368 S. O. MAST 



of orientation is probably due to some specific change or state in 

 the physiological processes of the organisms which can be induced 

 by any one of the factors mentioned, i.e., alkahs, anesthetics, 

 temperature, or light. 



In this paper we shall deal primarily with the effect of illumi- 

 nation on the sense of orientation, but we shaU also briefly con- 

 sider some other factors. 



MATERIALS AND METHODS 



The specimens used in this investigation were all collected at 

 Woods Hole in a fresh-water pond. Volvox appeared early in 

 July and continued for nearly a month. It was found only at 

 the edge of the pond in a few small puddles, but in these it was 

 very abundant most of the time. Pandorina appeared early in 

 August, shortly after Volvox had disappeared, and it continued 

 nearly to the end of the month. It was not found in all parts 

 of the pond, but was much more widely distributed than Volvox 

 and equally abundant . 



Pandorina thrived much better in the laboratory than Volvox, 

 but neither lived more than a week or two. Most of the obser- 

 vations were consequently made on specimens within a few days 

 after they had been collected. 



The observations were all made in a large basement dark-room 

 in which there was remarkably little variation in temperature 

 throughout the season and practically none during the time occu- 

 pied by any given experiment. The dark-room was so fitted up 

 that either natural or artificial light could be used. Aside from 

 ordinary electric lamps, there were two gas-filled stereopticon 

 lamps, one 250 and the other 1,000 watt. These two lamps were 

 mounted in an adjoining room from which light was admitted 

 to the dark-room through a hole in the wall, which could be 

 varied in size (fig. 1). Thus a horizontal beam of hght of the 

 desired size was produced. This extended through the dark-room 

 parallel with the surface of a series of black tables 7 m. long 

 The two lamps could readily be interchanged in position. By 

 this means and by varying the distance between the organisms 



