The Cirripedia present some examples of sexual relationships which are only paralleled, in the animal kingdom, among the para sitic Myzostomida.
The Myzostomida are protandric hermaphrodites, being functional males when small, nermaphrodite later, and finally functional females (Wheeler).
The structure and development of the Myzostomida seem to show that they are nearly related to Polychaeta (see Chaetopoda), though highly modified in relation to their parasitic mode of life.
There is a definite Annelid group (see Annelida), including the Archiannelida, the bristleworms (see Chaetopoda), of which the earthworm (q.v.) is the most familiar type, the Myzostomida, Hirudinea (see Leech) and the armed Gephyreans (see Echiuroidea).