The Collaborative International Dictionary of English v.0.48:

Syllidian \Syl*lid"i*an\, n. [From NL. Syllis, the typical
   genus.] (Zool.)
   Any one of numerous species of marine annelids of the family
   Syllidae.
   [1913 Webster]

   Note: Many of the species are phosphorescent; others are
         remarkable for undergoing strobilation or fission and
         for their polymorphism. The egg, in such species,
         develops into an asexual individual. When mature, a
         number of its posterior segments gradually develop into
         one or more sexual individuals which finally break away
         and swim free in the sea. The males, females, and
         neuters usually differ greatly in form and structure.
         [1913 Webster]