---
title: "Classifying Animals"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 21
exercises: 11
source: https://one-course.com/books/biology/1/en/chapter/21-classifying-animals
---

# Chapter 21 — Classifying Animals

A natural history museum keeps millions of [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) — drawers of beetles, halls of [skeletons](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton), jars of [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups). Yet a curator can walk to any one of them in minutes. The secret is not memory but *order*: every [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) has its place in a great system of groups within groups, built from the sorting rules of [Chapter 14](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#ch-g3-sorting-living-things) — and this year we learn how the system works.

## 21.1 Groups inside groups

**Definition 21.1 (Classification).**

To *classify* [living things](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) is to sort them by shared body features into groups, and to sort those groups into bigger groups — *groups nested inside groups*, like boxes in boxes. The whole arrangement is a *classification*.

**Example 21.2 (The cat’s addresses).**

One cat, several nested addresses: the cat is a *mammal* ([fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering), [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) for the young); every mammal is a *[vertebrate](#def-g4-classifying-animals-vertebrate)* (a [skeleton](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) with a [backbone](#def-g4-classifying-animals-vertebrate) inside); every [vertebrate](#def-g4-classifying-animals-vertebrate) is an *[animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal)*. Cat inside [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups), [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) inside [vertebrates](#def-g4-classifying-animals-vertebrate), [vertebrates](#def-g4-classifying-animals-vertebrate) inside [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal): each box sits inside a bigger one.

**Definition 21.3 (Vertebrates and invertebrates).**

A *vertebrate* is an [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) with an inner [skeleton](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) built around a *backbone* — like the [spine](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) you felt in [Example 17.2](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#ex-g3-bones-and-muscles-feel). An [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) without a backbone is an *invertebrate*: [insects](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups), spiders, snails, worms, jellyfish — the vast majority of all [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal).

![Boxes in boxes: the five classes of vertebrates together form one box, sitting beside the far more crowded invertebrates, inside the box of all animals.](https://one-course.com/images/onecourse/chapters/biology-1/g4-classifying-animals/fig-4338b98aaa8c.svg)

*Boxes in boxes: the five classes of [vertebrates](#def-g4-classifying-animals-vertebrate) together form one box, sitting beside the far more crowded [invertebrates](#def-g4-classifying-animals-vertebrate), inside the box of all [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal).*

## 21.2 The five classes of vertebrates

**Proposition 21.4 (The five vertebrate classes).**

[Vertebrates](#def-g4-classifying-animals-vertebrate) are classified into five great groups, called *classes*:

1. *[fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups)* : scales, fins, breathe in water — trout, shark, sardine;
2. *amphibians* : moist bare [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) , [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) in water, tadpole childhood — frog, toad, newt;
3. *reptiles* : dry scaly [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) , most lay shelled [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) on land — lizard, snake, tortoise, crocodile;
4. *[birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups)* : [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) , beak, shelled [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) — blackbird, ostrich, penguin;
5. *[mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups)* : [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) or hair, young fed on [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) — cat, whale, bat, human.

**Proof.** *Admitted at this level.* ∎

**Example 21.5 (Sorting a zoo delivery).**

Six crates arrive: a python (dry scaly [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense), shelled [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) — reptile), a toad (moist bare [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) — amphibian), a penguin ([feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) — bird), a shark (fins and gills — [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups)), a dolphin ([milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) for its calf — mammal), and a crate of giant snails (no [backbone](#def-g4-classifying-animals-vertebrate) at all — [invertebrates](#def-g4-classifying-animals-vertebrate), in no [vertebrate](#def-g4-classifying-animals-vertebrate) class whatever).

**Remark 21.6 (Reptile or amphibian?).**

Lizards and newts look alike at a glance — four [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), long tail. The [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) decides: the lizard’s is dry and scaly, the newt’s moist and bare; the lizard’s [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) have [shells](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) and are laid on land, the newt’s are laid bare in water. Same silhouette, different classes.

## 21.3 Using a classification

**Method 21.7 (Placing a vertebrate in its class).**

Ask the questions in order, stopping at the first yes:

1. [Feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) ? — a bird.
2. [Fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) or hair (or [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) for the young)? — a mammal.
3. Dry scaly [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) ? — a reptile.
4. Moist bare [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) and a tadpole childhood? — an amphibian.
5. Fins and gills, life in water? — a [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) .

No [backbone](#def-g4-classifying-animals-vertebrate) at all? Then it was an [invertebrate](#def-g4-classifying-animals-vertebrate), and the questions do not apply.

**Proposition 21.8 (What a class tells you for free).**

Knowing an [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal)’s class is knowing a bundle of facts before looking: told only “it is a mammal”, you already expect [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering), [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) for the young, and — like nearly all [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) — young [born alive](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born). That saving is the whole point of [classification](#def-g4-classifying-animals-classification): learn the group once, and every member arrives half-known.

**Proof.** *Admitted at this level.* ∎

**Example 21.9 (A new species arrives).**

Naturalists still discover [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) never seen before. When a new bat-like [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) is found with [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) and [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born), it goes straight into the [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) — and at once biologists expect a [backbone](#def-g4-classifying-animals-vertebrate), a four-chambered heart and warm blood, before any of it is checked. The [classification](#def-g4-classifying-animals-classification) pays out predictions.

**Remark 21.10 (Toward the family tree).**

Why do [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering), [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) and [backbones](#def-g4-classifying-animals-vertebrate) travel together so faithfully? The full answer — that members of a group inherit their shared features from common ancestors — belongs to the story of kinship and evolution, taken up in later years. For now, the nested boxes simply work, and work marvellously.

![The classifier’s favorite headache: fur and milk, but a bill, webbed feet — and eggs. The platypus (see the last exercise) tests the whole method.](https://one-course.com/images/onecourse/chapters/biology-1/g4-classifying-animals/img-e0f6aaf3a6cb.jpg)

*The classifier’s favorite headache: [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) and [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born), but a bill, webbed feet — and [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born). The platypus (see the last exercise) tests the whole method.*

## 21.4 Exercises

**Exercise 21.1 ★.**

What is a [vertebrate](#def-g4-classifying-animals-vertebrate)? What is an [invertebrate](#def-g4-classifying-animals-vertebrate)? Give two examples of each.

**Solution of Exercise 21.1.**

A [vertebrate](#def-g4-classifying-animals-vertebrate) has an inner [skeleton](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) built around a [backbone](#def-g4-classifying-animals-vertebrate) (the trout, the cat); an [invertebrate](#def-g4-classifying-animals-vertebrate) has no [backbone](#def-g4-classifying-animals-vertebrate) (the snail, the bee, the earthworm, the jellyfish).

**Exercise 21.2 ★.**

Name the five classes of [vertebrates](#def-g4-classifying-animals-vertebrate) with their defining features.

**Solution of Exercise 21.2.**

[Fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups): scales, fins, breathe in water. [Amphibians](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups): moist bare [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense), [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) in water, tadpole childhood. [Reptiles](#prop-g4-classifying-animals-classes): dry scaly [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense), most laying shelled [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) on land. [Birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups): [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering), beak, shelled [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born). [Mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups): [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) or hair, young fed on [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born).

**Exercise 21.3 ★.**

Place each [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal): the sardine, the crocodile, the newt, the ostrich, the whale, the earthworm.

**Solution of Exercise 21.3.**

Sardine: [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups). Crocodile: reptile. Newt: amphibian. Ostrich: bird. Whale: mammal. Earthworm: [invertebrate](#def-g4-classifying-animals-vertebrate) — no [backbone](#def-g4-classifying-animals-vertebrate), so no [vertebrate](#def-g4-classifying-animals-vertebrate) class at all.

**Exercise 21.4 ★.**

Write the cat’s nested addresses, smallest box to biggest, as in [Example 21.2](#ex-g4-classifying-animals-cat).

**Solution of Exercise 21.4.**

The cat, inside the [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups), inside the [vertebrates](#def-g4-classifying-animals-vertebrate), inside the [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal).

**Exercise 21.5 ★.**

A lizard and a newt look alike. Which two features separate their classes?

**Solution of Exercise 21.5.**

The [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) — dry and scaly for the lizard, moist and bare for the newt — and the [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born): shelled and laid on land for the lizard, bare and laid in water for the newt.

**Exercise 21.6 ★.**

Which group holds the vast majority of all [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal)? Name four of its members.

**Solution of Exercise 21.6.**

The [invertebrates](#def-g4-classifying-animals-vertebrate). For example: [insects](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups), spiders, snails, worms (and jellyfish).

**Exercise 21.7 ★.**

Run [Method 21.7](#met-g4-classifying-animals-place) aloud on the penguin and on the dolphin, question by question.

**Solution of Exercise 21.7.**

Penguin: [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering)? Yes — a bird; stop at the first question. Dolphin: [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering)? No. [Fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) or [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born)? It feeds its calf [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) — a mammal; stop there.

**Exercise 21.8 ★.**

You are told only: “this [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) is a bird.” List three things you now expect without looking, by [Proposition 21.8](#prop-g4-classifying-animals-free).

**Solution of Exercise 21.8.**

For example: [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering), a beak, wings, shelled [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) — and a [backbone](#def-g4-classifying-animals-vertebrate), since [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) are [vertebrates](#def-g4-classifying-animals-vertebrate).

**Exercise 21.9 ★★.**

The whale has almost no hair and lives like a [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups). Using [Method 21.7](#met-g4-classifying-animals-place) and [Example 14.5](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#ex-g3-sorting-living-things-surprises), explain why it is nonetheless a mammal — and why “lives in the sea” never was a [classification](#def-g4-classifying-animals-classification) feature.

**Solution of Exercise 21.9.**

The questions stop at “[milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) for the young”: the whale nurses its calf, so it is a mammal, near-hairless or not. “Lives in the sea” is a [habitat](https://one-course.com/books/biology/1/en/chapter/11-where-animals-live#def-g2-where-animals-live-habitat), not a body feature — and [classification](#def-g4-classifying-animals-classification) was built on what an [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) *has* precisely because habits and homes can be shared by strangers: the shark and the dolphin share an address, not a class.

**Exercise 21.10 ★★.**

The snake has no [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts), and neither has the earthworm. Why is one a [vertebrate](#def-g4-classifying-animals-vertebrate) reptile and the other an [invertebrate](#def-g4-classifying-animals-vertebrate)? Which single feature outranks the missing [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts)?

**Solution of Exercise 21.10.**

The [backbone](#def-g4-classifying-animals-vertebrate). The snake has a [spine](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) of hundreds of small [bones](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) — a [vertebrate](#def-g4-classifying-animals-vertebrate), with the dry scaly [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) of a reptile; the earthworm has no [backbone](#def-g4-classifying-animals-vertebrate) at all — an [invertebrate](#def-g4-classifying-animals-vertebrate). Missing [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) are just a missing feature; the [backbone](#def-g4-classifying-animals-vertebrate) question is asked first.

**Exercise 21.11 ★★★.**

The platypus puzzled naturalists: [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) and [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born), yet it lays shelled [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born). Which class won, and why? What does the puzzle show about classifying by *bundles* of features rather than by a single one?

**Solution of Exercise 21.11.**

The [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) won: [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) and [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) for the young are the mammal bundle, and the platypus has both — the [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) is one feature out of step. [Classification](#def-g4-classifying-animals-classification) weighs the whole bundle rather than letting a single feature decide; an [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) odd in one respect still sits where most of its features point. (The platypus is odd enough that [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) with [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) form their own small sub-group.)