RENAL ORGAN IN THE NUDIBRANCHIATE MOLLUSCA. 513 
less branched than in the preceding species, and on the left side there is an angular lobe, 
the margins of the whole being somewhat sinuous. In all three the chamber extends 
from one extremity of the liver to the other; it is rather wide, and its walls are richly 
supplied with an arterial network composed of twigs from numerous trunks passing from 
both sides of the aorta, which runs forward in the roof of the chamber. In D. repanda 
these trunks are small and very numerous, and branch off from each side of the aorta 
with a considerable degree of symmetry. Two trunks, larger than the rest, extend back- 
wards parallel to each other. In D. pilosa and D. bilamellata the arterial plexus is 
arranged much in the same manner, but scarcely so symmetrically. In the former the 
network in the roof of the chamber is remarkably rich and minute. 
The chamber is lined throughout with a spongy glandular tissue which is thickest over 
the traeks of the blood-vessels, and presents, particularly in 2. repanda, a very peculiar 
honeycombed appearance, caused by slightly elevated lines of membrane enclosing five- 
or six-sided spaces, each holding a single large, clear, globular vesicle, containing a few 
smaller cells of different sizes, together with some granules. 
The so-called pericardium (Pl. LIV. fig. 1, a) lies, as has been already stated, imme- 
diately above the renal chamber and directly below the dorsal skin in front of the 
branchial circle. It is, with the exception of the opening leading into the pyriform 
vesicle, a closed membranous sac, formed apparently by what has been designated the 
peritoneum, and is just”sufficiently large for the accommodation of the dilated auricle and 
. ventricle. Tt is lined with its own proper membrane, which is closely adherent to, and 
intimately confounded with, the peritoneal membrane, but can be observed reflected 
upon the heart at the root of the aorta. It therefore apparently encloses that viscus in 
à fold somewhat in the manner of a serous membrane. It has just been stated that this 
cavity is closed. In the paper on Doris before alluded to it was described as communi- 
cating with the great abdominal chamber or peritoneal cavity by numerous minute 
punctures in its floor. I am now satisfied, however, that this was a mistake arising from 
the specimens examined having been slightly decomposed. The walls of the pericardium, 
as we have seen, are composed of the peritoneum and the lining membrane: the former 
in the floor of the organ is comparatively robust, and is provided with muscular fibres 
mostly transverse; the latter is exceedingly delicate, and the consequence 1s that, when 
the specimen is in a soft state and the slightest strain is used, the fibres separate a little 
from each other, and the lining membrane gives way. Thus small openings occur which 
have much the appearance of being natural. 
The pyriform vesicle (Pl. LIV. figs. 1, 2, 3) in some species is nearly half the size of 
the contracted ventricle, and, as already stated, connects the pericardial chamber with 
the renal chamber proper. It lies transversely between these two organs towards the 
right side, with the rounded extremity opening upwards through the floor of the peri- 
cardium immediately in front of the right angle of the auricle, close to the point where 
the latter receives the great vein from the skin. The aperture is circular, and is appa- 
rently capable of being closed as if by a sphincter. The narrow extremity of the vesicle 
seems to penetrate the roof of the renal chamber on the right side towards the posterior 
end, and, bulging into the organ, becomes cemented as it were to the floor of the chamber. 
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