ADAPTATION OF OYSTER TO CHANGES IN SALINITY 
351 
tremendous variation in the degree of openness. The rate of pumping of water is the 
activity which primarily is being studied, for on this depends the rate of feeding; 
but this activity may not be isolated from the influence of the adductor muscle save 
by propping the valves open ; in which case the results would be of little significance 
with respect to the reactions of the oyster as a whole to environmental factors. 
In a previous publication (Hopkins, 1933) it was shown that, at constant tem- 
perature, the rate of flow of water through the gills increases as the valves become 
wider open. By plotting the results of certain series to show the relationship between 
S and F it is possible to find whether observed effects are due to changed activity of 
the gills or only to the effect of the position of the valves. Such a graph is shown 
in figure 4, in which the results for all of the tests with a salinity of 17.85 parts per 
mille are given in solid points and those taken with a salinity of about 28 parts per 
mille in open circles. In spite of the fact that the points are considerably dispersed, 
they fall into relative alinement. 
Figure 4.— Relationship between S (degree of openness of shell) and F (rate of flow) at salinity of 17.85 p. p. m. (solid points) as 
compared with 28 p. p. m. (open circles). Series II. 17°-19° C. 
Most of the points resulting from low salinity represent low values of both S and 
F, but they fall into general alinement with values obtained with the higher salinity. 
This indicates that at least a large part of the low F readings at the lower salinity 
(series II, table 1) may be due to the effect of the position of the valves. Yet, also, 
a good many of these points are high on the S scale but much lower on the F scale 
as compared with the values obtained at high salinity. The trend of the low salinity 
curve appears to be much steeper than the other. The significance of the graph is 
that both the adductor muscle, by controlling shell position, and the gill mechanism 
were in this case responsible for the lack of complete adaptation to the lower salinity. 
Tests with other specimens throw further light on the subject. 
SERIES III 
The preceding two series of tests were concerned with changes in salinity between 
about 14 and about 29 parts per mille. It was noted that adaptation to a reduction 
in salinity is quite slow as compared to that following a rise. The question presented 
itself as to whether this difference was due to the fact that the specimens had been 
