Like all flatworms, Trematoda are acoelomates and aprocti. They are more or less all endoparasites. It is believed that they derive from progenitors similar to the current rhabdocelids, turbellarians with a straight cavity. They are hermaphrodites with indirect development, they have the larval stage.
They are all parasitic and have suckers or hooks with which they adhere to the host, they can be divided into monogeneous and digeneous, depending on whether their parasitic life cycle is completed within one or two hosts.
The best known species is Fasciola hepatic Compared to turbellarians, they present structural modifications: loss of epithelial cilia, greater development of glandular cells, presence of structures that allow the parasite to adhere to the host, such as suckers, hooks or both, and loss of eyes.
The biodiversity of marine trematodes: then, now and in the future, research published on the International journal for parasitology, explained: "Trematodes are the richest class of platyhelminths in the marine environment, infecting all classes of marine vertebrates as sexual adults and many phyla of marine invertebrates as part of their life cycles.
Despite the cryptic nature of their existence (almost all marine trematodes are internal parasites ), they have been the focus of study for almost 250 years, with the first species described in 1774. Here we review progress in the study of the "biodiversity" of these parasites, contrasting the progress made in the last 50 years (post- 1971) to that in the almost 200 years before it (pre-1972).We consider an understanding of biodiversity to require knowledge of the species present in the system, an understanding of their evolutionary relationships (which informs higher classification), and, specifically for trematodes, an understanding of their complex life cycles.The fauna is now large, comprising well over 5,000 species.
Although species description continues, we see evidence of a slow down in all aspects of discovery. There has been only one completely new family identified since 1984 and the proposal of new genera is in decline as is the description of new species, especially for those of tetrapods.
However, the extent to which this slow-down reflects an approach to the richness asymptote is made uncertain by changes in the field; reduced effort and difficulty of study may be important components of the effect. Regardless of how close we are to a complete description of the fauna, we infer that the outline is well-understood although the details are not.
Adoption of molecular methodologies over the last 40 years have complemented morphometric analyzes to facilitate objective recognition of species; however, despite these objective data, there is still inconsistency between authors on species delimitation.
Molecular methodologies have also completely revolutionized inference of relationships at all levels, from within genera to between orders, and underpinned elucidation of novel life cycles. We expect the next 50 years to produce further dividends from technological innovations.
The backdrop to the field will be global environmental concerns and the growing problem of funding for basic biodiversity studies."