Year 7 Science · Unit 1 · Lesson 3

Vertebrates vs Invertebrates

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Learning Goals

Place an animal in its vertebrate class or invertebrate group using its body features.
Explain how an exoskeleton affects the way an animal grows.
Build a dichotomous key that gives every animal exactly one path.

Warm Up

Match each invertebrate to its group

Write the letter of the correct group next to each animal. Each letter is used once.

Animal	Letter	Group and its key features
Blowfly		A. Arachnid: 8 legs, 2 body parts
Yabby		B. Worm: long, soft body, no legs
Tick		C. Insect: 6 legs, 3 body parts
Squid		D. Crustacean: hard exoskeleton, many legs, mostly lives in water
Leech		E. Mollusc: soft body, often inside a shell

1. Some classes have one feature that no other class has, so finding it settles the class on its own. Name that feature for (a) birds and (b) mammals.

Recall 2 marks

Your Turn

Cause and effect: how a crab grows

Fill the two empty boxes so that each box explains the one after it.

A crab’s exoskeleton is not living tissue.

For a few hours the crab is soft and slow, and cannot hold up its own weight.

2. Use your chain to explain why the biggest land animals are all vertebrates.

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Scenario: a new rainforest animal

Scientists find a new animal with a backbone in the rainforest of Far North Queensland. It has moist, smooth skin with no scales. Its young hatch from eggs laid in a creek and live in the water as tadpoles. The adults live on land but always stay close to the creek.

3. Which vertebrate class does the new animal belong to? Give two pieces of evidence from the scenario, and explain why the adults must stay close to water.

Apply 3 marks

4. A student says the platypus cannot be a mammal because it lays eggs. Do you agree? Use the features of mammals to explain.

Apply 2 marks

Go Deeper

Read the data

How many kinds of animals?

Scientists have named about 1.37 million animal species. Table 1 shows how they split.

Table 1. Approximate number of named animal species

Group	Named species (approximate)
Vertebrates	70,000
Invertebrates	1,300,000
All animals	1,370,000

Source: rounded figures from the lesson.

5. Calculate the percentage of named animal species that are vertebrates. Round to the nearest whole number.

Apply 2 marks

Answer %

6. Most of the animals people notice every day, such as dogs, birds and fish, are vertebrates. Use Table 1 to explain why “most animals have a backbone” is a wrong conclusion to draw from what we see.

Analyse 2 marks

Show What You Know

7. Build a dichotomous key that identifies these five animals: emu, koala, eastern water dragon (a lizard), huntsman spider and earthworm. Write it in the table, then explain why your first question is a good one to start with.

Analyse 6 marks

Your key

Step	Question	If yes	If no
1
2
3
4

Answer Key (for teachers)

Match each invertebrate to its group

Blowfly → C; Yabby → D; Tick → A; Squid → E; Leech → B.

Question 1

(a) Feathers (1). (b) Fur or hair together with feeding young milk (1). [2 marks]

Cause and effect: how a crab grows

Box 1: once it has hardened it cannot get any bigger (1). Box 2: to grow, the crab climbs out of its old exoskeleton and swells up before the new one sets (1).

Question 2

An exoskeleton animal must moult to grow and cannot hold its weight until the new one sets, which is riskier the heavier it is (1); a vertebrate’s skeleton is living tissue that grows with it (1). [2 marks]

Question 3

Amphibian (1). Any two: moist skin with no scales; eggs laid in water; tadpoles (1). Its thin, moist skin dries out fast, so the adult stays near water (1). [3 marks]

Question 4

Disagree (1): it has fur and feeds its young milk, which define a mammal; laying eggs does not change that (1). [2 marks]

Read the data

5. 70,000 ÷ 1,370,000 × 100 = 5.1 (1), so about 5% (1). [2 marks]

6. About 95% of named species are invertebrates (1); we notice big, familiar vertebrates, but what we see is not a fair sample of all species (1). [2 marks]

Question 7

Marking guide: 1 all questions yes/no; 1 all answerable by looking (no opinions or colours); 2 every animal reaches exactly one end point (1 if one is lost or reached twice); 1 four questions; 1 explains the first split. [6 marks]

Model key: 1. Does it have legs? Yes, go to 2; no: earthworm. 2. Does it have exactly 8 legs? Yes: huntsman spider; no, go to 3. 3. Does it have feathers? Yes: emu; no, go to 4. 4. Does it have fur? Yes: koala; no: eastern water dragon. Explanation: legs are visible without handling the animal, and this first question immediately separates the earthworm from the four animals that have legs. Accept any correct key.

Common error: “Can it fly?” An emu is a bird that cannot fly, so it goes down the wrong branch; use features it has (feathers).