BEHAVIOR UNDER NATURAL CONDITIONS 183 



water which contains decaying vegetable or animal matter, and therefore 

 swarms with bacteria. Hence the usually ciliary current brings food 

 continuously, and little selection is necessary. The animals take, 

 within wide limits, all that the ciliary current brings. Bits of scot, 

 India ink, carmine or indigo, chalk granules, and the like are swallowed 

 along with the bacteria, though of course they are useless as food. They 

 are merely passed through the body and ejected along with the indi- 

 gestible remains of the food. They do no harm, and the animal may 

 continue to take them indefinitely, provided it receives in addition a suffi- 

 cient amount of real food. If the ciliary currents do not bring food, 

 of course the organisms die after a time. It is well known that infusoria 

 appear suddenly in immense numbers, or disappear with equal rapidity, 

 according as the conditions are favorable or unfavorable. 



But the animals do determine for themselves, to a certain extent, 

 what things they shall take as food, and what they shall not. This is 

 not done, so far as can be observed, by a sorting over of the food by the 

 cilia, as the water current carries it to the mouth. It is true that not 

 all the particles in the vortex produced by the cilia pass into the mouth. 

 But this is due to the simple mechanical conditions. The vortex is 

 very extensive, and the mouth is very small, so that only a fraction of the 

 water in the vortex can ever reach .the mouth. Hence inevitably a 

 large share of the particles in the vortex are whirled away. But this is 

 true of particles which are valuable for food as well as of those which are 

 not. If Stentor is placed in water containing immense numbers of small 

 algal cells which are useful as food, it is found that as many of these pass 

 through the vortex without being taken as happens in the case of worthless 

 particles of soot or carmine. 



Choice of food occurs in a somewhat cruder fashion than through a 

 sorting of the individual particles by the cilia. It takes place through 

 the reaction with which we have become familiar in studying the behavior 

 of the organisms under various stimuli. Thus in Paramecium the re- 

 jection of unsuitable food takes place through the avoiding reaction. If 

 the ciliary current brings water containing various chemicals in solution, 

 or if large solid objects are brought to the mouth, or too great a mass 

 of smaller particles, the Paramecium shifts its position in the usual way. 

 It backs more or less, turns toward the aboral side, and moves to another 

 place. The avoiding reaction is in itself always an expression of choice, 

 in so far as it determines the rejection of certain conditions of existence. 

 In Stentor and Vorticella choice of food occurs in a similar manner, 

 though in these fixed infusoria there is, as we have seen, usually more 

 than one way of rejecting unsuitable conditions. 



In Stentor the following behavior is at times observed. The animal 



