96 THE BEHAVIOR OF LOWER ORGANISMS. 



It will be important to keep in mind the nature of the reactions to 

 mechanical stimuli, especially in the infusoria, in considering the 

 reactions which are more usually classed among the tropisms. 



REACTIONS TO CHEMICALS.* 



The reactions to chemicals have been studied by the present author 

 and those associated with him in many Ciliata, in certain Flagellata, 

 in the Bacteria, the Rotifera, and the flatworm ; further studies, not 

 yet published, have been made on other organisms. Now, in regard 

 to our first question, as to orientation, the following must be said : 

 In no case has the typical reaction been found to take the form of an 

 orientation, such as is demanded by the theory of tropisms. In the 

 ciliates, flagellates, and rotifers the reaction has been found to take the 

 form of a " motor reflex," a backing followed by a turning toward a 

 certain structurally defined side, without regard to the direction from 

 which the chemical is diffusing. It is this motor reflex that causes the 

 organisms to collect in the region of certain chemicals, and to avoid 

 others. (Details in Jennings, "Studies," Nos. I-X.) 



In the Bacteria the results of our work (Jennings & Crosby, 1901) 

 are in agreement with those of Rothert (1901). Here, again, in the 

 gatherings in certain chemical solutions, or in the avoidance of others, 

 there is nothing resembling an orientation in the lines of diffusion. 

 The phenomena are brought about through a reaction of the same 

 essential character as the motor reflex of the infusoria, but still simpler. 

 The essential point is that the Bacteria, when stimulated chemically, 

 reverse the direction of movement. (Details in the papers just cited.) 



In the flatworm the results of the thorough study of the chemical 

 reaction by Pearl (1903) maybe given in that author's own words: 



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 reac- 

 tions 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 toward the source of stimulus with considerable pre- 

 cision, but it does not bring about an orientation of the sort defined above. (Pearl 

 loc. ci/., p. 701.) 



For details, the original paper of the author quoted must be consulted. 

 It may be added that this positive reaction, by which the anterior end 

 is directed toward the source of stimulus, is identical with that which 

 takes place in response to a single mechanical stimulus. This is analyzed 

 above (p. 95). 



Are there any precise and detailed observations which support the 

 idea that the reaction to chemicals is ever a typical tropism ? Before 



*For a statement of the theory of tropisms as applied to chemicals, see Loeb 

 (1897, p. 442) and Garrey (1900, pp. 292, 293). 



