Phylum Platyhelminthes and Phylum Aschelminthes

Phylum Platyhelminthes. Commonly called flatworms because the body is characteristically dorsoventrally flattened, this phylum may be free-living (mostly aquatic, e.g. Planaria) or parasitic in other animals, including humans (e.g. liver flukes and tapeworms). The body is bilaterally symmetrical, triploblastic and acoelomate — there is no body cavity between the gut and the body wall, this space being filled instead by a loose mesodermal tissue called parenchyma — and organisation has reached the organ level. Free-living forms such as Planaria show an extraordinary capacity for regeneration; a small fragment cut from the body can regrow the entire animal. Parasitic forms are structurally specialised for their mode of life: hooks and suckers are commonly present for attachment to the host's tissues, the outer body surface is usually covered by a resistant tegument that resists the host's digestive enzymes, and, since a parasite absorbs much of its nourishment directly from the host, the digestive system is often reduced or entirely absent. Most flatworms are hermaphrodite, and specialised excretory cells called flame cells help regulate the internal fluid balance. Fasciola (the liver fluke) and Taenia (the tapeworm) are the classic parasitic examples; Planaria is the classic free-living example. …