throw from the shell by rather suddenly closing the shell, to free 

 the mantle chamber of the dirt that gains entrance. The ability 

 to form a strong current is much better developed in some forms 

 than in others, the difference apparently depending upon the 

 ease with which dirt can be dislodged. Thus in the fresh-water 

 mussel which lives near the surface of the mud, with at least a 

 portion of the shell valves protruding above- the surface, the 

 current produced is not nearly so strong as in the case of the 

 soft-shelled clam, Mya arenaria, which lives deeply buried in the 

 mud where the dirt that gains entrance has to be elevated some 

 distance, and where mud in suspension, caused by waves and 

 tidal currents combined, is frequently considerable. 



Many forms of lamellibranchs when put into aquaria may 

 move some distance along the bottom by thus squirting water 

 from the shell. This is true with Yoldia, Angulus, Mya and a 

 large number of other forms, and at least one form, Solenomya, 

 (6) swims freely in the water by producing a somewhat similar 

 current of water. As such a current of water is primarily of 

 service in cleaning the mantle chamber and is used habitually for 

 locomotion by only a few forms, it seems quite possible that 

 those forms that do use it for locomotion may have simply per- 

 fected an already existing mechanism primarily designed for 

 another purpose. 



Pecten is one of the ablest swimmers among lamellibranchs. 

 The whole structure of the animal is modified for this purpose. 

 The valves have become rounded in outline, flattened, and com- 

 paratively light. The anterior adductor muscle has been lost, 

 and the posterior adductor muscle, which is very powerful, is 

 situated near the middle of the body. The cartilage has become 

 well developed so the shell may be opened quickly when the 

 muscle relaxes, and the hinge line is straight so there may be no 

 unnecessary strains in opening and in closing the shell. Each 

 gill is attached by one lamella only, so water in the temporary 

 cloacal chamber may be thrown out without injuring the gills, 

 and the gills and margins of the mantle are provided with 

 muscles to withdraw them from the margins of the shell when 

 the shell is closed. Furthermore the margins of the mantle are 

 provided with in-folded ridges and with circular muscles so it is 

 possible to direct the current of water which issues from the 

 shell in the required direction. 



4 



