LIGHT REACTIONS OF THE MEDUSA GONIONEMUS. 355 



cence of the sex products the animals lie expanded on the bot- 

 tom or suspended, and may remain so as long as the light con- 

 ditions are the same, full darkness having set in. It is probable 

 that much of their prey is captured at this time. As the light 

 grows brighter above, they again move about until they get into 

 weaker light which generally takes them into lower regions or 

 into shaded places, in weak or subdued light. Whenever dis- 

 turbed, especially by change of light intensity, this medusa 

 swims about in all directions, stopping to float down with ex- 

 panded tentacles and inverted bell. Again it displays a most 

 striking behavior; it swims almost vertically to the surface of the 

 water by pulsations of the muscular bell, turning over at the 

 surface, expanding and gracefully floating down. Much has 

 been made of this particular behavior by nearly all those who 

 have observed Gonionemus for some time. It has been referred 

 to as "fishing" and "surface reaction," the latter term being 

 more satisfactory because less anthropomorphic. The main fea- 

 ture of this behavior is the swimming up toward the surface and 

 floating passively down again after turning over at the surface. 

 This may be renewed as long as the same stimulus acts, or until 

 the condition of the medusa changes so that it no longer responds. 



From the foregoing it will be readily understood that these 

 medusae are sensitive to light influences, getting away from 

 strong light especially during the earlier part of the day and 

 afternoon later again moving up toward the fading light. In 

 general, Yerkes, 2 who has made most observations on the reac- 

 tions of Gonioncimis, says : " Clearly, the animals are attuned, so 

 to speak, to a certain range of light intensity, and are negative 

 in their reactions to higher intensities." Any marked change in 

 this intensity causes locomotion which under natural conditions 

 brings the medusa into the light suitable for its life processes. 

 Whether this optimum intensity is constant or changes with the 

 activities of the animal has not been determined, but ordinary 

 observation indicates that its range is not very wide ; it may be 

 called weak light. 



On account of the influence of light on these medusae they 

 collect in the weaker light of an ordinary glass aquarium placed 



2 Amer. Jour. PkysioL, 1903, Vol. IX., page 286. 



