228 MESSRS. HANCOCK AND EMBLETON ON THE ANATOMY OF DORIS. 
This cavity is circumscribed by a delicate membrane, which is found beneath the 
pericardium, and has the aorta running along its roof, and the great hepatic vein in 
the liver beneath it. It is intimately adherent to and undistinguishable from the 
capsule of the liver-mass. In the wall of the cavity lies the network from the two 
sources already named. The inner surface of the whole of this membrane is covered 
with a fine spongy-Iooking tissue*, which is most abundant over the tracks of the 
blood-vessels ; examined under the microscope, this tissue presents, particularly in 
D. 7'epanda^, a remarkable honeycombed appearance, produced by slightly elevated 
lines of membrane, enclosing irregular 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. In D. tuherculata a similar spongy tissue exists, but not so 
strongly marked. Considering that we have here a branched tube with a fine net- 
work of arterial and venous twigs on its wall, and the lining membrane evidently of 
a glandular nature, though of unusual form, there seems little room for doubt that 
this is an apparatus for the elaboration of some fluid from the blood ; further, we find 
that this tube opens externally, and that the position of the orifiee is close to the 
anus : hence we infer that this organ is one for excretion, and we have little hesitation 
in pointing it out as the renal organ of Doris. To return for a moment to the vesicle 
of Cuvier, it is now evident that it may with propriety be termed an accessory renal 
or hepatic heart ; for its function is to propel venous blood, first to the renal and then 
to the hepatic organ ; but if we may reason from what has been positively ascertained 
in the Vertebrata, we are inclined to consider it rather as belonging to the latter than 
to the former organ, particularly as we find the renal organ in Doris has not acquired 
that complete speciality and independence of other organs which it has attained in 
higher animals. 
In recapitulation of what has been said, we will now endeavour to follow the course 
of the circulation in Doris. The principal or systemic heart propels a mixed stream 
of blood, which has come partly from the skin and partly from the branchial circle, 
and which therefore is not completely aerated, to all the viscera and the foot. This 
blood, with the exception of a small portion sent to the pericardium, passes from the 
viscera (except the liver, ovarium and kidney) and from the foot through the inter- 
visceral sinuses, the common visceral cavity and the network of canals in the skin, 
by two lateral veins into the auricle. The liver, ovary and kidney are supplied with 
a current of blood brought to them, partly by the branches of the aorta, and partly 
by those of the portal heart, to which the pericardium acts as an auricle. This current, 
which on its passage to the liver acquires an additional venous character by traversing 
the walls of the renal sinus, penetrates the liver either through sinuses or capillary 
vessels, and is conveyed along a system of hepatic veins into the great hepatico- 
branchial trunk or afferent branchial vein ; by this it reaches the branehiae, whence, 
after having undergone the influence of the surrounding medium, it is returned to 
* Plate XIV. fig. 5. t Plate XIV. fig. 6. 
