360 C. M. Child 



judging from the form of the turbellarian intestine in relation 

 to the form and structure of other parts it is difficult not to believe 

 that functional and particularly mechanical conditions are impor- 

 tant factors in its development. In the rhabdoccels where no 

 strands of parenchyma or dorso-ventral muscles oppose it, it 

 forms simply a sac, filling the pseudocoel in part or wholly accord- 

 ing to conditions. In the polyclads, on the other hand, the intes- 

 tine might be compared roughly to an elastic sac placed in a space 

 in the axis of the body and then gradually distended so that parts 

 of it are forced mto the parenchymal spaces toward the periphery 

 of the body. If the fluid contents of the intestine move and exert 

 pressure in typical directions it seems to me that the effect of these 

 movements must necessarily appear in the direction and size of 

 the intestinal branches. All the facts seem to indicate that the 

 general direction and arrangement of the intestinal branches in 

 the various parts of the body is determined, at least in large part, 

 by the mechanical conditions resulting from movements and pres- 

 sures of the fluid contents. By altering these conditions the arrange- 

 ment of the intestinal branches can be altered, as I showed for 

 Leptoplana (Child '04a). In the triclads conditions are similar 

 but the intestine develops in different form because of the position 

 and form of the pharynx. The almost infinite variations in type 

 of the "normal" turbellarian intestine in a given species simply 

 show, in my opinion, how largely its form as regards details is a 

 matter of chance, determined often by the presence, absence, or 

 position of spaces, or dorso-ventral muscular fibers in the paren- 

 chyma, by slight individual differences in movement or consti- 

 tution of other parts, etc. 



If the functional conditions connected with the movements and 

 pressures of fluid contents are essential factors in determining the 

 form of the intestine, we may expect to find changes of form occur- 

 ring when these factors change and the facts justify our expecta- 

 tions. Starvation of a planarian results in degeneration and total 

 disappearance of the most distal portions of the intestine in succes- 

 sion: feeding results in the redevelopment of branches, but not 

 necessarily in the same pattern, and increased distension of a 

 normal animal results in the formation of new intestinal branches. 



