242 THE APODID^: PART u 



comparative smallness of the metastoma. Thus then 

 the entrance to the mouth may have come to have 

 almost an anterior-ventral instead of a posterior- 

 ventral aspect. This explains the enormous man- 

 dibles developed by the ventral parapodia of the 

 first trunk limb. In Pterygotus, Fig. 55, we have, in 

 fact, an arrangement almost exactly the opposite of 

 that found in the other primitive Crustaceans ; the 

 under lip forms the analogue of the upper lip, 

 the masticatory ridges of the first trunk limbs are 

 analogous to the mandibles, while those of the four 

 posterior head limbs probably function as maxillae, 

 their dorsal parapodia doubtless helping in the catch- 

 ing and holding of prey. And lastly, the first antennae 

 developed into large chelate feet. It is almost as if we 

 had the typical mouth formula of a modern Crustacean 

 turned quite round. 



These changes clearly went hand in hand with the 

 acquisition of more rapid motion in feeding. A spring- 

 ing or darting movement forward is most suitable 

 for an arrangement of mouth and jaws facing ante- 

 riorly, for the sudden seizure of the prey which 

 comes in the way. Further, it seemed to us that 

 the more rapid the movement the more delicate 

 should be the sensory organs for the rapid percep- 

 tion of what was food and what was not. A com- 

 parison of the rowing limbs of Eurypterus and 

 Pterygotus quite confirmed this supposition, and lent 

 unexpected support to this method of explaining 

 the morphology of these animals. Eurypterus (Fig. 

 56), which has all its anterior head limbs developed as 



