool Biol. Soc,
vol. vii., p. 212.

45

length. The abdomen only lengthens a very little. The
elongation takes place during the development of the
horns and before the eyes and the other organs disappear.
This condition is shown on Plate lY., fig. U ; the nat.
size is 11'4 mm. The next phase, represented on Plate
IV., fig. 7. shows that the development of the horns, the
disappearance of various appendages, and the great
lengthening of the genital segment is followed by a loop-
ing of the posterior region of the latter. This loop
gradually expands, and finally takes on the adult
condition.

In the metamorphosis of the cephalo-thorax the ovaries
are thrust into the genital segment, and take up a position
on the dorsal aspect of the posterior region of that seg-
ment, in such a manner that the more anteriorly placed
portion of the ovary in the adult is what was the posterior
part in the cyclops stage (see Plate V., figs. 1 and 4).

The cyclops stage of Lernu^a was first found in situ by
Metzger,* who published a short note on the observations
made and the conclusions arrived at early in 1868.
Clausf later on in the same year, from specimens supplied
by Metzger and fresh material, confirmed the observations
of that Zoologist.

Concluding Remark.s.

In the account set forth on the above pages, it will be
seen that there are remarkable differences between the
changes that take place in the life history of the two cope-
pods before they reach maturity. In the one case
{Leijeojyhtheirus) the life history exhibits a series of pro-
gressive developments. In the other [Lermea), although

* Ueber das Mannchen ii. Wcibc-hen der Gattvmg Leincea, vor dum
Eintritt der sog. ruckscln'citenden ^Metamorphose. Jany., 1868.

t Beobachtuiigen ueber Lenueoccru, Peniculus, und Lertioea. 180b.

46

for a time it advances, yet after a particular period has
arrived the remainder of its development is retrogressive.
The various appendag-es in each parasite are developed in
the same order. In the one they become perfected when
the creature is fully developed. In the other, long before
the animal has reached maturity some have disappeared,
the remainder continue in a rudimentary condition, and
it is incapable of further movement. The internal organs
of both copepods are developed in the same way. In one
they continue advancing until perfected, and the animal
is thus capable of living for considerable periods apart
from its host. In the other, such organs as the digestive
gland, the brain and nerves, and the blood system become
rudimentary, if they do not altogether disappear. The
ovary loses its original position and passes into the genital
segment. The animal dies when removed from its host.

If only the adults were known, it would practically be
impossible to recognise that siich a form as Lerncea was in
any way related to such a typical free-swimming Copepod
as Calanus, and it would therefore still occupy
an uncertain position. l^ut when the whole life
history of both copepods is known, tracing the connection
becomes comparatively easy- Both originate from a free
larval stage known as the nauplius, which has been
regarded as the representative of a far back common
ancestor. Both pass through a cyclops stage. The
one ancestral cyclops form, we may supjjose, by maintain-
ing a free swimming life, gradually acquired more perfect
appendages, and became at last the form now known as
Cdlanus. The other cyclops form by adopting a sedentary
life, and depending on other animals for its food, became
semi-parasitic like many of the ascidian- and sponge-fre-
quenting forms of copepoda. The transition from
Lichomolgus-likG copepods to such forms as Bomolochus

47

and Krgdsilua became simple. Further chaiig'e in form
and habit continiied as the various appendages, through
constant rest, degenerated. The animal became in con-
sequence more and more dependent on its host for food.
Such changes extending over a long period of time,
have apparently resulted in such a form as Lerncca.

Some Zoologists divide the fish parasites into blood-
suckers and mucus-eaters, on account of the apparent
presence or absence of blood in the alimentary canal. It
is doubtful if such a division is really satisfactory. The
probability is that they are all blood-suckers in different
degree, and that the presence of blood is only obvious
because certain organs are absent. Lcemargus muricatus,
one of the CaJigidce, appears to make excavations into the
skin of its host, Orfhagoriscus mola (the short sun-fish).
Several individuals are usually found in each excavation.*
No obvious appearance of blood can be observed even in
these parasites.

One or two parasites on a fish may not be hurtful, but
when the numbers increase they probably have an irri-
tating effect, and finally, when they remain in one position
fo] some time, the skin and tissues become lacerated.

Consequently even such external parasites as have been
regarded as harmless mucus-eaters may really have an

injurious effect upon the fish.

There is much opportunity for investigating the

internal structures of the various families of fish parasites.

The most of the literature hitherto published deals with

the external characters only.

The specimens necessary for the work connected with

this memoir have been almost entirely collected from fish

caught in the vicinity of Piel. The author is indebted to

Mr. R. Xewsham, Jun., the Laboratory Assistant at Piel,
* A. Scott. Tian.'j. Kat. Hist. Soc, Glasgow, vol. iii., part '6, p. 2GG. 16'J'2.

48

for help m collecting. The important stage of Lerncea,
shown on Plate lY., tig. 6, is drawn from a specimen sent
by Mr. T. Scott, F.L.S., the author's father. It was
fcnnd on the gills of a whiting caught in the Bay of Nigg,
Aberdeen, in 1900, and was the only one met with in the
course of these investigations.

49

Explanation of PiiAXEs.

Eeference Letters.

a. antennule.
a*- antenna.

«"■ lateral frontal sucker.
ail. anus.

b. median frontal sucker.
bis. blood space.

c. filament duct.
eg. filament gland.
en. chitin.

D. dorsal.

d. opening of spermatophore.

e. eyes.

f. filament.

g. ganglia.
gl. gland

i. intestine.

K^ left anchor process.

if 2 median ,,

K^ right ,,

L. left.

Id. duct of digestive gland.

Ins. lens.

Iv. digestive gland.

31. mouth.

m. mandible.
ml. muscle.

ml.a. antennule muscles.
ml.an. anal muscles.
ml./, frontal muscles.
Dili, intestine muscles.
nil.l. lateral muscles.
7nl.m. mandible muscles.
ml.mxp. first maxillipede

muscles.
inl.pt. posterior cephalic

muscles.
ml.rt. rectum muscles.
ml.st. stomach muscles.
mx. first maxilla.
mx^- second ,,
vixp. first maxillipede.
mxp^- second ,,
n. nerves.
nc. nucleus.
0. ovary.
od. oviduct.
ce. oesophagus.
og. optic lobe.

50

oni. muscles of the oviduct.

on. ovisacs.

ov. ova.

p'- first pail' of feet.

j)'^- second , ,

p"»- third

j)''"'- fourth , ,

/;'■• fifth

p(j. pigment.

R. right.

i\ rostrum.

rej). receptaculum seminis.

/■/. rectum.

yt)t. retina.

;■//. fin ray.

S. spermatozoa.

.s6;/. suhoesophageal
ganghon.

sf. sternal fork.

sg. cement gland.

sp. spermatophore.

spg. supra-oesophageal
ganghon.

at. stomach.

t. testis.

V. ventral.

va. vagina.

erf. vas deferens.

vu. vulva.

y. opening of digestive duct

into the stomach.
z. pore canals.
Nos. 1 to 13 nerves, as
follows : —

1. optic.

2. antennules.

3. antennae,
-i. mandibles.

4a. lateral frontal margins.
4b. first maxilUe.
J. second maxillte.
oa. lateral cephalic muscles.
0. first maxillipedes.
7. second maxillipedes.
6'. first feet.
8a. stomach muscles.
.V. second feet.
'Ja. posterior cephalic
muscles.

10. third feet.

11. fourth feet.

12. abdomen.

13. fifth feet.

Plate I.
Fig. 1. Lcpeoplitheiruii pectoralis, mature female, dorsal

view. X 17.
Fi<^ 2. LcpeoplUheirKs pectoralis, mature male, dorsal

view. X 17.
Fig. 'd. LepeoplitJu'iru.i pectoralis, nauplius stage, newly

hatched, x 52.

51

Fio-. 4. rjepcoplitlicirK.s perfm-alis, " chalimus " stage.

X 26.
Fig. 5. Caligus rapax. early "chaliinuK" stage attached

to tin ray of young cod, the Hue d' c' represents

the surface of the skin, x 54"4.
Fig. 6. CaUijus rapax, "chaHmus" stage, previous to

throwing off the tilament attachment. On tail

of young lunipsucker. X 15*24.
Fig. 7. Caligus rapax, mature, part of the frontal plate

showing a lateral sucker, x 2-2.

Plate II.

Lt'jit'ojjJitlwirHs pectoral is.

Fig. 1. Female, ventral view, showing tlie various
appendages and their muscles. x IT.

Fig. 2. Female, dorsal view, showing the chief muscles
and hlood currents. The arrows indicate the
course of the blood. x 17.

Fig. -i. Female, ventral view, showing the digestive
gland, its duct and alimentary canal. x 17.

Fig. 4. Female, ventral view, showing the reproduc-
tive organs. x 17.

Fig. 5. Male, ventral view, showing the reproductive
organs. x 26.

Fig. G. Grenital segment and abdomen of an immature
female, ventral view, showing vulva (vu.). x 40.

Fig. 7. Spermatophores detached from genital openings
of a female. x 25.

Fig. 8. Mouth from the anterior base, with the
mandibles inside, showing the muscles and
ducts of digestive gland. x 25.

Fig. 9. Digestive gland. x 77.

Fig. 10. Longitudinal section of the ovary. x 50.

Fig. 11. Transverse section of pore-canal at the base
of the mouth. x ."350.

Plate III.

Le jieophtheiriis pectoral is.

Fig. 1. Female, ventral view, showing the nervous
system ifi situ. x 17.

Fig. 2. The nervous system from the ventral aspect.
X ;j8.

Fig. •'{. Female, nearly median longitiidinal section.
X IT.

Fig. 4. One of the antennules, showing the nerve
endings. x 76.

Fig. 5. Median longitudinal section of the ganglia,
showing the " pinhole "" (jesophagus passing-
through between the supra and sub-cesophageal
parts. X 77.

Fig. (i. Transverse section in the region of the eyes.
X 38.

Fig. 7. Transverse section in the region of the supra-
and sub-(esophageal ganglia. x 35.

Fig. 8. Transverse section in the region of the second
maxillipedes. x 30.

Fig. 9. Transverse section through the genital segment,
female, x 30.

Fig. 10. Transverse section through the genital segment,
male. x 38.

Fig. 11. Part of a transverse section of the intestine,
x 76.

Fig. 12. Horizontal section of the dorsal aspect of the
supra-cesophageal ganglion, showing the cross-
ing of the fibres of the optic nerves. x 152.

Fig. 13. Transverse section of the eyes. x 152.

Fig.

2.

Fig.

:;

Fig.

4.

Fi-.

5

58
Plate IV.

Lerruea hranchialis.

Fig. 1. Mature female, from the right side. The line
/' g' shows how much of the anterior portion
is buried in the branchial arch. x 4'''1
Nauplius stage, newly hatched. x 50'8.
Very young female, unfertilised, dorsal view.
From gills of flounder. x 51'5.
Fertilised female, dorsal view. Just after
leaving the gills of flounder. x 27'6.
Mature male, dorsal view. From gills of
floiinder. x 28"5.
Fig. 0. Fertilised female, " Penella" stage, dorsal view.
Just after settling on gills of Gadns (whiting).
X 16-5.
Fig. 7. Later stage than Fig. 6, from the left side.

The folding has just finished. Nat. size.
Fig. 8. Apex of gill filament of flounder, showing mal-
formation caused by the young Lermeo. x 18'6.
Fig. 9. Apex of gill filament of flounder, normal, x 18' 6.

Plate V.
Lerncea hranchialis.

Fig. 1. Fertilised female, ventral view, showing the
appendages, the reproductive organs, and
nervous system. x 47"6.

Fig. 2. Nearly median longitudinal section of the same,
x 47-6.

Fig. '). Transverse section in the region of the eyes,
x 80.

Fig. 4. Mature female, from the right side, showing
the first maxillipede and the four pairs of feet,
the alimentary canal and the reproductive

54

organs. The specimen was cleared in xylol,
and the right anchor process removed. x 4.

Fief. 5. Transverse section through oc j3, (Fig. 4.)
shoAving the muscular wall, the ovary,
oviduct, cement gland, and intestine. x 9.

Fig. (i. Transverse section through x Q, (Fig. 4,) just
anterior to the rectum. X 20.

Fig. T. Portion of the cement gland. x 20.

Fig. S. Front view of the anchor processes of an adult
female. x 4.

C. Tiuling & Co., Printers, 58, Victoria Street, Liverpool.

L.M.B.C. Memoir VI.

Plate I.

^-Sc<}ft, c/e/

SB. /it/?

LEPEOPHTHEIEUS & LERN^A.

L.M.B.C. Memoir VI.

Plate II.

A Sco/t de/

S B /ir/l

LEPEOPHTHEIEUS & LEEN^A.

L.M.B.C. Memoib VI.

Plate III,

n /I

A Sc-o/^</e/

S B lit/!

LEPEOPHTHEIRUS & LERN^A.

L.M.B.C. Memoie VI.

Plate IY.

FiG.5

FjG.5

A Scott ae/

LEPEOPHTHEIKUS & LERN^A.

L.M.B.C. Memoir VI.

Plate V

Scott, cfe/

S B lifh

LEPEOPHTHEIRUS & LERN^A.

3 2044' 093 "366 409

Date Due

B