590 RAYMOND PEARL. 



stimulation of the sides of the middle region of the body 

 causes, in the first instance, a small local contraction at the 

 place stimulated, without any effect on the general direction 

 of movement of the whole organism. Under favourable 

 circumstances, however, it is frequently, possible to get a 

 quite different result by the use of a weak stimulus on the 

 lateral margins of this region. Specimens of Planaria may 

 be induced to give the characteristic positive reaction 

 described above by stimulation of a point as far back as the 

 middle of the pharyngeal region. The stimulus inducing it 

 is of the same intensity and character as that which will pro- 

 duce the same I'esult in the head region. 



Some experiments bearing on this point will be reported 

 in full, on account of their important bearing on theoretical 

 questions to be taken up later. These experiments were to 



Tig. 15. — Diagram to show the portion of the side of llie body {a b) 

 within which weak slimulation produces the positive reaction. 



test the effects of weak stimuli on the sides of the middle 

 region of the body, especially with reference to the relation 

 of the physiological condition of the individual to its 

 reactions. The experiments were performed on large 

 specitnens of P. maculata, and the region of the body 

 stimulated was that from a to h in Fig. 15. Most of the 

 stimuli were applied in the region between the auricles and 

 the origin of the pharynx. The stimuli were given b}- 

 moving the point of a fine scalpel along the bottom of the 

 dish till it came into contact with the margin of the body. 

 In this way no general disturbance was produced. The 

 attomi)t was made to make the stimuli of as nearly as 

 possible the same intensity each time. The results were 

 classed as positive, negative, or indifferent, according as the 



