134 BEHAVIOR OF THE LOWER ORGANISMS 



subjected to a sudden, powerful increase in the intensity of the light 

 falling upon them, they often contract (Mast, 1906), and later bend in 

 various directions, till they have become accustomed to the light. 



To sum up, the orientation of the free Stentor in line with the light 

 rays, with its anterior end directed away from the source of light, is due 

 to the fact that an increase of illumination at the sensitive anterior end 

 induces the avoiding reaction. As a necessary result the oriented 

 Stentor swimming in a spiral path tries new directions of movement until 

 it finds one where such changes of illumination no longer occur. Such 

 a direction is found only in orientation with the anterior end directed 

 away from the source of light. From a knowledge of the spiral course 

 and the fact that increase of illumination at the anterior end causes 

 the avoiding reaction, this result could be predicted. The reaction to 

 light, like that to most other stimuli, is based on the method of trial of 

 differently directed movements, till one puts an end to the stimulation. 



B. Positive Reaction to Light: Euglena viridis 



Euglena is not closely related to Stentor; it is a flagellate, while 

 Stentor is a ciliate. If we find similar principles governing the reaction 

 to light in these widely separated organisms, it is probable that these 

 principles are valid for the infusoria in general. 



Euglena viridis (Fig. 74) is a fish-shaped green organism, often 

 found abundantly in the water of stagnant roadside pools, giving them 

 a green color. At the anterior end is a notch from which there extends 

 a single long flagellum, by the lashing of which Euglena swims. Within 

 the body are chlorophyll masses, giving the organism its green color. 

 Near the anterior end, close to the side bearing the larger lip of the 

 notch, — the "dorsal" side, — is a red pigment spot, usually known as 

 the eye spot. As we have seen previously, the "action system" of 

 Euglena resembles in essentials that of Paramecium. It swims in a 

 spiral (Fig. 94), and to most stimuli it responds by an avoiding reaction 

 which consists in stopping or backing, then turning more strongly than 

 usual toward the "dorsal" side. 



If the light is not too strong, Euglenae gather in lighted areas, and 

 when the light comes from one side, they swim toward the source of 

 light. Thus in the culture jar the organisms are usually found on the 

 side next the window or other source of light. In very powerful light, 

 such as the direct rays of the sun, however, Euglena swims away from 

 the source of light. How is this behavior brought about ? 



Let us first study the effect of changes in the intensity of the light. 

 The Euglenae are placed on a slide in a thin layer of water, and are ex- 



