Types of Worms - What is a worm?

Worms have soft, thin bodies with no legs. There are thousands of different kinds, or species, of worm, some are so small that you need a microscope to see them and others grow as long as 10 feet (3 meters). Biologists sort worms into four very different groups.

Roundworms

The biggest group is that of roundworms. Roundworms, such as hookworms and vinegar eels, have bodies like pieces of thread.
Most roundworms are very small—scientists once counted 90,000 in one rotting apple! 
Some roundworms live in water, some live on land, and others live inside animals, plants, or humans.
Animals which live in or on other living things are called parasites.
Hookworms are parasites that can live in your body and will make you ill.

Ribbon worms

Most ribbon worms live in the sea. 
They are long and flat, like ribbons.
Ribbon worms eat other small animals, including worms and mollusks. A ribbon worm, such as the bootlace worm, catches its food by shooting out a long tube called a proboscis from the top of its head.

Segmented worms

The bodies of segmented worms, such as the earthworm and the leech, are divided into sections, called segments.
These make the worm look as though it has rings around its body.
Most leeches live in water.
Leeches have a flat body with a sucker at each end. They use the front sucker to feed on the blood of fish and other animals that live in the water.

Flatworms

Another group of worms is called flatworms. 
Some flatworms look like oval-shaped leaves, and others look like long ribbons. Many species of flatworm live in the sea, but some flatworms, called planarians, live among the stones and tiny plants found in freshwater lakes and streams.
Flukes and tapeworms are flatworms that are parasites. These flatworms commonly live inside the bodies of humans, where they can cause serious illnesses.