7 6 



AN INTRODUCTION TO ZOOLOGY 



ever, there is a definite strength which seems to suit the Para- 

 mecium better than any other, and no reaction takes place on 

 entering it. Passage from such a solution to either a weaker or 

 a stronger calls forth the avoiding reaction. The concentration 

 is therefore called the "optimum." " For each chemical there is 

 a certain optimum concentration in which the Paramecia are not 



caused to react. Passage from 

 this optimum to regions of 

 either greater or less concentra- 

 tion causes the avoiding reac- 

 tion, so that the animals tend 

 to remain in the region of the 

 optimum, and if this region is 

 small, to form here a dense col- 

 lection " (62, p. 66, Fig. 35). 



Paramecia may give any one 

 of three reactions to contact 

 stimuli; the first two are nega- 

 FIG. 36. Paramecium at rest with an- tive, the third positive, (i) If 

 terior end against a mass of bac- Paramecium swims against an 

 terial zoogloea (a), showing the obstacle, or if the anterior end, 

 currents produced by the cilia. ^^^ ^ sensitive than 



(From Jennings.) 



the other parts of the body, is 



touched with a glass rod, the avoiding reaction is given. (2) When 

 any other part of the body is stimulated in a like manner, 

 the animal may simply swim forward. (3) Frequently a Para- 

 mecium, upon striking an object when swimming slowly, comes to 

 rest with its cilia in contact with the object (Fig. 36). This 

 positive reaction often brings the animals into an environment 

 rich in food. 



Paramecia do not respond in any way to ordinary visible light, 

 but give the avoiding reaction when ultra-violet rays are thrown 

 upon them; if unable to escape, death ensues in from ten to fifty 

 seconds (58). 



The optimum temperature for Paramecium lies, under ordinary 



