Polyclads are among the largest animals in the taxon Plathelminthes (flatworms), generally reaching sizes in the centimeter range. Unlike many free-living flatworms of the smaller variety, polyclads are actually flat animals, flattened in the dorsoventral axis. As flatworms are lacking respiratory organs and blood, they are dependent on their epidermis for oxygen supply. Oxygen passively diffuses to all body regions, thus making a high surface to volume ratio necessary in these animals.
Polyclads can be found in marine waters all over the world. Some species, e.g. Thysanozoon brocchi, appear to be distributed almost ubiquitously: in the Mediterranean (see picture above), the Japanese Sea and also in the Atlantic. This is made possible by mobile larval stages, such as Müller's Larvae, Goette's Larvae and Kato's Larvae.
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