The Reactions of Planarians to Light 



119 



upon examining Table XXXI, the same result did not occur 

 when Dendroccelum lacteum was used. 



TABLE XXXI 



The average rate of locomotion in mm. per sec. of 5 small and 5 large individuals of each of four species, 

 subjected to identical light conditions 



Species 



Large 



Size 

 mm. 



Average 

 rate 



Small 



Size 



Average 



Dendrocoelum lacteum (first trial) . . 

 Dendroccelum lacteum (second trial 



Phagocata gracilis 



Planaria maculata 



Planaria gonocephala 



695 



74 

 58 

 57 

 17 



The worms selected for the experiments detailed in Table XXXI 

 were carefully chosen as to length and did not vary more than a 

 millimeter in any case from the size recorded in the table. The 

 result obtained with Dendroccelum was so unexpected that the 

 same ten individuals were put aside and tried a week later under 

 as nearly identical conditions as possible. As will be seen by the 

 table, the result of the second experiment was in general the same, 

 though not so pronounced, as that obtained in the first set of trials. 

 In the cases of Phagocata gracilis, Planaria gonocephala and 

 Planaria maculata, the larger worms traveled faster than the 

 smaller ones. Why the factor of size should give a different 

 result in the case of Dendroccelum lacteum from that common 

 to the dark-pigmented planarians is by no means clear. It is 

 conceivable that a planarian with undeveloped sexual organs or 

 one whose size was reduced through starvation might have a better 

 ciliary equipment in proportion to its mass than a normally adult 

 animal and that in consequence it could travel faster. This sup- 

 position explains the behavior of Dendroccelum lacteum, but it 

 does not throw light on that of the other species, of which the 

 smaller individuals, instead of traveling faster than their larger 

 associates, moved at a slower rate. It is possible that in the case 

 of the dark-colored worms reduction in size is accompanied by a 



