HELIOTROPISM OF ANIMALS 43 
just touched two larvee lying upon it. I also placed two 
larvae on top of the cylinder. If the animals were negatively 
geotropic, the upper animals should have buried themselves 
more quickly than the lower. But the opposite was the 
case. After forty-five minutes the lower animals had 
burrowed upward so that they were completely out of 
sight; the upper were not buried until an hour later. There- 
fore, even though they may be negatively geotropic, for 
which I have as yet no proof, the contact-irritability of these 
animals determines that they shall burrow into the ground. 
XI. THE DISTRIBUTION OF HELIOTROPIC PHENOMENA IN THE 
ANIMAL KINGDOM 
The experiments which have thus far been described 
were carried out on insects. 
So far as experiments on representatives of the other 
divisions of the animal kingdom are concerned, I have con- 
firmed the identity of animal with plant heliotropism on 
crabs (Gammarus locusta, Cuma Rathkii), naked snails and 
worms (leeches, planarians, earth-worms and others). Experi- 
ments on infusoria are already sufficiently complete to show 
that Sachs’s laws of heliotropism also hold good for them.’ 
Investigations have not yet been made on Ccelenterates 
and Echinoderms; Trembley’s experiments on Hydra, how- 
ever, show that in their case also the relation is the same; at 
least it seems to me that Trembley’s experiments cannot be 
interpreted unless we assume that the progressive movements 
of Hydra are determined by the direction of the rays of 
light. 
I used the following method with aquatic animals: To 
prove that the direction of the rays determines the direction 
of the progressive movement, I used a long, four-cornered 
glass box, one wall of which was made of a watch-glass. The 
1See the papers of Strasburger, Engelmann, and Stahl cited in the introduction. 
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