o2 A HISTORY OF RECENT CRUSTACEA 
CHAPTER IV 
ON STRUCTURE 
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 cephalo-thoraz 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 Hntomos- 
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 
