CHAPTER VIII 



OBSERVATIONS ON THE RESPONSES INVOLVED IN THE 



REGULATION OF MOVEMENT TOWARD THE 



SOURCE OF LIGHT IN COELENTERATES 



Only a few of the animals belonging to this group orient 

 in light. Many do not respond to light at all. In others 

 the general activity depends upon the light intensity. Some 

 get into regions of optimum light intensity by means of 

 orientation, others by random wandering movements. In 

 both cases they come to rest in this region. The former 

 method is much more effective than the latter. If animals 

 have the power to orient they can move directly toward 

 the region of optimum intensity and consequently get there 

 much more quickly than do those which reach such regions 

 by random movements. 



In this paper we shall consider only a few species, all of 

 which show some evidence of orientation. 



I. Hydra viridis 



a. Historical review. — Trembley (1744) seems to have 

 been the first to record experimental observations on the 

 effect of light on the movements of Hydra. He exposed 

 the animals in a glass jar covered with an opaque case con- 

 taining an opening on one side, and found that they migrated 

 toward the opening. He did not however record the details 

 in the method of migration. Loeb (1905, p. 73) referring 

 to these experiments says, " Trembley 's experiments on 

 Hydra, however, show that in their case also the relation 

 is the same," i.e., "that Sachs's ^ laws of heliotropism . . . 



^ Sachs, it will be remembered, claimed that orientation is controlled by 

 the direction in which the rays pass through the organism. 



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