MOLLUSCA). The Cephalopoda are mainly
characterized by the concrescence of the foot and head. The foot grows
forward on each side so as to surround the mouth, the two upgrowths
meeting on the dorsal side of the head--whence the name Cephalopoda. The
perioral portion of the foot is drawn out into paired arm-like
processes; these may be beset with sheathed tentacles or with suckers or
hooks, or both. The epipodia are expanded into a pair of muscular lobes
right and left, which are bent round towards one another so that their
free margins meet and constitute a short tube--the siphon or funnel. The
hind-foot is either very small or absent. A distinctive feature of the
Cephalopoda is their bilateral symmetry and the absence of anything like
the torsion of the visceral mass seen in the Anisopleurous Gastropoda.

The anus, although it may be a little displaced from the median line,
is approximately median and posterior. The mantle-skirt is deeply
produced posteriorly, forming a large sub-pallial chamber around the
anus. By the side of the anus are placed the single or paired
apertures of the nephridia, the genital apertures (paired only in
_Nautilus_, in female Octopoda, female _Ommatostrephes_ and male
_Eledone_), and the paired ctenidia. The visceral hump or dome is
elevated, and may be very much elongated in a direction almost at
right angles to the primary horizontal axis of the foot.

A shell is frequently, but not invariably, secreted on the visceral
hump and mantle-skirt. The shell is usually light in substance or
lightened by air-chambers in correlation with the free-swimming habits
of the Cephalopoda. It may be external or internal, that is, enclosed
in folds of the mantle. Very numerous minute pigmented sacs, capable
of expansion and contraction, and known as chromatophores, are usually
present in the integument. The sexes are separate.

The ctenidia are well developed as paired gill-plumes, serving as the
efficient branchial organs (figs. 4, 24),

The vascular system is very highly developed; the heart consists of a
pair of auricles and a ventricle (figs. 12, 28). Branchial hearts are
formed on the afferent vessels of the branchiae. It is not known to
what extent the minute subdivision of the arteries extends, or whether
there is a true capillary system.

The pericardium is extended so as to form a very large sac, passing
among the viscera dorsalwards and someti