While some of the species are bisexual, others are protandrous, self-fertilizing hermaphrodites.
Hydra is, moreover, bisexual, in contrast with what is known of hydroid colonies.
Both male and female gonoducts open through acornmon atrium to the exterior by tkis pore, but in three bisexual genera the male and female ducts are developed in separate individuals (Bilharzia, Didymozoon, Koellikeria).
These "psychically decadent" individuals appear to represent the entire male sex of a bisexual species, and become unnecessary owing to the grafting of hermaphroditism on the female sex.
In bisexual or hermaphrodite flowers, that is, those in which both stamens and pistil are present, though self-pollination might seem the obvious course, this is often prevented or hindered by various arrangements which favour cross-pollination.