28 



the muscle are also asyiumetrical, and the right mantle 

 lobe is of much larger extent hekjw the adductor than the 

 left. 



There is an obvious separation of the single adductor 

 into two parts (fig. 1, ^4. v., A. s.) one of which is of 

 different structure from the other. In the fresh or living 

 animal these two regions are easily distinguished by their 

 different appearance, but they are quite distinct even in 

 preserved specimens. 



The greater part of the muscle {Add. s.) has a colour- 

 less, semi-translucent appearance, and this part is 

 cylindrical in section near the right valve, but elongates 

 and increases in area as it approaches the left valve, where 

 the muscle impression is slightly larger. Lying against 

 the posterior surface of this main portion, but clothed by 

 the same connective tissue sheath that passes round the 

 two parts and binds them together, is a narrow bundle 

 (Add. u.), crescent shape in section and made up of white, 

 more opaque looking fibres. Coutance (13) and Thoring 

 showed that the larger part serves only for the rapid 

 spasmodic closing of the shell, while the small portion 

 serves for slower but more forcible and sustained activity. 

 If one valve is taken away, which means that the attach- 

 ments of the adductor are cut through, the small white 

 portion falls into a state of permanent contraction 

 ("tonus") and thus in fixed preparations this portion of 

 the muscle is generally much more strongly contracted, 

 and, therefore, shorter than the larger part of the muscle. 



The other part contracts and relaxes rapidly if stimu- 

 lated. It is obvious that this development is correlated 

 with the function of swimming, and that the clapping of 

 the valves of the shell is due to the large translucent 

 portion of the adductor, whereas the more permanent 

 closing of the shell is due to the much smaller part. P. 



