Phylum: Platyhelminthes

Flatworms

The members of this phylum are among the simplest of invertebrates, and even the simplest of worms. Their bodies are quite flat, or ribbon shaped, with a digestive tract that opens only on one end—the mouth. Flatworms can be either aquatic, free-living organisms (such as the planaria), or parasites which feed on the blood and tissues of their host (such as the tapeworm).

Free-living flatworms belong to the class Turbellaria. Parasitic flukes belong to the class Trematoda. Tapeworms belong to the class Cestoda—these parasites have a mouth ringed by suckers and hooks and inhabit the intestines of their host, reaching a length of over fifty feet in humans!

A marine flatworm. NOAA