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.
There are many different phyla of worms. The following three are the
best-known. Some worms live on land in burrows, feeding on plant matter; others
live in the sea or fresh water, filtering food from water.
(flatworms
)
Features: about 20,000 species flat,
unsegmented bodies, with a mouth but no anus, many live in water
(segmented
worms)
Features: about 15,000 species segmented
bodies, mostly burrowing, gut with mouth and anus, live on land and in
water
(roundworms)
Features: about 25,000 species
unsegmented bodies, gut with mouth and anus
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 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.
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, 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.
This group divides into earthworms, bristleworms, and
leeches. All have segmented bodies. The worms’ bodies are fluid-filled,
but the leeches are solid.
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.
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.
