THIGMOTAXIS OR STEREOTROPISM 33 



equilibrium is by no means general. Such organs occur in 

 medusae, 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 in 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 hi 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 hi 

 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 Amoeba and the heels of a mule are 

 very different kinds of behavior although, on reflection, it 

 is evident that they possess certain features hi 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 hi 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- 



