No. 385. ] THE PROBLEM OF GEOTAXTS. +. 37 
surface of .the water in which they were kept. He therefore 
suggested that the negative geotaxis they thus manifested 
should also be tested in solutions of gum arabic of differing 
densities. 
I found that between the lengths of 9% mm. and 12 mm., 
the specific gravity of the tadpoles decreased from 1.044 for 
the smaller, to 1.017 for the larger. The smaller tadpoles, 
which showed marked geotactic tendency, were placed in a 
phial filled to the brim with a solution of gum arabic heavier 
than the tadpoles, and the phial was then inverted with care 
that no bubbles of air were admitted. The tadpoles, which 
could not have been attracted upwards by the light, since light 
was admitted at all sides through the glass of the phial, imme- 
diately swam upwards, as they had done in water. As there was 
no air between the bottom of the inverted bottle and the upper 
surface of the solution, it was equally improbable that the tad- 
poles were attracted upwards by the air. 
When placed in solutions of the same specific gravity as 
- themselves, it was found that the tadpoles still swam to the 
upper surface of the solution, which as before was in immedi- 
ate contact with the bottom of the inverted phial. From these 
experiments it appears that the attraction upwards is neither 
air nor light, and that the direct action of gravity as expressed 
in the weight of this organism does not act as the incentive to 
negative geotaxis. It is possible that gravity may act, however, 
on some internal organ which is unaffected by the change in 
the density of the surrounding medium, and may by this means 
induce the movement upwards. Loeb (91) has shown that 
the semicircular canals of the ear in sharks act as balancing 
organs. Should this also be true for tadpoles, we might find 
here the: explanation of their constant motion upwards, even 
when the direction of the action of gravity on the organism 
as a whole has been changed by placing the organism, normally 
heavier than water, in a medium of greater specific gravity than 
itself. 
It is, moreover, by no means impossible that the action of 
gravity, which determines the direction in which the Infusoria 
move, may be through the internal organization of the animal. 
