PHYSIOLOGICAL STATES AS DETERMINING FACTORS. I 15 



observed that Amoeba verrucosa may at first begin to ingest an Alga 

 filament, then later, before the ingestion is complete, the filament may 

 be ejected. This involves a change in the physiological condition of 

 the Amoeba ; otherwise it would not now reject a certain object which 

 it before ingested. 



PHYSIOLOGICAL STATES IN THE LOWER METAZOA (THE 

 FLAT WORM AS A TYPE). 



Passing now to the Metazoa, we find in the flatworm, Planaria, as 

 described by Pearl (1903), a dependence of the reaction of the organism 

 on its physiological condition similar to that which we saw above for 

 Stentor. The flatworm may be considered typical of the lower bilat- 

 eral Metazoa, so that it will be worth while to subject some features 

 in its behavior to a brief analysis from our present point of view. 



We may examine for a simple typical case the reactions to mechani- 

 cal stimuli applied to one side of the anterior part of the body. The 

 flatworm is touched on one side with the tip of a hair or of a fine glass 

 rod. The resulting response is one of two reactions the worm turns 

 either toward the point stimulated (positive reaction) or away from it 

 (negative reaction). Typically, the positive reaction is given to a 

 weak stimulus, while the negative reaction results from a strong 

 stimulus. The words " weak" and " strong" have, as we shall see, 

 only a relative meaning when used in this connection. 



When, now, we ask which of these reactions shall be given as a 

 response to a certain stimulus, we find that this depends upon the 

 physiological condition of the organism. Pearl finds the reactions 

 determined by the following definitely marked physiological states : 



1. Individuals are frequently in what may be called a resting con- 

 dition. The tonus is lowered ; the animals are very inactive and do 

 not respond readily to stimuli. This condition is compared by Pearl 

 with that of sleep in higher animals. When a flatworm in this con- 

 dition is given a light stimulus, such as would in an active specimen 

 induce a positive reaction, it does not respond at all. If the strength 

 of the stimulus is increased, the animal finally responds with a negative 

 reaction, turning away from the point stimulated. We may call this 

 the condition of lowered tonus. 



2. In the more usual active condition the flatworm gives the positive 

 reaction to a very light touch, a negative reaction to a stronger stimulus. 

 We may call this the normal condition. 



3. After the animal has been repeatedly stimulated it seems to become 

 excited ; it moves about rapidly, and now gives always the negative 

 reaction to any mechanical stimulus to which it reacts at all. It behaves 

 much as many higher animals do under the influence of fear. We may 

 call this the excited condition. 



