The Cestoda, or tapeworms, are intestinal parasites in vertebrates, and they also show anatomical and life history modifications for parasitism. Recent molecular studies suggest that the Platyhelminthes as a whole may even be polyphyletic , having arisen as two independent groups from different ancestral groups. If this latter view is correct, then most of the flatworms may belong to the Lophotrochozoa , a large group within the animal kingdom that includes molluscs and earthworms, while the rest belong near the base of animal diversity. Polycladids get their name from their highly branched digestive cavity. These individuals were photographed on a reef near the island of Guam.
Platyhelminths have practically no fossil record.
Trematode larvae that parasitize molluscs may leave pits or thin spots on the inside of the shell, and these pits may be recognized on fossil shells. If the mollusc is irritated by the presence of trematode larvae, it may be able to surround them with layers of shelly material - and thus do parasites become natural pearls. More detailed classification of platyhelminths is available from the Tree of Life at the University of Arizona. To find out more about tapeworms and flukes that cause human disease, read this handbook published by the U.S. Food and Drug Administration, view these pages produced by the World Health Organization . Images of polycladid marine flatworms taken by Allen G. Collins.