It receives three large venous trunks, two of which pass from the skin backwards to the
sides of the auricle (fig. 1 Z, Z), and the third joins it from behind on the median line.
These three trunks result from the union of veins from the skin. Two small branches also
unite with the posterior trunk from below. We have not been able to trace satisfactorily the
means by which the blood is conveyed from the arteries to the veins, but there can be
little doubt that, in this respect, Eolis resembles the other nudibranchs.
The respiratory function appears to be partially specialised in the dorsal papillae, which,
usually exposing a large surface, are covered with strong and vigorous vibritile cilia; but as
the blood, in its return to the heart, appears to pass almost entirely through the skin, which is
thin and delicate, and also covered with cilia, there can be little doubt that the whole surface of
the body assists in its aeration. The cilia of the dorsal tentacles, which are also very strong,
we consider to be connected with the sense of smelling.
The central part of the nervous system consists of a series of ganglions and connecting
cords (fig. 3), placed round the commencement of the oesophagus. Four large principal
ganglions rest on the buccal mass immediately in front of the oesophagus : the two next the
median line are much the larger, and are of an irregular ovate form; the other two are
circular, and are closely attached to the outer sides of the central pair. These ganglions are
connected together by three collars that pass under the oesophagus. The inner one is much
shorter and thicker than the other two, and passes from the posterior margins of the lateral
ganglions; the two outer ones pass from the sides of the inferior surface of the central ganglions,
and lie nearly in contact with each other. One is much smaller than the other, and is placed
immediately in front of it,—this is the central collar: the larger or outer collar has two
small oval ganglions, which are connected together on the median line, and are usually
called the buccal ganglions. The first pair of nerves, which we consider olfactory, are large,,
and rise from the anterior and upper surface of the central ganglions near the median line,
and passing into the bases of the dorsal tentacles, swell into two well-defined oval ganglions,
sending off several nerves into these organs. The second, third, fourth, and fifth pairs of
nerves supply the different organs of the head, in front of the buccal mass; the sixth
and following pairs to the ninth go to the skin on the sides of the body and foot. All
these, excepting the first pair, arise from the anterior lateral margin of the central ganglions.
The tenth and eleventh pairs, which arise from the outer side of the lateral ganglions, supply
the foot and sides of the back. The twelfth pair originate from the posterior margin of the
central ganglions, and go to the back on each side of the median line. Besides these other
small nerves go to the skin* from different parts of the central ganglions^. The two small
inferior buccal ganglions give off each a large nerve, which sinking into the muscles of the
buccal mass, are lost there. These ganglions have attached to their inner inferior margins
two small ganglions, from which the stomach and oesophagus are supplied with nerves-; and
imbedded in the muscles of the buccal mass there is on each side another small ganglion.
These ganglions give off each a large nerve, which, passing within the outer and central
collars, is united to the former by a short, but. rather strong branch of communication. These
nerves supply the glands of the papillae, and shortly after their origin give off minute branches
to the oesophagus. The central collar gives off on each side a nerve that goes to the heart
and generative organs.. These details are taken from E. papillosa, but the nervous system
differs very little, in other members of the genus. In some the olfactory ganglions are round
EOLIS.
and larger, sending off only one large nerve into the tentacles. In E. cornata and E. Drummondi
the genital nerve is supplied with a small triangular ganglion at its junction with the collar.
The eye (PI. 8, fig. 3 b, and fig. 7) is situated on a small elongated process at the
outer side of the central ganglion immediately in front of its junction with the lateral
ganglion. It is composed of a thin capsule inclosing a black pigment cup, which receives
the optic nerve from below ; in front of the cup, and half buried in it is a spherical lens,
which is protected by a cornea a little in advance of it.
The auditory capsule is placed directly behind the eye, and is attached to the central
ganglion. The otolithes are inclosed in two thin membranous bags, exceedingly delicate and
transparent. In E. papillosa (fig. 4) and E. coronata, the otolithes are small, elliptical, and
very numerous, but in E. aurantiaca, E. olivacea, and E. pic ta (fig. 6) there is only one
large spherical otolithe.
The organs of generation (PI. 8, fig. 2) are very voluminous, filling by far the greater
portion of the cavity of the body. The intromittent organ is capable of much prolongation,
and terminates in a fine, perforated, tapering point. In its contracted state it lies in front
immediately within the orifice, and behind it are the female channel, and a small orifice
opening on its anterior wall. The testicle (fig. 2 c) is a long convoluted tube situated in
front of the other parts, and is connected with the intromittent organ at one end, and at the
other with the oviduct. The ovarium (fig, 2 d) is very bulky, and, during the breeding
season, almost entirely fills the posterior portion of the cavity of the body; it is deeply fissured
in the dorsal median line, and divided into transverse lobes. The oviduct (fig. 2 e) is a
delicate tube, receiving branches on each side from the various lobules, after which it dilates,
and is twice or thrice bent upon itself, before it passes into the general channel. In connexion
with the female parts is a large laminated gland (fig. 2 g , g ) formed of two lateral
lobes, occupying the greater portion of the cavity in front of the body. It is composed of
two portions, an outer homogeneous, white, semi-pellucid part, and an inner and anterior part,
which is granular, opake, and of a pale flesh colour. The ducts of the two lobes unite and
form the common female channel. This gland secretes the mucus by which the mass of eggs
is enveloped. In addition to the parts already described there is a small globular sack
(fig. 2 h), which lies between the lobes of the great gland and at its posterior margin. This
is the purple bag of Swammerdam, and there can be little doubt that it is a true spermatheca.
A duct from this receptacle, having been joined by the duct of a small elliptical accessory
gland, is united with the oviduct after the junction of the latter with the testicle, and
immediately afterwards a branch of communication opens into the female channel. The duct
of the spermatheca is then continued on to the small orifice before mentioned immediately
within the common aperture.
From an examination of the anatomy of these organs, it would appear that impregnation
is effected by the channel of the spermatheca, which, receiving the semen from another
individual, retains it till required; but from the connexion of the testicle with the oviduct,
each individual would also appear to have the power of self-impregnation. The ova after
impregnation reach the common female channel, where they receive the gelatinous envelope
previous to expulsion from the body of the parent.