SECT. Ill THE APPENDAGES 43 



rule, that the more the forehead is pressed against the 

 ground, the antennae pointing backwards, the more 

 Hkely woiikl the latter be to degenerate, as in Apus, 

 and in the Trilobites (?), or by losing the sensory 

 function to become modified as in Limulus. On the 

 other hand, the more the animal adopted the swimming 

 manner of life, the more the antennae would tend to 

 point forwards as sensory organs, and would then 

 undergo further development as such. This we see in 

 some Eurypteridse, Branchipus, and Nebalia. 



The Apodidae thus furnish us with a complete 

 explanation of the way in which the typical Crustacean 

 head is composed of five fused Annelidan segments 

 with their parapodia, and why the limbs of the head 

 differ from those of the trunk throughout the whole 

 class. The former (leaving out the antenna) develop 

 chiefly the ventral parapodia for mastication, the 

 latter the dorsal for locomotion and for other functions 

 which will be discussed in the following pages. In 

 Apus we find a transition between the two, the last 

 head segment having both dorsal and ventral para- 

 podia about equally developed. 



The important modifications, however, which the 

 cephalic limbs may present, will be referred to again 

 when discussing Limulus, the Trilobites and the 

 Eurypterida^, and are tabulated p. 250. 



