Dorids, and consists of three pail's of large ganglia set on the upper side of the oesophagus
and united by connectives (long or short) to various smaller ganglia. The cerebro-pleural
ganglia are always fused into a single mass, but whereas this mass appears in many
species as an undivided egg-shaped body, in JE. papillosa the cerebral and pleural portions
are separated by a distinct furrow so that the oesophagus when seen from above appears
to bear six white lumps. The pedal ganglia are round and set more at the side of the
oesophagus than the others, to which they are united by two very short connectives on
either side, one going to the cerebral and the other to the pleural portion. These ganglia
are fastened round the oesophagus by three bands. The first unites the cerebral ganglia
to the buccal and to the gastro-oesophageal ganglia which are set as in Doris on the
lower side of the oesophagus. The second (called the pleural or visceral commissure)
unites the right and left pleural ganglia. I t bears no ganglia, but from its right hand
portion it gives off an unpaired nerve which runs to the genitalia. This arrangement
points to the disappearance or diffusion of an unpaired visceral ganglion such as is
found in Doris.1 The third band is shorter and thicker than the others. I t connects
the two pedal ganglia and generally contains within a common sheath the pedal and subcerebral
commissures. But in JE. pcipillosa, I have not been able to find these two commissure^
distinctly marked. As in Doris the cerebral ganglia are connected with the eyes,
otocysts, and olfactory ganglia. The last named lie in the dorsal integuments at the base
of the rhinophores, into which they send several nerves. They are large and are united
to the cerebral ganglia by rather long connectives. The eyes are not quite sessile but
are set on very short processes. The otocysts contain numerous otoliths. The cerebral
ganglia also innervate the oral tube, lips, and tentacles. The pleural ganglia each send a
nerve along the side of the body, and the pleural commissure gives off the visceral nerve.
The pedal ganglia give off two nerves on either side which innervate the foot and also the
cerata.2 The buccal ganglia innervate the buccal mass, and the gastro-oesophageal ganglia
send nerves to the oesophagus and also backwards to the viscera. The accessory (so-called
sympathetic) system of nerves has not been described for any iEolid so definitely as for
Doris, since being colourless it is most difficult to detect. But in several JEolids fragments
of a network of nerves, with minute ganglia situated on the junctions, have been
found, and it is probable that the oesophagus, stomach, liver, and genitalia are all covered
with this reticulate system which forms a series of plexuses around the principal organs.
R espiratory and Circulatory System.
The arrangement of this system is not entirely the same in the ^Solids and Dorids.
The latter have general dermal respiration and also branchiae localized in a special region.
But though the cerata of iEolids may be regarded as branchiae, they are distributed over
the dorsal surface, so that this surface taken as a whole acts as one large respiratory
organ, thickly studded with points of special activity.
1 At the point where the genital nerve leaves the commissure are found, according to Bergh, in
Facelina ganglion cells and in RizzoUa modeata a small ganglion.
2 The innervation of the cerata is not the same in all ASolids. The nerves come sometimes from
the pedal ganglion only, sometimes from the pleural as well. See Herdman and Clubb, Quart.
Joum. Micr. Sci., 1891, pp. 550—555.
About the middle of the back can be seen externally a large oval prominence known
as the pericardial bulb. Within it is the thin pericardium containing the heart, which
consists as usual of a ventricle and auricle. The former, which lies in front, is conspicuous
in preserved specimens as a small (4—5 mm. long), opaque, fleshy organ, oval or
pear-shaped, with muscular walls. The lumen is reduced by the presence of anastomosing
fleshy columns. I t opens into the wider but much more delicate auricle which lies behind
and a little below it, the communication between the two chambers being controlled by a
valve consisting of two flaps. When seen in outline the auricle appears not as a simple
bag but as a ramified organ, since it receives broad venous trunks (with branches) on
either side and behind. The aorta is large and divided much as in Doris. The posterior
branch runs above the hermaphrodite gland into which it sends numerous branches ;
the anterior branch sends the pedal artery into the foot, while in the upper part it
divides into the cephalic and genital arteries which supply blood to the mouth parts and
anterior genital mass respectively. The cephalic artery is again subdivided into two
branches. The arteries terminate in the various viscera and in the foot from which the
blood is returned into the general body-cavity or hasmocele. Thence the blood passes
into the integuments and from them into the cerata. These latter have on either side of
the column of liver substance which they contain a blood-passage, and the two blood-
passages are connected by numerous delicate cross-canals. I t appears that the blood
circulating in the skin and specially in the cerata supplies the hepatic diverticula and
at the same time is purified itself, being divided from the surrounding water by membranes
no thicker than the branchial of Doris.1 The blood passes from the cerata and
integuments into various small veins which ultimately unite to form three main trunks.
These enter the auricle one on each side and one behind. The posterior trunk receives
two small branches from below.
T he E xcretory S ystem.
The kidney of JEolidia is an. elaborately ramified tubular organ. It has been
described and figured in greatest detail by Hecht,2 who was able to follow its ramifications
by injecting living animals with coloured fluids which are absorbed by the kidney,
causing it to appear as a coloured pattern. I have not been able to follow in preserved
specimens all the ramifications figured by Hecht, but dissection shows that under the pericardium
is a renal organ consisting of numerous minute colourless tubules lined with cells
containing concretions. The general structure of this organ, though not its superficial
appearance, is as in Doris. I t lies between the pericardium and the ramifications of the
alimentary canal, and, since the latter do not afford a solid support like the liver of
Doris, its free portions , tend towards the dorsal integuments rather than downwards. The
whole organ has roughly the shape of the letter Y surrounded by a mass of branching
tubules, the limbs of the Y forming the chambers where the excreted products
collect in order to be expelled through the renal pore. The renal apparatus connects, as
1 In jFiona the cerata bear a distinct branchial membrane along their whole length.
2 Hecht 1, pp. 686—642, and PI. iv, figs 42—45.