invertebrate ĭn˝vûr´təbrət, –brāt˝ [key], 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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