240 THE APODID^C PART n 



ing the modifications they show will not be far 



wrong. 



The earliest Crustacean- Annelids possessed large 

 labra or prostomia projecting backwards, still retained 

 in the Apodidse and Trilobites. This labrum almost 

 necessitated a very deliberate manner of browsing. 

 The animal would creep along, and would have to run 

 some way over its food before it could get it into its 

 mouth, the whole process, it seems to us, necessitating a 

 number of small movements backwards and forwards. 

 Small living prey would very often escape, owing 

 to the fact that the animal's mouth and jaws were 

 not ready in position for them when first perceived. 

 The labrum necessitates the animal passing forwards 

 over its prey, then darting backwards to follow it with 

 its jaws. We here see how useful the gnathobases of 

 Apus must be in catching and holding prey which 

 has been thus passed over. Indeed the whole arrange- 

 ment of the limbs of Apus with the sensory cndites, 

 forms an excellent trap to catch prey over which the 

 labrum has passed. The legs and pleura of the 

 Trilobites, and the large vaulted shield of the Xiphos- 

 uridse may serve the same purpose, although in the 

 latter case the labrum is much modified. In this re- 

 spect, however, the Trilobites were not so well equipped 

 as are the Apodidas ; hence perhaps the development of 

 the large locomotory limb, which enabled the animal 

 to dart backwards after prey thus run over, with great 

 rapidity. We here see the use of the two kinds of 

 limbs figured in Walcott's restoration, ambulatory 

 crawling limbs for slow and deliberate forward move- 



