Cirrate octopuses possess a well-developed internal shell that supports their muscular swimming fins. This is in contrast to the more familiar, finless, incirrate octopuses, in which the shell remnant is either present as a pair of stylets or absent altogether. The cirrate shell is quite unlike that of any other living cephalopod group and has its own dedicated set of descriptive terms. It is usually roughly arch- or saddle-shaped and is rather soft, being similar in consistency to cartilage. Each of the eight extant cirrate

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial ...

is characterised by a distinct shell morphology: *

Cirroteuthidae Cirroteuthidae is a family of cirrate octopuses comprising three species in two genera. Species *Genus ''Cirroteuthis'' **'' Cirroteuthis muelleri'' *Genus '' Cirrothauma'' **'' Cirrothauma magna'' **''Cirrothauma murrayi ''Cirrothauma murray ...

**''

Cirroteuthis ''Cirroteuthis muelleri'' was the first cirrate octopus species (and genus) to be scientifically described (in 1836). It is closely related to the genus ''Cirrothauma'' within the family Cirroteuthidae. At present the genus contains a single reco ...

'' — saddle-shaped, with large wings **''

Cirrothauma ''Cirrothauma'' is a genus of deep water octopuses from the cirrate family Cirroteuthidae. The species ''Cirrothauma'' are fragile, gelatinous deep-sea octopods with a shell, a moderate saddle, and triangular wings. Their eyes either have lenses ...

'' — butterfly-shaped *

Opisthoteuthidae Umbrella octopuses (family Opisthoteuthidae) are a group of pelagic octopuses. Umbrella octopuses are characterized by a web of skin between the tentacles, causing them to somewhat resemble an opened umbrella when the tentacles are spread. De ...

**''

Cirroctopus ''Cirroctopus'' is a genus of four species of octopuses within the monotypic family Cirroctopodidae. Members of this genus have larger fins than other cirrate octopuses, and tend to be more muscular.Vecchione, Michael and Richard E. Young. 2016 ...

'' — V-shaped, lateral wings tapering to fine points **'' Cryptoteuthis'' — U-shaped, each lateral wing ending in broad lobe with pointed projection **'' Grimpoteuthis'' — U-shaped, lateral wings ending bluntly **'' Luteuthis'' — W-shaped **''

Opisthoteuthis ''Opisthoteuthis'' is a genus of cirrate octopuses, sometimes known as flapjack octopuses, which are found in all the world's oceans. Behavior Like other cirrates, octopuses in Opisthoteuthis are generally small, and many dwell in the deep sea ...

'' — U-shaped, lateral wings usually tapering to fine points but termination complex in certain speciesVillanueva, R., R.E. Young & M. Vecchione (2008)
''Opisthoteuthis'' Verrill 1883

Tree of Life Web Project The Tree of Life Web Project is an Internet project providing information about the diversity and phylogeny of life on Earth. This collaborative peer reviewed project began in 1995, and is written by biologists from around the world. The site ...

. * Stauroteuthidae **''

Stauroteuthis ''Stauroteuthis'' is a genus of deepwater octopus, a cephalopod mollusk. This is the only genus in the family Stauroteuthidae, and only two species have been described in this genus. The organisms live below water depth; although sometimes foun ...

'' — U-shaped The comparatively simple shells of

and Stauroteuthidae are thought to approximate the

ancestral An ancestor, also known as a forefather, fore-elder or a forebear, is a parent or ( recursively) the parent of an antecedent (i.e., a grandparent, great-grandparent, great-great-grandparent and so forth). ''Ancestor'' is "any person from w ...

shape, with those of

being more derived.Bizikov, V.A. (2004)
Evolution of shell in Octopodiformes (Cephalopoda)
. In: Abstracts of the conference ''Mollusks of the Northeastern Asia and Northern Pacific: Biodiversity, Ecology, Biogeography and Faunal History''. October 4–6, 2004, Vladivostok, Russia. pp. 21–23. The shell of ''

'' appears transitional in form between those of incirrate octopuses and other cirrates, and resembles the reduced shell of the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

'' Palaeoctopus newboldi''. The paired, rod-shaped stylets of incirrates are evolutionarily derived from the lateral wings and horns of the cirrate shell.

References

Further reading