A loose grouping of acoelomate, parasitic flatworms of the phylum Platyhelminthes formerly accorded class rank and containing the subclasses (or orders) Digenea, Monogenea, and Aspidobothria. Although neither the name Trematoda nor the classification scheme appears as valid as formerly in demonstrating helminth phylogenetic relationships (in light of recent information, covered in the Taxonomy section below), a discussion of the three older “trematode” groups just mentioned is profitable. These organisms commonly occur as adults in or on all vertebrate groups. They exhibit cephalization, bilateral symmetry, and well-developed anterior and ventral, or anterior and posterior, holdfast structures. Electron-microscope studies of some parasitic platyhelminths by D. L. Lee in 1966 and K. M. Lyons in 1970 indicate that the outer covering of these worms, once thought to be a secreted, nonliving cuticle, is probably a living epidermis. The mouth is anterior, and usually a blind, forked gut occurs, as well as three muscle layers. The excretory system consists of flame cells and collecting tubules. These animals are predominantly hermaphroditic and oviparous with operculated egg capsules. The life histories of the Digenea are complex, while those of the Monogenea and Aspidobothria are simple.