856 PROF. G, C. BOURNE ON THE [Nov. 17, 



add that, contrary to Lenssen's statement, the genital nerve in 

 -JV. jiuviatilis is given off" from and not in front of the first 

 ganglionic swelling, and that the characters of the genital 

 ganglion &c. are practically identical with those described above. 

 The nerves given off" from the enlargement of the visceral 

 commissure in the vicinity of the in'opore are distributed, as all 

 previous authors have described, to the kidney and pericardium. 



If I have described the origin and distribution of these nerves 

 at some length, it is because they are of importance in determining 

 the homologies of the first ganglion on the visceral commissure 

 which Boutan and Haller have identified with the subintestinal 

 ganglion. Now this ganglion, whatever its size and position, 

 never gives rise to nerves supplying the viscera and gonads. It 

 is essentially the ganglion of the right side of the mantle, and in 

 the primitive dibranchiate Aspidobranchia supplies the nerve 

 going to the post-torsional right ctenidium and osphradium. 

 When these disappear, as in the Trochid* and Pecbinibranchia, 

 either there is no definite subintestinal ganglion or it tends to 

 approximate itself to the left pleural ganglion as in the Ceiithiidse, 

 or, as is more often the case, it may enter into close i-elations with 

 the right symmetrical pallial nerve and innervate the right side 

 of the mantle. On the other hand, the genital and visceral nerves 

 always issue from the visceral ganglion (or ganglia if more than 

 one is present) at the hinder end of the visceral commissure, and 

 this original connection is maintained with great persistence even 

 in the short-looped euthyneurous forms. Hence it would be 

 contrary to what is observed in other gastropods if the genital, 

 intestinal, and stomach nerves issued from the subintestinal 

 ganglion ; and the conclusion is that Boutan and Haller were 

 w^rong in their identification, and that the ganglion in question 

 is a member of the visceral series. This is the more likely when we 

 -consider that in other Rhipidoglossa — in Trochus, for instance — 

 the abdominal ganglion is an elongated and ill-defined enlarge- 

 ment occupying a considerable section of the posterior part of 

 the visceral commissure. As I have stated, there is a continuous 

 and thick cortex of nerve ganglion-cells investing all that part of 

 the visceral commissure of the Neritidse lying between the 

 ganglion from which the genital nerve proceeds and the ganglion 

 adjacent to the uropore. The whole of this thickened section is 

 to be compared with the elongated visceral ganglion of Trochus, 

 and the swellings at its two ends, which do not in fact form such 

 distinctly separate ganglia as might be inferred from figures and 

 descriptions, may be regarded as concentrations of nerve-cells — in 

 ■other words, incipient but not yet separate ganglia at the two 

 ends of a long and ill-defined tract of ganglion-cells. This view 

 is strengthened by the fact that the swelling at the right end 

 from which the genital and other nerves proceed bears the same 

 relations to the oviduco-coelomic funnel (which is evidently a 

 relic of the right kidney) as the swelling from which the renal 

 -and pericardial nerves proceed bears to the reno-pericardial funnel 



