Chapter XIII: Front Matter (13)

CRUSIUS, CHRISTIAN AUGUST (1715-1775), German philosopher and theologian, was born on the 10th of January 1715 at Lenau near Merseburg in Saxony. He was educated at Leipzig, and became professor of theology there in 1750, and principal of the university in 1773. He died on the 18th of October 1775. Crusius first came into notice as an opponent of the philosophy of Leibnitz and Wolff from the standpoint of religious orthodoxy. He attacked it mainly on the score of the moral evils that must flow from any system of determinism, and exerted himself in particular to vindicate the freedom of the will. The most important works of this period of his life are _Entwurf der nothwendigen Vernunftwahrheiten_ (1745), and _Weg zur Gewissheit und Zuverlassigkeit der menschlichen Erkenntniss_ (1747). Though diffusely written, and neither brilliant nor profound, Crusius' philosophical books had a great but short-lived popularity. His criticism of Wolff, which is generally based on sound sense, had much influence upon Kant at the time when his system was forming; and his ethical doctrines are mentioned with respect in the _Kritik of Practical Reason_. Crusius's later life was devoted to theology. In this capacity his sincere piety and amiable character gained him great influence, and he led the party in the university which became known as the "Crusianer" as opposed to the "Ernestianer," the followers of J. A. Ernesti. The two professors adopted opposite methods of exegesis. Ernesti wished to subject the Scripture to the same laws of exposition as are applied to other ancient books; Crusius held firmly to orthodox ecclesiastical tradition. Crusius's chief theological works are _Hypomnemata ad theologiam propheticam_ (1764-1778), and _Kurzer Entwurf der Moraltheologie_ (1772-1773). He sets his face against innovation in such matters as the accepted authorship of canonical writings, verbal inspiration, and the treatment of persons and events in the Old Testament as types of the New. His views, unscholarly and uncritical as they seem to us now, have had influence on later evangelical students of the Old Testament, such as E. W. Hengstenberg and F. Delitzsch.

There is a full notice of Crusius in Ersch and Gruber's _Allgemeine
Encyclopadie_. Consult also J. E. Erdmann's _History of Philosophy_;
A. Marquardt, _Kant und Crusius_; and art. in Herzog-Hauck,
_Realencyklopadie_ (1898). (H. St.)

CRUSTACEA, a very large division of the animal kingdom, comprising the familiar crabs, lobsters, crayfish, shrimps and prawns, the sandhoppers and woodlice, the strangely modified barnacles and the minute water-fleas. Besides these the group also includes a multitude of related forms which, from their aquatic habits and generally inconspicuous size, and from the fact that they are commonly neither edible nor noxious, are little known except to naturalists and are undistinguished by any popular names. Collectively, they are ranked as one of the classes forming the sub-phylum ARTHROPODA, and their distinguishing characters are discussed under that heading. It will be sufficient here to define them as Arthropoda for the most part of aquatic habits, having typically two pairs of antenniform appendages in front of the mouth and at least three pairs of post-oral limbs acting as jaws.

As a matter of fact, however, the range of structural variation within the group is so wide, and the modifications due to parasitism and other causes are so profound, that it is almost impossible to frame a definition which shall be applicable to all the members of the class. In certain parasites, for instance, the adults have lost every trace not only of Crustacean but even of Arthropodous structure, and the only clue to their zoological position is that afforded by the study of their development. In point of size also the Crustacea vary within very wide limits. Certain water-fleas (Cladocera) fall short of one-hundredth of an inch in total length; the giant Japanese crab (_Macrocheira_) can span over 10 ft. between its outstretched claws.

The habits of the Crustacea are no less diversified than their structure. Most of them inhabit the sea, but representatives of all the chief groups are found in fresh water (though the Cirripedia have hardly gained a footing there), and this is the chief home of the primitive Phyllopoda. A terrestrial habitat is less common, but the widely-distributed land Isopoda or woodlice and the land-crabs of tropical regions have solved the problem of adaptation to a subaerial life.

Swimming is perhaps the commonest mode of locomotion, but numerous forms have taken to creeping or walking, and the robber-crab (_Birgus latro_) of the Indo-Pacific islands even climbs palm-trees. None has the power of flight, though certain pelagic Copepoda are said to leap from the surface of the sea like flying-fish. Apart from the numerous parasitic forms, the only Crustacea which have adopted a strictly sedentary habit of life are the Cirripedia, and here, as elsewhere, profound modifications of structure have resulted, leading ultimately to a partial assumption of the radial type of symmetry which is so often associated with a sedentary life.

Many, perhaps the majority, of the Crustacea are omnivorous or carrion-feeders, but many are actively predatory in their habits, and are provided with more or less complex and efficient instruments for capturing their prey, and there are also many plant-eaters. Besides the sedentary Cirripedia, numbers of the smaller forms, especially among the Entomostraca, subsist on floating particles of organic matter swept within reach of the jaws by the movements of the other limbs.

Symbiotic association with other animals, in varying degrees of interdependence, is frequent. Sometimes the one partner affords the other merely a convenient means of transport, as in the case of the barnacles which grow on, or of the gulf-weed crab which clings to, the carapace of marine turtles. From this we may pass through various grades of "commensalism," like that of the hermit-crab with its protective anemones, to the cases of actual parasitism. The parasitic habit is most common among the Copepoda and Isopoda, where it leads to complex modifications of structure and life-history. Perhaps the most complete degeneration is found in the Rhizocephala, which are parasitic on other Crustacea. In these the adult consists of a simple saccular body containing the reproductive organs and attached by root-like filaments which ramify throughout the body of the host and serve for the absorption of nourishment (fig. 1).

Many of the larger species of Crustacea are used as food by man, the most valuable being the lobster, which is caught in large quantities on both sides of the North Atlantic. Perhaps the most important of all Crustacea, however, with respect to the part which they play in the economy of nature, are the minute pelagic Copepoda, of which incalculable myriads form an important constituent of the "plankton" in all the seas of the globe. It is on the plankton that a great part of the higher animal life of the sea ultimately depends for food. The Copepoda live upon the diatoms and other important microscopic vegetable life at the surface of the sea, and in their turn serve as food for fishes and other larger forms and thus, indirectly, for man himself.

A, Group of _Peltogaster socialis_ on the abdomen of a small
hermit-crab; in one of them the fasciculately ramified roots, r, in
the liver of the crab are shown (Fritz Muller).
B, Young of _Sacculina purpurea_ with its roots. (Fritz Muller.)]

_Historical Sketch._--In common with most branches of natural history, the science of Carcinology may be traced back to its beginnings in the writings of Aristotle. It received additions of varying importance at the hands of medieval and later naturalists, and first began to assume systematic form under the influence of Linnaeus. The application of the morphological method to the Crustacea may perhaps be dated from the work of J. C. Fabricius towards the end of the 18th century.

In the first quarter of the 19th century important advances in classification were made by P. A. Latreille, W. E. Leach and others, and J. Vaughan Thompson demonstrated the existence of metamorphosis in the development of the higher Crustacea. A new epoch may be said to begin with H. Milne-Edwards' classical _Histoire naturelle des crustaces_ (1834-1840). It is noteworthy that even at this late date the Cirripedia (Thyrostraca) were still excluded from the Crustacea, though Darwin's Monograph (1851-1854) was soon to make them known with a wealth of anatomical and systematic detail such as was available, at that time, for few other groups of Crustacea. About the same period three authors call for special mention, W. de Haan, J. D. Dana and H. Kroyer. The new impulse given to biological research by the publication of the _Origin of Species_ bore fruit in Fritz Muller's _Fur Darwin_, in which an attempt was made to reconstruct the phylogenetic history of the class. The same line of work was followed in the long series of important memoirs from the pen of K. F. W. Claus, and noteworthy contributions were made, among many others, by A. Dohrn, Ray Lankester and Huxley. In more recent years the long and constantly increasing list of writers on Crustacea contains no name more honoured than that of the veteran G. O. Sars of Christiania.

_Morphology._

_External Structure: Body._--As in all Arthropoda the body consists of
a series of segments or somites which may be free or more or less
coalesced together. In its simplest form the exoskeleton of a typical
somite is a ring of chitin defined from the rings in front and behind
by areas of thinner integument forming moveable joints, and having a
pair of appendages articulated to its ventral surface on either side
of the middle line. Frequently, however, this exoskeletal somite may
be differentiated into various regions. A dorsal and a ventral plate
are often distinguished, known respectively as the tergum and the
sternum, and the tergum may overhang the insertion of the limb on each
side as a free plate called the pleuron. The name epimeron is
sometimes applied to what is here called the pleuron, but the word has
been used in widely different senses and it seems better to abandon
it. The typical form of a somite is well seen, for example, in the
segments which make up the abdomen or "tail" of a lobster or crayfish
(fig. 2). The posterior terminal segment of the body, on which the
opening of the anus is situated, never bears appendages. The nature of
this segment, which is known as the "anal segment" or telson (fig. 3,
T), has been much discussed, some authorities holding that it is a
true somite, homologous with those which precede it. Others have
regarded it as representing the fusion of a number of somites, and
others again as a "median appendage" or as a pair of appendages fused.
Its morphological nature, however, is clearly shown by its
development. In the larval development of the more primitive
Crustacea, the number of somites, at first small, increases by the
successive appearance of new somites between the last-formed somite
and the terminal region which bears the anus. The "growing point" of
the trunk is, in fact, situated in front of this region, and, when the
full number of somites has been reached, the unsegmented part
remaining forms the telson of the adult.

C, carapace covering the cephalothorax.
Ab, abdominal somites.
T, telson, having the uropods or appendages of the last abdominal
somite spread out on either side of it, forming the "tail-fan."
l, labrum, or upper lip.
m, metastoma, or lower lip.
1, eyes.
2, antennule (the arrow points to the opening of the so-called
auditory organ).
3, antenna.
4, mandible.
5, maxillula (or first maxilla).
6, maxilla (second maxilla).
7-9, first, second and third maxillipeds.
ex, exopodite.
ep, epipodite.
g, gill.
10, sixth thoracic limb (second walking-leg) of female.
11, last thoracic limb of male. In 10 and 11 the arrows indicate the
genital apertures.
13, sterna of the thoracic somites, from within.
14, third abdominal somite, with appendages or "swimmerets."]

In no Crustacean, however, do all the somites of the body remain
distinct. Coalescence, or suppression of segmentation ("lipomerism"),
may involve more or less extensive regions. This is especially the
case in the anterior part of the body, where, in correlation with the
"adaptational shifting of the oral aperture" (see ARTHROPODA), a
varying number of somites unite to form the "cephalon" or head. Apart
from the possible existence of an ocular somite corresponding to the
eyes (the morphological nature of which is discussed below), the
smallest number of head-somites so united in any Crustacean is five.
Even where a large number of the somites have fused, there is
generally a marked change in the character of the appendages after the
fifth pair, and since the integumental fold which forms the carapace
seems to originate from this point, it is usual to take the fifth
somite as the morphological limit of the cephalon throughout the
class. It is quite probable, however, that in the primitive ancestors
of existing Crustacea a still smaller number of somites formed the
head. The three pairs of appendages present in the "nauplius" larva
show certain peculiarities of structure and development which seem to
place them in a different category from the other limbs, and there is
some ground for regarding the three corresponding somites as
constituting a "primary cephalon." For practical purposes, however, it
is convenient to include the two following somites also as cephalic.

C, cephalon.
Th, thorax. (Only seven of the eight thoracic somites are visible,
the first being fused with the cephalon.)
Ab, abdomen.

The numbers appended to the somites do not correspond to the
enumeration adopted in the text. 21 is the telson.]

A remarkable feature found only in the Stomatopoda is the reappearance
of segmentation in the anterior part of the cephalic region. Whether
the movably articulated segments which bear the eye-stalks and the
antennules in this aberrant group correspond to the primitive head
somites or not, their distinctness is certainly a secondarily acquired
character, for it is not found in the larvae, nor in any of the more
primitive groups of Malacostraca.

The body proper is usually divisible into two regions to which the
names _thorax_ and _abdomen_ are applied. Throughout the whole of the
Malacostraca the thorax consists of eight and the abdomen of six
somites (fig. 4), and the two regions are sharply distinguished by the
character of their appendages. In the various groups of the
Entomostraca, on the other hand, the terms thorax and abdomen, though
conveniently employed for purposes of systematic description, do not
imply any homology with the regions so named in the Malacostraca.
Sometimes they are applied, as in the Copepoda, to the limb-bearing
and limbless regions of the trunk, while in other cases, as in the
Phyllopoda, they denote, respectively, the regions in front of and
behind the genital apertures.

1, _Ceratiocaris papilio_, U. Silurian, Lanark.
2, _Nebalia bipes_(one side of carapace removed).
3, _Lepidurus Angassi_: a, dorsal aspect; b, ventral aspect of head
showing the labrum and mouth-parts.
4, larva of _Apus cancriformis_.
5, _Branchipus stagnalis_: a, adult female; b, first larval stage
(Nauplius); c, second larval stage.
6, Nauplius of _Artemia salina_.]

A character which recurs in the most diverse groups of the Crustacea,
and which is probably to be regarded as a primitive attribute of the
class, is the possession of a carapace or shell, arising as a dorsal
fold of the integument from the posterior margin of the head-region.
In its most primitive form, as seen in the _Apodidae_ (fig. 5, 3) and
in _Nebalia_ (fig. 5, 2), this shell-fold remains free from the trunk,
which it envelops more or less completely. It may assume the form of a
bivalve shell entirely enclosing the body and limbs, as in many
Phyllopoda (fig. 6) and in the Ostracoda. In the Cirripedia it forms
a fleshy "mantle" strengthened by shelly plates or valves which may
assume a very complex structure. In many cases, however, the
shell-fold coalesces with some of the succeeding somites. In the
Decapoda (fig. 3), this coalescence affects only the dorsal region of
the thoracic somites, and the lateral portions of the carapace
overhang on each side, enclosing a pair of chambers within which lie
the gills. The arrangement is similar in Schizopoda and Stomatopoda
(fig. 7), except that the coalescence does not usually involve the
posterior thoracic somites, several of which remain free, though they
may be overlapped by the carapace.

FIG. 6.--_Estheria_, sp.; D from Dubuque, Iowa; (e) the eye. L from
Lynn, Massachusetts (nat. size). S presents a highly magnified section
of one of the valves to show the successive moults. B an enlarged
portion of the edge of the shell along the back, showing the overlap
of each growth.]

In the Isopoda and Amphipoda, where, as a rule, all the thoracic
somites except the first are distinct (fig. 4), there seems at first
sight to be no shell-fold. A comparison with the related Tanaidacea
(fig. 8) and Cumacea (or Sympoda), however, leads to the conclusion
that the coalescence of the first thoracic somite with the cephalon
really involves a vestigial shell-fold, and, indeed, traces of this
are said to be observed in the embryonic development of some Isopoda.
It seems likely that a similar explanation is to be applied to the
coalescence of one or two trunk-somites with the head in the Copepoda,
and, if this be so, the only Crustacea remaining in which no trace of
a shell-fold is found in the adult are the Anostracous Phyllopoda such
as Branchipus (fig. 5, 5).

_General Morphology of Appendages._--Amid the great variety of forms
assumed by the appendages of the Crustacea, it is possible to trace,
more or less plainly, the modifications of a fundamental type
consisting of a peduncle, the protopodite, bearing two branches, the
endopodite and exopodite. This simple biramous form is shown in the
swimming-feet of the Copepoda and Branchiura, the "cirri" of the
Cirripedia, and the abdominal appendages of the Malacostraca (fig. 3,
14). It is also found in the earliest and most primitive form of
larva, known as the _Nauplius_. As a rule the protopodite is composed
of two segments, though one may be reduced or suppressed and
occasionally three may be present. In many cases, one of the branches,
generally the endopodite, is more strongly developed than the other.
Thus, in the thoracic limbs of the Malacostraca, the endopodite
generally forms a walking-leg while the exopodite becomes a
swimming-branch or may disappear altogether. Very often the basal
segment of the protopodite bears, on the outer side, a lamellar
appendage (more rarely, two), the epipodite, which may function as a
gill. In the appendages near the mouth one or both of the protopodal
segments may bear inwardly-turned processes, assisting in mastication
and known as gnathobases. The frequent occurrence of epipodites and
gnathobases tends to show that the primitive type of appendage was
more complex than the simple biramous limb, and some authorities have
regarded the leaf-like appendages of the Phyllopoda as nearer the
original form from which the various modifications found in other
groups have been derived. In a Phyllopod such as _Apus_ the limbs of
the trunk consist of a flattened, unsegmented or obscurely segmented
axis or corm having a series of lobes or processes known as endites
and exites on its inner and outer margins respectively. In all the
Phyllopoda the number of endites is six, and the proximal one is more
or less distinctly specialized as a gnathobase, working against its
fellow of the opposite side in seizing food and transferring it to the
mouth. The Phyllopoda are the only Crustacea in which distinct and
functional gnathobasic processes are found on appendages far removed
from the mouth. The two distal endites are regarded as corresponding
to the endopodite and exopodite of the higher Crustacea, the axis or
corm of the Phyllopod limb representing the protopodite. The number of
exites is less constant, but, in _Apus_, two are present, the proximal
branchial in function and the distal forming a stiffer plate which
probably aids in swimming. It is not altogether easy to recognize the
homologies of the endites and exites even within the order Phyllopoda,
and the identification of the two distal endites as corresponding to
the endopodite and exopodite of higher Crustacea is not free from
difficulty. It is highly probable, however, that the biramous limb is
a simplification of a more complex primitive type, to which the
Phyllopod limb is a more or less close approximation.

, showing the
orifice of entrance (x) into the cavity overarched by the carapace in
which an appendage of the maxilliped (f) plays. On four feet (i, k, l,
m) are the rudiments of the lamellae which subsequently form the
brood-cavity. (Fritz Muller.)]

The modifications which this original type undergoes are usually more
or less plainly correlated with the functions which the appendages
have to discharge. Thus, when acting as swimming organs, the
appendages, or their rami, are more or less flattened, or oar-like,
and often have the margins fringed with long plumose hairs. When used
for walking, one of the rami, usually the inner, is stout and
cylindrical, terminating in a claw, and having the segments united by
definite hinge-joints. The jaws have the gnathobasic endites developed
at the expense of the rest of the limb, the endopodite and exopodite
persisting only as sensory "palps" or disappearing altogether. When
specialized as bearers of sensory (olfactory or tactile) organs, the
rami are generally elongated, many-jointed and flagelliform. This
modification is usually only found in the antennules and antennae, but
it may exceptionally be found in the appendages of the trunk, as, for
instance, in the thoracic legs of some Decapods (e.g.
_Mastigocheirus_). Very often one or other of the appendages may be
modified for prehension, the seizing of prey or the holding of a mate.
In this case, the claw-like terminal segment may be simply flexed
against the preceding in the same way as the blade of a penknife shuts
up against the handle. The penultimate segment is often broadened, so
that the terminal claw shuts against a transverse edge (fig. 4), or,
finally, the penultimate segment may be produced into a thumb-like
process opposed to the movable terminal segment or finger, forming a
perfect chela or forceps, as, for instance, in the large claws of a
crab or lobster. This chelate condition may be assumed by almost any
of the appendages, and sometimes it appears in different appendages in
closely related forms, so that no very great phylogenetic importance
can in most cases be attached to it. A peculiar modification is found
in the trunk-limbs of the Cirripedia (fig. 9), in which both rami are
multiarticulate and filiform and fringed with long bristles. When
protruded from the opening of the shell these "cirri" are spread out
to form a casting-net for the capture of minute floating prey.

Gills or branchiae may be developed by parts of an appendage becoming
thin-walled and vascular and either expanded into a thin lamella or
ramified. Some of the special modifications of branchiae are referred
to below.

_Special Morphology of Appendages._--In many Crustacea the eyes are
borne on stalks which are movably articulated with the head and which
may be divided into two or three segments. The view is commonly held
that these eye-stalks are really limbs, homologous with the other
appendages. In spite of much discussion, however, it cannot be said
that this point has been finally settled. The evidence of embryology
is decidedly against the view that the eye-stalks are limbs. They are
absent in the earliest and most primitive larval forms (nauplius),
and appear only late in the course of development, after many of the
trunk-limbs are fully formed. In the development of the Phyllopod
_Branchipus_, the eyes are at first sessile, and the lateral lobes of
the head on which they are set grow out and become movably
articulated, forming the peduncles. The most important evidence in
favour of their appendicular nature is afforded by the phenomena of
regeneration. When the eye-stalk is removed from a living lobster or
prawn, it is found that under certain conditions a many-jointed
appendage like the flagellum of an antennule or antenna may grow in
its place. It is open to question, however, how far the evidence from
such "heteromorphic regeneration" can be regarded as conclusive on the
points of homology. The fact that in certain rare cases among insects
a leg may apparently be replaced by a wing tends to show that under
exceptional conditions similar forms may be assumed by non-homologous
parts.

The antennules (or first antennae) are almost universally regarded as
true appendages, though they differ from all the other appendages in
the fact that they are always innervated from the "brain" (or preoral
ganglia), and that they are uniramous in the nauplius larva and in all
the Entomostracan orders. As regards their innervation an apparent
exception is found in the case of _Apus_, where the nerves to the
antennules arise, behind the brain, from the oesophageal commissures,
but this is, no doubt, a secondary condition, and the nerve-fibres
have been traced forwards to centres within the brain. In the
Malacostraca, the antennules are often biramous, but there is
considerable doubt as to whether the two branches represent the
endopodite and exopodite of the other limbs, and three branches are
found in the Stomatopoda and in some Caridea. In the great majority of
Crustacea the antennules are purely sensory in function and carry
numerous "olfactory" hairs. They may, however, be natatory as in many
Ostracoda and Copepoda, or prehensile, as in some Copepoda. The most
peculiar modification, perhaps, is that found in the Cirripedia
(Thyrostraca), in the larvae of which the antennules develop into
organs of attachment, bearing the openings of the cement-glands, and
becoming, in the adult, involved in the attachment of the animal to
its support.

The antennae (second antennae) are of special interest on account of
the clear evidence that, although preoral in position in all adult
Crustacea, they were originally postoral appendages. In the nauplius
larva they lie rather at the sides than in front of the mouth, and
their basal portion carries a hook-like masticatory process which
assists the similar processes of the mandibles in seizing food. In the
primitive Phyllopoda, and less distinctly in some other orders, the
nerves supplying the antennae arise, not from the brain, but from the
circum-oesophageal commissures, and even in those cases where the
nerves and the ganglia in which they are rooted have been moved
forwards to the brain, the transverse commissure of the ganglia can
still be traced, running behind the oesophagus.

The functions of the antennae are more varied than is the case with
the antennules. In many Entomostraca (Phyllopoda, Cladocera,
Ostracoda, Copepoda) they are important, and sometimes the only,
organs of locomotion. In some male Phyllopoda they form complex
"claspers" for holding the female. They are frequently organs of
attachment in parasitic Copepoda, and they may be completely pediform
in the Ostracoda. In the Malacostraca they are chiefly sensory, the
endopodite forming a long flagellum, while the exopodite may form a
lamellar "scale," probably useful as a balancer in swimming, or may
disappear altogether. A very curious function sometimes discharged by
the antennules or antennae of Decapods is that of forming a
respiratory siphon in sand-burrowing species.

The mandibles, like the antennae, have, in the nauplius, the form of
biramous swimming limbs, with a masticatory process originating from
the proximal part of the protopodite. This form is retained, with
little alteration in some adult Copepoda, where the biramous "palp"
still aids in locomotion. A somewhat similar structure is found also
in some Ostracoda. In most cases, however, the palp loses its
exopodite and it often disappears altogether, while the coxal segment
forms the body of the mandible, with a masticatory edge variously
armed with teeth and spines. In a few Ostracoda, by a rare exception,
the masticatory process is reduced or suppressed, and the palp alone
remains, forming a pediform appendage used in locomotion as well as in
the prehension of food. In parasitic blood-sucking forms the mandibles
often have the shape of piercing stylets, and are enclosed in a
tubular proboscis formed by the union of the upper lip (labrum) with
the lower lip (hypostome or paragnatha).

The maxillulae and maxillae (or, as they are often termed, first and
second maxillae) are nearly always flattened leaf-like appendages,
having gnathobasic lobes or endites borne by the segments of the
protopodite. The endopodite, when present, is unsegmented or composed
of few segments and forms the "palp," and outwardly-directed lobes
representing the exopodite and epipodites may also be present. These
limbs undergo great modification in the different groups. The
maxillulae are sometimes closely connected with the "paragnatha" or
lobes of the lower lip, when these are present, and it has been
suggested that the paragnatha are really the basal endites which have
become partly separated from the rest of the appendage.

The limbs of the post-cephalic series show little differentiation
among themselves in many Entomostraca. In the Phyllopoda they are for
the most part all alike, though one or two of the anterior pairs may
be specialized as sensory (_Apus_) or grasping (_Estheriidae_) organs.
In the Cirripedia (Thyrostraca) the six pairs of biramous cirriform
limbs differ only slightly from each other, and in many Copepoda this
is also the case. In other Entomostraca considerable differentiation
may take place, but the series is never divided into definite
"tagmata" or groups of similarly modified appendages. It is highly
characteristic of the Malacostraca, however, that the trunk-limbs are
divided into two sharply defined tagmata corresponding to the thoracic
and abdominal regions respectively, the limit between the two being
marked by the position of the male genital openings. The thoracic
limbs have the endopodites converted, as a rule, into more or less
efficient walking-legs, and the exopodites are often lost, while the
abdominal limbs more generally preserve the biramous form and are, in
the more primitive types, natatory. These tagmata may again be
subdivided into groups preserving a more or less marked individuality.
For example, in the Amphipoda (fig. 4) the abdominal appendages are
constantly divided into an anterior group of three natatory
"swimmerets" and a posterior group of three limbs used chiefly in
jumping or in burrowing. In nearly all Malacostraca the last pair of
abdominal appendages (uropods) differ from the others, and in the more
primitive groups they form, with the telson, a lamellar "tail-fan"
(fig. 3, T), used in springing backwards through the water. In the
thoracic series it is usual for one or more of the anterior pairs to
be pressed into the service of the mouth, forming "foot-jaws" or
maxillipeds. In the Decapoda three pairs are thus modified, and in the
Tanaidacea, Isopoda and Amphipoda only one. In the Schizopoda and
Cumacea the line of division is less sharp, and the varying number of
so-called maxillipeds recognized by different authors gives rise to
some confusion of terminology in systematic literature.

_Gills._--In many of the smaller Entomostraca (Copepoda and most
Ostracoda) no special gills are present, and respiration is carried on
by the general surface of the body and limbs. When present, the
branchiae are generally differentiations of parts of the appendages,
most often the epipodites, as in the Phyllopoda. In the Cirripedia,
however, they are vascular processes from the inner surface of the
mantle or shell-fold, and in some Ostracoda they are outgrowths from
the sides of the body. In the primitive Malacostraca the gills were
probably, as in the Phyllopoda and in _Nebalia_, the modified
epipodites of the thoracic limbs, and this is the condition found in
some Schizopoda. In the Cumacea and Tanaidacea only the first thoracic
limb has a branchial epipodite. In the Amphipoda, the gills though
arising from the inner side of the bases of the thoracic legs are
probably also epipodial in nature. In the Isopoda the respiratory
function has been taken over by the abdominal appendages, both rami or
only the inner becoming thin or flattened. In the Decapoda the
branchial system is more complex. The gills are inserted at the base
of the thoracic limbs, and lie within a pair of branchial chambers
covered by the carapace. Three series are distinguished,
_podobranchiae_, attached to the proximal segments of the appendages,
_pleurobranchiae_, springing from the body-wall, and an intermediate
series, _arthrobranchiae_, inserted on the articular membrane of the
joint between the limb and the body. The podobranchiae are clearly
epipodites, or, more correctly, parts of the epipodites, and it is
probable that the arthro- and pleurobranchiae are also epipodial in
origin and have migrated from the proximal segment of the limbs on to
the adjacent body-wall.

Adaptations for aerial respiration are found in some of the
land-crabs, where the lining membrane of the gill-chamber is beset
with vascular papillae and acts as a lung. In some of the terrestrial
Isopoda or woodlice (Oniscoidea) the abdominal appendages have
ramified tubular invaginations of the integument, filled with air and
resembling the tracheae of insects.

_Internal Structure: Alimentary System._--In almost all Crustacea the
food-canal runs straight through the body, except at its anterior end,
where it curves downwards to the ventrally-placed mouth. In a few
cases its course is slightly sinuous or twisted, but the only cases in
which it is actually coiled upon itself are found in the Cladocera of
the family _Lynceidae_ (_Alonidae_) and in a single
recently-discovered genus of Cumacea (Sympoda). As in all Arthropoda,
it is composed of three divisions, a fore-gut or stomodaeum,
ectodermal in origin and lined by an inturning of the chitinous
cuticle, a mid-gut formed by endoderm and without a cuticular lining,
and a hind-gut or proctodaeum, which, like the fore-gut, is ectodermal
and is lined by cuticle. The relative proportions of these three
divisions vary considerably, and the extreme abbreviation of the
mid-gut found in the common crayfish (_Astacus_) is by no means
typical of the class. Even in the closely-related lobster (_Homarus_)
the mid-gut may be 2 or 3 in. long.

In a few Entomostraca (some Phyllopoda and Ostracoda) the chitinous
lining of the fore-gut develops spines and hairs which help to
triturate and strain the food, and among the Ostracods there is
occasionally (_Bairdia_) a more elaborate armature of toothed plates
moved by muscles. It is among the Malacostraca, however, and
especially in the Decapoda, that the "gastric mill" reaches its
greatest perfection. In most Decapods the "stomach" or dilated portion
of the fore-gut is divided into two chambers, a large anterior
"cardiac" and a smaller posterior "pyloric." In the narrow opening
between these, three teeth (fig. 10) are set, one dorsally and one on
each side. These teeth are connected with a framework of movably
articulated ossicles developed as thickened and calcified portions of
the lining cuticle of the stomach and moved by special muscles in such
a way as to bring the three teeth together in the middle line. The
walls of the pyloric chamber bear a series of pads and ridges beset
with hairs and so disposed as to form a straining apparatus.

The mid-gut is essentially the digestive and absorptive region of the
alimentary canal, and its surface is, in most cases, increased by
pouch-like or tubular outgrowths which not only serve as glands for
the secretion of the digestive juices, but may also become filled by
the more fluid portion of the partially digested food and facilitate
its absorption. These outgrowths vary much in their arrangement in the
different groups. Most commonly there is a pair of lateral caeca,
which may be more or less ramified and may form a massive
"hepato-pancreas" or "liver."

1a, Stomach of common crab, _Cancer pagurus_, laid open, showing b,
b, b, some of the calcareous plates inserted in its muscular coat;
g, g, the lateral teeth, which when in use are brought in contact
with the sides of the median tooth m; c, c, the muscular coat.
1b' and 1b", The gastric teeth enlarged to show their grinding
surfaces.
2, Gastric teeth of common lobster, _Homarus vulgaris_.
3a and 3b, Two crustacean teeth (of _Dithyrocaris_) from the
Carboniferous series of Renfrewshire (these, however, may be the
toothed edges of the mandibles).]

The whole length of the alimentary canal is provided, as a rule, with
muscular fibres, both circular and longitudinal, running in its walls,
and, in addition, there may be muscle-bands running between the gut
and the body-wall. In the region of the oesophagus these muscles are
more strongly developed to perform the movements of deglutition, and,
where a gastric mill is present, both intrinsic and extrinsic muscles
co-operate in producing the movements of its various parts. The
hind-gut is also provided with sphincter and dilator muscles, and
these may produce rhythmic expansion and contraction, causing an
inflow and outflow of water through the anus, which has been supposed
to aid in respiration.

In the parasitic Rhizocephala and in a few Copepoda (_Monstrillidae_)
the alimentary canal is absent or vestigial throughout life.

_Circulatory System._--As in the other Arthropoda, the circulatory
system in Crustacea is largely lacunar, the blood flowing in spaces or
channels without definite walls. These spaces make up the apparent
body-cavity, the true body-cavity or coelom having been, for the most
part, obliterated by the great expansion of the blood-containing
spaces. The heart is of the usual Arthropodous type, lying in a more
or less well-defined pericardial blood-sinus, with which it
communicates by valvular openings or ostia. In the details of the
system, however, great differences exist within the limits of the
class. There is every reason to believe that, in the primitive
Arthropoda, the heart was tubular in form, extending the whole length
of the body, and having a pair of ostia in each somite. This
arrangement is retained in some of the Phyllopoda, but even in that
group a progressive abbreviation of the heart, with a diminution in
the number of the ostia, can be traced, leading to the condition found
in the closely related Cladocera, where the heart is a subglobular
sac, with only a single pair of ostia. In the Malacostraca, an
elongated heart with numerous segmentally arranged ostia is found only
in the aberrant group of Stomatopoda and in the transitional
Phyllocarida. In the other Malacostraca the heart is generally
abbreviated, and even where, as in the Amphipoda, it is elongated and
tubular, the ostia are restricted in number, three pairs only being
usually present. In many Entomostraca the heart is absent, and it is
impossible to speak of a "circulation" in the proper sense of the
term, the blood being merely driven hither and thither by the
movements of the body and limbs and of the alimentary canal.

A very remarkable condition of the blood-system, unique, as far as is
yet known among the Arthropoda, is found in a few genera of parasitic
Copepoda (_Lernanthropus_, _Mytilicola_). In these there is a closed
system of vessels, not communicating with the body-cavity, and
containing a coloured fluid. There is no heart. The morphological
nature of this system is unknown.

_Excretory System._--The most important excretory or renal organs of
the Crustacea are two pairs of glands lying at the base of the
antennae and of the second maxillae respectively. The two are probably
never functional together in the same animal, though one may replace
the other in the course of development. Thus, in the Phyllopoda, the
antennal gland develops early and is functional during a great part of
the larval life, but it ultimately atrophies, and in the adult (as in
most Entomostraca) the maxillary gland is the functional excretory
organ. In the Decapoda, where the antennal gland alone is
well-developed in the adult, the maxillary gland sometimes precedes it
in the larva. The structure of both glands is essentially the same.
There is a more or less convoluted tube with glandular walls connected
internally with a closed "end-sac" and opening to the exterior by
means of a thin-walled duct. Development shows that the glandular tube
is mesoblastic in origin and is of the nature of a coelomoduct, while
the end-sac is to be regarded as a vestigial portion of the coelom. In
the Branchiopoda the maxillary gland is lodged in the thickness of the
shell-fold (when this is present), and, from this circumstance, it
often receives the somewhat misleading name of "shell-gland." In the
Decapoda the antennal gland is largely developed and is known as the
"green gland." The external duct of this gland is often dilated into a
bladder, and may sometimes send out diverticula, forming a complex
system of sinuses ramifying through the body. The green gland and the
structures associated with it in Decapods were at one time regarded as
constituting an auditory apparatus.

In addition to these two pairs of glands, which are in all probability
the survivors of a series of segmentally arranged coelomoducts present
in the primitive Arthropoda, other excretory organs have been
described in various Crustacea. Although the excretory function of
these has been demonstrated by physiological methods, however, their
morphological relations are not clear. In some cases they consist of
masses of mesodermal cells, within which the excretory products appear
to be stored up instead of being expelled from the body.

_Nervous System._--The central nervous system is constructed on the
same general plan as in the other Arthropoda, consisting of a
supra-oesophageal ganglionic mass or brain, united by
circum-oesophageal connectives with a double ventral chain of
segmentally arranged ganglia. In the primitive Phyllopoda the ventral
chain retains the ladder-like arrangement found in some Annelids and
lower worms, the two halves being widely separated and the pairs of
ganglia connected together across the middle line by double transverse
commissures. In the higher groups the two halves of the chain are more
or less closely approximated and coalesced, and, in addition, a
concentration of the ganglia in a longitudinal direction takes place,
leading ultimately, in many cases, to the formation of an unsegmented
ganglionic mass representing the whole of the ventral chain. This is
seen, for example, in the Brachyura among the Decapoda. The brain, or
supra-oesophageal ganglion, shows various degrees of complexity. In
the Phyllopoda it consists mainly of two pairs of ganglionic centres,
giving origin respectively to the optic and antennular nerves. The
centres for the antennal nerves form ganglionic swellings on the
oesophageal connectives. In the higher forms, as already mentioned,
the antennal ganglia have become shifted forwards and coalesced with
the brain. In the higher Decapoda, numerous additional centres are
developed in the brain and its structure becomes extremely complex.

_Eyes._--The eyes of Crustacea are of two kinds, the unpaired, median
or "nauplius" eye, and the paired compound eyes. The former is
generally present in the earliest larval stages (nauplius), and in
some Entomostraca (e.g. Copepoda) it forms the sole organ of vision in
the adult. In the Malacostraca it is absent in the adult, or persists
only in a vestigial condition, as in some Decapoda and Schizopoda. It
is typically tripartite, consisting of three cup-shaped masses of
pigment, the cavity of each cup being filled with columnar retinal
cells. At their inner ends (towards the pigment) these cells contain
rod-like structures, while their outer ends are connected with the
nerve-fibres. In some cases three separate nerves arise from the front
of the brain, one going to each of the three divisions of the eye. In
the Copepoda the median eye may undergo considerable elaboration, and
refracting lenses and other accessory structures may be developed in
connexion with it.

The compound eyes are very similar in the details of their structure
(see ARTHROPODA) to those of insects (Hexapoda). They consist of a
varying number of ommatidia or visual elements, covered by a
transparent region of the external cuticle forming the cornea. In most
cases this cornea is divided into lenticular facets corresponding to
the underlying ommatidia.

As has been already stated, the compound eyes are often set on movable
peduncles. It is probable that this is the primitive condition from
which the sessile eyes of other forms have been derived. In the
Malacostraca the sessile eyed groups are certainly less primitive than
some of those with stalked eyes, and among the Entomostraca also there
is some evidence pointing in the same direction.

Although typically paired, the compound eyes may occasionally coalesce
in the middle line into a single organ. This is the case in the
Cladocera, the Cumacea and a few Amphipoda.

Mention should also be made of the partial or complete atrophy of the
eyes in many Crustacea which live in darkness, either in the deep sea
or in subterranean habitats. In these cases the peduncles may persist
and may even be modified into spinous organs of defence.

_Other Sense-Organs._--As in Arthropoda, the hairs or setae on the
surface of the body are important organs of sense and are variously
modified for special sensory functions. Many, perhaps all, of them
are tactile. They are movably articulated at the base where they are
inserted in pits formed by a thinning away of the cuticle, and each is
supplied by a nerve-fibril. When feathered or provided with secondary
barbs the setae will respond to movements or vibrations in the
surrounding water, and have been supposed to have an auditory
function. In certain divisions of the Malacostraca more specialized
organs are found which have been regarded as auditory. In the majority
of the Decapoda there is a saccular invagination of the integument in
the basal segment of the antennular peduncle having on its inner
surface "auditory" setae of the type just described. The sac is open
to the exterior in most of the Macrura, but completely closed in the
Brachyura. In the former case it contains numerous grains of sand
which are introduced by the animal itself after each moult and which
are supposed to act as otoliths. Where the sac is completely closed it
generally contains no solid particles, but in a few Macrura a single
otolith secreted by the walls of the sac is present. In the _Mysidae_
among the Schizopoda a pair of similar otocysts are found in the
endopodites of the last pair of appendages (uropods). These contain
each a single concretionary otolith.

Recent observations, however, make it very doubtful whether aquatic
Crustacea can hear at all, in the proper sense of the term, and it has
been shown that one function, at least, of the so-called otocysts is
connected with the equilibration of the body. They are more properly
termed statocysts.

Another modification of sensory setae is supposed to be associated
with the sense of smell. In nearly all Crustacea the antennules and
often also the antennae bear groups of hair-like filaments in which
the chitinous cuticle is extremely delicate and which do not taper to
a point but end bluntly. These are known as olfactory filaments or
aesthetascs. They are very often more strongly developed in the male
sex, and are supposed to guide the males in pursuit of the females.

_Glands._--In addition to the digestive and excretory glands already
mentioned, various glandular structures occur in the different groups
of Crustacea. The most important of these belong to the category of
dermal glands, and may be scattered over the surface of the body and
limbs, or grouped at certain points for the discharge of special
functions. Such glands occurring on the upper and lower lips or on the
walls of the oesophagus have been regarded as salivary. In some
Amphipoda the secretion of glands on the body and limbs is used in the
construction of tubular cases in which the animals live. In some
freshwater Copepoda the secretion of the dermal glands forms a
gelatinous envelope, by means of which the animals are able to survive
desiccation. In certain Copepoda and Ostracoda glands of the same type
produce a phosphorescent substance, and others, in certain Amphipoda
and Branchiura, are believed to have a poisonous function. Possibly
related to the same group of structures are the greatly-developed
cement-glands of the Cirripedia, which serve to attach the animals to
their support.

_Phosphorescent Organs._--Many Crustacea belonging to very different
groups (Ostracoda, Copepoda, Schizopoda, Decapoda) possess the power
of emitting light. In the Ostracoda and Copepoda the phosphorescence,
as already mentioned, is due to glands which produce a luminous
secretion, and this is the case also in certain members of the
Schizopoda and Decapoda. In other cases in the last two groups,
however, the light-producing organs found on the body and limbs have a
complex and remarkable structure, and were formerly described as
accessory eyes. Each consists of a globular capsule pierced at one or
two points for the entrance of nerves which end in a central
cup-shaped "striated body." This body appears to be the source of
light, and has behind it a reflector formed of concentric lamellae,
while, in front, in some cases, there is a refracting lens. The whole
organ can be rotated by special muscles. Organs of this type are best
known in the _Euphausiidae_ among the Schizopoda, but a modified form
is found in some of the lower Decapods.

_Reproductive System._--In the great majority of Crustacea the sexes
are separate. Apart from certain doubtful and possibly abnormal
instances among Phyllopoda and Amphipoda, the only exceptions are the
sessile Cirripedia and some parasitic Isopoda (_Cymothoidae_), where
hermaphroditism is the rule. Parthenogenesis is prevalent in the
Branchiopoda and Ostracoda, often in more or less definite seasonal
alternation with sexual reproduction. Where the sexes are distinct, a
more or less marked dimorphism often exists. The male is very often
provided with clasping organs for seizing the female. These may be
formed by the modification of almost any of the appendages, often the
antennules or antennae or some of the thoracic limbs, or even the
mandibular palps (some Ostracoda). In addition, some of the appendages
in the neighbourhood of the genital apertures may be modified for the
purpose of transferring the genital products to the female, as, for
instance, the first and second abdominal limbs in the Decapoda. In the
higher Decapoda the male is generally larger than the female and has
stronger chelae. On the other hand, in other groups the male is often
smaller than the female. In the parasitic Copepoda and Isopoda the
disparity in size is carried to an extreme degree, and the minute male
is attached, like a parasite, to the enormously larger female.

The Cirripedia present some examples of sexual relationships which are
only paralleled, in the animal kingdom, among the parasitic
Myzostomida. While the great majority are simple hermaphrodites,
capable of cross and self fertilization, it was discovered by Darwin
that, in certain species, minute degraded males exist, attached within
the mantle-cavity of the ordinary individuals. Since these dwarf males
pair, not with females, but with hermaphrodites, Darwin termed them
"complemental" males. In other species the large individuals have
become purely female by atrophy of the male organs, and are entirely
dependent on the dwarf males for fertilization. In spite of the
opinion of some distinguished zoologists to the contrary, it seems
most probable that the separation of the sexes is in this case a
secondary condition, derived from hermaphroditism through the
intermediate stage represented by the species having complemental
males.

The gonads, as in other Arthropoda, are hollow saccular organs, the
cavity communicating with the efferent ducts. They are primitively
paired, but often coalesce with each other more or less completely.
The ducts are present only as a single pair, except in one genus of
parasitic Isopoda (_Hemioniscus_), where two pairs of oviducts are
found. Various accessory structures may be connected with the efferent
ducts in both sexes. The oviducts may have diverticula serving as
receptacles for the spermatozoa (in cases where internal impregnation
takes place), and may be provided with glands secreting envelopes or
shells around the eggs. The male ducts often have glandular walls,
secreting capsules or spermatophores within which the spermatozoa are
packed for transference to the female. The terminal part of the male
ducts may be protrusible and act as an intromittent organ, or this
function may be discharged by some of the appendages, as, for
instance, in the Brachyura.

The position of the genital apertures varies very greatly in the
different groups of the class. They are farthest forward in the case
of the female organs of the Cirripedia, where the openings are on the
first thoracic (fourth postoral) somite. The most posterior position
is occupied by the genital apertures of certain Phyllopoda
(_Polyartemia_), which lie behind the nineteenth trunk-somite. It is
characteristic of the Malacostraca that the position of the genital
apertures is constantly different in the two sexes, the female
openings being on the sixth, and those of the male on the eighth
thoracic somite.

Very few Crustacea are viviparous in the sense that the eggs are
retained within the body until hatching takes place (some Phyllopoda),
but, on the other hand, the great majority carry the eggs in some way
or other after their extrusion. In some Phyllopoda (_Apus_) egg-sacs
are formed by modification of certain of the thoracic feet. The eggs
are retained between the valves of the shell in some Phyllopoda and in
the Cladocera and Ostracoda, and they lie in the mantle cavity in the
Cirripedia. In the Copepoda they are agglutinated together into masses
attached to the body of the female. Among the Malacostraca some
Schizopoda, the Cumacea, Tanaidacea, Isopoda and Amphipoda (sometimes
grouped all together as Peracarida) have a marsupium or brood-pouch
formed by overlapping plates attached to the bases of some of the
thoracic legs. In most of the Decapoda the eggs are carried by the
female, attached to the abdominal appendages (fig. 11). A few cases
are known in which the developing embryos are nourished by a special
secretion while in the brood-chamber of the mother (Cladocera,
terrestrial Isopoda).

_Embryology._