If ectoparasitic and attached to the skin, apertures or gills, the Trematode adopts more elaborate adhesive organs and undergoes a less complex development than are required for the endoparasitic members of the class.
In most endoparasitic Trematodes the accessory gonopore is a median and dorsal structure.
It is usual to consider the ectoparasitic habit as leading up to the endoparasitic one.
The rapid multiplication that takes place in the larval stage of nearly all endoparasitic forms affects the tissues of the "intermediate" host in which they live.
In the endoparasitic trematodes the uterus is the only passage by which fertilization can be effected, and in cases of cross and selfimpregnation this duct is physiologically a vagina.
