Bonellia

:  a genus of marine worms (group Echiuroidea) that exhibit marked dimorphism and size disparity between the sexes, the male living parasitically in the nephridium of the female and that have what appears to be a unique mechanism of sex determination, the indifferent larva becoming a male if it settles on the proboscis of a mature female and becoming a female if it develops independently on the sea bottom