INVERTEBRATE

any animal that lacks a vertebral column, or backbone, in contrast to vertebrates, all of which possess some form of cartilaginous or bony internal skeleton. More than 90 percent of living animals are invertebrates. Worldwide in distribution, they range in size from minute protozoans to giant squids. Apart from the absence of a vertebral column, invertebrates have little in common. They are generally soft-bodied animals without a rigid internal skeleton for the attachment of muscles but often with a hard outer skeleton (as in most mollusks, crustaceans, and insects) that serves, as well, for body protection. The taxonomic term Invertebrata, formerly used in contrast to Vertebrata (still a valid subphylum name), has no validity in modern classifications.