A pretty full account of the anatomy of Tritonia, is given in the ‘ Memoires des Mollusques’
of Cuvier; but, requiring more complete details, we have thought it desirable to re-investigate
the entire subject, and have in consequence carefully dissected T. Hombergii, the anatomy of
which we purpose giving at considerable length; Tritonia being the type of our second family.
The oral opening is placed on the inferior surface of the head, and is guarded by a
wrinkled lip, which is divided by a fissure into two lateral lobes. The channel leading from
thence into the buccal organ is very short. The buccal organ itself (fig. 2a, and f. 3) is of
vast magnitude, exceeding in this respect that of all other Nudibranchs. It is very muscular,
somewhat depressed, of an irregular quadrate form, and provided with powerful corneous
jaws, which extend the whole length of the organ, imbedded in the muscles. The jaws
(figs. 7, 8) are much elongated, rather narrow, with the anterior extremities arched upwards,
and firmly united by an elastic cartilage. The inner margins are thick, forming serrated
cutting blades (figs. 73, 86):
Three sets of muscles are employed in the movement of the jaws; one of which (fig. 4d,
and fig. 3e) passes across in front of the cutting blades above, and another (4d' and 3e') below,
having the oral opening between them; their extremities are inserted into the external surface
of the jaws towards their outer margin. These two sets, acting in unison, will bring the
cutting blades together, whilst the elasticity of the hinge, aided by the third set (4e and 3f) will
separate them. This last-named set lies transversely over the hinge, and is attached by its
two extremities to the jaws; thus it will not only assist in closing them, but also in binding
them together. A very similar arrangement of muscles for moving the jaws exists in Eolis.
The buccal mass and jaws in T. alba and T. plebeia, are like those of T. Hombergii, but in both
the former the cutting blades are furnished with a peculiar denticulated belt (fig. 10c, and fig.
113) on the outer surface. It is likely that in these instances the jaws are principally
prehensile organs, holding the prey by the aid of the denticulated belt, while the tongue is
employed in tearing off piece by piece, and so transporting it to the oesophagus. A similar
belt has been observed in the young of T. Hombergii, but not satisfactorily in the mature animal.
The tongue of T. Hombergii is very large, filling up the greater part of the buccal cavity,
and is so placed that the anterior or upper portion of the dentigerous membrane (fig. 4f) is
opposed to the oesophagus. This membrane (fig. 5) is formed on the plan of that of Doris
tuberculata, being broad and tubular, with the anterior portion {a) expanded somewhat like the
mouth of a trumpet. The expanded portion only, which forms, as it were, two lateral lips, is
brought to act upon the food; and the lingual pouch does not extend backwards beyond the
buccal mass. The operative part is divided by a fleshy process from that which is more
distinctly tubular, and within which the spines are developed.
The dentigerous membrane in T. Horribergii, when removed from the lingual muscles
and spread out, is broader than long; and the teeth or spines (fig. 6) are long, simple, and
very numerous. They are arranged in eighty-four transverse rows, each containing four
hundred and forty spines, and a central (3) and two lateral plates (c, c). The central plate is
rather large and of a quadrate form, with three not very distinct, obtuse denticles. The lateral
plates are irregularly ova], each bearing a blunt projection. In T. alba the membrane is
considerably longer than broad, and is supplied with twenty-nine rows of about seventy-two
spines each, together with a central and two lateral plates. The spines are long, arched, and
denticulated: the central plate has three stout denticles; the lateral plates bear an elevated
ridge on their outer margin. The membrane of T. plebeia is broader than in T. alba, but
not by any means so broad as in T. Hombergii. In it there are thirty-two rows of simple
spines with the points much and suddenly hooked: each row contains forty-two of these, and
a central and two lateral plates like those of T. alba.
The muscular support of the dentigerous membrane is composed of two sets or layers of
fibres,—an external and an internal layer; these move over each other freely, there being
no union between their surfaces. The inner layer (fig. 43) forms the walls of the cavity
within which the posterior portion of the dentigerous membrane is placed; and being
attached in front to a dense, semi-cartilaginous body, upon which the spiny membrane is
expanded, and behind to the posterior part of the inner surface of the jaws, this layer will,
when brought into action, draw together the sides of the tongue, bringing the spines to close
upon the food. The outer layer is made up of three strata of fibres, by far the thickest of
which (*’, i) underlies the other two, and is formed of numerous transverse laminae, arranged
in regular parallel order. These laminae or fibres act also upon the anterior semi-cartilaginous
support, and have their opposite extremities attached to the inner surface of the jaws,
immediately in front of the attachment of the inner layer. This stratum is for the purpose
of rotating the tongue backwards, and upwards in the direction of the gullet, and will,
with the aid of the inner layer, withdraw the whole organ. Immediately overlying this
stratum is another (j) the fibres of which are most developed in front or below, and cross
those of the under stratum. These fibres pass likewise from the semi-cartilagenous support,
and go to the root of the tongue, and are apparently for the purpose of rotating the organ
forward. The third or superficial stratum is very thin; its fibres are transverse, and have a
tendency to compress the tongue, and perhaps in this way to elongate it.
The lingual muscles in Doris and Eolis are arranged much in the same manner, modified
only to suit the altered condition of the parts. In Doris tuberculata the semi-cartilagenous
support of the dentigerous membrane is developed to a much greater extent; and the inner
and outer layers of muscles, which are not so distinctly marked, have both their extremities
attached to it, and the muscular fibres move over it as cords over a pulley. The semi-
cartilagenous support does not appear to exist in Eolis, and the muscles are no longer divided
into inner and outer layers. But a powerful mass of fibres is provided for drawing the tongue
back, which is attached by one end to the posterior extremity of the dentigerous membrane,
and by the other to the jaws. The stratum of fibres, for rotating the tongue forward, appears
to be represented in that genus by the muscles, which, forming the sides of the lingual mass
in front, are attached by one end to the anterior portion of the spinous membrane, and, inclining
backwards have their other extremity inserted into the jaw behind. By the interlacement of
the fibres of these latter muscles, a dense mass is formed in the centre of the tongue, which
gives support to the spiny ridge, and firmness to the whole organ, and acts as a sort of
fulcrum about which the retractor muscles work.
The oesophagus (fig. 23) in T. Hombergii is rather wide, of moderate length, and passes
from the upper surface of the buccal mass further forward than usual. On each side of it
a long, tubular, folliculated salivary gland (c, c) opens into the mouth. The stomach (d), of no
great size, is irregular in form, and is partially concealed by the liver, the left lobe of which
lies over it. The intestine (figs. 1 and 2e) is a short widish tube of equal diameter throughout;
it passes from the upper aspect of the gastric organ, and advancing a little forwards, bends