Cephalopods became dominant during the Ordovician period, represented by primitive nautiloids. The class now contains two, only distantly related, extant subclasses: Coleoidea, which includes octopuses, squid, and cuttlefish; and Nautiloidea, represented by Nautilus and Allonautilus. In the Coleoidea, the molluscan shell has been internalized or is absent, whereas in the Nautiloidea, the external shell remains. About 800 living species of cephalopods have been identified. Two important extinct taxa are the Ammonoidea (ammonites) and Belemnoidea (belemnites). Extant cephalopods range in size from the 10 mm Idiosepius thailandicus to the 700 kilograms heavy colossal squid, the largest extant invertebrate.

There are over 800 extant species of cephalopod, although new species continue to be described. An estimated 11,000 extinct taxa have been described, although the soft-bodied nature of cephalopods means they are not easily fossilised.

Cephalopods are found in all the oceans of Earth. None of them can tolerate fresh water, but the brief squid, Lolliguncula brevis, found in Chesapeake Bay, is a notable partial exception in that it tolerates brackish water. Cephalopods are thought to be unable to live in fresh water due to multiple biochemical constraints, and in their >400 million year existence have never ventured into fully freshwater habitats.

Cephalopods are widely regarded as the most intelligent of the invertebrates and have well-developed senses and large brains (larger than those of gastropods). The nervous system of cephalopods is the most complex of the invertebrates and their brain-to-body-mass ratio falls between that of endothermic and ectothermic vertebrates. Captive cephalopods have also been known to climb out of their aquaria, manoeuvre a distance of the lab floor, enter another aquarium to feed on captive crabs, and return to their own aquarium. Cephalopods have advanced vision, can detect gravity with statocysts, and have a variety of chemical sense organs. Octopodes use their arms to explore their environment and can use them for depth perception.

Despite its name, the vampire squid (which is eaten in some cultures) is closer to the octopus than to true squid. Nautiloids are the most ancient extant species, with octopodiformes (octopodes and vampire squid) evolving subsequently. The decapodiformes, which include the cuttlefish and various groupings of squid are the most recent to evolve.

Unlike most modern cephalopods, most ancient varieties had protective shells. These shells at first were conical but later developed into curved nautiloid shapes seen in modern nautilus species. Which is why cephalopods are often culinarily classed amongst the shellfish. Indeed, Nautiluses are the only extant cephalopods with a true external shell. However, all molluscan shells are formed from the ectoderm (outer layer of the embryo); in cuttlefish (Sepia spp.), for example, an invagination of the ectoderm forms during the embryonic period, resulting in a shell (cuttlebone) that is internal in the adult. The same is true of the chitinous gladius of squid and octopuses. Cirrate octopods have arch-shaped cartilaginous fin supports, which are sometimes referred to as a "shell vestige" or "gladius". The Incirrina have either a pair of rod-shaped stylets or no vestige of an internal shell, and some squid also lack a gladius. The shelled coleoids do not form a clade or even a paraphyletic group. The Spirula shell begins as an organic structure, and is then very rapidly mineralized. Shells that are "lost" may be lost by resorption of the calcium carbonate component.

Cephalopods, particularly octopodes and squid have been used as human food for millennia. The most commonly consumed and cooked cephalopods are octopodes (octopus), squid and cuttlefish (squid and cuttlefish are also used for their ink). However, the nautilus is also consumed, particularly in the Pacific islands. The small vampire squid is also eaten, particularly in Southeast Asia where it's cooked in light soups and broths (but is not commonly seen outside fishing villages).

The alphabetical list of all the Cephalopod-based recipes on this site follows, (limited to 100 recipes per page). There are 36 recipes in total:

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Aliter Sepias (Cuttlefish, Another Way)      Origin: Roman	In Polypo (Of Octopus)      Origin: Roman	Ragoût de chatrou créole (Creole Chatrou Stew)      Origin: Guadeloupe
Alubias blancas con calamar y aceite de romero (White Beans with Squid and Rosemary Oil)      Origin: Spain	In Sepia Farsili (Sauce for Stuffed Cuttlefish)      Origin: Roman	Salata od hobotnice na Prigorski (Prigorski-style octopus salad)      Origin: Croatia
Asado de Tenera (Roast Veal)      Origin: Spain	Indonesian Black Squid Curry      Origin: Indonesia	Seafood Amok      Origin: Cambodia
Caldeirada de Lulas a Madeirense (Madeira Squid Stew)      Origin: Portugal	Isicia de lolligine (Squid Rissoles)      Origin: Roman	Sepias Elixas a Balineo (Boiled Cuttlefish from the Tank)      Origin: Roman
Cari Ourite (Octopus Curry)      Origin: Mauritius	Kalamar Tava (Turkish Fried Calamari)      Origin: Turkey	Shattoo Water      Origin: Dominica
Chipirones à la Criolla (Creole-style Squid)      Origin: Mexico	Kibeba (Cassava and Cuttlefish Stew)      Origin: Mozambique	Sic Farcies eam Sepiam Coctam (Stuffed and Cooked Cuttlefish)      Origin: Roman
Daello Thiyal (Sri Lankan Cuttlefish Curry)      Origin: Sri Lanka	Kibeba de Angola      Origin: Angola	Spagetti biz-zalza tal-Qarnit (Spaghetti with Octopus Sauce)      Origin: Malta
Dhallo Black Curry (Cuttlefish Black Curry)      Origin: Sri Lanka	Octopus Curry      Origin: Seychelles	Squid Salad with Chicory, Anchovies and Sun-dried Tomatoes      Origin: Britain
Embractum Baianum (Baian Stew)      Origin: Roman	Ojinguh Jut (Korean Pickled Squid)      Origin: Korea	Stir-fried Squid with Herbs and Sea Aster      Origin: Fusion
Fricassé de chatrou (Chatrou Fricassee)      Origin: Guadeloupe	Pisum Indicum (Indigo Peas)      Origin: Roman	Stuffat Tal-Qarnit (Octopus Stew)      Origin: Malta
In Lolligine Farsili (Stuffed Squid)      Origin: Roman	Polvo à São Tomé (Sao Tomean Octopus)      Origin: Sao Tome	Taro aux Fruits de Mer (Taro with Seafood)      Origin: Cote dIvoire
In Lolligine in Patina (A Dish of Squid)      Origin: Roman	Polvo a Modo ze de Lino (Octopus Stew)      Origin: Cape Verde	Tom Yum Talay (Fish Stew)      Origin: Thailand

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