1919] Esterly: Reactions of Various Plankton Animals 51 



TABLE 18 



The Phototropism of Metridia at Room Temperature and at 10°C or Less 



Number of observations 



per cent show 



animals negative positive' positive negative of dis- 



observed V IV III II I Total % % tribution 



Temperatures 15°-20°C 



391 47 8 9 10 26 56 25 70 3.9 



Temperature 10°C or Less 



78 10 6 3 90 13 100 1.2 



So far as my observations go, the animals will remain positive indefi- 

 nitely providing the water is kept cooled. There is more or less swim- 

 ming back and forth under any circumstances ; but most of the animals 

 are on the side toward the light in cold water, and on the side away 

 from the light in the warmer water. The same animals are positive, 

 in cold water, to the light of a 15-watt lamp and also to that of a 

 100-watt that is 200 times as intense. The movements away from the 

 light when its intensity is low (the 15-watt at 50 or 100 cm.) do not 

 begin as promptly as when the animals are exposed to the light from 

 the window, and progress is slower at the lesser intensities. 



When individual animals are used the results (table 19, p. 53) are 

 similar to those set forth for the higher temperatures in table 18. 



The table shows that the negative greatly outnumber the positive 

 responses, while the distance covered is practically all in the negative 

 direction. 



GEOTROPISM 



Table 20 (p. 53) gives the summary of results obtained when several 

 animals were under observation at once. It shows the difference in 

 behavior in water of low and high temperature. In all the experi- 

 ments with cold water the entire tube or cylinder was in ice water, at 

 temperatures ranging from 7°C to 9°C. 



In diffuse light there is strong tendency for the animals to descend 

 or to stay at lower levels when the temperature is that of the room. 

 It is suggested by the results that negative geotropism is somewhat 

 increased in the cold water. 



In darkness the geotropism changes as the temperature changes. 

 It is strongly positive at the higher temperatures, and strongly nega- 

 tive in cold water. If the animals are at the bottom in cold water when 

 the light is extinguished they ascend and remain at or near the top ; 



