MOVKMRNTS, ETC., OF FRKSII-WATKIJ I'LANAUFANS. 705 



behaviour was composed of invavial)le reflexes is foniicl in tlie 

 behaviour foHowing repeated stroug stimuli applied to tlie 

 anterior end (vide pp. 580, 581). In this case the organism 

 shows an evident modihability in reaction, for after giving 

 for some time the ordinary negative reaction, and not thereby 

 getting away from the stimulus, it finally turns directly 

 towards the source of the stimulus. Again, in the righting 

 reactions of pieces of the body we see entirely new forms of 

 reaction appearing (pp. 680 — 683). 



In order to give a concrete idea of the psychological 

 position of Planaria it may be well to present in parallel 

 columns the principal factors which make for simplicity in 

 the behaviour on the one hand, and for complexity on the 

 other hand. 



r actors which tend to make Factors which tend to make 

 tlie Behaviour Simple. the Behaviour Complex. 



A. Essential reflex character at the A'. Comparatively large number of 



basis of all the reactions. qualitatively different general 



reactions, 



B. General lack of modifiability of B'. Marked qualitatively different re- 



reactions, actions to differing intensities of 



stimulus. 



C. Comparatively small number of C. Definite relations of reactions to 



qualitatively different reflexes location of stimulus, 



composing the general reac- 

 tions. 



D'. Rather close dependence of reac- 

 tions on the physiological condi- 

 tion of tlie individual. This brings 

 about variation in the reactions. 



The behaviour of Planaria is evidently much more com- 

 plex than that of the Infusoria, as described by Jennings 

 (loc. cit.). In the case of the Infusoria, all the factors A', B', 

 C, D', which make the behaviour of Planaria so complicated, 

 are nearly or quite absent ; and in respect to C these organ- 

 isms are at a much lower stage than Planaria. The 

 Infusoria have practically but one purely reflex reaction to 

 nearly all stimuli, and this reaction is not localised with 



