heteronereis

from The Collaborative International Dictionary of English v.0.48

Heteronereis \Het`e*ro*ne*re"is\, n. [NL. See {Hetero-}, and
   {Nereis}.] (Zool.)
   A free-swimming, dimorphic, sexual form of certain species of
   Nereis.
   [1913 Webster]

   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.
         [1913 Webster]