NAKED-GILLED MOLLUSKS. 101 
shell or chorion, may be observed changing form. 
The time required to mature the embryo varies in 
different species, and probably in the same species 
under different circumstances; in general from ten 
to fifteen days is sufficient for this purpose. 
The development of the embryo is highly in- 
teresting, especially as, owing to the transparency 
of all the organs, nothing is easier than to watch its 
progress. I have kept many specimens in glass vases 
of sea water; and as all the species of this Order 
are hermaphrodite—that is, the sexual functions are 
united in the same individual—every specimen ob- 
tained is pretty sure to spawn during the season. 
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 ob- 
liquely; 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 larve 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 
