Turbellaria are mainly free-living in fresh and coastal waters some live in tropical, terrestrial habitats, and others are commensals or parasites of aquatic invertebrates or, rarely, fish. Some zoologists include Temnocephalida in Turbellaria. Platyhelminthes are subdivided into 5 classes: Turbellaria, Temnocephalida, Monogenea, Trematoda and Cestoidea. Probably thousands of species occur in Canada, but only a fraction of them have been discovered. Most of the roughly 15 000 known species are parasitic. Most flatworms are hermaphroditic however, cross-fertilization between 2 individuals is the rule. With few exceptions among the flukes, the gut has only a ventral-surface opening, often close to the front end, serving both as mouth and anus. A simple alimentary tract (gut) occurs in most forms, but is absent in tapeworms and one group of free-living flatworms (Acoela). Flatworms lack circulatory and respiratory systems, but have well-developed reproductive and excretory ones. A body cavity is lacking internal organs are suspended in a spongy matrix (parenchyma). Flatworms vary in shape from leaflike to ribbonlike size ranges from microscopic to over 15 m long (some parasitic forms).įlatworms have an identifiable 'head,' sometimes highly modified as an attachment organ in parasitic forms. Flatworm (Platyhelminthes), phylum of soft, bilaterally symmetrical invertebrates.

Flat worm phylum