272 BEHAVIOR OF THE LOWER ORGANISMS 



is a simple fact of observation, which leaves open the possibility of many 

 different explanations. Is the simple explanation given by the local 

 action theory of tropisms one that is of general applicability to the 

 directed reactions of lower and higher Metazoa ? 



In considering the evidence on this question, we find that even in 

 symmetrical Metazoa the direction of movement with reference to ex- 

 ternal agents is by no means always brought about by a simple, direct 

 turning. On the contrary, in many of the Metazoa, trial movements are 

 as noticeable and important as in the Protozoa. This we have illustrated 

 in detail for many invertebrates in the section devoted to this subject 

 (Chapter XII, Section 2). For such behavior the local action theory 

 of tropisms fails to give determining factors. 



In some cases the turning movements are directly toward or from 

 certain stimuli. But the question here is, whether this turning is pro- 

 duced by the local action of the agent in question on the part of the 

 body against which it impinges, as is asserted by the theory which we 

 are considering, and illustrated in Fig. 144. 



In a few instances this is apparently the case. The medusa escapes 

 unfavorable stimulation by contracting most strongly on the side on 

 which the stimulus impinges. In Hydra local stimulation by chemicals, 

 heat, or electricity often produces limited local contraction, causing the 

 animal to bend toward the side stimulated. In various sea anemones 

 the tentacles, and sometimes the body, may bend toward the side stimu- 

 lated, as this theory demands. Yet this direct contraction plays very 

 little part in the behavior of these animals. In Hydra it is only injurious 

 agents to which the animal responds in this way, and the result is to still 

 further subject the animal to the action of the injurious agent. In order 

 to escape the action of injurious stimuli, Hydra has recourse to behavior 

 of quite a different character, and in its natural life there seems to be 

 no indication that behavior ever occurs in accordance with this theory 

 of direct local action. In sea anemones the direct turning toward the 

 region stimulated is at once supplemented by movements determined in 

 quite a different way, — through the structure of the organism, — the 

 tentacles bending toward the mouth. Without this supplementary re- 

 action the local bending would be of no service. In the hydroid Cory- 

 morpha it is only this second method of bending that occurs at all. 

 Throughout the Ccelenterata the part played by trial movements, not 

 directly determined by the position of the stimulating agent, is most 

 striking and important. 



In the echinoderms we have, as in Amoeba, organisms which are 

 as a rule without a definite body axis, so far as the direction of locomo- 

 tion goes; there is usually no permanent anterior, posterior, right, or 



