246 CASWELL GRAVE 



group of retinula cells, is not located in the median sagittal plane 

 of the body, but is displaced to the right and so oriented that 

 those rays of light only which enter the series of lenses from the 

 upper right side of the body will reach the pigment cup and be 

 effective in stimulating the sensitive ends of the retinula cells 

 (fig. 1). In its natural habitat, light rays of greatest intensity 

 reach the tadpole from the direction of the water surface and, 

 as a consequence of the axial rotation of the body during loco- 

 motion, the tadpole receives its maximum stimulations from 

 light at one instant only in each revolution when the body is so 

 oriented that its long axis is parallel to the water surface. It is 

 thus obvious that a series of frequent orienting responses is 

 provided for. 



RESPONSE TO GRAVITY 



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

 tend to seek and to remain at or near the surface of the water, 

 but later on they swim away from the surface and seem to seek 

 the deeper strata of water and, at the close of the free-swimming 

 period, usually become attached to the less illuminated places on 

 or near the bottom. 



The tendency of tadpoles to remain at the surface during the 

 first part of their active period is subject to the interpretation 

 that it is simply one of the results of their positive reaction to 

 light, while their behavior a little later, when they exhibit a 

 very definite negative response to light yet continue to remain 

 at the surface of the water, warrants the conclusion that tad- 

 poles are sensitive to conditions other than light, probably to 

 their position with reference to gravity, but possibly to changes 

 in density or water pressure or to difTerences in the oxygen or 

 carbon-dioxide content of the upper and lower strata of water. 



I incline to the view that the response of the tadpole is to 

 gravity, because of the presence of a statocyst-like structure 

 within the sensory vesicle and the similarity of this organ to 

 sensory structures in other animals known to be end-organs for 

 the perception of the position of the body with reference to 

 gravity. 



