Source : Webster's Revised Unabridged Dictionary (1913)

Heteronereis \Het`e*ro*ne*re"is\, n. [NL. See {Hetero-}, and
   {Nereis}.] (Zo["o]l.)
   A free-swimming, dimorphic, sexual form of certain species of
   Nereis.

   Note: In this state the head and its appendages are changed
         in form, the eyes become very large; more or less of
         the parapodia are highly modified by the development of
         finlike lobes, and branchial lamell[ae], and their
         set[ae] become longer and bladelike.