50 University of California Publications in Zoology [Vol. 19 



(table 15) is in line with the distribution observed in the sea, and the 

 negative reactions to light account for the small numbers at the sur- 

 face during the day. But the inconstant results obtained in experi- 

 ments on the geotropism make difficult their application to the be- 

 havior of the species under natural conditions. 



Reactions of the copepod Metridia lucens 



The occurrence of this copepod as material for experiment was 

 erratic, but the animals were obtained in fairly large numbers and at 

 opportune times so that it was possible to work with them to advan- 

 tage. The locomotion of these organisms is slow unless they are sud- 

 dently disturbed ; there is nothing of the jerky character in the 

 swimming; it is what may appropriately be called "sailing." This 

 sort of movement is continual, but if. the vessel is jarred the animals 

 dart about with great rapidity. The sailing of Metridia is different 

 from the "fluttering" or "hovering" of Ldbidocera, but the difference 

 is hard to express in words. When specimens are in diffuse light in a 

 vertical tube it is especially common when .they are descending to 

 see them sail in small circles that together make up a close spiral 

 carrying them lower and lower. They also travel in vertical circles, 

 something like "looping-the-loop" in an aeroplane, so that they grad- 

 ually descend. Very rarely a specimen will drop down like Calanus. 

 but practically always the animals are sailing about in circles. If a 

 light is placed at the top of the tube it is not uncommon for the descent 

 to be accomplished by swimming down head foremost. 



Metridia lives very well under laboratory conditions, and I have 

 not observed any difference in the behavior soon after they have been 

 taken from the sea as compared with that several hours later. 



PHOTOTROPISM 



The reactions have been observed in the light of the window 50 

 by 40 cm., in the 15-watt, at distances of ten, fifty, and one hundred 

 centimeters from the end of the tube or side of a dish next the lamp, 

 and in the light of the 100-watt lamp about 200 times the intensity of 

 the 15-watt. Metridia is negative to each of these intensities at room 

 temperatures and in water of usual salinity. Positive responses pre- 

 dominate, however, if the water is cooled to 10° C. Table 18 shows a 

 summary of the results when several animals are under observation 

 at a time. 



