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KAY:\rOXIi I'KAKL. 



to survive, its individuals uiust liuvo some sort oC reaction 

 whereby they may avoid tlie harmful and take advantage of 

 the beneficial. In the case of planarians, the reactions to 

 cliemicals seem to be of about equal importance with the re- 

 actions to contact stimuli in the general activities. Since the 

 reactions to food substances are a special case of the reactions 

 to chemicals in general, they may be discussed first. 



a. Food Reactions. — The nature of the things used as 

 food by fresh-water planarians has been discussed ali-eady in 

 the section on '^Natural History," and hence need not detain 

 us here. 



A typical case of the food reactions to a bit of crushed 



Fig. 2G. — Diaj^ram showiiif^ tlio successive stages in llie normal food 

 reaction of PI an a via. A represents a small bit of meat. 



mollusc may first be described, to serve as a basis for the 

 account.^ If a piece of the body of Phy.sa which has jnst 

 been extracted from the shell and crushed between the points 

 of a pair of forceps is placed in a small dish containing a 

 number of active planarians, it will result from chance alone 

 that some of the flat-worms must in course of time pass near 

 the food material. For a very short time after the food has 



' The food reael ions of Planaria have; been l)iie(ly de^criljed i)_v Hardcen 

 (:01,«;. 



