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Marine Worms - Platyhelminths (Flatworms)

Polyclad Flatworms

The Polyclad Flatworms belong to a large group of the Flatworms called the Turbellaria. There are about 4,500 species of Turbelliara, most of these are free-living, and range from 1 mm to 60 cm long.

Most are predators or scavengers, and terrestrial species are mostly nocturnal and live in shaded humid locations such as leaf litter or rotting wood. However some are symbiotes of other animals such as crustaceans, and some are parasites. Free-living turbellarians are mostly black, brown or gray, but some larger ones are brightly coloured.

Main features of Flatworms


Most turbellarians are carnivores, either preying on small invertebrates or protozoans, or scavenging on dead animals. A few feed on larger animals, including oysters and barnacles, while someare commensal on the gills of horseshoe crabs. Other species swallow cells of green alga and do not feed as adults, relying on the algae to provide nourishment (similar to corals).

Many turbellarians clone themselves by splitting their bodies into pieces and others reproduce by budding. All turbellarians are hermaphrodites, having both female and male reproductive organs, and fertilize eggs internally by copulating. Some of the larger aquatic species mate by "penis fencing", a duel in which each tries to fertilize the other, and the loser adopts the female role of developing the eggs. In most species "miniature adults" emerge when the eggs hatch, but a few large species produce plankton-like larvae.


Polyclad Flatworm
Image © Paul Flandinette




This is an overview of the planarian, its body structure and its functions.


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