352 
BULLETIN OF THE BUREAU OF FISHERIES 
living in water of the higher salinity (28-29 parts per mille) and consequently any 
change from this level produced a pronounced effect, while restoration of this level 
merely permitted normal activity. 
The reactions were studied when the specimen was transferred into water of a 
salinity of 39.10, after 2 days of study at 28.55 and 28.27 parts per mille. Since 
pure ocean water has a salinity of only 35 to 36 parts per mille, there is no possi- 
bility that the specimen could already be adapted to water of even higher salinity. 
The results of the entire series are summarized in table 1, series III. It may readily 
be observed that the specimen became adapted quickly to the abnormally high level. 
In figure 5 the values of S and F are given in detail to show the reactions during the 
first 2 days in this high salinity. After about 3 hours the specimen was pumping 
water at a rate only a little below normal, and within the following 4 hours it appeared 
to become completely adapted, as judged by degree of openness and rate of pumping. 
Tests of the following day are given on the graph for comparison. 
Figure 5.— Adaptation of 5-minute values of S and F to salinity of 39.10 p. p. m. following change from 28.27 p. p. m. Series III. 
17°-19° C. 
On the other hand, adaptation proceeded at the same slow rate as previously 
observed when the salinity was lowered to 28.04 parts per mille, as shown in figure 
6. At least 3 days were required for restoration of the rate of pumping previously 
observed, although the valves were much wider open than at the higher salinity. 
The effect of lowering the salinity in this case appears to be entirely upon the gill 
mechanism, rather than upon the organs controlling position of the valves. 
Since the specimen appeared to function so well in a salinity of 39.10 parts per 
mille, a short test was made to give a general idea of the upper limit to which it 
could become adapted. A salinity of 56.32 parts per mille, twice that of the 
ordinary bay water, was used and records kept for 4 days (series III, table 1). During 
the first day the shell remained entirely closed, and on the second it opened only to 
a small degree. However, as the valves became sufficiently far apart the gills were 
