It is colourless in the oxidized, blue in the
deoxidized state, and contains copper as a chemical constituent.

The renal sacs and renal glandular tissue are closely connected with
the branchial advehent vessels in _Nautilus_ and in the other
Cephalopoda. The arrangement is such as to render the typical
relations and form of a renal tube difficult to trace. In accordance
with the metamerism of _Nautilus_ already noticed, there are two pairs
of renal organs. Each assumes the form of a sac opening by a pore to
the exterior. As is usual in renal tubes a glandular and a
non-glandular portion are distinguished in each sac; these portions,
however, are not successive parts of a tube, as happens in other
cases, but they are localized areae of the wall of the sac. The
glandular renal tissue is, in fact, confined to a tract extending
along that part of the sac's wall which immediately invests the great
branchial afferent vein. The vein in this region gives off directly
from its wall a complete herbage of little venules, which branch and
anastomose with one another, and are clothed by the glandular
epithelium of the renal sac. The secretion is accumulated in the sac
and passed by its aperture to the exterior. Probably the nitrogenous
excretory product is very rapidly discharged; in _Nautilus_ a
pink-coloured powder is found accumulated in the renal sacs,
consisting of calcium phosphate. The presence of this phosphatic
calculus by no means proves that such was the sole excretion of the
renal glandular tissue. In _Nautilus_ a glandular growth like that
rising from the wall of the branchial vessel into its corresponding
renal sac, but larger in size, depends from each branchial afferent
vessel into the viscero-pericardial sac and forms the pericardial
gland--probably identical with the "appendage" of the branchial hearts
of Dibranchs.

The chief difference, other than that of number, between the renal
organs of the Dibranchs and those of _Nautilus_, is the absence of the
accessory growths depending into the viscero-pericardial space just
mentioned, and, of more importance, the presence in the former of a
pore leading from the renal sac into the viscero-pericardial sac (y,
y' in fig. 29). The external orifices of the renal organs are also
more prominent in Dibranchs than in _Nautilus_, being raised on
papillae (np in fig. 29; r in fig. 25). In _Sepia_ the two r