360 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
the muscle are also asymmetrical, and the right mantle 
lobe is of much larger extent below the adductor than the 
left. 
There is an obvious separation of the single adductor 
into two parts (fig. 1, A. w., A. s.) cne 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 1n 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 raped 
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. 
