Flatworm
The
flatworms, or
Platyhelminthes,
Plathelminthes, or
platyhelminths (from the
Greek πλατύ,
platy, meaning "flat" and ἕλμινς (root: ἑλμινθ-),
helminth-, meaning worm) are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Unlike other bilaterians, they are acoelomates (having no body cavity), and have no specialized circulatory and respiratory organs, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both the ingestion (intake of nutrients) and egestion (removal of undigested wastes); as a result, the food cannot be processed continuously. In traditional zoology texts, Platyhelminthes are divided into
Turbellaria, which are mostly nonparasitic animals such as planarians, and three entirely parasitic groups:
Cestoda,
Trematoda and
Monogenea; however, since the turbellarians have since been proven not to be monophyletic, this classification is now deprecated.