REACTIONS TO LIGHT 251 



on its side in a shaded aquarium and a beam of sunlight 

 was suddenly thrown upon it, it would usually respond by 

 one or two vigorous locomotor leaps, after which it might 

 come to rest even in the sunlight. If now the sunlight was 

 suddenly cut off, no response followed. That this failure 

 to respond was not due to exhaustion from over-exposure 

 to light was easily shown by quickly throwing on the sun- 

 light a second time, whereupon a reaction much like the 

 first one usually followed immediately." 



The jellyfish Sarsia contracts, according to Romanes 

 (1885, p. 41), if suddenly illuminated while it is at rest, 

 and Yerkes (1902) observed similar reactions in Gonione- 

 mus. A decrease in illumination produces no response in 

 either of these forms if they are at rest, but in case of 

 Gonionemus in the active state the movements are imme- 

 diately checked either by an increase or by a decrease of 

 intensity, after which the medusae turn over and sink to 

 the bottom. While a sudden decrease of intensity does not 

 call forth a response in Gonionemus when at rest, prolonged 

 exposure to light of low intensity causes it to become active. 

 Thus its light reactions are such as to keep it in moderately 

 illuminated regions. In Sarsia in the active state, on the 

 other hand, increase in illumination tends to cause increase 

 in activity. It does not inhibit movement as it does in 

 Gonionemus. A number of animals respond to both a 

 decrease and an increase of light intensity. Most of these 

 respond more definitely to the former than to the latter, 

 but there are some which appear to respond to both in the 

 same way and equally definitely. Nagel says (1896, p. 74), 

 e.g., that Helix hortensis draws into its shell when suddenly 

 illuminated, much as it does when suddenly shaded. Such 

 reactions are of considerable theoretic importance : they will 

 be referred to again later. 



