As the common name suggest, these worms have a very thin, flattened body that lacks segmentation. Many flatworms are parasites, while the free-living species include some marine groups that are brightly coloured and are often mistaken for nudibranchs (which are molluscs). Some of these conspicuous marine flatworms are several centimetres long and are often observed in rock pools gliding over the substratum, or swimming freely in the water. These flatworms are generally carnivores that feed on soft corals, bryozoans or other invertebrates. Flatworms have only one body opening that serves as both mouth and anus, and oxygen and carbon dioxide and other wastes diffuse through the thin body. Reproduction is sexual, with most flatworms being hermaphrodites that produce both eggs and sperm.