300 INVERTEBRATE PHYSIOLOGY 



clocks. Maynard Johnson (1939), working a few years ago at Harvard 

 University, showed that mice, in constant darkness, tended to run at the 

 same time every day. But when they were kept in continuous light the clock 

 which controlled the activity of the mice ran fast about an hour a day. The 

 mice would run about an hour later every day. We have found that we can 

 similarly speed up the internal clock of the rat by keeping it in continuous 

 light, and have shown that another clock, the imposed one, is continuing 

 to run unaltered. Both clocks regulate the running activity. An "inner" 

 clock regulates the repeated day by day running; an "outer" clock con- 

 trols the amount of running at any given hour of the day. 



Two other experiments are very instructive. One of them was performed 

 upon oysters. The oysters were collected in New Haven Harbor in 

 Connecticut and shipped to Evanston, Illinois in a lightproof container. 

 The container was opened in a photographic darkroom. There the oysters 

 were found to have a daily rhythm in opening their shells and the daily 

 rhythm, in turn, showed a monthly variation. The oysters also had a tidal 

 rhythm of opening ; for the first two weeks they opened their shells most at 

 the time of high tide in New Haven Harbor. The form of the tidal rhythm 

 then changed, and for each of the next two fortnights they opened their 

 shells most at the times of lunar zenith and lunar nadir in Evanston, which 

 occurred more than three hours later in the day than the time of high tide 

 in their home waters. One interpretation of this change in the tidal cycles 

 in Evanston is that the opening of the shells at the time of high tide, ad- 

 vantageous for their normal feeding, was a learned behavior using the 

 lunar-day clock. The learned behavior was forgotten in a couple of weeks 

 away from the ocean tides and the oysters reverted to a simple direct re- 

 sponse either to barometric pressure change itself or to some other factor 

 whose intensity varied with the pressure. 



The second experiment was one designed to discover whether a fiddler 

 crab could measure a 24-hour period by means of an internal clock even 

 when there was no possible external indication of this period. There was 

 only one way to provide the proper experimental conditions. A group of 

 fiddler crabs was sealed in each of two opaque wooden tubs at Woods Hole, 

 Massachusetts (Brown, Webb, and Bennett, 1955). One tub was left at 

 Woods Hole, while the second was carried by airplane to Berkeley, Cali- 

 fornia. On the next day, at a prearranged time, the two lots of crabs were 

 synchronously opened in darkrooms in the two parts of the country. The 

 crabs were studied for six days to determine the settings of their color- 

 change clocks. The clocks of the crabs in California remained essentially 

 synchronized with those of the Woods Hole crabs. This they had been able 

 to do even though every conceivable factor varying normally in a 24-hour 

 cycle as a result of the rotation of the earth had, during the day of the trip. 



