Patterns in Life: Diversity and Classification
Animals without a backbone
Explore animals without a backbone.
Kingdom Animalia: many-celled, eat other living things (heterotrophic), usually move about, and react quickly. Animals can't make food, so they must move to find it, and to escape from predators.
The big split: the notochord. A notochord is a flexible rod along the back. Animals without one are non-chordates (invertebrates); animals with one are chordates.
Invertebrates as a ladder: each step adds something new.
| Group | Examples | What's new |
|---|---|---|
| Porifera | Sponges | Many cells, but no tissues; water flows in through pores, bringing food and oxygen; fixed in one place, in water |
| Cnidaria | Hydra, jellyfish, corals | Tissues; tentacles to catch prey; but one opening for both food in and waste out |
| Platyhelminthes | Flatworms | Bilateral symmetry: a head end and a tail end, so they move in one direction; still one opening |
| Nematoda | Roundworms | Two openings: a mouth and an anus |
| Annelida | Earthworms | A body in segments, like many rings; muscles, a nerve cord, a body cavity |
| Arthropoda | Insects, crabs, spiders | Jointed legs; a hard outer skeleton |
| Mollusca | Snails, squids, octopuses | A soft body, often in a shell, with a head, a muscular foot and a hump |
| Echinodermata | Starfish, sea urchins | Spiny skin; a hard inner skeleton of calcium carbonate, but no notochord |
A sponge is a great filter: 1 kg of sponge can filter up to 24,000 litres of sea water a day.
Worms with your hand: a flat palm is a flatworm; a round arm is a roundworm; an arm wearing many bangles is a segmented worm (an earthworm).
Bilateral symmetry means only one cut, down the middle, gives two matching halves, as in your body. (In a circle, any cut through the centre gives matching halves: that is radial symmetry.)
Stay safe from parasitic worms. Many flatworms live inside other animals, holding on with hooks and suckers. Head lice are not worms; they are insects. To avoid worms: wash your hands, eat cooked food, and drink boiled or filtered water.
The arthropods' hard outer skeleton protects them, cuts down water loss, and gives their strong muscles something to pull on.
Practise this lesson
Questions that get harder as you improve, a Tutor that helps without giving the answer away, and progress you can see. Free for every learner.
No account, no email address. Opens the real product.