Animalia

Did You Know?

Although the majority of invertebrates are small, a few reach impressive sizes. The true heavyweights of the invertebrate world are giant squid, which can be over 18 m (59 ft) long and can weigh more than 2,000 kg (4,000 lb). The longest are ribbon worms, also known as nemerteans, whose pencil-thin bodies can grow up to 55 m (180 ft) from head to tail.

What are Invertebrates?

Invertebrates-- animals lacking a backbone--are by far the most numerous animals on Earth. Nearly 2 million species have been identified to date. These 2 million species make up about 98 percent of all the animals identified in the entire animal kingdom.

Invertebrates live in a vast range of habitats, from forests and deserts to caves and seabed mud. In oceans and lakes they form part of the plankton—an immense array of miniature living organisms that drift in the surface currents. Invertebrates are also found in the soil beneath our feet and in the air above our heads. Some are powerful fliers, using wings to propel themselves, but others, particularly the smallest invertebrates, float on the slightest breeze. These tiny invertebrates form clouds of aerial plankton that drift unseen through the skies.

Due to their numbers and variety, invertebrates share only a single trait in common: the absence of a backbone. Many invertebrates have no hard body parts at all. These soft-bodied invertebrates, which include earthworms, keep their shape by maintaining an internal pressure, similar to the air pressure within an inflated balloon. However, having a soft body has disadvantages, one of which is that it leaves animals vulnerable to attack from predators.

To defend against predators, other invertebrates have evolved exoskeletons, hard outer coverings such as the shells found in clams and mussels and the body cases that surround adult insects. As well as protecting the animal, these exoskeletons also provide anchorage for muscles. On land, a body case is also useful because it prevents the water that bathes internal structures from evaporating. As a result the animal does not dry up and die. Arthropods, animals with a hard, outer skeleton and a jointed body and limbs, make up the single largest group of invertebrates. Arthropods include insects, crustaceans, and arachnids, such as spiders and ticks.

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