MUSCULAR SYSTEM. 



349 



Muscular system.— The shell is closed and kept closed 

 by the action of the two adductor muscles. When these are 

 relaxed under nervous control, the elasticity of the hinge 

 ligament opens the valves. A book with an elastic binding, 

 stretched when the book is closed by clasps, would in the 

 same way open when unclasped. It is easier for the mussel 

 to open the valves of its shell than to keep them shut. The 

 foot is a muscular protrusion of the ventral surface, under 

 the control of three muscles — a retractor and a protractor 

 anteriorly, and a posterior retractor. Its upper portion 



Fig. 152. — 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 ; p. , pedals ; v., viscerals ; t . , cere- 

 brals ; pi., pleurals ; b., buccals ; j., stellate ganglion. 



contains some coils of gut and the reproductive organs ; its 

 lower region is very muscular. The protrusion or extension 

 of this locomotor organ is mainly due to an inflow of blood, 

 which is prevented from returning by the contraction of a 

 sphincter muscle round the veins. In moving, the animal 

 literally ploughs its way along the bottom of the pond or 

 river pool, and leaves a furrow in its track. The muscle 

 fibres, as in the snail, are of the slowly contracting non- 

 striped sort. 



Nervous system. — There are three pairs of nerve- 

 centres : — 



