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 larve 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. Larve 
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- 
