4 THE LIFE OF CRUSTACEA 



afterwards to note the divergences from this type 

 presented by other members of the group. Speaking 

 very generally, it may be said that these divergences 

 are of two kinds. On the one hand there are char- 

 acters that have no apparent relation to the animal's 

 habits and mode of life, and on the other hand there 

 are modifications of structure which are more or less 

 plainly of use to the animal. It is to characters of 

 the former class that we look for evidence of an 

 animal's affinities, and it is upon them that our 

 systems of classification are chiefly based. The 

 characters of the second class — " adaptive " char- 

 acters, as they are called — become of importance 

 when we study the animal ''as a going concern," 

 so to speak, and endeavour to understand how its 

 life is carried on in relation to its surroundings. 



In pursuance of this plan of study, the next chapter 

 will be devoted to a description of the Common 

 Lobster as a type of the Crustacea. In the third 

 chapter a survey of the classification of the group 

 will be given ; since, however, the characters on 

 which the classification is based cannot be explained 

 fully without entering into technical details which 

 are beyond the scope of this work, this survey will 

 be restricted to what is necessary for comprehension 

 of the succeeding chapters. In the fourth chapter 

 some account is given of the young or larval stages 

 of Crustacea, and of the changes they undergo in the 

 course of development. 



