segmentation, comparable to the segmentation of
the ringed worms and Arthropods. The only other example which we have
of this metamerism in the Mollusca is presented by the Chitons. There
we find not two pairs of ctenidia merely, but sixteen pairs (in some
species more) accompanied by a similar metamerism of the dorsal
integument, which carries eight shells. In _Chiton_ the renal organs
are not affected by the metamerism as they are in _Nautilus_. It is
impossible on the present occasion to discuss in the way which their
importance demands the significance of these two instances among
Mollusca of incomplete or partial metamerism; but it would be wrong to
pass them by without insisting upon the great importance which the
occurrence of these isolated instances of metameric segmentation in a
group of otherwise unsegmented organisms possesses, and the light
which they may be made to throw upon the nature of metameric
segmentation in general.

[Illustration: FIG. 4.--View of the postero-ventral surface of a
female Pearly Nautilus, the mantle-skirt (c) being completely
reflected so as to show the inner wall of the sub-pallial chamber
(drawn from nature by A.G. Bourne).

a, Muscular band passing from the mid-foot to the integument.

b, The valve on the surface of the funnel, partially concealed by
the inrolled lateral margin of the latter.

c, The mantle-skirt retroverted.

an, The median anus.

x, Post-anal papilla of unknown significance.

g.n., Nidamental gland.

r.ov, Aperture of the right oviduct.

l.ov, Aperture of the rudimentary left oviduct (pyriform sac of
Owen).

neph.a, Aperture of the left anterior renal sac.

neph.p, Aperture of the left posterior renal sac.

viscper, Left aperture of the viscero-pericardial sac.

olf, The left osphradium placed near the base of the anterior
gill-plume.

The four gill-plumes (ctenidia) are not lettered.]

The foot and head of _Nautilus_ are in the adult inextricably grown
together, the eye being the only part belonging primarily to the head
which projects from the all-embracing foot. The fore-foot or front
portion of the foot has the form of a number of lobes carrying
tentacles and completely surrounding the mouth (figs. 2, 3). The
epipodia incline towards each other posteriorly so as to form an
incomplete siphon (fig. 4), a condition which is comp