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 

 ganglia (b) and (c\ by long paired connectives, 

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



FIG. 1 1 8. Nervous system of Molluscs. 



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

 middle that of Helix. In the last two the position of the gullet is 

 shown. 



c.p.) Cerebro pleural ganglia; /., pedals; v., yiscerals; c., cere- 

 brals; //., pleurals ; ., 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 with 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. 



