32 



BULLETIN OF THE BUREAU OF FISHERIES 



stimulus by closing its shell. My observations on the oysters attached to the record- 

 ing apparatus and kept in a glass tank show that ejection of the material accu- 

 mulated in the palhal cavity takes place at irregular intervals and that rhythmic 

 contractions of the adductor muscle (figs. 11 and 12) were not accompanied by 

 a discharge of any material. Such different factors as changes in illumination, 

 mechanical stimulation, changes in the pH or gas content in sea water, presence 

 of certain chemicals, and so on, may cause the contraction of the muscle and tem- 

 porary closing of the shell. The frequency of the contractions depends also on the 

 physiological condition of the oyster. It has been found in the present investiga- 

 tion that after spawning the female oyster makes many less contractions than it 

 does before spawning. The kjTnograph tracings represented in Figure 13 show 

 this very plainly. During this experiment, in both cases the shell of the oyster 

 was wide open and the temperature of the water and other external factors were 



I P.M. 



1AM. 



Fig. 12.— Part ot "the record of shell movements of two oysters made at 5° C. Dotted line indicates the position of 

 the pen when shell is closed. In lower line the oyster was nearly closed and showed very slight motion 



ahke. At present we understand very little the conditions that control the periodi- 

 cal contractions of the adductor muscle, but it appears obvious that they can not 

 be correlated directly with the rate of feeding, and the estimation of the latter can 

 not be based on the frequency of the contractions. \r.: 



It has been suggested by Nelson (1921) that there may be a certain amount of 

 correlation between the times of opening and closing and the hour of the day and 

 night. Nelson divides a day into four arbitrary periods, from 11.01 p. m. to 4.30 

 a. m., from 4.31 to 11 a. m., from 11.01 a. m. to 7.30 p. m., and from 7.30 to 11 p.m. 

 Then he counts on the kymograph records the number of closures and openings that 

 occur during each period. According to his data 50 per cent of closures take place 

 in the 534 hours between 11 p. m. and 4.30 a. m. (dawn). Nelson makes two some- 

 what contradictory conclusions — first, that "the period from lip. m. to dawn may 

 ahnost be looked upon as a tune of rest, or at least of greatly lessened activity", 

 and second, that "the hours of inactivity on 50 per cent of all the days were confined 



