CHAPTER VIII 

 REACTIONS OF INFUSORIA TO LIGHT AND TO GRAVITY 



i. Reactions to Light 



Like Paramecium, most colorless infusoria do not react at all to 

 light of ordinary intensity. But many species of infusoria are colored, 

 and these commonly react in a decided manner even to the light supplied 

 by the natural conditions of existence. Some react positively; they 

 gather in lighted regions or swim toward the source of light. Others 

 are negative, avoiding light regions and swimming away from the source 

 of light. We shall take up as examples the behavior of a negative or- 

 ganism, Stentor cceruleus, and of a positive organism, Euglena viridis. 



A. Negative Reaction to Light: Stentor ccerulens 



The blue Stentor is a trumpet-shaped organism, with a circle of large 

 adoral cilia or membranelke surrounding the large end or peristome. 

 This circle leads to the mouth, lying at one side of the disklike peristome. 

 The remainder of the body is covered with finer cilia. 1 The animal is 

 colored a deep blue. Stentor is often attached to solid objects by its 

 pointed end or foot, but it is likewise found at times swimming freely. 



We shall have occasion to study the general features of the behavior 

 of Stentor, particularly when attached, in a later section (Chapter X). 

 Here we need to recall only the facts that in response to strong stimula- 

 tion it may contract, becoming shorter and thicker, and that when free 

 swimming it has an avoiding reaction similar to that of Paramecium. 

 When stimulated, it stops or swims backward, turns toward the right 

 aboral side, and continues forward in the new direction (Fig. 76). This 

 is the reaction produced by mechanical stimulation, by heat, and by 

 chemical stimulation acting either on the anterior end or on the body as 

 a whole. The results of localized stimulation have shown clearly that 

 the anterior end or peristome is more sensitive than the remainder of 

 the body surface. 



1 For a figure of another species of Stentor, resembling in essentials the present one, 

 see Fig. 31, b. 



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