From the external characters, it is evident that Idalia and Goniodoris are closely related
to each other; but the intimacy of their connexion is only made manifest by the study of
their anatomy.
The mouth of Idalia opens almost at once into a buccal organ, which is rather small,
and is provided with a muscular gizzard attached to the upper surface, and a pair of short
tubular, salivary glands, with their extremities enlarged and folliculated. The tongue of
I. LeacMi is narrow, of a brown-amber colour, and formed of numerous rows of spines, four
in each row; those next the centre being large and minutely denticulated at the sides; the
external ones are simple and rudimentary: there is no central spine. The buccal lip is
armed with a prehensile collar, not interrupted above, and having the surface covered with
numerous minute simple spines.
In Idalia elegans the large lingual spines are similar in form to those of I . Leachii, but
have the sides smooth. In I. aspersa and I pvlchella, belonging to the second section, these
spines are broader, stouter, and more hooked, with the margins strongly denticulated;
and moreover the prehensile collar (fig. 5 a), in these two latter species, is very imperfect,
being composed of two rounded, minutely-spinous lobes, one on each side of the median line
on the lower portion of the buccal lip.
The oesophagus is a long slender tube without dilatation: it opens from below into the
stomach, which is small, and placed in a cleft in the anterior margin of the liver. The
intestine proceeds from above, and passes backwards along the right side of the liver.
towards its termination it is considerably enlarged. The liver is bulky and of an irregular
oval form.
The reproductive apparatus differs only very slightly from that of Goniodoris, The
testis is composed of three portions, but here the gland-like part cannot be called fusiform,
but is irregular in shape and very large. The vaginal tube, though not so long as in
Goniodoris nodosa, is very wide, and tapers as it approaches the spermathecse, of which there
are two; one being much larger than the other: both are elliptical. The ovary is spread
over the upper surface of the liver, and the anterior surface of the oviduct is much dilated.
The auricle and ventricle are placed within a distinct pericardium, below which is the
portal heart; and in every respect the circulation and respiration are exactly in the same
condition as in Goniodoris. The renal organ has been only imperfectly observed, but the
external orifice is conspicuous enough in front and a little to the right of the anal nipple. A
glandular organ of a folliculated structure is in connexion with the arterial system in front of
the generative apparatus.
The nervous system presents the same high degree of concentration observed in
Goniodoris; there being four principal oesophageal ganglia; but the upper pair in Idalia are
bilobed, indicating the position of the cerebroid and branchial, of which they are composed.
The eyes are sessile on the outer margin of the cerebroids; the auditory capsules have not
been observed, neither have the olfactory ganglia. The nerves are distributed much in the
same manner as in Goniodoris.
The skin is smooth and soft, with imbedded spicula, which, in /. aspersa, are not crowded;
they are fusiform, tuberculated, and bent in the centre. Cilia were detected on the tentacles
and branchiae, but nowhere else.
Genüs 14. EMBLETONIA, Alder and H ancock.
Corpus elongatum, limaciforme, non palliatum. Caput terminale, lateribus utrinque in lobum
planum extensis. Tentacula 2, linearia, sublateralia. Maxilla corneae. Branchia papillose, ad latera
dorsi, ut plurimum in simplici serie, utrinque dispositae. Anus et orificium generations ad latus
dextrum.
We described this genus in the ‘Annals of Natural History’ for November, 1844, under
the name of PterocTiilus; but having since ascertained that the name has been already used
for a genus of Hymenopterous insects, we now propose to substitute for it that of Embletonia,
in honour of oqr friend Dr. Dennis Embleton, from whom we have received such able
assistance in the anatomical part of this work. The genus is closely allied to the Eolides of
our fourth section; but differs from them in having only two tentacles, the oral pair being
here supplied by an expansion of the sides of the head into flattened lobes. The dorsal
tentacles, too, are placed much more laterally than in Eolis, and at a greater distance from the
eyes. There is at present only one ascertained British species, unless the small nudibranch,
noticed under the name of Eolidia minuta by Professor E. Forbes at the Birmingham meeting
of the British Association in 1839,* should prove to be distinct from that here described.
Two American species of the genus have been discovered by Dr. Gould of Boston, which, as
far as we know, remain yet unpublished. The Tergipes adspersusf of Nordmann, which ' is
described as having the oral tentacles triangular, appears to be closely allied to Embletonia.
The species are all minute, and belong to the littoral and laminarian zones. There can be
little doubt that they are carnivorous.
The body in this genus is elongated and nearly linear, tapering behind. The head is
terminal and furnished with two flattened lobes, broadly expanded laterally, and forming a
kind of veil. The mouth is inferior; it has large corneous jaws, and a narrow denticulated
tongue. The tentacles are two in number, linear, generally short, and placed very much
apart towards the sides of the head. The eyes are situated considerably behind them. The
branchiae are papillose, as in Eolis, and are generally placed in single series down each side o f
the back, alternating posteriorly. The great central hepatic vessel, partaking of the colour
of the branchiae, may usually be seen undulating down the back. In the American species
there is more than one row of papillae on each side. The foot is linear, truncated in front,
and without any lateral expansion. The anus is placed on the right side, a little behind
the generative organs, which are situated below and between the first and se cond papillae.
Its anatomical characters, as far as they have been ascertained, are as follow:—The
oral opening is beneath in the form of a longitudinal slit between two narrow lateral lips.
The buccal organ is provided with a pair of large corneous jaws (PI. 38, fig. 5) of a
* See ‘Athenæum’ for August 31,1839.
t Ann. des Seien. Nat., 3d ser. v, p. 110.