Classification means sorting living things into groups based on features they share. Scientists choose features that can be observed and that do not change from one individual to the next.

Animals are first split into vertebrates (animals with a backbone) and invertebrates (animals without a backbone). Most animal species are invertebrates.

The vertebrates are divided into five classes: fish, amphibians, reptiles, birds and mammals. Large invertebrate groups include arthropods (jointed legs and a hard outer skeleton, such as insects, spiders and crabs), molluscs (soft bodies, often with a shell, such as snails and octopuses) and segmented worms (such as earthworms).

A dichotomous key identifies an organism through a series of steps. Each step gives exactly two choices, and you follow the choice that fits until you reach a name.

Animal key

1a. Has a backbone ... go to 2

1b. Has no backbone ... go to 6

2a. Has feathers ... bird

2b. Has no feathers ... go to 3

3a. Has hair or fur ... mammal

3b. Has no hair or fur ... go to 4

4a. Has fins and breathes with gills as an adult ... fish

4b. Has no fins as an adult ... go to 5

5a. Has dry, scaly skin ... reptile

5b. Has moist skin without scales ... amphibian

6a. Has jointed legs ... go to 7

6b. Has no jointed legs ... go to 8

7a. Has six legs and three body parts ... insect

7b. Has eight legs and two body parts ... arachnid

8a. Has a soft, unsegmented body, often with a shell ... mollusc

8b. Has a long body divided into ring-like segments ... segmented worm