666 



ZOOLOGY 



SECT. 



it is a delicate streak (pi. I.) caused by the insertion into the shell 

 of muscular fibres from the edge of the mantle : the streak is hence 

 called the pallial line. Beneath the anterior end of the hinge the 

 pallial line ends in an oval mark, the anterior adductor impression 

 (a. ad.), on to which is inserted one of the muscles which close the 

 shell. A similar but larger posterior adductor impression (p. ad.) 

 lies beneath the posterior end of the hinge. Two smaller markings 

 in close relation with the anterior adductor impression mark the 

 insertion of the anterior retractor (a. r.), and of the protractor (prc.) 

 of the foot : one connected with the posterior adductor impression 

 is that of the posterior retractor (p. r.). From all these im- 

 pressions faint converging lines can be traced to the umbo : they 

 mark the gradual shifting of the muscles during the growth of the 



animal. 



The shell consists of three 

 layers. Outside is a brown 

 horn-like layer, the peri- 

 ostracum (Fig. 574, prc.), 

 composed of conehiolin, a 

 substance allied in composi- 

 tion to chitin. Beneath this 

 is a prismatic layer (prs.) 

 formed of minute prisms of 

 calcium carbonate s e p a - 

 rated by thin layers of con- 

 chiolin ; and, lastly, forming 

 the. internal part of the 

 shell is the nacre (n.), or 

 " mother-of-pearl," formed 

 of alternate layers of car- 

 bonate of lime and con- 

 chiolin arranged parallel to 

 the surface. The perio- 

 stracum 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-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. 573, B) 

 is seen to be elongated from before .backwards, narrow from side 

 to side, produced on each side into a mantle-lobe (m.) and con- 

 tinued ventrally into a keel-like visceral mass (Fig. 575, v.m.), which 

 passes below and in front into the foot (ft.). Thus each valve of 

 the shell is in contact with the dorso-lateral region of the body 

 of its own side together with the corresponding mantle-lobe, and 

 it is from the epithelium (Fig. 574, ep.l) covering these parts that 

 the shell is formed as a cuticular secretion. The whole space 



FIG. 574. Vertical section of shell aiid mantle of 

 Anodonta. c.t. connective-tissue layer of 

 mantle ; ep.l, its outer epithelium ; ep. 2, its 

 inner epithelium ; n. nacreous layer of shell ; 

 prc. periostracum ; prs. prismatic layer. (After 

 Glaus.) 



