before and behind the ganglia. The functions of this gland have not been ascertained
with precision. There are indications that it provides some of the constituents of the
blood, as the colourless circulating fluid may be termed for brevity, and that it produces
phagocytic cells which destroy pathogenic microbes.
The structure of the gills or branchise varies greatly in the different families of
Nudibranchs, but in all it seems clear that they only undertake part of the work of
respiration which is also performed by the integuments generally. Respiration means
the process by which the blood is passed through vessels where it is separated from the
water only by a thin membrane, and it is clear that in a thin-skinned shell-less mollusc
this can be done on any part of the dorsal or even lateral surface. But since the majority
of Nudibranchs (especially large Nudibranchs) have specialized gills, it must be necessary
or advantageous to supplement this dermal respiration without suppressing it. In Doi'is
the branchiae, though more highly organized than in any other family, deal only with the
blood going to the main visceral mass (liver, kidney, and hermaphrodite gland): the rest
of the blood passes into a system of intervisceral blood-spaces, thence into the lacunas in'
the integuments, and from them is returned direct to the auricle by two lateral veins.
The pericardium, which contains the heart and communicates with the renal system
through a special valvular apparatus, is placed symmetrically on the upper surface of the
posterior visceral mass. Its inner wall bears some raised folds which have received the name
of pericardial glands. They are not equally developed in all specimens. The heart consists
of an anterior roundish chamber, or ventricle, with thick walls, from which issues the aorta,
and a broader posterior chamber, the auricle, with thin walls, which communicates with the
efferent part of the branchial system, and receives the great lateral blood-sinuses. The
branchiae consist of several (seven to nine) plumes whose structure is somewhat complicated
in this and other species owing to their elaborate subdivision, and is, seen more clearly
in forms like D. verrucosa where each plume is only simply pinnate. But the essence
of the arrangement is the same in all. The impure blood arriving from the hepatic
trunk-vein passes up the inside of the plumes in the afferent blood-vessel and is
distributed throughout the membranous pinnae. I t is then collected again in the
outer or efferent blood-vessel and returns to the auricle through the round efferent
sinus at the base of the branchial circuit. The arrangement of the afferent and efferent
systems in the branchiae is, like so many details in the anatomy of Doi'is, not perfectly
symmetrical (see Text-figure 3). The afferent system does not form a circle but resembles
a horse-shoe bent somewhat asymmetrically round the anal papilla but remaining
open behind,1 and sending branches to the plumes. I t communicates with the lateral
blood-sinuses and the haemoccele. The efferent system describes a complete but still not
quite symmetrical circuit round the anal papilla, and its two ends, though uniting
posteriorly, do so only at one point and not with their whole diameter. The branchiae
can be retracted into their pocket by the action of numerous retractor muscles inserted
partly in the dorsal integuments and partly in the foot. Their protrusion from the
pocket seems to be effected by blood-pressure from the inside.
The aorta, issuing from the ventricle, runs both forwards and backwards. The
1 Its form suggests that it is derived from the arrangement found in many Polycerid® in which
the branchial plumes form a short curved line in front of the anal papilla, cf. Gtenodoris.
posterior aorta divides into two and supplies the visceral mass. The anterior aorta runs
to the anterior end of the body, giving off branches right and left on its course. I t passes
under the buccal mass, into which it sends one branch, and is continued in another branch
as the pedal artery in the floor of the body-cavity. This, after sending off branches in all
directions, finally opens into the great abdominal blood-space. Thence the blood passes
into the system of blood-sinuses in the integuments and is returned to the auricle through
the main lateral blood-sinus. The blood in the visceral mass, however, passes into various
small veins which unite with the main hepatic venous trunk opening into the afferent
system of the branchiae, where the blood is purified as described.
R enal S ystem.1
The general principle underlying the very diverse renal organs of the Mollusca is that
they are canals opening internally into the pericardium and also externally, which extract
and excrete waste products from the blood. In the Nudibranchiata this simple structure
apparently disappears as the organ becomes subdivided and ramified, but the essential
character of a cavity (though often of a most complicated and elaborate shape) provided
with special walls and communicating with the pericardium and the water still remains.
The renal organ in D. tuberculata (see Text-figure 4) is not so complicated as in some of its
allies. It lies on the visceral mass beside the intestine, under the pericardium and over
or among the follicles of the hermaphrodite gland. In shape it is somewhat dendritic,
consisting of a central trunk and five or six main lateral branches as well as numerous
smaller ones. The substance of the kidney consists of fine tubes lined with cells of a
special type, which are also found in the delicate membrane forming the dorsal wall
of the whole organ. Down its main trunk runs a cavity which communicates with
the pericardium through a pear-shaped vesicle (sometimes described as a funnel,
Nierenspritze, or syrinx renalis) with a laminated interior. The description of this
organ given in the Monograph is somewhat confusing, because Alder and Hancock term
it a portal heart, a mistake corrected in subsequent papers. It is really a valve which
allows liquid to pass from the pericardium into the kidney but not to flow in the opposite
direction. Pericardial excretory fluid is drawn off.by the valve and sent through a duct
to the walls of the kidney, which are also traversed by a network of veins in communication
with the hepatic trunk-vein which lies below. Hechta found by experiment that if a
coloured liquid is injected into the blood of Nudibranchs, the colour, at first spread
throughout the tissues, is in a period varying from ten to forty minutes in different forms
localized in the renal organ, and completely eliminated in about forty-eight hours.
The renal cells are large and cubical. Near their base they show a concretion formed
of solid matter consisting of impurities derived from the blood. Near the upper surface
are vacuoles containing a liquid which is excreted by the cells into the renal chamber and
thence carried out of the body by the renal opening, which is a minute orifice within the
branchial circuit close to the anal papilla. According to some authorities other organs
1 See Hancock, “ On the Structure and Homologies of the Renal Organ in the Nud. Mollusca/*
Trans. Linn. Soc. Lond., xxv, 1865.
8 Hecht 1, pp. 648—658.