214 
BULLETIN OP THE BUREAU OF FISHERIES 
countered. Attempts to rear them artificially were unsatisfactory, and because of 
the extended breeding season and the numerous similar young of other mollusks the 
direct observations of the larvae were not decisive. 
Belding (1910 and 1912) concluded that the quahog {Venus mercenaria) re- 
mained a free-swimming larva from a week to 12 days, while the pecten {Pecten 
irridans) passed through the corresponding stage in about a week. It was con- 
cluded by Field (1922) that the common mussel {Mytihis edulis) remained in 
the free-swimming stage about two months when living under natural conditions. 
He compared the specimens taken in nature with those raised in an aquarium and 
arrived at quite definite conclusions. 
This length of time agrees more closely with the larval period of the razor clam. 
The larvae differ in one respect, however, as the mussel is actually swimming 
during the greater part, if not all, of the period, while the razor clam is more 
inclined to remain in one place. The mussels are scattered over a great area and 
are dispersed by this method. The habitat of the razor clam is more limited, and 
the larvae do not scatter to a like degree. 
TIME OF SETTING AND GROWTH OF YOUNG 
Eight weeks after spawning large clam larvae visible to the unaided eye were 
found in a quiet lagoon. None of this age was taken with the plankton net in 
the surf, although repeated attempts to do so were made at several places. It is 
probable that the majority had already passed through this stage. The larval 
shell was now dense enough to hinder observation of the organs, and the posterior 
end had a marked tendency toward the elongation which is so marked in the adult 
clam. Prominent gills were present, but no velum was seen. 
While under observation the live specimens would extend the characteristic 
long foot from the shell. Five days later small clams in the adult stage were found 
in the surface of the sand. They averaged 0.25 centimeter in length and were the 
forerunners of a very heavy set. Distribution was slightly uneven, although they 
were everywhere abundant, averaging approximately 1,450 per square foot. At 
this size they were found in the surface layer of the sand, and if a small hole was 
dug in the wet sand it quickly filled with water in which there were a number of 
small clams. If the water in this depression were given a circular motion with the 
hand, a number of small clams, as well as sand from the edge, were dislodged. The 
clams tended to be whirled to the center, but as they are very active, even when 
being carried quite rapidly over the surface of the sand, they will catch their foot 
in the sand and disappear from sight. 
At intervals large numbers of small clams were taken by screening the sand. 
These were measured and the median size determined. (See fig. 10.) During 
the month of August the small clams increased in average size from 0.25 to 0.88 
centimeter. The following month they grew to 1.39 centimeters in average length. 
A marked decrease in the rate of growth was found in October, during which 
month an increase of only 0.24 centimeter was shown. At this time the average 
length was 1.63 centimeters, and no further growth was observed. 
