Due to their soft bodys Platyhelminthes do not preserve very well as fossils. Scientists believe the earliest Platyhelminthes arrived 550 million years ago. There are about 20,000 species of flatworms.
This is a planarian (Dugesia) and are free-living flatworms. They inhabit freshwater, and are carnivores or scavengers. Most are less than a centimeter long.
The picture above is a fluke. It has a complex life cycles often involving two or more host organisms. Flukes and other parasitic platyhelminthes pass through many juvenile stages that require multiple hosts before reaching adulthood, at which time they parasitize a definitive host.
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The marine flatworms (polycladids) are the largest of the free-living flatworms, sometimes reaching lengths of 15 centimeters. Polycladids get their name from their highly branched digestive cavity.
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