126
VICTOR L. LOOSANOFF AND HARRY C. DAVIS
and the maximum 117 p. After 5 days larvae averaged 120 p in length
and the largest individuals were 1 4 0 p long (Fig. 42). After 10 daye
some of the largest larvae approached a length of 180p at which
setting is possible. By the 15th day many individuals had already
set, and the average size of the larvae in the cultures had i n c r e w
to about 175 p. A few large larvae, measuring about 225 p in length
but still free swimming, were also found in our samples. In some
cultures setting began when the larvae were about 10 days old and
continued until the end of the 35th day.
As has already been mentioned, the smallest straight-hinge larvae
of M . arenaria in our cultures measured about 86 x 71 p. The largest
free-swimming individuals were 228 x 207 p, although most of them
metamorphosed before that size was reached. Our measurements,
therefore, differ from those given by Stafford (1912), who stated that
the smallest straight-hinge larva of M . arenaria that he found wzu only
75.9 x 6 2 p , while the largest measured 414 x 3 4 5 ~ . Of the latter
he said, “The largest measurement I have is 64 x 53, and I have
seen them attached by a byssus-thread, their siphons protruded, and
the big hinge-tooth on the left valve.” Since, according to Stafford,
each unit of his measurements was equal to 6*9p, the dimensions of
his larvae were as given above. It is quite possible, if his measurements
were correct, that Stafford was working with larvae of a species other
than M . arenaria and, possibly, his large individuals were already
juvenile mollusks and not free-swimming larvae. Another possibility
is that Stafford’s microscope was not correctly calibrated.
The dimensions of larvae in our cultures were not too different
from Sullivan’s (1948). Nevertheless, her smallest size of 105 x 90 p
is considerably larger than we found and her largest, 250 x 230p,
also somewhat exceeds ours. Yoshida’s (1938) observation that larvae
of M . arenaria in Japanese waters metamorphose upon reaching a size
ranging from 240 to 300 p also disagrees with ours because, while the
length of 240 p does not differ radically from the measurements of ow
largest larvae, the maximum size of 3 0 0 p given by Yoshida exceeds
ours by about 70 p.
The length of early straight-hinge larvae of M . arenaria given by
Jsrgensen (1 946) is similar to ours. This is to be expected because his
early larvae were laboratory-reared, as were ours, and, therefore,
there is no doubt that we and Jsrgensen were working with the same
species. However, we disagree with Jarrgensen that larvae of M . arenaria
may reach 300p in length before metamorphosis. His conclusion
regarding the setting size is based not on laboratory-reared M . arenaria,
but on specimens collected in the field and only assumed to be this
VICTOR L. LOOSANOFF AND HARRY C. DAVIS
and the maximum 117 p. After 5 days larvae averaged 120 p in length
and the largest individuals were 1 4 0 p long (Fig. 42). After 10 daye
some of the largest larvae approached a length of 180p at which
setting is possible. By the 15th day many individuals had already
set, and the average size of the larvae in the cultures had i n c r e w
to about 175 p. A few large larvae, measuring about 225 p in length
but still free swimming, were also found in our samples. In some
cultures setting began when the larvae were about 10 days old and
continued until the end of the 35th day.
As has already been mentioned, the smallest straight-hinge larvae
of M . arenaria in our cultures measured about 86 x 71 p. The largest
free-swimming individuals were 228 x 207 p, although most of them
metamorphosed before that size was reached. Our measurements,
therefore, differ from those given by Stafford (1912), who stated that
the smallest straight-hinge larva of M . arenaria that he found wzu only
75.9 x 6 2 p , while the largest measured 414 x 3 4 5 ~ . Of the latter
he said, “The largest measurement I have is 64 x 53, and I have
seen them attached by a byssus-thread, their siphons protruded, and
the big hinge-tooth on the left valve.” Since, according to Stafford,
each unit of his measurements was equal to 6*9p, the dimensions of
his larvae were as given above. It is quite possible, if his measurements
were correct, that Stafford was working with larvae of a species other
than M . arenaria and, possibly, his large individuals were already
juvenile mollusks and not free-swimming larvae. Another possibility
is that Stafford’s microscope was not correctly calibrated.
The dimensions of larvae in our cultures were not too different
from Sullivan’s (1948). Nevertheless, her smallest size of 105 x 90 p
is considerably larger than we found and her largest, 250 x 230p,
also somewhat exceeds ours. Yoshida’s (1938) observation that larvae
of M . arenaria in Japanese waters metamorphose upon reaching a size
ranging from 240 to 300 p also disagrees with ours because, while the
length of 240 p does not differ radically from the measurements of ow
largest larvae, the maximum size of 3 0 0 p given by Yoshida exceeds
ours by about 70 p.
The length of early straight-hinge larvae of M . arenaria given by
Jsrgensen (1 946) is similar to ours. This is to be expected because his
early larvae were laboratory-reared, as were ours, and, therefore,
there is no doubt that we and Jsrgensen were working with the same
species. However, we disagree with Jarrgensen that larvae of M . arenaria
may reach 300p in length before metamorphosis. His conclusion
regarding the setting size is based not on laboratory-reared M . arenaria,
but on specimens collected in the field and only assumed to be this