268 



MANUAL OF ZOOLOGY 



SECT. 



The shell consists of three layers. Outside is a brown 

 horn-like layer, the periostracum (Fig. 148, prc.\ composed of 

 cjmchiolin, a substance allied in composition to chitin. 

 Beneath this is a prismatic layer (prs,\ formed of minute 

 prisms of calcium carbonate separated by thin layers of 

 conchiolin : and, lastly, forming the internal part of the 

 shell, is the nacre (n), or "mother-of-pearl," formed of 

 alternate layers of carbonate of lime and conchiolin, arranged 



jorc 



%prs 



FIG. 148. Vertical section of shell and mantle of Anodonta. c. t. connective-tissue 

 layer of mantle ; ep.i, its outer epithelium ; ep.2, its inner epithelium ; n. 

 nacreous layer of shell ; prc. periostracum; prs. prismatic layer. (After Claus.) 



parallel to the surface. The periostracum and the prismatic 

 layer are secreted from the edge of the mantle only, the 

 pearly layer from the whole of its outer surface. 



By the removal of the shell the body of the animal (Fig. 

 147, B) is seen to be elongated from before backwards, 

 narrow from side to side, produced on each side into a 

 mantle-lobe (m.) and continued ventrally into a keel-like 

 visceral mass (Fig. 149, v.m.\ which passes below and in 



