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—TI 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 larve of Limu- 
lus polyphemus — the 
horseshoe crab—are ener- 
getically negatively helig- 
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 
brought near a window, FIG. 63 
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 larve 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 SS, 
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