Biologists have described more than 1 million species of animals, but they think that several million remain to be discovered and classified. Biologists divide the kingdom Animalia into 35 phyla. Familiar animals such as birds, fish, frogs, snakes, and mammals are vertebrates and part of the phylum Chordata. They comprise less than 5% of the animal species. Most animals lack a backbone (invertebrates), such as sponges, jellyfish, insects, and worms.

Taxonomists classify animals into different phyla based on body symmetry, type of body cavity (coelom), and pattern of early development. (For more information on this system, refer to Table 28-1, "Overview of the Animal Kingdom," on page 602 of your textbook.)