|Pseudobiceros bedfordi, (Persian Carpet Flatworm)|
Lang, 1884 
The Polycladida represents a highly diverse clade of free-living marine flatworms. They are known from the littoral to the sublittoral zone (extending to the deep hot vents), and many species are common from coral reefs. Only a few species are found in freshwater habitats.
Polyclads range from 3 to 20 millimetres (0.12 to 0.79 in) in length with a flattened, roughly oval, body shape and, in many cases, a pair of short tentacles on the head. They are distinguished from other related animals by the presence of a folded pharynx, an elongated intestine with numerous complex diverticula, and multiple ocelli.
The etymology of the order name Polycladida corresponds to the two ancient Greek words πολύς (polús), meaning "numerous", and κλάδος (kládos), meaning "branch". It refers to the ramified shape of the intestine in these flatworms.
Most polyclads hide away from direct light. However, some of the brightly colored species often are active during the day. With their flamboyant coloring they advertise their potential toxicity to visual predators such as fish.
Some polyclads develop through a Müller's larval stage.
Often polyclads are associated with other invertebrates, such as bivalve mollusks, sponges, corals, or ascidians. In such associations, the worms may use the invertebrates as a source of food, or they may find protection from predators inside the structural framework of these "hosts".
The order Polycladida is divided into the two suborders Cotylea and Acotylea, based on the character "presence/absence of a cotyl or sucker". Of the two, the Acotylea is the larger group with over 26 families worldwide. Acotyleans are major predators of sessile marine invertebrates such as all commercial bivalves species (including pearl and rock oysters), mussels, scallops and giant clams. Many acotyleans are dull in coloration, and cryptic in their behavior, hiding in crevices and under coral during the day.
Cotyleans, on the other hand, with as many as 16 families, are prominent members of tropical coral reef communities. Although cotylean flatworms are conspicuous predators in subtropical and tropical ecosystems, they are difficult to study. These worms are very fragile and when disturbed can break apart.
Some examples of Polycladida families are as follows:
- Lang, Arnold (1884). Die Polycladen (Seeplanarien) des Golfes von Neapel und der angrenzenden Meeresabschnitte. Eine Monographie. Fauna und Flora des Golfes von Neapel und der angrenzenden Meeresabschnitte, herausgegeben von der Zoologische Station in Neapel. Leipzig: W. Engelmann. p. 688.
- Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. p. 229. ISBN 0-03-056747-5.
- Bailly, Anatole (1981-01-01). Abrégé du dictionnaire grec français. Paris: Hachette. ISBN 2010035283. OCLC 461974285.
- Bailly, Anatole. "Greek-french dictionary online". www.tabularium.be. Retrieved November 25, 2018.
- Bulnes, Veronica (2010-01-01), Polycladida-Marine Flatworms, pp. 356–368, ISBN 9789563322446, retrieved 2018-11-25
- Bahia, Juliana; Padula, Vinicius; Schrödl, Michael (2017-05-11). "Polycladida phylogeny and evolution: integrating evidence from 28S rDNA and morphology". Organisms Diversity & Evolution. 17 (3): 653–678. doi:10.1007/s13127-017-0327-5. ISSN 1439-6092.
- Aguado, M. T.; Noreña, C.; Alcaraz, L.; Marquina, D.; Brusa, F.; Damborenea, C.; Almon, B.; Bleidorn, C.; Grande, C. (2017). "Phylogeny of Polycladida (Platyhelminthes) based on mtDNA data". Organisms Diversity & Evolution. 17 (4): 767–778. doi:10.1007/s13127-017-0344-4. ISSN 1439-6092.