44 U. S. BUREAU OF FISHERIES 
diameter to active veliger larvae 180 in length, at which stage they 
are discharged. 
The free-swimming period of the larvae is 30 or more days, per- 
mitting wide dispersal and large mortality. It was found that mature 
larvae attach, or set, most frequently to the under side of horizontal 
surfaces, and as the angle of the surface departs from the horizontal 
fewer spat are caught. Several closely-connected bays were studied 
for the purpose of finding when the most profuse setting takes place. 
In all bays except one the typical setting season consists of two dis- 
tinct periods, 6 to 8 weeks apart, which fall within periods of spring 
tides when tidal range is greatest. Secondary setting periods may 
occur between these two or after the second. 
Further observations were made on the circulatory system of 
oysters following discovery of the “accessory hearts” which appar- 
ently pump blood from the excretory organs to the mantle. A descrip- 
tion of these observations was published in a scientific journal. 
A series of experiments on the effects of changes in salinity on the 
feeding activities of O. gigas was brought to a close and the results. 
prepared for publication. Adaptation of the feeding mechanism to: 
changes in salinity was studied by recording on the kymograph the 
degree of openness of the valves and the relative rate of flow of water 
pumped by the gills. Both the activity of the gills and that of the 
adductor muscle, which by controlling the position of the valves 
determines the size of the inhalant and exhalant apertures, are mark- 
edly affected by any considerable change in salinity. The initial 
effect of such change is to cause partial or complete contraction of 
the adductor muscle and slowing or cessation of the flow of water. 
Recovery, or adaptation, following a rise in salinity is very rapid as 
compared with adaptation following the same change in the opposite 
direction. The former may require a few hours, while several days 
may be necessary in the latter case. 
Because of the great variability in activity of the gills and in 
degree of openness even under conditions of constant salinity and 
temperature, the results do not justify an exact statement of the 
optimum salinity. However, the optimum is probably not greatly 
different from that of ocean water, for salinities between about 25 and 
59 parts per thousand appear to produce similar effects. The lower 
limit of tolerance, or the minimum salinity at which water is pumped 
effectively, is between 10.5 and 13 parts per thousand. 
YORK RIVER INVESTIGATIONS 
At Yorktown, Va., where a temporary laboratory has been estab- 
lished, work was carried forward throughout the year to deter- 
mine the causes for the decline in quality and volume of produc- 
tion of oyster beds in the upper York River. Laboratory analyses and 
experiments were conducted by Dr. Walter A. Chipman, Jr., and 
Dr. Arthur D. Hasler under the direction of Dr. Galtsoff. The oyster- 
cultural aspects of the problem in the field were handled by James 
B. Engle. During the first few months of the year physiological 
experiments were also carried on by Dr. Nelson A. Wells. Labora- 
tory assistance was furnished by Lloyd R. Garriss and Orlin K. 
Fletcher, Jr. 
