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



Lenssen. The disposition of the coils of the intestine and their 

 relations to the stomach, oesophagus, and radula-sac are indicated 

 in fig. 2, as is also the position of the heart and the fact that the 

 ventricle laps completely round the rectum. The radula-sac is 

 large and usually of considerable length, but varies considerably 

 in diiferent specimens. When long it is involved in the coils of 

 the intestine and its posterior part always passes ventrad of the 

 oesophagus but dorsad of the stomach. 



The Nervous System. 



The main features of the nervous system have already been 

 described by Bouvier (9 cfc 10) and Boutan (7). The latter 

 author, correcting and amplifying the earlier account of Bouvier, 

 describes a supra-intestinal nerve completing the streptoneurous 

 condition of the visceral nerve, and gives an amended figure of 

 Bouvier's drawing of the pleuro-pedal nei-ve- centres. In my 

 earlier dissections I failed to identify the supra-intestinal nerve, 

 but have been able to follow its course more or less completely in 

 my serial sections, and am able to verify Boutan's statements as 

 far as they go. In one particular I can add to them. Boutan 

 traced the supra-intestinal nerve from its origin from the right 

 pleural ganglion along the right side of the body, whence it turns 

 over the gut towards the left side and courses, as he says, " dans 

 la cavite branchiale, au niveau du tiers inferieur de la branchie." 

 It is hardly correct to say that the nerve passes into the branchial 

 cavity. After a considerable amount of trouble I have been able 

 to trace the nerve as far as the osphradium, the precise character 

 and position of the latter organ having been overlooked by 

 Boutan. The supra-intestinal nerve on arriving at the left side 

 of the body passes obliquely forward in the connective tissue 

 underlying the integument on the dorsal side of the left 

 columellar muscle. Near the anterior end of this muscle the 

 nerve passes upward, and without any ganglionic enlargement on 

 its course, it joins the elongated ganglion underlying the osphra- 

 dium in the left anterior corner of the mantle-cavity. The 

 osphradial ganglion is also supplied, as is the case in Ner'ita and 

 Neritina, by the symmetrical left branchio-pallial nerve, emana- 

 ting from the left pleural ganglion. This large nerve traverses 

 the columellar muscle and passes almost direct to the osphradium, 

 where it enlarges to form the above-mentioned ganglion. From 

 the ganglion a branch passes along the anterior border of the left 

 suspensory fold of the ctenidium and may be traced without 

 difiiculty nearly to the tip of the latter organ. Another branch 

 passes backwards, nearly parallel to the columellar muscle. I 

 have not been able to trace this nei've in its entirety, but have no 

 doubt' that it is the continuation of the supra-intestinal nerve, 

 and joins the visceral ganglion in the vicinity of the uropore, 

 thus completing the visceral loop. If this is the case the strepto- 

 nevuy is complete, as it is in Nerita and the larger tropical sjjecies 

 of Neritina. 



