172 MANUAL OF ZOOLOGY. 



is generally from five to seven. They all pass through a series 

 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. 



The classification of the Crustacea is extremely complicated, 

 and hardly any two writers adhere to the same arrangement. 

 The tabular view which follows embodies the arrangement 

 which appears to be most generally adopted, and the dia- 

 gnostic characters of each order will be briefly given, a more 

 detailed description being reserved for the more important divi- 

 sions of the class. Before proceeding 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 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 

 cephalothorax 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, diifers 

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

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

 and six abdominal somites. 



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 num- 

 ber 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, how- 

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

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

 Milne-Edwards : 



