TRANSFORMATION OF HELIOTROPIO ANIMALS 285 



pendently of the light, Polygordius larvae usually swim at 

 the surface of the water when positive, while in their nega- 

 tively heliotropic condition they usually creep along the 

 bottom. When I introduced a large number of Polygordius 

 larvse into a eudiometer tube filled with sea-water, and set 

 it in a vertical position in a dark room, the larvae were not 

 distributed equally in the tube after some time in about 

 twenty-four hours- but one collection was usually found at 

 the surface of the water, and a second at the bottom. If the 

 tube was now carefully and suddenly brought to the light, 

 all the larvse at the surface layer showed themselves to be 

 positively heliotropic, while all the larvse at the bottom were 

 negatively heliotropic. It could further be easily shown 

 that positively heliotropic Polygordius larvse went to the 

 surface of the tube when put into the dark room, while 

 negatively heliotropic Polygordius larvse went to the bottom. 

 Further, positively heliotropic Polygordius larvse may, as 

 has already been mentioned, be made negatively heliotropic 

 by warming. When a eudiometer tube containing Polygor- 

 dius larvse at its surface was warmed in a dark room, the 

 larvae went to the bottom of the tube. When the tube was 

 cooled in the dark room, below 7 C., all the larvse left the 

 bottom and collected at the surface. Both the ascent and 

 the descent of the larvse are brought about by active swim- 

 ming motions. It seems to me probable that the animal is 

 not only heliotropic, but also geotropic, and that the sense 

 of geotropism is always changed under the same conditions 

 as the sense of heliotropism. Negative geotropism is asso- 

 ciated with positive heliotropism, and positive geotropism 

 with negative heliotropism. Nevertheless, this difference 

 persists, that the positively heliotropic animals (which at the 

 same time are negatively geotropic) always swim; while the 

 negatively heliotropic animals (which are also positively geo- 

 tropic) lie or creep upon the bottom, and, it seems to me, 



