2O4 s , - MAST AND MARY COVER. 



Phacus pleuronectes (Miiller). It was kept in the laboratory in 

 fairly good condition for several months. All of the specimens 

 used in these observations were taken from the same culture jar. 

 The experiments were all carried out in a large dark room. The 

 apparatus used was arranged as follows : A rectangular box 

 180 cm. long, 15 cm. wide and 15 cm. deep, open at one end and the 

 top, was placed on a table with a microscope near the open end. An 

 electric lamp in the box was attached to the under surface of a short 

 board which fitted into grooves on the top of the box, and could 

 be moved back and forth so as to rapidly change the luminous 

 intensity at the microscope. The lamp was so adjusted and 

 screened that it produced a horizontal beam of light which crossed 

 the stage of the microscope. The beam of light, before reaching 

 the organisms, passed through 3 cm. of water in a glass container. 

 Thus the longer waves from the lamp were absorbed by the water 

 and any possible effect of heat on the behavior of the organisms 

 was greatly reduced, if not entirely eliminated. 



All of the observations were made under the low power of a com- 

 pound microscope with the organisms on the stage in an aquarium, 

 about 3 cm. square, constructed of the best plate glass obtainable. 

 The eye-piece of the microscope contained a micrometer scale 

 which divided the field into equal divisions, each .017 mm. long. 



In making the observations, 30 to 40 specimens of Phacus were 

 taken from a culture jar and put into the aquarium, which con- 

 tained clear water about i cm. deep taken from the same jar. 

 The aquarium was then put into the beam of light on the stage 

 of the microscope and the distance from the lamp adjusted so as 

 to produce the luminous intensity desired. An individual which 

 was accurately oriented was now selected and carefully observed 

 as it proceeded toward the light. (All of the observations were 

 made on positive individuals.) When the anterior end reached 

 a cross-bar of the micrometer scale the stop-watch was started, 

 and when it reached the cross-bar 20 divisions, .34 mm., beyond, 

 the watch was suddenly stopped and the time consumed recorded. 

 The intensity was then suddenly raised or lowered and the same 

 individual again timed, in the manner just described. Thus there 

 were two readings made on each individual. In some cases the 

 readings were made first in the higher, and in other cases first in 



