PHOTOTAXIS 53 



selves to light by bending directly toward the more stimu- 

 lated side. These larvae swim by means of two bands of cilia 

 around the body, and for a short time after hatching are 

 markedly positive, swimming toward the light in nearly 

 straight lines. If while the larvae are swimming toward the 

 light the position of the light is changed they bend toward 

 the light until oriented in the direction of the rays. Certain 

 salts were found by Lillie to inhibit muscular action while 

 they did not interfere with the action of the cilia. Larvae 

 in solutions of these salts would continue swimming, but their 

 phototaxis was entirely destroyed. Mast who has studied 

 the orientation of the same form finds that "by using two 

 sources of light so situated that the rays cross at right angles 

 in the region where the specimen is located, and then alter- 

 nately intercepting the light from each of the two sources, 

 it can be seen clearly that the larva, by muscular movement, 

 turns its anterior end toward the source of light directly. 

 There is no trial in this process. It is an asymmetrical 

 response to an asymmetrical stimulation." 



Light has a certain orienting effect on Planaria maculata, 

 even in specimens devoid of eyes although it is masked by a 

 large amount of random activity. Planaria turns directly 

 away from strong mechanical and chemical stimuli, as shown 

 by Pearl, by lengthening the side stimulated instead of by 

 contracting the opposite side. The effect of the stimulus 

 is upon the muscles in the vicinity of the stimulated point. 

 There is no ventral nerve cord with its numerous cross 

 connectives such as we find in annelids and there seems to be 

 no mechanism by which an impulse set up on one side can 

 be transmitted more strongly to the muscles of the opposite 

 side of the body than to any other region. The turning away 

 is therefore effected in planarians in a very different way 

 than in annelids, probably by the contraction of the dorso- 



