1 6 C. M. CHILD. 



bulk than those from shallow sand. It is scarcely probable 

 that the difference was due to difference in nutrition, for so far as 

 it was possible to judge, the region of deep sand was much more 

 barren of life and organic matter than the other. I think there 

 can be no doubt that growth in the longitudinal direction is stim- 

 ulated " functionally " so long as the aboral end does not come 

 into contact with some solid surface to which it can attach. 



In animals which possess no hard parts and in which muscles 

 and supporting tissues form an important part of the body, the 

 shape of the body, the direction of growth, and the relative size 

 of various parts, must be determined, at least in large measure, 

 by the functional conditions accompanying muscular contraction 

 and by other mechanical factors. These factors may act either 

 directly, as deforming factors, or indirectly through the " func- 

 tional" stimulus, or they may act in both ways. Such species 

 determine their own form in greater or less degree by their be- 

 havior. If we alter experimentally the character or energy of 

 their reactions some more or less extensive morphological change 

 is likely to occur. In such cases it is merely the capacity for re- 

 action that is given in the germ, i. c., inherited. The actual 

 result in any given case depends on the internal and external 

 conditions of development. 



In the "Studies on Regulation" I have discussed the signifi- 

 cance of certain mechanical and "functional' 1 factors for the 

 shape and certain other features of the structure of turbellaria, and 

 in "Form Regulation in Cerianthus" for this form. The regula- 

 tion of Harenactis in altered environment shows very clearly that 

 similar factors are important in determining shape and other 

 structural relations in this species, and the importance of these 

 factors will appear still more clearly from the data of restitution. 



HULL ZOOLOGICAL LABORATORY, 

 UNIVERSITY OF CHICAGO, 

 October, 1908. 



