254 BEHAVIOR OF THE LOWER ORGANISMS 



individual that reacts to a given stimulus positively may a little later 

 react negatively, and vice versa. After long study of Planaria, Pearl 

 concludes that "it is almost an absolute necessity that a person should 

 become famihar, or perhaps better, intimate, with an organism, so that 

 he knows it in something the same way that he knows a person, before 

 he can hope to get even an approximation of the truth regarding its 

 behavior." This remark might be extended to most lower animals. 



As we have seen in a previous section (p. 236), the behavior of the 

 flatworm shows certain well-defined reaction types, which might, taken 

 separately, be called reflexes. But when we consider the various factors 

 which determine the production and combination of these reaction types, 

 we cannot consider the behavior of the flatworm as "purely reflex," 

 if we mean by reflexes invariable reactions to the same external stimuli. 

 On the contrary, the behavior is extremely variable in accordance with 

 many conditions, internal as well as external. 



A detailed analysis of the behavior of almost any of the lower inver- 

 tebrates would show as many different physiological conditions on which 

 behavior depends as we find in the flatworm. In the earthworm, for 

 example, the conditions are still more complicated than in the flatworm, 

 so that the same external stimulus, acting with the same intensity, and 

 applied to the same spot on the body, may produce any one of at least 

 six different reactions. The variations of internal state as the animal 

 moves about are what condition the "random movements" described 

 by Holmes in the reactions to light, and by Smith in the reactions to other 

 stimuli (see p. 247). 



Of special interest are changes in state that lead to more or less per- 

 manent modifications in behavior. These are httle known in the lower 

 organisms. Most of the changes of physiological state described in the 

 foregoing paragraphs are not known to last more than a short time. 

 In Vorticella, Hodge and Aikins (1895) state that the modified behavior 

 endured for five hours ; this perhaps needs confirmation. In the lowest 

 organisms it is difficult to carry out experiments that shall determine 

 how long modifications last. Perhaps the lowest animal in which an 

 enduring modification of behavior has been demonstrated is the flatworm 

 Convoluta roscojjensis. This is one of the lowest of the group, belong- 

 ing to the division Acoela, which includes the simple forms having no 

 alimentary canal. The behavior of Convoluta, as described by Gamble 

 and Keeble (1903), and by Bohn (1903 a), presents many features of 

 the greatest interest ; into only a few of these can we enter. Convoluta 

 is a small green worm that lives in immense numbers on the sand of the 

 seacoast of Brittany, just above the water Une. It forms thus large 

 green patches. When the tide rises the water covers the region where 



