along the whole base-line
of each lateral fin of the mantle (fig. 15), is a "basi-pterygial
cartilage." It is worthy of remark that we have, thus developed, in
Dibranch Cephalopods a more complete internal cartilaginous skeleton
than is to be found in some of the lower vertebrates. There are other
instances of cartilaginous endo-skeleton in groups other than the
Vertebrata. Thus in some capito-branchiate Chaetopods cartilage forms
a skeletal support for the gill-plumes, whilst in the Arachnids
(_Mygale_, _Scorpio_) and in _Limulus_ a large internal cartilaginous
plate--the ento-sternite--is developed as a support for a large series
of muscles.

[Illustration: FIG. 8.--Cartilaginous skeleton of Cephalopoda (after
Keferstein.)

A, Capito-pedal cartilage of _Nautilus pompilius._

a points to the ridge which supports the pedal portion of the
nerve-centre.

B, Lateral view of the same--the large anterior processes are sunk
in the muscular substance of the siphon.

C, Cephalic cartilages of _Sepia officinalis_.

D, Nuchal cartilage of _Sepia officinalis_.]

_Alimentary Tract._--The buccal cone of _Nautilus_ is terminated by a
villous margin (buccal membrane), surrounding the pair of beak-like
jaws, of which the ventral projects over the dorsal. These are very
strong and dense in _Nautilus_, being calcified. Fossilized beaks of
Tetrabranchiata are known under the name of rhyncholites. In Dibranchs
the beaks are horny, but similar in shape to those of _Nautilus_. They
resemble in general those of a parrot, the lower beak being the larger
and overlapping the upper or dorsal beak. The lingual ribbon and
odontophoral apparatus have the structure which is typical for
Glossophorous Mollusca. In fig. 9, A is represented a single row of
teeth from the lingual ribbon of _Nautilus_, and in fig. 9, B, C, of
other Cephalopoda.

In _Nautilus_ a long and wide crop or dilated oesophagus (fig. 10, cr)
passes from the muscular buccal mass, and at the apex of the visceral
hump passes into a highly muscular stomach, resembling the gizzard of
a bird (fig 10, gizz). A nearly straight intestine passes from the
muscular stomach to the anus, near which it develops a small caecum.
In other Cephalopods the oesophagus is usually narrower and the
muscular stomach more capacious, whilst a very important feature in
the alimentary tract is formed by the ca