60 A BIOLOGY OF CRUSTACEA 



darkening and the coincidence of maximum darkening with the 

 period just before low tide are aids to concealment when the crab 

 is active. Foraging and locomotory activity of Uca are at their peak 

 just before low tide. 



The time of low tide varies from locality to locality, and crabs 

 collected from beaches which differ in the times of low tide main- 

 tain their own particular rhythm of melanophore expansion when 

 kept in darkness in the laboratory, so that their cycle corresponds 

 with that of their fellows remaining on the beaches. 



It is evident that these crabs have an internal clock of consider- 

 able accuracy. We do not know how they measure their time, but 

 it does not seem to be due to ordinary metabolic processes, because 

 the mechanism is, to a large extent, independent of temperature. 

 The crabs maintain a steady rhythm at all temperatures between 

 6 and 26° C. However, if a crab is cooled to 2 or 3 ° C. its clock is 

 slowed by about the duration of the chilling. In this way it is 

 possible to reset the clock, but once the crab is returned to normal 

 temperatures its natural rhythm of colour change reappears with 

 its accuracy unimpaired apart from being behind that of its 

 untreated fellows. 



One final factor involved in the precise degree of expansion of 

 the chromatophores of Uca is the incidence of cosmic radiation. 

 This factor is rarely taken into consideration in biological experi- 

 ments, but if Uca is shielded from cosmic rays by sheets of lead 

 its melanophores expand more than they do if the crab is not so 

 shielded. The atmosphere also acts as a shield against cosmic rays. 

 When barometric pressure is high fewer rays penetrate to the sur- 

 face of the earth. An apparent correlation between melanophore 

 expansion and changes in barometric pressure can be found, but 

 the effect is due to cosmic radiation, not to the direct effect of air 

 pressure. This has been shown by experiments in which crabs were 

 kept in chambers at constant pressure; the rhythm of the crabs was 

 not affected by the local constancy of pressure. 



Is it possible that the crabs use a diurnal cycle of cosmic radiation 

 to adjust their own clocks? A recent experiment, made in America, 

 was designed to test whether the crabs could maintain their rhythm 

 when there was no possible external factor which could give them 

 a clue. A group of crabs was collected from the Atlantic coast, 

 and half were sealed in an opaque tub and flown to the Pacific 

 coast. In making this trip the normal diurnal cycle of factors due 

 to rotation of the earth was extended to over 27 hours. In spite 



