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Medicine LibreTexts

2.05: The Animal Kingdom

So what are animals? If we were suddenly confronted with an animal we had never seen in our lives before, how would we know it was not a plant or even a fungus? We all intuitively know part of the answer to this.


  • eat organic material (plants or other animals)
  • move to find food
  • take the food into their bodies and then digest it
  • and most reproduce by fertilizing eggs by sperm

If you were tempted to add that animals are furry, run around on four legs and give birth to young that they feed on milk you were thinking only of mammals and forgetting temporarily that frogs, snakes and crocodiles, birds as well as fish, are also animals.

These are all members of the group called the vertebrates (or animals with a backbone) and mammals make up only about 8% of this group. The diagram on the next page shows the percentage of the different kinds of vertebrates.

Proportions of different kinds of vertebrate.JPG

However, the term animal includes much more than just the Vertebrates. In fact this group makes up only a very small portion of all animals. Take a look at the diagram below, which shows the size of the different groups of animals in the Animal Kingdom as proportions of the total number of different animal species. Notice the small size of the segment representing vertebrates! All the other animals in the Animal Kingdom are animals with no backbone, or invertebrates. This includes the worms, sea anemones, starfish, snails, crabs, spiders and insects. As more than 90% of the invertebrates are insects, no wonder people worry that insects may take over the world one day!

Fraction of vertebrates within the animal kingdom.jpg


  • Ruth Lawson (Otago Polytechnic; Dunedin, New Zealand)