198 BULLETIN OF THE 



of ganglia and an azygos ganglion. Together these form three complete 

 rings surrounding the oesophagus. 



The relative positions of the ganglia are best appreciated from cross 

 sections. In passing from behind forward, they are encountered in the 

 following order: (1) the pair of pedal ganglia, which lie under the 

 radula sac, and are joined to each other by an anterior and a posterior 

 comniissure ; (2) one abdominal ganglion a little to the right of the 

 median plane ; (3) a pair of visceral ganglia occupying the posterior 

 angle formed by the outgrowth of the radula sac from the oesophagus. 

 They are separated by the abdominal ganglion, from which connectives 

 pass to them ; (4) a pair of pleural ganglia, not joined by a com- 

 missure, and not giving off nerves. They are united by means of con- 

 nectives to the pedal, visceral, and cerebral ganglia of the same side ; 

 (5) a pair of cerebral ganglia, with their supra-oesophageal commissure 

 and connectives to the pleural, pedal, and buccal ganglia ; (6) a pair of 

 buccal ganglia, with a commissure under the oesophagus posterior to its 

 connection with the sac of the radula. 



The mantle ganglion lies far back, and is joined to the abdominal 

 ganglion by a large nerve. 



It seems as if there could be no doubt that the infolding of the ecto- 

 derm of the anterior wall of the respiratory cavity on the right side of 

 the body gives rise to the special-sense organ discovered by Lacaze- 

 Duthiers ('72, pp. 483-494). It corresponds in its position and its 

 connection with the right visceral ganglion to his description of the 

 adult, and also to Fol's description ('80, pp. lGG-168) of the origin and 

 position of that organ in the aquatic pulmonates. 



As is well known, Limax belongs to that group of Gastropods in which 

 all the nerve centres, except the cerebral and buccal, lie on the ventral 

 side of the intestinal tube ; not to the group in which the connection 

 between the right pleural and right visceral ganglia passes above the 

 oesophagus, and in which that of the left lies below it. Limax, there- 

 fore, is not directly referable to Von Jhering's group of Chiastoneura, 

 although the want of symmetry in the position of its ganglia does not 

 allow one to say that it is orthoneuric. 



The Gastropod in which the details of the origin and fate of the 

 nervous centres have been most carefully studied is Bithynia, a chias- 

 toneuric form, in which Sarasin has found that the abdominal ganglion 

 is joined to the right visceral ganglion only, and is located at the fundus 

 of the gill cavity. The relation is different from that found in Limax 



