REACTIONS OF INFUSORIA TO LIGHT AND TO GRAVITY 149 



A change in the intensity of hght does not as a rule produce its 

 characteristic effect immediately, but requires a definite interval of time. 

 When the light is faint and the organisms are swimming toward it, if 

 the light is suddenly increased to an intensity to which they are nega- 

 tive, the swarm spores continue to swim toward it for some time. The 

 interval may amount to as much as half a minute. At the end of this 

 period they turn and swim away from the light. Again, when the or- 

 ganisms are swimming away from a strong light, a sudden decrease 

 in illumination causes them to become positive only after some seconds. 

 But in some species there is no such delay in the effects of a change of 

 illumination. 



To sum up, we find that the reactions to Hght occur in the infusoria 

 in essentially the same way as do the reactions to most other stimuU, 

 through the avoiding reaction ; that is, by the method of trying movements 

 in different directions. The cause of reaction is a change in the intensity 

 of light, — primarily that affecting the sensitive anterior end. Changes 

 in intensity may be produced either (i) by the progression of the or- 

 ganism into a region of greater or less illumination, or (2) by the swinging 

 of the sensitive anterior end toward or away from the source of Hght, 

 so that it is shaded at one moment and strongly Ughted the next. Usually 

 these two classes of changes work in unison ; when they are opposed, the 

 organism reacts in accordance with that which is stronger. When the 

 second class of changes above mentioned is the determining factor, 

 the organism continues to react by trial till these changes cease. This 

 results in producing orien'ation with anterior end directed toward or 

 away from the source of hght. In strong light the effect of an increase 

 or decrease of intensity is often the reverse of that observed in weak 

 light. 



2. Reaction to Gravity and to Centrifugal Force 



A considerable number of infusoria have been found to react to 

 gravity in much the same way as does Paramecium (Jensen, 1893). 

 As a rule, when placed in vertical tubes, they rise to the upper end. 

 The following infusoria have been found to behave in this way : Among 

 the flagellates: Euglena, Chlamydomonas, Hasmatococcus, Polytoma, 

 ChromuHna; among the ciliates: Paramecium bursaria, Urostyla. 

 Spirostomum ambiguum takes at times a vertical position in the water 

 a short distance above the bottom, with anterior end upward. Under 

 these circumstances it is anchored by an invisible thread of mucus, as 

 may be observed by passing a glass rod between it and the bottom 

 (Fig. 82), The stationary position oriented with reference to gravity 



