Platyhelminths

Platyhelminths, or flatworms, are a diverse phylum of soft-bodied invertebrates that include free-living and parasitic species. With about 20,000 described species, they inhabit marine, freshwater, and damp terrestrial environments. They are among the simplest bilaterally symmetric animals with organ systems, yet show a wide range of lifestyles and body plans.

They are dorsoventrally flattened and generally lack a true body cavity, making them acoelomates. Most have a blind gut; some parasites have reduced or no digestive system and absorb nutrients across the body surface. They lack circulatory and respiratory organs; gas exchange occurs by diffusion. The nervous system is ladder-like, with a cerebral ganglion and two nerve cords, and they excrete via protonephridia.

The phylum includes four major groups: Turbellaria (mostly free-living), Monogenea (ectoparasites on fish), Trematoda (flukes), and Cestoda (tapeworms). Turbellarians are mostly aquatic predators or scavengers with ciliated epidermis. Trematodes and cestodes are parasitic and possess a syncytial tegument; they have complex life cycles with one or more intermediate hosts. Trematodes often produce miracidia, sporocysts, rediae, and cercariae; cestodes form larval cysts within intermediate hosts.

Reproduction is predominantly sexual and hermaphroditic in many species, with cross-fertilization common. Asexual reproduction by fragmentation occurs in several free-living forms. Some flatworms show remarkable regenerative abilities.

Ecology and significance: Platyhelminths play key roles as grazers and parasites. Several species are important to human and veterinary health, including Schistosoma (trematodes) and Taenia and Echinococcus (cestodes). Planarians are model organisms for regeneration studies.