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The turbellarian body is elongated, relatively soft, and usually tapered at the ends. Sometimes a short tail-like section or lateral flaps are present near the cerebral region. With the exception of triclads, flatworms are generally not flat- despite their common name. Most microturbellaria are cylindrical in cross-section, with some differentiation of shape of the dorsal and ventral surfaces. Moreover, species that reproduce asexually may be composed of several zooids, giving a chain-like appearance to an individual. Turbellarians may be colourless, white, red, bluish, green, black, brown, or yellowish depending on epidermal and parenchymal pigments, gut content, and symbiotic algae. Anatomically, the most prominent turbellarian features are a ciliated epidermis, rhabdoids, an intestine without anus, ventral mouth, and complex reproductive system.
In most species, miniature replicas of the adult hatch directly from eggs; these juveniles differ from adults chiefly by the absence of reproductive systems. Some freshwater turbellarians, however, may be ovoviviparous (Mesostoma) or may have a larval stage distinctly different from the adult (Rhynchoscolex). Many seasonally occurring species are univoltine, particularly those associated with temporary habitats or at the extremes of geographical ranges. Most other species are multivoltine, with the number of generations depending on habitat availability. A similar diversity of life cycles is observed in triclads. As a rule, turbellarians are hermaphroditic. Asexual reproduction by means of paratomy, that is transverse division of the body, is common in several genera of microturbellaria.
The ecological distribution of both microturbellarians and triclads has been studied more intensively in Europe but most of the results are directly relevant to North American fauna. Ecological differentiation of microturbellarians in running waters is more pronounced than in lakes.
|Approx. number of species||Average body size (mm)||Comments and special features|
|Catenulida||60||0.5-1||Thin chains of zooids; common in various habitats|
|Acoela||3||0.5-1||Most species marine|
|Macrostomida||50||1.0-3||Common in various habitats|
|Prolecithophora||5||5.0-10||Many marine species|
|Lecithoepitheliata||10||3.0-10||Many marine species; some are terrestrial or semiaquatic|
|Proseriata||4||2.0-5||Many marine species|
|Dalyellioida||100||0.8-1||Common in various habitats|
|Dalyelliida||0.8-1||Common in various habitats|
|Temnocephalida||1.0-14||Commensals on crustaceans, snails, turtles, one parasitic|
|Typhloplanida||150||0.5-6||Common in various habitats including terrestrial and semiaquatic|
|Kalyptorhynchia||15||1.0-2||Rare; most species marine|
|Tricladida (planarians)||100||5-20||Greatest diversity associated with karst habitats|
High densities of both triclads and microturbellarians suggest that their role in biotic interactions of benthic communities may be greater than their contribution to the diet of other organisms. In some cases, microturbellaria may regulate population dynamics of zooplankton in ponds. More important, perhaps, is the functional role of microturbellaria as consumers of protozoans, rotifers, and algae.
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