ILEAICING OF I % I V A I , V E MOI,I,lJSKY
87
Larvae of A . transversa are quite distinctive in appearance resembling, in general, their adults, particularly as they grow larger. The
smallest normal straight-hinge larva measured 70 p long and 55 p
wide. Thus, even in the youngest larvae, the difference between length
and width is quite considerable. However, when larvae are near
setting, the length may exceed the width by about 70 p (Fig. 29).
As they grow larger, their
color becomes darker and one end of their shells becomes more pointed
than the other. Under the microscope larvae appear quite variable
in outline because their odd shapes cause them t o rest in a variety of
positions.
The first indication of an umbo begins to appear soon after the
larvae pass 105 p in length, and at 130 p it usually is quite conspicuous.
When the larvae reach approximately 140 p in length the pointed, or
less rounded end of the shell becomes distinctly reddish-brown, while
the opposite, more rounded end remains almost brown. This color
difference is quite pronounced in many individuals.
An " eye " spot appears when the larvae are approximately 225 p
long. In some individuals this spot is not too conspicuous at first, hut
as the larvae approach metamorphosis it becomes quite prominent.
Slow-growing larvae may develop the " eye " spot at a much smaller
size. For example, in at least one case it was observed in larvae that
were only 205 p in length.
As is usual in rearing larvae of bivalves, considerable variation
was noticed in setting sizes. The smallest, virtually metamorphosed
individuals measured only 215 p in length, while one larva in the
same culture measured about 310 p and, although it had a large foot,
was observed actively swimming. This may have been a somewhat
abnormal individual because a majority of the normal larvae in our
cultures set when only 240 to 260 p long.
Upon reaching metamorphosis recently set clams develop a strong
byssus by which they attach to particles of sand, shells or other
material.
Unlike larvae of M . mercenariu and several other species, which are
easily affected by fungi, no epizootics that could be ascribed to these
organisms were observed in our cultures of A . transversa.
The larvae are dark brown in color.
D. ,Vodiolus demissus (Dillwyn)
M . demissus, the common ribbed mussel of our waters, is a180
referred to by some authors as Volsellu demissa, Volsella ylicalula
and Modiolus plicatulus. A review of the literature shows that, except
87
Larvae of A . transversa are quite distinctive in appearance resembling, in general, their adults, particularly as they grow larger. The
smallest normal straight-hinge larva measured 70 p long and 55 p
wide. Thus, even in the youngest larvae, the difference between length
and width is quite considerable. However, when larvae are near
setting, the length may exceed the width by about 70 p (Fig. 29).
As they grow larger, their
color becomes darker and one end of their shells becomes more pointed
than the other. Under the microscope larvae appear quite variable
in outline because their odd shapes cause them t o rest in a variety of
positions.
The first indication of an umbo begins to appear soon after the
larvae pass 105 p in length, and at 130 p it usually is quite conspicuous.
When the larvae reach approximately 140 p in length the pointed, or
less rounded end of the shell becomes distinctly reddish-brown, while
the opposite, more rounded end remains almost brown. This color
difference is quite pronounced in many individuals.
An " eye " spot appears when the larvae are approximately 225 p
long. In some individuals this spot is not too conspicuous at first, hut
as the larvae approach metamorphosis it becomes quite prominent.
Slow-growing larvae may develop the " eye " spot at a much smaller
size. For example, in at least one case it was observed in larvae that
were only 205 p in length.
As is usual in rearing larvae of bivalves, considerable variation
was noticed in setting sizes. The smallest, virtually metamorphosed
individuals measured only 215 p in length, while one larva in the
same culture measured about 310 p and, although it had a large foot,
was observed actively swimming. This may have been a somewhat
abnormal individual because a majority of the normal larvae in our
cultures set when only 240 to 260 p long.
Upon reaching metamorphosis recently set clams develop a strong
byssus by which they attach to particles of sand, shells or other
material.
Unlike larvae of M . mercenariu and several other species, which are
easily affected by fungi, no epizootics that could be ascribed to these
organisms were observed in our cultures of A . transversa.
The larvae are dark brown in color.
D. ,Vodiolus demissus (Dillwyn)
M . demissus, the common ribbed mussel of our waters, is a180
referred to by some authors as Volsellu demissa, Volsella ylicalula
and Modiolus plicatulus. A review of the literature shows that, except