102 NUDIBRANCHIATA. 
ereat 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 cur- 
rents, others fallg 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 pro- 
bably 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 preject 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 
* Alder and Hancock’s Monograph. (Doris.) 
