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102 great agility and ceaseless perseverance, the cilia all the while vigorously vibrating on the margins of the outstretched lobes. The membranous chorion, 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."

As the larva increases in age, the foot acquires considerable dimensions. The operculum becomes delicate and transparent, so as to be with difficulty examined; it is, however, seen to be circular, and concentric lines seem to indicate that its growth takes place in the ordinary manner. Besides the long cilia of the lobes, the action of which is under the control of the larva, "the whole surface of the exposed parts is covered with minute cilia, whose vibrations never cease. These cilia serve most probably for respiratory purposes, and may also assist in bringing food to the mouth.

"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