MOVKMI'INTS, ETC., OF FRESH-WATER PLANARFANS. 701 



4. The reactions of Planaria to a variety of chemicals, in- 

 cluding- representatives of several of the most important 

 chemical groups, were studied. It was found that to a weak 

 solution of any substance, regardless of its chemical composi- 

 tion, the organism gave a positive reaction identical with the 

 positive reaction to mechanical stimuli. To strong solutions 

 of the same substances (with a single exception, see p. 657) 

 the organisms responded by a negative reaction identical with 

 that caused by strong mechanical stinmli. 



Planaria does not orient itself to a diffusing chemical in 

 such a way that the longitudinal axis of the body is parallel 

 to the lines of diffusing ions. Its reactions to chemicals are 

 motor reflexes identical with those to mechanical stimuli. The 

 positive reaction is an orienting reaction in the sense that it 

 directs the anterior end of the body towards the source of the 

 stimulus with considerable precision, but it does not bring 

 about an orientation of the sort defined above. 



5. Several important features in the normal behaviour of 

 the flat-worm are found upon analysis to have their explana- 

 tion in the positive and negative reactions to mechanical and 

 chemical stimuli. 



The method by which the organism gets its food is simply 

 a special case of the positive reaction. From substances 

 which serve as food for the planarians, various juices diffuse 

 into the surrounding water. When the planarian meets any 

 of these diffusing substances it gives the positive reaction, — 

 that is, turns in the direction from which the stimulus comes. 

 The food substance acts as a weak chemical stimulus, to 

 which the animal reacts in the same way as to all other weak 

 chemicals. 



The direction of the planarian's movement, and its behaviour 

 with reference to obstacles in its path, are usually deter- 

 mined by its reactions to mechanical stinuili. 



The behaviour of the organism with reference to the 

 surface film is determined by its reactions to mechanical 

 stimuli. 



0. Strong stinmlation — either mechanical or chemical — of 



VOL. 46, PAKT 4. — NKW SERIES. Z Z 



