In this topic you will learn how to tell whether an organism is an animal.

All animals share common characteristics. They are made of cells that reproduce; they move in some way; they grow and change, and eat food.

All living things need energy to stay alive. Animals get energy from food. A food chain shows the steps in which organisms get the food they need to survive. An ecosystem can have different food chains. Combined, they form a food web. A food web shows how food chains in an ecosystem are related.

Animals are also different in many ways. One major difference is whether they have a backbone. An animal with a backbone is called a vertebrate. An animal without a backbone is called an invertebrate.

Another difference between animals is the way their body parts match up around a point or central line. This is known as symmetry. Body parts with symmetry match up as mirror images of each other when they are folded together.

Some animals, such as the sponge, have no symmetry. An animal such as a starfish has radial symmetry, with body parts that extend outward from a central point. You could fold a starfish through its center five ways and its parts would match up. A sea urchin has spherical symmetry. You could fold a sea urchin any way through its center and it would match up.

An animal with bilateral symmetry has two sides, which are mirror images of each other. You could fold a butterfly only one way through its center to have its parts match up. Organisms with bilateral symmetry have a definite front end, tail end, upper side, and lower side. Invertebrates most commonly have radial and bilateral symmetry.

As you study different animals, you will get to know what characteristics to look for in order to tell the difference between them.