The latter are most developed in the Eoridida, but are never large, and are not always
present; the eyes being frequently supplied by simple nerves. The olfactory ganglia are
never absent, and are occasionally of considerable magnitude; in some of the Eolidida they
are half the size of the cerebroids, and in Tritonia too they are very large. In the Eoridida
they are sessile; but they are removed to a considerable distance from the cerebroids,
by the interposition of long nerves or commissures, in most of the Tritoniada and Eolidida.
In these cases, the olfactory ganglia are placed just within the base of the tentacles.
The auditory sacs are also in connexion with the cerebroids, upon which they are seated ; in
the Eoridida and Tritoniada, on the under side near the commissure uniting them to the
pedials; in the Eolidida, on the upper surface immediately behind the eye. In this respect it
would appear that the animals of this order disagree with the other gasteropods, in which
the auditory sacs are attached to the pedial ganglia. In the Heteropods, however, they are
always, as in the Nudibranchs, connected with the cerebroids. The cerebroids, as already
stated, give off the cords or commissures, which unite the oesophageal ganglia to the buccal.
The branchial ganglia distribute nerves to the mantle or dorsal skin, and to the
branchial ganglia of the sympathetic. system; and, in the Eolidida, give off from the under
side, a nerve or two in connexion with the sub-oesophageal commissure or branchial collar,
which go to the viscera. In the Eoridida there are three or four visceral nerves which
originate in the visceral ganglion, attached to the right branchial. All these nerves go to the
chief centres of the sympathetic system of the viscera. In some of the Polycerina, however,
the genital nerve comes off from the side of the right branchial ganglion.
The pedial ganglia send all their nerves to the foot.*
The buccal set of ganglia are generally two pairs:—the buccal proper, and the gastro-
cesophageal. The buccal pair, as already stated, are connected by commissure with the
cerebroids. The gastro-cesophageal are united to the buccal, upon which they are generally
sessile. The latter give their nerves to the buccal mass and tongue, and are connected with
the sympathetic plexus on the buccal organ. The gastro-cesophageal supply the salivary
glands, and send two large nerves—the par vagum,—down the oesophagus, which terminate in
two of the principal ganglia of the gastro-hepatic plexus of the sympathetic system.
Sympathetic System. This is composed of numerous minute, variously formed ganglia,
connected together by open plexuses of nerves, spread out over the viscera. The principal of
these plexuses are,—the oesophageal, the gastro-hepatic, the pyloric, the intestinal, the
branchial, and the genital. Similar ganglia and nerves have also been found in connexion
with the heart, arteries, buccal mass, and skin. This system, as we have seen, is connected
with both sets of cephalic ganglia, with the oesophageal, through the visceral and branchial
centres, and with the buccal by the par vagum, and gastro-oesophageal ganglia.
This portion of the nervous system I has been fully examined only in the Eorides, but
extensive traces of it have been found in the Polycerina, Tritoniada, and Eolidida; and in the
latter family a very striking modification of the buccal plexus exists. In Eolis and Fiona, a
large nerve is given off on either side from a small ganglion, buried in the muscles of the
buccal mass, and is also in connexion with the buccal ganglia. These nerves supply the
* In our description of the anatomy of Eolis, one of the pedial nerves is erroneously stated to
supply the skin of the sides of the back.
hepatic glands of the branchial papillae, and probably represent, along with the ganglia from
which thev originate, a portion of the gastro-hepatic plexus of the Dorides.*
The Senses. All the Nudibranchs are provided with auditory capsules, which contain
numerous vibratile otolithes in all the genera, except Embletonia and a portion of Eolis
(Cavolina and Tergipes), where there is only a single large, spherical otolithe; the auditory
organs thus, in these, retaining their embryonic condition. Eyes are also universally present,
and are only a little inferior in organisation to those of the higher gasteropods. The dorsal
tentacles are the organs of smell, and, judging from their great development, this sense must
be more acute in most of the Nudibranchs than it is in any other Mollusk, with the
exception, perhaps, of Nautilus. Olfaction, however, in these animals, probably is not so much
to assist in the discovery of alimentary matters, as to give warning of the unhealthy state of the
surrounding medium, arising from putrescence or other causes; for the Nudibranchs,
breathing more or less by the whole surface, and being entirely unprovided with covering
of any kind, are exceedingly liable to be affected by external influences. The sole object of
vision appears to be that of ascertaining the presence of light, and thus directing the animal
in its search for shelter in dark and concealed places. Touch undoubtedly resides everywhere
in the skin, but is specialised in the oral tentacles and parts about the mouth. The lips and
channel of the mouth are probably the seat of taste.
The Skin. This varies very much in thickness in the several groups; it is delicate and
soft in .the Eolididce, and in most of the Polycerince; while in the Tritoniada and Eorides, it is
thick and coriaceous, and generally more or less roughened with tubercular excrescences-
In thejdoridida it is always stiffened with imbedded calcareous spicula of various forms, which
in the Polycerina are for the most part scattered, and not very numerous; but in the Eorides
are generally abundant, much crowded, and always more or less symmetrically arranged.
The skin consists of a layer of muscular fibres, covered by a tegumentary envelope or cutis,
which is provided with an epithelium. The epithelium of the whole surface, not even
excepting the pedial disc in the Eorides, and perhaps in the other members of the order, is
provided with vibratile cilia. The dermal layer is thin and continuous with the inner or
muscular layer, which is amply supplied with muscular fibres, principally longitudinal and
transverse. Outside of, and amidst this muscular stratum, which is densest next the viscera,
is the cell-work or system of dermal sinuses, through which the blood flows on its return to
the heart.
The dermal layer appears to secrete the tenaceous fluid that so abundantly exudes from
these animals; though that which lubricates the foot is probably provided by a special
gland. The two large glands in Fiona, which lie beneath the stomach, and the ducts of
which pass into the anterior margin of the foot, seem to be for this purpose. The so-called
salivary glands of Eoto are, in all likelihood, of a similar nature (see note, p. 12); and JEgirus
has a glandular body placed transversely under the channel of the mouth, which appears to
open externally in front of the crawling disc. Goniodoris is likewise furnished with a similar
gland, less perfectly developed, and the thickened margin of the foot of Polycera ocellata
contains numerous secreting follicles. The thickening of this margin, the transverse groove,
and lateral prolongations, so common in these animals, are perhaps universally connected with
the production and suffusion of the mucus over the pedial disc.
The cloak or mantle, which is characteristic of the Eoridida and Tritoniada,- and