268 



MANUAL OF ZOOLOGY 



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

 into which is inserted one of the muscles which close the 



h.t 



aad 



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; //, 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; /. ad, posterior adductor or its impression; pc, 

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

 p. r, posterior retractor or its impression ; pre, 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 periostracinn (Fig. 160, prs), composed 



