682 



ZOOLOGY 



SECT. 



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 deli-i 

 cate, striated plates, the gills (Figs. 566-568). 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. 



The Shell. When the body of the mussel is removed from 

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



n.i 



a. ad 



FIG. 563. Anodonta cygnea. A, interior of right valve ; B, the animal removed from th 

 shell, a. ad. anterior adductor or its impression ; a. r anterior retractor or its impression ;| 

 d. gl. digestive gland, seen through mantle ; ex. sph. exhalant siphon ; ft. foot ; gl. gills, seem 

 through mantle ; h. 1. hinge-line ; in. sph. inhalant siphon ; kd. kidney, seen through mantle ; 

 k. o. Keber's organ, seen through mantle ; m. mantle ; p. ad. posterior adductor or its 

 impression ; pc. pericardium', seen through mantle ; pi. 1. pallial line ; pi. m. pallial muscles; 

 . r. posterior retractor or its impression ; prc. protractor or its impression. 



hinge-line (Fig. 563, A, h.L), by a tough, elastic substance, the 

 hinge-ligament (Figs. 562 and 568, lg.) passing transversely from valve 

 to valve. It is by the elasticity of this ligament that the shell isi 

 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 liga-j 

 ment. but in Unio each is produced into strong projections and] 





