TYPE MOLLUSC A. 



325 



troch bauds which obscure the Trochophore characters, a still earlier larva 

 preseutiug greater similarities to the annelid larva. 



By the earlier writers the Scaphophods were considered more closely 

 related to the Pelecypoda than to the other Molluscau groups, this relation- 

 ship being indicated more especially by the symmetrical form, the apparent 

 lateral arrangement of the mantle-folds and the absence of eyes. On the 

 other hand, there are a large number of differences between the members 

 of the two groups, as for instance the univalve character of the shell, and 

 especially the occurrence of a radula and jaw. This latter feature suggests 



Fie. 146. DIAGRAMS TO SHOW THE ORIGIN OP THE SCAPHOPODS FROM A Fis- 



suretla-Uke ANCESTOR (after PLATE). 

 ct = ctenidium. go = reproductive organ. 



/ = foot. ra = uiouth. 



sh = shell. 



the Gasteropods, and it seems most probable that it is to this group that 

 the Scaphopods should be considered as related. They must, however, be 

 referred to the more primitive Gasteropods, those in which the rotation of 

 the mantle-chamber had not occurred. An elongation of the dorsal hump 

 of a Fissuretta-like ancestor unaccompanied by a twisting to one side, as 

 represented in Fig. 146, would bring about a condition from which it does 

 not seem a great step to reach the Scaphopods. 



Among recent Gasteropods it is with the Diotocardiates that the 

 Scaphopods seem to be most affiliated, and, as we shall later see, it is from 

 the primitive members of this order that the Pelecypods have probably 

 been derived, and thus any similarities which may exist between the 

 Scaphopods and Pelecypods is readily explicable on the basis of a similar 

 ancestry, both groups being derived from Prosobranch-like forms. The 

 absence of a larva corresponding closely to the Gasteropod Veliger would 

 seem to oppose such a view, but it must be remembered that the Veliger is 

 characteristic only of the more highly-differentiated Prosobranchs such 



