374 ELEMENTS OF BIOLOGY 



electricity when treated experimentally, but their results only indi- 

 rectly aid in the understanding of the normal and usual behavior of 

 animals. 



In the first chapter (p. 25) it was shown that colloidal particles 

 bear electrical charges. If such a suspension of fine particles in 

 water is connected with the poles of a battery, positively charged 

 particles will move toward the negative pole and negatively charged 

 particles toward the positive pole. This is quite understandable, 

 since unlike charges attract and like charges repel. Similarly, if bac- 

 teria are placed in an electric field they behave as do colloidal par- 

 ticles, migrating toward the pole that bears the charge opposite to 

 their own. But this comparatively simple explanation cannot be 

 extended to the behavior of larger animals when exposed to elec-J 

 tricity, without becoming involved in some complicated assump-W 

 tions. Small animals such as insects, worms, frog tadpoles, and 

 small fish do respond to an electric current; some types move 

 toward the positive pole but the majority move toward the negative. 

 Large animals, Man included, appear to be indifferent so far as 

 definite orientation in an electric current is concerned. 



Stereotropism. This term is applied to a type of response in 

 which the animal seeks to place itself in contact with surfaces. It is 

 highly characteristic of the behavior of burrowing worms such as 

 the sandworm, Nereis (p. 140) and the earthworm. If earthworms 

 are placed in containers with square corners they presently arrange 

 themselves so that as much as possible of the body surface is in 

 contact with the sides and floor of the container. It is probable that 

 this reaction is of considerable importance in other phenomena of 

 animal behavior, for example in the bunching together of animals 

 for mutual protection (p. 384). 



Thermotropisin. All animals, whether warm- or cold-blooded 

 (homoiothermal or poikilothermal) are affected by temperature and 

 either seek out or accumulate in environments in which the tem- 

 perature is best suited for their metabolic processes. But homoio- 



