4 C. M. CHILD. 



extreme cases reduces the length of the whole body to a small 



fraction of what it is when fully extended. 



In the condition of extreme contraction following 

 some violent stimulus almost all of the water is ex- 

 pelled from the enteron, largely through the cinclides, 

 and the body-wall contracts until the enteric cavity is 

 so reduced that further contraction is practically im- 

 possible on account of the bulk of the mesenteries, 

 F muscles, filaments, and gonads, if present, which now 



fill the enteron. 



II. REGULATION FOLLOWING REMOVAL FROM SAND. 



In the laboratory the animals were kept in dishes, containing 

 water several inches in depth, but without sand. This species is 

 exceedingly hardy and will live for months under these condi- 

 tions with occasional change of water. In my experiments I kept 

 many specimens under these conditions and without food during 

 four and a half months, and at the end of this time they were ap- 

 parently in good condition, though smaller than when collected. 

 It is under these conditions that the changes to be described occur. 



I . The Earlier Stages of Regulation. 



As long as the animal is in its burrow in the sand the pressure 

 upon the body-wall of the water in the enteron is supported in 

 large measure by the walls of the burrow, not by the body-wall 

 alone. After removal from sand the body never attains the de- 

 gree of extension and distension which it may attain in the burrow. 

 As in Ceriant/ins ccstnarii (Child, '08), so here, a regulation un- 

 doubtedly occurs both as regards the amount of water usually in 

 the enteron, and the ability of the body-wall to sustain the pres- 

 sure unaided. 



The effect of the altered conditions appears first chiefly in a de- 

 crease in length and frequently a slight increase in diameter of 

 the column. Figs. 8, 9 and 10 show three common types of the 

 shape of the body after twenty-four hours in water without sand- 

 There is great variation in the shape of both body and foot at this 

 time. In case the foot was originally attached to a bit of shell 

 or other solid body it usually frees itself soon after the animal is 



