XI PHYLUM MOLLUSCA 267 



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. 159, A, h. I), by a tough, elastic substance, the 

 hinge-ligament (Fig. 158,^) 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. 158) parallel with the free edge or gape, and 

 starting from a swollen knob or elevation, the umbo {urn), 

 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 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. 159, 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 

 of the mantle ; the streak is hence called the pallial line. 

 Beneath the anterior end of the hinge the pallial line ends 



