lighted side, others in the opposite direction, and the majority perhaps pay no attention to the light.
This condition changes instantly if we add to the water some acid, preferably carbonic acid, which easily penetrates the cells. This is done by slowly adding to every 50 c.cm. of the fresh water a few cubic centimeters of water charged with . If the correct amount of carbonic acid is added all the individuals become actively positively heliotropic in a few moments and they move in as straight a line as the primitiveness of their swimming movements permits, toward the source of light, and remain there closely crowded together on the lighted side of the vessel. If the vessel is turned around 180°, they go directly back again to the lighted side of the vessel. Every other acid acts like carbonic acid and alcohol acts in the same manner, only more feebly and much more slowly. Animals which were previously indifferent to light become, under carbonic acid treatment, complete slaves of the light.
How does the acid produce this result? We will assume that it acts as a sensitizer. The light produces chemical changes, for instance, oxidation on the surface of the animal, especially in the eye, as was suggested in the case of the aphids. The mass of photochemical substance which is acted upon by the light is often relatively small, so that even when the light strikes the crustacean (copepod) on one side only, the difference in the chemical changes on the two sides of the body remains still too small to call forth a difference in tension or action, in the muscles of the two sides of the body, sufficient to turn the animal toward the source of light. But if we add an acid this could act as a catalyzer, as, for instance, in the catalysis of esters. In the catalysis of esters, the acid acts, according to Stieglitz, only to the extent of increasing the active mass of the substance which undergoes a chemical change. In order to provisionally fix our ideas, we will assume that the acid makes the animal more strongly positively heliotropic by increasing the active mass of the photosensitive substance. By this means it becomes possible for the same intensity of light which before produced no heliotropic reaction now to cause a very pronounced positively heliotropic reaction; because if now the animal is struck on one side only by the light, the difference in the reaction product in both retinæ becomes rapidly great enough to cause automatically a difference in the action of the muscles of both sides of the body and a turning of the head towards the source of light.
A second consideration must also be mentioned here. In certain forms, for instance, in daphnia and in certain marine copepods, a decrease in temperature also increases the tendency to positive heliotropism. If the mere addition of acid is not sufficient to make the
- Loeb, Pflügers Archiv, Bd. 115, S. 564, 1906.