Royal Society. 227 



respect, on a par with the higher Crustaceans ; and from the blood 

 arriving at the heart in the same condition, according to the re- 

 searches of Garner and Milne-Edwards, in Ostrea and Pinna, the 

 great Triton of the Mediterranean, Haliotis, Patella and Helix, it can 

 scarcely be doubted that this arrangement will be found throughout 

 the Mollusca. 



From a consideration of the facts cited in the paper, it may be 

 deduced that the skin or mantle is in the Mollusca the fundamental 

 organ of respiration, and that a portion of that envelope becomes 

 evolved into a speciality as we trace upwards the development of 

 the respiratory powers. 



Upon the dorsal aspect of the liver- mass is a branched cavity, 

 that of the renal organ, lined with a spongy tissue, and opening 

 externally at the small orifice near the anus. 



Organs of Innervation. — These are in two divisions, one corre- 

 sponding to the cerebro-spinal division, the other to the sympathetic 

 or ganglionic system of the Vertebrata. The existence of the latter, 

 it is stated, is now for the first time fully established. The centres 

 of the first system are seven pairs and a half of ganglia. Of the 

 seven pairs, five are supra-oesophageal, two, infra- oesophageal : the 

 single ganglion belongs to the right side and has been named visceral. 

 There are three nervous collars around the oesophagus, one of which 

 connects the infra- with the supra-oesophageal. The total 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 demonstrable, 

 in the skin, the buccal mass, and on all the internal organs. It 

 consists of a vast number of minute distinct ganglia, 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 papiUosa, and in Arion ater, believe 

 it to exist in all the more highly organized Mollusca. 



The supra-oesophageal 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 repre- 

 sentative 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 re- 

 presented in Aplysia by a pair of ganglia, situated in the posterior 

 part of the body near the root of the branchiae. The supra-oesopha- 

 geal ganglia in the Lamellibranchiata 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 oesophageal nervous 



15* 



