280 THE APODID^E PART n 



morphology which stand obstinately in the way of 

 attempts to establish a close relation between them. 



Has not our derivation of Apus and the Crustacea 

 from a bent Annelid supplied us with the clue as to 

 the essential morphological difference between the 

 Crustacea and the Tracheata, leaving out of sight for 

 the moment the tracheae and the Malpighian tubules 

 which are confined to the latter ? 



The Annelid which gave rise to the Tracheata 

 started, as did the Crustacean-Annelid, by using its 

 anterior parapodia as mouth parts, but, unlike the 

 latter, it did not bend round its anterior segments to 

 browse in the manner described in the opening sen- 

 tences of this essay, but remained straight. The fusion 

 of segments to form the head was, in the Tracheatan- 

 Annelid, axial, the mouth remaining at the anterior 

 end of the body. 



In such an axial fusing there is nothing to fix the 

 number of segments to form a head common to all 

 the Tracheata, whereas in the Crustacea the bending 

 round of the five segments marked off this region of 

 the body as the head for all time. 



The difference between the number of the cephalic 

 appendages of the Crustacea and the Tracheata is to 

 be referred to the fact that with the mouth at the 

 anterior end of the Annelidan body it was impossible 

 to bring so many pairs of parapodia into the region 

 of the mouth to function as mouth parts as in the 

 Crustacea, where its ventral position allows of the 

 arranging on each side of a large number of para- 

 podia as jaws. 



