42 SCIENCE PROGRESS. 



But the novelties begin with Thiele's interpretations of 

 the nervous system of Gastropoda and Pelecypoda. We 

 have already pointed out Thiele's view that the epipodium 

 of Gastropods represents the primitive body-edge. Now 

 at the base of the epipodium in Fissurella and Haliotis there 

 lies a ganglionic plexus ; and this plexus, which takes the 

 form of an incomplete ring, is regarded as the homologue 

 of the lateral cords of Turbellarians and Amphineura. The 

 series of epipodial nerves which connect the epipodial plexus 

 with the upper half of the pedal cords in Rhipidoglossa is 

 compared with the series of connectives between the lateral 

 and ventral cords in Amphineura. 



This seems very plausible until one recollects (i) that, 

 the epipodium being infra-rectal, the epipodial plexus is 

 also infra-rectal and thus difficult to compare with the 

 lateral cords of Amphineura, whose "commissure" is supra- 

 rectal ; and (2) that, whereas in Amphineura the lateral 

 cords innervate practically the whole of the pallium and 

 viscera, in Rhipidoglossa the epipodial plexus has nothing 

 to do with any other organs except the sense-organs of the 

 epipodium. If the pallium of the Gastropoda is really, as 

 Thiele maintains, a secondary differentiation of the primary 

 pallium of the Amphineura, one would expect that its 

 innervation would also be effected by progressive differen- 

 tiation of the nerve-centres which supplied the primary 

 pallium, viz., from the lateral or epipodial centres. So far 

 from this being the case, however, Thiele himself (xxv., pp. 

 587-9) adopts the view that the pallial nerves as well as the 

 pleural ganglia of Gastropoda are secondary derivatives of 

 the ventral or pedal cords. 



The recklessness of Thiele's comparisons reaches its 

 high-water mark, perhaps, in his remarks on the nervous 

 system of Pelecypoda. Correlated with the existence of 

 numerous sense-organs (eyes, tentacles, etc.) along the 

 mantle edge, there exists in many forms {Area, Pecten, 

 Pinna, etc.) a nervous ring around the mantle which may 

 take the form either of a complete ring of peripheral ganglia 

 united by a plexus, or of a circumpallial ganglionated nerve, 

 as was recognised by Duvernoy (5) more than thirty years 



