CRUSTACEA 333 



to the other. The head of the Crustacea is unlike, and less specialized 

 than, those of other arthropods in that its limbs are not entirely re- 

 stricted to sensory and alimentary functions but often have also other 

 uses, such as swimming, the setting up of currents, or prehension. 



The head, though it varies in extent, is of the same nature through- 

 out the group, being primarily, like the heads of other animals, the 

 seat of the principal organs of special sense and of rtianducation. On 

 the other hand, the two tagmata known as the thorax and abdomen, 

 which usually can be recognized in, and together compose, the post- 

 cephalic part of the body or trunk, vary much more in extent, and each 

 of them has in the several groups no constant feature save its position 

 relative to the other. The precise boundary between thorax and ab- 

 domen is sometimes difficult to fix. The names, as they are com- 

 monly used, are in this respect inconsistently applied, denoting in 

 some groups limb-bearing and limbless regions, in others the sections 

 of the trunk which lie before and behind the genital openings. For 

 the sake of consistency we shall adopt the convention that the somite 

 which bears the genital openings (or the hinder such somite when, as 

 sometimes happens, the male opening is on a somite behind that of 

 the oviduct) is always the last somite of the true thorax. In this sense, 

 in certain cases (copepods, cladocera), somites which are commonly 

 called abdominal are strictly to be reckoned as thoracic. In respect of 

 segmentation the trunk varies from the condition of a limbless stump 

 in certain ostracods to the possession of more than sixty somites in 

 some of the Branchiopoda. 



A structure very commonly found in crustaceans is the shell or 

 carapace, a dorsal fold of skin arising from the hinder border of the 

 head and extending for a greater or less distance over the trunk. Its 

 size varies greatly. In the Ostracoda (Fig. 248) and most concho- 

 stracans (Fig. 243) it encloses the whole body, extending forwards 

 at the sides so as to shut in the head. In other cases (cirripedes, Fig. 

 255, most cladocera, Fig. 244), it only leaves part or the whole of the 

 head uncovered. In typical malacostraca it covers the thorax (Fig. 

 283), but in some it is a short jacket, leaving several thoracic somites 

 uncovered (Fig. 272), and in some (the Syncarida, Isopoda, and Am- 

 phipoda, Figs. 269, 274, 278) it has disappeared. In the Anostraca 

 (Fig. 236) and Copepoda (Fig. 249) it was perhaps never present. It 

 may be a broad, flat shield over the back, as in Apus (Fig. 242), but 

 is usually compressed, and in the Conchostraca and Ostracoda be- 

 comes truly bivalve, with a dorsal hinge. In the Cirripedia it is an 

 enveloping mantle, usually strengthened by shelly plates (Fig. 257). 

 In the Conchostraca, Ostracoda, Leptostraca, and Cirripedia it has 

 an adductor muscle, but the adductors of these groups vary in position 

 and are not homologous. The carapace may fuse with the dorsal side 



