PHYLUM MOLLUSCA 245 



layers: (1) an outer thin, horny layer, the periostracum, which 

 is secreted by the edge of the mantle, — it serves to protect the 

 underlying layers from the carbonic acid in the water, and gives 

 the exterior of the shell most of its color; (2) a middle portion 

 of crystals of carbonate of lime, called the prismatic layer, which 

 is also secreted by the edge of the mantle; and (3) an inner na- 

 creous layer (mother-of-pearl), which is made up of many thin 

 lamellae secreted by the entire surface of the mantle, and pro- 

 duces in the light an iridescent sheen. 



Anatomy and Physiology. — General Account. — The 

 valves of the shell are held together by two large transverse 



14. £ 



Fig. 174. — Right side of Anodonta mutabilis with mantle cut away and 

 right gills folded back. /, mouth ; 2, anus ; 3, cerebro-pleural ganglion ; 

 4, anterior adductor muscle; 5, anterior protractor muscle of shell; 6, re- 

 tractor muscle; 7, dorsal siphon; 8, inner labial palp; g, foot; 10, external 

 opening of nephridium; //, opening of genital duct; 12, outer right gill- 

 plate; 1 j, inner right gill-plate ; 14, ventral siphon; 15, epibranchial chamber; 

 16, posterior protractor muscle. (From Shipley and MacBride, after Hatschek 

 and Cori.) 



muscles which must be cut in order to gain access to the internal 

 organs. These muscles are situated one close to either end near 

 the dorsal surface; they are called anterior adductors (Fig. 174, 

 4; Fig. 175, a.ad) and posterior adductors (Fig. 175, p. ad). 

 As the shell grows, they migrate outward from a position near 

 the umbo, as indicated by the faint lines in Figure 173. When 

 these muscles are cut, or when the animal dies, the shell gapes 



