32 A HISTOEY OF EECENT CEUSTACEA 



CHAPTER IV 



ON STRUCTUEE 



The body of a crustacean is externally divided into a theo- 

 retically constant number of segments and paired append- 

 ages. In the Malacostraca, it generally exhibits a more or 

 less clear partition into head, trunk, and tail, or, as these 

 parts are sometimes called, cephalon, thorax, and abdomen. 

 There is indeed a rather bewildering supply of alternative 

 names, which it is needless for the moment to discuss. 

 The objections to the use of the word thorax have been 

 already mentioned. Throughout the sub-class, however, 

 the head is found to be united to some portion of the trunk, 

 and to denote this variable combination the convenient 

 word cejjhalo-tJiorax is very frequently employed for the 

 region covered by the carapace. It is a matter of opinion 

 whether the full number of true segments should be 

 reckoned as twenty or twenty-one, since that which is 

 called the telson, and which is regarded by many authors 

 as the terminal segment, is considered by others not to be 

 a true twenty-first segment, but a median outgrowth of the 

 twentieth. Scarcely ever can the whole number be dis- 

 tinguished in one and the same animal. As the Entomos- 

 traca always have more or fewer than the theoretical num- 

 ber, there is some need for the eye of faith to include them 

 in the reckoning. The foundation of the integument 

 which forms the external skeleton of a crustacean is a 

 tolerably flexible substance called chitin. This is hardened 

 and consolidated by being impregnated with calcareous 

 salts, and the absence of these leaves the skin the requisite 

 flexibility for acting as a joint between one hardened part 



