XII 



PHYLUM MOLLUSCA 



633 



lies between the dorsal ends of the valves : it is prodiiced 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 (Fig. 530). 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 re- 

 mainder 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 



h.i 



a act. 



-A' 



/ 



a-cui 



FIG. 525. Anodonta cygnea. A, interior of right valve ; B, the animal removed from the 

 shell, a. ml. anterior adductor or its impression ; a. /. anterior retractor or its impression : 

 d. g. digestive gland, seen through mantle ; t.c. spit, exhalant siphon ; .ft. foot ; ,<//. gills, seen 

 through mantle ; /;. /. hinge-line ; in. spit., 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 ; j>/. 1. pallial line ; pi. m. pallial muscles ; 

 p. }. posterior retractor or its impression ; pro. protractor or its impression. 



hinge-line (Fig. 525, A, li. /.), by a tough, elastic substance, the 

 hinge-ligament (Figs. 5 24 and 530,/#.) 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 Anoclonta 

 the only junction between the two valves is afforded by the liga- 

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



