76 C. M. Child 
correspondingly restitution at the aboral end in pieces kept under 
the same conditions does not proceed much beyond closure of 
the wound, though when the animals are allowed to burrow in 
sand it is much more nearly complete. 
In my experiments with Harenactis I endeavored to place pieces 
like Fig. 12 under natural conditions, i. e., | embedded them in 
sand with only the oral ends protruding, but their movements 
always resulted in their gradual emergence from the sand. Their 
emergence was due to the fact that since the aboral end was not 
firmly fixed in the sand contraction often drew the aboral end up- 
ward instead of the oral end downward and the sand gradually 
filled the space beneath the body. Longer pieces are more likely 
to remain in the sand since the more proximal portions are less 
likely to be dislodged by the contractions. I have no doubt, 
however, that if these shorter pieces can be kept in the sand aboral 
restitution will at least proceed much further; in all probability 
they will attain the usual form of the species. I hope to obtain 
further data on this point at a future time. 
As noted above aboral restitution in the subcesophageal region 
does not show any marked regional differences. So far as I can 
determine, the amount of redifferentiation is in some degree pro- 
portional to the size of the portion removed, i. e., short pieces like 
Fig. 12 representing the more distal portions of the body proceed 
somewhat further in the restitution of the proximal parts than do 
longer pieces. If restitution consists in an approach to the condi- 
tion of physiological equilibrium characteristic for the existing 
conditions, as I believe it does, this difference is to be expected, 
for when we remove the whole subcesophageal region we remove 
some portions which are characteristic of the condition of equi- 
librium under the experimental conditions. 
One regional difference in aboral restitution does occur, how- 
ever, though, macroscopically it is physiological rather than mor- 
phological. ‘This difference concerns the functioning of the aboral 
end of the piece as a foot. As I pointed out in an earlier paper 
- (Child ’o8b) the region of attachment shows no marked differences 
in structure from other regions of the body-wall. As a matter of 
fact, however, the more proximal the level of section the more 
