5 TMK BEHAVIOR OF LOWEK ORGANISMS. 



the organism is necessarily turned with anterior end toward the source 

 of stimulus ; then its usual forward movements take it toward the 

 source of stimulus (positive taxis or tropism). Loeb lays especial 

 stress on the direction from which the stimulus comes, as it is this 

 that determines which side shall be most strongly affected by the 

 stimulus ; otherwise the theory as he sets it forth is essentially like that 

 held by Verworn. Both these authors apply this schema to the move- 

 ments of organisms to and from many sorts of stimuli, making it a 

 general formula for taxis or tropisms. Verworn says (1899, p. 503): 



Thus the phenomena of positive and negative chemotaxis, thennotaxis, photo- 

 taxis and galvanotaxis, which are so highly interesting and important in all or- 

 ganic life, follow with mechanical necessity as the simple results of differences 

 in biotonus, which are produced by the action of stimuli at two different poles of 

 the free living cell. 



In the present series of papers the writer proposes to examine the 

 behavior of a number of lower organisms, in order to determine 



FIG. 2.* 



whether the reactions to the usual stimuli take place in accordance 

 with this tropism schema or not, and if not, to determine the real 

 nature of the reaction method. In this first paper we shall deal with 

 reactions to heat and cold. 



In his recent series of papers on the reactions of infusoria to heat and 

 cold, Mendelssohn (1902, a, b, c} develops a theory of thermotaxis in 

 accordance with the general theory of tropisms, above set forth. In an 

 earlier paper (Jennings, 1899) the present author, on the other hand, 



* FIG. 2. Diagram of a positive reaction, according to the tropism schema. 

 A stimulus coining from the direction indicated by the arrows to the right acts 

 upon the organism a. The effect of the stimulus is to cause the motor organs 

 directly affected by it to contract less strongly, as indicated by the lighter shade 

 on the right side of a. As a result the animal is turned as shown by the curved 

 arrows, occupying successively the positions , &, c, d. At d the stimulus 

 affects the two sides alike, hence there is no cause for further turning, and the 

 usual forward movement of the organism takes it toward the source of stimulus. 



