What Makes an Animal an Animal?

Five groups of life forms live on our planet, divided into what scientists call "kingdoms." They are: protists, bacteria, fungi, plants, and animals.

Animals share several important traits:

  1. they eat other living things
  2. they can usually move from place to place
  3. they react quickly to their environment
  4. their bodies have multiple cells
  5. they usually reproduce sexually.

That All-Important Backbone!

According to scientists, up to 99% of all animal species are invertebrates, which means they lack a backbone. These include worms, jellies, anemones, snails, crabs, insects, and spiders.

Only 1% of all animal species are vertebrates (have a backbone). These are:

Animals are the most abundant living things on Earth. Scientists estimate there are about 9 or 10 million species. They exist in a staggering array of forms and sizes, ranging from a few cells to creatures that weigh several tons. Though most animals live in the seas, they're found in every habitat on the planet.