398 THE MUSSEL CHAP. 



plates, constituting a gill or ctenidium, as it is often 

 called. Thus the whole animal has been compared to a 

 book, the back being represented by the hinge, the 

 covers by the valves, the fly-leaves by the mantle-lobes, 

 the two first and the two last pages by the gills, and the 

 remainder of the leaves by the foot (Fig. 1 03). 



When the body of the mussel is removed from the 

 shell the two valves are seen to be united, along a 

 straight hinge-line, by a tough, elastic substance, the 

 hinge-ligament (Fig. 103, Ig) passing transversely from 

 valve to valve. It is by the elasticity of this ligament 

 that the shell is opened ; it is closed, as we shall see, by 

 muscular action : hence the mere relaxation of the 

 muscles results in opening the shell. In Anodonta the 

 only junction between the two valves is afforded by the 

 ligament, but in Unio each is produced into strong 

 projections and ridges, the hinge-teeth, separated by 

 grooves or sockets, and so arranged that the teeth of 

 one valve fit into the sockets of the other. 



The valves are marked externally by a series of con- 

 centric lines parallel with the free edge or gape, and 

 starting from a swollen knob or elevation, the umbo, 

 situated towards the anterior edge of the hinge-line. 

 These lines are lines of growth. The shell is thickest at 

 the umbo, which represents the part first formed, and 

 new layers are deposited under and concentrically to 

 this original portion as secretions from the mantle, the 

 shell being, like the armour of the crayfish, a ciiticular 

 exoskeleton (p. 369). As the animal grows each layer 

 projects beyond its predecessor, and in this way suc- 

 cessive outcrops are produced, giving rise to the markings 

 in question. In the region of the umbo the shell is 

 usually more or less eroded by the action of the carbonic 

 acid in the water. 



The inner surface of the shell also presents character- 

 istic markings. Parallel with the gape, and at a short 



