GENERAL CONSIDERATIONS ON THE MOLLUSCA. 321 



The above may, on the whole, be described as possible, but this 

 origin has the disadvantage of starting from very highly differentiated 

 animals ; and, a point which appears to us as very important, affords 

 no clear explanation of the striking resemblance existing between the 

 larvae of the Mollusca and those of the Annelida. This resemblance 

 is made possible if we go back farther, to a form from which may be 

 derived both the Trochophore and the ancestors of the Turbellaria. 

 Since, in such speculations, it is only right to try to start from existing 

 forms, the common ancestral form of the Trochophore and the Tur- 

 bellaria has been sought among the Ctenophora, but it is impossible 

 to construct from them the desired ancestor without unduly forcing 

 the comparisons. It is true that the Ctenophora, on account of their 

 locomotion by means of cilia, their possession of the apical plate and 

 the condition of the entoderm and mesoderm may be brought into 

 relation with the Turbellaria and perhaps even with the Trochophore, 

 but it appears to us still more certain that in them we have forms 

 already strongly specialised in a definite direction and thus no longer 

 suited to serve as the ancestral forms of the Mollusca. Instead then 

 of attempting to derive these forms from known and specialised groups, 

 it seems to us simpler and at least equally justifiable to fall back 

 upon some form with a more primitive organisation. 



In considering the ontogeny of the Annelida, we started from a 

 gastrula-like form, ciliated all over, which developed at the cephalic 

 pole a ciliated tuft, and in which also the locomotory cilia were 

 specially arranged in the form of a ring running round the body (the 

 later pre-oral ciliated ring or velum). The primitive mouth (blasto- 

 pore), originally lying at the posterior end of the body, becomes dis- 

 placed forward owing to the development of a ciliated apparatus in 

 this very limited region ; thus the mouth shifts towards the locomotory 

 apparatus which at the same time serves for conducting food to the 

 mouth, as is still seen to be the case with the ad-oral zone and the 

 post-oral ring of the Trochophore. In the primary body-cavity of this 

 form, mesodermal elements are already found ; among these lie the 

 gonads which no doubt are derived from the entoderm as well as from 

 the mesoderm [probably from distinct blastomeres which are neither 

 mesodermal nor entodermal but are the germ-teloblasts], and which 



doubt, the foot arose (No. 20). The transformation of the sucker into the 

 foot and the relation of the latter to the rest of the body is treated in detail 

 by this author. It may here also be mentioned that Thiele ascribes to the 

 Ctenophora a very important part, not only in the phylogeny of the Mollusca 

 but in that of the bilateral animals generally. 



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