THIGMOTAXIS OR STEREOTROPISM 33 
equilibrium is by no means general. Such organs occur in 
meduse, ctenophores, turbellaria, crustacea, mollusks and 
vertebrates, and have been evolved along many independent 
lines of descent. They may be absent in many species of 
all these groups, excepting the ctenophores and vertebrates, 
without entailing any loss of the sense of equilibrium. 
Geotropic irritability m many cases seems to be quite 
generally distributed throughout the body. Where special 
organs of equilibration have been evolved orientation is 
generally only partially dependent upon them. In but 
few cases is orientation to gravity entirely destroyed when 
these organs are removed. 
THIGMOTAXIS OR STEREOTROPISM 
The lives of all animals are spent in more or less per- 
manent contact with solid objects and reactions to the 
stimuli thus afforded are universal. Contact of various 
kinds means food, enemies, shelter and many other things 
of interest to the organism or its posterity. Any animal in 
order to stand the least chance of survival must be endowed 
with the power to react to contact with various solid ob- 
jects in an adaptive manner. There are such varied modes 
of reaction to contact that the limitation of our descriptive 
terms is a matter of unusual difficulty. A contact reaction 
by a pseudopod of an Amceba and the heels of a mule are 
very different kinds of behavior although, on reflection, it 
is evident that they possess certain features in common. 
One class of reactions to contact has been termed by Loeb 
stereotropism, which he defines as “the peculiarity, possessed 
by some animals, of orienting their bodies in a definite way 
toward the surface of other solid bodies,” and he distinguishes 
stereotropism from other responses, such as locomotor 
movements, which follow the application of a contact stimu- 
3 
