EXPERIMENTAL STUDY OF THE OYSTER GILL 



83 



to periods 1 and 2" (i. e., from 11.01 p. m. to 11 a. ni.). In another paper (Nelson, 

 1923) he states that from 60 to 70 per cent of the hours of mactivity (closure) occur 

 during darkness. The examination of the niunber of closures and openings occur- 

 ring during a given period of time does not convey a true idea of the activity or 

 inactivity of the oyster. A better understanding can be gained by counting the 

 number of hours the oyster was closed or open during a given period of a day. These 

 observations should be made under controllable conditions, inasmuch as it has been 

 shown by Nelson for Barnegat Bay (1921) and confirmed by Prytherch for Milford 

 Harbor (unpublished report) that in these bays the oysters close their shells at ebb 

 tide. We know that every change in the environment may cause the oyster to close 

 its shell, and therefore the problem of whether oysters exhibit daily periodicity in 



BEFORE. 5PAWNIN6;t° 25, 0.C. 



AFTER SPAWNING; r25,O.C 



Fig. 13. — Kymograpu leourds of the movement of the shell one hour before spawning (upper Imej and one hour 

 and a half after spawning. Oyster was open all the time. The marks under the line indicate one-minute 

 intervals. 



behavior should be studied without any possible interference of other factors that 

 may produce the same reaction. The examination of the material obtained by the 

 author diu-ing July-September, 1926, fails to disclose any correlation between the 

 periods of inactivity (closure) and darkness. Analyzing the data, all incomplete 

 records (i. e., those covering less than 24-hours periods) and all those showing that 

 the oyster was either closed or open continuously for 24 hours were excluded, so that 

 in each of 103 records taken into consideration the oyster was closed for a part of a 

 day. The period of darkness was determined as beginning half an hour after 

 sunset and ending half an hour before sunrise. The results of the examination are 

 given in the following table: 



Table 10. — Opening and closing of the shell in relation to the time of day '■"-' 



Total number of hours oysters were under observa 



, tion --- 



Number of hours oysters were closed during day 



time 



Number of hours oysters were closed during night 



time -- - 



Number 

 of hours 



2,472 

 565 

 206 



Number of days when shells were closed during day 

 hours only _ _ 



Number of days when shells were closed during night 

 hours only ._ 



Number of days when shells were closed during night 

 and day hours 



Number 

 of days 



-.7 



S3 



