Studies on Regulation 397 



and complete degeneration in the redifFerentiating region and are 

 replaced in the longer pieces by new branches, corresponding in 

 arrangement with those of a normal postpharyngeal region. 



3 In headless pieces which have not sufficient regulatory capac- 

 ity to give rise to a new pharynx (short prepharyngeal pieces and 

 most postpharyngeal pieces) the old intestinal branches undergo 

 rapid and complete degeneration, but after two months or more a 

 new" set of short and slender intestinal branches arise, which per- 

 sists, but undergoes gradual reduction as time goes on. 



4 In all other pieces undergoing regulation without food intes- 

 tinal reduction occurs and usually proceeds from the peripheral 

 towards the middle regions, though special modifications occur 

 with special conditions. 



5 The intestine of polyclad and triclad turbellaria is not merely 

 a digestive organ, but functions also as a reservoir for the tempo- 

 rary accumulation and storage of undigested food-material and 

 also, to a considerable extent, as a circulatory system. Its con- 

 tents are largely fluid and undergo movement in consequence of 

 the muscular contractions of the body-wall. The presence and 

 movements of these contents must produce characteristic mechan- 

 ical effects upon the intestinal wall. 



6 The facts of intestinal regulation indicate that these mechan- 

 ical conditions play an important role in determining the outline 

 of the intestine and the direction and arrangement of the branches. 

 Total disappearance of the old branches occurs when the mechan- 

 ical conditions are widely altered, even though nutritive material 

 be present in excess. The rapidity of degeneration depends on 

 the degree of change in the mechanical conditions. The develop- 

 ment ol new branches after degeneration of the old is determined 

 primarily, not by the presence of nutrition, but by mechanical con- 

 ditions, though of course nutritive material is necessary for such 

 development. 



Undoubtedly certain features of intestinal regulation are deter- 

 mined by other functional factors, but the general outline and the 

 arrangement and direction of branches are very evidently closely 

 correlated with mechanical factors. 



