MOLLUSCA 



63 



protractor, posterior adductor, and posterior retractor, the 

 present position of each being marked by the broad termina- 

 tions of triangular-shaped areas which indicate the migration 

 and growth of the muscles. The adductors close the shell, 

 the others are related to protruding or withdrawing the fleshy 

 foot and are paired. The attachment of the mantle fold, 

 which in life lies close to each valve, may be traced in a line 

 near the ventral border of the shell, which it follows from the 

 ventral aspect of the anterior adductor to that of the posterior 

 adductor. This is the pallial 

 line, and in species in which 

 the posterior openings are carried 

 out on a siphon it is indented 

 posteriorly, forming the so-called 

 pallial bay. The bay is associ- 

 ated with the provision for the 

 retraction of the siphon. 



A section of the shell shows 

 that it consists of three layers, 

 the outer chitinous layer, perio- 

 stracum, which is uncalcified, 

 a prismatic layer both of these 

 are formed at the margin of 

 the mantle and a nacreous or 

 pearly layer which is secreted by 



the mantle as a whole. It is to be observed that the shell 

 is altogether outside the body and is secreted by the action 

 of the external ectoderm of the mantle and the continuation 

 of the external ectoderm over the dorsal part of the body. 



The mantle which produces the nacreous layer of the 

 shell forms pearls in various Mollusca, notably in certain 

 Pelecypoda. The pearl is initiated by an irritant, and usually 

 in nature by a larval trematode or cestode. The parasite 

 comes to lie between the mantle and the shell, and the outer 

 ectodermal layer of the former begins to secrete layers of 

 nacre around it, but the process is a slow one. According 

 to the position of the parasite, the resulting pearl will be free 

 or more or less attached to the shell. A perfect or fine pearl 

 is formed in an ectodermal pocket of the mantle. Lyster 



Periostracum 



'^Prismatic 

 layer 



Nacreous 

 layer 



External 



epithelium 

 Mesenchyme 



Internal 



epithelium 



FIG. 29. Anodonta cygnea. Sec- 

 tion of shell and mantle. 



