The Reactions of Planarians to Light 47 



Bureau at Wood's Hole, Mass., and I wish here to express my 

 thanks to the director, Dr. F. B. Sumner, as well as to others in 

 authority there, for their uniform courtesies. The bulk of the 

 investigation, however, was made at Harvard University. I am 

 deeply indebted to Professor Mark for the privilege of having 

 a place in his laboratory and particularly to Prof. G. H. Parker, 

 under whose immediate direction the work was done and whose 

 daily counsels and generous suggestions were indispensable. 



IV Criteria for Measuring Behavior 



Both the form and the structure of an animal set a limit to the 

 character and degree of its movements, which no combination of 

 stimuli, external or internal, can force it to overstep. In estimat- 

 ing the influence of light upon planarians, therefore, it is necessary 

 to know not only the normal behavior of the worms but also the 

 possible range of their reactions under any circumstances. For 

 example, the ordinary gliding locomotion of planarians is accom- 

 plished by means of cilia beating in a mucus track and augmented 

 by muscular contraction. It is physically impossible for this 

 sort of locomotion, even under the most favorable conditions, to 

 exceed a certain rate. By the use of excessive stimuli, however, a 

 worm may be forced to abandon this accustomed gliding for a 

 somewhat faster method of progression known as "crawling" or 

 "humping," in which the muscles are used more than the cilia. 

 But when this is done the limit of possible rate of locomotion has 

 been reached, at least for fresh water planarians, which cannot 

 be urged to abandon entirely contact with some support and to 

 swim freely in water, although the marine form, Bdelloura, does 

 have this addition to its repertory of behavior. 



The following observations may illustrate more specifically what 

 is meant by range of behavior. Planaria maculata, when gliding 

 on the bottom of a dish, was lightly touched on the anterior end 

 with a hair mounted on a glass rod. During one hundred trials 

 of this kind eight different responses resulted, which may be indi- 

 cated as follows: 



