Xi PHYLUM MOLLUSCA 269 



of conchiolin, a substance allied in composition to chitin. 

 Beneath this is a. prismatic layer {pre), 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 (») or " mother-of-pearl," formed of 

 alternate layers of carbonate of lime and conchiolin arranged 

 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. The hinge 



Fig. 160. — Vertical section of shell and mantle of Anodonta; ct, connective-tissue 

 layer of mantle; ep.l, its outer epithelium; ep.2, its inner epithelium; w, nacreous 

 iayer of shell; pre, prismatic layer; prs, periostracum. (After Claus.) 



ligament is continuous with the periostracum, and is to be 

 looked upon simply as a median uncalcified portion of the 

 shell, which is therefore, in strictness, a single continuous 

 structure. 



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

 159, 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 



