136 OILMAN A. DREW. 



clapping the shell valves together, as in the case of Pccten (3), or 

 by the more complicated method used by Solenomya and the 

 form under consideration. It is likely that the jets thrown by 

 this form are of very secondary importance, so far as swimming 

 is concerned, and that their chief function is to aid the animal in 

 burrowing. This is indicated by the fact that the jets are thrown 

 from the anterior end of the shell, while in forms that use them 

 for cleaning the mantle chamber only, they are thrown from the 

 siphons. 



Leaping. Leaping may consist simply of a sudden, powerful 

 protrusion of the foot, in which case the animal generally turns 

 so as to lie somewhat nearly on its dorsal margin and catches the 

 tip of its foot in the mud as it is protruded. The shell is thus 

 thrown posteriorly. Generally, however, the foot is bent back 

 under the shell, which is turned partly over towards its dorsal 

 margin (Fig. 3) and is then suddenly made rigid with the result 

 that it straightens out with great rapidity. This may result in 

 projecting the animal backward, or in certain cases the foot may 

 catch so as to turn the shell more or less completely end for end. 

 Leaping movements are usually rapidly repeated several times 

 when they are once begun. In many ways they resemble similar 

 movements in Yoldia and Solenomya (2), but the foot of this form 

 is so much longer that the impression of much greater activity is 

 left with the observer. 



The perfection of the movements of burrowing by a form that 

 lives in the mud, so it may be able to escape its enemies, is of 

 so much importance as to need no comment. When combined 

 with sense organs that give immediate information of the presence 

 of enemies, and with protective coloration that hides it from its 

 enemies until they shall have given it warning, the rapid burrow- 

 ing movements form a striking adaptation. 



The uses of the swimming and leaping movements are not quite 

 so evident. Small razor-shell clams have been taken in tow nets 

 at the surface of the sea. The ability to swim is, then, sufficient 

 to make- it possible for the young specimens, at least, to change 

 their positions after settling to the bottom, and after the larval 

 locomotor organ, the velum, has been lost. If the first location 

 does not offer the necessary food or bottom conditions, it is pos- 



