HELIOTROPISM OF ANIMALS 73 



just touched two larvse lying upon it. I also placed two 

 larvse 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 (Grammarus locusta, Cuma Kathkii), 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. 1 



Investigations have not yet been made on Coslenterates 

 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 



i See the papers of Strasburger, Engelmann, and Stahl cited in the introduction. 



