In connexion with the vascular system in the Doridida y there is a gland-like organ which
generally overlies the buccal mass. This is copiously supplied with blood by a branch from
the aorta. Its function is unknown, but it may probably be analogous to some of the vascular,
ductless glands of the higher animals.
The flow of the blood is rapid; the pulsations of the heart varying, in the different
species, from fifty to a hundred in the minute.
It is evident, from the state of the circulatory apparatus, that respiration is performed only
in part by the branchiae. In all the three families, the skin, which is covered with vibratile
cilia, acts as an imperfect accessory breathing organ, and thus the blood is returned to the
heart in a partially aerated condition. The branchiae are most highly developed in the
Doridida, in which they are always laminated; the blood is brought to them by a distinct
afferent vein, and, after circulating over the respiratory surface in a definite course, is returned
to the auricle by another distinct or efferent vein, there to be mixed up with that brought
from the skin. In the Tritoniada the gills have a similar laminated structure, and the
branchial circulation is apparently as complete as in the Doridida; but the efferent branchial
veins are not distinct, but communicate with the sinuses of the skin, and therefore transmit to
the auricle a mixed stream. The Eolidida have the respiratory organs less perfectly
organised; they are generally papillose, though sometimes branched. Here there are
apparently no afferent branchial veins; the blood being at once drawn from the sinuses of the
skin into a vessel which passes up one side of the papilla: it then filters through fine cellular
tissue, under the aerating surface, into another vessel on the opposite side, and so passes on in
distinct vessels to the great efferent, branchio-cardiac trunks, which conduct it, mingled with
the blood received from the sinuses of the skin, to the auricle. The gills, whether laminated
or papillose, are clothed with vibratile cilia.
Renal Organ. An excreting organ, which, from analogy there can be no doubt is of a
renal character, exists in all the Nudibranchs, though little is known of its anatomy, except in
the Doridida; in all, however, the portal heart and the excretory orifice have been
detected. This orifice is invariably associated with the anus. The renal organ is, in the
Doridida, a large membranous sac, more or less branched, lying immediately below the
pericardium, and having its floor firmly attached to the liver-mass. It is supplied with
arterial and venous blood, by the systemic and portal hearts; both of which send numerous
vessels to ramify in its walls.
Nervous System. This presents a high degree of concentration,—perhaps higher than in any
other group of Mollusks,—and is divided into two very distinct portions;—one, the cephalic
or excito-motor; the second, the splanchnic or sympathetic: these two portions intercommunicate
at several points. The cephalic ganglia are situated at the origin of the oesophagus,
and naturally divide themselves into two sets, which may be denominated respectively
oesophageal and buccal, or supra- and infra-oesophageal. The latter are attached to the
buccal mass; the former are more immediately related to the oesophagus, upon which they
are always seated, and about which they, with their commissures, form one or more constricted
collars. The two sets of ganglia are also interconnected.
The oesophageal set is composed essentially of three pairs of principal ganglia, howsoever
they may be fused or blended together, and of one or two accessory pairs; and, in the
Doridida, there is an- additional visceral ganglion. The principal ganglia are the cerebroid,
the branchial, and the pedial; the accessory,—the olfactory and the optic. They are placed
symmetrically with regard to the median line, there being one of each pair on either side.
The cerebroids are invariably above the oesophagus; they hold a central position, and
are united by a very short commissure across the median line, being usually in contact with
each other, and, when distinct, are connected by commissures with the branchial and pedial:
these two latter of the same side also intercommunicate. The branchials, as well as the
cerebroids, are generally above the alimentary tube, and are frequently fused with them, so as
to form a single mass on each side of the median line. This is not uncommonly the case in
the Doridida, but seems to be universally so in the Tritoniada and Eolidida. The pedial
ganglia are also occasionally placed above the oesophagus; but more frequently they are at
the sides, and sometimes, though rarely, they are below it, as in Doris pilosa, Ancula
cristata, Lomanotus marmoratus, and Doto fragilis. In the Doridida, the visceral ganglion is
attached to the under side of the right branchial; it is always small. In the Tritoniada and
Eolidida this ganglion is apparently absorbed into the branchial. The fusion of the
oesophageal ganglia is most complete in Tetliys leporina. In this species all the three pairs are
situated above the alimentary tube, and are so perfectly united into one mass, that it would be
scarcely possible to ascertain the boundaries of its component parts, were it not for the manner
in which the nerves are distributed.
The oesophageal ganglia are united to the buccal, by two long commissural, cords, which
pass from the under side of the cerebroids and embrace the gullet, forming a wide collar—the
buccal—around that tube. A large sub-oesophageal commissure, passing between the
oesophageal ganglia of either side, completes a second or great posterior oesophageal collar.
This commissure varies in length, in accordance with the position of the ganglia: if they are
all above the alimentary tube, it is long; if partly below, very short. It is composed of either
two or three cords. In the Doridida and Tritoniada there are generally three, two of which
belong to the pedial ganglia, one to the branchial and visceral. This great oesophageal collar
is, therefore, in these two families really both a pedial and branchial commissure. In the
Eolididay however, it generally consists of only two cords, and they both come from the pedial
ganglia; the branchial commissure is nevertheless invariably present, but is quite distinct
from that of the pedial, so that here the great posterior collar is represented by two separate
collars. Still there is apparently another sub-oesophageal commissure. Doris tuberculata has
a fine cord, uniting the cerebroid ganglia below the buccal mass. Thus, it seems probable
that in the Nudibranchs all the three principal pairs of oesophageal ganglia are united by
separate cords, below the alimentary tube.* But at present we must be satisfied with
enunciating the general facts; that in the Eolidida the oesophagus is encircled by three
collars, namelyr^first, the great oesophageal—the pedial; second, the branchial; third, the
buccal; and that in the Doridida there are usually two; first, the great oesophageal,
comprising the pedial and branchial commissures; and second, the buccal.
The cerebroid ganglia give nerves to the channel of the mouth, to the lips and oral
tentacles, also to the veil, when it is formed by the fusion of these latter organs : they have
likewise attached to them the two pairs of accessory ganglia,—the olfactory and the optic.
* This additional or cerebroid collar does not appear to be peculiar to the Nudibranchs, for we
have also detected it in Pleurobranchidium Meckelii.
a