Invertebrates are animals that lack a backbone. Invertebrates account for more than 97% of all species alive today. They include animal groups such as sponges, cnidarians, flatworms, molluscs, arthropods, insects, segmented worms, and echinoderms as well as many other lesser-known groups of animals.
The first animals to have evolved were invertebrates. Fossil evidence of invertebrates dates back to the late Precambrian, 600 million years ago. Invertebrates evolved from single-celled microorganisms.
Since invertebrates do not have bones, a bony skeleton, or a backbone, they must instead gain structural support for their bodies in different ways. For example, sea anemones have a hydrostatic skeleton that produces support via sheets of muscles and an internal cavity filled with fluid. Other invertebrates such as insects and crustaceans have a hard outer shell or exoskeleton.
Some species of invertebrates form large colonies. Colonies are groups of animals of the same species that remain together throughout most of their life cycle. Members of a colony are often closely related and benefit from living together by dividing up the work of obtaining food, protecting themselves, and reproducing. Invertebrate colonies are most common in marine habitats where the members of the colony are often physically joined. Marine invertebrate colonies include corals, hydrozoans, Portuguese man-of-war, and sea squirts. Invertebrate colonies that occur on land have individuals that are separated. The best known terrestrial colonial invertebrates are the social insects—bees, ants, termites, and wasps.