What does "heteronereis" mean?

Heteronereis: 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 lamellæ, and their setæ become longer and bladelike.