KIXE TO GENESIS. 



263 



may be retained, reduced, or lost, according as the 

 persistence or changes in correlated anatomical fea- 

 tures retain in use or bring into disuse the muscles in 

 question. 



"Let us look at examples of the retention or loss of 

 the adductors. In typical dimyarian pelecypods, as 

 in Mya (Fig. 60) or Venus, the adductors lie toward 

 either end of the longer axis of the shell. As the hinge 

 occupies a position on the borders of the shell about 

 midway between the adductors, both muscles are nearly 

 or quite in a position to be equally functional in clos- 



Fig. 60. Mya arenaria. Lettering: ap ax, antero posterior axis; h ax, 

 hinge axis; a ad, anterior, and/a<f, posterior adductor muscle; in. mouth; 

 //, palps ; a, anus ; g, gills ; fid, pedal muscle ; f, foot ; />, byssus ; h, heart. 



ing the valves. As a result, both muscles are of about 

 the same size. The condition described is that existent 

 in the completed prodissoconch stage in all pelecy- 

 pods, as far as known. In later life, however, a revo- 

 lution of the axes of the soft parts may take place, so 

 that the antero-posterior axis (represented by a line \ 

 drawn through the mouth and middle of the posterior 

 adductor muscle), instead of being parallel to the 

 hinge-axis (the axis of motion of the valves) as in 

 dimyarians, may present a greater or less degree of 

 divergence from the parallel. In progressive series, 



