There are well over fifty thousand different kinds of worm, divided into three main groups. The annelids have bodies that are divided into segments. They include earthworms and leeches. Roundworms, also known as nematodes, do not have segments. Many of these are crop pests, eating plants and making crops prone to diseases. Others are parasites, living on or in other animals, some causing serious diseases in humans. Finally, flatworms also include several parasites, including some with a complicated life cycle that involves them living in two different animal hosts one after the other.
There are about one million species of worm, living in a wide range of habitats. They have a long, thin body, and have no legs. Many worms are parasites that live on or in another animal and use strong mouthparts to feed off that animal. Others are predators, and can move quite quickly. The three main groups are Flatworms, Roundworms, and Segmented Worms.
FLATWORMS
There are about 20,000 species of flatworm. They have a solid, flat body that does not contain blood. Most flatworms are parasitic, but some are free-living.
MARINE FLATWORM: Marine flatworms absorb oxygen through the surface of their very thin, flattened body. They creep along, rippling their body to help them move. Eyespots enable them to find their way around. Most are predators, eating tiny animals with the mouth situated on the underside of their body.
TAPEWORM: Tapeworms are parasites that live in other animals, including humans. They have hooks and suckers on their head to attach themselves to the animal’s gut wall. They have no digestive system but absorb food through the surface of their body. They are hermaphrodites – they produce both eggs and sperm.
ROUNDWORMS
Roundworms, or nematodes, are found almost anywhere and exist in huge numbers. As many of the roundworms are transparent, few people are aware of them. The roundworm has a long, round body that tapers towards the tail. The outer layer, or cuticle, is smooth. Muscles run along its body, but not around it. To move along, the worm contracts these muscles, thrashing backwards and forwards in a single plane, making C or S shapes.
SEGMENTED WORMS
This group divides into earthworms, bristleworms, and leeches. All have segmented bodies. The worms’ bodies are fluid-filled, but the leeches are solid.
EARTHWORM: Earthworms are formed from many segments. Only the gut runs through the whole body from head to tail. Worms have a circulatory system with blood vessels but no heart. The thickened area towards the front of their body secretes mucus, which binds mating worms together and forms a cocoon for eggs.
LEECH: Leeches are parasites that live on the outside of other animals. They have specialized cutting jaws to bite through skin so that they can suck the animal’s blood. Substances in their saliva prevent the blood from clotting and make the bite painless so that the animal is unaware it has been bitten. Leeches move by shifting one sucker forwards and then bringing the other one up behind it.
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