CRUSTACEA 297 



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. 237) and most concho- 

 stracans (Fig. 232) it encloses the whole body, extending forwards 

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

 244, most cladocera, Fig. 233), it only leaves part or the whole of 

 the head uncovered. In typical malacostraca it covers the thorax 

 (Fig. 269), but in some it is a short jacket, leaving several thoracic 

 somites uncovered (Fig. 259), and in some (the Syncarida, Isopoda, 

 and Amphipoda, Figs. 256, 260, 264) it has disappeared. In the 

 Anostraca (Fig. 225) and Copepoda (Fig. 238) it was perhaps never 

 present. It may be a broad, flat shield over the back, as in Apus 

 (Fig. 231), but is usually compressed, and in the Conchostraca 

 and Ostracoda becomes truly bivalve, with a dorsal hinge and an 

 adductor muscle. In the Cirripedia it is an enveloping mantle, 

 usually strengthened by shelly plates (Fig. 246). It may fuse with 

 the dorsal side of some or all of the thoracic somites (the Cladocera, 

 most of the Malacostraca): such somites are not on that account 

 alone to be regarded as included in the head, though they may become 

 so. The chamber enclosed by the carapace is known in various 

 cases by various names as gill chamber, mantle cavity, etc., and 

 performs important functions in sheltering gills or embryos, directing 

 currents of water which subserve feeding or respiration, etc. In front, 

 the carapace is continuous with the dorsal plate which represents the 

 terga of the head, the cervical groove, if present, markingthe boundary 

 between them. We shall apply the term dorsal shield to the structure 

 composed of the dorsal plate of the head vnth. the carapace, if the 

 latter be present.^ The dorsal plate of the head may be prolonged in 

 front as a projection which is called the rostrum (Fig. 269, rs.). 



A glandular patch or patches on the dorsal surface of the head, near 

 its hinder limit, in many of the Branchiopoda, in Anaspides, and in the 



* These terms have been used in various senses. In the usage here pro- 

 posed, when there is no carapace fold the dorsal shield is the dorsal plate 

 of the head together with the terga of the somites (if any) that are fused with 

 the head. 



