364 C. M. Child 



of the broader polyclads corresponds very closely to what may be 

 expected if hydrodynamic factors play an important part in their 

 formation. 



On the following pages the various regulatory changes in the 

 intestine of Cestoplana under various conditions are described 

 and their bearing on the above dynamic hypothesis of intestinal 

 development is discussed. 



Perhaps it should be added in order to forestall objections that 

 hydrodynamic factors are not considered as the only factors 

 involved in determining intestinal outline and arrangement ot 

 parts in the turbellaria. It seems very probable that other factors 

 must also play some part, though the facts seem to me to indicate 

 that hydrodynamic factors are certainly of great importance. 



II THE NORMAL INTESTINE AND THE TYPICAL COURSE OF INTES- 

 TINAL DEGENERATION IN THE ABSENCE OF FOOD 



I Descriptive 



The appearance of the intestine in newly captured animals 

 differs to some extent, apparently according to the previously 

 existing conditions. In Fig. i the terminal and middle regions of 

 the intestine in a normal newly captured specimen are shown, 

 somewhat diagrammatically. The intestine in this case is only 

 moderately distended by its contents: in many cases it is so dis- 

 tended that no spaces between the branches are visible and it 

 appears as in Fig. 2. 



In uninjured animals kept without food a gradual reduction or 

 degeneration of the intestinal branches occurs, though much more 

 slowly than under certain experimental conditions. Intestinal 

 reduction proceeds from the peripheral or terminal region of the 

 intestine toward the middle. 



The first parts to disappear are the tips of the branches at the 

 anterior and posterior end and as reduction of these branches 

 continues branches nearer the middle region are affected until a 

 condition resembling that shown in Fig. 6 is attained. In this 

 case which represents a normal animal after about four and a half 

 months without food, only short stumps of the lateral branches 



