CHAPTER 14 



THE CRAYFISH AND THE ARTHROPODA 



The crayfish is a member of the Class Crustacea in the Ph} lum 

 Arthropoda. In studying the Arthropoda, we find again a type of 

 structure resembling that of the Annulata, since the bilaterally 

 symmetrical body is metameric and there are paired appendages. 

 The general organization is, however, far more specialized than 

 that which is found among the annulates. In two other great 

 invertebrate phyla, the Mollusca and Echinodermata, which are 

 not described in the present volume, the body is non-metameric. 

 Crustacea are for the most part aquatic animals, although a few 

 species are terrestrial in their mode of life. Taking the Crustacea 

 alone among the Arthropoda, it appears that the members of this 

 class present the same general conditions of structure and habitat 

 as that indicated for the Annulata. The sea is the home of the vast 

 majority of existing crustaceans and was presumably their original 

 home. From this primaeval habitat the ancestors of forms like the 

 crayfish and other crustaceans now living in lakes and stream,s 

 seem to have migrated into fresh water at various times in the past. 

 The few species of land-dwelling crustaceans are clearly offshoots 

 of a group that is primarily aquatic, just as the earthworms are 

 fresh-water oligochsetes which have become adapted for terrestrirJ 

 life in moist soil. Two other great classes of Arthropoda, the 

 Insecta, or insects, and the Ai'achnida, which includes the spiders 

 and scorpions, are, however, thoroughly adapted to terrestrial 

 life. The distinctive characteristics of the Arthropoda will 

 become apparent during the study of the representative types that 

 follows. 



The Crayfish 



Occurrence and Distribution. — Crayfish have been widely 

 used in zoological study to illustrate certain general principles as 

 well as the structure of a crustacean, since Huxley, in 1880, made 



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