There are more than 9 million species of animals on Earth. They range from tiny creatures made of just a few cells to huge animals like the Polar Bear and the giant Blue Whale. All animals are multicellular, which means they are made of many cells. Animals are also heterotrophs, meaning they must get food by eating other organisms.

Animals digest food inside their bodies using a digestive system or digestive tract.

Unlike plants, animal cells do not have cell walls. Animal cells can work together to form tissues that perform special jobs. These tissues can then form organs such as the heart, brain, skin, and lungs.

Most animals can move at some point in their lives. Most also reproduce using sperm and eggs.

Scientists organize animals into large groups called phyla (singular: phylum). There are more than 35 different phyla in the animal kingdom.  Seven of the most common ones are: Annelida (segmented worms), Chordata (vertebrates and their relatives), Echinodermata (starfish and sea urchins), Porifera (sponges), Arthropoda (insects, spiders, and crustaceans), Cnidaria (jellyfish, corals, and sea anemones), and Mollusca (snails, clams, and squids).