Drilophagus and Albertia are parasitic on the surface or within the gut of Naid Oligochaete worms: Seisonaceae are ectoparasitic on the Crustacean Nebalia, Proales werneckii forms galls within the Conferva Vaucheria, and P. parasita infests the central jelly of the Phytoflagellate Volvox; P. petromyzon is a frequent commensal in the gill cavity of some Cladoceran Crustacean Eurycereus lamellatus.
The only organs that exhibit any sign of degeneration are those of sense, but in the ectoparasitic Trematodes simple eye-like structures are present and perhaps serve as organs of temperature.
In the ectoparasitic Trematodes this post-oral sucker is a complex disk placed near the hinder end and provided With suckerlets, hooks and a musculature arising from a special skeleton.
The excretory pore is terminal and posterior in endoparasitic forms: paired, anterior and dorsal in the ectoparasitic class.
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.