All animals are motile, meaning they can move spontaneously and independently at some point in their lives.

The major group of animals are classified under the Kingdom Animalia, also known as Metazoa. There are around 9 to 10 million species of animals, but only about 800,000 species are identified.

Fossil records of animals were found in the era of the Cambrian explosion, about 540 million years ago. Animals are divided into various sub-groups called phyla. Biologists have identified about 36 phyla within the animal kingdom including birds, mammals, reptiles, fish, amphibians, etc.

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Most animals can be divided into four basic groups.

The Porifera (sponges) diverged from other animals early. They lack the complex organization found in most other phyla. Their cells are differentiated, but in most cases not organized into distinct tissues. Sponges are sessile and typically feed by drawing in water through pores. Archaeocyatha, which have fused skeletons, may represent sponges or a separate phylum.