THE LAMELLIBRANCHIATA. 



407 



face is usually marked by concentric lines of growth, which 

 indicate the boundaries of the successively added new layers 

 of shell-substance. 



Fig. 117.— Sectional diagram of a fresh- water Mussel (Anodonta).— A, A. mantle, the 

 right lobe of which is cut away; B, foot; C, branchial chamber of the mantle 

 cavity; 7), anal chamber; I, anterior adductor muscle; II, posterior adductor 

 muscle ; III, retractor muscle of the foot ; a, mouth ; b, stomach ; c, intestine, 

 the turns of which are supposed to be seen through the side- walls of the meso- 

 soma; d, rectum; e, anus; /, ventricle; g, auricle; k, gills, except i. ri^ht exter- 

 nal gill, largely cut away and turned back; k, labial palpi; I, cerebral: m, pedal; 

 n, parieto-splanchnic ganglia; o, aperture of the kidney or or^an of Bojanus; p, 

 pericardium. 



The applied edges of the two valves are very often pro- 

 duced into elevations and depressions which interlock with 

 one another. The form and arrangement of these teeth and 

 sockets are of much use in systematic conchology. 



