22O MANUAL OF ZOOLOGY. 



The body of a typical Crustacean may be divided into three 

 regions a head, a thorax, and an abdomen, each of which is 

 composed of a certain number of somites, though opinions 

 differ both as to the number of segments in each region, and as 

 to their number collectively. By the majority of writers the 

 body is looked upon as being typically composed of twenty-one 

 segments, of which seven belong to the head, seven to the 

 thorax, and seven to the abdomen. In many Crustacea, how- 

 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 abdomen. 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 Glaus 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. Accord- 

 ing to Milne-Edwards : 



" In the embryo these segments are formed in succession 

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

 the latter, 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 



* 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 what is 

 strictly called the " post abdomen " of the Crustacea. 



