404 PROCEEDINGS OF THE AMERICAN ACADEMY. 



An animal is dependent upon certain reactions to its environment to 

 keep it in the proper conditions for its best welfare, and in many cases 

 the response to light is without doubt of great importance in this re- 

 spect. It is questionable whether a Bipalium, for example, were it 

 positively phototropic, could long survive unless it had developed in 

 addition to its phototropism some other form of response that would 

 prevent its going, under the influence of the light, directly into the 

 most unfavorable conditions. Loeb ('90, p. 51) stated that, so far as he 

 had found, all larvae of Lepidoptera were positively phototropic, even 

 including the willow-borer, which, as previously mentioned, like most 

 negative animals, lives naturally in a situation with little or no light. 

 It would seem that we have here a case of secondary adaptation ; the 

 willow-borer, we may infer, has become adapted to living in a dark 

 situation in spite of its positive phototropism. It comes from a group 

 of animals uniformly positive in their reactions to light, and has prob- 

 ably inherited this character from its positively phototropic ancestors. 

 We have seen that as a rule animals like the earthworm and Bipalium, 

 which live in dark situations, lack image-forming eyes ; at least such 

 is the inference from the reponses of these forms to equal illumination 

 from areas of different size. It has been pointed out that the posses- 

 sion of such eyes could be of no use to them in the dark ; and it is 

 likewise obvious that the simpler organs are equally effective in pre- 

 venting the animals from straying out into the light. All that is 

 necessary for them is to avoid light altogether. This can be done, 

 however, with greater precision when the organism is able to appreciate 

 more exactly the direction from which the light comes. The earthworm 

 probably has little power of discrimination in this respect beyond recog- 

 nizing upon which side the source of light lies, — on its right or on its 

 left, — according to whether its right or its left side is illuminated. 

 Bipalium, on the other hand, by the arrangement of many of its eyes 

 around the anterior margin of the semicircular head, is able to deter- 

 mine the direction of the light more accurately, and consequently shows 

 a (quicker and more precise response by moving directly away from it. 

 Similarly, in the mealworm, the sow bug, and the cockroach, the eyes 

 are probably little more than "direction eyes." 



The experiments on Helix failed to give any decisive evidence as 

 to the ability of these animals to discriminate between the two areas. 

 Nor do the observations of Mitsukuri (:0l) and Bohn (:05) help us 

 much in this respect, since neither of these authors discriminated 

 between the size of the areas and the relative intensity of the light 

 received from them. Bohn studied the reactions of Littorina when 

 vertical screens, both black and white, were placed in the illuminated 



