, Cestodes (tapeworms) and digeneans (flukes) cause diseases in humans and their livestock, whilst monogeneans can cause serious losses of stocks in fish farms. Omissions? Original article on Live Science. However, some long species have an anus and some with complex, branched guts have more than one anus, since excretion only through the mouth would be difficult for them. The adults of all 3,400 cestode species are internal parasites. Thirty-six or more fluke species have been reported as parasitic in humans. In the Far East, S. japonicum is the important blood fluke. This category only includes cookies that ensures basic functionalities and security features of the website. (Image: © Scripps Oceanography/Greg Rouse).  While poorer countries still struggle with unintentional infection, cases have been reported of intentional infection in the US by dieters who are desperate for rapid weight-loss. Aspidogastrea are from a few millimetres to 100 millimetres in length. A very few species feed on algae. The two species - the blue-lined flatworm and the purple-spotted flatworm - lay a smaller number of eggs than the others, and their offspring take a longer time to develop. Surface appearances aside, this genus — Xenoturbella — has proved surprisingly difficult to position on the tree of life, ever since the first species, Xenoturbella bocki, was discovered in 1950, according to the study researchers. This species is considered extremely invasive, and it has been characterised as the most threatening flatworm species to the soil ecosystem and native soil organisms presently in Europe. It is clear that the broad tapeworm (Diphyllobothrium latum) can occur only where an intimate ecological association exists among the three host groups. Despite this difference in environments, most platyhelminths use the same system to control the concentration of their body fluids. There was a problem. The class Turbellaria (planarians) is free-living. A larva of a Schistosoma invades the blood vessels of humans. Some turbellarians are gray, brown, or black, with mottled or striped patterns. Some marine species occur at relatively great depths in the sea; others are pelagic (i.e., living in the open sea). The majority view is that Platyzoa are part of Lophotrochozoa, but a significant minority of researchers regard Platyzoa as a sister group of Lophotrochozoa. The group contains many unpleasant internal parasites of almost every vertebrate species including man. They are divided into the Monogenea and Cestoda groupings. Flatworm, any of the phylum Platyhelminthes, a group of soft-bodied, usually much flattened invertebrates. Flatworms are members of the phylum Platyhelminthes. Unlike other bil… Or view hourly updated newsfeeds in your RSS reader: Keep up to date with the latest news from ScienceDaily via social networks: Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. "Phylogenetic relationships within the Plathelminthes", pp 143–158 A mouth opening at one end leads to a gut sack, but there is no anal opening in the back end. These analyses had concluded the redefined Platyhelminthes, excluding Acoelomorpha, consists of two monophyletic subgroups, Catenulida and Rhabditophora, with Cestoda, Trematoda and Monogenea forming a monophyletic subgroup within one branch of the Rhabditophora. Endemic (local) centres of infection occur in virtually all countries, but widespread infections occur in the Far East, Africa, and tropical America. In free-living forms, the body covering is typically an epidermis consisting of one layer of ciliated cells—i.e., cells with hairlike structures—the cilia being confined to specific regions in some species. Turbellaria are adapted to a wide range of environments, and many species are resistant to extreme environmental conditions.  Their eggs produce ciliated swimming larvae, and the life cycle has one or two hosts. The definitive host in which adults develop is a land vertebrate; the earliest host of juvenile stages is usually a snail that may live on land or in water, whilst in many cases, a fish or arthropod is the second host. There are four major classes of flatworms such as Cestoda (tapeworms), Turbellaria (planarians), Trematoda (flukes), and Monogenea. , These have about 4,500 species, are mostly free-living, and range from 1 mm (0.04 in) to 600 mm (24 in) in length. The biggest species, X. monstrosa, measured 8 inches (20 centimeters) long, while tiny X. hollandorum was a mere 1 inch (2.5 centimeters) in length. Classification and evolutionary relationships, Ehlers U. However, a few are internal parasites. With about 25,000 known species they are the largest phylum of acoelomates. Cave-dwelling species tend to show loss of eyes and pigment. Adult monogeneans have large attachment organs at the rear, known as haptors (Greek ἅπτειν, haptein, means "catch"), which have suckers, clamps, and hooks. , Planarians, a subgroup of seriates, are famous for their ability to regenerate if divided by cuts across their bodies. These statocysts are thought to function as balance and acceleration sensors, as they perform the same way in cnidarian medusae and in ctenophores. Suckers are used to remain attached to the internal body surface of the host. It consists of two main types of cell: fixed cells, some of which have fluid-filled vacuoles; and stem cells, which can transform into any other type of cell, and are used in regenerating tissues after injury or asexual reproduction. Their bodies are blobs that look more like empty socks than animals, and are wrinkled by muscular folds and propelled by cilia. A larval stage of the gid parasite of sheep (Multiceps multiceps) usually lodges in the sheep brain. But where sanitation is poor and meat eaten undercooked, the incidence of tapeworm infestations is high.  Beyond that, they are "defined more by what they do not have than by any particular series of specializations. Financial support for ScienceDaily comes from advertisements and referral programs, where indicated. The redefined Platyhelminthes is part of the Lophotrochozoa, one of the three main groups of more complex bilaterians. , The relationships of Platyhelminthes to other Bilateria are shown in the phylogenetic tree:, The internal relationships of Platyhelminthes are shown below. Since then, molecular phylogenetics, which aims to work out evolutionary "family trees" by comparing different organisms' biochemicals such as DNA, RNA and proteins, has radically changed scientists' view of evolutionary relationships between animals. , The oldest confidently identified parasitic flatworm fossils are cestode eggs found in a Permian shark coprolite, but helminth hooks still attached to Devonian acanthodians and placoderms might also represent parasitic flatworms with simple life cycles. Embedded in the epidermis of turbellarians are ovoid or rod-shaped bodies (rhabdoids) of several sorts; of uncertain function, the bodies frequently are concentrated dorsally or may be clustered anteriorly as rod tracts opening at the apex. Because they do not have internal body cavities, Platyhelminthes were regarded as a primitive stage in the evolution of bilaterians (animals with bilateral symmetry and hence with distinct front and rear ends). Trematodes are mostly between about one and 10 millimetres (0.04 to 0.4 inch) long; members of some species, however, may grow to several centimetres. The ability of these flatworms to live in artificial containers demonstrated the potential of placing these species in popular mosquito breeding sites, which would ideally reduce the amount of mosquito-borne disease.