Ova of the Nudibranchiate Mollusca. 383 
days later. The case-membrane previous to the escape of the em- 
bryo becomes gradually thinner, and at last either entirely disap- 
pears or is reduced to shreds. This change in the case-membrane 
may probably be in some measure caused by the incessant strokes 
of the long cilia of the ciliated discs wpon its inner surface during 
the active revolutions of the embryo round its interior. ‘The em- 
bryo at the time of its liberation 1s provided with a shell (figs. 25 
and 20 a), considerably longer in its antero-posterior than in its 
transverse diameter, from which it can protrude the upper part 
of its body and retract it at pleasure. Fig. 20 is a representation 
of the ae when protruded from its shell, and fig. 21 when en- 
tirely drawn into its interior. The embryo with its shell is at this 
period considerably larger than the ovum at the time of its extru- 
sion. Some of them measured about 1-145th of an inch in length 
and 1-200th in the antero-posterior diameter ; others 1-170th in 
the former and 1-250th in the latter direction. The parts which 
ean be protruded fromthe shell are two large and prominent ciliated 
dises (fig. 20 c), and a projecting process* attached to the wpper 
part of the anterior surface of the body (fig. 20 d). The lower 
surface of this projecting process or foot is covered by a hard 
plate (fig. 20g), which closely adheres to it and moves along with 
it. The ciliated discs are higher in front than behind, and are 
separated in front by a deep notch and by a shallower one behind. 
They are very contractile, and present very different appearances 
at different times; and their superior surface is provided with a 
thickened margin, to the upper and outer edge of which a row of 
long and strong cilia is attached, by whose movements they can 
swim rapidly in various directions through the water. When 
these discs are elevated and in the vertical position, as repre- 
sented in figs. 20 and 18, the parts connecting their margins to 
the body of the embryo are translucent, and they may now be 
contracted from before backwards and a number of the central 
cilia collected into a tuft; or if the embryo be about to retire 
into the shell, they are pressed together, the translucent tex- 
ture connecting their thickened margins to the body contracts 
and pulls them downwards, and the foot with its hard plate is 
raised, as is represented in fig. 21. This plate now acts as an im- 
perfect operculum. When the embryo, on the other hand, is 
about to swim, the ciliated discs are thrown apart and flattened, 
as is represented in figs. 16 and 19; and in this position each dise 
approaches the circular form, is hollow on the upper surface, and 
their thickened margins are prolonged inwards along the edges 
of the anterior notch, at the bottom of which they are continuous, 
* To an exactly similar structure in the embryo of the Asplysia Van Be- 
neden (Annales des Sciences Naturelles, tom. xv. p. 123, 1841) has given the 
name of foot, and we have here retained the appellation. 
