COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 391 



allowed to crawl out into the light. Trials were made alternately with 

 the animals headed in one direction and then in the opposite. 



It soon became apparent that these snails were by no means uni- 

 formly positive to light, so it was necessary each time to test them 

 first with a light at one side only in order to be certain of the state 

 of the animal. It was furthermore found that they appeared to become 

 fatigued very quickly, and apparently this was often" accompanied by 

 a change in the character of the phototropic response. Finally, the 

 method was adopted of making only a few trials with each individual 

 each day. The first trials were made with both lights, one trial in 

 each of the two orientations, and then trial was made with a single 

 light to ascertain whether the response was positive or negative to that. 

 The records for animals that were positive were kept separate from 

 those of animals that were negative. The responses were so irregular 

 and apparently so dependent upon the physiological state of the animal 

 that no very definite conclusions could be drawn. As far as they 

 showed anything, however, they seemed to indicate a fairly indifferent 

 reaction to the two lights in the case of animals that were negative as 

 well as in those that were positive. This leads to the inference that 

 the eyes of the snail do not aid greatly, if at all, in the discrimination 

 of two lights differing in area as the two used. If, as is maintained, 

 the general integument is sensitive to light-stimulation, a result simi- 

 lar to that obtained in the reactions of the earthworm to the influence 

 of light on the skin would be expected, since, without the eyes, the 

 condition would be comparable to that of the earthworm. These ex- 

 periments on the snail, as far as they go, point to the conclusion that 

 the ability to discriminate differences in the size of luminous areas is 

 aided but little, if at all, by the eyes. 



10. European Garden Slug {Limax maximus Linn.). 



A few experiments were also made with the slug Limax maximus, 

 as an example of a mollusk normally negative in its reaction to 

 light. As in the case of Helix, however, the animals were found to be 

 so inconstant in their responses that it was exceedingly difficult to 

 obtain consistent results fi:om them. Frandsen ( : Ol), in his work on 

 the reactions of Limax to direction stimuli, found much individual 

 difference in the reactions to light, as well as to gravity. Further- 

 more, he found that the reactions of the same individual vary in in- 

 tensity, and even in character, with variations in the intensity of the 

 light, and probably also in response to undeterminable physiological 

 states. These variable conditions introduce so many elements of un- 



