MOVEMENTS, KTC, OF FKESH-WATER PLANAKIANS. 703 



reflexes, more or less complex. What the animal will do 

 after a given stimulus, or in a given situation, can be predicted 

 with reasonable certainty. There is, however, some variation 

 in the behaviour, depending on the physiological or tonic 

 condition of the individual at the time of stimulation. Thus 

 a stinuilus sufficiently weak to induce the positive reaction in 

 one specimen may cause the negative reaction in another ; or 

 at different times the same individual may show different re- 

 actions — either the positive or negative' — to the same stimulus. 



11. Psychological Position of Planaria. — The objec- 

 tive psychological position of any organism is evidently deter- 

 mined by the relative simplicity or complexity of what it 

 does. With a view of determining what the position of 

 Planaria in the psychological scale is, it may be well to 

 make a catalogue of the things which it does in the course 

 of its ordinary existence. 



The animal performs the following acts : 



a. It moves progressively by two methods, a ciliary motion 

 and a muscular motion. 



h. It turns, by a complex of simple reflex acts, towards all 

 weak stimuli investigated. 



c. It turns, by another set of simple reflex acts, away from 

 all strons: stimuli investisj'ated. 



d. It comes to rest in certain definite environmental 

 situations. 



e. When stimulated in a certain way it extends the pharynx 

 and feeds. 



/. Wlien its ventral surface is removed from contact with 

 a solid body (or the surface film), a reflex of essentially the 

 same character as that of c brings this surface again into 

 contact with the solid. 



From these essential factors is composed a behaviour 

 whose complexity one has only to study to realise. 



The behaviour is thus seen to be, in the main, what may 

 be characterised as reflex. It is very simple to say that an 

 animal's activity is composed of a series of invariable reflex 

 acts in response to stimuli, but I doubt whether the full 



