268 



MANUAL OF ZOOLOGY 



SECT. 



in an oval mark, the anterior adductor impression (a. ad), 

 into which is inserted one of the muscles which close the 



FIG. 159. Anodonta cygnea. A, interior of right valve; B. the animal removed 

 from the shell, a. ad, anterior adductor or its impression; a. r, anterior retrac- 

 tor or its impression; d. g, digestive gland, seen through mantle; ex. sph, exhal- 

 ant siphon; ft, foot; gl, gills, seen through mantle: h. 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; pc, 

 pericardium, seen through mantle; //. /, pallial line; //. m, pallial muscles; 

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



shell. A similar but larger posterior adductor impression 

 (p. ad) lies beneath the posterior end of the hinge. 



The shell consists of three layers. Outside is a brown 

 horn-like layer, the pcriostracum (Fig. i6o,prs), composed 





