Tricladida

Tricladida is an order of free-living flatworms in the phylum Platyhelminthes. Members are best known for their distinctive digestive system, which consists of a three-branched gut that arises from the pharynx. The tri-branched intestine is the defining trait that gives the group its name and helps distinguish them from other flatworms.

Anatomy and physiology: Triclads are dorsoventrally flattened and typically have a ciliated epidermis. They lack specialized circulatory and respiratory organs and rely on diffusion for gas exchange. The nervous system includes a bilobed brain (cerebral ganglia) connected to two longitudinal nerve cords and a range of peripheral nerves. Excretion involves protonephridial systems with flame cells. Most species are hermaphroditic and reproduce sexually, though many freshwater forms can also reproduce asexually by fission.

Habitat and diversity: The order includes freshwater, marine, and some moist terrestrial species. Freshwater planarians (for example, those in the genera Dugesia and Girardia) are common in streams and ponds and are well represented in science due to their accessibility. Marine triclads inhabit coastal and benthic marine environments, while a few species live on damp soils and leaf litter in tropical and subtropical regions.

Ecology and research importance: Triclads are typically predators or scavengers on small invertebrates. They have become famous in developmental biology for their remarkable regenerative abilities; many species can regenerate entire bodies from fragments, making them important model organisms for stem cell research and regeneration studies.

Taxonomy: Within Tricladida, diversity is organized into suborders that reflect habitat and gut structure, including groups such as Maricola (marine) and Continenticola (freshwater and terrestrial), among others.