Form Regulation in Harenactis 75 
periods without sand in which to burrow. Briefly stated, these 
changes consist in a very great decrease in length with consider- 
able atrophy of the more proximal regions, including the retractor 
muscles. Pieces undergoing regulation were kept under the same 
conditions, and since under these conditions reduction and atrophy 
of the more proximal portions of the body occurs to a considerable 
degree in unmutilated individuals it cannot be expected that resti- 
tution of these parts shall be complete. In short the condition 
of physiological equilibrium differs widely from that existing under 
the usual conditions in nature, i. e., when the animal is in its bur- 
row in the sand. Consequently restitution under these altered 
conditions consists in an approach to the new condition of equi- 
librium determined by these conditions, not in an approach to the 
original condition of equilibrium determined by conditions which no 
longer exist. In such cases as Fig. 12 then, restitution is approx- 
imately complete for the conditions under which the individual 
is living, though it is not complete with reference to the animal 
as we usually find it in nature. 
There is nothing inherent in Harenactis which determines the 
restitution of these missing parts irrespective of the environmental 
conditions. As regards certain features, at least the form and 
structure of the species are the product of its constitution and the 
environment and with respect to these same features the same is 
true for restitution. Under the conditions of the experiments 
in question there is not and cannot be any stimulus for the resti- 
tution of the proximal regions of the body in the form character- 
istic of the species in nature, since these conditions determine re- 
duction and partial disappearance of these parts in individuals 
which originally possessed them. 
In Cerianthus solitarius (Child ’03, etc.) the shape and struc- 
ture of the body are much less dependent upon external conditions, 
though not wholly independent of them, and there removal of a 
proximal region of the body is followed by at least partial resti- 
tution. Cerianthus zstuarii (Child ’o8a) occupies a position 
intermediate between C. solitarius and Harenactis in this respect. 
When kept without sand marked decrease in length occurs and 
the more proximal portions undergo some degree of reduction, and 
