\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.