SKIN— MUSCULAR SYSTEM. 365 



(a) The anterior adductor. 



(^) The posterior adductor. 



{c) The anterior retractor of the foot continuous 



with (fl). 

 {d) The protractor of the foot a httle below {a), 

 {e) The posterior retractor of the foot continuous 

 with {b). 

 As the shell grows the insertion of the muscles and the 

 attachment of the mantle change, and the traces of this 

 shifting are visible. 



Skin. 



There is much ciliated epithelium about Anodonta, especi- 

 ally on the internal surface of the mantle, on the gills, and on 

 the labial palps ; and little pieces cut from an animal incom- 

 pletely dead {e.g., from the oyster which many of us swallow 

 half alive) have by means of their cilia a slight power of 

 motion. The skin of the foot is not ciliated but glandular ; 

 on the mantle edge sensitive and glandular cells are abun- 

 dant, but usually in inverse ratio to one another. 



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 contains some 

 coils of gut and the reproductive organs ; its lower region 

 is very muscular. The protrusion or extension of this loco- 

 motor 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 are of 

 the slowly contracting non-striped sort. 



