An octopus is a soft-bodied, eight-limbed mollusc of the order Octopoda. The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefish, and nautiloids. Like other cephalopods, an octopus is bilaterally symmetric with two eyes and a beaked mouth at the centre point of the eight limbs. The soft body can radically alter its shape, enabling octopuses to squeeze through small gaps. They trail their eight appendages behind them as they swim. The siphon is used both for respiration and for locomotion, by expelling a jet of water. Octopuses have a complex nervous system and excellent sight, and are among the most intelligent and behaviourally diverse of all invertebrates.
A squid is a mollusc with an elongated soft body, large eyes, eight arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida. Like all other cephalopods, squid have a distinct head, bilateral symmetry, and a mantle. They are mainly soft-bodied, like octopuses, but have a small internal skeleton in the form of a rod-like gladius or pen, made of chitin.
A cephalopod is any member of the molluscan class Cephalopoda such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot. Fishers sometimes call cephalopods "inkfish", referring to their common ability to squirt ink. The study of cephalopods is a branch of malacology known as teuthology.
The vampire squid is a small cephalopod found throughout temperate and tropical oceans in extreme deep sea conditions. The vampire squid uses its bioluminescent organs and its unique oxygen metabolism to thrive in the parts of the ocean with the lowest concentrations of oxygen. It has two long retractile filaments, located between the first two pairs of arms on its dorsal side, which distinguish it from both octopuses and squids, and places it in its own order, Vampyromorphida, although its closest relatives are octopods. As a phylogenetic relict, it is the only known surviving member of its order.
Vampyroteuthidae is a family of vampyromorph cephalopods containing the extant vampire squid, Vampyroteuthis infernalis, and the extinct genera Necroteuthis, Provampyroteuthis and Vampyronassa.
Coleoidea or Dibranchiata is one of the two subclasses of cephalopods containing all the various taxa popularly thought of as "soft-bodied" or "shell-less". Unlike its extant sister group Nautiloidea, whose members have a rigid outer shell for protection, the coleoids have at most an internal shell called cuttlebone or gladius that is used for buoyancy or as muscle anchorage. Some species, notably incirrate octopuses, have lost their internal shell altogether, while in some it has been replaced by a chitinous support structure.
Belemnoids are an extinct group of marine cephalopod, very similar in many ways to the modern squid. Like them, the belemnoids possessed an ink sac, but, unlike the squid, they possessed ten arms of roughly equal length, and no tentacles. The name "belemnoid" comes from the Greek word βέλεμνον, belemnon meaning "a dart or arrow" and the Greek word είδος, eidos meaning "form".
Cirrina or Cirrata is a suborder and one of the two main divisions of octopuses. Cirrate octopuses have a small, internal shell and two fins on their head, while their sister suborder Incirrina has neither. The fins of cirrate octopods are associated with a unique cartilage-like shell in a shell sac. In cross-section, the fins have distinct proximal and distal regions, both of which are covered by a thin surface sheath of muscle.
Cephalopod intelligence is a measure of the cognitive ability of the cephalopod class of molluscs.
Incirrata is a suborder of the order Octopoda. The suborder contains the classic "benthic octopuses," as well as many pelagic octopus families, including the paper nautiluses. The incirrate octopuses are distinguished from the cirrate octopuses by the absence in the former of the "cirri" filaments for which the cirrates are named, as well as by the lack of paired swimming fins on the head, and lack of a small internal shell.
Octopodiformes is a superorder of the subclass Coleoidea, comprising the octopuses and the vampire squid. All living members of Octopodiformes have eight arms, either lacking the two tentacles of squid or modifying the tentacles into thin filaments. Octopodiformes is often considered the crown group of octopuses and vampire squids, including all descendants of their common ancestor. Some authors use the term Vampyropoda for the same general category, though others use "Vampyropoda" to refer to the total group. Another term is Octobranchia, referring to cephalopods without prominent tentacles.
Cephalopods, which include squids and octopuses, vary enormously in size. The smallest are only about 1 centimetre (0.39 in) long and weigh less than 1 gram (0.035 oz) at maturity, while the giant squid can exceed 10 metres (33 ft) in length and the colossal squid weighs close to half a tonne (1,100 lb), making them the largest living invertebrates. Living species range in mass more than three-billion-fold, or across nine orders of magnitude, from the lightest hatchlings to the heaviest adults. Certain cephalopod species are also noted for having individual body parts of exceptional size.
Trachyteuthis is a genus of extinct octopodiform cephalopods, comprising six species: T. hastiformis, T. latipinnis, T. nusplingensis, T. teudopsiformis, T. covacevichi and T. chilensis.
Proteroctopus is an extinct genus of cephalopod that lived in the Middle Jurassic, approximately 164 million years ago. It is only known from a single species P. ribeti. The single fossil specimen assigned to this species originates from the Lower Callovian of Voulte-sur-Rhône in France. It is currently on display at the Musée de Paléontologie de La Voulte-sur-Rhône. While originally interpreted as an early octopus, a 2016 restudy of the specimen considered it to be a basal member of the Vampyropoda, less closely related to octopus or vampire squid than either of the two groups are to each other. A phylogenetic analysis by Kruta et. al indicates that Proteroctopus may be more closely related to the Vampyromorpha based on its unique morphology: two fins, head fused to the body, eight arms, two rows of oblique sucker, a gladius and absence of an ink sac. A 2022 phylogenetic analysis also found it to be more closely related to vampire squid than to octopuses.
Vampyronassa rhodanica is an extinct vampyromorph cephalopod known from around 20 fossils from the Lower Callovian of La Voulte-sur-Rhône, Ardèche, France.
Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks. Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The number of additional fossil species is estimated between 60,000 and 100,000, and the proportion of undescribed species is very high. Many taxa remain poorly studied.
Cephalopod ink is a dark-coloured or luminous ink released into water by most species of cephalopod, usually as an escape mechanism. All cephalopods, with the exception of the Nautilidae and the Cirrina, are able to release ink to confuse predators.
Cuttlefish, or cuttles, are marine molluscs of the suborder Sepiina. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of buoyancy.
Belemnitida is an extinct order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous. Unlike squid, belemnites had an internal skeleton that made up the cone. The parts are, from the arms-most to the tip: the tongue-shaped pro-ostracum, the conical phragmocone, and the pointy guard. The calcitic guard is the most common belemnite remain. Belemnites, in life, are thought to have had 10 hooked arms and a pair of fins on the guard. The chitinous hooks were usually no bigger than 5 mm (0.20 in), though a belemnite could have had between 100 and 800 hooks in total, using them to stab and hold onto prey.
Argonautoidea is a superfamily of the suborder Incirrata containing all known argonautoids.
