MOVRMRNTS, ETC., OF FRRSH-WATRR PLANARTAXS. 537 



vegetal)le food is not alone sufficient to keep the animals in 

 good condition, for specimens kept in an aqnarinm dish with 

 plenty of living- plant material, on which they stay the 

 greater part of the time during the day, will steadily grow 

 smaller unless animal food is given them. 



The food is in part digested, or at any rate softened, and 

 physicall}^ changed outside the body. A piece of mollusc, 

 on which a number of worms have been feeding for some 

 time, has a white, fluffy appearance, similar to that of meat 

 after partial gastric digestion. This is apparently brought 

 about by a secretion poured ont of the end of the pharynx. 

 The necessity for some such action is apparent, because the 

 flat-worm has no teeth or other means of separating a portion 

 of ordinary tough fibrous tissue off from a mass so that it can 

 be swallowed. This can be done, however, if the mass is first 

 softened and partially dissolved. There are certain other 

 evidences that a secretion is poured out from the pharynx 

 during the feeding process. These will be taken up in 

 another connection. 



After the worms have fed undisturbed for a certain length 

 of time they will leave the meat, and, after a short period of 

 activity, come to rest. 



The worms are able to live for a considerable length of 

 time (at least two months) Avithout food, although they 

 continually grow smaller during tliis time. This marked 

 decrease in size while starving has been noted by several 

 observers, and especially studied by Lillie (.-00). This 

 author finds that the decrease in size is accompanied by a 

 simplification of structure — a sort of "development back- 

 ward," such as has been described by Patten ('90) for ab- 

 normal embryos of Limulus. 



IV. Defecation. 



The process of defecation has been observed by Bardeen 

 (:01). The process consists of three or four general con- 

 tractions involving the whole body, during which the 



