A Guide to Vertebrates and Invertebrates

Animal classification is a matter of sorting out similarities and differences, of placing animals in groups and then breaking those groups apart into subgroups. The whole endeavor creates a structure—a hierarchy in which the large high-level groups sort out bold and obvious differences, while the low-level groups tease apart subtle, almost imperceptible, variations. This sorting process enables scientists to describe evolutionary relationships, identify shared traits, and highlight unique characteristics down through the various levels of animal groups and subgroups.

Among the most basic criteria by which animals are sorted is whether or not they possess a backbone. This single trait places an animal into one of just two groups: the vertebrates or the invertebrates and represents a fundamental division among all animals alive today as well as those that have long ago disappeared. If we are to know anything about an animal, we should first aim to determine whether it is an invertebrate or a vertebrate. We'll then be on our way to understanding its place within the animal world.

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