HELIOTROPISM OF ANIMALS 23 
would be wrong to attribute their tendency to creep into the 
folds of the cloth to their negative heliotropism. When I 
experimented on these animals with the glass plate, I found 
that they wedged themselves under it, and remained there 
exposed to broad daylight, rather than creep away from it. 
Inside of a box the animals collected in the concave edges; 
and it was very noticeable that the animals rarely ran over 
the free surfaces, but nearly always along the edges, as if it 
were ever necessary for them to have their sides in contact 
with solid bodies. 
I believe that this form of contact-irritability is identical 
with the important phenomenon, observed by J. Dewitz,’ 
that spermatozoa are compelled to turn a certain side of their 
bodies toward solid bodies. Because of this contact-irrita- 
bility a spermatozoon is never able to leave a cover-glass or 
a glass slide when once it comes in contact with it. I have 
observed the same phenomena in hypotrichal Infusoria. 
These always turn one side of their bodies, the ventral, 
toward solid bodies. They further resemble the spermatozoa 
observed by Dewitz in that they alter the direction of their 
movement always in the same sense, so that on the cover- 
glass of a microscopical preparation are found only Infu- 
soria which move in one direction, while on the glass slide 
they seem to move in the opposite direction. 
In order to distinguish this form of contact-irritability 
f-om other forms of contact-irritability (such as the rolling up 
or progressive or retrogressive movements when touched), I 
shall call the peculiarity, possessed by some animals, of 
orienting their bodies in a definite way toward the surface 
of other solid bodies, stereotropism. 
The co-operation of other forms of animal irritability with 
heliotropism is so simple as to be self-explanatory wherever 
we may encounter it in our experiments. 
1J,Drwitz, Pfliigers Archiv, Vol. XXXVIII (1886). 
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