Coleoid

Coleoid is a subclass of cephalopods within the phylum Mollusca, comprising octopuses, squids, and cuttlefish, along with several extinct groups such as belemnites. It is contrasted with the nautiloids, which retain external shells.

Living coleoids are characterized by the loss of an external shell. Most have an internal shell or none at all: octopuses have no shell; cuttlefish possess a small internal cuttlebone; squids have a flexible internal gladius or pen. They share a prominent head, a ring of arms or tentacles, and a siphon used for jet propulsion. They typically have well-developed eyes and a complex nervous system. Many species can change color and pattern rapidly via chromatophores and other structures, and most produce ink as a defensive display.

Coleoids are carnivorous and occupy a wide range of marine habitats, from shallow coastal waters to the open ocean and deep sea. They capture prey with their arms and, in many species, with two elongated feeding tentacles. Reproduction generally involves transfer of spermatophores from the male to the female; many species show complex mating behaviors and relatively short life spans.

Fossil coleoids include belemnites and other extinct groups; the living coleoids comprise the octopuses, squids, and cuttlefishes. The group is highly diverse, with species adapted to a variety of ecological niches. Coleoids are distinguished from nautiloids by their typically reduced or absent internal shell and by distinctive reproductive and anatomical traits evolved in the coleoid lineage.