Flatworms

Flatworms

Ilyella gigas, from Cornell Collection of Blaschka Invertebrate Models. Photo: David O. Brown, used with permission from Cornell University.

Flatworms (Platyhelminthes) are a group of relatively simple, bilaterally symmetrical soft-bodied animals that live in marine, freshwater, and terrestrial environments. They are referred to as acoelomate because they lack an internal body cavity and have no specialized circulatory or respiratory organs. Their flat shape and usually small size allow oxygen to diffuse directly into their bodies. The digestive cavity has only one opening for both feeding and elimination of waste. Several groups of flatworms are parasitic on other animals.

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