Vertebrates vs Invertebrates
In 2022, CSIRO researchers catalogued 640 new invertebrate species in a single survey of the Great Barrier Reef — yet more proof that 95% of all animals get through life with no backbone at all.
Printable Worksheets
Print or save as PDF — or build a custom worksheet from any module's questions.
Q1 · List 3 animals you definitely have backbones and 3 animals you think don't. What clue did you use?
Q2 · A jellyfish, a centipede and an oyster all look very different. What feature do you think groups them together?
● Know
- Vertebrate = has a backbone; invertebrate = no backbone
- The 5 vertebrate classes (fish, amphibian, reptile, bird, mammal)
- 5 invertebrate groups (insects, arachnids, crustaceans, molluscs, worms)
● Understand
- Why "backbone or not" is a useful first split
- Why invertebrates make up about 95% of all animals
- How a dichotomous key uses yes/no questions to identify a species
● Can do
- Sort animals into vertebrate or invertebrate
- Match a vertebrate to its class using one or two features
- Use a simple dichotomous key to identify an unknown animal
- Vertebrate
- Invertebrate
- Exoskeleton
- Mammal
- Dichotomous key
- Warm-blooded vertebrate that feeds its young milk
- An animal without a backbone
- A yes/no question tool used to identify species
- An animal with a backbone
- A hard outer skeleton on the outside of the body
Hold a freshly caught prawn in your hand and squeeze it gently — it squishes, because there is no hard internal spine, only a shell on the outside. This single question divides every animal on Earth into two groups.
| Vertebrate | Invertebrate | |
|---|---|---|
| Backbone? | Yes — internal spine made of vertebrae | No |
| Skeleton | Inside the body (endoskeleton, usually bone) | Often outside (exoskeleton) or none at all |
| How many species? | About 5% of all animals (~70,000) | About 95% (~1.3 million named) |
| Examples | Tuna, frog, snake, kookaburra, koala | Ant, snail, jellyfish, octopus, prawn |
It's easy to think vertebrates are "the main animals" because we see them most, but invertebrates are far more common. There are more species of beetle alone than all the vertebrate species put together.
Vertebrates are split into five groups called classes:
| Class | Body covering | Body temp | Reproduction | Aussie example |
|---|---|---|---|---|
| Fish | Scales; gills | Cold-blooded | Eggs in water | Murray cod |
| Amphibian | Moist, smooth skin | Cold-blooded | Eggs in water; tadpoles → adults | Green tree frog |
| Reptile | Dry scales | Cold-blooded | Eggs on land (or live young) | Eastern brown snake |
| Bird | Feathers; beak | Warm-blooded | Hard-shelled eggs | Kookaburra |
| Mammal | Fur or hair | Warm-blooded | Live young (mostly); feed milk | Red kangaroo |
Quick rules to remember:
- Feathers → bird (only birds have feathers).
- Fur + milk → mammal.
- Moist skin + spends life partly in water → amphibian.
- Dry scales → reptile.
- Gills + fins → fish.
Invertebrates are split into many groups. Here are the five biggest you should know:
| Group | Key features | Examples |
|---|---|---|
| Insects | 6 legs; 3 body parts (head, thorax, abdomen); exoskeleton; often wings | Ant, bee, blowfly, mosquito |
| Arachnids | 8 legs; 2 body parts; exoskeleton | Spider, scorpion, tick, mite |
| Crustaceans | Hard exoskeleton; many legs; mostly live in water | Crab, prawn, yabby, slater |
| Molluscs | Soft body, often in a shell | Snail, oyster, octopus, squid |
| Worms | Long, soft, no legs | Earthworm, leech, tapeworm |
A handy spider/insect rule: count the legs. 6 legs = insect; 8 legs = arachnid. So a Sydney funnel-web is an arachnid, not an insect.
An ant has legs and is an . A spider has legs and is an . Both have a hard outer skeleton called an .
Wrong: "A whale is a fish — it swims." Whales swim, but they have lungs (not gills), feed milk to their young and have warm blood. That makes them mammals.
Right: Whales and dolphins are mammals. They breathe air, are warm-blooded and feed their young milk.
Wrong: "A spider is an insect." Insects have 6 legs and 3 body parts. Spiders have 8 legs and 2 body parts — they are arachnids.
Right: Spiders are arachnids. Insects have 6 legs; arachnids have 8.
Wrong: "An octopus is a fish — it lives in the sea." An octopus has no backbone and a soft body, so it can't be a fish. It's a mollusc, related to snails and squid.
Right: An octopus is a mollusc. Soft body, no backbone, often (but not always) has a shell.
A dichotomous key ("di" = two, "chotomy" = cut) is a series of yes/no questions that ends with the species name. At each step you have two choices. Here's a simple example for a few Aussie animals:
| Step | Question | Yes → go to | No → go to |
|---|---|---|---|
| 1 | Does it have a backbone? | Step 2 | Step 5 |
| 2 | Does it have feathers? | Kookaburra | Step 3 |
| 3 | Does it have fur and feed milk? | Red kangaroo | Step 4 |
| 4 | Does it have dry scales? | Blue-tongued lizard | Green tree frog |
| 5 | Does it have 8 legs? | Sydney funnel-web | Ant (6 legs) |
To use the key, start at step 1 and follow the yes/no path. You always end at one species. Scientists use much longer keys (hundreds of steps) to identify things like leaves, fish or insects.
A platypus has fur, feeds its young milk, swims in rivers and lays eggs. Predict: which vertebrate class is it in? Explain in one sentence.
How close was your prediction?
Nice — fur + milk is the strongest signal for "mammal", egg-laying or not.
Good — the egg-laying tricked you. Fur + milk are the defining mammal features.
Earlier you were asked about a whale, a tuna and a snake. Which is the odd one out and why?
Now that you've worked through the lesson, write a fuller answer. Name each animal's vertebrate class.
Q1. List the five vertebrate classes and give one Australian example of each. (3 marks)
Q2. Explain two ways spiders differ from insects. Use specific features. (4 marks)
Q3. A student claims: "If an animal swims in water, it must be a fish." Evaluate this claim using two examples from the lesson. (4 marks)
Answers
▾MCQ 1
B — A vertebrate is defined by having an internal backbone (spine). A is wrong (most vertebrates don't have wings). C is for arthropods, not vertebrates. D is not unique to vertebrates.
MCQ 2
D — The Sydney funnel-web is an arachnid (8 legs, 2 body parts). Bees, blowflies and ants are all insects (6 legs, 3 body parts).
MCQ 3
C — Dolphins are mammals. They breathe air, are warm-blooded, and feed milk to their young, just like other mammals. They are not fish even though they live in the sea.
MCQ 4
A — About 95% of all named animal species are invertebrates. Vertebrates make up only about 5%, even though they include the animals we see and think about most.
MCQ 5
B — "Di-chotomous" literally means "two cuts". Each step gives exactly two options, and you choose one to move to the next step.
Short Answer 1
Model answer: Fish — Murray cod (or any native fish). Amphibian — green tree frog. Reptile — eastern brown snake, blue-tongued lizard. Bird — kookaburra, magpie. Mammal — red kangaroo, koala, platypus. Any sensible Aussie example per class is fine.
Short Answer 2
Model answer: (1) Number of legs — spiders have 8 legs, insects have 6. (2) Body parts — spiders have 2 main body parts (cephalothorax and abdomen), while insects have 3 (head, thorax and abdomen). Other valid differences: spiders never have wings or antennae; insects often do.
Short Answer 3
Model answer: The claim is wrong. Just living in water doesn't make something a fish — fish are vertebrates with gills, fins and scales. A dolphin swims in water but breathes air with lungs, feeds milk and has fur as a baby — it is a mammal. An octopus swims in water but has no backbone and a soft body — it is a mollusc. So the test for "fish" is having gills, fins and scales, not just living in water.