62 



BEHAVIOR OF THE LOWER ORGANISMS 



in Fig. 46, but in the opposite direction ; it is, as it were, whirled about 

 in its own whirlpool. The resulting path is shown in Fig. 48. This 



circular movement, with the oral side 

 directed toward the centre of the 

 circle, is seen only in specimens show- 

 ing the contact reaction to objects of 

 minute size. 



The contact reaction modifies 

 strongly the reactions to most other 

 stimuh ; this is a matter which will 

 be taken up later. 



Thus when Paramecium comes 

 in contact with a sohd object, it may 

 react in three different ways. First, 

 it may react either positively or 

 negatively, this depending partly on 

 the intensity of the stimulus, partly 

 on the physiological condition of the 

 Paramecium. If it reacts negatively, 

 this reaction may take either one of two forms. If the stimulation 

 occurs at the anterior end, the animal gives the avoiding reaction; 

 if it occurs elsewhere, the animal merely moves forward. 



Fig. 48. — Circular path followed by 

 Paramecium in reacting to contact with a 

 minute particle. 



2. REACTIONS TO CHEMICAL STIMULI 



The reactions to chemical stimuh occur through the avoiding reaction 

 described in the preceding chapter. As we have seen, the avoiding 

 reaction is produced as a rule by a change from one chemical to another. 

 With regard to this relation, there are certain facts of importance. 



In all cases a certain amount of change is necessary to produce 

 reaction; that is, the chemical must be present in a certain concentra- 

 tion before reaction is produced. The sensitiveness of different indi- 

 viduals varies greatly, and even that of given individuals changes much 

 with changes in the conditions. It is therefore not possible to estabhsh 

 for any given chemical the weakest concentration that causes the avoid- 

 ing reaction. But the animals when in ordinary water are very 

 sensitive to the common inorganic chemicals, reacting to very weak solu- 

 tions. Thus the weakest solutions causing reaction have been found to 

 be for various chemicals about as follows : — 



Sodium chloride, ^ to ^ per cent (-^ to y^ normal) ; potassium 

 bromate, about -^ per cent ; sodium carbonate, about ^^ to -g^ per 

 cent ; copper sulphate, about -g-^-g- per cent ; potassium hydroxide, about 



