Thomas Roscoe Rede Stebbing.

A history of crustacea: recent malascostraca online

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maxillipeds helps the respiration, as in the Mysidae and
cheliferous Isopods, but with the important addition of
sessile branchial sacs. In the swimming branches on
some of the peraeopods there is a weighty resemblance to
the Mysidae. The mouth-organs make ^ome approaches
to those of the Isopoda, and, as with them, the young are
hatched before the development of the last pair of peraeo-
pods. The pleon recalls the palaeozoic Phyllocarida and
their existing representative Nehalia,

It will have been seen by the number of genera and
species to which the name of G. 0. Sars is attached that
he has made a special study of this sub-order, and this
sketch of it is deeply indebted to his numerous and lumi-
nous works upon the subject, to which, indeed, must be
credited the clearness and accuracy with which this small
but very interesting group is now known.



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314



A HISTORY OF RECENT CRUSTACEA



CHAPTER XXI



ISOPODA



Tribes,
Chelifera •



Flabellifera



Talvifera

Asellota .
Phreatoicidea .



Epicaridca



Oniscoidea



Fa m ill eg
I Apseudidse
\ TanaidaB

/Anthuridae
G-nathiidas
Cirolanidae
CorallanidsB
Alcironidae
Barybrotidae
^gidas
Cjmothoidae
Serolidae
SphaBromidae

\LimnoriidaB

f Arcturidae
\, Idoteidae

f Asellidae
\ Munnopsidas

Phreatoicidae

/ MicroniscidaB
Cyproniscidae
DajidsB
Cabiropsidae
Cryptoniscidse
Entoniscidaa

^Bopyridae

' Ligiidaa
TylidsB
Helleriidas
Oniscidae

^Armadillididas



o o
S bo

5"



Suborder 2. — Isopdda.

The Isopoda form a vast and widely distributed army.
In contrast witk the distinctive uniformity of the Cumacea,
they exhibit an extreme diversity of shape. The name was



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KEEPING DAEK 815

formed by Latreille from the Greek words ta-os, equal, and
TTom^ a foot, but, so far from the legs being all alike or
equal as the name would imply, these appendages often
have two or three very different developments in a single
animal. Tliere is, to be sure, a typical form of limb which
prevails very widely, but the exceptional forms are nume-
rous and remarkable. With these Latreille was unac-
quainted, and therefore naturally gives no clue to them in
the namelsopoda, which he himself interprets as signifying
' tiOus les pieds simples et uniquement propres a la locomo-
tion ou ^ la prehension.' In proportion to their importance
in the economy of the world the Isopoda have hitherto
attracted little of popular notice. They enjoy still less of
popular favour. They are all of retiring habits, never
needlessly courting attention, but in general clinging as
closely as possible lo whatever shelter or holdfast they
have adopted. Amidst enormous disparities of size and
strength and shape and temper, this prudent love of ob-
scurity, the one feature of the moral character which all of
them possess in common, is strong evidence that all of
them must have sprung from a common origin. They
have never tempted mankind to search for them as food.
The services which they doubtless often render as effective
scavengers are in some measure counterbalanced by the
damage which some of them inflict on submarine structures
and the depredations committed by others on the fruits
of the garden. Several of the species treat their fellow-
inhabit>ants of the sea with little ceremony, and make up for
smallness of size by ferocity of behaviour. It is only to be
hoped, as indeed it may be considered certain, that their
living victims are immeasurably less sensitive to pain than
ourselves.

Normally the members of this sub-order have an elongate
ventrally flattened body, divided into a head of six seg-
ments under a carapace, a trunk or perason of seven arti-
culated segments, and a pleon usually limited to six
segments. As in all the Edriophthalma, there is no ap-
preciable ocular segment. The carapace is occasionally
in coalescence with one or two of the segments of the



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316 A HISTORY OF RECENT CRUSTACEA

peraeon. The six segments of the pleon may be all
articulated, or some or all of them fused together. The
seventh segment or telson is scarcely ever free, but its in-
dependent existence is indicated in the peculiar genus
PhreatoiauSy and in one tribe, the Flabellifera, its presence
may generally be inferred from the attachment of the
uropods high up on the sides of the terminal segment,
while a few of the genera, as Faranthura, have it distinctly
articulated.

A pair of sessile compound eyes, remote or contiguous,
are usually present, but may be entirely wanting. Occa-
sionally they project from the head, but never on movable
foot-stalks.

The first antennaB are never very elongate, and rarely
have a secondary flagellum. The second antennas very
seldom carry an exopod, the articulated scale so conmion
among the Macrura. The upper lip usually forms a plate
projecting from the top of the oral aperture over the cutting
edges of the mandibles, and may have an inner plate lying
parallel to the outer. The lower lip is bilobed, or forms two
pairs of lobes, of which the inner pair is much the smaller.
The mandibles are very variable, the dentate cutting edge,
secondary plate, spine-row, molar tubercle, and three-jointed
* palp,' being sometimes strongly developed, at others, some
or even all of them disappearing. The first maxillas very
rarely have a backward directed ' palp^' The second
maxillae have neither ' palp ' nor exopod ; the outer plate
is divided, the inner undivided. The maxillipeds, of which
the first are in this and the next sub-order the only pair,
often have an epipod on the first joint. This first joint as
a rule stands free in each maxilliped, the pair not being
fused together. The plate developed from the second
joint is provided with coupling spines. For the limbs of
the trunk it is difficult to find any characters that are at
all constant through the sub-order. Dr. Boas in his defi-
nition (1883) says that these seven pairs are strong running-
feet with a spine at the point of the terminal joint ; that
the basal joint is always small, the second joint long, the
third in the Tanaidas smaller than those which follow,



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STRUCTURE OF THE ISOPODA 817

elsewhere almost of the same size as these. But the ex-
ceptions to this statement are exceedingly numerous.
Occasionally the first pair are opercular ; often they have
the form of gnathopods, that is to say. they are prehensile
instead of ambulatory. Sometimes they are weak and
slender, and agree in character with the three following
pairs, at others they agree with only the next two pairs.
Rarely the last three pairs are adapted for swimming in-
stead of walking. The comparative lengths of the joints
are also subject to much variation, although the second
joint is scarcely ever shorter than the first or the third, the
seventh pair of legs in Phreatoicus typicus, Chilton, and
the first pair of the male in one or more species of Munna,
offering perhaps rare exceptions. Marsupial plates are de-
veloped in the females in varying number. The pedun-
cles of the pleopods often carry coupling spines. The two
first pairs of the pleopods are often modified in the male
as sexual organs. Usually the inner branch in some of
the pairs is branchial. All the pairs are sometimes want-
ing. The uropods vary greatly both as to shape, function,
and their position on the sixth segment. The liver is said
to consist of four or six backward-directed tubes. The
heart is generally but not always elongate, and usually in
part situated in the pleon. The ovaries and testes are with-
out a median section. The young usually quit the brood-
pouch with the last peraeopods still undeveloped.

While in the Cumacea the pleon is in general very
considerably narrower than the trunk, in the Isopoda this
is only rarely the case, and sometimes the pleon even ex-
ceeds the trunk in breadth.

DeJlnHicn of ihe Sub-order Isopoda,

The cephalon or carapace with rare exceptions leaves
free seven segments of the trunk. There are no branchial
sacs in connection with the first maxillipeds, although these
occasionally assist the respiration. Usually some of the
pleopods have a branchial character. None of the last five
pairs of trunk-limbs have natatory exopods. The first



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318 A HISTORY OF RECENT CRUSTACEA

three and geoerally the first five segments of the pleoa are
short, whether fused or articulated ; the sixth comprising
the telson is usually the largest, and its appendages con-
stitute the only pair of uropods. The pleopods when
present are almost always closely overlapping.

Instead of being contented with the single genus
Oniscus^ as in the first edition of Linnaeus's ' System of
Nature,' the Isopoda now occupy seven tribes, several of
which are of great extent. These are named respectively
Chelifera, Flabellifera, Valvifera, Asellota, Phreatoicidea,
Epicaridea, and Oniscoidea.

Tribe 1. — Chelifera.

This first tribe, the ' claw-bearing ' Isopods, is a com-
paratively small one, but it has many peculiarities and is
controversially interesting. It has to do with narrow,
subdepressed, or subcylindrical animals, in which the head
is united with the first or even the first and second seg-
ments of the peraeon. The segments of the pleon are
sometimes fused together, and in that case the pleopods
are wanting. Otherwise these appendages are generally
present, and consist of a two-jointed stem and two un-
jointed branches, but differing from those of the Isopoda
in general in being swimming rather than branchial
organs. In this tribe respiration is carried on by means
of a branchial chamber situated under the sides of the
carapace to the rear. The first maxillae have a backward-
directed ' palp ' similar to that described in the Cumacea,
and no doubt fulfilling the same function of cleansing the
branchial chamber from obstructions. It is supposed to
be a part of the endopod turned backwards. The maxilli-
peds have also a remarkable structure directed backward
into the branchial chamber. This by its attachment to
the first joint is recognised as the epipod. By its rhyth-
mical movement to and fro within the cavity it maintains
a constant influx of water for the oxygenation of the
animal's blood. In evident connection with this respira-
tory process in the front part of the body is the circum-



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ISOPODS WITH FINGER AND THUMB 319

stance that the heart occupies the earlier segments of the
peraeon, whereas in those Isopoda which have branchial
pleopods, the heart is correspondingly situated in and
near the pleon. The tribal name refers to the fact that
the first pair of trunk-limbs, or gnathopods (corresponding
to the second maxillipeds of preceding descriptions), are
furnished with a chela, the two terminal joints forming an
opposed thumb and finger as in the familiar chelipeds of
crabs and lobsters, a character not met with in the rest of
the Isopoda. The uropods are slender, the branches often
flagelliform. The eggs are carried in a marsupium of thin
plates, which either form one pair attached to the fourth
free segment alone, or four pairs attached to four segments.
The tribe includes only two families, the Apseudidae
and Tanaidae, distinguishable by numerous characters both
external and internal.

Family 1. — Apseudidoe.

The body is depressed, the carapace generally having
a well-developed rostrum and carinate sides, with pyriform
or spine-like ocular lobes. The segments of the pleon are
well defined, narrower than those of the peraeon.

The first antennae, placed at the front corners of the
carapace, are remarkable in this sub-order by having two
many-jointed flagella. The second antennae with the
bases contiguous are placed between and below the first,
and are often furnished with an articulated scale, ciliated
all round. The flagellum is many-jointed.

The mandibles have a three-jointed ' palp.' The first
maxillae have two incisive lobes and a two-jointed back-
ward-directed ' palp,' ending in two or more setae. The
second maxillae are furnished with setae and spines. The
maxillipeds have a large laminar epipod, branchial in
function. The first gnathopods are strong and chelate,
the inner margins of the finger and thumb being usually
tuberculate in the male and serrate in the female. The
second gnathopods are fossorial, having the last three
joints, especially the penultimate or * hand,' flattened, the



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820 A HISTORY OF RECENT CRUSTACEA

flattened spines being of use for specific distinctions. One
or both of the pairs have usually small two-jointed exopods.
The peraeopods are slender, with many setae and spines.
The pleopods have two long setiferous branches, and the
uropods two that are many-jointed and filiform, the inner
elongate.

The small exopods above mentioned, when seen only
in cabinet specimens, might be considered rudimentary or
of little importance. But this would not be a right way
of regarding them. The branchial chamber has two little
openings, one for the entrance, the other for the exit of
the water used in breathing. In front of the latter of
these orifices in the living animal, M. Yves Delage points
out that the exopods of the first gnathopods with their
terminal setae vibrate so rapidly that the machinery can
with difficulty be seen, while those of the second gnatho-
pods behave in the same manner in front of the entrance
apertures. By this contrivance a lively current of water
is introduced to the branchial chamber, and, when its
virtue has been exhausted, the parting guest is speeded to
a convenient distance. When there is only one pair of
exopods, it is those at the gate of egress which remain,
and it is no doubt of less importance that the entering
current should be rapid than that the used-up water
should be effectively dismissed.

Five genera are at present assigned to this family.
The ocular lobes are not movable, as they wero at one
time supposed to be. #

Apseudes, Leach, 1814, has six segments of the trunk
free, a well-developed scale on the second antennae, a well-
developed ^ palp ' on the mandibles, exopods on both pairs
of gnathopods, and five pairs of pleopods, with both
branches usually one-jointed.

Parapsevdes, Sars, 1880, has six segments of the trunk
free, the antennal scale rudimentary, the mandibular
* palp ' very small, exopods on both pairs of gnathopods,
and only four pairs of pleopods, which have one of the
branches two-jointed. The single species is Parcups^eudes
latifrons (Grube) from the Mediterranean. It was at one



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THE APSEUDIDiE . 821

time supposed to be without exopods on the first two pairs
of limbs.

Sphyrdpus, Norman and Stebbing (in Sars), 1880, has
only five segments of the trunk free, no antennal scale,
exopods on the first, and sometimes if not always on the
second, pair of gnathopods, and five pairs of pleopods.

Typhlapseudes, Beddard, 1886, * the blind Apseudes*
shares a want of eyes with the genus ISphyrapiis and with
some species of Apseudes, Six segments of the trunk are
free. It has a small antennal scale, but is said to be with-
out exopod on either first or second gnathopods. There
are five pairs of pleopods, in which one of the branches is
two-jointed.

Leidpiis, Beddard, 1886, *the smooth-footed,' has six
free segments of the trunk, a rudimentary antennal scale,
a minute three-jointed exopod on the gnathopods of both
pairs, and five pairs of pleopods, in which one of the
branches is two-jointed. The type-species, Leiopns lepto^
daciylvs, is described as having three joints to the palp of
the first maxillae, but *the very short median joint'
seems open to some suspicion.

The earliest known species of this family is Apseudes
talpa (Montagu), first recorded from Devonshire, but also
occurring in the Mediterranean. From Apseudes Latreillii
(Milne-Edwards), also a Biitish species, it may be distin-
guished by the serrate first joint of the first antennae, the
spines on the epistome and on the ventral surface of the
peraBon-segments, and the elongation of the last segment
of the pleon. Other species, such as Apsettdes sirnplici''
rostris, Norman and Stebbing, 'taken in 1,263 fathoms,
about one hundred miles directly south of Rockall, Porcu-
pine Expedition, 1869,' and Apseudes grossimdmis, Norman,
from 90 fathoms, long. 11° 40' W. off the south-west
coast of Ireland, may be technically regarded as British,
but these and others in like circumstances seldom come
into the hands of any students but those who will prefer
to consult the original memoirs for their characteristics.
Sphyrapus malledlus, Norman and Stebbing, alludes both
by its generic and its specific name to the hammer-like^
24



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322 A HISTORY OF BECENT CRUSTACEA

appearance of the first gnathopods, especially in the male,
in which sex the huge hand is set on at right angles to
the preceding handle-like joint. Like Sphyrapus tvdes, of
the same authors, it has been taken off the west of Ireland.
From the latter species it is distinguished by the great
extension of the lateral processes on the second segment
of the pleon.

In both of these species there are exopods on both
pairs of gnathopods, but, according to Sars, they are absent
from the second pair in his species, anomalas and serratus.

Family 2. — Tanaidce,

The body is nearly parallel-sided, the carapace being
truncate in front or with a minute rostrum, with or without
ocular lobes and eyes. The peraeon and pleon are without
spiny armature. The pleon is seldom narrower and some-
times broader than the peraeon. The first antennae are
contiguous, with a single flagellum, which is rudimentary
or sometimes wanting or rarely well developed in the
female, but multiarticulate in the male. The second
antennae are smaller than the first and below them, with-
out scale, and with the flagellum rudimentary or rarely
well developed. The mandibles have no * palp.' The first
maxillae have one incisive lobe and a one-jointed 'palp'
usually ending in two setae. The second maxillae form
minute rudimentary unarmed lobules. The maxillipeds
are fused at the base, and usually have a narrow falci-
form epipod. The masticatory organs are, however, often
evanescent in the adult male. The gnathopods are with-
out exopods. In the first pair the second joint is large
and tumid, and in the adult male the sixth joint or hand
becomes so much developed that it seems to crowd the
mouth-organs out of existence. The second gnathopods
are ambulatory like the following limbs. The pleopods
have two short setiferous branches, or are rarely absent.
The uropods are either simple or furnished with two fila-
ments, of which the outer is always short, having one, two,
or at most three, joints.



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THE TANAID^ 323

Within recent years the genera of this family have
become tolerably numerous. Many of the species are
quite microscopic, and in some instances an impatient
observer may be tempted to suppose that the distinguish-
ing features even of the genera have been invented to
match the size of the specimens. But in most cases such
a criticism will be withdrawn upon a more careful in-
spection.

Only a few selected characters can here be offered to
discriminate the fifteen (or rather, the fourteen) genera
that have been established : —

Tanais, Audouin and Milne-Edwards, 1829, has eyes,
several setae on the ' palp ' of the first maxillae, the marsu-
pium of the female consisting of two large plates aflSxed
to the fourth free segment of the peraeon, only three pairs
of pleopods, and the uropods simple, the single branch
being two- or three-jointed.

Leptochelia, Dana, 1849, has two setae on the ' palp ' of
the first maxillae, the first gnathopods of the adult male
with very elongate thumb and finger, the marsupium of
the female consisting of a pair of plates on each of the
first four free segments, five pairs of pleopods, and in the
uropods a multiarticulate inner and a one-jointed outer
branch.

Heterotanais, Sars, 1880, scarcely differs from Lepto^
chelia^ except in the first gnathopods of the male, which
are incompletely chelate, the so-called thumb being very
little instead of enormously produced. In the uropods the
outer branch is little but two-jointed, the inner filiform,
with four or five joints.

Pcf/ratanais, Dana, 1852, has eyes, but without distinct
ocular lobes. The mandibles are strong in the female,
with cylindrical molar process, whereas in the male all the
mouth-organs, except the maxillipeds, are lost. The first
gnathopods are similar in the two sexes. Of the uropods
both branches are two-jointed.

Psevdotanaisy Sars, 1880. The eyes are imperfect or
absent, and there are no ocular lobes. The mandibles
have a stiliform molar. The maxillipeds are fused at the



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324 A HISTORY OF RECENT CRUSTACEA

base, a single central plate issuing between the 'palps*
from the fused second joints. The marsupium is like that
in Tanais, There are five pairs of pleopods in the male,
and five or none in the female. The uropods have both
branches two-jointed.

TyphlotanaiSy Sars, 1880, is without eyes or ocular
lobes. The mandibles have a thick cylindrical molar in the
female. In the male the mouth-organs disappear as in
Paraiaimis. The marsupium has the usual four pairs of
plates. There are five pairs of pleopods in both sexes.
Both branches of the uropods are two-jointed, or one or
other of them may be one-jointed.

Leptognathia, Sars, 1880, has no eyes or ocular lobes.
The mandibles are weak, with a feeble pointed molar in
the female. The mouth-organs of the male and marsu-
pium of the female and the pleopods are as in the preced-
ing genus. Of the uropods the inner branch is two-
jointed in the female, three-jointed in the male, the outer
much smaller, two- or one-jointed, sometimes rudimentary.

Alaotanais, Norman and Stebbing, 1886, has minute
ocular lobes, but no eyes. The manibles are strong, with
large molar, at least in the female, the full-grown male
being probably content with the maxillipeds for mouth-
organs. The marsupium and pleopods are as in the pre-
ceding genus. The first gnathopods are powerful, espe-
cially in the male, much like those of Leptochelia. Of the
uropods the inner branch is eight- or nine-jointed, the
outer two-jointed.

Neotanais, Beddard, 1886, is no doubt the same genus
as the preceding. Both were published by the Zoological
Society of London in the same year. The description of
Alaotmiais was received Nov. 5, 1884, read Dec. 2, 1884,
and published in October 1886. Neotanais was, I believe,
received and read considerably later, but actually published
earlier. Mr. Beddard mentions ' mandibles with the usual
structure, with a slender extremity and a stout molar
process.' His only specimens were males, but not perhaps
at the stage when the mandibles would be lost.

Bathytanais, Beddard, 1886, has distinct ocular lobes,



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SOME BLIND GENERA 325

with eyes well developed, and the first and second antennae
four-jointed apparently in the male as well as in the
female. The male is thought to be distinguished chiefly
by a more elongate pleon. The single species is repre-
sented by one small specimen supposed to come from
2,050 fathoms in Mid Pacific, and other specimens firom
bet\<reen 2 and 10 fathoms at Port Jackson in Australia.

Tanaelkb, Norman and Stebbing, 1886, has no eyes
and no distinct ocular lobes. There are no pleopods in
the female, and the uropods are simple, the single branch
being also single-jointed. The male is unknown.

Strongylura, Sars, 1880, has no eyes and no distinct
ocular lobes. The mandibles have the molar moderately
large, laminar. There are no pleopods in the female. In
the uropods the inner branch is two-jointed, the outer
one-jointed. .

Gryptocope^ Sars, 1880, like several of the preceding
genera is without eyes. The mandibles have the molar
sublaminar, unarmed, and on the right hand mandible
there is a very prominent secondary plate. The segments
of the peraeon are divided by deep instrictions. The pleo-
pods are well developed in the male, rudimentary in the
female. The uropods in the male are long, with three-
jointed inner, and two-jointed outer, branch ; in the
female short, with two-jointed inner, and one-jointed outer,
branch.

Haphcope, Sars, 1880, agrees nearly with the preced-
ing genus, except that the mandibles have a cylindrical
molar, of which the truncate apex is encircled with teeth,



Online LibraryThomas Roscoe Rede StebbingA history of crustacea: recent malascostraca → online text (page 28 of 41)