1 92 MANUAL OF ZOOLOGY. 



In addition to these characters, the body in the Crustacea 

 is always protected by a chitinous or sub-calcareous exoske- 

 leton, or " crust," and the number of pairs of articulated limbs 

 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 permanent 

 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 diagnostic 

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

 description being reserved for the more important divisions 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 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.* 



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, 



* 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. 



