174 THE GENESIS OF SPECIES. [CHAP. 



ample of serial homolpgy. The body (except at its two 

 ends) consists of a longitudinal series of similar segments 

 Each segment supports a pair of limbs, and the appendages 

 of all the segments (except as before) are completely alike 

 A less complete case of serial homology is presented by 

 Crustacea (animals of the crab class), notably by the squilla. 

 and by the common lobster. In the latter animal we have 



SQtTILLA. 



a six-jointed abdomen (the so-called tail), in front of which 

 is a large solid mass (the cephalo-thorax), terminated ante- 

 riorly by a jointed process (the rostrum). On the under 



