By Dr. Kate Rawlinson, Smithsonian Marine Station, Fort Pierce, Florida
The Platyhelminthes , or flatworms, are a highly diverse phylum of over 25,000 species making them the largest phylum of acoelomates. They are dorsoventrally flattened and bilaterally symmetrical, and depending on species and age, individuals can range in size from almost microscopic to over 20 m long. Traditionally they’ve been divided into four classes; the free-living Turbellaria and the parasitic Cestoda (tapeworms), Trematoda (flukes such as the shistosomes – the cause of shistosomiasis or bilharzia) and Monogenea. The three parasitic classes make up the Neodermata.
The evolution of the flatworms has been a controversial topic in evolutionary biology in recent years. They have been promoted from being a basal metazoan group to a position within the protostomes and subsequently split into on the one hand the Catenulida plus Rhabditophora (Neodermata, plus Haplopharyngida,â€¨Lecithoepitheliata,â€¨Macrostomida,â€¨Polycladida,â€¨Prolecithophora,â€¨Rhabdocoela,â€¨Seriata,â€¨Temnocephalida) which remain in this elevated position and on the other hand the Acoelomorphs (Acoels and Nemertodermatida) which have been shunted back to the base of the tree.
Platyhelminthes have a more complex level of organization than the acoels. Rhabditophorans possess complex nervous, reproductive and excretory systems. In addition to the acoels, the catenulids are distinguished by their relative simplicity. The catenulids are small and roundish, with an anterior mouth. Contrary to acoels, they have a nephridium and a ciliated gut. The Acoela are very small and do not have a gut, digestion is accomplished by means of a syncytium that forms a vacuole around ingested food. There are no epithelial cells lining the digestive vacuole. Acoels are almost entirely marine, living between grains of sediment, swimming as plankton, or crawling on algae.
Flatworms are hermaphroditic and the Neodermata and some members of the Polycladida have complex life histories involving larval forms. Most free-living flatworms live in aquatic environments. Polyclads are diverse and often colorful members of the marine benthic fauna, found in habitats ranging from the intertidal to the deep sea. They are characterized by their highly branched gut and they predate on other marine invertebrates. They are generally cryptic and negatively phototactic.