Marine triclads belong to the Class Turbellaria of the Phylum Platyhelminthes or Flatworms.
Platyhelminths are Bilateria with weakly multicellular epidermal cells, the cilia being devoid of an accessory centriole at the basal body. The bilaterally symmetrical protonephridia are provided with more than one terminal cell; terminal and canal cells are biciliar, both cilia without accessory centriole and without an inner circle of 8 microvillar rods. Epidermal cells and other somatic cells do not undergo any mitosis.
The Class Turbellaria comprises the free-living flatworms. Because it has been shown that the Turbellaria are a paraphyletic group the class name Turbellaria does not feature in modern classifications. However, for convenience the name Turbellaria is still used frequently to designate the free-living, non-helminth flatworms.
Turbellarians are acoelomate, bilaterally symmetrical animals with a blind gut and a cellular, ciliated epidermis. Usually, turbellarians are free-living but there are some commensal and parasitic species. Most turbellarians are hermaphrodites, but asexual reproduction is common in several groups. In most turbellarians the sperm has two flagellae, each flagellum generally containing a circle of nine peripheral doublet microtubules with in its centre only one electron dense core. Cleavage and cell lineage are very similar to the spiralian pattern.
Turbellarians are softbodied worms living in aquatic or marine environments, sometimes on land in most environments. Most turbellarians are very small (less than 1 mm) and all turbellarians are difficult to identify and require careful fixation.
Marine triclads, or Maricola, represent one of the three main groups (infraorders) of planarians. The other two groups are the freshwater planarians, or Paludicola, and the land planarians, or Terricola. Together, these three groups constitute the suborder Tricladida.