254 C. M. Child. 



any "redifferentiation" as in the case of Gonionemus (Morgan, 



'99)- 



Some term is necessary to denote those changes of form in the 



Turbellaria and other groups which are primarily mechanical 

 and connected with motor activity. There is no fundamental 

 difference between such changes in the new parts and in the old. 

 Both, as well as many other regulative phenomena, may be 

 included under the head of mechanical regulations (Child, '02). 

 The fact that the typical proportions usually result is merely 

 incidental. I have shown ('02) that in the absence of the loco-' 

 motor tensions the result may be exactly the reverse. Until 

 opportunity offers for a more extended discussion of this matter 

 I prefer to designate these changes merely as changes in propor- 

 tion. 



During form-regulation in Leptoplana changes in proportion 

 smiilar to those occurrmg m Planana and Stenostoma occur. In 

 pieces containing the cephalic ganglia changes are considerable, 

 though not as great nor as rapid as in Planaria and Stenostoma, 

 a difference which is evidently due to the fact that the tissues of 

 Leptoplana are less soft and plastic than those of the other forms 

 mentioned. The changes consist in relative elongation and re- 

 duction of the transverse diameter, especially toward the posterior 

 end. 



In order to make clear my point of view in these experiments 

 it is necessary to refer briefly to earlier experiments on Stenos- 

 toma and Leptoplana (Child, '02, '03a). In the case of Stenos- 

 toma I found that the changes in form and proportion of the pieces 

 during regulation, /. e., the elongation and the change from 

 cylindrical to conoidal form were due primarily to the tension 

 upon the tissues consequent upon the use during locomotion of 

 the posterior end as an organ of attachment. It was possible to 

 inhibit or retard the change in proportions by preventing the 

 pieces from attaching themselves to the substratum (Child, '03a). 

 In Stenostoma the changes in proportion are much more rapid than 

 in other forms examined, being completed in many cases in twenty- 

 four to thirty hours. 



In my first paper on Leptoplana (Child, '04a) a brief descrip- 

 tion of the method of locomotion was given, certain points of 

 which must be recalled to mind. In creeping, Leptoplana uses 

 the margins of the head region for drawing the body forward. 



