1919]- Esterly: Reactions of Various Plankton Animals 17 



GEOTEOPISM 



It has been shown (Esterly, 1917&) that Acartia tonsa and A. 

 clausi have a marked tendency to remain at or near the top of a 

 column of water when in the diffuse light of a room. And specimens 

 of both forms are to be found largely toward the bottom when kept 

 in the dark during the day. But from 6 p.m. to 8 p.m. they both show 

 a noteworthy increase in the upper levels even if they have been for 

 hours continually in darkness except for occasional observations. This 

 rhythm is less marked in tonsa at temperatures below 16° C, but it is 

 clearly shown in clausi at those temperatures. In view of the tables 

 published in the paper cited it is unnecessary to repeat the figures here. 



Acartia tonsa is positively geotropic if the animals are obtained in 

 deep water and tested within a short time ; nothing could be more 

 striking by contrast with the surface animals than the way in which 

 the others swim down in diffuse light, or from the 15-watt lamp at' 

 the top of a tube, or in darkness. Some details of experiments may be 

 useful at this point, although my material dealing with the geotropism 

 of deep-water Acartia is not sufficient to warrant tabulation. 



On November 1, 1917, a haul from about 100 feet was brought into 

 the laboratory at 11:30 a.m. Fifteen animals (eleven females and 

 four males) were tested one by one and all were negative to light 

 from the window of the dark room. Ten of these animals (the four 

 males and six of the females) were put, one by one, into a vertical 

 tube 50 by 3 cm., beginning at 2 :30 p.m. They all swam to the bottom 

 in the light of the north windows in the main room and all were at or 

 close to the bottom when observations were taken at 2 :40, 3 :00, 3 :30, 

 and 4:20 p.m. The next observation was at 6:20 p.m.; at that time 

 there were three animals at the top while at 7 :20 there were five at 

 the top. 



Another lot of animals was obtained from about 120 feet on Decem- 

 ber 1, 1917, and was brought into the laboratory at 10:15 a.m. Eight 

 animals were tested in the light of the large window in the dark room, 

 by releasing each individual at the center of a shallow dish and noting 

 the direction of travel. I have records for seventeen such trips, and 

 only two were positive. The two were made by an animal that also 

 made one negative trip. At 11 :15 a.m. the animals were put into a ver- 

 tical tube (50 by 3 em.), one by one. The tube was left standing in the 

 dark room one meter from the window. The length of the tube was 

 mai-ked off into five equal divisions, and in giving the distribution of 

 the animals the number in the top section is set down at the left, and 



