318 GENERAL CONSIDERATIONS ON THE MOLLUSCA. 



form which also in several ways throws light upon the relationships 

 of the Molluscs. 



Although the larvae of the different divisions, e.g., those of the 

 Amphineura, the Solenoconchae, the Lamellibranchia, and perhaps 

 also those of the Heteropoda and the Opisthobranchia appear with 

 very different forms, all of them may without any difficulty be traced 

 back to the Trochophore, the typical form of which was met with 

 in the Annelida. In some cases, such, for instance, as the larvae of 

 Donderxia and Dentalium (Figs. 10, pp. 17 and 138, p. 291) as well as 

 in those of a few Gastropoda (perhaps of the Gymnosomatous Ptero- 

 poda, Fig. 77, p, 172) this is less evident, while in the larvae of other 

 Gastropods such as Patella, Patudina and most Lamellibranchia, the 

 resemblance is exceedingly striking (Figs. 51-53, p. 127 ; 56, p. 135 ; 

 14, p. 28 ; 18, p. 36). But even in larvae in which the resemblance 

 is not so great (Figs. 66 and 67, p. 155 ; 72, p. 162 ; 75, p. 168), com- 

 parison with other forms or the examination of the younger stages 

 enable us easily to trace back the larval form to the Trochophore 

 {Figs. 64 and 65, pp. 153 and 154 ; see also p. 166). In the greatly 

 modified ontogeny of the Cephalopoda, traces of the larval form have 

 hitherto not been established with certainty. 



The development of the Molluscan Trochophore closely resembles 

 that of the Annelidan larva. It arises from the gastrula-stage, 

 several rows of cells at the anterior region becoming covered with 

 strong cilia and thus yielding the pre-oral ciliated ring, the velum 

 of the Molluscan larva. The blastopore, which at first lies at the 

 posterior end, usually becomes slit-like ; it probably closes from 

 behind forward and generally, at its anterior end, passes into the 

 mouth through the formation of an ectodermal depression, the 

 stomodaeum. A post-oral ciliated ring appearing behind the mouth 

 heightens the resemblance to the typical Trochophore, the external 

 form of the Molluscan larva now also agreeing with that of the 

 latter, the anterior end being much widened and the posterior 

 narrowed. 



At the posterior end of the body, and thus at the point where the 

 blastopore at first lay and from which its closure commenced, the 

 anus now forms. It thus appears as if it also bore some relation 

 to the blastopore ; indeed, an attempt has been made to prove this 

 to be the case in some Molluscs (e.g., the Opisthobranchia), and in 

 another Gastropod, Paludina, the direct transformation of the blasto- 

 pore into the anus has been assumed. We thus find in the Mollusca 

 conditions altogether similar to those met with in the Annelida and 



