ghtly shrunk, and gas is secreted from
the dorsal integument so as to fill up the space previously occupied by
the animal. A certain stage is reached in the growth of the animal when
no new chambers are formed. The whole process of the loosening of the
animal in its chamber and of its slipping forward when a new septum is
formed, as well as the mode in which the air-chambers may be used as a
hydrostatic apparatus, and the relation to this use, if any, of the
siphuncular pedicle, is involved in obscurity, and is the subject of
much ingenious speculation. In connexion with the secretion of gas by
the animal, besides the parallel cases ranging from the protozoon
_Arcella_ to the physoclistic fishes, from the hydroid _Siphonophora_ to
the insect-larva _Corethra_, we have the identical phenomenon observed
in the closely allied _Sepia_ when recently hatched. Here, in the pores
of the internal rudimentary shell, gas is observable, which has
necessarily been liberated by the tissues which secrete the shell, and
not derived from any external source (Huxley).

The coiled shell of _Nautilus_, and of the majority of extinct
Tetrabranchiata, is peculiar in its relation to the body of the animal,
inasmuch as the curvature of the coil proceeding from the centro-dorsal
area is towards the head or forwards, instead of away from the head and
backwards as in other discoid coiled shells such as _Planorbis_; the
coil is in fact absolutely reversed in the two cases. Such a shell is
said to be exogastric. But in some extinct forms, e.g. _Phragmoceras_,
_Cyrtoceras_, _Ptenoceras_, the shell is coiled towards the ventral
side, when it is termed endogastric. Amongst the extinct allies of the
nautilus (Tetrabranchiata) we find shells of a variety of shapes, open
coils such as _Scaphites_, leading on to perfectly cylindrical shells
with chamber succeeding chamber in a straight line (_Orthoceras_),
whence again we may pass to the corkscrew spires formed by the shell of
_Turrilites_. In some extinct genera, e.g. _Gomphoceras_, among the
Nautiloidea the aperture of the shell is contracted and the edge of the
aperture is lobed. In these cases the animal was probably able only to
protrude its appendages and not its whole head. The ventral part of the
aperture corresponding to the funnel is separated from the dorsal part
by a constriction. Hence it is possible to distinguish the ventral and
dorsal sides of the shell and to decide whether it was exogastri