Class Turbellaria has approximately 3000 species in 12 orders. Most are marine and benthic, but some also inhabit fresh water and moist temperate and tropical terrestrial habitats. The order Temnocephalida is entirely commensal or parasitic, but some members of other orders are commensal as well. Classification within the Turbellaria is in a state of flux as it is unclear whether the class is truly monophyletic. Currently orders are defined by the type of pharynx, organization of the reproductive system, and branching pattern of the gut. Orders of the class Turbellaria recognized by Ruppert and Barnes (1994) are: Nemertodermatida, Acoela, Catenulida, Haplopharyngida, Macrostomida, Polycladida, Lecithoepitheliata, Prolecithophora, Proseriata, Temnocephalida, Rhabdocoela, and Tricaldida. Brusca and Brusca (2003) include Temnocephalida as a suborder of Rhabdocoela, and recognize an additional class, Proplicastomata, which includes known specimens from Greenland.
Like other members of the phylum Platyhelminthes (flatworms), those of the class Turbellaria are dorsoventrally compressed, with high surface area to volume ratios. Marine species can be quite colorful, but the interstitial and terrestrial turbellarians tend to be drab. Turbellarians generally locomote by coordinated waves of cilia on a secreted mucus trail, though some species can swim by rhythmic muscle contractions. Their ciliated epidermis, the presence of sub-epidermal rhabdites, and their free-living condition distinguish turbellarians from members of the other classes of Platyhelminthes. The visual impression of their epidermal ciliary activity gives this group its name, as 'Turbellaria' is derived from the Latin term for "whirlpool."
Turbellarians lack fluid transport systems, and are acoelomate. The gut in turbellarians has only a mouth opening. There is usually a pharynx for introducing food into the gut, and undigested food particles are ejected through the mouth. The final stages of digestion are intracellular. Most members of class Turbellaria are predatory on invertebrates smaller than themselves; the rest are herbivores, ectoparasites, or scavengers. Turbellarians use protonephridia scattered throughout their epidermis for the excretion of metabolic waste.
Turbellarians sense their environment with statocysts, chemoreceptors, and photoreceptors. They do not have image-forming eyes, but many species have pigment cells and photoreceptors concentrated into eyespots. The complexity of the turbellarian peripheral nervous system varies from a simple nerve net based on pairs of longitudinal nerve cords to an interlacing web of nerves capable of fine muscular control. Turbellarians are simultaneous hermaphrodites and lay eggs bundled into cocoons. The young undergo direct development and hatch as juveniles. Spiral cleavage is prevalent. Some turbellarians can reproduce asexually by fission. Regeneration of somatic parts is well documented in Dugesia.
Some of the colorful marine polycladids are in demand in the aquarium trade, and Dugesia is a common laboratory animal, but the vast majority of turbellarians are of little economic importance to humans. Humans can threaten turbellarian populations through pollution of aquatic habitats, but there have only been a few case studies documenting this (New 1995).
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Andrew Campbell (author).