HABITS AND MOVEMENTS OF THE RAZOR-SHELL CLAM. 135 



tracted ; (5) each movement of the animal as a whole, corre- 

 sponds to the period of retraction of the foot. 



In describing the movements of burrowing it has already been 

 mentioned that water is thrown from the shell, through the 

 opening through which the foot is protruded, every time the foot 

 is retracted into the shell. Each jet is caused by closing the 

 other openings into the mantle chamber and driving the water 

 out by pulling the foot in, by closing the shell by the contraction 

 of the adductor muscles and the united margins of the lobes of 

 the mantle, and by drawing the mantle margins, with the shell 

 cuticle to which they are attached, into the mantle chamber 

 (Fig. 8). The resultant action is to drive out most of the water 

 that was between the valves of the shell, as nearly all of the 

 space is now occupied by organs of the body. As all of the 

 openings except the one around the foot are held closed, a very 

 strong jet of water must be forced out around the sides of the 

 foot. This is sufficient to cause the movement of the animal in 

 the opposite direction. Many muscles, all of which are power- 

 ful, are used in this action, and as the water is thrown through 

 a small opening between the muscular collar and the foot, the 

 resulting force is considerable. The action, so far as movement 

 is concerned, is similiar to what is so well known in the squid, 

 and differs from the movement of Solenomya only in direction (2). 

 Here the movement is posterior, in Solenomya the movement is 

 anterior. Here the water is admitted through the siphons, and 

 possibly also around the foot, and then, with the siphons closed, 

 the water is thrown from the anterior end of the animal. In 

 Solenomya the water is admitted around the foot, which opening is 

 then closed and the water is thrown through the posterior 

 opening. The method of forming the jet is quite the same 

 in both animals. In both forms the same organs are used, but 

 in Solenomya more use is made of the mantle margins and less of 

 the retraction of the foot than is the case in this form. 



Throwing strong jets of water from the siphons must aid 

 lamellibranchs in keeping their mantle chambers clean. Some 

 forms need to throw more powerful jets than others because the 

 conditions under which they live demand it. A diversion of this 

 use is apparently to be seen in the forms that swim either by 



