MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 563 



tinned for some tiinOj tlic ciuiinal will b(!couiu generally 

 stirred up and glide or erawl away. This condition of 

 lowered tonus in the resting animal reminds one of the con- 

 ditions found in sleep in the higher animals. There, as in 

 this case, the general sensory and muscular tonus is greatly 

 reduced, and there seems to be no good reason why the 

 resting condition of these lo.wer organisms may not be con- 

 sidered and called '' sleep." The two things appear to be 

 fundamentally the same physiologically, and would appear 

 to serve the same purpose. Furthermore, there is no 

 apparent reason why the lower organisms should nut have as 

 great a need as the higher for periods of rest or sleep, 

 during which the anabolic processes are in considerable 

 excess over the katabolic. The fact that some lower 

 organisms are so balanced physiologically that they 

 apparently do not require such periods of rest is not con- 

 clusive evidence that other low organisms must be similarly 

 balanced. So far as is known to the writer, there has been 

 comparatively little attention paid to the physiological con- 

 dition of lower organisms during different phases of their 

 activities. An animal which is not moving is loosely said 

 to be in a " resting condition," Avhen in nuiny instances, 

 as in Clepsine, the quiet animal is in a condition of 

 heightened rather than lowered tonus (cf. Whitman, loc. 

 cit.). 



As was noted ia the section on " Natural History," the 

 periods of activity of PI an aria are separated by periods of 

 rest of greater or less length. The time spent in the rest- 

 ing condition, at least during the daytime, is considerably 

 greater, on the average, than that spent in movement. 

 Probably, however, this is reversed during the night, when 

 the activity is greater than during the day. This periodicity 

 in the activity is just what would be expected if there is a 

 necessity for rest at intervals as in the higher animals. 



The causes which immediately induce the coming to rest 

 may now be considered. The principal cause, as has been 

 indicated above, is that the animal becomes fatigued by 



