||Chæ*top"o*da (?), n. pl. [NL., from Gr. &?; hair + -poda.] (Zoöl.) A very extensive order of Annelida, 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.