264 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



as in Modiola (Fig. 61), Perna, etc., as the adductor 

 muscle is brought nearer and nearer to the hinge-line, 



Fig. 61. Modiola plicatula. Lettering same as in Fig. 60. 



where its mechanical action is less and less effectual 

 in closing the valves, we find that it is more and more 



reduced until it finally dis- 

 appears from disuse and 

 atrophy, as in Ostrea (Fig. 

 62), and Pecten. Con- 

 versely, the posterior ad- 

 ductor in the same series in 

 the revolution of the axes is 

 pushed farther and farther 

 from the hinge -line and 

 nearer to the central plane 

 of the valves, where its me- 

 chanical action is most ef- 

 fectual in closing the valves. 

 With its increase in func- 

 tional activity the muscle 

 increases in size. The revolved position of the axes, 

 and the consequent reduction or loss of the anterior 

 adductor and increase of the posterior adductor, is 



Fig. 62. Ostrea inrginiana. Let- 

 tering same as in Fig. 60. 



