328 THE CRAYFISH AND THE ARTHROPODA 



plan that was recognized in the appendages of the abdomen; but 

 a part of the structure has disappeared in the course of the devel- 

 opment, just as one or more digits may be present in the embryo 

 and not in the adult of a vertebrate animal. 



The homologies of the first and second maxillipeds, the first 

 and second maxillw, the mandibles and the antennce may be sim- 

 ilarly estabUshed by comparisons between their adult structure 

 and development. The mandible, for example, is a biramous 

 appendage in the embryo but the exopodite is lost during later 

 stages, while the protopodite and endopodite become modi- 

 fied. The antennules are the only pair of appendages regarding 

 which there may be question as to the fundamental structure. 

 They are in fact biramous; yet it is possible that they are not 

 body appendages, with protopodite, exopodite, and endopodite, 

 Uke all the others, but were originally sense-organs of the head 

 like the cirri of an annulate (cf. Fig. 152 A, p. 313). The antennm, 

 on the other hand, are probably a pair of appendages that were 

 originally posterior to the mouth and have shifted anteriorly to 

 function as sense-organs, while the next pair posteriorly have 

 shifted to a position on either side of the mouth to become the 

 mandibles. 



Thus, within the body of a single animal, are found the modi- 

 fications of a similar plan of structure to suit the needs of func- 

 tions so diverse as sense-organs, jaws, walking legs, sexual organs, 

 and tail-fins. Although this serial homology is confined to the 

 single animal, it illustrates the same principle of structural resem- 

 blance underlying seeming diversity that is found in comparing 

 the limbs of vertebrates {cf. Fig. 264, p. 501). In the one instance, 

 the resemblance can be explained as due to the antero-posterior 

 differentiation that has gradually arisen during the evolution of a 

 metameric animal from ancestors whose somites were more nearly 

 similar throughout the body. In the other, the resemblance is 

 due to the descent of different types of animals from a common 

 ancestral race from which they have inherited their common 

 structural organization. 



General Activities. — In its natural surroundings the crayfish 

 is found crawling upon the bottom, or concealed among the rocks 

 and within the burrows that it excavates in the banks of streams 

 and ponds. It walks slowly forward by means of the pereiopods, 

 holding the great claws menacingly in front of the body; or swims 



