220 BEHAVIOR OF THE LOWER ORGANISMS 



them a chance to find the food if it is very near. If they do not find it, 

 "there soon follows a general contraction or series of contractions of 

 the bell, which may take the animal either toward or away from the 

 source of the stimulus." Thus the medusa is induced by the presence 

 of food to swim about, and it usually in this way sooner or later comes 

 in contact with the food (Yerkes, 1902 a, p. 438). The behavior is 

 throughout not a definitely directed action, but an excellent example 

 of the method of trial — of what we call searching, in higher animals. 



When the tentacles actually come in contact with food, they con- 

 tract and twist about each other in such a way as to hold it. The group 

 of contracting tentacles then bends toward the mouth, and that portion 

 of the margin of the bell bearing them contracts, drawing them nearer 

 the mouth. The manubrium bends toward the food, placing the mouth 

 against it, and the food is enveloped by the hps and swallowed. 



What are the determining factors in this behavior? Doubtless, as 

 in Hydra, internal conditions play a part in determining the reaction to 

 food bodies, but this matter has not been studied in the medusa. As 

 to external factors, Yerkes (1902 a) has brought out the following: In 

 Gonionemus the entire food reaction may be produced by chemicals 

 alone. If with a pipette a strong infusion of fish meat is applied to the 

 tentacles, they twist and contract, bending toward the mouth, while the 

 manubrium as usual bends toward the tentacles stimulated. Solutions 

 of common inorganic chemicals do not produce this result ; the tentacles 

 merely contract from them, remaining straight. If the infusion of fish 

 meat is made very weak, the animal begins the food reaction, contract- 

 ing and twisting the tentacles ; but the reaction goes no farther. In rare 

 cases Yerkes (1902 a, p. 439) found that the animal begins the food re- 

 action when a very weak inorganic chemical, such as an acid, is applied 

 to it. But this quickly ceases, before it has gone far. The medusa in 

 such cases makes what we call in higher animals a mistake, but changes 

 its behavior as soon as it discovers the mistake. 



Mechanical stimuli of a certain sort may likewise produce the food 

 reaction. With regard to this we find in Gonionemus certain pecuKar 

 and most suggestive relations. 



If the tentacles come in contact with some quiet object, or are touched 

 with a rod or a needle, they merely contract, remaining straight, as 

 when they are affected by inorganic chemicals. The response is clearly 

 a negative reaction, not a food reaction. But if the tentacles are touched 

 in a peculiar way, by drawing the rod quickly across them, they behave 

 differently. They quickly react and twist, just as when they touch a 

 piece of meat. Then they bend toward the mouth, the margin bearing 

 them contracting inward as usual, while the manubrium bends toward 



