232 THE APODID/1. PART n 



The development of the ventral parapodium into 

 mandibles and gnathobases is a common specialisa- 

 tion in all the groups, this being the most primitive 

 modification according to our deduction of the Crus- 

 tacea from a carnivorous Annelid, which caught prey 

 between its ventral parapodia and forwarded it on into 

 the mouth, bent round to receive it. The dorsal para- 

 podia, being chiefly used for locomotion, have how- 

 ever been differently developed according to the 

 different methods of locomotion adopted. In Apus 

 they are specialised as rowing plates (except a few 

 anteriorly for raking prey together), in the Trilobites 

 as ambulatory legs. The former modification requires 

 no description ; it results simply in a further develop- 

 ment of the flat leaf-shaped parapodia, the sensory 

 cirrus alone, perhaps, requiring to change its form 

 from a cirrus into the flat flabcllum. The ambulatory 

 leg of the Trilobite may be supposed to have arisen 

 as follows. A strip running from the tip of the 

 parapodium, where it rested on the ground, to the 

 body, would tend to be strengthened, and would 

 eventually bear the weight of its share of the 

 bod\'. On each side of this strip the leaf-like para- 

 podium would be useless, and would gradually dis- 

 appear, this disappearance being accelerated in the 

 Trilobites by other and special causes which we have 

 already described, such as the necessity of having limbs 

 which, in the rolled-up body, would occupy as little 

 space as possible. Thus we may safely assume that 

 the parapodia, if used for walking or crawling, would, 

 by a simple biological law, turn into ambulatory legs. 



