NERVES OF MOLLUSKS. 



11 



Fig. 2. 



NERVES OF A 

 JANTHINA. 



(a) are sometimes considerably separated from 

 each other, and only united by an intermediate 

 cord, and sometimes glued one to the other, or 

 even confounded in a single mass : they are 

 situate above the O3sophagus, and far from the 

 posterior ganglia (6), which are placed beneath 

 the intestine towards the posterior part of the 

 body. In some mollusks, the nervous system 

 is still more simple in its composition ; but, in 

 general, the ganglia tend to approximate the 

 oesophagus more and more (Jig- 2), and in 

 most of these animals we find a greater num- 

 ber of small medullary masses, some of which 

 form a sort of nervous centre, and the others 



are distributed in different parts 



of the body. Thus, in the snails 



and most mollusks constructed on 



the same plan, and named gastero- 



pods (from the Greek, gaster, 



belly, and pous, foot), on account 



of their mode of crawling on the 



ventral surface of the body, there g 



exists, above and in front of the p "' 



oesophagus, a medullary mass (Jig. 



3, c.), which is the representative d 



of the anterior pair of ganglia 



above mentioned, and which is re- a"" 



garded by most anatomists as the Fig. 3. NERVES OF A SNAIL. 



generally called the brain ; here, these two ganglia are considerably sepa- 

 rated from each other, and are united by a transverse band ; c. nerves 

 which arise anteriorly to be distributed to the tentacula, to the mantle, &c. ; 

 6. ventral pair of ganglia, which are united in a single mass, placed be- 

 neath the intestine, and joined to the anterior ganglia by two very long 

 nervous cords ; d. nerves which arise from the posterior ganglia to t>e dis- 

 tributed to the mantle, &c. 



Explanation of Fig. 2. Nervous system of a Janthina ; a gasteropod 

 mollusk, in which the posterior ganglia (&) as well as the anterior ganglia 

 (a) are separated from each other ; but they are more approximated to the 

 latter, so that the inter-ganglionic cords are very short, and form a sort of 

 closely fitting collar around the oesophagus. 



Explanation of Fig. 3. Central portion of the nervous system of the 

 snail : c. ganglia situate in front of the oesophagus, and constituting, by 

 their union, the mass which is called the brain in mollusca ; o. nerves of 

 the anterior parts of the body, the eyes, &c. ; g. the nervous mass formed 

 by the union of the ganglia of the posterior pair ; the oesophagus passes 

 through the collar or ring which unites it to the brain ; p. the nerves of 

 the foot ; n. nerves which go to the pulmonary cavity, &c. ; a. the nerve 

 which accompanies the principal artery ; d. the nerve which goes to the 

 diaphragm, &c. 



