Classification and Dichotomous Keys — Answers
Answer sheet with model answers and success criteria for the Year 7 cells & classification worksheet.
Success TutoringWorksheetsAnswer sheetYear 7 · Science · Cells & ClassificationClassification and Dichotomous Keys — Answers
Part A: The five vertebrate classes
Classification means sorting living things into groups based on features they share. Scientists choose features that can be observed and that do not change from one individual to the next.
Animals are first split into vertebrates (animals with a backbone) and invertebrates (animals without a backbone). Most animal species are invertebrates.
The vertebrates are divided into five classes: fish, amphibians, reptiles, birds and mammals. Large invertebrate groups include arthropods (jointed legs and a hard outer skeleton, such as insects, spiders and crabs), molluscs (soft bodies, often with a shell, such as snails and octopuses) and segmented worms (such as earthworms).
A dichotomous key identifies an organism through a series of steps. Each step gives exactly two choices, and you follow the choice that fits until you reach a name.
- Fish
- Amphibians
- Reptiles
- Birds
- Mammals
- Hair or fur; feed their young on milk
- Feathers and a beak; lay eggs with hard shells
- Dry, scaly skin; lay eggs with leathery shells on land
- Gills, fins and (usually) scales; most lay eggs in water
- Moist skin without scales; young usually live in water and breathe with gills
Answer: Fish → Gills, fins and (usually) scales; most lay eggs in water; Amphibians → Moist skin without scales; young usually live in water and breathe with gills; Reptiles → Dry, scaly skin; lay eggs with leathery shells on land; Birds → Feathers and a beak; lay eggs with hard shells; Mammals → Hair or fur; feed their young on milk
Part B: Use the key
Use the animal key to identify the group each animal belongs to. Write the group name and the steps you followed (for example 1a, 2b, 3a).
Animal key
1a. Has a backbone ... go to 2
1b. Has no backbone ... go to 6
2a. Has feathers ... bird
2b. Has no feathers ... go to 3
3a. Has hair or fur ... mammal
3b. Has no hair or fur ... go to 4
4a. Has fins and breathes with gills as an adult ... fish
4b. Has no fins as an adult ... go to 5
5a. Has dry, scaly skin ... reptile
5b. Has moist skin without scales ... amphibian
6a. Has jointed legs ... go to 7
6b. Has no jointed legs ... go to 8
7a. Has six legs and three body parts ... insect
7b. Has eight legs and two body parts ... arachnid
8a. Has a soft, unsegmented body, often with a shell ... mollusc
8b. Has a long body divided into ring-like segments ... segmented worm
Answer: Mammal (1a, 2b, 3a)
Answer: Insect (1b, 6a, 7a)
Answer: Amphibian (1a, 2b, 3b, 4b, 5b)
Answer: Mollusc (1b, 6b, 8a)
Answer: Reptile (1a, 2b, 3b, 4b, 5a)
Answer: Arachnid (1b, 6a, 7b)
Answer: Fish (1a, 2b, 3b, 4a)
Answer: Segmented worm (1b, 6b, 8b)
Part C: Tricky cases
Circle the best answer.
- AIt can swim
- BIt feeds its young on milk
- CIt lives in water
- DIt has a tail
Answer: It feeds its young on milk
Feeding young on milk is a feature of mammals. Whales also breathe air with lungs and have some hair, at least when young. Swimming and living in water are shared with many unrelated animals.
- AIt flies at night
- BIt has fur and feeds its young on milk
- CIt eats insects
- DIt has wings
Answer: It has fur and feeds its young on milk
Bats have fur and feed their young on milk, and they have no feathers. Flying is not a feature used to define birds.
- AIt swims
- BIt has feathers
- CIt lives in cold places
- DIt walks on two legs
Answer: It has feathers
All birds have feathers, including birds that cannot fly.
- AIt has eight legs and two main body parts
- BIt has no backbone
- CIt has jointed legs
- DIt is very small
Answer: It has eight legs and two main body parts
Insects have six legs and three body parts. Spiders are arachnids, with eight legs and two main body parts. Both are arthropods, so both have jointed legs and no backbone.
Part D: True or false?
Write T for true or F for false.
Answer: True
Invertebrates, especially insects, make up the great majority of known animal species.
Answer: True
Dichotomous means dividing into two parts.
Answer: False
Colour can vary between individuals of the same species, with age or with the season, so features such as the number of legs or the type of body covering are more reliable.
Part E: Make your own key
Answer: Flying does not match the groups scientists use: bats (mammals), most birds and many insects can fly, while some birds, such as penguins, cannot. A question about flying would put unrelated animals together and split up birds. Features such as feathers, fur or a backbone are more reliable.
Model answer: 1a. Has a backbone ... go to 2. 1b. Has no backbone ... butterfly. 2a. Has feathers ... eagle. 2b. Has no feathers ... go to 3. 3a. Has fins and breathes with gills as an adult ... shark. 3b. Has moist skin and no fins as an adult ... frog. (Other correct keys are possible, as long as every step has two choices based on observable features and each animal ends up on its own.)
Success criteria:
- Every step has exactly two contrasting choices
- Choices use observable features (not colour, size or behaviour alone)
- Each of the four animals is reached at the end of a path
- The key works correctly for every animal
Marking tips
- Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
- For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
- Celebrate what went well first, then pick one thing to work on next.
Great work. Here's what to try next: