ADAPTATIONS 199 



structures which, with growth, lose their generalized structure 

 and become specialized. In different regions of the same 

 organism and in different organisms, therefore, the same general- 

 ized type may become adapted for quite diverse activities. 

 Thus the fourth pair of thoracic appendages in Mysis (Schizopod) , 

 Squilla (Stomatopod), and the lobster (Decapod) are, at some 

 period in development the same in structure and resemble one 

 another, but in Mysis they remain biramous and primitive, 

 in Squilla they become modified into characteristic maxillipeds 

 or food getting organs, and in the lobster they change into the 

 powerful offensive and defensive chelate walking legs character- 



FIG. 87. A schizopod (Gnath'ophausia gigas) with permanent biramous tho- 

 racic appendages (cf. schizopod stage in development of the lobster, Fig. 78). 

 (From Sars.) 



istic of the Decapods. Such differences form the basis of 

 animal species. 



A. ANIMAL DESCENT 



The lesson taught by the structures and functions of the 

 lobster's appendages may be extended to all animals. As the 

 specialized appendages of different Crustacea may be traced 

 back to generalized structures, so may animals no matter how 

 modified, be traced back more or less accurately, to more 

 generalized types from which they have descended. This 

 descent is often difficult to make out for some of the primitive 

 structures may have become so modified through functional 

 activity or otherwise, as to be unrecognizable; others, like the 



