VII SHELL 383 



ctenidia, as they are 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. 95). 



When the body of the mussel is removed from the shell 

 the tAvo valves are seen to be united, along a straight kinge- 

 'line, by a tough, elastic substance, the hinge-ligament (Fig. 

 95, 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 concentric 

 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 

 aiticular exoskeleton. As the animal grows each layer projects 

 beyond its predecessor, and in this way successive 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 characteristic 

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

 from it, is a delicate streak caused by the insertion into the 



