\Syl*lid"i*an\, n. [From NL. Syllis, the typical
genus.] (Zo["o]l.)
Any one of numerous species of marine annelids of the family
{Syllid[ae]}.
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.