heteronereis

‖n. [ NL. See Hetero-, and Nereis. ] (Zool.) A free-swimming, dimorphic, sexual form of certain species of Nereis. [ 1913 Webster ]

☞ 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 lamellæ, and their setæ become longer and bladelike. [ 1913 Webster ]