196 LIGHT AND PROTOPLASM [Ch. VU 



A study of this table reveals the fact that, in general, organ- 

 isms which live in shady places or in the dark are negatively 

 pliototactic or photopathic, while those living in the light are 

 positively phototactic or photopathic. Thus most fresh-water 

 plaiiarians and leeches are inhabitants of shady pools. Polynoe 

 is generally found in dark retreats, the earthworm and fly 

 larvae are lovers of the dark, and shell-molluscs are for the 

 most part enclosed in cases impervious to light. On the other 

 hand, Daphnia is found largely in open pools, larvce of Lepi- 

 doptera and many adult insects live in the sun. Into this 

 general rule there are some cases which do not so obviously fall. 

 But we have little data concerning the habits of the races em- 

 ployed and the absolute intensity of light used, so that these 

 cases may perhaps be only apparent exceptions. One clear 

 exception is that of the mud-inhabiting Diastylus, which is -t- 

 phototactic. 



3. The General Laws of Phototaxis and Photopathy. — Under 

 this head we shall consider : (a) the sense of the response ; 

 (6) the effective rays ; (c) prototaxis vs. photopathy ; (ci) the 

 mechanics of response to light. 



a. The Sense of the Response. — ■ In considering, now, more 

 generally, the effect of daylight upon the direction of loco- 

 motion of organisms, we must recognize that the sense of 

 response (whether -|- or — ) depends upon internal conditions 

 and external conditions — upon the quality of the protoplasm 

 and the nature of the environment. Let us consider, first, the 

 dependence upon internal conditions. 



We find that, under similar external conditions, different 

 organisms respond differently. For example, many Oscillarise 

 (p. 184) are positively phototactic even in direct sunlight; 

 whilst even moderately strong light will repel many diatoms. 

 In fact, we find that a positively phototactic or photopathic 

 organism is such only in the presence of a certain intensity of 

 light. When the intensity is diminished below a certain point, 

 no response will occur. When, on the other hand, the intensity 

 is increased above a certain point, the organism moves away 

 from the source of light. There is a certain range of intensity 

 in which alone the positive responses occur. The position and 

 the extent of this positively phototactic range vary for the 



