If you can read this page, you’re an animal. Kingdom Animalia is an extremely diverse group with an elaborate set of defining characteristics, but as with plants, you know one when you see one. Because we are animals, it is the most studied and described group of living organisms.

Characteristics
Animals evolved from colonial flagellated protists that developed division of labor and then dependence among their specialized cells. Animals have a number of shared traits:

Germ Layers
Early in the development of animal embryos, distinct cell layers begin to differentiate. These are called germ layers or embryonic cell layers, and they go on to form specific tissues, organs, and systems. The number of germ layers dictate the complexity of the animals, and is an important taxonomic tool.

Animals with only one germ layer are extremely simple and lack true tissues and organs.

Animals with two or three germ layers are more specialized. Those with two layers are called diploblastic. They have an endoderm layer that develops into the gut and an ectoderm that becomes the outer, protective layer of tissue.

Triploblastic animals have a third, middle layer of germ cells called the mesoderm. This layer produces most of the internal tissues and organs.

Symmetry
Symmetry is another important descriptive characteristic of animals. Irregularly-shaped animals with no symmetry are simple and unspecialized. Animals that have radial symmetry have an axis centered around the mouth. Any plane that contains the axis divides these animals into two mirror images. These creatures don’t have distinct heads or paired body parts. They have an upper surface, and a lower surface, but no unique sides.

Animals with bilateral symmetry are the most sophisticated. There is only a single plane that can pass through the body to produce mirror images. These animals have a head and tail end and a left and right side.

Backbones
The kingdom Animalia can be broken into two informal groups—animals that have a backbone, called the vertebrates, and those that do not, called invertebrates. The earliest forms of animals were all invertebrates. This group is still in existence and comprises 95% of all living animal species.