Invertebrate

Invertebrates are animals that lack a vertebral column, or backbone, and a cranium. The term covers every animal phylum except the vertebrate branch of Chordata, so it groups together sponges, jellyfish and corals, flatworms, roundworms, segmented worms, mollusks, arthropods such as insects, spiders and crabs, and echinoderms such as sea stars. Some chordates are invertebrates too, including sea squirts and lancelets.

Invertebrates make up the vast majority of animal species, commonly estimated at more than nine in ten, and live in almost every habitat, from the deep-sea floor to soil and the air. Without bones, they support their bodies in other ways: insects and crustaceans have a hard exoskeleton, many mollusks build shells, and earthworms and sea anemones use a hydrostatic skeleton, in which muscles squeeze against a fluid-filled body cavity.

Invertebrate is a grouping of convenience rather than a natural group, because it is defined by a missing trait and does not contain all the descendants of one ancestor. Invertebrates play vital roles as pollinators, decomposers, food for other animals, parasites and carriers of disease.