flatworms

Flatworms, or Platyhelminthes, are a diverse phylum of invertebrates that are typically soft-bodied. They are bilaterally symmetrical and dorsoventrally flattened. They lack specialized respiratory and circulatory systems and rely on diffusion across a moist body surface. They are acoelomates with three germ layers (triploblastic).

Most flatworms possess a gastrovascular cavity with a single opening, but many are highly branched and can digest food in cells. Some, like tapeworms, have no digestive tract and absorb nutrients through their syncytial tegument. They occur in marine, freshwater, and terrestrial environments.

The nervous system forms a ladder-like structure with cephalization and paired eyespots in many species. Excretion and osmoregulation occur via protonephridia. Reproduction is usually hermaphroditic; sexual reproduction often involves cross-fertilization, while many free-living species reproduce asexually by fission and regeneration.

Flatworms include free-living groups, such as Turbellaria (planarians), and parasitic groups, including Trematoda (flukes) and Cestoda (tapeworms), with Monogenea also parasitic on fish. They play roles as scavengers and predators and are important in medical and veterinary contexts as parasites.

In biology, flatworms are studied for regenerative capacity and early tissue organization, providing insights into nervous system and organ development. The phylum has a fossil record dating to the Precambrian and shows diverse adaptations to a wide range of habitats.