M(WrL\[KN'l'.S, KTC, OF FRRST[-\VATKIi PLANARTAXS. 53/ 



veg'ofcable food is not alone sufficient to keep tlie 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 dni'ing the day, will steadily grow 

 smaller niiless animal food is given them. 



The food is in part digested, or at an}' rate softoiied, and 

 physically changed ontside the body. A piece of mollusc, 

 on which a nnmber of worms have been feeding for some 

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

 after partial gastric digestion. 'J'his is ajiparently brought 

 about by a secretion poured out 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, liowever, 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 

 ano-ther 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) without food, although the}- 

 continually grow smaller dui-ing this 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 ('06) for ab- 

 normal embryos of Limulus. 



IV. Defecation. 



The process of defecation has been observed by Bai-deen 

 (:0l). The process consists of three or four general con- 

 tractions involving the whole body, during which the 



