tricladids

Tricladids are flatworms belonging to the order Tricladida, within the class Rhabditophora of the phylum Platyhelminthes. They are mostly free-living and inhabit freshwater, marine, and terrestrial environments. The order is divided into two main suborders: Maricola, which comprises marine tricladids, and Continenticola, which includes freshwater and terrestrial species.

A distinguishing feature is a tripartite, three-branched digestive system. After the pharynx, the gut divides into three main branches that run through the body, a trait from which the group derives its name. In addition to the gut, tricladids are dorsoventrally flattened and typically have a broad, often triangular head with eyespots and paired anterior sensory lobes (auricles). The nervous system includes a pair of cephalic ganglia and longitudinal nerve cords, characteristic of flatworms.

Tricladids are predators or scavengers, feeding by everting a muscular pharynx to ingest small invertebrates or detritus. They reproduce both sexually and asexually; most are simultaneous hermaphrodites, and several species can reproduce by transverse fission. They also exhibit regenerative abilities, with some species capable of forming complete individuals from fragments.

Beyond their biology, tricladids have played a prominent role in scientific research. Planarians, particularly freshwater species, are model organisms for studying regeneration, stem cell biology (neoblasts), and development. Taxonomically, the group has undergone reclassification as molecular data clarified relationships within Platyhelminthes, leading to placement in Tricladida rather than traditional Turbellaria.