THE CRUSTACEA 



iintennules. Whether or not these rings correspond to the primitive 

 somites, their distinctness in the highly specialised Stomatopoda is 

 clearly a secondary condition since it is not found in their larvae nor 

 in any of the more primitive Malacostraca. For the same reasons 

 no great morphological significance can be attributed to the less 

 distinctly marked skeletal areas described as representing the 

 ophthalmic, antennular, and antennal sternites in the higher 

 Decapoda. 



In nearly all cases the post-cephalic somites can be further 

 grouped into regions or tagmata distinguished by the shape of the 

 somites or the character of their appendages. In descriptive 

 carcinology two such regions are commonly distinguished as tliorax 

 and ahdomcn, but it must be pointed out that there is no morpho- 

 logical equivalence lietween the tagmata so named in difi'erent 

 groujjs. Throughout the Malacostraca, the thorax of eight and the 

 abdomen of six somites are sharply distinguished by the appendages. 

 In the other sub-classes the same names are sometimes applied to 

 the limb-bearing and limbless regions of the trunk, while in the 

 Branchiopoda they may denote respectively the regions in front of 

 and behind the genital apertures. 



The total number of post-cephalic somites varies within very 

 wide limits. In the Ostracoda, where the body is not distinctly 

 ■ segmented, the number of trunk-limbs does not exceed two pairs. 

 In some Branchiopoda the number of trunk-somites exceeds forty. 



A structure which, from its occurrence in the most diverse 

 groups of Crustacea, is probably a primitive attribute of the Class, 

 is the dorsal shield or carapace, originating as a fold of the 

 integument from the posterior margin of the cephalic region. In 

 its simplest form, as seen in Apus among the Branchiopoda, the 

 carapace loosely envelops more or less of the trunk. In many 

 Branchiopoda and in the Ostracoda it forms a bivalve shell 

 completely enclosing the body and limbs. In the Cirripedia it 

 forms a fleshy " mantle " usually strengthened by shelly plates. 

 In many cases among the Malacostraca the carapace coalesces with 

 the tergites of some or all of the thoracic somites, though it may 

 project freely at the sides, overhanging, as in the Decapoda, the 

 branchial chambers. 



It is possible that, in those cases where some of the post-cephalic 

 somites seem to be simply fused with the head-region, a reduced 

 shell-fold is also involved in the coalescence. This is most probably 

 the case in tlie Isopoda and Amphipoda, where the fusion of the 

 first thoracic somite with the head is clearly the last vestige of a 

 shell-fold, traceable, with progressively diminishing extent, from the 

 primitive Mysidacea through the Cumacea and Tanaidacea. In the 

 Copepoda, on the other hand, in which one or two trunk-somites 

 coalesce with the head, there is less evidence that the dorsal 



