52 THE THEORY OF EVOLUTION 



posterior pairs are walking legs, which correspond 

 to the internal division of the swimmerets. In front 

 of the legs is the pair of great claws, which are com- 

 posed of the same number of joints, arranged in the 

 same way, only very much larger, heavier and more 

 powerful. In front of the claws come three pairs 

 of appendages called :< foot-jaws' (maxillipeds) 

 which are legs with their basal joints forming organs 

 of mastication; the hindmost pair of the foot- jaws 

 differs but little from the walking legs, while in the 

 foremost pair it is difficult to make out the corre- 

 sponding parts. Next in order come three pairs of 

 jaws, still more modified, then the very long and 

 many jointed feelers (antennae) and the anterior 

 pair of much shorter feelers (antennules) . The ap- 

 pendages of the head, body and tail are thus seen 

 to be modifications of a common plan and grade into 

 each other almost imperceptibly. 



If representatives of the many families and orders 

 of the Crustacea be compared, it is seen that there 

 is great variety in the number of segments which 

 are united to form the head and trunk, as well as 

 in the form and function of the appendages. The 

 appendages of the same segment, counting from the 

 anterior end, are in one group a pair of jaws, in 

 another foot- jaws and in a third walking legs. The 

 comparison immediately suggests the derivation of 

 all the Crustacea from ancestors in which all the 

 segments, except the head, were similar and were 

 provided with appendages which were similar 



