The Reactions of Planarians to Light 125 



possibly be interpreted as the connecting link between undifferen- 

 tiated light-receiving organs and the normal eyes of the planarians. 



The part that pigment plays in the reception of light is not as 

 yet clearly defined. It is not probable that pigment in itself con- 

 stitutes a photoreceptor, though it is usually found associated 

 with sensory cells which are directly concerned with light reception. 

 That it is not an essential factor of a photoreceptor is evident, 

 inasmuch as it is absent from the eyes of albino animals. The 

 secondary role of pigment in the reception of light by organisms is 

 admirably pointed out and discussed by Beer ('01). 



The presence of pigment in a planarian may, however, modify 

 the animal's response to light stimulation by shielding the sen- 

 sory cells from light, and since its distribution in general is near 

 the exterior, it may afford some clue to the relative receptivity 

 of internally and externally situated photoreceptors. In other 

 words, if pigmented and non-pigmented worms, for example, ex- 

 hibited the same behavior in light, it might reasonably be assumed 

 that the photoreceptors were not located internally, since they 

 would be partially shielded from light in the case of the pigmented 

 forms and consequently would give rise to a different response. 



It is of interest, therefore, to contrast the behavior of dark- 

 pigmented worms with those in which the dark pigment is absent 

 except in the eyes. This is done in Table XXXIV, but it by no 

 means follows that the contrasts there given between the behavior 

 of dark and light worms are due to the presence of dark pigment 

 in the one case and its absence in the other. Other factors than 

 pigment may very probably have been influential in bringing 

 about variations in the light reactions tabulated. Furthermore, 

 it is inaccurate to refer to a white worm as being non-pigmented, 

 since in that case it would be entirely transparent. The question, 

 then, so far as planarians are concerned, is confined not to differ- 

 ences between pigmented and non-pigmented but to differences 

 between dark-pigmented and light-pigmented forms. 



It will be seen from Table XXXIV that when subjected to light 

 stimulation dark-pigmented worms in general show more activity 

 than light-pigmented forms. A single exception to this rule 

 occurred in the case of the cave planarian experimented upon. 



