ĭnˌvûrˈtəbrət, –brātˌ, any animal lacking a backbone. The invertebrates include the tunicates and lancelets of phylum Chordata, as well as all animal phyla other than Chordata. The major invertebrate phyla include: the sponges ( Porifera), coelenterates ( Cnidaria), echinoderms ( Echinodermata), flatworms ( Platyhelminthes), roundworms ( Nematoda), segmented worms ( Annelida), mollusks ( Mollusca), and arthropods ( Arthropoda). Invertebrates are tremendously diverse, ranging from microscopic wormlike mezozoans (see Mezozoa) to very large animals such as the giant squid. Approximately 95% of all the earth's animal species are invertebrates; of these the vast majority are insects and other arthropods. Invertebrates are important as parasites and are essential elements of all ecological communities.

See A. Kaestner, Invertebrate Zoology (3 vol., 1967–70); R. D. Barnes, Invertebrate Zoology (5th ed. 1987); R. Buchsbaum et al., Animals without Backbones (3d ed. 1987).

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