74 Herbert Eugene Walter 



10 cm. in diameter and marked off plainly into arcs measuring 

 5 degrees each. An incandescent lamp (/), placed on the 

 table at approximately the height of the inner aquarium floor, 

 could be manipulated at any desired distance, while a diaphragm 

 (J) prevented much of the light from reaching either the upper 

 surface of the water contained in the two vessels or the floor of the 

 outer aquarium whence it would be reflected. A biconvex lens was 

 then so interposed as to make the light rays practically parallel 

 upon their emergence from it. Their course through the inner aqua- 

 rium was kept parallel by means of the medium of water on both 

 its inner and outer sides. A nearly uniform intensity over the 

 entire floor of the swinging aquarium was thus obtained and the 

 objection arising when the inner aquarium is used in air, viz: that 

 it acts as a converging lens, was obviated. Side reflections were 

 eliminated by enclosing the light (/), together with the interven- 

 ing space between it and the diaphragm, with black screens. 



When a worm introduced into the inner aquarium began to 

 glide, it could with slight mechanical disturbance be quickly rotated, 

 by means of moving this inner aquarium, into any desired posi- 

 tion with reference to the light, and then swung so as to bring its 

 posterior end exactly over the center of the stationary circle 

 below. 



Various species of planarians were started in this manner at 

 right angles to the light. Out of 386 cases, 371, or 96 per cent, 

 emerged from the 10 cm. circle at a point farther away from the 

 light than that toward which they were originally directed. This 

 is taken to mean that 96 times out of a hundred the worms were 

 negatively phototactic. If, however, the method of reckoning 

 negativeness employed by Parker and Arkin ('01) on the earth- 

 worm is used, the foregoing per cent would be somewhat less. 

 These authors assume ('01, p. 28) that the apparently positive 

 responses of a normally negative animal, such as the earthworm, 

 may be due to causes other than light, in which case an equal 

 number of responses of like nature might be expected to occur on 

 the negative side as well as on the positive. A number equal to 

 the sum of these apparently positive responses should therefore 

 be subtracted from the total of the apparently negative responses 



