In certain genera the margins are infolded either along their whole length (the male of Schistostomum haematobium; fig.
The most important of the Trematodes in its effect on man is Schistostomum (Bilharzia).
Fourteen species belonging to five genera have been found in man, but only .one [Schistostomum (Bilharzia) haematobium] is of serious medical importance, the others being rare and occasioned by want of cleanliness and close association with infected domestic animals.
Schistostomum (Bilharzia) haematobium in which the male is larger than the female and encloses the latter in a ventral canal; Koellikeria filicolle Rud (Distomum okenii, K01l) which also occurs in pairs, a large female and a small male being found together encysted in the branchial chamber of Brama raja: and Didymozoon thynni (Monostomum bipartitum) which occurs in pairs fused for the greater part of their length and only free anteriorly; the larger individual is the female.
A, Schistostomum (Bilharzia) haematobium, the thin female in the gynaecophoric canal of the stouter male.
The life-history of Schistostomum haematobium is still unknown, but the difficulty in obtaining developmental stages in any of the numerous intermediate hosts that have been tried suggests that the ciliated larvae may develop directly in man and either gain access to him by the use of impure water for drinking or may perforate his skin when bathing.