112 THE BEHAVIOR OF LOWER ORGANISMS. 



PHYSIOLOGICAL STATES IN THE PROTOZOA (STENTOR 

 AS A TYPE). 



We will take up first the lowest organisms in which the matter has 

 been studied in detail, that is, the unicellular animals. These are of 

 special interest in view of their entire lack of a nervous system. As 

 the best-known case we may take the behavior of Stentor. This has 

 been described in full in a former paper by the present author (Jennings, 

 1902, a) ; for details this paper may be consulted. 



When a quiet, extended Stentor is stimulated by lightly touching it 

 or the support to which it is attached with a rod, it reacts by giving a 

 definite reflex, that is, by contracting into its tube. 



After this has taken place once or twice we find that the Stentor no 

 longer reacts as before. All the external conditions remain the same ; 

 the stimulus applied is the same. Nevertheless, the Stentor does not 

 react. Therefore, we must conclude that the Stentor itself has changed. 

 Its physiological condition is now different from what it was originally. 

 What the nature of the change in its condition is we do not know, save 

 in the fact that the Stentor in this second condition does not react as 

 does the Stentor in the first condition. For the sake of convenience 

 we may number the different physiological conditions, in order that we 

 may determine, if possible, how varied these conditions are. We will 

 call the physiological condition of the undisturbed extended Stentor, 

 before the stimulation, No. i, or the first condition. The condition 

 in which the Stentor no longer responds to the slight stimulus we will 

 call No. 2. 



We may frequently distinguish still a third condition in the behavior 

 under this simple stimulus. At first the Stentor reacts by contraction 

 (condition i). Then it no longer reacts (condition 2). Later, or in 

 other cases, it may react to the stimulus, but by a different method 

 from the first reaction. It now bends over to one side when touched 

 with the rod. As set forth in my previous paper, "The impression 

 made on the observer is very much as if the organism were at first 

 trying to escape a danger, and later merely trying to avoid an annoy- 

 ance." As conditioning this third method of behavior, when all out- 

 ward conditions are the same, we must postulate a third physiological 

 state differing from the other two ; this we may call condition No. 3. 



We may thus distinguish at least three different physiological states in 

 the reactions to very weak stimuli, where the initial marked response 

 becomes a weak one or disappears. We may now analyze in the same 

 way the behavior under stimuli of a different character, when there is 

 a series of reactions which may be considered of increasing rather than 



