366 



MOLLUSC A. 



Nervous System. 



There are three pairs of nerve centres : — 



(a) Cerebro-pleural ganglia, lying above the mouth on 

 each side on the tendon of the anterior retractor 

 of the foot, connected to one another by a com- 

 missure, connected to the two other pairs of 

 gangha (b) and (<:), by long paired connectives, 

 and giving off some nerves to mantle, palps, &c. 



Fig. ii8. — Nervous system of Molluscs. 



To.the left that of Ancdonta. ; to the right that of Octopus ; in the 

 middle that of Helix. In the last two the position of the g:ullet is 

 shown, 



c.p., Cerebro pleural ganglia ; /., pedals; zr., viscerals ; c, cere- 

 brals; //., pleurals ; d.^ buccals; s., stellate ganglion. 



{b) Pedal ganglia, lying close together about the middle 

 of the foot, united by connectives to {a), giving 

 off nerves to the foot, and having beside them 

 two small ear sacs, each w^ith a calcareous 

 otolith, and with a nerve said to be derived 

 from the connective between {a) and {b). 



(c) Visceral ganglia (also called parieto-splanchnic or 

 osphradial), lying below the posterior adductor, 

 connected to {a) by two long connectives, and 

 giving off nerves to mantle, muscles, &c., and 

 to a patch of " smelling cells " at the bases of 

 the gills. 



