The Digenetic Trematodes
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The vast majority of digeneans are endoparasitic, except for a few such as Transversotrema spp. which are ectoparasitic. The preferred site of infection is the gut; there are also reports from the urinary bladder (gorgoderids), air bladder (Elongoparorchis) and blood vessels (aporocotylids). The didymozoid trematodes are exceptional in that they occur encysted in pairs in almost all the organs of the fish body where they appear as prominent yellowish cysts. Wide variations also exist in the life cycle patterns. As an adaptation to the site of infection, the digenean body organization varies in different species, while at the same time reflecting similarities to the basic structure of the group.