indrical sheaths.

v, The dorsal "hood" formed by an enlargement in this region of the
annular lobe of the fore-foot (m in figs. 2, 3).

V, A swelling of the mantle-skirt, indicating the position on its
inner face of the nidamental gland (see fig. 4, g.n.).]

_Visceral Hump and Shell._--The visceral hump of _Nautilus_ (if we
exclude from consideration the fine siphuncular pedicle which it trails,
as it were, behind it) is very little, if at all, affected by the coiled
form of the shell which it carries, since the animal always slips
forward in the shell as it grows, and inhabits a chamber which is
practically cylindrical (fig. 1). Were the deserted chambers thrown off
instead of being accumulated behind the inhabited chamber as a coiled
series of air-chambers, we should have a more correct indication in the
shell of the extent and form of the animal's body. Amongst Gastropods it
is not very unusual to find the animal slipping forward in its shell as
growth advances and leaving an unoccupied chamber in the apex of the
shell. This may indeed become shut off from the occupied cavity by a
transverse septum, and a series of such septa may be formed, but in no
Gastropod are these apical chambers known to contain a gas during the
life of the animal in whose shell they occur. A further peculiarity of
the nautilus shell and of that of the allied extinct _Ammonites_,
_Scaphites_, _Orthoceras_, &c., and of the living _Spirula_, is that the
series of deserted air-chambers is traversed by a cord-like pedicle
extending from the centro-dorsal area of the visceral hump to the
smallest and first-formed chamber of the series. No structure comparable
to this siphuncular pedicle is known in any other Mollusca. The
siphuncle does not communicate with the coelomic cavity; it is a simple
vascular process of the mantle, whose cavity consists of a venous sinus,
and whose wall contains a ramification of the pallial artery. There
appears to be no doubt that the deserted chambers of the nautilus shell
contain in the healthy living animal a gas which serves to lessen the
specific gravity of the whole organism. This gas is said to be of the
same composition as the atmosphere, with a larger proportion of
nitrogen. With regard to its origin we have only conjectures. Each
septum shutting off an air-containing chamber is formed during a period
of quiescence, probably after the reproductive act, when the visceral
mass of the nautilus may be sli