1919] Esterhj: Reactions of Various Plankton Animals 41 



tropic and phototropie reactions is beset by too many difficulties to 

 be readily acceptable in view of the results of field work. The matter 

 of the rhythm affords a simpler way of accounting for the habit, but 

 it ought to be supported by more evidence than that brought forward 

 here. 



In discussing the results of field work (Bsterly, 1911, p. 14S, 1912, 

 p. 292) it was stated, as a matter of opinion, that light appears to be 

 the chief factor in the diurnal movement. There was no evident cor- 

 relation to be found between abundance of organisms anil temperature 

 and salinity (Esterly, 1912, pp. 292, 29-1). It must be said that on the 

 basis of the laboratory results the effects of variation in intensity of 

 light (apart from direction) are negligible so far as vertical move- 

 ments are concerned. There is some change in behavior when the tem- 

 perature is lowered and the source of light is at the top or at the bot- 

 tom of the column of water. The reactions do not change significantly 

 when the salinity is increased. It is evident, therefore, that the causal 

 interpretations based on field results do not conform to the laboratory 

 results. Likewise, it is difficult to arrive at an understanding of what 

 Calanus does in nature if we use the results of these experiments as a 

 basis for judgment. 



It is not improbable that a reexamination of the field data, which 

 are now more extensive than when the papers referred to were written, 

 will show correlations between abundance of animals and external 

 conditions that come into line with the results of experiment. It is 

 also not improbable that continued experimentation will show that 

 certain of the responses are due to the artificial conditions of the 

 laboratory, and will reveal others that conform to the facts shown by 

 the field data. 



Eeactions op the copepod Eucalanus elongatus 



This is an unusually large copepod, the length often being from six 

 to eight millimeters. The body is of glass-like transparency, but in 

 many animals there is a reddish or yellowish oil drop outside the diges- 

 tive tube in the posterior part of the cephalothorax. The animals of 

 this species are unique, so far as my experience goes, in that they do 

 not have the incessant jerky movement so characteristic of copepods. 

 A specimen of Eucalanus rests quietly for several minutes at a time, 

 theu suddenly darts away at such speed that it can hardly be followed 

 with the eye. While resting the long anterior antennae are extended 

 at right angles to the body, but when the animal swims they are folded 



