While some of the species are bisexual, others are protandrous, self-fertilizing hermaphrodites.
While the great majority are simple hermaphrodites, capable of cross and self fertilization, it was discovered by Darwin that, in certain species, minute degraded males exist, attached within the mantle-cavity of the ordinary individuals.
Since these dwarf males pair, not with females, but with hermaphrodites, Darwin termed them " complemental " males.
The Myzostomida are protandric hermaphrodites, being functional males when small, nermaphrodite later, and finally functional females (Wheeler).
Small "males" are in some species constantly associated with large hermaphrodites, but according to Beard there are in some cases true dwarf males, comparable to the complementary males described by Darwin in the Cirripedia.