polyclads

Polyclads are a diverse group of free-living marine flatworms belonging to the order Polycladida within the phylum Platyhelminthes. They are characterized by a flattened body and a highly branched gut, with many diverticula arising from a single pharynx. The name derives from the branched gut structure implied by the term polyclad.

Most polyclads are dorsoventrally flattened and range in size from a few millimeters to several centimeters. They possess a ciliated, often syncytial epidermis and lack specialized respiratory or circulatory systems, relying on diffusion for gas exchange and waste removal. The body plan is typically soft and smooth, with some species displaying bright coloration or patterns that may serve as warning or camouflage.

Polyclads are predominantly marine and occupy a variety of benthic habitats, including coral reefs, rocky shores, seagrass beds, and soft sediments, from shallow subtidal zones to deeper waters. They feed on a range of invertebrates, including small mollusks, crustaceans, and other invertebrates, using their eversible pharynx to ingest prey.

Taxonomically, Polycladida is divided into two main suborders: Acotylea and Cotylea. Acotylea lack a ventral attachment organ, while Cotylea possess a ventral sucker-like structure called the cotyl, used for attachment. Reproduction is typically hermaphroditic, with internal fertilization and egg cocoons; development can be direct or involve free-living larval stages in some taxa.

Polyclads are an important component of marine biodiversity and are often used in studies of flatworm evolution and benthic ecosystems.