TRITONIA.
posterior portions or branchial ganglia give off, from the upper surface behind, a pair of large
nerves (8) which supply the dorsal skin; and the right branchial furnishes an additional single
nerve (7) to the root of the male intromittent organ. This is the only one we have traced of
the visceral nerves though doubtless others exist. The pedial ganglia (c, c) are round, and
united by short commissures to the cerebroid and branchial. They give off three pairs of
stout nerves (9,10, 11), which are distributed to the foot. The great sub-oesophageal collar
\h) is wide, and is composed of three filaments, two of which belong to the pedial ganglia,
one to the branchial.
The buccal ganglia (ƒ) are situated as usual on the buccal organ, immediately behind the
gullet; they are rather large, broadly elliptical, and united across the median line by a short
commissure, and to the under side of the cerebroids by two long cords (i, i) forming the
buccal collar. These ganglia give two pairs of nerves (12, 13) to the buccal organ, and a
single one (14) to the tongue ; the latter comes off from the commissure uniting the ganglia.
They have also appended to them by much longer cords than usual, a pair gf gastro-
oesophageal ganglia ; these supply, besides the oesophageal nerves (17) (the par vagum),
two other pairs, one of which (15) goes to the salivary glands, the other (16) to the root of the
oesophagus.
The senses are in the same condition as in Doris. The eye { j ) is small, and is removed
to a considerable distance from the cerebral ganglia. The optic nerve is consequently long ;
it is very delicate and arises from a small ganglionic enlargement (e) seated above and behind
on the outer margin of the cerebroids. The auditory capsules have not been examined in
T. Hombergii, but in T. plebeia, in which the cerebral ganglia are similar, they contain
numerous otolithes (fig. 12 e), and are sessile on the under surface of the cerebroid ganglia, a
little in advance of the commissures (d ), which unite these ganglia to the pedial. Taste
probably resides in the lips, touch everywhere, but is speciallised in the tentacular points of
the veil.
The skin is devoid of spicula, and is thick and coriaceous.
EXPLANATION OF THE PLATE.
Fig. 1. General view of the viscera of Tritonia Hombergii, the dorsal skin laid open:—a, buccal organ;
b, oesophagus ; c, c, salivary glands; d, anterior portion of the stomach; e, intestine; ƒ, liver
enveloped in the ovary; g,g, portions of the mucus-gland in connexion with the female
organs; h, retracted penis; i, portion of the testis; j, pericardium; k, ventricle, seen through
the transparent wall of the pericardium; l, auricle; m, m, lateral venous trunks, or efferent'
branchial veins, passing from the skin to.the auricle; n, r£, branches of the same laid open,
showing orifices communicating with the gills and cellular tissue of the skin; o, o, o, afferent
branchial veins from the liver-mass; p,p, branches of the same exposed, giving twigs to the
branchial plumes; q, portal hea*t seen through the transparent walls of the pericardium; r,
oesophageal ganglia, giving nerves to the various organs; s, s, the eyes.
2. General view of the viscera, the various organs partially separated:— a, buccal organ; b,
oesophagus; c, c, salivary glands; d, stomach; e, e, intestine cut through; f , liver enveloped
in the ovary; g, root of the aorta, the ventricle removed; h, anterior branch of the same; i,
hepatic branch of the same; j J J J , afferent branchial veins from the liver- mass; k, retracted
penis; l, testis; m, dilated portion of the oviduct; n, constricted portion of the same; o, o,
mu - . cl.^d in connexion with the female parts; p, female channel; q, spermatheca; r,
vagina, leading from the external orifice to the same; s, oesophageal ganglia, distributing
nerves to the various organs; t, t, the eyes.
Fig. 3. Side view of the buccal mass:—«, channel of the mouth; b, oesophagus; c, salivary glands;
d, outer edge of jaw; e, e', two sets of muscles for closing the jaws; ƒ, muscles to aid in opening
the same; g, lingual muscles.
4. Side view of the buccal mass, with one jaw and part of the muscles removed to exhibit t e
tongue -.— a, oesophagus; b, cutting edge of jaw; c, hinge cut through; d, d', two sets of muscles
: for closing the jaws, cut through; e, muscles to aid in opening the same; ƒ, anterior or
expanded portion of the dentigerous membrane; g, end of the posterior or tubular portion of
the same; h, portion of the inner lajerMUngual muscles for drawing the sides of the tongue
together; i, i, principal stratum of the outer layer of muscles for drawing the tongue upwards
and backwards, the two outer strata being almost entirely removed; j , a portion of one of
the outer strata of the same, for rotating the tongue forward, the outer or external one being
removed.
5. Dentigerous membrane removed from its muscular support :—a, anterior or expanded portion
of the membrane; 4, posterior or tubular portion of the same.
6. A portion of one of the rows of lingual spines :— a, a, lateral spines; 4, central plate; c, c, the
two lateral plates.
7. Inside view of jaws:— a, hinge ; 4, cutting edges.
8. Outside view of a single jaw:— a, point of articulation; 4, cutting edge.
9. Cerebral ganglia:— a, a, cerebroid ganglia; 4,4, branchial; c,c, pedial; d, olfactory ; e, optic;
ƒ, buccal; g,g, gastro-resophageal; h, great sub-ffisophageal collar; i,i, buccal collar; 1,
olfhetory nerves; 2, 8, 4, nerves of the veil, 2 sends a branch to the channel of the mouth
and 4 one to the tentacular process on the under side of the veil; 5, 6, nerves to the skin on
the sides of the body; 7, genital nerve; 8, paUial nerves; 9, 10, 11, pedial nerves ; 12, 18,
buccal nerves; 14, lingual nerve; 15, nerves to the salivary glands; 16, nerves to the root
of the oesophagus; 17, oesophageal nerves.
10. Inside view of the jaw of T. plebeia.— a, point of articulation; 4, cutting edge; c, denticulated
belt.
* 11. A portion of the jaw of the same more highly magnified :—«, cutting edge; b, denticulated belt.
12. Under view of a portion of the oesophageal ganglia of the same:—a, cerebroid ganglion; b,
branchial; c, pedial; d, commissures uniting the pedial to the cerebroid and branchial
ganglia; e, auditory capsule.