Smallwood and Rogers, Mollusc an Nerve Cells. 49 



nerve passing to that ganglion. The nervous system of gastero- 

 pods does not permit of any such treatment, as the following 

 description and diagram shows. The nervous system of Limax 

 may be taken as typical of 

 the common snails. It 

 makes its first appearance on 

 the sixth or seventh day after 

 the eggs are laid (Hench- 

 man '90) and is derived en- 

 tirely from the ectoderm. 

 The several ganglia which 

 constitute the nervous sys- 

 tem of Limax arise separ- 

 ately to become secondarily 

 joined by commissures. 



In the adult stage, the 

 central nervous system con- 

 sists of five pairs of ganglia 

 and a single ganglion asym- 

 metrically placed. The rel- 

 ative position of the ganglia 

 can be appreciated from the 



view shown in Fig. i 



In 



passing from behind for- 

 ward, the ganglia are en- 

 countered in the followino; 



Fig. I. Nervous system 

 of Limax agrestis dissected 

 and drawn from nature by 

 H. S. Cadmus. I, pedal 

 ganglion; 77, abdominal; 

 777, visceral ; 7F, pleural ;F, 

 order: (l) The pair of pedal cerebral; r7, buccal; 7,2,5, 



ganglia, which lie under the r"':*'' '^' ^T'/' ^'^^ 



00' _ _ 7, 7 J, mner\'ate body wall; 



radular sac, and are joined s, to aorta; 9, pulmonary 

 to each other by an anterior '^"^ f ' Pf"">' /f' f"^'- 



J . palleal wall; 72,to mtestme; 



and a posterior commissure; 14, optic; 75, copuiatory; 

 (2) one abdominal ganglion ^^' ''f^'^'' ^7- buccal; iS, 



^ . . 1 buccal commissures; ig, la- 



a little to the right of the me- biai; 20, oesophageal. 

 dian plane (which is inti- 

 mately fused with the right 

 visceral, and is also in close 

 connection with the left vis- 

 ceral ganglion, p. 199); (3) 

 a pair of visceral ganglia occupying the posterior angle formed by 

 the outgrowth of the radular sac from the oesophagus. They are 



