Studies on Regulation. 255 



while the margins of the body from about the middle or a little 

 anterior to it, to the posterior end are employed as organs of 

 attachment, the most posterior part, the "tail" being most fre- 

 quently used in this manner. The body of the animal is fre- 

 quently subjected to tension during creeping and I believe that, 

 as in Stenostoma, this tension is of considerable importance in 

 determining the general form. Its effect upon the newly formed 

 regenerating parts has already been discussed (Child, '04b, '04c). 

 We have now to consider the changes in proportion of the old 

 parts during regulation and to discuss the role of mechanical 

 factors in these changes. Since the changes are slow it is imprac- 

 ticable to control them by preventing the animals from attaching 

 themselves to the substratum, as was done in the case of Stenos- 

 toma, but modification of the changes is possible by certain meth- 

 ods which will be described below. 



In Stenostoma (Child, '02) the posterior region of the body is 

 subjected most frequently and in greatest degree to tension, but in 

 Leptoplana the lateral margins of the body as well as the posterior 

 end are used for attachment, so that very frequently only the 

 anterior portions of the body are subjected to tension, the whole 

 posterior portion being attached. However, the posterior end is 

 usually the first part to attach itself and the last to be released, 

 hence in the long run it is undoubtedly more stretched than other 

 parts. It is probable that the outline of the body of Leptoplana 

 is due in large part to the mechanical factors connected with 

 attachment and locomotion. A form which like Stenostoma uses 

 only the posterior end and the median ventral region for attach- 

 ment must be slender and only slightly tapering in the definitive 

 condition, while on the other hand a species which uses the mar- 

 gins of the body for attachment will be broader and the decrease 

 in breadth toward the posterior end will depend upon the relative 

 frequency with which the lateral margins and the posterior end 

 alone are used for attachment. In Leptoplana the posterior end 

 is still the principal organ of attachment and the body possesses 

 a tapering form as it must according to mechanical principles if 

 it is plastic. But in such forms as Stylochus where the whole 

 margin is used as an organ of attachment and the posterior end 

 is used with no greater frequency than other parts the form of the 

 body becomes ovoid or almost circular. (See Child, '04a.) The 



