244 THE BEHAVIOR OF LOWER ORGANISMS. 



In the Metazoa behavior is usually not that of trial and error in 

 so elementary a form as is found in the organisms thus far considered. 

 The higher animals, with the development of a nervous system and 

 other bodily differentiations, have usually acquired the power of react- 

 ing more precisely with reference to the localization of the source of 

 stimulus. They more often, therefore, turn directly toward or away 

 from the source of stimulus, a preliminary trial of different directions 

 being unnecessary. But with the acquirement of many reaction possi- 

 bilities, the field for the operation of the method of trial and error is 

 greatly broadened. This could be amply illustrated from the behavior 

 of certain of the lower Metazoa. I hope to develop this point in detail 

 at some later time. Since at present we are interested chiefly in the 

 lowest organism, I shall mention here only a few cases in the Metazoa. 



In the Rotifera behavior is, under many conditions, precisely similar 

 to that which I have described above for the infusoria (details in the 

 third of these contributions) ; that is, the behavior is an example of the 

 method of trial and error in a very pure form. 



In Hydra we find the method of trial and error in a number of 

 features of the behavior. Since a general paper on the behavior of 

 Hydra is in preparation,* I will mention only one or two points that 

 have already been described. If we observe a living, unstimulated 

 green Hydra, we find that it does not remain at rest. If the Hydra is 

 extended in a certain direction, after one or two minutes it contracts, 

 bends over to a new position, then extends in a new direction. After 

 about two minutes it contracts again, bends into a still different posi- 

 tion, and again extends. This process is repeated at fairly regular 

 intervals, so that after a time the Hydra has tried every position possi- 

 ble in its present place of attachment. This exploration of all parts of 

 the surrounding region, of course, aids greatly in finding food. 



Mast (1903) finds that when Hydra is heated from one side it does 

 not move directly away from the source of heat, but merely moves in 

 some random direction. In other words, the animal when heated 

 merely tries a new position. 



McEbius (1873) describes the reaction of a large mollusk (Nassa) to 

 chemical stimuli, as shown when a piece of meat is thrown into the 

 aquarium containing them. They do not orient themselves in the lines 

 of diffusion and travel directly toward the meat, but move " now to the 

 right, now to the left, like a blind man who guides himself forward by 

 trial with his stick. In this way they discover whether they are com- 

 ing nearer or going farther away from the point from which the attrac- 

 tive stimulus arises" (/. c, p. 9, translation). The reaction is thus a 

 clear case of the method of trial and error. Experiments on the leech, 



* By Mr. George Wagner. 



