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 sometimes containing the ovary or testis—the viscero-pericardial sac—which opens to the exterior either directly or through the renal organs. It has no connexion with the vascular system. The renal organs are always paired sacs, the walls of which invest the branchial afferent vessels (figs. 28, 29). They open each by a pore into the viscero-pericardial sac, except in Nautilus. The anal aperture is median and raised on a papilla. Jaws (fig. 6, e) and a radula (fig. 9) are well developed. The jaws have the form of powerful beaks, either horny or calcified (Nautilus), and are capable of inflicting severe wounds.
Cerebral, pleural and pedal ganglia are present, but the connectives are shortened and the ganglia concentrated and fused in the cephalic region. Large special ganglia (optic, stellate and supra-buccal) are developed. Sense-organs are highly developed; the eye exhibits a very special elaboration of structure in the Dibranchiata, and a remarkable archaic form in the nautilus. Otocysts are present in all. The typical osphradium is not present, except in Nautilus, but other organs are present in the cephalic region, to which an olfactory function is ascribed both in Nautilus and in the other Cephalopoda.
Hermaphroditism is unknown in Cephalopoda; male and female individuals always being differentiated. The genital aperture and duct is sometimes single, when it is the left; sometimes the typical pair is developed right and left of the anus. The males of nearly all Cephalopoda have been shown to be characterized by a peculiar modification of the arm-like processes or lobes of the fore-foot, connected with the copulative function. The term hectocotylization is applied to this modification (see figs. 6, 24). Elaborate spermatophores or sperm-ropes are formed by all Cephalopoda, and very usually the female possesses special capsule-forming and nidamental glands for providing envelopes to the eggs (fig. 4, g.n.). The egg is large, and the development is much modified by the presence of an excessive amount of food-material diffused in the protoplasm of the egg-cell. Trochosphere and veliger stages of development are consequently not recognizable.
The Cephalopoda are divisible into two orders, Tetrabranchiata and Dibranchiata, the names of which (due to Sir R. Owen) describe the number of gill-plumes present; but in fact there are several characters, of as great importance as those derived from the gills, by which the members of these two orders are separated from one another.
Order 1. Tetrabranchiata (= Schizosiphona, Tentaculifera).
Characters.—The inrolled lateral margins of the epipodia are not fused, but form a siphon by apposition (fig. 4). The circumoral lobes of the fore-foot carry numerous retractile tentacles, not suckers (fig. 6). There are two pairs of ctenidial gills (hence Tetrabranchiata), and two pairs of renal organs, consequently four renal apertures (fig. 4). The viscero-pericardial chamber opens by two independent apertures to the exterior, and not into the renal sacs. There are two oviducts (right and left) in the female, and two sperm-ducts in the male, the left duct in both sexes being rudimentary. A large external shell, either coiled or straight, is present, and is not enclosed by reflections of the mantle-skirt. The shell consists of a series of chambers, the last-formed of which is occupied by the body of the animal, the hinder ones (successively deserted) containing gas (fig. 1). The pair of cephalic eyes are hollow chambers (fig. 14. A), opening to the exterior by minute orifices (pin-hole camera), and devoid of refractive structures. A pair of osphradia are present at the base of the gills (fig. 4, olf). Salivary glands are wanting. An ink-sac is not present. Branchial hearts are not developed on the branchial afferent vessels.