Platyhelminthes: a surprisingly diverse group
There are some 3000 species varying in size from microscopic to 600 mm, and although most are marine some species have managed to inhabit humid terrestrial environments and move on a bed of mucus. Many species in this phylum have become parasitic and live on the surface and inside bodies of other animals including man. Some of these parasitic forms such as liver flukes still resemble a basic flatworm form whereas others such as the tape worm have a highly modified morphology with hooks on their heads and an ability to detach egg-bearing sections of their posterior body parts.
In biology, Cestoda is the class of parasiticflatworms, called tapeworms, that live in the digestive tracts of vertebrates as adults and often in the bodies of various animals as juveniles. In a tapeworm infection, adults absorb food predigested by the host, so the worms have no need for a digestive tract or a mouth. Large tapeworms are made almost entirely of reproductive structures with a small "head" for attachment.