Year 7 Science · Unit 1 · Lesson 3
Challenge Worksheet
Learning Goals
1. State the leg-count rule for telling an insect from an arachnid. Then name one animal with a hard exoskeleton that the rule would get wrong, and say which group it really belongs to.
Odd one out
In each row, circle the animal that belongs to a different class or group from the other three. Judge by body features, not by where the animal lives. Give the reason.
| Animals | Odd one out and reason |
|---|---|
| dolphin · red kangaroo · Murray cod · platypus | |
| slater · prawn · bee · yabby | |
| jellyfish · octopus · snail · oyster |
Find the faults in a key
Scenario: Leo’s key
Leo wrote Key 1 to identify six Australian animals: a magpie, a wombat, a carpet python, a barramundi, a redback spider and a bull ant. His key has two faulty steps.
| Step | Question | If yes | If no |
|---|---|---|---|
| 1 | Does it have a backbone? | go to step 2 | go to step 5 |
| 2 | Does it have feathers? | magpie | go to step 3 |
| 3 | Does it have fur and feed its young milk? | wombat | go to step 4 |
| 4 | Does it have scales? | carpet python | barramundi |
| 5 | Is it dangerous to people? | redback spider | bull ant |
2. Follow a barramundi through Key 1 and write where it ends up. Then name the two faulty steps and explain what is wrong with each (for example, which rule it breaks, or which animal it can never reach).
3. Rewrite both faulty steps so that every animal has exactly one path through the key. Keep the same step numbers.
Read the data
Leaf-litter survey
A class marks out five squares, each 1 m², on the floor of the school bushland. They count every animal they find in the leaf litter and sort each one into a group.
| Group | Number of animals counted |
|---|---|
| Insects | 46 |
| Crustaceans (slaters) | 18 |
| Arachnids | 12 |
| Worms | 9 |
| Molluscs (snails) | 5 |
| Vertebrates (skinks) | 1 |
| Total | 91 |
Source: sample class results.
4. Calculate the percentage of the animals counted that were invertebrates. Give your answer to one decimal place.
Answer %
5. A student writes: “Our survey proves that 99% of animal species are invertebrates, so the figure of about 95% in the lesson is wrong.” Evaluate this conclusion. Give two separate reasons.
A student wrote this answer
“Invertebrates are less advanced than vertebrates because they don’t have a backbone. This means they are simpler animals that are less successful. Insects, for example, are basically just tiny vertebrates without bones.”
6. Evaluate the student’s answer. Identify the scientific errors, correct them using evidence from the lesson, and explain what having an exoskeleton instead of a backbone really means for an animal.
Question 1
6 legs means insect; 8 legs means arachnid (1). A slater (or prawn, crab, yabby) has a hard exoskeleton and more than 8 legs: it is a crustacean (1). [2 marks]
Odd one out
Row 1: Murray cod, a fish; the rest are mammals (fur or hair, milk). Row 2: bee, an insect (6 legs); the rest are crustaceans. Row 3: jellyfish, not a mollusc; the rest are molluscs. 1 mark per row with reason. [3 marks]
Question 2
Backbone yes, feathers no, fur and milk no, scales yes: it ends up as “carpet python” (1). Step 4: both animals have scales, so the question does not separate them and the barramundi can never be reached (1). Step 5: “dangerous” is an opinion, not something you can see, and people would disagree about a bull ant, which stings (1); a key question needs two answers anyone can check by looking (1). [4 marks]
Question 3
Step 4: “Does it have gills and fins?” Yes: barramundi; no: carpet python (1). Step 5: “Does it have 8 legs?” Yes: redback spider; no: bull ant (1). Accept equivalents. [2 marks]
Read the data
4. Invertebrates = 91 − 1 = 90 (1); 90 ÷ 91 × 100 = 98.9% (1). [2 marks]
5. 1 each for two reasons, 1 for judging it unsupported: they counted animals, not species (46 insects may be a few species); one habitat that suits small invertebrates; a small 5 m² sample; the lesson’s figure is named species worldwide. [3 marks]
Question 6
Marking guide: 1 each for two errors (invertebrates are not less successful; insects are not vertebrates of any kind, having no vertebrae; “no backbone” does not mean no skeleton); 1 species evidence (about 95%, or more beetle species than all vertebrates); 1 an exoskeleton is a hard skeleton outside the body that muscles pull against; 1 its limit: not living tissue, so the animal must moult and is soft for a few hours, which is why the largest land animals are vertebrates; 1 clear judgement. [6 marks]
Model answer: The student is wrong. About 95% of animal species are invertebrates, so they are not less successful. Insects have no backbone at all; they have an exoskeleton, a hard outer skeleton that muscles pull against. It cannot grow, so they must moult, which limits size on land but does not make them simpler.
Common error: reading “invertebrate” as “no skeleton”; arthropods have one outside.