134 GILMAN A. DREW. 



squirts the water against the mud ahead of the shell, the shell is 

 decreased in diameter by being closed, and the mud is dislodged 

 and washed up the sides of the shell where it may be seen raising 

 after each downward movement of the shell. The action is 

 similar to the pile driver that opens a way for the pile by a 

 somewhat similar stream of water. 



The burrowing movements may follow each other quite rapidly 

 but the extension of the foot is never very rapid, as it must be 

 carefully worked into the mud to keep from forcing the shell 

 back. The opening of the shell just before the extension of the 

 foot, tends to embed it more firmly and thus to hold it in position 

 while the foot is being worked into the mud. 



A specimen laid on its side on mud, has no difficulty in gaining a 

 hold with its foot that enables it to right itself and start the anterior 

 end into the mud. Burrowing is then normal, and the shell is soon 

 completely buried. The time necessary for a specimen to com- 

 pletely bury itself varies with the character of the mud. In soft 

 mud the thrusts may be rapid and few are needed, in hard sand 

 the thrusts will necessarily be slower and more movements are 

 required, but even in such material the animal will disappear very 

 promptly. When the animal is laid on its side on such sandy 

 mud as that in which it usually lives, one movement will fre- 

 quently suffice for it to right itself, and four or five more will 

 carry it out of sight. The time necessary for this may be less 

 than half a minute. Embedded as the animal usually lives, a 

 single retraction takes it out of sight and away from enemies. 



Swimming. It is more difficult to study the movements of 

 swimming, as animals swim only occasionally, and then generally 

 immediately after being dug, and the movements of parts of the 

 animal, and the animal as a whole, are so rapid as to make ac- 

 curate observations difficult. The following points have been 

 determined however and, from these, conclusions may be drawn : 

 (i) The animal progresses posterior end foremost; (2) move- 

 ments are by jerks, each jerk carrying the animal one or more 

 times its length ; (3) the foot is very active, being thrust out and 

 withdrawn repeatedly. The outward thrust is comparatively 

 slow but the withdrawal is extremely rapid ; (4) apparently the 

 valves of the shell are drawn together every time the foot is re- 



