236 BEHAVIOR OF THE LOWER ORGANISMS 



be such a multiplicity of these definite units of reaction as in the sea 

 urchin. In the starfish the extension and withdrawal of the tube feet, 

 and the extrusion and withdrawal of the stomach in feeding, may be 

 considered examples. In free-swimming rotifers we find an avoiding re- 

 action similar in all essentials to that of the ciliate infusoria, the animals 

 when stimulated turning toward a structurally defined side. There 

 is the same variability in this reaction that we find in the infusoria. In 

 planarians, the earthworm, and many other worms, reactions of a fairly 

 well-defined character are seen in the turning of the head toward certain 

 stimuli and away from others. These reactions play a large part in 

 the behavior of Planaria, according to Pearl (1903). Weak stimuli of 

 all sorts affecting one side of the body cause the positive turning ; stronger 

 ones, the negative turning. 



Such reactions often depend closely on the localization of the stimu- 

 lus. This may be illustrated from the behavior of the flatworm just 

 mentioned. A weak stimulus at the side of the head, near the anterior 

 tip, causes the head to turn only a little toward the side touched. If 

 the stimulus is farther back, the turning is greater. In each case the 

 turning is so regulated with reference to the point stimulated as to direct 

 the animal very accurately toward the region from which the stimulus 

 came ; this aids it much in finding food. If something touches the flat- 

 worm lightly at the middle of the upper surface of the head, the reaction 

 is much modified. The head is sharply raised and twisted, so as to 

 direct the anterior tip toward the stimulating object, and in such a way 

 that the ventral surface will first come in contact with this object as the 

 animal moves forward. Similar regulatory changes occur in the nega- 

 tive reaction. A strong stimulus at the side of the anterior end causes 

 a quick turning away. A similar stimulus at one side behind the middle 

 causes no turning away, but only a movement forward. At intermedi- 

 ate regions there is a combination of the two reactions, the animal glid- 

 ing forward and at the same time turning away. The farther back the 

 stimulus is given the greater is the tendency to react by moving forward 

 in place of turning away. This change of reaction with a change in 

 the point stimulated is of course regulatory. An intense stimulus at 

 the anterior end is best avoided by turning away, while one near the 

 posterior end is most easily escaped by moving rapidly forward. 



In most animals there are found a certain number of these relatively 

 fixed reaction types which are determined by the usual conditions of 

 existence, — gravity, light, temperature changes, contact with solids, 

 etc. We have examined a considerable number of these in the Protozoa 

 and Ccelenterata. In such reactions the organism often turns or bends 

 directly toward or away from the source of stimulation, as in the posi- 



