Books and Current Literature. 249 



a day then these animals, in response to the rythmic move- 

 ment of the tides, alternately expose themselves to the light 

 on the surface of tlie sand and bury themselves in it. How- 

 ever when the tide is out at night the animals do not come 

 to the surface unless there is strong moonlight. This oc- 

 curs every other week, and it is at this time that the ani- 

 mals deposit their eggs, another periodic phenomenon con- 

 trolled by the movements of the tides. 



In the laboratory, although not subjected to the action 

 of the tides, the animals keep up their periodic movements 

 for nearly a week if exposed to light, but in total darkness 

 only a dav or less. These movements the author holds are 

 due to the tonic effect of light on the response to gravity. 

 Light he maintains induces certain chemical changes which 

 make the animals positive to gravity and darkness pro- 

 duces the opposite effect. However, the fact that periodic 

 movements continue in darkness, even if for but a short 

 time, appears to militate directly against this theory. The 

 periodic movements must depend in some way upon inter- 

 nal periodic processes which in some instances continue in 

 the absence of immediate external stimuli for considerable 

 periods of time, sixty days and more as recently discovered 

 bv Menke. 



There are according to Keeble four different methods 

 of response to light — tropoisms, taxes, back-ground reac- 

 tions and phototonic responses. The assumption, however, 

 that the back-ground reactions are essentially different 

 from the others mentioned does not appear to be well 

 founded. Taxes are defined as movements in a definite 

 direction and tropisms as "purposeful curvatures," — an- 

 other addition to the multifarious definitions alreadv devot- 

 ed to this term. 



Keeble concludes that the reactions are highly adap- 

 tive. After opposing the idea that reflexes are "unalterable 

 and inevitable" he says p. 43 : "They are but servants, and 

 tropistic reflexes serve the master-organisms, to draw it this 

 way or that according as it is well that, this or that route 

 be taken." And again, p. 44: "The organism takes the 



