66 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



direction. That the motion of the animal is determined by the direction of the 

 rays of light, and not by the differences in the intensity of the light, can be very 

 easily shown. Positively heliotropic animals, like plant lice, move towards the source 

 of light even then, if by certain experimental arrangements their path goes from 

 lighter to darker places; and negatively heliotropic animals go from the light even 

 then, if measures are taken so that their way leads them from the dark to the light. 

 But in both cases the animal migrates in the direction of the rays of light. An illus- 

 tration will show this influence of the direction of the rays of light better than a long 

 theoretical expl anation . 



Spirographs spallanzani, a marine annelid, lives in a tube which it fastens to the 

 ground. The tube is not transparent and only the tentacles of the animal, which pro- 

 ject from the tube, are exposed to the light. The tentacles are arranged in a circle. If 

 we put these animals into an aquarium in which the light falls in from only one side 

 the animal bends its tube in such a way that the symmetry axis of the circle of ten- 

 tacles is parallel to the direction of the rays of light. As long as we do not change 

 anything in the direction of the rays of light which strike the animal, it does not 

 change its orientation, but as soon as we cause the light to fall into the aquarium from 

 another direction the Spirographis changes the direction of its tube so that the sym- 

 nietrjr axis of its tentacles is again parallel to the rays of light. Whether the intensity 

 of the light is great or small, whether we use direct sunlight or diffused daylight, the 

 result remains the same. 



The larvae of Limulus at a certain period of their life are negatively heliotropic 

 If we put such larvae into a glass dish and place it near a window through which the 

 rays of sunlight fall in obliquely, the Limuli migrate with mathematical exactness in the 

 direction of the sunlight away from the window as far as the sides of the dish allow 

 them to go, and then they remain at rest. If, however, we turn the dish 180 degrees 

 around its vertical axis the animals move again in the direction of the rays of light 

 going away from the window. It is not possible to give you here a full account of the 

 experiments by which it can be proved that the direction of the rays of light alone 

 determine the direction of the motions of the animals, nor is it possible for me to give 

 you here the theory of animal heliotropism. For both I must refer you to my former 

 publications. I only wish to show how this heliotropism determines the periodical 

 depth migrations of pelagic animals. 



It is known that the nauplii of Balanus appear at the surface of the sea at 

 night and go down during daytime to a depth of 00 fathoms and more. Groom 

 and I found that the nauplii of Balanus p&rforatm show a very peculiar helio- 

 tropism. They are positively heliotropic when the light is very weak; but when 

 they are exposed to strong light they very soon become negatively heliotropic. This 

 explains the periodical migration. In the evening, and very often during the night, 

 the light which is reflected from the sky is very weak, and in this light the nauplii 

 are positively heliotropic. As only the vertical components of the rays of light can 

 take effect, the horizontal ones annihilating each other, the animal is forced to move ver- 

 tically upwards to the surface. At daybreak, as soon as the light becomes sufliciently 

 intense, the animal, by the influence of the light itself, becomes negatively heliotropic 

 and must go vertically downwards. But why does it not go to the bottom of the sea 1 ? 

 You know that the water absorbs light and the deeper we go downwards into the 

 ocean the less the intensity of the light becomes. 



