All members of the animal kingdom can be conveniently divided into invertebrates or vertebrates. Invertebrates are animals that lack a backbone, while vertebrates have a backbone. Invertebrates are found in all the animal phyla (see Chapter 21), including the phylum Chordata.

The phylum Chordata includes animals that have a notochord, a hollow nerve cord, and gill slits. Within this phylum, the vertebrates are classified in the subphylum Vertebrata. There are more than 40,000 living species of vertebrates. Vertebrates are the largest animals ever to inhabit Earth; the dinosaurs were vertebrates, as is the blue whale. The vertebrates are divided into several classes, encompassing fishes, amphibians, reptiles, birds, and mammals.

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