All members of the Animalia kingdom, colloquially known as animals, are multicellular. They are also exclusively heterotrophic, meaning that they rely on exterior sources for food.
Animalia is the largest of the six kingdoms. It has an estimated 9 to 10 million members. The exact number of animals on Earth can only be estimated since new species are consistently being discovered while projections show that others are being eradicated.
In addition to being the largest of the six kingdoms, Animalia is also the most diverse. Members range from very small creatures consisting of just a few cells to those that have an untold number.