The animal kingdom is one of the five kingdoms which scientists use to classify living things. With millions of species, it is the largest of the five kingdoms. Animals are multicellular organisms with eukaryotic cells. Unlike plants, animals get nutrients by eating – they can’t make energy themselves.
The first known person to classify living things into kingdoms was Aristotle, a Greek philosopher. He categorized all living things into two kindoms: plants and animals. Scientists still use the animal kingdom as a way to group together living things. However, as scientists learned more about animals, they split up the kingdoms into even smaller groups.
One important feature of animals is that the type of cells they have. Their cells are eukaryotic, which means they have a nucleus and organelles. Additionally, animal cells do not have cell walls. Animals are also multi-cellular, which means they have more than one type of cell.
Another important feature of animals is that they cannot make their own food like plants can. Instead, they have to eat to get energy. Animals also have the ability to move, to respond to their surroundings and to reproduce sexually.
In the animal kingdom, there are many smaller groups based on similar features. All animals can be split into two groups: vertebrates and invertebrates. Vertebrates are animals with a backbone and invertebrates are animals without a backbone.
Additionally, each kingdom is split into smaller categories called phylum. All of the species in one phylum share some common characteristics. You can visit the animal classification article to learn more about the smaller groups within the animal kingdom.