StiiJit's 077 Regulation 363 



Local contractions and extensions of course cause local changes 

 in the distribution of intestinal contents, but these follow the 

 same rules as the more general movements. 



Evidently then, the intestinal branches in the middle region are 

 filled and distended by the intestinal contents which accumulate 

 in the middle region during contraction and the branches in the 

 terminal regions by the contents during their flow away from the 

 middle region. In the regions between the middle and end all 

 intermediate conditions exist. Those branches which are filled 

 and distended chiefly by the fluid accumulating in the pharyngeal 

 region arise at right angles to the axial intestine since their for- 

 mation is correlated essentially to lateral pressure of the intes- 

 tinal contents, escape in other directions being impossible. But 

 toward the ends of the body the intestinal branches are filled and 

 distended by fluid which is moving anteriorly or posteriorly. If 

 mechanical conditions are factors in determining the form of the 

 intestine, the intestinal branches in these regions may be expected 

 in accordance with the laws of hydrodynamics to be directed 

 more or less obliquely in the direction in which the fluid is moving. 

 The intestine in Cestoplana seems to me to possess exactly the 

 form which might be expected if movements and pressures of 

 fluid contents are the chief factors in producing it. The fact that 

 a gradual change in direction of the branches between the middle 

 and the ends of the body exists is due simply to the gradual change 

 in conditions. In the regions between the middle and terminal 

 regions the branches are filled and distended in part by the fluid 

 moving away from the pharynx, and in part by standing contents 

 escaping laterally from pressure in other directions. The nearer 

 the pharyngeal region, the more exclusive the latter condition of 

 filling and distension, the farther away the more exclusive the 

 former. Hence we may expect to find with increasing distance 

 from the pharynx a gradual change in the direction of the 

 branches from a position at right angles to the axis to one oblique 

 tow^ard the direction of movement of the contents. 



Similar conditions in general, with of course various specific 

 difi^erences, exist in other polyclads and triclads, and the form of 

 the intestine as a whole and of each of the long branches in many 



