A very extensive order of Annelida (segmented worms), characterized by the presence of lateral setæ, or spines, on most or all of the segments. They are divided into two principal groups: Oligochæta, including the earthworms and allied forms, and Polychæta, including most of the marine species.
In some systems of zoölogical classification, a prime division or branch of a phylum of the animal kingdom called Appendiculata, consisting of two classes, Oligochæta and Polychæta: in this sense contrasted with Rotifera (alone) and Gnathopoda (Arthropoda indiscriminately). [Little used.]
Ordinarily, an order or subclass of the class Annelida, with dorsal branchiæ and non-suctorial mouth.