82 CASWELL GRAVE 



Although the structure and organization of the eye are perhaps 

 sufficient to support the interpretation that the eye is the organ 

 by whic'i the tadpole orients with reference to rays of light, it 

 may be worth while to state the physiological evidence, secured 

 since the publication of the paper on the activities and reactions 

 of the tadpole larva (Grave, '20), which shows conclusively that 

 the eye is a functional light-perceiving organ. 



During the latter part of their free-swimming period, tadpoles 

 cease to swim continuously, and intervals of rest, when they 

 lie quiescent upon one side, become longer and longer. While 

 examining a tadpole during one of its resting periods with the 

 microscope, the mirror was so turned as to cut off the transmitted 

 light. Immediately the light was cut off the tail began to vibrate. 

 Repeated experiments of the same kind with light reflected from 

 the mirror, alternately turning it off and on, showed that when 

 the tadpole was so lying that light from the mirror entered the 

 pigment cup (on its right side), the tail almost invariably began 

 to vibrate at the instant the light was turned off and in no case 

 when the light was turned on. The actual stimulus to muscular 

 contraction is not transmitted from the eye to the muscle bands 

 of the tail during the illumination of the visual rods of the retina, 

 but unmediately after the pigment cup is darkened, follow- 

 ing its illumination. This takes place in the course of normal 

 locomotion at the moment in each revolution of the body on its 

 axis when the pigment cup is carried to a position in which rays 

 of light no longer enter its cavity. 



It has been noted from the beginning of the investigation that 

 the shadow of the hand when passed over resting tadpoles almost 

 invariably causes immediate renewal of locomotor activity. 

 The observations just described, which show that the eye is a 

 functional light-perceiving organ, incidentally explain this 

 shadow reaction. 



THE STATIC ORGAN 



The static organ consists of a single sensory cell, at the distal 

 end of which is borne a relatively large, subspherical, black 

 statolith, and a small number of large nerve cells which form 



