350 THE FRESH-WATER MUSSEL LESS. 



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. 



When the body of the mussel is removed from the shell 

 the two valves are seen to be united, along a straight hinge- 

 line (Fig. 92, A, h. /), by a tough, elastic substance, the 

 hinge-ligament (Fig. 93, B, lig] 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 opens the 

 shell. In Anodonta the only junction between the two 

 valves is afforded by the ligament, but in Unio each is pro- 

 duced 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 end of the hinge-line. These lines are lines of 

 growth. The shell is thickest at the umbo, which represents 

 the part originally formed, and new layers are deposited 

 under this original portion, as secretions from the mantle, 

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

 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 (Fig. 92, A). Parallel with the gape, and at a 



