DK Nature: Invertebrates

Invertebrates include more than 30 different phyla (major groups) of animals. Some of the most important ones are shown here.

6,000 species include starfish, sea urchins, and sea cucumbers.

10,000 species include corals, sea anemones, and jellyfish.

10,000 species include tube sponges and glass sponges.

800,000 species include beetles, flies, and ants.

70,000 species include slugs, snails, mussels, and squid.

9,000 species include earthworms and leeches.

Insects, crustaceans, and many other invertebrates have a hard outer case called an exoskeleton. This protects them against blows and predators, and keeps them from drying out. Slugs, leeches, and jellyfish have soft bodies and no exoskeleton. The pressure of fluids inside their bodies maintains their shape.

Like most invertebrates, sea urchins have relatively complex internal organs. Food is broken down using five teeth contained in a central structure known as Aristotle’s lantern, before passing into the intestine.

The hard exoskeletons of insects and creatures such as crabs do not grow with the rest of the body. As the animal grows, its exoskeleton gets too tight—so it sheds it from time to time. Underneath is a new, slightly bigger, and looser case. The animal pumps itself up with fluid before the case has time to harden.