4 1 2 ^[^sce lla n eo us . 



The nervous system of AcUeon * is decidedly chiastoneurous, like 

 that of the Prosobranchia. The commissural ganglion on each side 

 is fused with the corresponding cerebral ganglion. One of the 

 branches of the visceral comraissure starts from the left cerebro- 

 commissural ganglion, travels obliquely from left to right and from 

 front to rear, passing beneath the long buccal mass, and ends in the 

 subintestinal ganglion, which is situated to the right close to the 

 body-wall ; the other branch is detached from the right cerebro- 

 commissural ganglion, travels from right to left and from front to 

 rear above the buccal mass, and ends in the supra-intestinal 

 ganglion. Starting from this ganglion, which is situated upon the 

 body-wall to the left, the commissural branch passes backwards, 

 and, shortly before reaching the anus, inclines to the right above 

 the oesophagus, and terminates in the visceral ganglion, which lies 

 between the latter and the oviduct. In this same ganglion likewise 

 terminates the prolongation of the subintestinal branch. 



The supra-intestinal ganglion innervates the gill and the left 

 portion of the mantle ; the subintestinal ganglion emits a nerve 

 which proceeds to the right portion of the latter organ. The inner- 

 vation of the mantle is, however, a little further complicated, owing 

 to the presence of two small accessory ganglia, which we may term 

 secomho')/ palJiaJ ganglia^ and which are the more important since 

 they will enable the chiastoneurous nervous system of Actwoti to 

 transform itself by degrees into an orthoneurous system. The 

 first of these ganglia is situated upon the subintestinal branch, 

 midway between the left cerebro-commissural ganglion and the 

 subintestinal ganglion ; it innervates the left portion of the 

 mantle : the second is found upon the supra-intestinal branch, in 

 the immediate neighbourhood of the right cerebro-commissural 

 ganglion ; it innervates the right portion of the mantle. Thus the 

 left portion of the mantle receives at the same time the nerves of 

 the supra-intestinal ganglion and of the left secondary pallial 

 ganglion, while the right portion is innervated by the subintestinal 

 ganglion and the right secondary pallial ganglion. 



From the foregoing it is clear that Actwon is directly connected 

 with the Prosobranchia, and, if we take into consideration the 

 characters of the bipectinated gill, with the diotocardiac division of 

 the latter. Vie now have to consider by what process it has been 

 possible for them to give rise to orthoneurous descendants, that is to 

 say to the other Opisthobranchia and to the Pulmonata. 



It has been shown by Biitschli that it would be possible to derive 

 ihe Gastropoda from a dibranchiate primitive form, the two gills of 

 which would have been situated symmetrically behind, the one to 

 the right, the other to the left of the anus ; this primitive form had 



* Our knowledge of the nervous system of Actccon is based upan a 

 figure by M. Pelseueer ('Challenger' Pteropoda, pi. ii. fig. 11). The 

 cerebral and pedal centres are distinctly shown, but the visceral commis- 

 sure is incompletely figured ; however, it is merely a repetition of the 

 scarcely twistt^d commissure of the normal Tectibraucliia. 



