Invertebrates are animals without
backbones, and they represent about 90% of all living creatures.
Three major invertebrate groups are
arthropods, annelids and molluscs.
The bodies of arthropods are protected by a
hard outer skin known as an exoskeleton. Their bodies are usually divided into
segments. There are four main arthropod groups:
Insect bodies have three segments, namely
head, thorax and abdomen. Three pairs of legs are attached to the thorax as are
the wings if the insect species has them. Sensing antennae, as well as eyes,
are on the head.
Crustaceans have two pairs of antennae and
eyes on stalks. Although most crustaceans live in water and breathe through
gills, there are some land species such as woodlice.
Spider bodies have two segments with four
pairs of legs. Some species have more than two eyes. Many spider species are
able to produce silk from which they construct webs.
Millipedes and centipedes are
multi-segmented arthropods, the distinction between them being that centipedes
have one pair of legs per segment and millipedes have two pairs.
Annelids are segmented worms, with each
segment containing a complete set of organs. Most worms burrow into the soil.
Molluscs have soft bodies and no skeleton,
although many species are protected by a hard shell. They include octopuses and
squids at sea and garden snails and slugs on land.
© John Welford
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