REARING OF BIVALVE MOLLUSKS
29
germinal vesicles intact and fertilization does not occur. However, as
elready mentioned, eggs of such species can be chemically treated
and thereby become ready for fertilizaticp.
As cleavage progresses and more micromeres are formed (Fig. 13e),
the embryo gradually develops into a swimming, ciliated larva which
eventually reaches trochophore stage (Fig. 13f ). Under favorable
conditions this stage may be reached in 12 to 16 hr, depending upon
the original condition of the eggs, culturing methods and, of course,
water temperature.
During late trochophore stage (Fig. 139) the cell gland begins to
secrete the shell. When the shell completely encloses the soft parts
the larva has reached early straight-hinge stage (Fig. 13h).
Development of larvae of Mactra 8olidissima, from early straighthinge stage until metamorphosis, and their length-width measurements during this entire period are shown in Fig. 14. Very early,
normal straight-hinge larvae measure only about 79 p in length and
usually between 63 and 65 p in width. In some cultures, composed
mostly of abnormal individuals, somewhat smaller, slightly deformed
straight-hinge larvae can be seen occasionally, but it is doubtful that
they survive to metamorphpsis.
Individual larvae of M . solidissima display considerable variations
aa to the size at which certain organs of their bodies begin to develop
and at which metamorphosis occurs. For example, in some individuals
the foot can be seen when they are only about 160 p long. Approximately 80% of the larvae show a well-developed foot by the time
they are 215 p long, and at a length of 240 p practically all possess
this organ.
Disappearance of the velum is another step in larval $evelopment
that is not strictly correlated with a definite size. In some larvae
measuring only 219 p in length the velum wm already completely
resorbed, while in extreme cases a diminishing, but still functional
velum was seen in larvae about 257 p long.
A few larvae begin to metamorphose when they are about 220 p
long, but the majority are between 230 and 250 p before metamorphosis occurs. At this time the velum is resorbed, rudimentary gills
develop, and a powerful ciliated foot, which when expanded is as long
as the young clam itself, serves as the only means of locomotion.
Individuals memuring 262 p in length were the largest true larvae
recorded. In this respect our observations are in agreement with those
of Imai et al. (1953), who found that in Mactra sachalinen& the foot
begins t o develop at a length of about 200 p and that larvae set at
about 270 p.
29
germinal vesicles intact and fertilization does not occur. However, as
elready mentioned, eggs of such species can be chemically treated
and thereby become ready for fertilizaticp.
As cleavage progresses and more micromeres are formed (Fig. 13e),
the embryo gradually develops into a swimming, ciliated larva which
eventually reaches trochophore stage (Fig. 13f ). Under favorable
conditions this stage may be reached in 12 to 16 hr, depending upon
the original condition of the eggs, culturing methods and, of course,
water temperature.
During late trochophore stage (Fig. 139) the cell gland begins to
secrete the shell. When the shell completely encloses the soft parts
the larva has reached early straight-hinge stage (Fig. 13h).
Development of larvae of Mactra 8olidissima, from early straighthinge stage until metamorphosis, and their length-width measurements during this entire period are shown in Fig. 14. Very early,
normal straight-hinge larvae measure only about 79 p in length and
usually between 63 and 65 p in width. In some cultures, composed
mostly of abnormal individuals, somewhat smaller, slightly deformed
straight-hinge larvae can be seen occasionally, but it is doubtful that
they survive to metamorphpsis.
Individual larvae of M . solidissima display considerable variations
aa to the size at which certain organs of their bodies begin to develop
and at which metamorphosis occurs. For example, in some individuals
the foot can be seen when they are only about 160 p long. Approximately 80% of the larvae show a well-developed foot by the time
they are 215 p long, and at a length of 240 p practically all possess
this organ.
Disappearance of the velum is another step in larval $evelopment
that is not strictly correlated with a definite size. In some larvae
measuring only 219 p in length the velum wm already completely
resorbed, while in extreme cases a diminishing, but still functional
velum was seen in larvae about 257 p long.
A few larvae begin to metamorphose when they are about 220 p
long, but the majority are between 230 and 250 p before metamorphosis occurs. At this time the velum is resorbed, rudimentary gills
develop, and a powerful ciliated foot, which when expanded is as long
as the young clam itself, serves as the only means of locomotion.
Individuals memuring 262 p in length were the largest true larvae
recorded. In this respect our observations are in agreement with those
of Imai et al. (1953), who found that in Mactra sachalinen& the foot
begins t o develop at a length of about 200 p and that larvae set at
about 270 p.