8o LIGHT AND THE BEHAVIOR OF ORGANISMS 



turn until both sides are equally illuminated, and symmet- 

 rically located points on the body equally stimulated. 



Such a method of orientation is in harmony with much 

 in Verworn's theory and also with the essentials in Loeb's. 

 It does not, however, support the idea connected with 

 these theories, that a constant intensity produces a constant 

 directive stimulation. 



Jennings (1904) has shown that certain amoebae roll over 

 and over in their movement. The protoplasm on the 

 underside in relatively low light intensity is constantly 

 coming to the surface into a greater intensity, and moreover 

 the beginning of every laterally directed pseudopod in those 

 forms which do not roll necessarily causes a change in the 

 light intensity of the protoplasm in it. Thus it is clear that 

 the protoplasm is being continuously subjected to changes 

 of intensity. And while the rate of movement in the animal 

 as a whole is no doubt influenced by constant light inten- 

 sity, much as it is by temperature, it may be that orienting 

 reactions are responses solely to changes in light intensity, 

 — in negative organisms to a rather sudden increase of 

 intensity. 



This method of orientation is opposed to the idea of 

 Sachs (see p. 14), that the direction in which the rays 

 penetrate the tissue is of importance in orientation, and 

 also to that of Loeb (see p. 28) with reference to the im- 

 portance of the angle between the rays and the surface. 



2. Euglena 



a. Description. — Euglena is a minute elongated or- 

 ganism. The posterior extremity ends in a spinelike 

 process; the anterior end is rounded off rather bluntly. 

 The different species vary greatly in size; some are not over 

 o.oi mm. long and o.ooi mm. in diameter, while others are 

 nearly fifty times as large. The forms most commonly met 

 with average about o.i mm. in length and 0.015 mm. in 

 diameter. Nearly all are green, having numerous chloro- 



