Grade by Grade
B. Life Systems · Biodiversity

Classifying Animals: Vertebrates and Invertebrates

B2.1 A2.1 A1.5About 50 minutes
Learning goals
  • I can sort animals into vertebrates and invertebrates.
  • I can describe the characteristics of fish, amphibians, reptiles, birds and mammals.
  • I can describe major groups of invertebrates, including insects, arachnids and crustaceans.
  • I can use and write a dichotomous key, and code one that asks for input.

Words to know

vertebrate
An animal with a backbone (spine).
invertebrate
An animal without a backbone.
arthropod
An invertebrate with jointed legs, a body in segments and a hard outer covering called an exoskeleton.
exoskeleton
A hard covering on the outside of an animal's body, like a suit of armour.
ectotherm
An animal whose body temperature changes with its surroundings (often called "cold-blooded").
endotherm
An animal that keeps a steady body temperature using heat it makes itself (often called "warm-blooded").
metamorphosis
A big change in body form as an animal grows, such as a tadpole becoming a frog.
dichotomous key
A tool for identifying organisms by answering a series of two-choice questions.

Which one doesn't belong?

A crayfish, a pill bug, a spider and a grasshopper. Which one is the odd one out?

Many people say the crayfish, because it lives in water. But scientists group the pill bug with the crayfish: both are crustaceans! Classifying by careful characteristics, not by where something lives, can surprise you.

A brownish-olive crayfish with two large claws on the pebbly bottom of a clear stream.
A crayfish has no backbone. Its hard outer shell and jointed legs make it an invertebrate, a crustacean like the pill bug. AI-generated image

Backbone or no backbone?

The animal kingdom is often split first into two big groups:

  • Vertebrates have a backbone made of small bones called vertebrae. Fish, amphibians, reptiles, birds and mammals are vertebrates. You are a vertebrate!
  • Invertebrates have no backbone. Insects, spiders, snails, worms, jellyfish and sea stars are invertebrates.

Most animal species on Earth are invertebrates.

Vertebrate groupDistinguishing characteristicsOntario examples
Fishlive in water; breathe with gills; have fins; most have scales; ectothermsbrook trout, lake sturgeon, yellow perch
Amphibiansmoist skin with no scales; most lay jelly-coated eggs in water; young usually live in water and go through metamorphosis; ectothermsspring peeper, American toad, mudpuppy (a salamander)
Reptilesdry, scaly skin; most lay eggs with leathery or hard shells on land (some, like the garter snake, give birth to live young); ectothermspainted turtle, snapping turtle, eastern garter snake
Birdsfeathers; beak; two legs and two wings; lay hard-shelled eggs; endothermscommon loon, black-capped chickadee, Canada goose
Mammalshair or fur; mothers feed young milk; most give birth to live young; endothermsbeaver, moose, little brown bat, human
Characteristics of vertebrate groups. Scientists actually split fish into several classes, but "fish" is a handy group to start with.

The invertebrates

Invertebrates come in many body plans. Here are some major groups:

  • Arthropods have jointed legs, a segmented body and an exoskeleton. They include:
  • Insects: 3 body parts (head, thorax, abdomen), 6 legs, usually antennae, often wings. Ants, beetles, butterflies, dragonflies.
  • Arachnids: 2 body parts, 8 legs, no antennae. Spiders, ticks, mites.
  • Crustaceans: usually 2 pairs of antennae, many legs, most live in water. Crayfish, lobsters, and the pill bug, which lives on land in damp places.
  • Centipedes and millipedes: long bodies with many segments and many legs.
  • Molluscs: soft bodies, often with a shell. Snails, slugs, clams, octopuses.
  • Segmented worms: soft bodies made of ring-like segments. Earthworms, leeches.
  • Cnidarians: soft bodies with stinging tentacles. Jellyfish, corals, sea anemones.
  • Echinoderms: spiny skin, body parts arranged around a centre, live in the ocean. Sea stars, sea urchins.
Insect3 body parts, 6 legsSpider (arachnid)2 body parts, 8 legs
An insect has 3 body parts and 6 legs attached to the middle part (the thorax). A spider is an arachnid: 2 body parts and 8 legs. Not to scale.
Worked example

Let's sort Ontario animals

Classify each animal: spring peeper, pill bug, painted turtle, garden slug, common loon, little brown bat, lake sturgeon, earthworm.

  1. Spring peeper: moist skin, eggs in water, tadpole stage → vertebrate, amphibian.
  2. Pill bug: jointed legs, exoskeleton, more than 8 legs, related to crayfish → invertebrate, arthropod, crustacean.
  3. Painted turtle: dry scaly skin, lays eggs on land → vertebrate, reptile.
  4. Garden slug: soft body, no legs → invertebrate, mollusc (a snail without an outside shell).
  5. Common loon: feathers, beak → vertebrate, bird.
  6. Little brown bat: fur, feeds young milk → vertebrate, mammal (it flies, but it is not a bird!).
  7. Lake sturgeon: gills, fins, lives in water → vertebrate, fish.
  8. Earthworm: soft body in rings → invertebrate, segmented worm.

Ask first: backbone or not? Then look for the distinguishing characteristics of each group.

Just for fun

Funny but true: a pill bug is a crustacean, so it is more closely related to a lobster than to an ant. It's basically a tiny land lobster living under your flowerpot!

Dichotomous keys

Dichotomous means "split in two". A dichotomous key asks one question with two choices at each step. Each answer sends you to the next question, until you reach a name.

Because a key is a set of yes/no decisions, it is easy to turn into code. Here is a key written as a program that asks the user for input:

ask "Does the adult animal have a backbone? (yes/no)" and save the answer as backbone
if backbone = yes then
ask "Does it have feathers? (yes/no)" and save the answer as feathers
if feathers = yes then say "Bird"
else
ask "Does it have hair or fur? (yes/no)" and save the answer as fur
if fur = yes then say "Mammal"
else
ask "Does it have fins? (yes/no)" and save the answer as fins
if fins = yes then say "Fish"
else
ask "Is its skin dry and scaly? (yes/no)" and save the answer as scaly
if scaly = yes then say "Reptile"
else say "Amphibian"
else
ask "Does it have jointed legs? (yes/no)" and save the answer as jointed
if jointed = yes then
ask "How many legs does it have?" and save the answer as legs
if legs = 6 then say "Insect"
else if legs = 8 then say "Arachnid"
else say "Another arthropod: maybe a crustacean, centipede or millipede"
else say "Another invertebrate: maybe a mollusc, worm, jellyfish or sea star"

A dichotomous key as code. Each "ask" gets input from the user, and each "if" is one split in the key.

Backbone?yesnoFeathers?yesBirdnoHair or fur?yesMammalnoFins?yesFishnoDry, scaly skin?yesReptilenoAmphibianJointed legs?yesnoHow many legs?68otherInsectArachnidother arthropodanother invertebrateVERTEBRATESINVERTEBRATESFollow one answer at a time until you reach a group name.
The dichotomous key from the code, drawn as a flowchart. Start at the top, answer each question, and follow the line to a group name.

Test your key

Scientists and coders test with tricky cases. Run the key in your head for a bat (backbone yes, feathers no, fur yes → Mammal: correct!) and a pill bug (backbone no, jointed legs yes, 14 legs → Another arthropod: correct!).

Hands-on activity

Build and code your own key

You need: paper, pencil, 8 index cards (or paper squares). A computer with a block-coding app (such as Scratch) is optional.

  1. Write or draw one animal on each card. Choose animals from at least three different groups, for example: robin, moose, garter snake, American toad, ladybug, spider, crayfish, snail.
  2. Find one yes/no question that splits the cards into two piles. Write it down.
  3. Keep splitting each pile with a new question until every card is alone.
  4. Draw your key as a branching chart.
  5. Test it: ask someone to think of one of your animals and answer your questions. Does the key reach the right animal?
  6. Code it (optional): in a block-coding app, use "ask" blocks to get the user's answers and "if / else" blocks for each split, like the example above.

What to look for: questions about characteristics you can observe ("Does it have 8 legs?"), not opinions ("Is it scary?"); each question has only two answers; every animal ends up alone.

Your turn

1

What is the difference between a vertebrate and an invertebrate?

2

Name the five vertebrate groups.

3

Which vertebrate group has feathers?

4

How many legs does an insect have? An arachnid?

5

Is a tick an insect or an arachnid? How do you know?

6

A salamander has moist skin with no scales. Is it a reptile or an amphibian?

7

Which group of invertebrates has soft bodies and often a shell?

More practice: warm-up

1

Is a beaver a vertebrate or an invertebrate?

2

Name an invertebrate that lives in soil.

3

What are the three body parts of an insect?

4

Which vertebrates feed their young milk?

5

What does dichotomous mean?

More practice: on your own

1

A whale lives in water and has flippers. Why is it a mammal and not a fish?

2

Explain why a pill bug is a crustacean and not an insect.

3

Ectotherms and endotherms: which vertebrate groups are endotherms? Why might endotherms need to eat more food?

4

Write a yes/no question that separates a spider from a ladybug.

5

In the coded key, what would the program say for a snail? Trace the answers.

Challenge

1

A mudpuppy is a salamander that keeps its gills as an adult and lives in water its whole life. A student says it must be a fish. Use the coded key (answering for the adult) to classify it, and explain.

2

Find a weakness in the coded key: name an animal it would classify wrongly or could not handle, and suggest a fix.

3

Both bats and moths fly at night. Using classification, explain why they are in different groups even though they live similar lives.

Check your understanding

Quiz

1.

Which animal is an invertebrate?

2.

An animal has 8 legs, 2 body parts and no antennae. It is...

3.

Which vertebrate group has dry, scaly skin and lays shelled eggs on land?

4.

A bat is classified as a mammal because it...

5.

A good question for a dichotomous key is...

6.

In a coding program, an "ask" block is used to...

For parents and teachers

Big idea: animals are classified using distinguishing characteristics. The first split is vertebrates (backbone) and invertebrates (no backbone). Each group has features that identify it.

How to help at home:

  • Play "20 questions" with animals using only yes/no questions about body features. This is a dichotomous key in action.
  • On a walk, look for invertebrates under stones and logs (put them back gently) and count legs and body parts together.
  • If your child uses a block-coding app like Scratch, help them turn their key into code using "ask" and "if/else" blocks. Paper versions are just as good for learning.

Common difficulties:

  • Thinking spiders are insects (count the legs!).
  • Classifying by habitat or behaviour ("it swims, so it's a fish"; "it flies, so it's a bird") instead of body characteristics.
  • Mixing up amphibians (moist skin) and reptiles (dry scales), especially salamanders and lizards.