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The turbellarian flatworms ('Turbellaria') are acoelomate bilateria without a definitive anus. They are largely free-living, with simple life cycles and a cellular, usually ciliated, epidermis. Turbellaria live in freshwater, seawater, wet terrestrial habitats and many are symbiotic (even parasitic). About 2/3 are small (1-2mm long or less), but some, the marine polyclads and the terrestrial triclads in particular, can grow to many centimeters in length.

The worms are often dorso-ventrally flattened with anterior sensory and glandular regions. Larger forms are often pigmented and with very few exceptions locomotion is by beating of the epidermal cilia which are often in restricted tracts in the larger or more specialized forms. Many turbellarians have epidermal rhabdoids (rod-like bodies secreted by the epidermis). Sub-epithelial longitudinal and circular muscles combine with larger muscles through the parenchyma (the cellular body substance in which the organ systems are embedded) to provide an extremely flexible body. There is usually an anterior nerve ganglion and main anterior-posterior trunks with cross fibres and sensory tracts forming an elaborate network. The digestive system usually consists of a sac-like gut (often ramified in larger forms) with a mouth and a muscular pharynx for food capture. As there is no anus, wastes are regurgitated. There is also no circulatory system; there is, however, in nearly all a protonephridial system with terminal flame cells.

Most turbellarians are hermaphrodites with cross fertilization following copulation; some have asexual stages and some are parthenogenetic, but most lay egg capsules from which one or more ciliated young emerge, grow and differentiate directly without metamorphosis to the adult. Reproductive systems are often very complex—in the male system one or several testes connect to one or paired sperm ducts, united these form an ejaculatory duct usually with a seminal vesicle interpolated; there is often prostatic tissue and then a copulatory apparatus frequently armed with hard, so called 'cuticular' or more appropiately 'sclerotic', processes. The female systems may be equally complex and the degree to which the yolk or vitelline material is separated from the oocytes forms the fundamental basis for distinguishing the higher orders of the group. In the more primitive forms the yolk is incorporated into the oocytes, there may or may not be discrete ovaries and oviducts. In the more highly evolved groups oocytes develop in separate regions or discrete organs (ovaries) and the yolk develops in vitelline tissue or glands. A seminal receptacle is often present, as is a region in which the egg capsule is formed. This is commonly called a uterus which may have one or more secondary storage spaces for capsules. The female gonopore, when present, is often, but not always, united with the male gonopore to form a common genital opening. The complexity and diversity of the reproductive systems form the main means of classifying the turbellarians.

Turbellarians are ubiquitous forms in fresh and marine habitats; they are common in moist terrestrial habitats and very many associate with other organisms to various degrees. Because they are the simplest of the triploblastic metazoa there is considerable interest in their form and function amongst those interested in the phylogeny of the invertebrates. Because of their relatively simple form, yet their adaptation to almost all modes of life and habitats, they are also excellent tools for experimental biology, e.g. embryology, cellular regeneration and differentiation, physiology and behavior. They are also of considerable value to ecological studies as work with freshwater triclads, for example, has shown.

Considered a class of the old phylum 'Vermes' by Ehlers (1864), the Turbellaria (a name coined by Ehrenberg (1831)) were placed in the phylum Plathelminthes (Platyhelminthes is etymologically correct) by Schneider (1873). Graff gave exhaustive accounts of the group in Bronn's Klassen und Ordnungen des Tier-Reichs published in 1904-8 and 1912-17. A more recent overview, also in German, was published by Bresslau (1933) and another in French by Beauchamp (1961). The only generalised account in English is by Hyman (1951), though Crezée (1982) has presented a synopsis of the families. Recent work employing ultrastructure studies and cladistic analysis (Ehlers, 1984; Ax, 1984) suggests the class Turbellaria is an invalid taxon consisting of several essentially non-parasitic groups of the Phylum Platyhelminthes. More work is clearly needed: Ax (1984) preferred to leave the Prolecithophora and the Lecithoepitheliata incertae sedis.