The Animal Kingdom has by far the most number of species among all kingdoms. In fact, you could add up all the known species of the other four kingdoms and this figure would still be less than those found in the Animal Kingdom. Although the Animal Kingdom is highly diverse, all organisms in this kingdom share three main characteristics: they rely on other animals and plants for nourishment, they are mobile, and are multicellular.
Most animals reproduce sexually and have sense organs that allow them to hear, touch, see, taste, and smell. Many are made of millions of cells. These millions of cells can divide into tissue based on their function. When there are many tissues working together, they form an organ. When the organ works with other organs on a common task, this is known as an organ system. These levels of organization are unique to the Animal Kingdom.
Because the Animal Kingdom is so large, it is subdivided into other categories which are then also further divided. These subdivisions are based on animals' similar characteristics or lack thereof.
The most common way to divide the Animal Kingdom is to divide based on the presence or absence of a backbone. Animals with a backbone are called vertebrates and include animals such as fish, birds, and mammals. Invertebrates such as insects and mollusks lack a backbone.