SECT. Ill THE APPENDAGES 45 



motory, arc the more highly speciaHsed. The max- 

 ilHpedes are secondary modifications only in the 

 degeneration of their dorsal parapodia which, as the 

 limbs of the first free segment, typically developed 

 into large locomotory limbs. 



In Apus, the gnathobases of the anterior trunk 

 limbs are retained, and are doubtless functional. In 

 Limulus those of the first pair even work in front of 

 the under lips ; the same is true of the Trilobites, and 

 of the Eurypteridae, in some of the latter they have 

 taken on almost the whole function of mastication. 



The extraordinary number of the limbs, and their 

 gradual simplification from before backward, i.e., from 

 complicated Crustacean limbs to parapodium-like 

 swimming plates, has been already discussed and 

 described. 



The following points are also of interest. Unlike 

 the limbs of the head, in which, for the purpose of 

 pushing food into the mouth, the ventral parapodia 

 were developed at the expense of the dorsal, the 

 trunk limbs develop the dorsal at the expense of the 

 ventral parapodia. 



Apus shows its primitive character in that the 

 ventral parapodia do not disappear, but are retained 

 as functional appendages to the limbs for pushing 

 food towards the mouth. In the higher Crustacea, 

 however, they have, as a rule, entirely disappeared, 

 except in a few anterior trunk limbs. 



The reason of the greater development of the 

 dorsal than of the ventral parapodia in the trunk 

 limbs of Apus may be conjectured ; the greater sweep 



