58 University of California Publications in Zoology [Vol. 19 



PHOTOTROPISM 



The evidence that I can present regarding the reactions of Sagitta 

 to light has all been obtained by observing individual specimens. So 

 far as my records go the animals are strongly positive to light if they 

 are in a horizontal tube that is parallel to the rays. In a stock dish 

 before the window it seems to me that the specimens are swimming 

 around indiscriminately, but the difficulty of observation under such 

 circumstances is great. Table 22 (p 57) is a summary of the observa- 

 tions. All were made upon animals in the 50-centimeter tube. As 

 soon as a specimen reached the end of the tube it was turned to the 

 vertical position and the geotropism noted. The observations were 

 made in intensities ranging from bright daylight, through the 50 by 40 

 centimeter window, to the 15-watt lamp at 10, 20, and 50 centimeters ; 

 the 100-watt lamp was not used. 



The table clearly shows that the animals react positively to light. 

 There is no reason to doubt that after moving as far as possible toward 

 the light they would remain there. Some observations have shown that 

 an animal may actually turn about and swim away from the light for 

 a short distance, and then turn and swim toward the light. The be- 

 havior toward light was not tested in water of low temperature or 

 high salinity. 



GEOTEOPISM 



When the individual records are summarized it appears that there 

 is a well defined negative geotropism at ordinary temperature and 

 salinity. There is evidence that the geotropism changes in diffuse 

 light of too great intensity, but in the dark room with the window 

 unshaded or in the light of the 15-watt lamp, the animals ascend and 

 remain at or near the top. The same is true of the behavior in dark- 

 ness and in vertical light from below and from above. Table 23 (p. 57) 

 gives the summary of results of observations on single specimens. 



It seems to me that the evidence is conclusive that Sagitta is nega- 

 tively geotropic in darkness as well as in light from any direction and 

 of the intensities used in the experiments. When once at the top the 

 animals keep bumping against the end of the tube, and if they stop 

 swimming they drop down for a longer or shorter distance and then 

 swim back to the top ; this passive descent accounts for most of the 

 records that show downward movement. When the tube is lighted 



