The eggs, watched day by day under a good microscopic power, as they
advance towards maturity, present a most interesting object of study.
The yolk, which at first nearly fills the egg-shell, soon becomes a
little elongated, with one end diagonally truncated, or, as it were,
cut off obliquely; the truncated end then becomes two-lobed, “each
lobe exhibiting an imperfect spiral, and having its margin ciliated.
The now animated being is seen to rotate within its prison. Shortly
the lobes enlarge, and a fleshy process, the rudimentary foot, is
observed to develop itself a little behind them, on the medial line; a
shell closely investing the inferior portion of the embryo, the lobes
and rudimentary foot being uppermost. The shell rapidly increases, and
assumes a nautiloid form; afterwards the foot displays, attached to its
posterior surface, a circular operculum, which is opposed to the mouth
of the shell. The lobes now expand into two large, flattened, ovate
appendages, with very long vibratile cilia around the margins; and the
larvæ are at length mature. The whole mass of spawn now presents the
utmost animation. Hundreds of these busy atoms are seen, each within
its transparent, membranous cell, rotating with great agility and
ceaseless perseverance, the cilia all the while vigorously vibrating
on the margins of the outstretched lobes. The membranous chorion
[or transparent egg-shell], which by this time has become enlarged,
ultimately gives way, no longer able to resist the perpetual struggle
within; and the liberated larva, wending its way through the shattered
shreds of the general envelope, boldly trusts itself to the open
trackless water, where, doubtless, thousands and tens of thousands
perish ere they find a fitting resting-place, some being swept away
by resistless currents, others falling a prey to ever-watchful and
innumerable enemies.
“When the larva is at rest, the oral lobes are pulled back into
the shell, and the foot being drawn down, brings along with it the
operculum, which closes the orifice. But when in action, the whole of
these parts project beyond the opening of the shell, the foot lying
back against the spire; and the oral lobes inclining forward, their
cilia commence to vibrate, and the larva, with the mouth of the shell
upwards, moves through the water with lively action, sinking or rising,
or advancing onwards at its pleasure.”[20]