298 INVERTEBRATE PHYSIOLOGY 



radiation towards the earth. Life as we know it would not survive without 

 such a protective screen. When the barometric pressure drops, the screen 

 is thinner and less effective and more radiation reaches the earth's sur- 

 face. Therefore, since there are daily and lunar rhythms of barometric 

 pressure, due partly to the gravitational pull of the sun and the moon on 

 the earth's atmosphere, so are there also daily and lunar rhythms in the 

 intensity of the cosmic-ray rain. Supporting but not proving an hypothesis 

 that these rhythms in cosmic radiation may be at least partly responsible 

 for the rhythms observed in organisms is the fact that we have been able 

 to show within the past year that, when the intensity of cosmic-ray rain 

 is experimentally altered through the use of screening lead plates, fiddler 

 crabs show a response to the change in intensity (Brown, Bennett, and 

 Ralph, 1955). Even more support has come from recent experiments with 

 a barostat. Under conditions of constant pressure the rhythms are still ob- 

 served. In addition, very clear 27-day cycles have been found in the oyster 

 and quahog (Brown, Bennett, Webb, and Ralph, 1956). The most 

 regular period in the rhythmic emission of cosmic radiation from the sun 

 is known to be a 27-day period (Bartels, 1934). Also, the physicist Simp- 

 son (1954) has very recently described 27-day cycles in cosmic radiation. 

 One final suggestion that cosmic radiation is an operating factor is the 

 fact that the observed, slow, multi-day drifts in rates of O^-consumption 

 in the several organisms studied (Brown, Webb, Bennett, and Sandeen, 

 1955; Brown, Freeland, and Ralph, 1955), show forms with a very 

 suggestive close relationship with the gradual barometric pressure changes, 

 but commonly anticipating the latter by one or two days. Barometric 

 pressure changes are now known to be significantly influenced by cosmic 

 radiation reaching the earth's atmosphere during the preceding day 

 or so. 



We do not yet know the nature of the external factor or factors which 

 are producing the change in metabolism. However, since there is a correla- 

 tion with barometric pressure, it is apparent that, under conditions which 

 biologists have hitherto considered as constant ones, living things are still 

 able to receive rhythmic signals with cycles of daily, lunar, monthly, and 

 even annual lengths. For example, there is a clear monthly rhythm of 

 barometric pressure. The moon is above the horizon at noon during that 

 half of a month straddling new moon, and below the horizon at noon during 

 that half straddling full moon. The daily rhythm of barometric pressure is 

 different when one calculates it using daily data for a fortnight straddling 

 full moon than when one uses a fortnight of data straddling new moon. 

 Over a full moon the pressure at night is relatively lower than for the 

 period over a new moon, and consequently the extent of the morning 

 pressure rise is greater, on the average. Correspondingly, the rhythms of 



