Sperm transfer in the monogenean gill parasite Gastrocotyle trachuri

Hypodermic impregnation is thought to be effected by a suction pressure in a muscular chamber bringing into contact with each other a penis tube and almost any part of the tegument of a co-copulant that is penetrated. The sperms may be deposited in vitelline spaces from which they have unobstructed access to the oviduct. This form of non-localized impreg­nation is thought to have led, phylogenetically, to the great variety in the architecture of the vagina(e) in closely related gastrocotylids and probably also in other polyopisthocotyleans. The convergences that result make the task for systematists particularly difficult.