In the Dibranchiata the shell shows various stages of degeneration,
culminating in its complete disappearance in _Octopus_. As in other
Mollusca, there is a tendency in Cephalopods for the mantle to extend
over the outside of the shell from its edges, and when these secondary
mantle-folds entirely cover the shell and meet or fuse together the
shell is surrounded by the mantle both externally and internally, and is
said to be internal, though it remains always a cuticular structure
external to the epidermis. This procebs is generally accompanied by a
reduction of the size of the shell in comparison with that of the body,
so that the relations of the two are gradually reversed, the body
outgrows its house and instead of the mantle being enclosed by the
shell, the shell is enclosed by the mantle. The earliest stage of this
process is shown in the recent _Spirula,_ though it is perhaps not
impossible that in some of the later fossil Ammonoids the shell was
becoming more and more internal. The shell of _Spirula_ (fig. 18) is
coiled somewhat like that of _Nautilus_, but the coils are not in
contact, the direction of the coil is endogastric or ventral instead of
exogastric, and the shell is very much smaller than the body. Like that
of _Nautilus_ it is divided by septa and traversed by a siphuncle. The
relation of the animal to the terminal chamber is as in _Nautilus,_ but
the body extends far beyond the aperture, and folds of the mantle grow
up over the shell and cover it everywhere except part of the dorsal and
ventral surfaces.
[Illustration: After Chun, from Lankester's _Treatise an Zoology_