FURTHER INVESTIGATIONS ON HELIOTROPISM 91 
gills fully unfolded takes them at first for plants bearing a 
palm-like crown (the gills) upon a long naked stem (the 
tube). A slight jar, however, causes the animals to draw 
back their gills rapidly into the tubes. 
When the animal is taken from the sea and kept in an 
aquarium, it is at first indifferent to the light. This con- 
tinues until the animal has attached itself by its foot to the 
bottom of the aquarium—a period often of several days. As 
soon as this has taken place, however, the orienting influence 
of the light begins to be noticeable. If light falls upon the 
animal from one side only, heliotropic curvatures make their 
appearance in the tube. The animal turns its oral pole 
toward the source of light and bends its tube until the axis 
of its radially expanded gills lies in the direction of the rays 
of light. The animal maintains this orientation as long as 
the direction of the rays of light remains unaltered. 
2. To test more accurately to what extent the direction of 
the rays of light determines the orientation of the animals, I 
put them into an aquarium which stood at the window, and 
which could be completely screened from the light by a zine 
box. The outlines of the aquarium are indicated in the 
drawings (Figs. 7 and 8) by black lines, the outlines of the 
zine box by dotted lines. The wall abcd of the zinc box 
could be moved vertically upward, so that the amount of 
light entering the aquarium could be regulated. The zinc 
box, the walls of which were painted black on the inside, 
was so placed over the aquarium that the movable wall was 
on the window side of the aquarium. If this wall was 
raised only slightly, as shown in Fig. 8, the rays entered 
the aquarium almost horizontally. When it was drawn 
farther up, as in Fig. 7, rays entered from above in addition 
to the horizontal rays. These were more intense than the 
rays entering horizontally. 
On December 14, 1889, I put nine vigorous specimens of 
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