1919] Esterhj: Reactions of Various Plankton Animals 31 



Conclusions. Calanus is negative to light of all intensities except 

 when the water is cooled. The phototropism becomes noticeably posi- 

 tive at about 13° C, and is pronouncedly so at temperatures below 

 10° C. There is no marked change in reaction when the salinity is 

 increased if the temperature is not lowered ; the phototropism is nega- 

 tive. But there is some indication that tbe positivity decreases, rela- 

 tively, in water of low temperature and high salinity. 



GEOTROPISM 



In sea water of ordinary temperature and salinity Calanus is 

 always positively geotropic except when lighted from below. The 

 animals descend to the bottom and remain there except for what may 

 be called accidental dashes upward. If put into a column of water 

 the specimens do not swim down, under ordinary conditions, but they 

 drop down passively. The descent is sometimes interrupted by swim- 

 ming to the side of the tube where the animal may cling for a short 

 time or bump against the glass, but it soon begins to drop again. An 

 animal may even make a short ascent, then drop, then ascend, and so 

 on, each upward movement being of less extent than one in the opposite 

 dh*ection so that the individual gets lower and lower in the tube. 

 When dropping passively the long axis of the body is not quite per- 

 pendicular but the anterior end of the animal is always uppermost. 

 The axis is inclined toward the dorsal side about 25° from the per- 

 pendicular, and the anterior antennae are always extended at right 

 angles to the body. Under certain conditions Calanus will swim down, 

 head foremost, with the axis of the body at an angle of 45° to 90°. 

 The dorsal surface is uppermost in swimming down unless the body 

 is perpendicular. 



It is necessary to state that in the experiments on geotropism a 

 temperature or salinity gradient was used as well as uniform condi- 

 tions of both these factors. The lower two-fifths of a tube containing 

 ordinary sea water was set into ice water, or this was done when the 

 water in the tube was of high salinity, or the entire container was 

 chilled when it held water of normal or of high salinity. And in some 

 cases the tube held water of high salinity in the lower part and normal 

 sea water in the upper part, with the lower two-fifths of the tube sur- 

 rounded by ice. There is no evidence from my results that the be- 

 havior in a gradient differs from that shown when the water in the 

 tube is of the same temperature or salinity in all parts. In tabulating 



