flatworm

Flatworms are animals of the phylum Platyhelminthes. They are bilaterally symmetric and dorsoventrally flattened, and they lack a true body cavity (acoelomates). The group includes free-living species such as planarians and numerous parasitic lineages, notably flukes (trematodes) and tapeworms (cestodes). They occur in marine, freshwater, and damp terrestrial environments, and many parasitic forms inhabit the bodies of vertebrates.

Anatomy and physiology: Flatworms have simple organ systems. Most have a gut with a single opening or a highly branched gut, while many parasitic species lack a gut altogether and absorb nutrients across the outer surface. Excretion and osmoregulation are performed by protonephridia with flame cells. The nervous system consists of cephalic ganglia and longitudinal nerve cords, forming a ladder-like arrangement; some species bear eyespots or other sensory organs. The body is often covered by a syncytial tegument in parasitic forms, facilitating nutrient uptake and protection from host defenses. Planarians are renowned for their regenerative abilities due to stem-like neoblasts.

Reproduction and development: Most flatworms are hermaphroditic and can reproduce sexually, though cross-fertilization is common. Many species also reproduce asexually by transverse fission. Parasitic groups have complex life cycles often requiring one or more intermediate hosts and multiple larval stages, such as miracidia, sporocysts, rediae, and cercariae, before reaching the adult stage in the final host.

Ecology and significance: Flatworms play diverse ecological roles as free-living predators and as parasites. They include important human and animal parasites, e.g., Schistosoma species and Taenia tapeworms. Phylogenetically, major groups are Turbellaria (mostly free-living), Trematoda, and Cestoda.