margin of the mantle-skirt, the middle letter d points
to that portion of the mantle-skirt which is reflected over a part
of the shell as seen in fig. 1, b; the cup-like fossa to which b and
d point in the present figure is occupied by the coil of the shell.

g.a. points to the lateral continuation of the nuchal plate b to
join the root of the mid-foot or siphon.]

[Illustration: FIG. 3.--Lateral view of the same specimen as that
drawn in fig. 2. Letters as in that figure with the following
additions--

e, points to the concave margin of the mantle-skirt leading into the
sub-pallial chamber.

g, The mid-foot or siphon.

k, The superficial origin of its retractor muscles closely applied
to the shell and serving to hold the animal in its place.

l, The siphuncular pedicle of the visceral hump broken off short.

v, v, The superior and inferior ophthalmic tentacles.]

The apertures of the two pairs of renal sacs, of the
viscero-pericardial sac, of the genital ducts, and of the anus, are
shown in position on the body-wall of the pallial chamber of
_Nautilus_ in figs. 4, 5. There are nine apertures in all, one median
(the anus) and four paired. Besides these apertures we notice _two_
pairs of gill-plumes which are undoubtedly typical ctenidia, and a
short papilla (the osphradium) between each anterior and posterior
gill-plume (see figs. 4, 5, and explanation). As compared with this in
a Dibranchiate, we find (fig. 25) only four apertures, viz. the median
anus with adjacent orifice of the ink-sac, the single pair of renal
apertures, and one asymmetrical genital aperture (on the left side)
except in female Octopoda and a few others, where the genital ducts
and their apertures are paired. No viscero-pericardial pores are
present on the surface of the pallial chamber, since in the
Dibranchiata the viscero-pericardial sac opens by a pore into each
nephridium instead of directly to the surface. A single pair of
ctenidia (gill-plumes) is present instead of the two pairs in
_Nautilus_. The existence of two pairs of ctenidia and of two pairs of
renal sacs in _Nautilus_, placed one behind the other, is highly
remarkable. The interest of this arrangement is in relation to the
general morphology of the Mollusca, for it is impossible to view this
repetition of organs in a linear series as anything else than an
instance of metameric