INTERNAL DIFFERENCES

47

while the posterior portion represents its appendages. This idea is still more strongly suggested in the genus Cheiroplatea, where the separation of the posterior from the anterior division is clearly defined by a distinct membranous articulation, and the posterior portion is divided into two lateral lobes.' The older genus Porcellana is even more to the purpose than Cheiroplatea, and Miss J. M. Arms, in the Manual before referred to, considers that it settles the question. Comparing a species of it with the lobster, "This curious little crab,' she says, 'possesses a telson with an unmistakable pair of appendages attached to it, proving that this part is really a ring whose appendages are wanting in the lobster.' It must, however, be remarked that neither in the Porcellanidæ nor in the Galatheidæ do these apparent appendages of the telson ever become freely articulated with it, and as they are the last to put in any appearance at all, and then only in a late stage of the animal's development, it remains a question whether they may not be dividing lines of the telson rather than appendages arising from it.

In the internal organs of crustaceans the differences are as great as in the external. One writer has even undertaken to classify the Brachyura according to the structure of their stomachs. Unless this part of the organism were tolerably complicated, it will be easily understood that it would not afford sufficient variations for such

a purpose. But though, for establishing a really natural system, every stage of an animal's development and all its parts ought to be studied and taken into account, surely a systematist ought to aim at founding his classification as far as possible on the most accessible stages and the parts most easily observed. At any rate the general student will have little inclination to arrange his collection by investigating in the different specimens the walls of the stomach and the teeth and hairs within it, although he may occasionally be pleased to observe in that of the lobster the three horny-looking grinders, the central one

1 Spence Bate. Report on the Macrura collected by H.M.S. Challenger, p. xlviii.

of which has from of old been fancifully called 'the lady in the chair.' The character of all the internal organs of

FIG. 1.-A lobster's stomach opened to show the teeth, the central one of which has been supplied with eyes, nose, and mouth, to represent the lady in the chair.' [Herbst]

a crustacean, as exemplified in the crayfish, has been already discussed in detail by Professor Huxley in a previous volume of this series. It may here, therefore, suffice to recall that in a crustacean the heart is dorsal, the nervechain, with the exception of the brain, ventral, and the alimentary canal central, having in proximity the hepatic lobes or liver, and the testes and ovaries. Some of the more or less striking peculiarities which prevail in different groups in regard to these organs are reserved for mention as occasion offers in the description of the several suborders and their families.

The following table supplies a synopsis of all the leading groups of the Crustacea. The literal meaning of the various names has been explained in the first chapter :

CHAPTER V

THE SUB-CLASS MALACOSTRACA

THE head and trunk are together composed of thirteen, or, if an ophthalmic ring be included, of fourteen segments. The caudal part or pleon is composed of six segments and a telson. The trunk is clearly distinguished from the pleon, but some part of it is always more or less closely united with the head. To every segment normally belongs a pair of jointed appendages. The eyes are either pedunculate, and limited to two in number, with rarely a pair of accessory ocelli, or they are sessile, and then generally two, but sometimes four, or with the components variously distributed. There are two pairs of antennæ, a pair of mandibles, and two pairs of maxillæ. Of the next eight pairs of appendages, from one to three are maxillipeds, organs of the mouth, the remainder, from seven to five in number, being prehensile or locomotive. All these are typically seven-jointed. Like the second antennæ and second maxillæ they may either have or be without an exopod on the second joint, and they may also have or be without an epipod on the first. The six pairs of appendages of the pleon, when present, generally have an exopod. The last pair almost always differs in character from the rest. The paired appendages of the mouth work from the sides, the oral aperture itself being fringed by the labrum or upper lip above, and the bifid labium or lower lip below. A short oesophagus leads up into the stomach. The intestinal tube terminates in the under side of the telson. The heart which is dorsally placed has lateral openings for the entrance of the blood that has been oxygenated in the branchiæ. These slits are in one, two,

or three pairs, only in the Squillidæ exceeding that number. The ganglia of the same pair are situated close to one another, though the commissures may stand a little apart. By the dorsal and lateral extension backwards and generally also forwards of one (or two) of the cephalic segments a shield or carapace is formed covering at least some part of the trunk and sometimes all of it.

in

The above characters will suffice for a descriptive definition of the Malacostraca, but it may be proper to remind the reader that the segments are sometimes so intimately coalesced that their separate identity is entirely obscured, and that moreover almost any pair of the appendages, even one so seemingly indispensable as the mandibles, may certain cases be missing. Absence of eyes is by no means infrequent, and the telson, though perhaps never properly speaking absent, is often, by its close union with the preceding segment, so withdrawn from recognition, that in practice it is spoken of as absent.

Order 1.-Podophthalma.

In this order there is normally a pair of compound eyes on movable stalks, the eyes being sometimes absent but never sessile; the dorsal shield or carapace extends back over the ninth segment or further.

Sub-order 1.-Brachyura.

The carapace extends over the whole head and trunk, with occasional exception of the trunk's ultimate and penultimate segments, and is longer than the pleon. In the carapace are excavated orbits and fossettes, hollows respectively adapted to receive the stalked eyes and the short first antennæ. The third maxillipeds have some of the joints broad and flat, and they form a more or less complete operculum to the well-defined mouth cavity. The following pair of appendages are perfectly chelate limbs, commonly called the chelipeds. The next four pairs are adapted for walking or swimming, or rarely may have a prehensile character. In the sternal plastron, or breast