80 



THE NERVOUS SYSTEM OF MOELUSKS. 



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 

 each side of the oeso- 

 phagus, with a long and 

 curved double ganglionic 

 mass (/»). This latter 

 body, situated beneath the 

 oesophagus, 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 respiratoi'y and other 



FiG. 20. —Head .and Nervous System of 

 the Common Garden Snail. (Owen.) /.Cere- 

 bral ganglia receiving nerves from smaller 

 ( a) and fi'om larger tentacles bearing ocelli 

 (b) ; m, sub-oesopbageal ganglionic ma^a, 

 representing a pair of pedal and a i)air of 

 brancbial ganglia. Two of the tentacles are 

 represented in different states of retraction. 



The nervous system of 

 one of the Nudibranch 

 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 jirin- 

 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 occui)y an inferior 

 rather than a superior and lateral position in regard to 



