ZOOLOGICAL POSITION OF THE TKILOBITES. 43 



pharynx, this, in time, becoming specialised as a radula. 

 And so I believe that the great phylum of the Crustacea 

 arose from Chaetopods also becoming adapted to take 

 advantage of the same food supply, but in a manner very 

 different from, and less efficient than, that adopted by the 

 Mollusca. The ancestor of the crustacean phylum used 

 its appendages as the instruments for obtaining supplies. 

 The food raked from each side by the parapodia into the 

 middle line, was then pushed forward towards the mouth, 

 bent round to receive it. This new function of the 

 parapodia, which were primitively protective on account 

 of their bristles, and locomotory as swimming plates, would 

 soon lead to modifications of structure, which we are not 

 yet in a position to describe in detail. The most primitive 

 phyllopodan limbs with which we are acquainted, viz., the 

 larval limbs of Apus, represent the earliest known stage in 

 the transformation of the annelidan parapodium into the 

 crustacean legs. They are fiat unjointed skin-folds, carrying 

 on their dorsal edges a pair of processes, one of which has 

 strong claims to be the typical gill of an annelidan para- 

 podium. Their ventral edges are deeply notched so as to 

 form a series of lobes. The proximal lobe of the series is 

 distinct from the rest, and can safely be taken to be the 

 original ventral parapodium. 1 We further find in Apus 

 that these innermost lobes (the ventral parapodia), forming 

 a longitudinal series alone each side of the middle line, are 

 twisted round in such a way as to show that in addition to 

 assisting to rake food together, they also serve for pushing 

 it forward towards the mouth. 



Having thus briefly described the new method of feed- 

 ing of the ancestor of the crustacean phylum we must re- 

 turn to the ridpe-like metastoma of Triarthrus and its 

 commencing modifications. 



It is not difficult to see that, in its most primitive, ridge- 

 like form, such a metastoma would be a barrier across the 

 ventral surface, hindering the pushing forward of food into 



1 1 have figured such a limb, 772., the last limb in Apus proditctus, in 

 The Apod idee, fig. 20, p. 48. 



