ÆGIRUS.
each row: the median spine is deficient, and the tongue is divided down the centre by a narrow
naked space. The tubular portion behind projects a little backwards from the buccal organ.
A pair of tubular salivary glands open into the mouth at the sides of the oesophagus. This
latter organ is rather short, and passes from the upper posterior end of the buccal mass : it is
of equal caliber throughout, with the exception of a slight swelling immediately after it passes
through the nervous collar : it enters the liver in front, to join the stomach, which is small, and
buried in that viscus. A portion of the upper surface of the gastric pouch is seen, however,
emerging upwards from the biliary organ, and gradually subsiding into the intestine, which
is short. At the junction of this tube with the stomach, there is a small oval sac—a rudimentary
pancreas—and at the other extremity, a gland-like body; but this latter does not
appear to be connected with the alimentary tube, and we have not succeeded in determining
its nature.
The reproductive organs are much like those of Polycera. The testis is composed of a
•tubular, and a large, irregularly rounded, glandular portion; the intermediate thin duct-like
tube not being present. The spermathecse are oval, the principal one being large, the other
very small.
There is nothing peculiar in the vascular system, so far as we have been able to examine
it, and the renal organ, though observed, has not been sufficiently made out.
The nervous system is more highly concentrated than in Polycera. There are only two
pairs of principal oesophageal ganglia,—the cerebroid and branchial being completely fused,
so as to form, above the alimentary tube, two large reniform masses, which are united to each
other by a very short commissure across the median line. The anterior portions of these
masses represent the cerebroids, and accordingly each gives off from the frontal margin three
nerves, which supply the channel of the mouth and the oral tentacular appendages. The
olfactory ganglia are large and oval, being equal in size to the buccal; they are closely
adherent to the frontal margin of the same ganglia, but a little nearer to the median line.
The nerves they supply are stout, and go direct to the dorsal tentacles. The posterior portions
of the large central masses are the branchial ganglia, and from the outer margin of each a couple
of nerves are sent off, which, branching forwards and backwards, go to the dorsal skin: the
anterior branch supplies the veil or mantle-border overhanging the head. From the right
branchial, two additional nerves are given off; one of which, coming from the outer margin,
passes to the male sexual organ; the other is given off from the under side. These are all we
have detected of the visceral nerves. The pedial ganglia are rounded in form, and are closely
attached to the outer margin, below, of the two large central masses : they are placed at the
sides of the oesophagus, and are united below that tube, by a rather long, stout cord,—the
great oesophageal collar, composed, in this instance, of three filaments, two of which are, as
usual, pedial—one branchial: three nerves are given by these ganglia to the foot.
The buccal ganglia present nothing unusual; they are joined together across the median
line by a short commissure. The gastro-cesophageal ganglia have not been observed.
The eyes are sessile on the outer margin of the anterior or cerebroid portions of the central
ganglionic masses. The auditory capsules are attached to the under side of the same portion»
immediately behind the eyes, and contain numerous elliptical otolithes, and a single spherical
one, much larger than the others.
The skin is tough and coriaceous, having the imbedded spicula much more crowded than
in any other of the Polycerina: they are fusiform, pointed at both ends, and bent in the Centre.
Genus 5. THECACERA,* Fleming.
Corpus limaciforme, læve, non palliatum, postice acuminatum. Caput suhinferius, vélo aut plano
aut lobatp sepissimè obtectum. Tentacula 2 dorsalia, laminata, intra vaginas retractilia. Branchioe
plumosæ, in lineà mediâ dorsi anum circumdantes, appendicibus lateralibus linearibus vel tubercularibus.
Apertura genitalis ad latus dextrum.
Thecacera is very nearly allied to Polycera, from which it chiefly differs in having sheathed
tentacles; the laminated character of these organs, together with the smooth surface of the
body, sufficiently distinguish it from JEgirus; and with other genera of the group it cannot
readily be confounded. The genus was proposed by Dr. Fleming in his ‘ History of British
Animals’ (1828) for the Boris pennigera of Montagu, a rare and curious Mollusk, then only
known by the description of the latter author, and even up to the present time involved in
much obscurity; but possessing characters, which, Dr. Fleming very properly observed,
“ mark it as the type of a genus.” Another species has lately been discovered by Mr. Cocks
at Falmouth; and more recently still, a third species has been dredged off St. Ives by
Mr. Barlee. These agree with that of Montagu, in the smooth body, and laminated and
sheathed tentacles,, but vary in almost every other respect from the typical form.
The only known foreign species of this genus was found by Mr. Charles Darwin in the
Chomos Archipelago, south of Chiloe, and is not yet published.
Thecacera has the body limaciform and smooth, swelling a little in the centre, and
terminating in a pointed tail. The head is sub-inferior and furnished, like Polycera, with
corneous jaws: it is generally covered by a velar expansion, more or less distinct, and of
variable character, being either even, lobed, or tuberculated. The tentacles are two in
number, dorsal, and laminated; retractile within even or foliated sheaths. There is no dorsal
area or rudimentary cloak. The branchiae are plumose, surrounding the vent in the centre
of the back posteriorly. They are furnished with one or more appendages on each side,
which in some species are large and well developed, in others tubercular and obscure. The foot
is of the usual form, and co-extensive with the body. The aperture of the sexual organs is
on the right side.
The anatomy of Thecacera appears to be very similar to that of Polycera; but on account
of the rareness of the genus, we have not been able to examine it in detail. We have
ascertained, however, some particulars respecting the buccal apparatus and the nervous
system.
The buccal organ and jaws of T. virescens closely resemble those of Polycera Lesson 'd,
the jaws having very imperfect cutting edges: in T. capitata the jaws are, however, more like
those of P. quadrilineata. The tongue of the former species has eighteen or nineteen rows of
From a sheath, and Ktpaç, a horn.