50 SCIENCE PROGRESS. 



the attention of its discoverer. In Paludina v. E danger 

 figures the pleural ganglia arising from the ectoderm on 

 each side of the body at a point just outside the velar area, 

 but in actual contiguity with the cells of the ciliated ring. 

 In Bithynia (7, Taf. xxvi., fig. 16) he figures the same 

 condition of things for the pair of visceral ganglia. The 

 only difference in origin between the two ganglia is that the 

 visceral ganglia arise behind the pleural ganglia. If the 

 Molluscan veliger possessed a nerve-ring beneath its proto- 

 troch (velum), as occurs in the trochosphere of the Annelida, 

 it is quite clear that the pleural and visceral ganglia of 

 Bithynia and Paludina would represent a series of gangli- 

 onic thickenings along the course of the nerve-ring. Apart 

 from this inference, however ; the topographical relations to 

 which I have called attention seem sufficient to establish 

 the proposition that the pleural and visceral ganglia, and, 

 as I shall show directly, the abdominal ganglion also, of 

 Gastropods — and, therefore, of other Mollusca — belong to 

 a group of dorso-lateral nerve-centres quite distinct from 

 that which is represented by the ventral or pedal cords. 

 Here again we are reminded of the direct continuity of the 

 pleural and visceral nerve-centres in the Amphineura. 



Development of the abdominal ganglion. — In Chiton, 

 as Kowalevsky has shown (15), the unpaired abdominal 

 ganglion, or, as it is often called, the visceral ganglion, 

 arises by a proliferation of the ectoderm at the hinder pole 

 of the embryo, dorsally to the site of the future proctodeum. 

 In the adult this ganglion is simply a special concentration 

 of ganglion-cells on the supra-anal portion of the pleuro- 

 visceral ring. 



The abdominal ganglion of Gastropods is also situated 

 at the hinder end of the visceral loop, but lies of course 

 ventral to the gut. Can these two ganglia be regarded as 

 homologous ? 



If Molluscs were mere mechanical models the answer 

 would be undoubtedly in the negative ; but embryology 

 points unhesitatingly to the opposite conclusion. Von 

 Erlanger has shown that in Bithynia as well as in Paludina 

 the abdominal ganglion develops as an ectodermal pro- 



