THE 
AMERICAN NATURALIST 
THE HELIOTROPISM OF HYDRA? 
BY EDMUND B. WILSON. 
I. /ntroductory—Every observer of Hydra is familiar with the 
fact that the animal possesses considerable power of locomotion, 
and under certain circumstances may creep restlessly about the 
aquarium; it is not so generally known that its wanderings, 
which on superficial examination seem vague and meaningless, 
are in reality directed towards a definite end, and play an im- 
portant part in the life of the animal. Trembley observed as long 
ago as 1791 that the movements of Hydra viridis show a definite 
relation to the source of light (heliotropism), the animal manifest- 
ing a marked tendency to collect on the illuminated side of the 
aquarium. Although this heliotropism is now well known, it has 
not received the attention it deserves; as far as I know, indeed, 
nothing has been added to Trembley’s account by later observers. 
I find no mention of the subject in any of the more recent papers 
on heliotropism, except in Loeb’s very interesting work,” and 
this gives no more than a brief review of Trembley’s results. 
The subject is, however, one of considerable interest for several 
reasons. Hydra is not known to possess any kind of differen- 
tiated visual apparatus; the animals can be kept under observation 
for a long time and their behavior closely studied ; the comparison 
of H. fusca with H. viridis enables us to determine how the 
l Read before the American Morphological Society, December, 1890. 
*Heliotropismus der Thiere. Würzburg, 1890. 
