APPENDAGES OF TRUNK AND TAIL

45

peræon, has proposed to substitute for it 'banosome,' a word of precisely the same sense. The epithets chelate and sub-chelate are of constant occurrence in descriptions of Crustacea. A limb is chelate when it has joints that will act together like a pair of tongs. Generally this character is produced by the hingeing of the seventh joint a considerable way down on the side of the sixth. When the seventh joint or finger can be folded back upon the sixth, although the latter is not produced into any thumblike process to oppose it, the limb is then said to be subchelate, the claw being in that case partial, though often extremely efficient. The possession of chelæ is not confined to the first pair of so-called peræopods, although it is seldom elsewhere that they attain a monstrous development. They may occur on any of the pairs, and on several in the same animal. In connection both with the maxillipeds and the peræopods there are developed in great variety of form the branchiæ or gills, also the plates of the marsupium, wherein, in some groups, the eggs are retained for a time after their discharge from the ovaries; and again, in some groups, the exopods are developed as swimming organs. The vulvæ, or uterine openings of the female, belong to the sternal, that is the ventral, side of the tweltth segment, while the genital openings of the male occupy a similar position in the fourteenth segment. In those Crustacea which have the basal joints of each pair of legs brought close together, the openings in question have been transferred from the wall of the trunk to the first joint in each of the last pair of legs in the male, and of the antepenultimate pair in the female.

15, 16, 17, 18, 19, 20. The remaining segments belong to the tail or caudal portion of the animal, which has been termed the pleon or swimming-part, a convenient and often a very appropriate name, although on the other hand there are plenty of crustaceans which do not and cannot use the pleon to swim with. The first five of these segments frequently have appendages that are really natatory and may properly be called pleopods, swimmingfeet. But some or all may be wanting, or rudimentary,

or devoted to other purposes. They may be partially or entirely branchial. Among the Stalk-eyed Crustacea they are often used in the female for retaining the eggs during an early period of development or hatching. In the Amphipoda the fourth and fifth pairs are more or less adapted for springing, and bear the name of uropods, or tail-feet. This name is also given to the appendages of the twentieth segment whenever they are present. These are prominent features in the Cumacea and most other Edriophthalma, and in the Macrura they combine with the telson to form the powerful tail-fan, for which Mr. Spence Bate has proposed the Greek name rhipidura (see Plate X.). In the Copepoda there is a 'caudal furca,' homologous with the caudal rami in the Nebaliidæ, which must be distinguished from the terminal uropods of the higher Crustacea, as being not true limbs, but more properly representing a bipartite terminal segment.'1

ence.

21. The telson is extremely variable in form and relative size, and sometimes by coalescence with the preceding segment shows little trace of independent existThe intestinal canal opens on its under side. It is sometimes deeply cleft, as though the two terga, or dorsal plates, of a body-ring had come apart. To prove its claim to be regarded as a segment, the most effective argument would be to show that it sometimes carries appendages after the fashion of all the other segments. Bell, in the British Stalk-eyed Crustacea,' says that he has frequently observed appendages to the telson of the common prawn, Leander serratus (Pennant), 'in the form of extremely minute points attached to the very extremity of the segment, and moveable.' Spence Bate says, 'In some genera, or even families, the telson is posteriorly rounded, as in the Astacidae; in others it is anteriorly hard and calcareous, and posteriorly soft and membranous, as in the Synaxidea, a circumstance that is suggestive of a distinct relationship of the two parts, the anterior which carries the anus belonging to the normal somite,

p. 35.

Sars. Report on the Phyllocarida collected by H.M.S. Challenger,

INTERNAL DIFFERENCES

4.7

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.'1 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 :