They are always moving. They move through the mud or sand eating plant or animal parts. They are dull pink in colour. Their bodies are not divided into ring-shaped segments like those of an earthworm. Most eat dead animal and plant parts thereby turning them into fertile soil.
Some round worms are parasites of crustaceans, insects and even other water worms.
Some nematoids eat other smaller living animals found in the water. Nematoids are 'hermaphrodites'. This means each animal has both male and female parts. Sperm from one animal must fertilize the eggs of another round worm. In a few species of round worms there are separate female and male animals.