SyllidianSyl*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.