38 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



leg. The endopodite consists of five primitive joints, 

 which are well developed and form the ordinary 

 claw ; these five parts are the ischiopodite (is.), the 

 mesopodite (me.}, the carpopodite (ca.\ the propodite 

 (pr.\ and the dadylopodite (da.}. The exopodite is 

 missing. No vestige or rudiment of it is to be 

 found in any phase of the development of the cray- 

 fish. In the lobster, however, which is closely 

 allied to the cray-fish, the exopodite is still to be 

 found during the larval period. The third funda- 

 mental part of the primitive member persists also 

 in prawns throughout the entire period of life, but 

 the organ is very small. At the extremity of the 

 first and second pairs of walking legs there is an 

 apparatus consisting of a fixed part an elongation 

 of the protopodite and of a moveable part the 

 dadylopodite. This furnishes the walking leg with 

 a prehensile organ which is well developed in the 

 first pair of walking legs, and which is enormously 

 increased in the true claws. In this evolution 

 degeneration is exhibited by the disappearance of a 

 joint, for in these appendages the basipodite and the 

 ischiopodite are immovably united. This morpho- 

 logical degeneration corresponds to a functional 

 change in the appendage. So long as the claw was 

 used for locomotion a joint at this point was in- 

 dispensable for progression. It is this joint which, 

 in six or eight-footed beasts allows of the horizontal 

 motion of the member necessary for locomotion, 

 which in six-footed beasts results from the general 



