TRANSFORMATION OF HELIOTROPIC ANIMALS 267 



tropic phenomena than long discussions, and as negatively 

 heliotropic animals are very rare indeed, much rarer than 

 I formerly assumed I will illustrate the more simple facts 

 of heliotropism in such an animal and on one which I have 

 had the opportunity of studying in America. 



2. The larvse of Limu- 

 lus polyphemus the 

 horseshoe crab are ener- 

 getically negatively helio- 

 tropic for some time after 

 they have escaped from 

 the egg. If these animals, 

 which live for months 

 without food in a small 

 vessel of sea water, are 4 

 brought near a window, 

 they collect during the 



day in a narrow zone on the room side of the vessel. If the 

 vessel is carefully turned through an angle of 180, so that 

 the animals are brought to the window side, they at once 

 return in perfectly straight lines to the room side of the 

 dish. The animals are clumsy in their walking movements, 

 and tumble over very easily a fact which must of course 

 be considered. 



It can easily be shown that the movements of the animals 

 follow the direction of the rays of light. Let AB in Fig. 

 63 represent the horizontal section of a window through 

 which direct sunlight falls obliquely. SS^ are the horizontal 

 projections of the sun's rays. The circle is the section of 

 the vessel in which the animals are contained. At the be- 

 ginning of the experiment the larvae are at C. Immediately 

 after being exposed to the light they begin to migrate, not, 

 however, in the direction CD, perpendicular to the plane of 

 the window, but in the direction of the sun's rays 



