Anatomy of Doris. 159 



number of pairs of nerves from the oesophageal centres is 

 twenty-one, and there are also four single nerves. 



The sympathetic system exists, and is more or less demon- 

 strable in the skin, the buccal mass, and on all the internal 

 organs. It consists of a vast number of minute distinct gan- 

 glia, varying in size and form, the largest quite visible to the 

 naked eye, of a bright orange colour, like the ganglia around 

 the oesophagus, and inter-connected by numerous delicate, 

 white nervous filaments, arranged in more or less open 

 plexuses. This beautiful system is connected with both sets 

 of oesophageal ganglia. 



The authors having found the sympathetic nervous system 

 in several species of Doris, in Eolis papillosa, and in Arion 

 ater, believe it to exist in all the more highly organised 

 Mollusca. 



The supra-cesophageal nervous centres in the Mollusca 

 are in some instances so concentrated as to have led to the 

 idea that they form only one mass ; in others the ganglia 

 are more or less distinct, and separated from each other. 

 Doris has been taken as the representative of one class, 

 Aplysia of the other ; and, on a comparison of both the supra 

 and infra-oesophageal ganglia of these with each other, there 

 has been found a close correspondence between them, with 

 the exception of the visceral ganglion. The single one in 

 Doris is represented in Aplysia by a pair of ganglia, situated 

 in the posterior part of the body, near the root of the bran- 

 chiae. The supra-cesophageal ganglia in the Lamellibran- 

 chiata appear homologous with those of Doris. 



Having determined the existence of a true sympathetic or 

 organic nervous system in Doris, the authors feel themselves 

 more in a position to trace a parallelism between the oeso- 

 phageal nervous centres of these Mollusca and the cerebro- 

 spinal system of the Vertebrata ; and accordingly they find 

 there is a strict analogy between them, even to the individual 

 pairs of ganglia of which they respectively consist ; the 

 general result being, that the whole of the ganglia grouped 

 around the oesophagus in these Mollusca answers to the en- 

 cepnalon, and a small portion of the enrachidion of the Ver- 

 tebrata. 



