HABITS AND MOVEMENTS OF THE RAZOR-SHELL CLAM. 133 



it, in which position the foot may remain quiet for some time. 

 When the movements are active they are essentially the same in 

 both cases. The foot is slowly protruded with the pointed tip 

 working as if trying to bore into the mud, ending each time with 

 a dorsal thrust. These movements are continued until the foot 

 is fully extended. During this extension the end of the foot is 

 kept small, the point is directed well forward, and the general 

 diameter of the protruded part of the foot is decidedly less than 

 its normal diameter when at rest. When the foot reaches its 

 greatest extension, the end is suddenly swelled into a great 

 bulb, more than twice the diameter of the remainder of the 

 foot (Figs, i and 4) and the whole foot becomes very rigid. 

 That this result is attained by injecting blood into the foot may 

 be readily proved by sticking spring forceps into the end of the 

 foot so the spring will hold the wound open, and stimulating the 

 foot to activity by stroking the tentacles as before described. 

 When the foot starts to become active the wound begins to 

 bleed rapidly, and when the final effort to swell the end of the foot 

 is made, the blood rushes out in a great jet, but the swelling is 

 slight. A simple incision does not answer as well, as the con- 

 tracting muscles seem to close the wound more or less perfectly. 



The instant that the swelling of the end of the foot is com- 

 plete, a process that takes place so rapidly as to be almost start- 

 ling, the retractor muscles pull the foot back to the shell with a 

 jerk, the end remaining swollen until it reaches the shell (Fig. 7). 

 It is then reduced in size and either withdrawn into the shell or 

 extended in the beginning of a new burrowing movement. 



While the foot is being extended the shell valves are allowed 

 to gap apart and the siphons and ventral opening in the mantle 

 are kept more or less widely open. Just before the final sudden 

 retraction, the siphons and ventral opening are all tightly closed, 

 and kept so until retraction is complete. The result is that the 

 water in the mantle chamber is discharged through the opening 

 through which the. foot is extended, between the collar and the 

 foot. Whether the water escapes all around the foot or only 

 ventral to it, where the contact of the collar is poorest, has not 

 been determined, but the jet of water is quite powerful. When 

 the shell is embedded in the mud, each retraction of the foot, 



