{{Short description|Internal shell of cirrate octopuses}} [[File:Grimpoteuthis innominata 73 mm ML.jpg|thumb|300px|Dorsal (left) and ventral aspects of a mature female ''Grimpoteuthis angularis'' (73&nbsp;mm ML).<!--The internal shell is visible through the animal's translucent skin??-->]]

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.<ref>Fuchs, D., C. Ifrim & W. Stinnesbeck (2008). A new ''Palaeoctopus'' (Cephalopoda: Coleoidea) from the Late Cretaceous of Vallecillo, north-eastern Mexico, and implications for the evolution of Octopoda. ''Palaeontology'' '''51'''(5): 1129–1139. {{doi|10.1111/j.1475-4983.2008.00797.x}}</ref>

The cirrate shell is quite unlike that of any other living cephalopod group and has its own dedicated set of descriptive terms.<ref>Young, R.E., M. Vecchione & K.M. Mangold (1999). [http://tolweb.org/accessory/Cephalopoda_Glossary?acc_id=587 Cephalopoda Glossary]. Tree of Life Web Project.</ref><ref>Young, R.E., M. Vecchione & K.M. Mangold (2002). [http://tolweb.org/accessory/Cirrate_Shell_Terminology?acc_id=1488 Cirrate Shell Terminology]. Tree of Life Web Project.</ref> It is usually roughly arch- or saddle-shaped and is rather soft, being similar in consistency to cartilage.<ref>Vecchione, M., K.M. Mangold & R.E. Young (2010). [http://tolweb.org/Cirrata/20086 Cirrata Grimpe, 1916]. Tree of Life Web Project.</ref> Each of the eight extant cirrate genera is characterised by a distinct shell morphology outlined below<ref>Collins, M.A. & R. Villanueva (2006). [https://books.google.com/books?hl=en&lr=&id=R7-TfdYeLEgC&oi=fnd&pg=PA277 Taxonomy, ecology and behaviour of the cirrate octopods]. In: Gibson, R.N., R.J.A. Atkinson & J.D.M. Gordon (eds.) ''Oceanography and Marine Biology: An Annual Review''. Aberdeen University Press, Aberdeen. pp.&nbsp;277–322.</ref> (below taxonomy updated per WoRMS):

*Superfamily Cirroteuthoidea **Cirroteuthidae ***''Cirroteuthis'' — saddle-shaped, with large wings ***''Cirrothauma'' — butterfly-shaped **Stauroteuthidae ***''Stauroteuthis'' — U-shaped *Superfamily Opisthoteuthoidea **Opisthoteuthidae ***''Opisthoteuthis'' (also ''Exsuperoteuthis'' & ''Insigniteuthis'')— U-shaped, lateral wings usually tapering to fine points but termination complex in certain species<ref>Villanueva, R., R.E. Young & M. Vecchione (2008). [http://tolweb.org/Opisthoteuthis/20106 ''Opisthoteuthis'' Verrill 1883]. Tree of Life Web Project.</ref> **Cirroctopodidae ***''Cirroctopus'' — V-shaped, lateral wings tapering to fine points **Grimpoteuthidae ***''Grimpoteuthis'' — U-shaped, lateral wings ends expanded in a broad lobe (with offset spike present or absent).<ref>{{Cite journal |last=Verhoeff |first=Tristan Joseph |last2=O’Shea |first2=Steve |date=2022-01-02 |title=New records and two new species of Grimpoteuthis (Octopoda: Cirrata: Grimpoteuthididae) from southern Australia and New Zealand |url=https://figshare.com/articles/journal_contribution/New_records_and_two_new_species_of_i_Grimpoteuthis_i_Octopoda_Cirrata_Grimpoteuthididae_from_southern_Australia_and_New_Zealand/19204753 |journal=Molluscan Research |volume=42 |issue=1 |pages=4-30 |doi=10.1080/13235818.2022.2035889 |issn=1323-5818|url-access= }}</ref> ***''Luteuthis'' — W-shaped, lateral wings ends expanded (with offset spike present). ***''Cryptoteuthis'' — U-shaped, each lateral wing ending in broad lobe with pointed projection.<ref>Collins, M., R. Villanueva & R.E. Young (2008). [http://tolweb.org/Cryptoteuthis_brevibracchiata/121985 ''Cryptoteuthis'' Collins, 2004]. Tree of Life Web Project.</ref>

The comparatively simple shells of Opisthoteuthidae and Stauroteuthidae are thought to approximate the ancestral shape, with those of Cirroteuthidae being more derived.<ref name="Bizikov2004">Bizikov, V.A. (2004). [http://rfems.dvo.ru/conference_2004/Malacological_Conference_2004_-_Abstracts.pdf#page=22 Evolution of shell in Octopodiformes (Cephalopoda)] {{Webarchive|url=https://web.archive.org/web/20131021220600/http://rfems.dvo.ru/conference_2004/Malacological_Conference_2004_-_Abstracts.pdf#page=22 |date=2013-10-21 }}. 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.&nbsp;21–23. <!--http://www.webcitation.org/6KWNFbt75--></ref> The shell of ''Cirroctopus'' appears transitional in form between those of incirrate octopuses and other cirrates, and resembles the reduced shell of the Late Cretaceous ''Palaeoctopus newboldi''.<ref name="Bizikov2004" /> The paired, rod-shaped stylets of incirrates are evolutionarily derived from the lateral wings and horns of the cirrate shell.<ref name="Bizikov2004" />

==References== {{reflist|2}}

==Further reading== {{refbegin|2}} * Bizikov, V.A. (2004). The shell in Vampyropoda (Cephalopoda): morphology, functional role and evolution. ''Ruthenica'' '''3''': 1–88. * Haas, W. (2002). The evolutionary history of the eight-armed Coleoidea. ''Abhandlungen der Geologischen Bundesanstalt, Wien'' '''57''': 341–351. {{refend}}

{{Cephalopod anatomy}}

Category:Cephalopod zootomy Category:Octopuses