Biodiversity and Classification, Grade 7
Arachnids do not have compound eyes.
You will find it written that they do, in a list headed "all arachnids have", with the spider diagram labelled to match. Spiders have simple eyes only, usually eight of them, and no compound eyes at all.
It is worth knowing because it is not the only one. This topic is full of sentences that begin "all" or "only", and most of them are wrong. This page is the classification a Grade 7 needs, with the absolutes fixed and a free worksheet and memo at the bottom.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 7 |
| Term | 1 |
| Strand | Life and Living |
| Topic | Biodiversity |
| Status | Class activities. Nothing here is formally assessed |
| Marks | The topic revision is 25. Our worksheet is 40 |
Five kingdoms or six? Your own exam paper answers it
You will be told six kingdoms in one unit and five kingdoms eleven pages later. They cannot both be right.
It is six, and the proof is in the revision section of the same topic, which asks learners to name the six kingdoms for six marks.
A learner who revises from the wrong page loses six marks on a question the material itself sets. If you teach one thing from this page, teach six.
| Kingdom | What is in it |
|---|---|
| Eubacteria | The ordinary bacteria, including the useful ones in yoghurt |
| Archaebacteria | Ancient bacteria living in extreme places: hot springs, salt pans |
| Protists | Mostly single-celled, like amoeba and paramecium |
| Fungi | Mushrooms, moulds and yeasts. Not plants, because they cannot photosynthesise |
| Plants | Make their own food using light |
| Animals | Cannot make their own food, and nearly all can move |
1,7 million or 50 million species? Both, and they answer different questions
You will meet two numbers in this topic that look like a contradiction.
| Number | What it actually counts |
|---|---|
| About 1,7 million | Species that have been DESCRIBED: found, named and written up by a scientist |
| 10 to 50 million | Species ESTIMATED to exist, including everything not yet found |
Add one word and the contradiction disappears. The gap between them is the honest answer to "how many kinds of living things are there?", and it is more interesting than either number on its own: most species on Earth have never been named.
The sentences with "all" and "only" in them
Five of them, and each one is worth a minute of class time.
| What you will read | What is actually true |
|---|---|
| "All arachnids have one pair of compound eyes" | Arachnids have NO compound eyes. Simple eyes only, usually eight. The diagram is labelled wrong too |
| "All crustaceans have five pairs of jointed legs" | That describes the decapods: crab, lobster, prawn. Barnacles and krill are in the same list and do not |
| "All molluscs have a shell and a radula" | Bivalves have no radula, and slugs, octopuses and nudibranchs have no shell |
| "Mammals give birth to live young" | The platypus and the echidnas lay eggs. They are still mammals |
| "Reptiles have teeth that do not have roots" | Crocodiles have rooted teeth in sockets, and a crocodile is usually the reptile in the photograph |
There is a pattern here worth teaching on its own.
Biology is full of groups that are mostly one thing. The moment a sentence says "all" or "only", there is usually an animal that has not read it.
"Most reptiles lay eggs" is true and useful. "All reptiles lay eggs" is wrong, and the puff adder, which gives birth to live young, is about as South African a counter-example as exists.
Linnaeus did not invent the seven levels
The order is kingdom, phylum, class, order, family, genus, species, and the usual mnemonic is "keep plates clean or family grows sick".
But Carl Linnaeus did not set up all seven. He used five: kingdom, class, order, genus and species. Phylum and family were added later, by other people.
What he did give us is binomial naming, the two-word scientific name that works in every language, and that is the bigger idea anyway.
How to write a scientific name, because it is an exam instruction:
| Rule | Example |
|---|---|
| Genus first, with a capital | Loxodonta africana |
| Species name second, lower case | Loxodonta africana |
| Both in italics, or underlined by hand | Panthera leo |
The species name is the whole two-word pair, not just the second word. A learner asked to "write the scientific name of the lion" who writes "leo" has answered half the question.
Vertebrates, invertebrates, and the 97 per cent nobody draws
A vertebrate has a backbone. An invertebrate does not.
The diagrams are badly out of proportion, and it is worth saying so.
The five vertebrate classes get a branch each. The invertebrates usually get three, all of them arthropods. But invertebrates are about 97 per cent of all animal species.
Missing from almost every diagram: molluscs, worms, jellyfish and corals, starfish and sea urchins. The backboned animals are the small, unusual corner of the animal kingdom, not the main part of it.
The five vertebrate classes, with the exceptions included
| Class | Skin | Young | Body temperature |
|---|---|---|---|
| Fish | Scales | Eggs, usually fertilised outside the body | Cold-blooded |
| Amphibians | Smooth, moist, and used for breathing | Eggs in water, then metamorphosis | Cold-blooded |
| Reptiles | Dry scales | Mostly eggs. Many snakes and lizards give birth to live young | Cold-blooded |
| Birds | Feathers, with scales on the legs | Hard-shelled eggs, incubated | Warm-blooded |
| Mammals | Hair, and mammary glands | Mostly live young. The platypus and echidnas lay eggs | Warm-blooded |
The invertebrate groups, done properly
| Group | Legs | Body | Eyes |
|---|---|---|---|
| Insects | 3 pairs | Head, thorax, abdomen | One pair of compound eyes |
| Arachnids | 4 pairs | Two parts: cephalothorax and abdomen | Simple eyes only, usually 8. No compound eyes |
| Crustaceans | Varies. Five pairs in crabs and prawns; barnacles and krill differ | Hard exoskeleton, two pairs of antennae | Often on stalks |
| Molluscs | A muscular foot | Soft body. Shell in many, but not slugs or octopuses | Varies hugely, from none to the octopus eye |
Legs are the quickest test in the room. Six legs is an insect, eight is an arachnid. It settles the spider argument in one second and it is almost always right.
Dichotomous keys, and the one rule that makes them work
A key is a series of paired choices. Each answer sends you to the next pair until only one organism is left.
The rule: every step must use something you can SEE.
"Has six legs" is a good step. "Has eight legs" is a good step. "Is a carnivore" is not, because you cannot see what an animal eats from a photograph, and keys that use it leave a learner stuck with no way to decide.
If you write your own key with a class, make them justify every step by pointing at the picture.
Monocots and dicots, with two South African plants
| Monocotyledon | Dicotyledon | |
|---|---|---|
| Seed leaves | One | Two |
| Leaf veins | Parallel | Net, branching |
| Flower parts | In threes | In fours or fives |
| Roots | Many similar roots, adventitious | One main tap root with side roots |
| Example to use | Clivia, which grows in shade all over the country | Plumbago, which is in half the hedges in South Africa |
Clivia and Plumbago are both indigenous and both easy to find, which makes them far better than a textbook drawing. Pick a leaf of each on the way into the lesson and the whole table is in your hand.
The cheapest practical in the topic: split a seed
Soak beans, maize, lentils and barley overnight. That is the entire preparation.
- Soak the seeds in water overnight so the coat softens
- Peel off the seed coat with a thumbnail
- Split the seed gently along its natural line
- Count the halves. Two fat halves means two cotyledons and a dicot. One means a monocot
- Check against the leaf: net veins with the dicots, parallel veins with the monocots
Beans and lentils split cleanly into two. Maize and barley do not split at all, and that refusal is the result. A hand lens helps but is not needed.
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| The seeds will not split | Not soaked long enough | Overnight, not an hour. Start them the day before |
| The seeds went mouldy and smell | Soaked for days in warm water | Twelve hours is plenty. Drain them in the morning |
| A learner splits a maize seed in half anyway | Cut with a blade rather than split | Splitting by hand is the test. Anything can be cut in two |
| Everyone says their spider has compound eyes | They read it in a list | Count the eyes on a photograph. Eight small separate ones, no mosaic |
| The key gets stuck on an animal | A step uses something invisible, like diet | Rewrite that step using a visible feature |
How the 40 marks are made up
| Part | Marks |
|---|---|
| Classification and the six kingdoms | 10 |
| The seven levels, and writing a scientific name | 8 |
| Vertebrates and invertebrates, including the exceptions | 12 |
| Using and writing a dichotomous key | 6 |
| Monocots and dicots | 4 |
Nothing in this topic is formally assessed. The topic revision is 25 marks. Our worksheet is 40 and the split above is ours.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks, including the six kingdoms, the five absolute statements and a key to build
- Marking memorandum, with what to accept for each answer and the five answers that look right and score nothing
Related practicals
- Plant and Animal Cells, Grade 9. The next step down, with a microscope
- Food Chains and Food Webs, Grade 8. What all these organisms do to each other
- Separating Mixtures, Grade 7. The Term 2 practical task in the same grade
What you need to run it
The seed practical costs nothing but a packet of beans. Everything else in this topic is looking closely at things you already have.
| Item | Price | What it adds |
|---|---|---|
| Insect anatomy slide set of 9 | R225 | The best value here. Mouthparts, legs and wings, ready-made |
| Basic botany slide set of 25 | R495 | Roots, stems and leaves, including monocot and dicot sections |
| Stereo microscope, 10 to 30x, LED | R3 950 | The right instrument for whole insects and shells, not slides |
| Biodiversity wall chart | R85 | The topic at a glance |
One honest note. A stereo microscope is what this topic actually wants, because it shows a whole spider rather than a slice of one. But a R3 950 instrument is not what most schools will buy for one term, and the seeds, the leaves and a photograph do almost all of the work.
And a gap we will not paper over: we do not currently stock hand lenses. A simple magnifier is the ideal tool for the snail and seed work, and ours are out of stock. Any cheap plastic one from a bookshop does the job.
Slides are in microscope slides, the instruments are in stereo microscopes, and the charts are in wall charts.