THE ODONTOPHORA. 423 



ceed from the buccal ganglia are distributed to the odonto- 

 phore and its muscles. 



In ether Odontophora^ the two cerebral and two pedal 

 ganglia, with their commissures, are alwaj's to be recognized ; 

 but the number of the ganglia which represent the parieto- 

 splanchnic system may be increased, and the anterior ganglia 

 of this system may attain a large size, and may come into 

 close relation not only with the cerebral but with the pedal 

 ganglia. 



In Lymnceiis palustris^ for example, there are five such 

 ganglia situated close to the cerebro-pedal ring. The most 

 anterior of these, on each side, is united with both the cere- 

 bral and the pedal ganglion of its side, and appears, indeed, 

 like an enlargement upon a second commissure between those 

 two ganglia. The ganglia which constitute the second pair 

 are united, in front, by a short commissure, with the preced- 

 ing; and, behind, with the fifth or azygos ganglion. The 

 second pair of ganglia give off the neives to the right and 

 left sides of the mantle respectively. 



In Limax^ and apparently in the terrestrial Pidmonata 

 generally, the arrangement is essentially the same, except 

 that all the ganglia of the parieto splanchnic S3'stcm coalesce 

 into one mass, between which and the pedal ganglia the aorta 

 passes. 



In Hallotis^ on the other hand, while the anterior parieto- 

 splanchnic ganglia are situated close to the pedal ganglia, 

 and are connected with them and with the cerebral ganglia 

 in such a manner as to give rise to an apparent second cere- 

 bro-pedal commissure, the ganglia which represent the second 

 pair in Lymnceus are situated at the base of the branchiae, 

 and are united by a long commissure with one another, and 

 also with the anterior paricto-splanchnic ganglia. Of the 

 latter commissures, that from the left branchio-pallial gan- 

 glion goes to the right anterior parieto-splanchnic ganglion, 

 and vice versa. 



With respect to the position of the cerebral and pedal 

 ganglia in the OdontopJiora^ the commonest arrangement is 

 that in which the cerebral ganglia are supra-oesophageal, and 

 are connected by two longer or shorter commissures, on each 



» Compare Lacaze-Duthiers, " Du svsteme nerveux des Mollusques gas- 

 teropodes pulraones aquatiques" ("Arch, de Zoologie," 1872), and ths numer- 

 ous figures of the arrangement of the cerebral ganglia of the nervous system 

 given in his memoir on the otocysts. (Ibid.) 



' See Lacaze-Duthiers, " Sur le systeme nerveux de Haliotide." 



