1919] Esterly: Reactions of Various Plankton Animals 5 



equal illumination on the two sides of the cardboard screen. The 

 light is diffused by the parafnne to such an extent that the edges of 

 the blocks show when both are equally lighted. The 15-watt lamp was 

 used as a standard of intensity. The intensity of the 100-watt is about 

 6, if that of the other lamp is called unity. The 50 by 40 centimeter 

 window gives an intensity about 3800 times that of the 15-watt, and 

 400 times that of the 100-watt. These figures are only approximations, 

 of course, but they serve for present purposes. 



Temperatures lower than those ever found at the surface of the 

 sea in the region about La Jolla were produced by putting a dish or 

 tube of sea water into ice. With a vertical container the whole column 

 or only the lower portion could be cooled, the latter case giving a 

 gradient with a difference of 8°C or 10°C between the top and bottom. 



Water of a salinity greatly above that of ordinary sea water was 

 obtained by allowing evaporation to continue for days in a warm 

 room. This highly concentrated water was diluted with normal ocean 

 water until a desired degree of salinity was reached. The actual 

 salinity was computed by means of Knudsen's tables (1901) from 

 the specific gravity as given by a hydrometer. It should be noted that 

 this method of increasing the salinity does not involve changing the 

 relative proportions of the salts in sea water as does the addition of 

 one salt, such as NaCl, to ordinary sea water. I did not attempt in 

 any case to ascertain whether a decrease in the salinity has an effect 

 on reactions. 



In many experiments a single animal was enclosed in a glass tube 

 500 by 30 mm., which was filled with water and closed with a rubber 

 stopper so that no air space remained. The tube could be clamped to 

 a ring support and held in position so that light might enter from 

 above or from below ; or the tube could be kept horizontal. It is 

 possible, furthermore, to arrange the test so that the animal must 

 start at the top or at the bottom of the column of water, by swinging 

 the tube through 180°. There was no apparent effect on the animals 

 from the mechanical stimulus caused by turning the tube; if there 

 was it could not have been as great as that caused by picking up the 

 specimen in a pipette. 



All the animals were collected by means of a conical plankton net. 

 A wide-mouth bottle or jar was tied into the peak of the net, so that 

 the organisms were never out of water and they were not handled 

 at all until in the laboratory. It was found that the specimens were 

 in better condition and lived longer if they passed from the net into 

 a large amount of water, and accordingly, for the greater part of the 

 collecting a two-quart jar was tied into the net. 



