124 



DYNAMICS OF LIVING MATTER 



directed upwards. If such a colony is put horizontally into an aqua- 

 rium which receives its light from above in the direction of the arrow cd, 

 Fig. 24, it will be observed that very soon the heads of the worms are 



turned upward so that the 

 axis of symmetry is in the 

 direction of the rays of light. 

 Very soon the tubes begin to 

 grow in front through the 

 deposition of new lime salts 

 (which are secreted by 

 glands). But the direction 

 of this growth is now at right 

 angles to the longitudinal 

 axis of the old tubes. This 

 again shows that the primary 

 effect of the light in the 

 heliotropic reactions is the 

 bending of the tip, or head, 

 of the animal through protoplasmic or muscular contraction. The bend- 

 ing of the tube or growth are secondary phenomena which follow the 

 former. I believe that the phenomena of heliotropism of sessile animals 

 and plants are essentially alike. The presence of nerves in animals 

 is no reason for denying this identity, especially since some botanists, 

 e.g. Hildebrandt, claim that tissues which functionally resemble nerves 

 also exist in plants. 



FlG. 24. Positive heliotropism of Serpula uncinata. 

 The light had originally struck the animal in the 

 direction of the arrow at, and their tubes were 

 parallel with the direction of the rays. When the 

 light fell in the direction cd the tubes began to grow 

 at right angles to their former direction. Partly 

 diagrammatic. 



2. HELIOTROPISM OF FREE-MOVING ANIMALS 



\ 



We have seen that the essential feature of the heliotropic reaction 

 consists in the fact that the light automatically puts the plant or the 

 animal (Eudendrium, Spirographis} into such a position that the axis 

 of symmetry of the body, or organ, falls into the direction of the rays 

 of light. In the case of positively heliotropic organs, the tip, or head, 

 is directed toward the source of light, while it is the reverse in the case 

 of negatively heliotropic organs. If we imagine that such a positively 

 heliotropic organ, e.g. the polyp of Eudendrium, or a worm, like Spiro- 

 graphis, be endowed with the power of spontaneous locomotion, and 

 if for some internal reasons the animal were compelled to be constantly 

 in motion (as is the case with many pelagic larvas), we should notice 

 that these animals had no choice left in regard to the direction of their 

 motion. The light would turn them automatically until their axis of 

 symmetry was in the direction of the rays of light, and the animal could 



