202 MANUAL OF ZOOLOGV. 



ever, the segments of the head and thorax are welded together 

 into a single mass, called the "cephalothorax;" in which case 

 the body shows only two distinct divisions, of which the cephalo- 

 thorax claims fourteen segments, whilst the remaining seven 

 are allotted to the abdomfen. By Professor Huxley, on the 

 other hand, the terminal joint of the abdomen, termed the 

 "telson," is regarded as an appendage, and not as a somite. 

 Upon this view, the body of a typical Crustacean will consist 

 of twenty segments only. Professor Huxley, further, differs 

 from the above-mentioned view in the allotment of the somites, 

 and he divides the body into six cephalic, eight thoracic, and 

 six abdominal somites.* Fritz Miiller and Claus deny that the 

 eyes are limbs, or that there is an ocular segment. The telson, 

 on the other hand, is regarded by the former as a true somite, 

 chiefly because the intestine usually opens in this piece. 



Whilst the normal number of segments in the body of any 

 Crustacean may thus be regarded as being twenty-one, or 

 twenty, there occur cases in which this number is exceeded, 

 and others in which the number of somites is apparently less. 

 In these latter cases, however, the apparent diminution in the 

 number of segments is really due to some having been fused 

 together, as is shown by the number of appendages, since each 

 pair of appendages indicates a separate somite. In other cases, 

 however, in which the number of somites is really less than the 

 normal, this is due to an arrest of development. According to 

 Milne- Edwards : — • 



" In the embryo these segments are formed in succession 

 from before backwards, so that, when their evolution is checked, 

 the later, rather than the earlier, rings are those which are 

 wanting ; and, in fact, it is generally easy to see in those speci- 

 mens of full-grown Crustaceous animals whose bodies present 

 fewer than twenty-one segments, that the anomaly depends on 

 the absence of a certain number of the most posterior rings of 

 the body." According to Dana, however, the abortion of 

 segments, with their appendages, almost always takes place at 

 the posterior end of the cephalothorax. 



In no single example can a general view be obtained of the 

 different segments and their appendages in the Crustacea., 

 " Indeed, the only segment that may be said to be persistent, 

 is that which supports the mandibles, for the eyes may be 



* In reality the five hindmost segments of the eight somites here allotted 

 to the thorax, should alone be regarded as constituting the abdomen proper, 

 — that is, the region corresponding to the " abdomen " of insects and Arach- 

 nida. The six somites allotted above to the abdomen belong to vphat is 

 strictly called the " fosl-abdomeit" of the Crustacea . 



