302 INVERTEBRATE PHYSIOLOGY 



cision of its daily cycles and simultaneously provides them, for reasons we 

 have explained, with an effective monthly pacemaker. In a sense, then, the 

 internal clock not only regulates the rhythmic phenomena of the organisms 

 but simultaneously functions as a self-correcting mechanism for retaining 

 its mean, very precise 24-hour periodicity. 



There is further suggestion from some of our analyses of fluctuations in 

 the rate of ventilation in quahogs, that the internal clock is used in the 

 mechanism responsible for metabolic homeostasis. There appears to be a 

 strong tendency for the rate of metabolism at any given period of a day 

 to be higher or lower in compensation for variations from the mean which 

 occurred for that daily period during the preceding two or three days. The 

 mean daily state so attained possesses a cycle form so characteristic of a 

 population for that period that two random samples, of nine quahogs each, 

 can in less than two weeks yield mean daily cycles exhibiting a coefficient 

 of correlation of more than 0.9. 



Summary 

 It seems highly probable at this time that all living things are con- 

 tinuously responding to some external factor or factors which have varia- 

 tions reflecting the positions of the sun and moon relative to the earth. 

 The patterns of these external factors are known to show variations of sev- 

 eral frequencies varying from the relatively short primary solar and lunar 

 cycles, on the one hand, to the much longer 27-day, monthly, and annual 

 cycles, on the other. At least many living things appear also to possess an 

 internal clock or clocks capable of measuring off at least a few primary 

 solar and lunar cycles independently of the external cycles. The internal 

 clocks appear to be the ones concerned with the regulation of those overt 

 rhythms in many activities which clearly repeat themselves with great 

 precision from one day to the next or from one low tide to the next. The 

 internal clocks may probably also have an essential role in that integration 

 of the several cycles of the external factors which is necessary to resolve 

 the rhythmic character of these latter. The evidence suggests that these 

 two kinds of clocks, internal and imposed, normally work hand in hand in 

 living animals and plants to help assure that they time their multifold ac- 

 tivities to permit them to live more successfully in their essentially hos- 

 tile, rhythmic environment. 



REFERENCES 



Bartels, J., 1934. Twenty-seven day recurrences in terrestrial-magnetic and solar 



activity, 1923-1933. Terr. Magn. 39, 201-202. 

 Belehradek, J., 1935. Te^nperature and living matter. Berlin. 

 Beling, I., 1929. Uber das Zeitgedachtnis der Bienen. Zeitschr. vergl. Physiol. 9, 239- 



2,Z7. 



