MOLLUSKS, ARTHROPODS AND VERTEBRATES 2 21 



(1909, p. 13), " Sind zwei gleich starke Lichtquellen in 

 gleichem Abstand vom Tier vorhanden, so bewegt sich das- 

 selbe senkrecht zur Verbindungslinie der beiden Licht- 

 quellen weil dann beide Augen in gleicher Weise vom Licht 

 beeinflusst werden Darin untersche det sich, wie Bohn 

 richtig bemerkt, die maschinenmassige heliotropische Reak- 

 tion der Tiere von der nicht durch Heliotropismus bedingten 

 Bewegung eines Menschen zu einer von zwei Lichtquellen." 

 Toads, exposed to light from two sources, as stated above, 

 do not proceed toward a point between them. Judged by 

 the criterion of Loeb and Bohn, their reactions under these 

 conditions are therefore like those of a human being under 

 similar conditions and not like those of other animals 

 ( Tiere) . 



c. Orientation with one eye destroyed. — Toads with 

 the lenses ^ removed from the eyes so that the power of 

 forming images is destroyed, frequently orient fairly accu- 

 rately and hop or walk toward a source of light. When 

 exposed to light from two sources they move toward a 

 point between them, contrary to what occurs in specimens 

 that can see. 



In specimens with one eye destroyed there is a slight 

 tendency to deflect toward the injured eye. The head in 

 such specimens is inclined toward the blind side as though 



^ No special precaution was taken to destroy the retina in the eyes from 

 which the lens was removed. This seemed unnecessary since I was not 

 primarily interested in the question of distribution of sensitive elements. 

 Parker (1903 and 1905) demonstrated very clearly that the skin of some 

 frogs and fishes is sensitive to light, and Pearse (19 10) showed the same for 

 various amphibia. Many of these creatures can orient fairly accurately 

 with the retina of both eyes entirely destroyed. My aim was to test the 

 effect on orientation of unequal stimulation on opposite sides. In the toad 

 with the lens removed, the eye fills with a substance which, while it may not 

 be absolutely opaque, certainly intercepts nearly all the light, so that even 

 if the retina is functional in both eyes, one receives much more light than 

 the other. The important point here is that when the toads with the lens 

 of one eye removed as described in the text are oriented and move toward 

 the light they are not equally stimulated on symmetrically located sensitive 

 points. 



