266 MANUAL OF ZOOLOGY SECT. 



the body lies between the dorsal ends of the valves ; it is 

 produced in the middle ventral line into the keel-like foot, 

 and on each side, between the foot and the corresponding 

 mantle lobe, are two delicate, striated plates, the gills. 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. 



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 147, A, /z./.), by a tough, elastic substance, the hinge- 

 ligament (Fig. 146, lg.\ 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 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 (Fig. 146) parallel with the free edge or gape, and 

 starting from a swollen knob or elevation, the umbo (um.\ 

 situated towards the anterior end of the hinge-line. These 

 lines are lines of groivth. The shell is thickest at the umbo, 

 which represent the part first formed in the young animal, 

 and new layers are deposited under this original portion, as 

 secretions from the mantle. 



The inner surface of the shell also presents characteristic 

 markings (Fig. 147, A). Parallel with the gape, and at a 

 short distance from it, is a delicate streak (//./.) caused by 

 the insertion into the shell of muscular fibres from the edge 



