282 MR. J. D. MACDONALD ON THE ANATOMY OF NAUTILUS UMBILICATUS 
Now that grave doubts have been cast upon the existence of the so-called epithe- 
lial investment of the Malpighian tufts of the kidney in Vertebrata, the office of these 
minute vascular bodies would seem to present a solitary example of the secretion of 
a peculiar fluid directly from the blood, or independently of the agency of nucleated 
cells. The glandular follicles of the Nautilus just described appear, as it were, to 
go one step further, the vascular and secreting portions having so far coalesced as 
to be almost undistinguishable the one from the other. When the Malpighian tufts 
are excessively distended with the contained blood, the albuminous elements pass away 
with the thinner parts, and doubtless congestion of the renal follicles of the Nautilus 
would be attended by a similar result*. 
These views may be still further supported by glancing at the relationships of the 
kidney, liver, and respiratory organs in the Vertebrata and Mollusca respectively. 
Thus, in vertebrate animals the biliary fluid is secreted from venous blood supplied 
by the portal system. In Mammals the secerning vessels of 'the kidney are chiefly 
arterial ; but in Fishes, which possess a distinct portal system in connexion with the 
kidney, the urine is principally separated from venous blood, which ultimately com- 
mingles with that returning from the liver before reaching the branchial heart. 
In Mollusca, on the contrary, the biliary secretion is furnished from arterial blood; 
and if the glandular follicles of the Nautilus above noticed are veritable renal organs, 
as they evidently appear to be, the kidney exchanges place, as it were, with the liver, 
lying between the great sinus system and the branchiae, which return their blood into 
a systemic heart. 
It will be perceived, therefore, that in these respects a remarkable difference exists 
between the Mammalia and Mollusca; but the steps of transition from the one to 
the other are so distinctly marked in the intervening classes, taken in their natural 
order, that we are enabled more fully to comprehend the nature of this disparity; 
the apparently anomalous position of the renal glands in Nautilus and Sepia, and in- 
deed also in certain Gasteropoda and Conchifera, in which their function has been 
more satisfactorily determined, being reconciled with the relative anatomy of those 
organs in animals of higher grades. 
The body of N. umbilicatus is larger and more elongated than that of N. Pompi- 
lins, as it occurs in the South Seas, although the specimens of the latter species 
brought from the Chinese Seas much exceed both in size. In the N. umbilicatus the 
longitudinal lamellse on the median lobe of the external labial processes are divided 
by a wide groove into two distinct lateral sets, and the corresponding lamellse be- 
tween the internal labial processes are about seventeen in number and of consider- 
able thickness. In N. Pompilius the latter lamellse are much thinner and more 
numerous, and the lateral sets of the former are united together in the median line, 
* On comparing these follicles with their spongy homologues in Sepia, for example, one cannot fail to ob- 
serve in them a relationship similar to that existing between the lobulated renal organ of the Porpoise and the 
more condensed and perfect kidney of Man. 
