146 HUGH WATSON. 



carnivorous snails with dextral heliciforin or depressed shells ; 

 therefore it seems possible that the above explanation of the 

 asymmetry of the nerves of the odontophore may be not very 

 far from the truth. 



The Venteal Group of Ganglia. — Beneath the odontophore 

 or the posterior part of the buccal mass, and a little further 

 back than the cerebral ganglia, there lie six ganglia very 

 close to each other. These are the two pedal, the two pleural, 

 and the two visceral ganglia. In A per a purcelli the limits 

 of each of these ganglia can be clearly seen, although they are 

 very near together ; but in some of the larger forms, such as 

 A. gibbonsi, the sepai-ate ganglia can only be distinguished 

 with difficulty, so closely are they aggregated (PI. XIV, fig. 

 67). These ganglia are joined to the cerebral ganglia by the 

 cerebro-pedal and cerebro-pleural connectives, the length of 

 which varies directly with the size of the buccal mass and 

 odontophore. Thus in A. gibbonsi these connectives are 

 very long, so as to permit of the protrusion of the enormous 

 odontophore (PI. XIII, fig. 52), while in A. burnupi and A. 

 sexangula they are fairly short. The connectives are 

 sometimes slightly, but rather abruptly, swollen at their 

 junctions with the pedal and pleural ganglia (PI. XIV, fig. 

 68). 



The most anterior of these ganglia are the pedal ganglia, 

 which are also more ventrally situated than the others. 

 These are the largest ganglia in the nervous system, being 

 even larger in Apera than the cerebral ganglia. They are 

 somewhat oval in shape, and very close together. Two short 

 commissures connect them, one anterior and dorsal, the other 

 further back and more ventrally situated (PI. XV, fig. 74). 

 The first of these is usually termed the pedal commissure, 

 and is shown in section in PI. XIV, fig. 68 ; the second is 

 knoAvn as the parapedal commissure, and is seen to be rather 

 thinner than the other in vertical section. Beutler^ has 

 found both commissures in Paryphanta hochstetteri 

 {Pfr.), and it is probable that they occur in all the Euthy- 



» Op. cit.. p. 402. 



