A cross-section below ground, revealing a number of invertebrate species living in the soil. In every house, garden, field or forest on Earth, there is a world of activity that we seldom notice. On the ground, in the soil, in the air and in the water live many small creatures such as insects, spiders, millipedes, worms and snails. They are all invertebrates, animals without backbones. Some are large, predatory creatures, big enough to feed on rodents and small lizards. Others are far too tiny for us to see without a microscope. Invertebrates are an extremely wide-ranging group of animals—far larger than vertebrates. Land invertebrates include insects, spiders, scorpions, worms, gastropods, millipedes and centipedes. There are even more marine invertebrates, animals such as crabs, sponges and molluscs, which live in the world's oceans.
Arthropods are invertebrates that have a hard outer skeleton, called an exoskeleton, instead of an internal skeleton. This is made of a light, strong material, called chitin, that supports and protects their soft inner parts. They have legs and antennae made up of jointed segments (the word arthropod actually means "jointed legs").
Insects, centipedes, spiders, scorpions and crustaceans (including woodlice) are all types of arthropod. As arthropods grow, they moult their outer skeleton—which cannot grow any bigger—and grow a new one.
More than 80% of all known living animal species are arthropods.
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