the veil, for we have seen the blood passing to and fro in each of these organs so as to leave
little doubt of their branchial nature. The foot is linear and slender, formed. as much for
clasping corallines and sea-weeds as for crawling on a flat surface, though from the thin and
pliant nature of its sides it may be used for either purpose. The sexual aperture is situated
on the right side below the first branchial tuft, and that of the anus further behind and a
little above, between the first and second branchiae.
The digestive system varies in some respects from that of Solis. The mouth and jaws
are the same, the latter varying only a little in form (f. 5). The tongue occupies in the
same manner the ridge of a wedge-shaped muscle that rises in the centre of the mouth, and
has a similar complicated muscular apparatus for its movement. In D. arborescens this
organ is composed of upwards of twenty transverse rows of curved, denticulated spines, with
a large central one, also denticulated. Each row contains twenty of the small spines, which
are brilliantly crystalline, the whole forming a very beautiful object for the microscope
(f. 6, 7, 8). On account of the smallness of our specimens we were not able to detect the
salivary glands, but as we did not find them outside the buccal mass, they are probably coh-
cealed by the jaws, as in Solis. The oesophagus is much larger than usual, and opens into
a well-defined stomachal pouch, which terminates in a short intestinal canal that opens on
the right side between the first and second branchial tufts. The hepatic organ, however,
shows the widest deviation from the structure of Solis. The central vessel in Sendronotus
is not a mere canal passing from the stomach and receiving the branches from the glands of
the papillse or branchial tufts, but is a large folliculated mass (f. 2, 3) occupying the centre
of the body—occupying, in fact, the very position of the liver in the Dorididce and
Tritoniadee, and communicating with the stomach by a constricted duct. From this mass
branches pass off into the branchiae and tentacular sheaths: these branches lose their
follicular structure and become mere tubes as they pass into the smaller ramifications of
these organs. They are lined, however» through their entire length, with the granular
substance observed in the other portions of the hepatic apparatus. The sides and upper
anterior portion of the stomach are covered with follicular masses, resembling in every
respect the great central trunk, which, as well as the stomach, is lined with vibratile cilia.
The central trunk of the digestive system lies above the ovarium, and not below it as in
Solis. In this respect, as well as in the glandular structure of the central trunk or mass,
and in its separation from the stomach by a constricted duct, Sendronotus shows a deviation
from the type of the Eolidida, and an approximation to the Sorididce and Tritoniadte, thus
supplying a connecting link between these two forms of gastric structure in the Nudi-
branchiata.
The vascular system is furnished with a well-developed heart, consisting of a ventricle
and auricle, and in other respects does not appear to differ from the rest of the family.
The nervous system is very similar to that of Solis. The cerebral ganglions are four
in number (f. 9,) and are placed symmetrically, giving off nerves much in the order observed
in that genus. Of these we have been able to determine ten pairs. The ganglions of the
dorsal tentacles (the olfactory ganglions) are larger than usual (9 a, «,) and are placed at the
base of the laminated portion of these organs, consequently at a considerable distance from
the cerebral ganglions, to which they are united by large nerves: these form the first pair.
The eyes are very small. They are composed of a well-formed pigment cup, a lens, and a
cornea, enveloped in a general capsule (f. 10.) The optic nerves are very long; the
eyes are therefore removed to some distance from the large ganglions. The auditory
capsules contain numerous otolites of an elliptic form (f. 11, 12.)
The generative organs do not appear to differ from those of Solis, with the exception
that the ovarium, as before stated, lies above the central mass of the digestive system.
EXPLANATION OF THE PLATE.
Fie. 1. General view of the viscera of Dendronotus a rb o resc en sa , buccal mass; b, oesophagus;
c, intestinal canal; d, anus; e, e, e, branches of the central mass passing into the
branchiae; f f branches of the same passing into the tentacular sheaths; g, ventricle
of the h eart; g , auricle; h, cerebral ganglions ; h', olfactory ganglions; i, i, portion of
the generative organs; j , ovarium.
2. Digestive system:—a, buccal mass; b, oesophagus; c, stomach; d, intestine; e, duct
connecting the alimentary canal with the liver; f f , ƒ, branches of the central mass
passing into the branchiae; g,g, branches from the same passing into the tentacular
sheaths; h, h, central portion of the liver.
3. A portion of the digestive system seen in profile, showing the duct connecting the liver
with the stomach. The letters of this figure correspond to those of fig. 2.
4. Buccal mass.
.5. Jaws deprived of their muscles.
6. Tongue.
7. A portion of the same more highly magnified.
8. Dorsal view of the same.
9. Cerebral ganglions showing the origin of the principal nerves:—a, ganglions of the
dorsal tentacles (olfactory ganglions;) b, b, the eyes.
10. The eye more highly magnified.
11. Auditory capsule with otolites.
12. Otolites more highly magnified.