FurTHER INVESTIGATIONS ON HELIOTROPISM 95 
This experiment was made in another aquarium, in which 
the light rays entered chiefly from above. The animals in 
the large aquarium of the zodlogical station at Naples are 
usually found mainly in this position; the light enters this 
aquarium chiefly from above. Here, however, where free- 
swimming forms easily disturb the orientation of Spiro- 
graphis, it is not always so perfect as when all possible dis- 
turbing causes are avoided, as in an aquarium used only for 
such experiments. 
5. It follows from these experiments that gravitation 
exerts only a slight effect, if any, upon animals which are 
subjected simultaneously to the effects of light and gravity. 
It was, however, necessary to discover whether a geotropic 
erection of the animals would not occur under the influence 
of gravity alone in a completely darkened room. 
On March 21, 1890, I placed a large number of Spiro- 
graphis in a horizontal position upon the floor of an aquarium 
in the dark room. On March 24 most of the animals had 
attached themselves by their aboral ends to the bottom of 
the aquarium. The oral ends of the tubes were then elevated 
until the gills no longer touched the bottom of the aquarium. 
The axis of the spiral did not stand vertically (as was the 
case when light fell vertically into the aquarium, or as should 
have been the case had the animals been geotropically irri- 
table), but only at a slight angle from the horizontal. The 
animals remained in this position until the end of the experi- 
ment, which was interrupted in the middle of April. Gravity 
therefore has no important influence upon the orientation of 
Spirographis Spallanzanii. 
6. The contact-irritability of the gills is manifested by 
the fact that they bend away from solid surfaces. This 
form of irritability can modify the result of the heliotropic 
experiments upon the animals. I placed several of the ani- 
mals upon the floor of an aquarium which was so shallow 
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