The Reactions of Planarians to Light 



59 



highest intensity upon rate, as well as with certain other evidence 

 to be discussed later. 



The small excess of clockwise over contra-clockwise turnings 

 is not explainable upon the ground of varying intensities of light. 

 A distribution of the cases under the several intensities of light 

 (Table VI) makes it plain that this peculiarity is due rather to 

 individual causes than to light intensities. Indeed it would be 

 difficult to conceive theoretically how varying intensities of non- 

 directive light could influence a worm in such a w^ay as to affect 

 the direction in which it turns. The natural expectation accord- 

 ing to chance would be an equal number of turnings in either 

 direction. The excess of clockwise turns seems, therefore, un- 

 doubtedly due to internal causes which render certain worms 

 more liable to go one way than another. In fact, when the 

 records were arranged according to individual behavior it was 

 found that of the ten worms seven averaged a majority of clock- 

 wise turns while only three fell in the contra-clockwise column. 



TABLE VI 



Character of turning of Planaria gonocephala in non-directive light of various intensities 



Change of Course. A greater number of "definite" changes 

 occur in the light than in the dark, but fewer "indefinite" changes. 

 This point requires no further exposition as its corollary has already 

 been given. 



The behavior of the worm in this respect seems to be more 

 closely correlated with the highest intensity (431 cm.) than with 

 any other. In the highest intensity employed there are indicated 

 (Table II, p. 52) 40 per cent of definite changes, which is con- 

 siderably in excess of the percentage of such changes made in any 

 other intensity. On the other hand indefinite, or S-shaped, changes 

 constitute only 9 per cent of all records taken at the highest 

 intensity, which is less than half the number of indefinite paths 

 made in any other intensity. 



