shaped gastric pouch, whose orifice in the stomacli-wall is closed with a more or less
distinctly developed flap. This pouch is often called the gall-bladder, but nothing indicates
that its functions correspond to this name. Its walls are glandular, and appear to
secrete globules of a glistening material which is also found in the intestine. It is
possible that this secretion subsequently dissolves and forms a membrane which is found
to cover the walls of the stomach and intestine, and probably serves to protect these
delicate surfaces against the spicules abounding in the sponges on which most Dorids
feed. The intestine is a long thin tube with longitudinally laminated internal walls. It
does not seem probable that in this species it takes any part in the work of digestion or
absorption. It runs forward almost to the level of the nerve-collar, makes an abrupt turn
to the right across the oesophagus, and runs backwards to the branchial circuit, within
which it terminates as a tubular papilla set not quite symmetrically but rather to the left
of the centre.
T he N ervous S ystem.
The nervous system consists of two parts, one known as the central nervous sj’stem
which takes the form of a collar round the oesophagus with peripheral nerves springing
from it, and another which is less conspicuous and distributed over the viscera. This
is commonly called the sympathetic nervous system, but perhaps accessory would
be a better name. As seen from above the central nervous system is enclosed in a
semi-transparent membranous capsule. When this is removed there are seen more
plainly three pairs of ganglia, divided by a median depression into two halves, one
ganglion of each pair being right and the other lefk The cerebral and pleural ganglia
are usually spoken of as a single group (cerebro-pleural) because they are more or
less united into one mass. The pedal ganglia, lying at the side, are more definitely
independent though partially fused with the others. "Seen from below (see Text-figure 1,
p. 42), the ganglia are found to be fastened round the cesophagus by three bands,
which are not, however, all similar. The most anterior is a simple thread or commissure
(ft) which unites the two cerebral ganglia and near its roots is fused with a nerve
coming from them.1 The second band (b) bears ganglia, and is to be described in
the greater part of its length not as a commissure uniting the right and left members of a
pair of ganglia, but as the cerebro-buccal connectives, running from the cerebral to the
buccal ganglia. These latter almost touch one another and are closely applied to the
lower surface of the oesophagus, just behind the buccal mass. To the posterior part of
them are attached the small gastro-oesophageal ganglia. The third band (c) consists of three
commissures united in a common sheath but showing a distinct origin at their roots; these
are called the subcerebral, pedal, and pleural commissures. Beneath the right pleural
ganglion lies a smaller unpaired ganglion (d), not visible from above, which may be called
visceral. Compared with Tectibranchs such as, Aplysia, Doris shows a double modification
in its nervous system. The visceral connectives are shortened, and the parieto-visceral
ganglion complex (itself formed by the fusion of nerve-centres) is drawn up into the
nerve-collar, and secondly the single ganglion representing this complex is put on one
1 This anterior commissure, sometimes called labial, appears to be recorded only for Archidoris.
Hi
side and almost obliterated. But the nerves which it sends to the genitalia and viscera
are studded with beads consisting of ganglionic cells.
The chief organs of sense are directly connected with the cerebral ganglia. From
their anterior extremity rise the almost sessile olfactory ganglia which send a nerve each
into the rhinophores or dorsal tentacles, highly sensitive pillars bearing on their upper
part numerous perfoliations and retractile into two pits. The rhinophores are in direct
communication with the water, but the eyes (whose functional value must be small) are subcutaneous.
They lie further back than the olfactory bulbs near the external margin of the
cerebral ganglia, to which they are attached by very short nerves. They consist of a
capsule containing a cup-like layer of black pigment set round a spherical crystalline lens.
Immediately behind the eyes lie the sessile auditory capsules which contain numerous
otoconia. The cerebral ganglia give off four other pairs of nerves which innervate the oral
tentacles, the mouth, and the lips. The pleural ganglia send off three pairs of nerves
innervating the mantle, and the hindmost of the- three communicates with the sympathetic
system on the right-hand side. The pedal ganglia, which lie a little below the others,
give off five pairs of nerves, three large ones whicli innervate the foot and two small ones
which run to the sides of the body. Four nerves arise from the unpaired visceral ganglion
and supply the genitalia, the whole alimentary canal, the heart, and the kidney. They
communicate with the sympathetic system and differ in structure from the other nerves,
since they are studded with beads of ganglionic cells along their whole length. The
ganglia beneath the oesophagus are mainly concerned with the nerve-supply of the buccal
mass. Each buccal ganglion sends off two rather large lateral nerves distributed over this
mass and a small one to the base of the tongue. The gastro-oesophageal ganglia also send
off three pairs of nerves, one to the salivary glands, one to the top of the oesophagus, and
a third pair, the largest of the three, running side by side down the under surface of the
oesophagus and eventually connecting with the sympathetic system.
An examination of several specimens of Doris will show that the symmetry of the
central nervous system is not absolute or consistent, and that one half is often larger than
the other, especially in the cerebral portion. The excess is most commonly in the right
half (as seen from above), but there is no rule and sometimes the left is larger.
The accessory or sympathetic system1 consists of numerous minute ganglia distributed
over the viscera and united by an elaborate plexus of nerve-fibres. I t is not concentrated
or co-ordinated according to any obvious principle, but falls into several divisions
corresponding to the parts of the body with which it is concerned, such as the genital
plexus and the gastro-hepatic plexus. It communicates as described above with the
nerves given off. by the central system. In a freshly killed specimen of D. tuberculata the
ganglia of this system appear as bright orange dots scattered over the viscera. In other
Nudibranchs they are not so conspicuous, but the system probably exists.
R espiratory and Circulatory S ystems.
These consist of the branchiae, heart, blood-vessels, blood-spaces or sinuses, and the
blood-gland. The meaning of the first three terms will be plain. In the Nudibranchiata,
1 See Bela Haller, Beiträge zur Kenntniss der Nerven im Peritoneum von Doris tuberculata,
Yienna, 1884. But the statements contained in this paper have been criticized.