86 W. H. TALIAFERRO 
specimens in the same illumination was 1.10 mm. per second; 
and that, after the removal of the anterior end, the rate was 
only 0.62 mm. per second. The table also shows that essentially 
the same results were obtained in the case of non-directive illum- 
ination. It shows, moreover, that in specimens tested two hours 
after the removal of the eyes the rate of locomotion actually 
increased. This is, no doubt, due to the mechanical stimulation 
of removing the eyes which has not yet had time to wear off, 
for, in normal specimens, shortly after small incisions are made 
in the dorsal surface, the rate of locomotion is similarly increased. 
These results show that the rate of locomotion is not appre- 
ciably affected by the removal of the eyes, whereas it is greatly 
affected by the removal of the anterior end, and they indicate 
very clearly that the photoreceptors which receive the orienting 
stimulus are not the ones which control the rate of locomotion. 
It has been shown by Walter (’07, p. 57) that, in general, plan- 
arians move faster in higher intensities than in lower. The 
photoreceptors involved in this increase of the rate of locomo- 
tion under increased intensity of illumination as well as in the 
experiments just described are other than the eyes—very probably 
the general body surface. 
3. Discussion of experiments in relation to former investigations. 
The results of the preceding experiments are in accord with those 
of Lillie (01) and Mast (710) in regard to the character of the 
response to light, but are at variance with the results of Loeb 
(94), Hesse (97), and Parker and Burnett (’00) both in regard 
to the nature of the responses and the rate of locomotion. The 
question immediately arises as to what causes the disparity be- 
tween these results and those of the latter investigators. 
In regard to the nature of the response in specimens with 
both eyes removed, the answer probably can be found in the 
fact that the species used by the former investigators did not 
normally orient with any great degree of precision to the directive 
influence of light. There is no doubt that many planarians do 
not orient to light, and, of course, one would not expect to find 
any great change brought about by the removal of the eyes, if 
the eyes did not function in the normal animal. 
