FORM REGULATION IN CERIANTHUS. 249 



two ends, though in general the oral end is slightly in advance of 

 the aboral end. 



DISTENSION OF THE PIECES WITH WATER. 



The piece remains completely collapsed during two or three 

 days in summer and five to six days in winter, and then gradu- 

 ally becomes distended. At this time the piece is completely 

 closed at both ends, no mouth or aboral pore being present. It 

 is probable that the accumulation of water in the enteron is the 

 result of diffusion through the walls, and especially through the 

 very thin membranes at the two ends, in consequence of the 

 accumulation of soluble products of metabolism in the closed 

 enteron. 



In the course of a day or two the piece becomes well filled 

 with water and attains a degree of distension approaching that of 

 the normal animal, though not as great. In some cases the ac- 

 cumulation of water in the enteron occurs so rapidly that the thin 

 membranes closing the ends are ruptured and collapse occurs 

 again, though usually the increase in thickness and strength of 

 the membrane is sufficient to prevent rupture. The piece is 

 usually well filled with water by the fourth day in summer and 

 usually by the seventh or eighth in winter. 



The immediate result of the renewed distension of the piece 

 with water is, of course, the resumption of the cylindrical form ; 

 the body wall becomes translucent and is elastic to the touch 

 like that of the normal animal. 



The most marked effect of the internal water pressure occurs, 

 however, at the ends of the piece. So long as the piece re- 

 mains collapsed the thin membrane closing the ends is not visible 

 since the infolded edges of the body-wall are in close contact. 

 As the body becomes distended with water, however, the in- 

 folded portions gradually spread apart and a central area cov- 

 ered by the new tissue becomes visible. Very small at first, it 

 gradually increases in size until its diameter is about one third 

 the diameter of the end. Fig. 10 shows the oral end of a piece 

 at about this stage. The area within the folded margin of the 

 old body-wall is covered by the thin membrane of new tissue. In 

 Fig. 12 the aboral end of a similar piece is shown. There is 



