There are in excess of 10,000 described species of parasitic platyhelminths, with the most speciose group being the Digenea, followed by Cestoidea, Monogenea and Aspidogastrea. The four major taxa are quite distinctive on morphological and life cycle features. The expression of diversity within the groups differs.
Aspidogastreans have the entire ventral surface modified into a multichambered sucking disk, and can mature in molluscs.
Monogeneans have a well developed posterior haptor, and all have direct life cycles in aquatic environments. Diversity within the group is evident most prominently in variation of the haptor: the size and number of subdivisions, and the presence, number and shape of hooks, anchors, suckers and clamps.
Cestodes lack a gut, and all species require 1 or 2 intermediate hosts. Diversity within the group involves the type of proglottidization, and structure of the scolex of the adult. Larval stages also vary, particularly in the presence and type of cyst that surrounds them.
The Digenea all have indirect life cycles, requiring a mollusc as the first intermediate host. The sequence of larval stages, structure of larvae, and life cycles have diverged considerably. Although there are some extreme morphological variants among adult digeneans, typical diversity of the basic body plan results from rearrangements of the internal organs, the presence of diverticula on the gut, the presence of peculiar accessory reproductive structures, the shape and location of external suckers, and the presence of spines, glands or fleshy protuberances on parts of the body.