OBSERVATIONS ON UNICELLULAR FORMS 89 



Under the conditions of the experiment described above, 

 the organisms mentioned do not move in a direction parallel 

 with the rays, neither do they necessarily orient so " that 

 symmetrical points of the photosensitive surface [are] 

 struck by the rays of light at the same angle," nor does 

 " the light with the strongest intensity determine the 

 orientation and direction of motion." 



Toads (Bufo americanus) were the only animals w^ith 

 image -forming eyes that were tested with reference to 

 orientation in light from two sources (see p. 87). If the 

 intensity from the two sources is unequal they usually hop 

 directly toward the stronger light and pay no attention to 

 the weaker. This is in accord with Loeb's explanation 

 given above. But if the intensity from the two sources is 

 equal, they go toward either one and not toward a point 

 between the two, as Loeb's explanation demands. In none 

 of the organisms studied are the orienting reactions such 

 as are demanded by Loeb's explanation. These results 

 will be referred to in connection with the discussion of the 

 importance of equal stimulation of symmetrical points on 

 the animal. 



Let us now return to our study of the reactions of Euglena 

 in the crawling state and to the problem suggested by 

 Torrey's criticism of Jennings referred to above. Is orien- 

 tation in Euglena due to light acting constantly as a direc- 

 tive stimulation similar to the effect of a constant electric 

 current, or to an intermittent effect, a response to change 

 of intensity only, in accord with Jennings' explanation? 



d. Material. — During the months of November and 

 December excellent material for this study was discovered 

 in a puddle of water fed by a drain from a dwelling house 

 at Windsor Hills, Baltimore. The bottom of the puddle 

 was covered with a dense green layer composed almost 

 entirely of two species of Euglena, — E. deses and another 

 species which was somewhat like viridis but could not be 

 positively identified. It will be referred to as Euglena x. 

 Most of the E. deses had fiagella, but the E. x with very 



