The React!0j2s of Planariaiis to Light 129 



be, and, secondly, to pass briefly in review some of the many ways 

 in which hght may change the physiological state of such a worm. 



a Classification of Physiological States 



It is by no means easy to define even a simple physiological state, 

 since the subtle changes form a continuous series of conditions 

 which pass imperceptibly into each other. 



An arbitrary classification for convenience may, however, be 

 made as follows: 



1 Relaxation, or rest. 



2 Slight activity, without locomotion. 



3 Normal activity. 



4 Violent activity. 



5 Rigor. 



6 Exhaustion. 



In the first of these states there is a minimum expenditure of 

 energy caused by the ebb of the katabolic processes. 



The second and fourth states indicate what are often referred 

 to as conditions of low and high "tonus," but as this term has a 

 technical significance with reference to muscle reactions, it will not 

 be used in this classification. The third state, thatof normal activ- 

 ity, is the average condition; it is the most desirable one to main- 

 tain in testing the animal's responses to different stimuli. By 

 rigor is understood a state wherein there may be an excessive outgo 

 of energy, but unaccompanied by movement, while under exhaus- 

 tion is included the condition when energy is not being released 

 because there is none to release. 



That excessive or continuous light stimulation may go beyond 

 the point producing rigor or exhaustion and may actually cause 

 death, has been repeatedly proven in the case of bacteria by a 

 long line of observers.^ The inhibitive eff"ect of excessive light 

 upon other organisms than bacteria has been pointed out by 

 Berger ('00) with reference to Cubomedusae; by Pearl and Cole 



*Tyndall ('78), Downes and Blunt ('77, '78), Jamieson ('82), Duclaux ('85a, '85b, *qo), Arloing 

 ('87a,'87b),Roux ('87), Dandrieu ('88), Raum ('89), Pansini ('8q), Janowski ('50), Buchner (V) , 

 and Ward ('94). 



