40 SCIENCE PROGRESS. 



compactness, and characteristic organs, such as mantle and 

 ctenidium, which have been regarded as homologous 

 throughout the Molluscan series, are interpreted in different 

 ways in the different types, as the exigencies of Thiele's 

 theory demand. One of the first propositions assumed by 

 this writer is that the foot of the Mollusca is simply a colossal 

 enlargementof the ventral sucker of the Polyclad; thesuctorial 

 function of the foot in Chiton and the lower Gastropoda is 

 pointed to in support of this comparison. A series of 

 more revolutionary propositions is then promulgated in 

 consequence of the necessity under which the author is 

 placed of discovering the primitive body-edge of the 

 Mollusca comparable to the edge of the body of the Tur- 

 bellaria. This primitive body-edge Thiele identifies by 

 means of the lateral sense-organs which characterise the 

 epipodium in the Rhipidoglossa and the margin of the 

 mantle in Pelecypoda. The epipodium in Gastropoda and 

 the mantle edge in Pelecypoda are thus taken by this writer 

 to represent the sides or edge of the body in the Tur- 

 bellarian ancestor. The epipodium in Gastropoda and the 

 mantle edge in Pelecypoda consequently separate the 

 dorsal from the ventral regions of the body in those groups. 

 It follows from this that the ctenidia of Gastropoda, which 

 are supra-epipodial in position, are not homologous with 

 the ctenidia of Pelecypoda, which are infra-pallial. How we 

 are to regard the anus, which is dorsal in the one group and 

 ventral in the other, is not explained. But since in oper- 

 culate Rhipidoglossa the operculum, like the shell, is 

 situated above the epipodium, we are told that the oper- 

 culum must also be regarded as dorsal in position, as well 

 as serially homologous with the shell proper. This, in 

 Thiele's eyes, compares well with the condition of affairs in 

 Chiton, whose shelly plates are without doubt serially 

 homologous. Moreover, although the existence of an 

 epipodium in Chiton has not been hitherto recognised, 

 Thiele argues that, since the pallial fold in this form re- 

 presents the primitive body-edge, it must also, together with 

 the series of ctenidia which are attached to its lower surface, 

 be regarded as the homologue of the epipodium of the 



