i, The siphon.]

_Head, Foot, Mantle and Mantle-cavity._--If we now compare the
fore-foot of the Dibranchiata with that of _Nautilus_, we find in the
first place a more simple arrangement of its lobes, which are either
four or five pairs of tapering processes (called "arms"), arranged in
a series around the buccal cone, and a substitution of suckers for
tentacles on the surface of these lobes (figs. 15 and 24). The most
dorsally placed pair of arms, corresponding to the two sides of the
hood of _Nautilus_, are in reality the most anterior, and are termed
the first pair. In the Octopoda there are four pairs of these arms
(fig. 38), in the Decapoda five pairs, of which the fourth is greatly
elongated (figs. 15, 16). In _Sepia_, _Sepiola_ and _Rossia_, each of
these long arms is withdrawn into a pouch beside the head, and is only
ejected for the purpose of prehension. In _Loligo_ they are completely
retractile, very slightly so in the majority of the Oigopsida, and in
_Rhynchoteuthis_ they are united to form a beak-like appendage. A
gradual reduction of the tentacular arms can be seen in the Decapoda,
leading to their total absence in Octopoda; thus in _Leachia_,
_Chaunoteuthis_ and others these arms are reduced to mere stumps. In
some _Cheiroteuthidae_ and _Cranchiidae_ the ordinary or sessile arms,
especially the dorsal pairs, are reduced. In the Octopoda they are not
unfrequently connected by a web, and form an efficient swimming-bell,
e.g. in _Cirrhoteuthidae_ and _Amphuretidae_. The suckers are placed
on the adoral surface of the arms, and may be in one, two or four
rows, and very numerous. In place of suckers in some genera, e.g.
_Veranya_, we find on certain arms or parts of the arms horny hooks;
in other cases a hook rises from the centre of each sucker. The hooks
on the long arms of _Onychoteuthis_ are drawn in fig. 23. In various
species of _Cheiroteuthis_ the suckers on the tentacular arms are very
feeble, but the bottom of the cup is covered by a number of
anastomosed epithelial filaments which are used as a fishing-net. The
fore-foot, with its apparatus of suckers and hooks, is in the
Dibranchiata essentially a prehensile apparatus, though the whole
series of arms in the Octopoda serve as swimming organs, and in many
(e.g. the common octopus or poulp) the sucker-bearing surface is used
as a crawling organ.

[Illustration: FIG. 25.--Vie