[Illustration: FIG. 15.--_Sepia officinalis_, L., about ½ natural size,
as seen when dead, the long prehensile arms being withdrawn from the
pouches at the side of the head, in which they are carried during life
when not actually in use. a. Neck; b, lateral fin of the mantle-sac; c,
the eight shorter arms of the fore-foot; d, the two long prehensile
arms; e, the eyes.]
_Characters_.--Cephalopoda in which the inflected margins of the
epipodia are fused so as to form a complete tubular siphon (fig. 24, i).
The circumoral lobes of the fore-foot carry suckers disposed upon them
in rows, _not_ tentacles (see figs. 15, 24). There is a single pair of
typical ctenidia (fig. 25) acting as gills (hence Dibranchiata), and a
single pair of renal organs, opening by apertures right and left of the
median anus (fig. 25, r) and by similar internal pores into the
pericardial chamber, which consequently does not open directly to the
surface as in _Nautilus_. The oviducts are sometimes paired right and
left (Octopoda, Oigopsida), sometimes that of one side only is developed
(Myopsida). The sperm-duct is always single except, according to W.
Keferstein, in _Eledone moschata._
A plate-like shell is developed in a closed sac formed by the mantle
(figs 20, 21), except in the Octopoda, which have none, and in _Spirula_
(fig. 17, D) and the extinct _Belemnitidae_, &c., which have a small
chambered shell resembling that of _Nautilus_ with or without the
addition of plate-like and cylindrical accessory developments (fig. 17,
A, C, fig. 19).
The pair of cephalic eyes are highly-developed vesicles with a
refractive lens (fig. 33), cornea and lid-folds,--the vesicle being in
the embryo, an open sac like that of _Nautilus_ (fig. 34). Osphradia are
not present, but cephalic olfactory organs are recognized. One or two
pairs of large salivary glands with long ducts are present. An ink-sac
formed as a diverticulum of the rectum and opening near the anus is
present in all Dibranchiata (fig. 25, t), and has been detected even in
the fossil _Belemnitidae._ Branchial hearts are developed on the two
branchial afferent blood-vessels (fig. 28, _vc'_, _vi_).
[Illustration: FIG. 16.--Decapodous Cephalopods.
A, _Cheiroteuthis Veranyi_, d'Orb. (from the Mediterranean).
B, _Thysanoteuthis rhombus_, Troschel (from Messina).