1. Kingdom Animalia: non-chordates and chordates
Animals are grouped by shared body features. The first split is on the notochord, a stiff rod running along the back.
| Feature | Non-chordates | Chordates |
|---|---|---|
| Notochord | Absent | Present at some stage of life |
| Nerve cord | Solid, on the belly side | Hollow, on the back (dorsal) side |
| Pharyngeal gill slits | Absent | Present at some stage (often only in the embryo) |
| Heart | On the back side, when present | On the belly side |
Non-chordates in this course form eight phyla: Porifera, Coelenterata, Platyhelminthes, Nemathelminthes, Annelida, Arthropoda, Mollusca and Echinodermata. Chordates here are the five classes of vertebrates (animals with a backbone): Pisces, Amphibia, Reptilia, Aves and Mammalia.
Key terms
- Notochord
A stiff supporting rod along the back of a chordate; in vertebrates it is replaced by the backbone.
- Phylum
A major group of animals sharing a basic body plan.
- Vertebrate
A chordate with a backbone.
2. Porifera and Coelenterata
Porifera (sponges) — "pore bearers".
- Mostly marine; fixed to a rock or shell and do not move about.
- Body is full of pores; water enters through the pores and leaves through a large opening, the osculum; the central cavity is lined with collar-bearing cells that capture food.
- Supported by spicules or fibres. Examples: Sycon, Spongilla (freshwater), Euspongia (bath sponge).
Coelenterata — aquatic, mostly marine.
- Body is a tube with two cell layers and a central cavity (the coelenteron); a single opening serves as mouth and anus.
- Tentacles around the mouth carry stinging cells (cnidoblasts) used to catch prey and for defence.
- Examples: Hydra (freshwater), jellyfish, sea anemone, coral.
Key terms
- Osculum
The large opening at the top of a sponge through which water leaves.
- Spicule
A tiny rod-like skeletal piece in the body wall of a sponge.
- Cnidoblast
A stinging cell on the tentacles of a coelenterate.
3. Platyhelminthes, Nemathelminthes and Annelida
| Phylum | Body | Body cavity | Other features | Examples |
|---|---|---|---|---|
| Platyhelminthes | Flat, leaf-like or ribbon-like | None (solid tissue) | Many are parasites with hooks and suckers | Planaria (free-living), liver fluke, tapeworm |
| Nemathelminthes | Round, unsegmented, pointed at the ends | False (pseudocoelom) | Separate sexes; many parasites | Ascaris, hookworm, filarial worm |
| Annelida | Long, divided into rings (segments) | True cavity (coelom) | Bristles; moist skin | Earthworm, leech, Nereis |
A coelom is a body cavity completely lined by tissue; a false cavity is not fully lined.
Key terms
- Coelom
A true body cavity, lined all round by tissue.
- Parasite
An organism that lives on or in another and takes food from it.
- Segmented
Having a body divided into repeated rings or parts.
4. Arthropoda, Mollusca and Echinodermata
Arthropoda — the largest phylum: a segmented body, jointed legs and a hard outer skeleton (exoskeleton) that is moulted as the animal grows. Insects have three body regions (head, thorax, abdomen) and three pairs of legs; spiders have four pairs of legs. Examples: cockroach, butterfly, prawn, crab, spider, scorpion, centipede.
Mollusca — soft, unsegmented body, usually covered by a mantle that makes a shell, and a muscular foot for creeping. Examples: snail, mussel, octopus, cuttlefish (octopus and cuttlefish have no shell on the outside).
Echinodermata — spiny skin; tube feet for moving and catching food; body in five parts in adults; all marine. Examples: starfish, sea urchin, sea cucumber.
Key terms
- Exoskeleton
The hard outer covering of an arthropod.
- Mantle
A fold of skin of a mollusc that secretes the shell.
- Tube feet
Small tube-like feet of an echinoderm used for movement.
5. Chordates: Pisces and Amphibia
| Feature | Pisces (fishes) | Amphibia |
|---|---|---|
| Habitat | Water only | Land and water |
| Skin | Scales | Moist, smooth, no scales |
| Breathing | Gills | Skin and lungs (gills in the larva) |
| Limbs | Fins | Four limbs; webbed feet in many |
| Heart | Two chambers | Three chambers |
| Eggs | Laid in water | Laid in water; the larva (tadpole) changes by metamorphosis |
| Examples | Rohu, shark, seahorse | Frog, toad, salamander |
Both classes are cold-blooded: body temperature changes with the surroundings. Frogs lay eggs in water, so amphibians depend on water to breed.
Key terms
- Metamorphosis
A change of body form from the larva to the adult, as from tadpole to frog.
- Cold-blooded
Having a body temperature that varies with the surroundings.
6. Chordates: Reptilia, Aves and Mammalia
| Feature | Reptilia | Aves | Mammalia |
|---|---|---|---|
| Body covering | Dry scales | Feathers | Hair |
| Limbs | Four short legs (none in snakes) | Forelimbs form wings | Legs, wings (bat) or flippers (whale) |
| Heart | Three chambers | Four chambers | Four chambers |
| Body temperature | Cold-blooded | Warm-blooded | Warm-blooded |
| Young | Shelled eggs on land | Hard-shelled eggs | Born alive and fed with milk from mammary glands (most) |
| Other | Lungs; dry skin | Beak; hollow bones; lungs | External ear (pinna); diaphragm |
| Examples | Lizard, snake, turtle, crocodile | Crow, pigeon, penguin | Human, bat, whale, kangaroo |
To classify a vertebrate, work through the clues: gills and scales mean fish; moist skin and a larva mean amphibian; feathers mean bird; hair and mammary glands mean mammal; dry scales and shelled eggs on land mean reptile.
Key terms
- Mammary gland
A gland of female mammals that makes milk to feed the young.
- Warm-blooded
Keeping a constant body temperature whatever the surroundings.
Exam tips
Common mistakes
- Writing the phylum without a feature as reason.
- Calling a spider an insect, or an octopus an echinoderm; a spider is an arthropod with eight legs, an octopus is a mollusc.
- Putting a bat or a whale in the wrong class: both are mammals.
- Confusing a false body cavity (Nemathelminthes) with no cavity (Platyhelminthes) and a true cavity (Annelida).
- Writing four chambers for every reptile: most reptiles have three (the crocodile has four; books differ on how far it is taught).
How marks are usually awarded
- 1 mark: a term, an odd one out with its category, a reason.
- 2 marks: two features or two differences matched point for point.
- 3 marks: a table of three differences, or a neat, labelled diagram.
- 10-mark structured question: a short situation or diagram with parts of 3, 3 and 4 marks; one part is usually a labelled diagram.