THE CLASSIFICATION OF CRUSTACEA
_Table of Classification of Crustacea_
CLASS CRUSTACEA.
Subclass BRANCHIOPODA - {Order Anostraca.
{ " Notostraca.
{ " Conchostraca.
{ " Cladocera.
" OSTRACODA - { " Myodocopa.
{ " Podocopa.
" COPEPODA - { " Eucopepoda.
{ " Branchiura.
" CIRRIPEDIA - { " Thoracica.
{ " Rhizocephala.
" MALACOSTRACA.
Series _LEPTOSTRACA_ - " Nebaliacea.
" _EUMALACOSTRACA._
Division _Syncarida_ - " Anaspidacea.
" _Peracarida_ - { " Mysidacea.
{ " Cumacea.
{ " Tanaidacea.
{ " Isopoda.
{ " Amphipoda.
" _Eucarida_ - { " Euphausiacea.
{ " Decapoda.
" _Hoplocarida_ - " Stomatopoda.
[Illustration: FIG. 10--THE "FAIRY SHRIMP" (_Chirocephalus diaphanus_),
MALE. x 2. (After Baird.)]
Occasionally there may be found in rain-water puddles and the like, in
the South of England, a beautiful, transparent, shrimp-like animal, an
inch or more in length, to which the name of "Fairy Shrimp" has been
given (Fig. 10). It is known in technical zoology as _Chirocephalus
diaphanus_, and is a representative of the subclass BRANCHIOPODA. The
members of this group are distinguished from other Crustacea by their
flattened, leaf-like feet, each of which is divided into a number of
lobes, and has a gill plate on the outer side. In _Chirocephalus_ there
is no carapace, and the head is followed by eleven distinct body
segments, each bearing a pair of leaf-like, or rather fin-like, feet.
The hinder part of the body has no appendages, and ends in a forked
tail. In the female a large pouch hangs from the under-side of the body,
just behind the limb-bearing part, and is often found filled with eggs.
In the male, a pair of remarkable-looking appendages, each shaped
somewhat like a hand with webbed fingers, hang in front of the head.
These are connected with the antennae, and are known as the "claspers,"
from their function in seizing and holding the female. The eyes are set
on movable stalks. Those Branchiopoda which, like _Chirocephalus_, have
no carapace, form the order ANOSTRACA.
[Illustration: _PLATE II_
_Apus cancriformis_, FROM KIRKCUDBRIGHTSHIRE. (SLIGHTLY ENLARGED)]
A second order, the NOTOSTRACA, is represented by _Apus cancriformis_
(Plate II.), which occurs in many places in Europe in ponds and puddles,
and very rarely indeed in Britain. In _Apus_ there is a large dorsal
shield, or carapace, covering the greater part of the body, which
consists of a large number of segments (about twenty-eight), and ends
behind in a pair of long antenna-like filaments. The fin-like feet are
also very numerous (about sixty-three pairs). The eyes are not stalked,
but are set close together on the upper surface of the carapace.
[Illustration: FIG. 11--_Estheria obliqua_, ONE OF THE CONCHOSTRACA.
(After Sars, from Lankester's "Treatise on Zoology.")
A, Shell of female, from the side; B, male, from the side, after removal
of one valve of the shell. (Enlarged.) _a'_, Antennule; _a''_, antenna;
_ad_, muscle which draws together the valves of the shell; _f_, tail
fork; _md_, mandible]
The third order of the Branchiopoda, the CONCHOSTRACA (Fig. 11), are not
represented in Britain, though several species occur on the Continent of
Europe. In these the carapace forms a bivalved shell, completely
enclosing the body and limbs, and closely resembling that of a small
Mollusc.
[Illustration: FIG. 12--_Daphnia pulex_, A COMMON SPECIES OF
"WATER-FLEA." MUCH ENLARGED. (From British Museum Guide.)
Female carrying eggs in the brood-chamber]
The fourth order, the CLADOCERA, comprises the so-called "Water-fleas,"
which are abundant everywhere in ponds and lakes (Fig. 12). They are
all of small size, almost or quite microscopic. The carapace, as in the
Conchostraca, forms a bivalved shell, but does not enclose the head.
There is a single large eye, which really corresponds to two eyes fused
together. A pair of large antennae, each with two branches, carrying long
feathered hairs, project at the sides of the head, and are used in
swimming with a peculiar jumping motion, from which the popular name of
the animals is derived. There are not more than six pairs of feet. The
"Water-fleas," of which _Daphnia pulex_ is one of the commonest species,
are very beautiful and interesting objects for microscopic examination,
on account of their transparency, which allows many details of their
internal structure to be studied in the living animal.
[Illustration: FIG. 13--SHELLS OF OSTRACODA. MUCH ENLARGED. (From
Lankester's "Treatise on Zoology," after Brady and Norman, and Mueller.)
A, _Philomedes brenda_ (Myodocopa); B, _Cypris fuscata_ (Podocopa); C,
_Cythereis ornata_ (Podocopa). _n_, Notch characteristic of the
Myodocopa; _e_, the median eye; _a_, mark of attachment of the muscle
connecting the two valves of the shell. A and C are marine species; B is
from fresh water]
The OSTRACODA (Fig. 13), which form the second subclass in the system
of classification here adopted, are nearly all microscopic animals, and
are found abundantly in fresh water as well as in the sea. The carapace
forms a bivalved shell, which completely encloses the body and limbs,
and is often sculptured in an elegant fashion. The Ostracoda are
remarkable for the very small number of their appendages. There are not
more than two pairs of limbs behind the maxilla. Most of the species are
included in two orders, the _Myodocopa_ and the _Podocopa_, of which the
former may generally be distinguished by a notch in the anterior part of
the margin of the shell (Fig. 13, A, _n_). In the _Podocopa_ the margin
is entire.
[Illustration: FIG. 14--_Cyclops albidus_, A SPECIES OF COPEPOD FOUND IN
FRESH WATER. (After Schmeil.)
Female specimen carrying a pair of egg-packets. The actual length is
about one tenth of an inch]
The subclass COPEPODA comprises animals, for the most part of
microscopic size, which are abundant in fresh water and in the sea. The
common fresh-water genus _Cyclops_ (Fig. 14) furnishes a good example of
the type of structure characteristic of the class. The body is somewhat
pear-shaped, with a narrow abdomen ending in a "caudal fork." The body
is divided into somites, and there is no overlapping carapace, although
the head and the first two thoracic somites are coalesced. There are
four pairs of two-branched, oar-like, swimming feet, and a fifth pair,
found in some other Copepoda, is represented in _Cyclops_ by minute
vestiges on the first segment of the narrow posterior part of the body.
The antennules are very large, unbranched and composed of numerous
segments; the antennae are much smaller. In addition to the usual
mandibles, maxillulae, and maxillae, there is a pair of maxillipeds which
really represent the first pair of trunk limbs. There is a single red
eye in the middle of the front of the head. This eye is not formed, like
the single eye of the Cladocera, by fusion of a pair of eyes, but it
corresponds to a median eye of simple structure which is found in the
Branchiopoda, Ostracoda, and many other Crustacea, in addition to the
paired compound eyes. From the fact that this median eye is the only one
present in the earliest larval stage of Crustacea, the Nauplius (see
Chapter IV.), it is sometimes known as the "nauplius eye." The female
_Cyclops_ carries her eggs until they hatch, in two oval packets
attached to the sides of the body.
Forming a separate order (BRANCHIURA) apart from the more normal
Copepoda (order EUCOPEPODA) is the little group of the Carp-lice, one of
which, _Argulus foliaceus_, is common in England, living as a parasite
on different species of fresh-water fish, and often found swimming free
in ponds and rivers. It has a broad, flat, and very transparent body,
about three-sixteenths of an inch in length. It differs from _Cyclops_
in a great many points, of which, perhaps, the most conspicuous is the
possession of a pair of true compound eyes in addition to the median
eye. On the under-side of the head are a pair of large round suckers, by
means of which the animal fixes itself on to its prey. A study of their
development shows that these suckers are really the maxillae, which in
the young animal are jointed limbs ending in a strong claw, but later
become changed into the suckers of the adult. A sharp spine, which can
be protruded in front of the mouth, is connected with what is believed
to be a poison-gland. The eggs are not carried in packets by the female
as in _Cyclops_, but are deposited on stones or water-weeds.
The fourth subclass, CIRRIPEDIA, comprises the Barnacles and
Acorn-shells. These are very unlike any of the other Crustacea, and, in
fact, they were long classed by naturalists with the Mollusca. It was
not until their larval development was made known that they were
recognized as Crustacea. The common Goose Barnacle (_Lepas
anatifera_--Plate III.) is found adhering to the bottoms of ships and to
floating timber. It has a fleshy stalk or peduncle which is fixed at one
end to the supporting object, and bears at the other end a shell, made
up of five separate plates, enclosing the body of the animal. The stalk
corresponds to the front part of the head, and careful examination may
discover at its end, among the hardened cement which fixes it to the
support, the remains of the antennules by which the attachment of the
young animal was first effected. The body of the animal within the
carapace or shell bears the usual mandibles, maxillulae, and maxillae,
close to the mouth, and six pairs of long, tendril-like feet. These feet
have each two branches, composed of numerous short segments and fringed
with long hairs. They can be protruded from the slit-like opening of the
shell, forming a sort of "casting-net" for the capture of minute
floating prey.
The Acorn-shells, of which one species (_Balanus balanoides_--Plate
III.) is abundant everywhere on our coasts, covering rocks and stones
just below high-water mark, differ from _Lepas_ and its allies in having
no peduncle. The shell is cemented directly to the rock, and is conical
in shape, like a small limpet, with a hole at the top which is closed
by four movable valves.
[Illustration: _PLATE III_
GROUP OF SPECIMENS OF THE GOOSE-BARNACLE (_Lepas anatifera_), ONE
SHOWING THE CIRRI EXTENDED AS IN LIFE. (NATURAL SIZE)
(_From Brit. Mus. Guide_)
GROUP OF A COMMON SPECIES OF ACORN-SHELL OR ROCK BARNACLE (_Balanus
balanoides_)(NATURAL SIZE)]
The Stalked Barnacles, like _Lepas_ (suborder _Pedunculata_), and the
Sessile Barnacles, or Acorn-shells, like _Balanus_ (suborder
_Operculata_), together form the order THORACICA. Of the other orders
which compose the subclass Cirripedia, the only one that need be
mentioned here is the RHIZOCEPHALA, which comprises strangely degenerate
parasites living on other Crustacea.
The Cirripedia are unlike nearly all other Crustacea in the fact that,
with few exceptions, they are hermaphrodite, having both sexes united in
each individual. In certain species of the Stalked Barnacles, however,
there are minute male individuals that are attached, like parasites, to
the large hermaphrodites. In a few species the large individuals only
possess female organs, so that the separation of the sexes is complete.
The remarkable larval metamorphoses of Cirripedes and the modifications
of structure presented by some parasitic forms will be described in
later chapters.
The fifth and last subclass, that of the MALACOSTRACA, is by far the
largest and most important, and will require to be considered in more
detail than any of the others. The animals composing the various orders
into which the subclass is divided differ very greatly in structure, but
they all agree in having typically the same number of appendages as the
Lobster--namely, nineteen pairs (or twenty, if the eye-stalks be
included). They also agree in the very important character that the
trunk limbs are divided into two sets, thoracic and abdominal, the
former of eight, and the latter of six pairs.
[Illustration: FIG. 15--_Nebalia bipes._ ENLARGED. (From Lankester's
"Treatise on Zoology," after Claus.)
_a'_, Antennule; _a''_, antenna; _ab_1-_ab_6, the abdominal limbs; _ad_,
muscle joining the two valves of the shell; _f_, tail-fork; _p_, palp of
maxillula; _r_, rostral plate; _t_, telson; 1-7, the seven somites of
the abdomen]
The first order of the Malacostraca, the NEBALIACEA, comprises a few
Crustacea of small size, which differ in some very important characters
from all the other orders. _Nebalia bipes_ (Fig. 15), which occurs on
the southern coasts of the British Isles, has a large bivalved carapace
enclosing most of the limbs. In front, a small "rostral plate" is joined
to the carapace by a movable hinge, and partly covers the stalked eyes.
The eight pairs of thoracic feet are all alike, and are flattened and
leaf-like in form, resembling those of the Branchiopoda. The first four
pairs of abdominal limbs are large two-branched swimming feet, but the
last two pairs are reduced to small vestiges. Two of the most important
points in which the Nebaliacea differ from all the other Malacostraca
are that there are seven instead of six somites in the abdomen (the last
somite has no appendages), and that the telson has connected with it a
pair of movable rods forming a "caudal fork" like that of the
Branchiopoda. On account of the leaf-like thoracic feet and the
possession of a caudal fork and other features, the Nebaliacea were
formerly classified with the Branchiopoda, but a closer examination of
their structure has shown that they are true Malacostraca. In having an
additional somite in the abdomen and in other points, however, they may
be regarded as forming a link between the Malacostraca and the lower
forms of Crustacea, and for this reason they are set apart as a series
LEPTOSTRACA, while the other orders form a series EUMALACOSTRACA.
The orders of the Eumalacostraca, again, are grouped, as shown in the
table of classification, into four divisions. The first of these, the
SYNCARIDA, includes only one order, comprising a few small Crustacea
(see Fig. 84, p. 264) which have recently been discovered in fresh water
in Tasmania and Australia. They have no carapace, and all the thoracic
somites, or all but the first, are distinct. The antennules are
two-branched, the antennae may have a scale-like exopodite, and the last
pair of abdominal appendages form, with the telson, a tail-fan. The eyes
are sometimes stalked, but in one species they are sessile. The thoracic
limbs, which are not clearly divided into maxillipeds and legs, carry a
double series of plate-like gills or epipodites. As will be shown later,
the living Syncarida are especially interesting on account of their
resemblance to certain very ancient fossil Crustacea.
The second division of the Eumalacostraca, the PERACARIDA, includes five
orders, the members of which differ very greatly in appearance. They all
agree, however, in certain important points of structure, of which the
most conspicuous is the possession, in the female sex, of a brood-pouch
for carrying the eggs and young. This brood-pouch is formed by a series
of overlapping plates attached to the bases of the thoracic limbs.
[Illustration: FIG. 16--_Mysis relicta_, ONE OF THE MYSIDACEA. ENLARGED.
(From Lankester's "Treatise on Zoology," after Sars.)
_cs_, Cervical groove of the carapace; _m_, brood-pouch]
The first order of the Peracarida, the MYSIDACEA, consists of small,
free-swimming, shrimp-like animals (Fig. 16). Many species are common in
the sea round the British coasts, and from their possession of a
brood-pouch, in which the young are carried, they are sometimes known as
"Opossum Shrimps." The eyes are stalked, and the carapace is well
developed, although it does not unite with all the thoracic somites. The
antennae have a flattened, scale-like exopodite, probably of use for
keeping the animal balanced in swimming. Only one pair of the thoracic
limbs are modified to form maxillipeds, and all the legs (as in the
larval Lobster) have exopodites which form the chief swimming organs.
The uropods and telson form a "tail-fan." One of the most curious points
in the organization of some Mysidacea is the possession of a pair of
statocysts in the endopodites of the uropods. Each statocyst consists of
a small cavity containing a cake-shaped concretion known as a
"statolith," resting on a group of sensory hairs. There is reason to
believe that these organs have the same function as the statocysts of
the Lobster, although they are placed at the other end of the body. The
statolith serves the same purpose as the sand-grains found in the
Lobster's statocyst, although, unlike these, it is not introduced from
outside, but is formed in position by secretion from the walls of the
sac.
[Illustration: FIG. 17--_Gnathophausia willemoesii_, ONE OF THE DEEP-SEA
MYSIDACEA. HALF NATURAL SIZE. (From Lankester's "Treatise on Zoology,"
after Sars.)
_gr_, A groove dividing the last abdominal somite]
Most of the Mysidacea have no special organs of respiration, that
function being discharged (as in many of the smaller Crustacea) by the
general surface of the body, and especially by the thin carapace; but
certain deep-sea Mysidacea (Fig. 17) have tufted gills attached at the
base of the thoracic legs. In all cases the maxilliped has a plate-like
epipodite, which lies under the side-fold of the carapace, and no doubt
assists respiration, causing by its movements a current of water to flow
under the carapace.
[Illustration: FIG. 18--_Diastylis goodsiri_, ONE OF THE CUMACEA.
ENLARGED. (From Lankester's "Treatise on Zoology," after Sars.)
_a'_, Antennule; _l_1-_l_5, the five pairs of walking legs; _m_,
brood-pouch; _ps_, "pseudo-rostrum" formed by lateral plates of the
carapace; _t_, telson; _ur_, uropods]
The members of the second order of the Peracarida, the CUMACEA (Fig.
18), are small marine Crustacea in which the anterior part of the body
is generally stout, while the abdomen is slender and very mobile. The
short carapace does not cover more than the first three or four of the
thoracic somites. The eyes are not stalked, and are usually fused
together to form a single organ on the front part of the head. Swimming
branches (exopodites) are usually present on some of the thoracic legs,
at least in the males, which are more active swimmers than the females.
In the males, also, the swimmerets of the abdomen are often more or less
developed, but they are always absent in the females. The uropods do not
form a tail-fan, but are slender forked rods carrying comb-like rows of
spines, said to be used in cleaning the anterior appendages from the mud
among which these animals generally live. The telson is often absent,
or, rather, it is coalesced with the last somite of the abdomen. Under
the side-fold of the carapace on each side lies, as in the Mysidacea,
the epipodite of the maxilliped; but in this order it forms a gill, and
usually carries a row of flattened gill lobes.
[Illustration: FIG. 19--_Apseudes spinosus_, ONE OF THE TANAIDACEA.
ENLARGED. (From Lankester's "Treatise on Zoology," after Sars.)
_ex_, Vestiges of exopodites on second and third thoracic limbs; _oc_,
the small and immovable eye-stalks; _sc_, scale or exopodite of antenna;
_ur_, uropod]
The third Order, that of the TANAIDACEA (Fig. 19), is of special
interest, since in many respects it forms a transition to the next. It
comprises a number of minute Crustacea, generally found burrowing in mud
in the sea. They have a small carapace, which only involves the first
two thoracic somites, the rest of the somites being distinct. The
side-folds of the carapace enclose a pair of small cavities, within
which lie, as in the case of the last two orders, the epipodites of the
maxillipeds. The eyes are not movable, although they are set on little
side-lobes of the head, representing the vestiges of eye-stalks. The
first pair of thoracic limbs are maxillipeds, and the second pair are
very large, and form pincer-claws (chelae). Minute vestiges of exopodites
are sometimes found on the second and third pairs, but they are not
used for swimming, and only help to keep a current of water flowing
through the gill cavities. The abdomen is very short, with small
swimmerets, and the telson is not separated from the last somite. The
uropods are generally very small, and do not form a tail-fan.
[Illustration: FIG. 20--A WOODLOUSE (_Porcellio scaber_), ONE OF THE
ISOPODA. ENLARGED. (From Lankester's "Treatise on Zoology," after Sars.)]
Unlike the Tanaidacea, the ISOPODA, which form the fourth order of the
Peracarida, are very numerous in species, and very varied in structure
and habits. The most familiar are the Woodlice, or Slaters, which are
commonly found in damp places, under stones and the like. Besides these,
however, the order includes a vast number of forms living in the sea and
a few that live in fresh water. The examination of a common Woodlouse,
such as _Oniscus_ or _Porcellio_ (Fig. 20), will give a general idea of
the form and structure of a typical Isopod, although many curious
modifications are found, some of which will be mentioned in later
chapters.
There is no distinct carapace, but the last vestige of one may be
indicated by the fact that the first thoracic somite is completely
fused with the head. All the other somites of the body are distinct (in
some Isopods, however, the abdominal somites are coalesced), but the
telson is not separate from the last somite. The eyes are not stalked,
but are sessile on the sides of the head. The antennules have only a
single branch, and in the Woodlice are very small. The antennae have no
exopodite, although in a few other Isopods a minute vestige is present.
The thoracic limbs never have any trace of exopodites. The first pair
are maxillipeds, and if they carry an epipodite it is never enclosed in
a gill cavity, as in Tanaidacea. The swimmerets form one of the most
characteristic features of the Isopoda, for they are always flattened
into thin plates, which act as gills. In the Woodlice, which breathe
air, certain curious modifications of the swimmerets are found, which
will be described in a later chapter. In some Isopods that live as
parasites on fish or on other Crustacea, each individual is at first a
male, and later becomes a female. They are almost the only Crustacea,
except the Cirripedes already mentioned, which are normally
hermaphrodite.
[Illustration: FIG. 21--AN AMPHIPOD (_Gammarus locusta_). ENLARGED.
(From Lankester's "Treatise on Zoology," after Sars.)
_a'_, Antennule; _a''_, antenna; _acc_, accessory (inner) flagellum of
antennule; _br_, gill plate; _cx_, coxal plate (the expanded first
segment of the leg); _gn_, the two pairs of gnathopods (prehensile
legs); _plp'''_, abdominal appendage of third pair; _prp'_, _prp''_,
first and second peraeopods, or walking-legs; _t_, telson; _ur_, uropod;
II, VIII, second and eighth thoracic somites; 1, 6, first and sixth
abdominal somites]
The fifth order of the Peracarida, the AMPHIPODA, is also a very large
one. The "Sand-hoppers," which are very common on sandy coasts, belong
to this order, as do also a very large number of other forms found in
the sea and in fresh water, which have no popular names. A common
species is _Gammarus pulex_, sometimes called the "Fresh-water Shrimp,"
which is found everywhere in streams and ditches. Several closely allied
species, such as _G. locusta_ (Fig. 21), are found in the sea. The body
is flattened from side to side, and the abdomen is generally bent upon
itself. There is no carapace, but, as in the Isopods, the first thoracic
somite is fused with the head. The eyes are sessile on the sides of the
head. The antennules have a small inner branch, and the antenna have no
exopodites. The thoracic limbs, of which the first pair form
maxillipeds, have no exopodites, and are partly hidden by a row of
shield-like plates along the sides of the thorax. These plates are
formed by the enlarged and flattened basal segments of the limbs
themselves, and on the inner side they carry a series of oval plates,
which are the gills. The abdominal appendages are divided into two
sets: the first three pairs have each two slender, many-jointed
branches, and are used in swimming; the last three pairs are short,
stiff, and directed backwards, and are used in pushing the animal
through mud or among water-weeds. In many Amphipods, such as the
Sand-hoppers, the last three pairs of abdominal limbs are used in
jumping by sudden backward strokes of the abdomen.
[Illustration: FIG. 22--TWO SPECIES OF CAPRELLIDAE. (From Lankester's
"Treatise on Zoology," after Sars.)
A, _Phtisica marina_, a species which retains the fourth and fifth pairs
of thoracic limbs (_prp'_, _prp''_); B, _Caprella linearis_, in which
these limbs are represented only by the gills (_br_). (Enlarged.) _a'_,
Antennule; _a''_, antenna; _abd_, vestigial abdomen; _gn_, gnathopods;
_m_, brood-pouch; IV, V, fourth and fifth thoracic somites]
[Illustration: FIG. 23--_Paracyamus boopis_, THE WHALE-LOUSE OF THE
HUMPBACK WHALE. (From Lankester's "Treatise on Zoology," after Sars.)
A, Male, dorsal view, enlarged; B, the maxillipeds detached and further
enlarged. _a'_, Antennule; _a''_, antenna; _abd_, vestigial abdomen;
_br_, gills; _gn_, gnathopods; IV, V, fourth and fifth thoracic somites]
Two families of the Amphipoda differ so much in general appearance from
the others that they deserve mention. The Caprellidae (Fig. 22) have the
body drawn out to a thread-like slenderness, and the abdomen reduced to
a mere vestige. The fourth and fifth pairs of thoracic limbs are
generally absent, though the corresponding gills remain. The animals
live in the sea, clambering among sea-weeds or zoophytes in a fashion
which recalls the movements of "looper" caterpillars. The _Cyamidae_, or
"Whale-lice" (Fig. 23), are, as the name implies, parasites on the skin
of whales, and are closely related to the Caprellidae. They have,
however, a broad, flattened body, more like that of an Isopod than an
ordinary Amphipod, and their legs have strong curved claws with which
they cling to the skin of their host.
[Illustration: FIG. 24--_Meganyctiphanes norvegica_, ONE OF THE
EUPHAUSIACEA. TWICE NATURAL SIZE. (From Lankester's "Treatise on
Zoology.")]
The third division of the Malacostraca, the EUCARIDA, consists of two
orders of very unequal interest and importance. The first of these, the
order EUPHAUSIACEA (Fig. 24), comprises only a single family of small,
shrimp-like Crustacea found swimming freely at the surface or in the
depths of the sea. In these the carapace fuses with all the thoracic
somites, the eyes are stalked, the antennules have two flagella, and the
antennae have a broad scale. None of the thoracic limbs are modified into
maxillipeds, and all carry swimming exopodites. The uropods and telson
form a tail-fan. A single series of feathery gills are attached to the
bases of the thoracic limbs. Nearly all the Euphausiacea possess the
power of emitting light, and are furnished for the purpose with a number
of organs which were formerly supposed to be "accessory eyes."
The second order of the Eucarida, the DECAPODA, is by far the largest of
the orders of Crustacea, and it includes all the larger and more
familiar members of the class. It is necessary, therefore, to give a
considerably fuller account of its subdivisions than has been given in
the case of the other orders. The typical characters of the Decapoda are
well illustrated by the Lobster, which has been already described. As in
the Euphausiacea, the eyes are stalked, and the carapace fuses with all
the thoracic somites. From the Euphausiacea the Decapoda differ in the
fact that three pairs of the thoracic limbs are modified as maxillipeds,
the remaining five pairs forming the "ten legs" to which the name of the
order alludes. Further, the gills are arranged in more than one series,
not all attached to the bases of the legs, as in the Euphausiacea, and
covered over by the side-flaps of the carapace instead of being freely
exposed. While agreeing in these essential characters, however, the
members of the order Decapoda differ very widely among themselves in
structure and in general form, and they are classified (in the
arrangement adopted here) in two suborders, which are again subdivided
into sections and tribes.
ORDER DECAPODA.
Suborder NATANTIA - { Tribe Penaeidea.
{ " Stenopidea.
{ " Caridea.
" REPTANTIA.
Section _Palinura_ - { " Scyllaridea.
{ " Eryonidea.
" _Astacura_ - " Nephropsidea.
" _Anomura_ - { " Galatheidea.
{ " Thalassinidea.
{ " Paguridea.
{ " Hippidea.
" _Brachyura_ - { " Dromiacea.
{ " Oxystomata.
{ " Brachygnatha.
Subtribe Brachyrhyncha.
" Oxyrhyncha.
The suborder NATANTIA includes the numerous species of what are commonly
known as Prawns and Shrimps. These are characteristically powerful
swimmers, with lightly armoured bodies, more or less flattened from side
to side, with a thin, saw-edged rostrum, and with large swimmerets which
are the chief organs of swimming; in addition, some of the more
primitive Natantia have swimming branches, or exopodites, like those of
the Euphausiacea, on the thoracic legs. This suborder is divided into
three tribes. The _Penaeidea_ include the large Prawns of tropical
seas (_Penaeus_--Plate IV.), which have the first three pairs of legs
provided with chelae, and not differing greatly in size. The _Stenopidea_
are a small group of forms resembling the _Penaeidea_ in having chelae on
the first three pairs of legs, but the third pair are much larger than
the others. The _Caridea_ comprise our common Prawns (_Leander,
Pandalus_) and Shrimps (_Crangon_), besides a host of less generally
known forms; in these the third legs are never chelate, although the
first and second often are.
[Illustration: _PLATE IV_
_Penaeus caramote_ FROM THE MEDITERRANEAN. (ABOUT HALF NATURAL SIZE)
(_From Brit. Mus. Guide_)]
The second suborder, that of the REPTANTIA, is much more diversified,
but the animals composing it are united by certain characteristics, of
which the most obvious are their creeping habits (although some species
can swim well), their heavily armoured bodies, often more or less
flattened from above downwards, with the rostrum never thin and
saw-edged, and the swimmerets not used to any great extent for swimming.
The first section of the Reptantia, the _Palinura_, includes the Spiny
Lobsters, Rock Lobsters, or Sea-Crawfish, and their allies, forming the
tribe _Scyllaridea_. They are distinguished by having no large
pincer-claws, though the last pair of legs may have small pincers in the
female sex. One species, the Common Spiny Lobster (Plate V.), is found
on the southern and western coasts of the British Islands. The other
tribe belonging to this section is the _Eryonidea_, comprising a number
of small lobster-like forms living in the deep sea. They have
pincer-claws on the first four, or on all five, pairs of legs, and they
are of special interest on account of their geological antiquity.
[Illustration: _PLATE V_
THE COMMON SPINY LOBSTER (_Palinurus vulgaris_). (MUCH REDUCED)
(_From Brit. Mus. Guide_)]
The section _Astacura_ contains only a single tribe, _Nephropsidea_,
formed by the true Lobsters and the fresh-water Crayfishes. They have
pincer-claws on the first three pairs of legs, and the first pair are
much larger than the others.
The third section of the Reptantia, the _Anomura_, comprises forms in
which the abdomen is variously modified, being either bent upon itself
or, if extended, more or less soft and feebly armoured. The last pair of
legs are commonly reduced in size, and not used in walking. The members
of the four tribes composing the section differ widely in their general
appearance.
The _Galatheidea_ (Plate VI.) are small, flattened, lobster-like animals
which have the abdomen bent under the body. In one family
(_Porcellanidae_) the animals have quite the appearance of little Crabs
(see Fig. 41, p. 113), but they may be distinguished from the true Crabs
(Brachyura) by the fact that there are only three pairs of walking legs
behind the great chelae, the last pair of legs being very small and
carried folded up at the sides of the body, or even within the gill
chambers.
[Illustration: _PLATE VI_
_Munida rugosa._ BRITISH. (REDUCED)]
The _Thalassinidea_ are small lobster-like animals which burrow in sand
and mud, and have generally a more or less soft abdomen (see Fig. 38, p.
103).
[Illustration: _PLATE VII_
THE COMMON HERMIT-CRAB, _Eupagurus bernhardus_, IN THE SHELL OF A
WHELK (REDUCED)
(_From Brit. Mus. Guide_)]
The tribe _Paguridea_ includes the Hermit Crabs (_Paguridae_) and their
allies. The typical Hermit Crabs (Plate VII.), which are familiar
objects in seaside rock-pools, live in the empty shells of Whelks and
other Gasteropod Molluscs, which they carry about with them as portable
shelters. The structure of the animals is modified in adaptation to this
curious habit. The abdomen, which is protected during life by the
borrowed shell, is soft and unarmoured, and is spirally twisted. The
swimmerets, which have only the function of carrying the eggs in the
female, are much reduced, and are usually present only on one side of
the body. The uropods no longer form a tail-fan, but are adapted for
firmly wedging the hind part of the body into the coils of the shell.
One of the chelipeds is much larger than the other, and serves to block
up the opening when the animal withdraws into its shelter. In tropical
countries certain Hermit Crabs (_Coenobitidae_) have become adapted to a
life on land, and one of these, the well-known Coconut Crab, or Robber
Crab (_Birgus latro_), which is the largest species of the tribe, has
given up the habit of protecting itself with a shell, and its abdomen
has again acquired a strong armour on the upper side. The marine
_Lithodidae_--to which the British Stone Crab, _Lithodes maia_ (Plate
VIII.) belongs--seem at first sight to have little resemblance to the
Hermit Crabs, for they have the abdomen very small, and tucked up under
the body as in the true Crabs. Like the Porcellanidae, mentioned above,
however, the Lithodidae have only three pairs of walking legs behind the
chelipeds, the last pair being feeble and usually folded out of sight
within the gill chambers. The relationship of the Lithodidae to the
Hermit Crabs is shown by the abdomen, which is more or less twisted to
one side, and has swimmerets only on one side in the female, and quite
wanting in the male.
[Illustration: _PLATE VIII_
THE "NORTHERN STONE-CRAB," _Lithodes maia_, MUCH REDUCED. THE LAST
PAIR OF LEGS ARE FOLDED OUT OF SIGHT IN THE GILL CHAMBERS
(_From Brit. Mus. Guide_)]
The _Hippidea_ are curious little Crabs found burrowing in sandy beaches
in the warmer seas. They have the abdomen tucked under the body, and the
legs flattened for shovelling the sand.
The BRACHYURA, or true Crabs, form the fourth section of the Reptantia,
and are distinguished by having the abdomen reduced to a tail-flap,
which is doubled up under the cephalothorax, and is usually without any
trace of the uropods which are present in all the groups already
mentioned, with the single exception of the Lithodidae. At the sides of
the head the side-plates of the carapace become firmly soldered to the
"epistome," a plate which lies in front of the mouth, and in this way
there is formed the "mouth-frame," within which lie the jaws, covered in
by a pair of "folding-doors" formed by the flattened third
maxillipeds.
[Illustration: _PLATE IX_
THE COMMON SHORE-CRAB (_Carcinus maenas_). (REDUCED)
_Dromia vulgaris_, CARRYING ON ITS BACK A MASS OF THE SPONGE _Clione
celata_. BRITISH. (REDUCED)]
The first tribe of the Brachyura, the _Dromiacea_, comprises a number of
Crabs that in many points of structure resemble the Lobsters, and are
regarded as the most primitive members of the section. _Dromia vulgaris_
(Plate IX.), a furry, clumsy-looking Crab, occasionally found on our
southern coasts, has the last two pairs of legs short and carried up
over the back, where they are used for holding a mass of living sponge
which the Crab uses as a cloak to protect and conceal itself. At the
sides of the abdomen, wedged in between the telson and the last somite,
a pair of small plates may be seen, which are the last vestiges of the
uropods. These are wanting in the other tribes of the Brachyura.
[Illustration: _PLATE X_
_Calappa flammea_, BRAZIL. (REDUCED)]
The _Oxystomata_ (Plate X.), which form the second tribe of the
Brachyura, are distinguished by the form of the mouth-frame, which is
narrowed in front so as to be triangular instead of square in outline.
The passages through which the water passes out from the gills, which in
other Crabs open at the front corners of the mouth-frame, are carried
forwards to the front of the head. The Oxystomata are most abundant in
tropical seas, but are represented on the British coasts by species of
_Ebalia_, small and compact Crabs which are not unlike pebbles of the
gravel among which they live.
The remaining Crabs form the tribe _Brachygnatha_, in which the
mouth-frame and the maxillipeds that close it are more or less
quadrilateral in shape. The tribe is divided into two subtribes, which
may be recognized by the general shape of the carapace. In the
_Brachyrhyncha_ this is generally rounded or square-cut in front,
without a projecting rostrum. In this subtribe are included the great
majority of Crabs. The Edible Crab and the Shore Crab (Plate IX.) are
familiar examples. In the _Oxyrhyncha_, on the other hand, the carapace
is generally narrowed in front, with a projecting rostrum, either simple
or forked, and is often armed with spines. In this subtribe are included
the long-legged Spider Crabs, several species of which are common on our
coasts. The Giant Spider Crab of Japan (Plate XI.) is the largest of
living Crustacea.
[Illustration: _PLATE XI_
THE GIANT JAPANESE CRAB, _Macrocheira kaempferi_, MALE. THE SCALE OF
THE FIGURE IS GIVEN BY A TWO-FOOT RULE PLACED BELOW THE SPECIMEN
(_From Brit. Mus. Guide_)]
The last division of the Eumalacostraca, the HOPLOCARIDA (Plate XII.),
is one of very small extent, comprising only a single order
(_Stomatopoda_) of very remarkable Crustacea which are common in
tropical seas, and of which at least one species, _Squilla desmarestii_,
is occasionally captured on the south coast of England. The Stomatopoda
are prawn-like Crustaceans, usually with a flattened body, and are
easily recognized by the form of the large claws (the second pair of
thoracic limbs), in which the last segment shuts down, like the blade of
a pocket-knife, on the preceding segment, and forms a very efficient
weapon, so that the larger species are not to be handled without
caution. The resemblance of these claws to those of the mantis-insect
of Southern Europe led to a common Mediterranean species receiving long
ago the name _Squilla mantis_ (Plate XII.).
[Illustration: _PLATE XII_
_Squilla mantis_, FROM THE MEDITERRANEAN. ABOUT ONE-HALF NATURAL SIZE
(_From Brit. Mus. Guide_)]
The Stomatopoda have a small carapace, which does not cover the last
four thoracic somites, and has in front a small flattened rostrum,
attached by a movable hinge, like that of the Leptostraca. The eyes are
stalked, and, like the antennules, are attached to a separate movable
segment of the front part of the head--a peculiarity not found in any
other Crustacea. There are small plate-like gills attached to the bases
of some of the thoracic limbs, but the chief organs of respiration are
large feathery gills attached to the pleopods or swimmerets.
The Stomatopoda are all found in the sea, generally in shallow water,
burrowing in sand or hiding in crevices of rocks or corals. Some species
are more than a foot in length.