46 THE TROPISMS 
process varies greatly in different animals. Orientation may 
be effected by the action of the cilia, flagella, muscles of the 
body wall, legs and wings; and the way in which special 
parts function to bring about orientation may be funda- 
mentally different in different forms. In some cases the 
whole body is sensitive to light, in other cases only certain 
regions, and in many forms the response is entirely depend- 
ent upon the eyes. 
There has been more or less controversy as to whether 
the phototactic reaction is due to the direction of the rays 
or to differences in light intensity. A good part of this 
discussion has been based on misunderstandings. If an 
animal is oblique to the rays of light it naturally becomes 
stimulated with unequal degrees of intensity on its two 
sides, which probably causes the orienting movements. 
There is little doubt that direction of light in the majority 
of cases, if not in all, causes orientation in this manner; it is 
possible, however, that in more or less transparent animals 
the direction the rays take through the tissues may deter- 
mine orientation in a way analogous to the directive effect 
of the galvanic current, but at present we have no convine- 
ing evidence that such is the case. 
It was shown by Loeb in all the animals he subjected to 
the experiment that it is the rays toward the violet end of the 
spectrum that are the most potent in producing the photo- 
tactic response. Red light has comparatively little orienting 
power and many phototactic animals react to it as to dark- 
ness. I have found, however, that the amphipod Orchestra 
agilis when brought into a photographic dark room illumin- 
ated by light through ordinary red glass is markedly positive. 
Specimens in a dark box held so as to get the red light re- 
flected from one wall still showed a positive response although 
the light was so dim I could scarcely see their movements. 
