Cnidaria
Anemones, jellies, corals, sea fans and bluebottles. Very different to look at — but every one is built from two tissue layers with a jelly between, every one is radially symmetrical, and almost every one can sting. "Cnidaria" comes from the Greek for nettles.
A shrimp brushes against a sea anemone's tentacle in a Cape Recife rock pool. Before it can flick away, thousands of microscopic harpoons have fired into it. Each harpoon launches in about 700 nanoseconds — less than a thousandth of a thousandth of a second. The anemone has no brain, and the stinging cell has no muscle to throw it. How does it fire anything that fast?
The harpoon is a nematocyst: a tiny capsule inside a stinging cell, holding a coiled, barbed thread under enormous pressure — like a loaded spring. The cell does not throw it; it simply opens the lid. A hair-like trigger on the cell responds to touch together with the chemical taste of prey, the lid flips open, and the thread turns inside out as it shoots, stabbing in and injecting venom.
Scientists filming nematocysts at more than a million frames a second measured accelerations of up to about 5 million times the force of gravity — among the fastest movements recorded in any living thing. Each capsule fires only once; interstitial cells then make a replacement.
The ability to sting is the one feature that unites the whole phylum. This topic is how the rest of the animal is built around it.
Two Layers, a Jelly, and a Sting
Cnidarians are diploblastic, radially symmetrical animals with a single body opening and a stinging way of life. They come in two forms — the attached polyp and the floating medusa — and they are almost all predators.
Where they sit. On the tree from Topic 28, cnidarians branch off right after the true tissues appear (branch point ②) — one step beyond the sponges. They have true tissues but few real organs, no mesoderm, no brain, and a gut with one opening.
Where they live. A large phylum found in every marine habitat, from the rocky shores of our coast to the deepest trenches: anemones, jellies, corals, bluebottles and many less familiar groups. They have muscle fibres, so they can move, stretch out and change shape; most have tentacles able to immobilise prey; and because they can make digestive enzymes, they can catch prey larger than a mouthful and break it down inside.
The worked example. CAPS asks for a sandy anemone as the example, and the descriptions below are based on an anemone — but they apply to all cnidarians.
Polyp and Medusa — One Plan, Flipped
The typical cnidarian is a polyp: a cylinder with one opening surrounded by tentacles, its other end anchored to a substrate. The medusa is the same plan turned upside down, with a thick layer of buoyant jelly in its body wall — every jellyfish is a medusa.
↑ colour code from Topic 28: blue = ectoderm, lilac = mesoglea, yellow = endoderm, cream = the coelenteron.
Ready in every direction
Cnidarians are radially symmetrical — a polyp can be cut into mirror halves along many planes through its mouth. That suits a sessile or planktonic life: prey can arrive from any side, so the animal can feel, stretch and hunt equally well in all directions.
One animal, or thousands
Polyps may live alone — like the sandy anemone — or as colonies of hundreds or thousands joined together, like corals and hydroids. Some classes have a life cycle with both forms: a polyp stage and a medusa stage.
The Body Wall, Cell by Cell
A slice through the body wall of a polyp, from the sea (left) to the coelenteron (right). Cnidarians are diploblastic: an outer ectoderm for protection and catching food, an inner endoderm for digestion, and a jelly mesoglea between — no mesoderm. The letters are your booklet's labelling question. Name each before you tap it.
Mesoglea is not mesenchyme. The booklet points out that the cnidarian's mesoglea is a true gel, different from the sponge's mesenchyme. In a polyp it is thin and gives only a little support; in a medusa it is thick and gives the bell its shape.
A Skeleton Made of Water
Hydrostatic support means the body is supported by a fluid-filled cavity. In a cnidarian three things provide it: the coelenteron, full of sea water held in by the closed mouth; two sheets of muscle squeezing it; and the mesoglea. Tap each state to see the muscles at work.
The two muscle sheets are antagonistic: one contracts while the other relaxes. Longitudinal fibres run lengthways in the musculo-epithelial cells of the ectoderm; circular fibres run around the body in the musculo-digestive cells of the endoderm. Because water cannot be squashed, squeezing it one way pushes the body out the other — the same principle as a water balloon. Nervous coordination comes from sensory cells, muscle fibres and nerve cells in both layers, linked by a nerve net lying against the mesoglea. With no brain, the net lets the animal sense, catch and digest food, and protect itself.
How an Anemone Eats a Shrimp
Your booklet asks for this as an eight-point paragraph. Here are the points in order.
Why folds — and why cnidarians stay thin
Folds. Cut an anemone across and the endoderm is not a smooth ring: it is pleated into many inward folds. More folds mean more digestive cells in the same body — more surface without a bigger animal. CAPS turns this into the investigation for this topic: corrugated paper inside a cylinder (practical sheet, Investigation 1).
Diffusion sets the limits. Oxygen diffuses in from the sea outside and from the water in the coelenteron, so both layers are in direct contact with a supply; carbon dioxide and wastes leave the same way, and each cell regulates its own water balance. No cell can be far from water — which is why a cnidarian's living layers stay thin. Even a large jellyfish is mostly mesoglea, which needs very little oxygen. (Your booklet asks why diffusion might limit a cnidarian's size; this is the answer, and Topic 28's jelly cubes are the evidence.)
Reproduction and the Jelly's Double Life
Asexual — budding and fragmentation
With plenty of food and ideal, stable temperatures, some species reproduce asexually: by fragmentation (breaking or splitting into pieces that each grow into a new individual) or by budding (smaller individuals growing from the base and breaking away). Fast and cheap — many identical copies.
Sexual — gametes and a planula
Gametes form from clusters of interstitial cells and are released. The fertilised egg grows into a planula larva — a small oval covered in cilia, which it uses to crawl or swim before settling. Sexual reproduction mixes genes, giving offspring that may cope with a changing environment.
The life cycle of a jelly — tap round the wheel
CAPS asks you to discuss the jelly's structure and life cycle stages using diagrams. Most jellies alternate between a tiny attached polyp and the big floating medusa.
The Four Classes — and a Look-Alike
| Class | Dominant form | Features | Examples |
|---|---|---|---|
| Hydrozoa | usually the polyp colony | Hydroids: microscopic polyps growing together from a fern-like or feathery stalk; some bud off a short-lived medusa (a hydromedusa). Tiny, but some sting people strongly. Polymorphic colonies: one apparent animal made of many individuals with different shapes and jobs. | hydroids; the bluebottle (Physalia sp.) — float, stinging tentacles, feeding and reproductive individuals, all one colony |
| Scyphozoa | the medusa | The larger jellies, from a few centimetres to over a metre across. Polyp and medusa stages; the medusa lives longest. Light- and balance-sensing organs; swim by squeezing the bell. Most sting their prey; one order filter-feeds with feathery tentacles. | compass jellies, moon jellies |
| Cubozoa | the medusa | Very like scyphozoans, but with a smaller, box-shaped bell and only four long tentacles (or four bunches). A similar life cycle. | box jellies |
| Anthozoa | the polyp only — no medusa | Six main divisions (below). | anemones, corals, sea fans |
Anthozoa — the six divisions
| Cnidaria | Ctenophora | |
|---|---|---|
| Shared | jelly-like bodies: an ectoderm layer, a mesoglea of jelly, and an endoderm-lined coelenteron; most have tentacles for catching food | |
| Stinging cells | yes — nematocysts | none; tentacles covered in sticky cells |
| Swimming | muscles squeeze the bell | eight rows of fused cilia — the "combs" — beat down the length of the body |
| Openings | one: the mouth | a mouth, and in most, small anal pores at the opposite end |
| Size | microscopic to metres | a few millimetres to, rarely, over a metre |
Why comb jellies shimmer — the booklet's explanation needs a fix
The booklet says a "bioluminescent strip behind the cilia rows gives them a flickering comb-like appearance (hence the name)". Two separate things have been run together. The rainbow flicker you see in daylight is not light being made — it is ordinary light scattered and split into colours by the beating comb rows, the way a CD shimmers. And the name comes from the combs themselves: ktenos is Greek for comb. Many comb jellies are also bioluminescent — in the dark they glow blue or green from light-producing cells — but that is a different light, and it is not what gives them their name.
Corals and Their Algae — and a Promise Kept
Tropical seas carry little plankton, so a hard coral cannot live by catching food alone. Most of its nutrition comes from zooxanthellae — microscopic, photosynthetic algae (dinoflagellates, Topic 18) living inside its cells in a mutualism. The coral gives them shelter, carbon dioxide and its nitrogen-rich wastes; they give the coral sugars made by photosynthesis — enough to power its growth. Colonies build on top of one another and on old, dead colonies: a coral reef. The Great Barrier Reef off eastern Australia stretches over 2 000 km.
Ectoderm or endoderm? The answer Topic 22 promised
In Topic 22 your sources — CAPS and that topic's booklet — said zooxanthellae live in the coral polyp's ectoderm, and that page warned you this topic would disagree. It does: this topic's booklet says they live in the endodermal cells, in both its text and its examinable definition. That is correct: the algae sit in the inner layer (the gastrodermis, which develops from endoderm), lining the coelenteron — exactly where the polyp's wastes and CO₂ are, and where they are safest.
📌 For your exams: in a Cnidaria question, use this topic's examinable definition — "the microscopic mutualistic algae which inhabit the endodermal cells of coral polyps." Watch for one slip: the booklet's own Test-your-knowledge question 1.5 still describes them in the "ectodermal" cells. If a question uses that wording, answer "zooxanthellae" — the term is what is being asked for.
And one more correction. The booklet says the zooxanthellae provide the coral with "minerals, like calcium carbonate". They do not hand over limestone: the coral builds its own skeleton. What the algae do is supply the energy that powers skeleton-building and remove CO₂, so corals with zooxanthellae calcify far faster in light — which is why reef-building corals live in the shallow, sunlit sea.
Bleaching. When the water stays too warm for too long, the partnership breaks down and the coral expels its zooxanthellae. Without its algae the coral's white skeleton shows through the clear tissue — it bleaches — and if the heat lasts, it starves. Bleaching is one of the clearest pieces of evidence for warming seas that you met in Topic 26.
Cnidarians on Our Coast
Local connection — from the sandy anemone to Africa's southernmost reefs
The sandy anemone, CAPS's chosen example, lives in very shallow water all round southern Africa, from Lüderitz to Durban. Look in pools on the lower shore at Cape Recife and in sandy gullies around boulders: up to 10 cm across, with more than 300 short tentacles, and a column covered in sticky knobs that hold sand and shell grit — the camouflage that gives it its name. It eats mussels, whelks and even small sea urchins, holding them with a powerful ring of muscle.
Bluebottles wash up on Algoa Bay's beaches after strong onshore winds. Do not touch them — their nematocysts can still fire after the animal is stranded, and the sting is painful. Colonial strawberry anemones carpet the walls of False Bay, and soft corals and sea fans grow on the reefs of the De Hoop Marine Protected Area — all photographed in your booklet.
Sodwana Bay, in the iSimangaliso Wetland Park at about 27–28 °S, has the southernmost coral reefs in Africa — the southern limit of the western Indian Ocean reefs. Kept warm enough by the Agulhas Current, they are a thin living layer on old sandstone rather than a deep reef, and scientists are watching them closely as possible refuges for corals as the tropics warm.
In the Lab — the CAPS Investigations
CAPS sets a specific investigation — "THEME: increasing surface area. Prepare cardboard cylinders from an A4 sheet … fold A4 paper sheets to corrugate them and see how many can be fitted around the inside of the cylinder" — as well as observing live examples in rock pools, and using labelled diagrams of the sandy anemone.
Four investigations · paper, a rock pool, and a pencil
- 🟢 kitchen The CAPS investigation: folds and surface area — a paper cylinder lined with corrugated sheets. Calculate how much surface the folds add, and why an anemone's gut is pleated.
- 🌊 shore What makes an anemone sting? — touch, taste, or both? Test sandy anemones in a rock pool with a clean stick, a stick dipped in mussel juice, and a scrap of mussel. Results supplied.
- 🟢 desk Life cycles and classes — draw the jelly life cycle, then sort twelve described cnidarians and look-alikes into their classes.
- 🟢 desk The booklet's questions — label the body wall A–H, give functions, compare Cnidaria with Ctenophora, and brief a dive class on corals and sea fans.
→ Open the Topic 30 practical sheet — printable, with supplied data for every task.
Key Vocabulary
Tap each card to flip it. The first fifteen are the booklet's examinable definitions in its own words; the last three are terms used throughout the topic.
Having the ability to glow in dark or low light settings.
A sac or space inside an animal performing the role of a body cavity (coelom) as well as a gut (enteron).
Having a body wall made up of two tissue layers.
Proteins which act as catalysts in organic processes, facilitating anabolic and catabolic reactions.
An intermediate phase in the lifecycle of the jelly, resembling a microscopic medusa.
The hard, horn-like protein in the supporting structure of a sea fan.
Clusters of unspecialised cells in the body walls of cnidarians capable of replacing dead or damaged cells — including used nematocysts — and of producing gametes.
The cnidarian body form typified by a jelly(fish).
The stinging cells of cnidarians. Strictly, the nematocyst is the capsule inside the stinging cell.
A muscular tube behind the mouth of an animal.
A species of organism exhibiting more than one body form — like the different individuals in a bluebottle colony.
The cnidarian body form typified by an anemone.
Having both halves identical either side of a central axis — in many planes through that axis (Topic 28).
The early polyp-like phase of the lifecycle of a jelly. Usually spelt scyphistoma; it is a small polyp rather than a true larva.
The microscopic mutualistic algae which inhabit the endodermal cells of coral polyps.
The jelly layer between the ectoderm and endoderm of a cnidarian — thin in a polyp, thick in a medusa. (Topic 28 spells it mesogloea.)
The small, ciliated larva that grows from a cnidarian's fertilised egg; it crawls or swims, then settles.
Performing opposite actions — like the longitudinal and circular muscles: one contracts while the other relaxes.
Spellings to know
The booklet writes Schyphozoa and schyphistoma; the usual spellings are Scyphozoa and scyphistoma (from the Greek skyphos, a cup). It also writes mesoglea here and mesogloea in Topic 28 — both are accepted. Sessile (general term): immobile or attached.
Quick Check
Question 1
Well done — that is all thirty!
You have reached the end of the course's thirty topics. Go back through the practical sheets you have not yet done — and keep looking in rock pools.