Invertebrates

Invertebrates are animal species that do not possess or develop a vertebral column, derived from the notochord. This in effect includes all animals apart from the subphylum Vertebrata. Familiar examples of invertebrates include insects, worms, clams, crabs, octopus, snails, and starfish.

Taxonomically speaking, "invertebrate" is no more than a term of convenience. The overwhelming majority of animal species are invertebrates, because only about 4% of animal species include a vertebral column in their anatomy. In other words all animals except those in the chordate subphylum Vertebrata (fish, amphibians, reptiles, birds, and mammals) are regarded as invertebrates. Furthermore, many individual invertebrate taxa have a greater number and variety of species than the entire subphylum of Vertebrata. In fact some of the so-called invertebrata, such as the Chaetognatha and Hemichordata, are more closely related to the Chordata than to other invertebrate phyla.