126 



DYNAMICS OF LIVING MATTER 



it occurs in the latter case at all. I have found that the ciliated larvae 

 of Eudendrium swim rapidly toward the source of light behind a blue 

 screen, while they react quite slowly, or not at all, behind a red screen.* 

 We will now show that the same ideas also hold for forms which, 

 like the insects, possess a central nervous system.t We may choose 

 for this purpose animals like the caterpillars- of Porthesia chrysorrhaa, 

 or the winged Aphides. When the young caterpillars of Porthesia, 

 which hibernate in a nest, are brought during winter into a warm room, 

 they leave the nest. If a large number of these larvae are put into a 

 test-tube which is placed upon a table with its longitudinal axis at 

 right angles to the plane of the window, all the caterpillars move toward 

 the window side of the tube, where they remain. If the test-tube be 

 turned carefully by an angle of i8o° in a horizontal plane, the animals 

 will go back at once to the window side, and the quicker, the stronger 

 the intensity of the light. They react in this way, whether the source 

 of light is sunlight, diffused daylight, or lamplight. The representa- 

 tives of the anthropomorphic viewpoint would say that the animals 

 go to the source of light because it is brighter at the window side of the 

 test-tube than at the room side. It can, however, be shown 

 that in this case the animal has no choice, but that its head 

 is turned mechanically toward the light by the latter, and 

 that it is compelled to move in this position. The proof 

 of the correctness of the mechanical, automatic, or 

 heliotropic view lies in the fact that the animals also 

 move toward the source of light, even if in so 

 doing they must pass from the light into the shade. 

 The experiment can be made in the follow- 

 ing simple manner: Let, through the upper 

 half of a window (ww, Fig. 25), direct 

 sunhght 5 fall upon a table, through 

 the lower half, the diffused daylight 

 (Z>). A test-tube ac is placed on 

 the table in such a way that its 

 long axis is at right angles with 

 the plane of the window; and 

 one half ab is in the direct 

 sunlight, the other half in the shade. If at the beginning of the 

 experiment the animals are in the direct sunlight at a, they promptly 



* These observations on the larvse ot Eudendrium were made in 1895 ^' Woods Hole, 

 but have not been published heretofore. 



t Loeb, Der Heliotropismus der Thiere und seine Uebereinstimmung mit dem Helio- 

 iropismus der Pflanun, Wiirzburg, 1889. Reprinted in Studies in General Physiology, Vol. I, 

 Chicago, 1905. 



Fig. 2S- 



