foot (figs. 11 e, 12 e)—is observed to develop itself a little behind them on the median line. A
shell (figs. 11c, 12 c) 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
(fig. 13c?), which is opposed to the mouth of the shell. The lobes (fig. 15/) now expand into
two large, flattened, ovate appendages, with very long vibratile cilia around the margins, and
the larvee 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, 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 waters, 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.
On close examination, the shell of the larva is found to be of excessive delicacy, exquisitely
brilliant, and so transparent that its exact form is not very easily made out. In all the species
it is of a nautiloid character, with the spire of scarcely one volution, and invariably a little
twisted to one side. It is, in fact, the commencement of the ordinary testaceous covering of
a gasteropod, and were its growth continued, it would assume the usual spiral form. In
D. tuberculata the shell is rather shallow, while, in some of the Eolididts (Fam. 3, PI. 1,
figs. 5, 6), it is of considerable depth; and in Eolis olivacea, it (fig. 9) may be said to be
altogether without a .spire. Weak acetic acid destroys the shell; but the operculum remains
unaffected by it. There can be little doubt, therefore, that the former is calcareous, and the
latter horny.*
The larva is somewhat opaque, and is enveloped in a very wide, thin, delicate, and transparent
membrane or cloak (PI. 3, -fig. 15 m), which partially lines the shell, being in contact
with it at the mouth, and at the bottom where the retractor muscle (n) is attached to it. This
is a strong band, with the end in contact with the shell, a little enlarged; it passes upwards,
and is lost in the midst of the larva, which, in D. tuberculata, is so opaque that the various
parts are not easily determined. In Eolis coronata, however, they can be readily distinguished.
In this species, as well as in Doris tuberculata, Doto coronata, and Eolis picta, the ciliated oral
lobes are large; but they are small in E. olivacea (Fam. 3, PI. 1, fig. 9), Hemusa dendritica
(Fam. 3, PI. 40, fig. 7), and Polycera quadrilineata (Fam. 1, PI. 22, fig. 11). The mouth
(Fam. 1, PI. 3, fig. 15^) is situated between and a little behind these lobes, and is a mere
circular opening of considerable dimensions: the oesophagus passes downwards from this
opening, and is a rather wide and straight tube, which, suddenly swelling out, is formed into
a rounded, in some species an elongated, pyriform, well-marked stomach (h). The intestine (i)
is considerably constricted, and leaves the stomach at the extremity opposite to the cardiac
* The same is the case with the larva of Aplysia : on placing it in weak acetic acid, air bubbles
were immediately formed, and on account of their rapid generation, the shells were jerked about. In
a short time these latter entirely disappeared, leaving exposed the animal enveloped in its cloak. The
operculum was not in the least changed.
end, and bending upwards, runs almost parallel to the oesophagus, and terminates at one side
not far from the base of the oral lobe and close behind an oval sac (k), probably the incipient
genitalia. The alimentary canal is ciliated within, and particles of matter may frequently be
seen rotating in the interior of the stomach, influenced by the action of the cilia.
Besides the parts just described, there is always a rounded, yellowish, and opaque
mass {j), sometimes divided into two, which is situated within the spiral portion of the shell,
and in contact with the stomach. This, according to M. Vogt, is the liver; and he is probably
’correct. On each side, near the commencement of the oesophagus, is a rather conspicuous
circular vesicle (/), containing a distinct nucleus; the vesicle and nucleus being very distinct
in Eolis coronata. They have been considered by most writers on the subject to be the
auditory capsule and otolithe; a view first adopted by Sars, and in which we are disposed to
agree, having observed the enclosed nucleus or otolithe to vibrate within its capsule in the
larva of Aplysia.
In the mature larva, the foot is of considerable dimensions; and the operculum, which
is attached to its posterior surface, is so delicate and transparent, that ijiuch care is required
to observe it accurately. Under favorable circumstances it is seen to be circular, extending
considerably beyond the fleshy support; and indistinct circular lines of growth may occasionally
be observed upon it.* We have failed to detect the operculum in Dendronotus
arborescens, Eolis picta, and E. olivacea. Our failure, however, in these cases may have arisen
from the excessive tenuity and transparency of the parts.
Besides the large, locomotive, vibratile cilia of the oral 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 are most probably for respiratory purposes, and
may also assist in bringing food to the mouth, as stated by M. Vogt.
When the larva is at rest, the oral lobes are pulled back into the shell, and the opercu-
ligerous lobe or foot, being drawn down, brings along with it the operculum, which closes the
orifice; but when in action (figs. 14, 15), 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.
We have not succeeded in tracing the development of the larva into the mature
form; but it is not difficult to understand how this change is effected. When the larva is
placed with the mouth of the shell downwards, the oral lobes in front (we are speaking
particularly of Eolis), the anal termination of the intestine, and the oval sac representing the
generative organs, will be found on the right side, close to the base of the oral lobe, and the
operculigerous lobe or foot will be seen to extend backwards in a medium position, occupying
the place of the crawling disc. Thus it is evident that the principal organs of the larva only
require to be slightly modified in form, and it is changed into the mature animal, the shell and
operculum being cast off, and the oral lobes either absorbed, or altered into a veil or oral
tentacles.
This view of the change of the larva into the mature animal is apparently verified by the
investigations of Professor Nordmann, who has traced the transformations in a species closely
allied to our Eolis dispecta.
* Circular lines of growth are perfectly distinct on the operculum of Aplysia.