Invertebrates, animals that have no backbones, or vertebral columns. They include such widely different kinds of animals as cockroaches, clams, sea anemones, and giant squids.

Invertebrates were the earliest animals, appearing on earth between three and five billion years ago. Nearly a million species have been classified, and some scientists believe there may be several million unknown species. They make up all the large groups, or phyla, of animals except the phylum Chordata, which is shared with the vertebrates (animals with backbones).

CockroachesCockroaches are invertebrates.

The many kinds of invertebrates have little in common except their lack of backbones and of internal temperature controls. Like such vertebrates as fishes and reptiles, they are cold-blooded: their body temperature approximates that of their environment.

Billions of individual invertebrates are found in water, on land, and living as parasites in the bodies of other animals or in plants.