THE BEHAVIOR OF INFUSORIA; PARAMECIUM 57 



rally results in the tendency of the animals to collect in distilled water. 

 But there are certain chemicals of which the optimum is a certain posi- 

 tive concentration, so that Paramecia give 

 the avoiding reaction at passing to weaker 

 solutions or to water containing none of 

 the substance in question. This is the 

 case with acids and with oxygen. If a 

 drop of very weak acid is introduced into 

 a slide of Paramecia (Fig. 43) that are in 

 ordinary water, the animals quickly gather 

 in the drop. This may be shown by the 

 use of about y^-g to Jjy per cent of the ordi- 

 nary laboratory solutions of hydrochloric 

 or sulphuric acid, or of -^ to ^5 per cent Fig. 44. — Path followed by a 

 acetic acid. In a short time^the drop is sin *= le Paramecium in a drop of 



1 acid. 



swarming with Paramecia. 1 



Observation shows that the method of collecting in the acid is the 

 same as in the cases before described. The rapid movements of the 

 animals in all directions are what carry them into the drop. They do 

 not react in any way at the moment of entering it, but swim across. 

 At the point where they would pass out into the surrounding water they 

 respond by the avoiding reaction ; hence they return to the acid. This 

 is repeated each time that they come to the boundary. Hence all that 

 enter the acid remain till it is crowded. The path of a single Parame- 

 cium within a drop of acid is shown in Fig. 44. 



In the formation of all these collections the natural roving move- 

 ments play an essential part. These movements cause any given speci- 

 men in the course of a short time to cross almost any given area in the 

 preparation, and hence bring the animals to the introduced drop. The 

 animals do not turn and swim in radial lines toward the drop of acid. 

 If a ring is marked on the upper surface of the cover-glass, as many 

 Paramecia will be found to pass beneath this ring before a drop of acid 

 is placed beneath it as after. But in the latter case all that pass beneath 

 the ring remain, and the collection results. If we wait, before introduc- 

 ing the acid, till all have become nearly quiet, no collection is produced. 



We may sum up the usual behavior of Paramecium under the vari- 

 ous stimuli of the environment in the following way. The natural 

 condition of the animal is movement. In constant external conditions 

 (unless destructive) the movements are not changed, — that is, there 



1 In all these experiments it is assumed, of course, that the preparation contains the 

 infusoria in very large numbers. With scattered specimens only, the results are slow and 

 not striking. 



