Cirrina

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.

The suborder is named for small, cilia-like strands (cirri) on the arms of the octopus, a pair for each sucker. These are thought to play some role in feeding, perhaps by creating currents of water that help bring food closer to the beak. Cirrate octopuses are noteworthy for lacking ink sacs.

The oldest known member of the group is Paleocirroteuthis from the Late Cretaceous of Japan and Canada.

Phylogeny
A molecular phylogeny based on mitochondrial and nuclear DNA marker sequences by Sanchez et al., 2018, shows that the Cirrina is paraphyletic, i.e. it is not a single clade. Instead, a clade containing Opisthoteuthidae and Cirroctopodidae is sister to the Octopodida, while a clade containing Cirroteuthidae and Stauroteuthidae is sister to the clade that contains those other groups. However, subsequent studies, using a greater coverage of species and genes, have found Cirrata and Incirrata to be monophyletic clades, consistent with earlier morphological and molecular studies.

Classification
The family level clades recognized within Cirrina has changed over time. Currently five families are recognized by the World Register of Marine Species (Cirroteuthidae, Stauroteuthidae, Opisthoteuthidae, Grimpoteuthidae, and Cirroctopodidae), with these groups also recognized in recent molecular work. The families Cirroteuthidae and Stauroteuthidae have been problematic, while they are distinct morphologically, molecular studies using mitochondrial genes revealed a single family (Stauroteuthidae being synonymized under Cirroteuthidae), but more recent analysis using nuclear genes does show separation. The family Grimpoteuthidae has also at times been synonymized under Opisthoteuthidae but is currently well supported.
 * CLASS CEPHALOPODA
 * Subclass Nautiloidea: nautilus
 * Subclass †Ammonoidea: ammonites
 * Subclass Coleoidea
 * Superorder Decapodiformes: squid, cuttlefish
 * Superorder Octopodiformes
 * Family †Trachyteuthididae (incertae sedis)
 * Order Vampyromorphida: vampire squid
 * Order Octopoda
 * Genus †Keuppia (incertae sedis)
 * Genus †Palaeoctopus (incertae sedis)
 * Genus †Proteroctopus (incertae sedis)
 * Genus †Styletoctopus (incertae sedis)
 * Suborder Cirrina: finned deep-sea octopus
 * Family Opisthoteuthidae (containing Opisthoteuthis)
 * Family Grimpoteuthidae (containing Grimpoteuthis, Cryptoteuthis, & Luteuthis)
 * Family Cirroctopodidae (containing Cirroctopus)
 * Family Cirroteuthidae (containing Cirroteuthis, & Cirrothauma)
 * Family Stauroteuthidae (containing Stauroteuthis)
 * Family uncertain: †Paleocirroteuthis
 * Suborder Incirrina