Spatially Determined Reactions 193 



58. Orientation to Light 



In some animals light is sought or avoided not simply 

 because of the fact that in certain intensities it stimulates to 

 restlessness and activity (photokinesis) , so that they come to 

 rest in regions illuminated by other intensities ; but through 

 a direct movement of the animal towards or away from 

 the source of light. It is this type of response to which 

 Loeb and his followers restrict the term " tropism." Plants 

 show it, both in the orienting of their stems with relation 

 to light, and in the movements of their freely swimming 

 " swarm spores." In the case of animals, it is illustrated 

 by the behavior of the sea-anemone Actinia cereus. Weak 

 light causes expansion of the tentacles of this organism 

 perpendicularly to the light rays. If the light is increased, 

 Bohn (86) says the tentacles "tend to orient themselves in 

 the direction of the rays, and finally converge in a bundle 

 parallel to that direction," a response which has the effect 

 of protecting them from the intense light. Again, the tube- 

 dwelling worm Spirographis spallanzanii gradually curves 

 its tube until its mouth end faces the direction from which 

 the rays of light come, and another marine worm, whose 

 tube is absolutely stiff, adapts itself to a change in the 

 direction of the rays by curving the newly formed portions 

 of the tube as it constructs them (42 2). 1 Sea-anemones and 

 tube-dwelling worms closely resemble plants in their mode 

 of living. In freely moving animals, where the oriented 

 movement is made in response to light, it is commonly 

 preceded by body orientation ; that is, the body first faces 

 or turns tail to the light, and the animal then moves for- 

 ward. Sometimes, however, there is no regular body 



1 Hargitt (288) finds no such constancy of orientation in Spirographis as 

 would warrant Loeb's calling the motion a tropism. 

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