682 



ZOOLOGY 



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- 

 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 



cad. 



jbrc/ 



Fio. 503.— 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 ; /(. foot ; gl. gills, seen 

 through mantle ; It. I. 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 ; pe. pericardium, seen through mantle ; pi. I. pallial line ; pi. m. pallial muscles ; 

 i>. r. posterior retractor or its impression ; pre. protractor or its impression. 



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

 hinge-ligament (Figs. 562 and 568,/^.) 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 liga- 

 ment, but in Unio each is produced into strong projections and 



