THE BEHAVIOR OF OTHER INFUSORIA 121 



direction. Thus the region containing the chemical is avoided. In 

 many cases this reaction takes place in a very pronounced manner; 

 the animal shoots far backward, whirls rapidly toward the one side, 

 and repeats the reaction many times. In other cases the reaction is 

 less pronounced, and motion merely becomes a little slower as long as 

 the chemical is received in the ciliary current, while at the same time 

 the animal quietly swings its anterior end about in a circle (as in Fig. 

 37 or 38). This continues until it finds a direction from which no more 

 of the chemical is received ; in that direction it swims forward. If the 

 movements of the animal are not precisely observed, the method by 

 which the reaction occurs may in such cases be easily misunderstood. 



There are various chemicals in which certain infusoria gather, pro- 

 ducing collections like those formed by Paramecium in acids (Fig. 43). 

 In all cases in which the facts are accurately known, these collections 

 are formed in the same way as are those of Paramecia. The animals 

 enter without reaction into the region where the substance is present, then 

 respond by the avoiding reaction whenever they come to the outer boun- 

 dary of the area containing the substance. Thus every individual that 

 enters the area of the chemical remains, and in the course of a longer 

 or shorter period a collection is formed here. In many cases this indirect 

 method of gathering together is strikingly evident, and the individuals 

 may be clearly seen to move about within the area containing the chemi- 

 cal, in the manner represented in Fig. 44. If the infusoria observed 

 are very minute, so that differentiations of the body are to be seen only 

 with great difficulty, if their movements are rapid, and if in the avoid- 

 ing reaction they do not swim backward, but merely stop and turn 

 toward one (structurally defined) side, at the same time revolving on 

 the long axis, then the reaction method is not so evident on a cursory 

 examination. In such cases, if the relation of the direction of turning 

 to the structural differentiations of the body and to the revolution on the 

 long axis are not carefully determined, the animal will be supposed to 

 turn directly, without variations of any sort, into the chemical. This 

 was formerly supposed to be the universal method of reaction to chemi- 

 cals. The cause for the turning was supposed to be found in the dif- 

 ference in the concentration of the chemical on the two sides of the 

 organism. The animal turned directly toward the side of greater con- 

 centration ("positive chemotaxis") or of less concentration ("negative 

 chemotaxis"). This method of reacting to chemicals is no longer sup- 

 posed to exist for infusoria by any one familiar with the reaction method 

 described in the foregoing pages, so far as I am aware, save in the case 

 of certain very minute organisms, — fern spermatozoids, Saprolegnia 

 swarm spores, and the flagellate Trepomonas agilis (Rothert, 1901, 



