2/6 MANUAL OF ZOOLOGY. 



of metamorphoses before attaining their adult condition, and 

 every part that is found in an embryonic form, even though 

 only temporarily developed, may be represented in a perma- 

 nent condition in some member of a lower order. 



As regards the classification of the Crustacea, the tabular 

 view which follows embodies the arrangement which is most 

 generally adopted, and the diagnostic characters of each order 

 will be briefly given, a more detailed description being reserved 

 for the more important divisions of the class. Before proceed- 

 ing further, however, it will be as well to give a description of 

 the morphology of a typical Crustacean, selecting the lobster 

 as being as good an example as any. 



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 authorities 

 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 ab- 

 dominal 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 



* 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. 



