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. 



(b) 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. 



