Animals are multicellular, eukaryotic organisms. They are different from plants in that they take in food and digest it into smaller components. They are heterotrophic rather than autotrophic. In addition to these characteristics, animals are able to move during some point of their life cycle. Also, the primary mode of reproduction in animals is sexual. An animal grows and changes from a single fertilized egg cell into a multicellular organism, passing through various stages of development during its life cycle.

Two major groups of animals exist in the world: invertebrates and vertebrates. Invertebrates are animals that have no backbones, while vertebrates are animals with backbones (see Chapter 22). Among the invertebrates are numerous phyla of animals comprising approximately 95 percent of all the animal species.