Sedentaria

Sedentaria is a diverse clade of annelid worms. It is traditionally treated as a subclass of the paraphyletic class Polychaeta, but it is also a monophyletic group uniting several polychaetes and the monophyletic class Clitellata. It is the sister group of Errantia.^[1]

Sedentaria
Examples of Sedentaria (clockwise from upper left corner): Sabellida, Echiura, Maldanomorpha, Clitellata, Terebelliformia, Siboglinidae.
Scientific classification
Kingdom:	Animalia
Phylum:	Annelida
Clade:	Pleistoannelida
Clade:	Sedentaria Lamarck 1818
Orders and families
Orbiniida Cirratuliformia Siboglinidae (=Pogonophora) Sabellida Spionida Opheliida Capitellida Echiura Capitellidae Terebelliformia Maldanomorpha Clitellata "Oligochaetes" Hirudinea Incertae sedis ?Myzostomida ?Spintheridae

Sedentaria are mainly found within marine environments that have low oxygen levels  and are specially adapted to these low oxygen environments by  increasing gill surface area and having high-affinity respiratory proteins. Furthermore, they go through a process of metabolic depression which lowers their energy use so that they can inhibit these low oxygen zones .^[2]

Phylogeny

The phylogeny of polychaetes is slowly being resolved. Sedentaria and Errantia are the two biggest clades of polychaetes, and together they compose clade Pleistoannelida. Sedentaria's most basal clade is Orbiniida.^[3] Other groups that are nested within Sedentaria are: Clitellata, the Sabellida/Spionida clade, Opheliida, Echiura, Cirratuliformia, Terebelliformia, Maldanomorpha and the families Siboglinidae and Capitellidae.^[4][1][5]

Pleistoannelida	Errantia Sedentaria Orbiniida Cirratuliformia Siboglinidae Sabellida Spionida Opheliida Capitellida Echiura Capitellidae Terebelliformia Maldanomorpha Maldanidae Arenicolidae Clitellata

Some taxa, such as Spintheridae and Myzostomida, are still difficult to place due to their long branching, but they likely belong to either Errantia or Sedentaria.^[1]