Tube-dwelling anemone

Tube-dwelling anemones or ceriantharians look very similar to sea anemones but belong to an entirely different class of anthozoans. They are solitary, living buried in soft sediments. Tube anemones live inside and can withdraw into tubes, which are composed of a fibrous material made from secreted mucus and threads of nematocyst-like organelles known as ptychocysts. Within the tubes of these ceriantharians, more than one polyp is present, which is an exceptional trait because species that create tube systems usually contain only one polyp per tube. Ceriantharians were formerly classified in the taxon Ceriantipatharia along with the black corals but have since been moved to their own class, Ceriantharia.

Ceriantharians have a crown of tentacles that are composed of two whorls of distinctly different sized tentacles. The outer whorl consists of large tentacles that extend outwards. These tentacles taper to points and are mostly used in food capture and defence. The smaller inner tentacles are held more erect than the larger lateral tentacles and are used for food manipulation and ingestion.

A few species such as Anactinia pelagica are pelagic and are not attached to the bottom; instead, they have a gas chamber within the pedal disc, allowing them to float upside down near the surface of the water.

Taxonomy

 * Order Spirularia
 * Family Botrucnidiferidae Carlgren, 1912
 * Genus Angianthula Leloup, 1964
 * Genus Atractanthula Leloup, 1964
 * Genus Botruanthus McMurrich, 1910
 * Genus Botrucnidiata Leloup, 1932
 * Genus Botrucnidifer Carlgren, 1912
 * Genus Calpanthula van Beneden, 1897
 * Genus Cerianthula Beneden, 1898
 * Genus Gymnanthula Leloup, 1964
 * Genus Hensenanthula van Beneden, 1897
 * Genus Ovanthula van Beneden, 1897
 * Genus Sphaeranthula Leloup, 1955
 * Family Cerianthidae Milne-Edwards & Haime, 1852
 * Genus Anthoactis Leloup, 1932
 * Genus Apiactis van Beneden, 1897
 * Genus Bursanthus Leloup, 1968
 * Genus Ceriantheomorphe Carlgren, 1931
 * Genus Ceriantheopsis Carlgren, 1912
 * Genus Cerianthus Delle Chiaje, 1830
 * Genus Engodactylactis Leloup, 1942
 * Genus Isodactylactis Carlgren, 1924
 * Genus Nautanthus Leloup, 1964
 * Genus Pachycerianthus Roule, 1904
 * Genus Paradactylactis Carlgren, 1924
 * Genus Parovactis Leloup, 1964
 * Genus Peponactis van Beneden, 1897
 * Genus Plesiodactylactis Leloup, 1942
 * Genus Sacculactis Leloup, 1964
 * Genus Solasteractis van Beneden, 1897
 * Genus Synarachnactis Carlgren, 1924
 * Genus Syndactylactis Carlgren, 1924
 * Genus Trichactis Leloup, 1964
 * Order Penicillaria
 * Family Arachnactidae McMurrich, 1910
 * Genus Anactinia Annandale, 1909
 * Genus Arachnactis Sars, 1846
 * Genus Arachnanthus Carlgren, 1912
 * Genus Dactylactis van Beneden, 1897
 * Genus Isapiactis Carlgren, 1924
 * Genus Isarachnactis Carlgren, 1924
 * Genus Isarachnanthus Carlgren, 1924
 * Genus Isovactis
 * Genus Ovactis
 * Genus Paranactinia

A 2020 integrative study incorporating molecular phylogenetic reconstructions and morphological assessment across the three families recovered Arachnactidae as a well-supported clade, but did not recover Cerianthidae and Botrucnidiferidae as monophyletic, drawing into question the validity of the Spirularia suborder