Cephalopods vary from elongate, streamlined oceanic organisms to saccular, slow-moving bottom and drifting forms. Their body plan is indicative in many ways of the habitat in which they dwell and their mode of life.

The viscera of a generalized cephalopod are covered by a dome-shaped or elongated sheath of muscle, the mantle, which is connected with the head anteriorly. Ventrally, the mantle is free and encloses the mantle cavity, the space into which the gills project and the excretory and reproductive systems open.

Anterior to the mantle is the head-foot, which bears a ventral muscular tube, the funnel. Surrounding the mouth are eight long, prehensile arms, provided with suckers in octopuses and argonauts (order Octopoda), or eight arms and two tentacles, equipped with horny ringed suckers bearing teeth or hooks in squids and cuttlefishes (orders Teuthoidea and Sepioidea). The primitive Nautilus has about 90 small suckerless appendages.

The fossil nautiloids and ammonites (represented today only by Nautilus) were primitive, less-specialized forms, probably leading a rather inactive sluggish life. The modern octopuses, squids, and cuttlefishes have acquired an active, vigorous life that has led to marked departures in structure and function from the type represented by Nautilus. Modern forms are divided into three basic life-styles: the sluggish life in the great depths of the sea, a floating life in the midwaters, and a more active, aggressive existence near the surface. The nautiloids and ammonites were probably shallow-water animals living near the bottom and, like the slow-moving Nautilus, relied for protection on a coiled or curved calcareous external shell.

All cephalopods have an internal cartilaginous covering of the consolidated ganglia of the nervous system. In all except ammonites and nautiloids, it constitutes a cranium. Various other skeletal supports are found at the base of the fins and in the “neck,” gills, and arms.

The alimentary system consists of a buccal mass with a pair of jaws (mandibles) and a rasping tongue (radula), esophagus, salivary glands, stomach, cecum, digestive gland (“liver”), intestine, and anus. In the Octopoda the esophagus may be expanded to form a crop, while in the deep-sea Cirrata the radula may be degenerate or lacking. Except in Nautilus and certain deep-sea Octopoda there is an ink sac located near the anus. This secretes a dark fluid, the sepia or ink, which is forcibly ejected through the funnel.

All members of the Coleoidea (octopuses, squids, and cuttlefishes) possess a closed circulatory system of blood vessels; in Nautilus it is partly lacunar (i.e., made up of sinuses). The blood contains a blue respiratory pigment, hemocyanin (a copper compound), dissolved in the plasma. There are three hearts, the main systemic heart and the two branchial hearts, one at the base of each gill. The rhythmical contractions and expansions of the mantle cause a circulation of water over the gills where gas exchange takes place between the seawater and blood. The featherlike gills, consisting of a central axis with a row of lamellae on either side, are suspended in the mantle cavity from the mantle wall.

The excretion of nitrogenous wastes is carried out exclusively by the kidneys. There are four kidneys in Nautilus and two in the coleoids.

The central nervous system is highly developed, the major ganglionic centres being concentrated in the head. In some of the cuttlefishes (Sepioidea) the cerebral centres are subdivided for specialization, with the pedal ganglia divided into brachial and epipodal elements that innervate the arms and funnel respectively. In active squids the mantle is innervated by giant paired dorsal axons. Much of the present knowledge of mechanisms of nerve impulse conduction has come from the study of these giant axons. The sense organs of the cephalopods are eyes, rhinophores (olfactory organs), statocysts (organs of equilibrium), and tactile organs. In Nautilus the eyes are open pits without lenses. In the Coleoidea the eyes are complex and approach those of some lower vertebrates in efficiency.

Evolution and paleontology

The first cephalopods were probably provided with a simple, caplike shell. With elongation of the shell and the formation of septa or partitions, the nautiloid shell could be formed (Late Cambrian, 510 million to 488.3 million years ago). The primitive elongate shell of Orthoceras became unmanageable and coiling resulted, as in the Gastropoda.

The Ammonoidea are usually considered to have evolved from Devonian (416 million to 359.2 million years ago) straight-shelled forms (Bactrites) with certain nautiloid traits. Coiled ammonites appeared in the Late Devonian Epoch (Goniatites). The subclass became extinct in the Cretaceous.

Modern cephalopods possess an internal and partly degenerate shell, straight except in Spirula. The state of the shell in modern forms is due to the progressive overgrowth of it by the mantle, probably accompanying the evolution of an active swimming life. The first evidence of the modification of the shell is in Aulococeras in the Triassic Period (251 million to 199.6 million years ago). The belemnites, with their modified, internal shell, gave rise to Spirula (coiled shell), to cuttlefishes (calcified phragmoconal septa, forming a thick shell), and to the squids (only the proostracum as a horny “pen”). These forms appeared in the Jurassic Period (199.6 million to 145.5 million years ago) and were probably derived from belemnite-like ancestors.

In the Octopoda the shell persists as cartilaginous stylets or fin supports. Palaeoctopus newboldi, the oldest known octopod, from the Cretaceous of Syria, was already too advanced to provide a clue to the derivation of the Octopoda. The Vampyromorpha are considered to be a possible connecting link between the Teuthoidea and the Octopoda.