The animal kingdom, from Three Domains and the Kingdoms, holds a huge range of creatures. The first big split is simple: does the animal have a backbone?
Key idea
Vertebrates have a backbone (a spine made of small bones called vertebrae): fish, amphibians, reptiles, birds and mammals. Invertebrates have none: insects, spiders, worms, snails, jellyfish and many more. About 95 of every 100 named animal species are invertebrates.
Why it works
A backbone protects the spinal cord and gives muscles a strong frame to pull on, which lets vertebrates grow large. But being large is not the same as being common. Insects alone count about a million named species, far more than all the vertebrates together. The animals we know best (pets, farm animals, zoo animals) are mostly vertebrates, which makes them seem typical.
Worked examples
Worked example: Backbone or not?
Does an octopus have a backbone?
- An octopus is a mollusk, like a snail.
- Mollusks have soft bodies and no bones.
Answer: No. It is an invertebrate.
Worked example: Most animals
Out of 1,000 named animal species, about how many are invertebrates?
- About 95% are invertebrates.
- $0.95 \times 1000 = 950$.
Answer: About 950.
Common mistake
Thinking most animals have backbones because the ones we know best do. Insects alone far outnumber all vertebrate species. Check: next time you are outside, count the kinds of animals you see. Most will be insects, spiders and other invertebrates.