1917] Esterly: Occurrence of Rhijthiiis in Geotropnm 397 



included in table 3, regardless of temperature. It should be stated 

 that animals from such depths are positively geotropic in the light 

 if not left too long in the laboratory before being tested, and their 

 positive reaction is more strongly marked than the m gative response 

 of surface animals. 



Comments on table 3 are needless, since the results arc in all 

 respects similar to those in table 2. 



It seems reasonable to conclude that the stimulus for the upward 

 movement shown in tables 2 and 3 is an inner iinknown factor, since 

 the periodic behavior takes place in the absence of known recurrent 

 changes in the environment. This statement may pass as a matter of 

 observation, aside from the question of how the rhythm may have been 

 impressed on the organisms in the first place. Ou account of the 

 mortalitj- among the animals, there is not much that can be said about 

 the duration of the tendency to ascend at certain times. In two 

 separate experiments on deep-water animals the upward movement 

 was manifested on two successive days. In each case the animals were 

 in constant darkness (except when the distribution was recorded) 

 from twenty-eight to twent.y-nine hours. In another, using surface 

 animals, the ascent took place on two days, the animals having been 

 in darkness for fifty-three hours and under intermittent observation 

 for twenty-nine hours. 



Acartia clausi, as well as A. tonsa, exhibits this sort of behavior, 

 but the former is not affected by colder water as is the latter. All 

 experiments vising clausi were at temperatures below 16° C. Table 

 4 is for the general comparison of the distribution of clausi in diffuse 

 light and in darkness. 



Acartia clausi, Adults of Both Sexes: Summary of Records of Distribution 

 IN A Column of Water in Diffuse Light and in Darkness 



The table shows that there is a strong tendency for the animals 

 to gather in the upper portions of the column of water when in the 

 light, and to congregate at lower levels in the dark if the time is 

 considered as a whole. 



