Source : Webster's Revised Unabridged Dictionary (1913)

Polychaeta \Pol`y*ch[ae]"ta\, n. pl. [NL., from Gr. poly`s many
   + ? hair.] (Zo["o]l.)
   One of the two principal groups of Ch[ae]topoda. It includes
   those that have prominent parapodia and fascicles of set[ae].
   See Illust. under {Parapodia}.

Annelida \An*nel"i*da\, n. pl. [NL. See {Annelid}.] (Zo["o]l.)
   A division of the Articulata, having the body formed of
   numerous rings or annular segments, and without jointed legs.
   The principal subdivisions are the {Ch[ae]topoda}, including
   the {Oligoch[ae]ta} or earthworms and {Polych[ae]ta} or
   marine worms; and the {Hirudinea} or leeches. See
   {Ch[ae]topoda}.

Source : WordNet®

Polychaeta
     n : marine annelid worms [syn: {class Polychaeta}]