Heliotropism of animals 265 
arthropods. All the animals were in a marked degree positively 
heliotropic. These authors found that if one cornea is blackened in 
such an animal, it moves continually in a circle when it is exposed to 
a source of light, and in these motions the eye which is not covered 
with paint is directed towards the centre of the circle. The animal 
behaves, therefore, as if the darkened eye were in the shade. 
(6) The production of positive heliotropism by acids and other 
means and the periodic depth-migrations of pelagic animals. 
When we observe a dense mass of copepods collected from a 
freshwater pond, we notice that some have a tendency to go to the 
light while others go in the opposite direction and many, if not the 
majority, are indifferent to light. It is an easy matter to make 
the negatively heliotropic or the indifferent copepods almost instantly 
positively heliotropic by adding a small but definite amount of carbon- 
dioxide in the form of carbonated water to the water in which the 
animals are contained. If the animals are contained in 50 cubic 
centimetres of water it suffices to add from three to six cubic centi- 
metres of carbonated water to make all the copepods energetically 
positively heliotropic. This heliotropism lasts about half an hour 
(probably until all the carbon-dioxide has again diffused into the 
air) Similar results may be obtained with any other acid. 
The same experiments may be made with another freshwater 
crustacean, namely Daphnia, with this difference, however, that it is 
as a rule necessary to lower the temperature of the water also. If 
the water containing the Daphniae is cooled and at the same time 
carbon-dioxide added, the animals which were before indifferent to 
light now become most strikingly positively heliotropic. Marine 
copepods can be made positively heliotropic by the lowering of the 
temperature alone, or by a sudden increase in the concentration of 
the sea-water. 
These data have a bearing upon the depth-migrations of pelagic 
animals, as was pointed out years ago by Theo. T. Groom and the 
writer. It is well known that many animals living near the surface 
of the ocean or freshwater lakes, have a tendency to migrate 
upwards towards evening and downwards in the morning and during 
the day. These periodic motions are determined to a large extent, if 
not exclusively, by the heliotropism of these animals. Since the 
consumption of carbon-dioxide by the green plants ceases towards 
evening, the tension of this gas in the water must rise and this must 
have the effect of inducing positive heliotropism or increasing its 
intensity. At the same time the temperature of the water near the 
surface is lowered and this also increases the positive heliotropism in 
the organisms. 
