576 EAYMOND PEARL. 



of tlie brain, and is, further, its most important function so 

 far as movement is concerned. 



3. The brain plays a certain part in the production of 

 spontaneous movements. 



F. Reactions to Stimuli. 

 I. Eeactions to Mechanical Stimuli. 



Since the reactions which are given by Planaria to 

 mechanical stimuli are in a sense the foundation on which 

 the reactions to other stimuli are based, it may be well to 

 consider them first. After thoroughly working out the 

 reactions to mechanical stimuli we have a very definite clue 

 to practically all the animal's behaviour. 



a. Methods. — For rough, general work with mechanical 

 stimuli a needle or a sharp-pointed scalpel may be used as 

 the stimulating agent. For the finer work in sharply local- 

 ising the stimulus, I at first made use of pieces of glass 

 tubing drawn out to capillary fineness. This method was 

 not, however, satisfactory, as the glass was too stiff to admit 

 of reaching all points of the body under some circumstances. 

 Furthermore, this stiffness, together with the sharpness of 

 the end, made it almost impossible to give the animal a 

 moderately strong stimulus without wounding it. A far 

 better plan was found to be to fasten with sealing-wax a 

 moderately stiff piece of human hair to a piece of glass 

 tubing, the latter to serve as a handle. With such an 

 arrangement the stiffness of the stimulating point can be 

 varied by varying the length of the hair. Danger of 

 wounding the animal is avoided, yet repeated strong stimuli 

 may be given, while, further, the flexibility of the hair makes 

 it possible to stimulate the animal at any point and from any 

 desired direction. 



An annoying difficulty in connection with this work was 

 the clinging of the slimy secretion of the body to the point 

 used for stimulating. Once coated with this slime the 



