30 



BULLETIN OF THE BUREAU OF FISHERIES 



graph. For the purpose of the present experiments the use of a time recorder has 

 several advantages over the ordinary kymograph — it ehminates the necessity of 

 having a special time signal, and after the apparatus has been set it can be left for 24 

 hours without any further attention. The oysters were kept attached to the appara- 

 tus for varying periods of time ranging from one to eight days. Altogether, during the 

 time between June 15 and October 15, 1926, there were obtained 132 daily records 

 written by 34 different oysters. The temperature of the water varied from 13° to 



24 

 22 



«2 





o ' 



i ' 



> 



A.M. 



MD 



2 4 6 8 10 12141618 20 22 24 



NUMBER OF HOURS OYSTERS WERE OPEN DURING 24- HOUR PERIOD. 



Fig. 10.— Frequency distribution of the duration of periods of shell opening 



22° C. Within this range there was no definite correlation between temperature 

 and the opening and closing of the shells. TJie results of all the observations are 

 given in Figure 10. They are grouped in 12 classes, each having two-hour intervals and 

 the frequencies are plotted as the ordinates. The number of oysters that were either 

 closed or open for a 24-hour period are shown in cross-hatched areas. An examination 

 of Figure 10 shows that the oyster has a tendency to keep its shell open as long as 

 possible. The arithmetic mean of the number of hom's the average oyster keeps its 



