80 



THE NERVOUS SYSTEM OF MOLLUSKS. 



Naturalists now generally admit that Snails and their 

 allies are endowed with a rudimentary sense of smell, 

 though hitherto they have heen unahle to locate the endow- 

 ment in any particular organ or surface -region. 



The brain of the Snail is connected, by means of a thick 



cord or commissure on 

 ft each side of the oeso- 

 phagus, with a long and 

 curved double ganglionic 

 mass (m). This latter 

 body, situated beneath the 

 ossophagus, represents the 

 pair of pedal and the pair 

 of branchial ganglia of the 

 bivalve Mollusks. Here 

 nerves are received from 

 the integument and given 

 off to the muscles of the 

 foot ; while they are also 

 received and given off from 

 the respiratory and other 



FIG. 26. Head and Nervous System of 



The nervous system of 

 of the Nudibranch 



the Common Garden Snail. (Owen.) I, Cere- 

 bral ganglia receiving nerves from smaller 

 (a) and from larger tentacles bearing ocelli 

 (6); m, sub-oesophageal ganglionic mass, 

 representing a pair of pedal and a pair of 

 branchial ganglia. Two of the tentacles are 

 represented in different states of retraction. Mollusks has been repre- 

 sented in fig. 17. It is also 



highly developed and concentrated, whilst its sensory and 

 motor ganglia are unusually distinct and separate from one 

 another. A somewhat analogous arrangement of the prin- 

 cipal nerve centres exists in the Common Slug (fig. 27), only 

 here the motor ganglia of the two sides are fused together, 

 as in the Snail, instead of being widely separated as in 

 Eolis and its allies. They consequently occupy an inferior 

 rather than a superior and lateral position in regard to 



