26 GILBERT C. BOURNE. 



ot the g-ill-filiiments, which, as has been exphiined above, run 

 back along this region of the mantle. At the anterior edge 

 of the colmnellar muscle the nerve expands to form the large 

 right osphradial ganglion. The right symmetrical pallial 

 nerve takes its origin from the ventral side of the right 

 pleural ganglion, just where the latter becomes fused to the 

 pedal gjinglion. It runs outward, traverses the columellar 

 muscle some way in front of the osphradial nerve, and takes 

 a direct course to the right osphradial ganglion, which it 

 crosses dorsally, and in so doing enlarges and makes an 

 intimate connection with it. Just in front of the osphradial 

 ganglion the ])allial nerve divides into two branches. The 

 posterior branch, which is slender, runs back along the 

 thickened border of the posterior part of the mantle. The 

 anterior branch runs forward to the mantle-slit, where it 

 expands to form a small ganglion, indicated by a distinct 

 accumulation of nerve-ganglion cells, and is here joined by a 

 slender nerve from the anterior end of tlie osphradial ganglion. 

 This little iranjjlion at the hinder border of the mantle- 

 slit gives otf an external branch supplying the posterior sense- 

 papilla of the mantle-slit, and a stout anterior branch which 

 passes round the mantle-slit and is continued forward as the 

 peripheral pallial nerve, meeting and uniting with its fellow 

 of the opposite side on the anterior border of the mantle. 

 There is thus a very intimate dialyneury on the right side. 

 These relations are veiy hard to make out, and require careful 

 study with high powers of the microscope, but I can vouch for 

 the correctness of the account here given of them. The rela- 

 tions in Fissurella are somewhat similar, but the proportions of 

 the lengths of the nerves differ greatly, and apparently differ 

 in different species, for in my sections of F. graeca the sub- 

 intestinal is close to the right osphradial ganglion, whereas 

 in F. reticulata Boutan figures them as far apart and con- 

 nected by a long slender nerve, as in Incisura. The origin 

 of the right symmetrical pallial nerve from the upper surface 

 of the pedal ganglion rather than from the right pleural 

 ganglion is identical in Incisura and Fissurella. 



