The Collaborative International Dictionary of English v.0.48:

Polychaeta \Pol`y*ch[ae]"ta\, n. pl. [NL., from Gr. poly`s many
   + ? hair.] (Zool.)
   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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Annelida \An*nel"i*da\, n. pl. [NL. See Annelid.] (Zool.)
   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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Polychaeta
    n 1: marine annelid worms [syn: Polychaeta, class
         Polychaeta]