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



enquiry on these lines would lielp to elucidate a number of 

 doubtful problems in gastropod morphology ; but the subject is a 

 large one, requiring much comparative study of different forms of 

 nervous systems and must be left for a future occasion. 



Respecting the supra-intestinal ganglion and its connections,^ 

 my observations serve to bring together and harmonise the 

 scattered descriptions of previous authors. The text-figure on 

 p. 855 shows the relations of the supra-intestinal ganglion, the 

 symmetrical left branchial and pallial nerves, the osphradium, 

 and the osphradial ganglion. The supra-intestinal ganglion is 

 always very small and liard to discover, but I have determined 

 its presence in Nerita fl'icata and Paranerita gagates. It is con- 

 nected by a very short branch with a long cord of nerve-fibres 

 ensheathed by nerve ganglion-cells which underlies the osphradium. 

 This cord must be regarded as a diflTuse osphradial ganglion. It 

 receives the bulk of its nerve-supply from the left symmetrical 

 branchial nerve, and this connection has been noted by several 

 authors ; Thiele (39) in particular gives a good figure of it as seen 

 in section. The posterior end of the osphradial ganglion is con- 

 tinued into the clearly defined nerve which passes along the edge 

 of the right or anterior suspensory membrane of the ctenidium 

 and is continued along the afferent side of the gill. I can con- 

 firm Bernard's statement that this nerve is continued round the 

 apex of the gill and down its efferent side. The position of the 

 osphradium has been correctly described by Bernard (3). It lies, 

 as indicated in the text-figure, in front of the attachment of the 

 suspensory membrane of the ctenidium to the roof of the inantle- 

 cavity, and may be distinguished by the naked eye as a small 

 furrow bounded by two ridges of thickened epithelium. Its. 

 minute structure does not differ in any important particular from 

 that of Septaria. 



The, Respiratory and Circulatory Systems. 



The ctenidium, heart, and blood-vessels require little or no 

 description. They are constructed on the same plan throughout 

 the ISTeritidje, and as Lenssen has treated the subject very fully 

 in his memoir on Neritina jiuviatilis and I have already written 

 all that I have to add to his account in dealing with Septaria, 

 there is no more to say. The right auricle is always present, its 

 position in Paranerita gagates being indicated in fig. 42. It 

 is somewhat less well-developed in marine forms (e. g. Nerita 

 peloronta) than in the freshwater forms, but always carries 

 blood back to the ventricle from lacuna? in the posterior and left 

 side of the body- wall. It is separated by the whole width of the 

 visceral pedicle from, and has no connection with, the little hollow 

 organ lying in the right-hand posterior corner of the mantle- 

 cavity which I have referred to as possibly being a relic of the 

 right ctenidium, and this fact must be admitted as evidence 

 against the suggested homology. The gill-lamellai of the ctenidium 



