resembles some of the Dorides, particularly Doris repanda, in which the testis has appended
to it, not only a vas deferens, but is likewise supplied with a much constricted duct, which
.unites it to the oviduct as in Fiona; and the testis, too, of the Doris is very similar to the
same organ in this animal.
The vascular system is very interesting, inasmuch as we have been able to trace the
branchio-cardiac vessels more completely than in any other member of the family. Indeed
nearly the whole of these vessels are distinctly visible on the skin of the back, rising above
the general surface, and exhibiting a very curious and novel spectacle, much resembling the
vascular appearance on the lateral expansions of Elysia. The heart (fig. 2 a, and fig. 10) is
situated about the middle of the back, where it forms a large oval swelling immediately below
the skin, having the generative organs beneath. From the posterior extremity of the
swelling, an elevated, broad, and rounded ridge, (fig. 2 d), passes down the median line of
the back to the termination of the body. This ridge is joined on either side by numerous
similarly elevated branches, (e), which divide and subdivide as they approach the cloak-like
expansion on the sides of the body. The whole of these branches, and their subdivisions,
standing boldly up from the general surface of the skin, have the branchial papillae set along
them, and they give off twigs, which pass up the broad membranous expansion of the
papillae, (fig. 3).
On opening the heart from above, the ventricle (fig. 10 a) and auricle (c) are found ta
occupy a well-defined oval pericardium. The ventricle is large and muscular, of an irregular,
elliptical form, giving off the aorta (b) in front, which, in the usual manner, supplies branches
to the various organs. The auricle is united to it behind, a little on the left side; it is delicate
in comparison with the ventricle, but is nevertheless abundantly supplied with muscular fibres;
it lies diagonally in the pericardium, having the left side advanced almost to the front of that
organ, where it receives a trunk-vein (y) from the skin. The right side of the auricle stretches
backward, and receives a similar trunk-vein from the skin of this side, almost at the posterior
extremity of the pericardium. On laying the dorsal wall of the auricle open, its cavity is
found to be continuous with that of the great posterior elevated median ridge or trunk-vein
(d) before alluded t o ; and on opening this, the various lateral branches (/ƒ ,) are observed
debouching into it on either side. It is therefore evident that these three trunk-veins, their
beautiful system of ramifications all lying imbedded in the skin, are branchio-cardiac veins,
and herein we find a clear proof also of the branchial character of the papillae.
The papillae (fig. 3) are, as we are already aware, of a very peculiar character in this
animal, being somewhat compressed, as in Eolis papillosa, and having a distinct, broad, frilled
membrane (b) extending up their inner margin. It is, as before remarked, from the border of
this membrane that the twigs of the efferent vessels are given, and they pass down its entire
length. When a transverse section of a papilla (fig. 9) is made, a widish canal (b) is seen to
pass up the opposite margin. This may be looked upon as an afferent branchial vein, and
deteriorated blood, passing from the skin up this canal, may be supposed to filter through the
cellular tissue (d), between the external skin and the glandular sheath of the papilla, and so
find its way to the efferent vessel (c) at the free border of the membrane. If this view be
correct, and it would seem scarcely possible to doubt it, the papillae are evidently specialised
breathing organs, and by no means so low in organisation as has been thought.
At the same time, from the arrangement of the branchio-cardiac, or efferent vessels, and
from their elevation above the general surfaceof the skin, thus exposing to the influence ofthe surrounding
medium nearly three fourths of their circumference, it is pretty clear that the dorsal skin
itself must act, to some extent, as a gill, especially when we consider further, that the whole of the
blood returned to the heart does not pass through the papillae: much of it, no doubt, circulating
in the spongy tissue of the skin, passes at once into the efferent vessels : and, indeed,
small orifices for this purpose are seen in the wall of the great median trunk-vein. Here,
then, as in Doris, the blood is partly aerated in specialised breathing organs and partly in
the skin.
In connexion with the vascular system, Fiona is provided with an additional propelling
organ similar to what, in Doris, we have called a portal heart. As in that genus, this propelling
organ (fig. 10 e) lies below the floor of the pericardium, and, in like manner, opens into that
organ. In this species it is considerably elongated, with the ends rounded, and is placed far
back on the right side of the pericardium. It is firmly, attached to the skin of the body, and
is internally longitudinally plicated. Judging from our knowledge of this heart in the Dorides, it
may .be supposed to throw venous blood from the pericardial cavity to the glands of the
papillae. Tn Fiona, it is certainly connected with the skin, and probably has some relationship
to the vascular apparatus therein.
A renal organ also probably exists, though we have failed to detect it. A small distinct
orifice (fig. 2 b), however, opens externally immediately above the anus, and close to the posterior
border of the heart. ’ There can be little doubt that this is of the same nature as the
minute opening by the side of the anus in Doris, and which, in that genus, leads into an extensive
renal apparatus.*
The cerebral ganglia of the nervous system resemble those of Doris, rather than of Eolis.
They are'placed at the commencement of the gullet: there are, as usual, three pairs of principal
oesophageal ganglia, though, at first sight, only two are apparent,’— the cerebroid
(fig. 11, a. a,) and branchial (5) being completely fused, forming two oval masses, resting upon
the upper surface of the gullet, one on each side of the median line, across which they are
united at the anterior extremity by a short but distinct commissure : their posterior extremities
diverge, and are slightly bilobed, marking the boundaries of the two ganglia of which each
mass is composed,—the anterior lobe indicating the cerebroid, the posterior the branchial.
The pedial ganglia (c, c), are irregularly rounded, being equal in bulk to the cerebroid and
branchial together. They lie against the sides of the gullet, and are united to the under
surface of the central masses. Besides these there is a pair of accessory ganglia,—the
olfactory (d, d).
The buccal ganglia, (e,e), are placed in the usual situation on the buccal mass below the
gullet. These are scarcely larger than the olfactory, and are of an oval form, with their inner
extremities connected across the median line by a short commissure; their outer extremities
receive a cord (y), of communication from each of the cerebroid ganglia. Two minute
elliptical, gastro-oesophageal ganglia (ƒ,ƒ), are almost sessile on the anterior border of the
buccal. Thus, in all, there are six pairs of ganglia; four above the gullet—the oesophageal;
and two below it-—the buccal.
* Since the publication of our anatomy of Eolis, we have detected a portal heart and renal
orifice in that genus.