Kingdom Animalia: invertebrates to vertebrates

Watch a gecko on a wall: it spots a moth, freezes, then darts. That is animal life in one scene. Animals are many-celled, they cannot make food so they must take it from other organisms, and most of them are built to move, sense and react fast, which is exactly what finding a meal and not becoming one demands.

To sort them, start with one structure: the notochord, a flexible rod-shaped support along the body. No notochord at any stage: non-chordata (invertebrata). A notochord: chordata. Inside the chordates there is a second fork, protochordata on one side and vertebrata on the other, and in some chordates the notochord is the forerunner of the backbone.

No notochord does not mean simple. Line the invertebrates up from sponge to starfish and you can watch the animal body being redesigned: a feature appears, it fixes one problem of feeding or moving or control, and it usually creates a new one.

Porifera (pore-bearers), the sponges, are many-celled but have no tissues or organs. Water flows through their many pores, bringing food and oxygen straight to each cell and taking waste away. They stay fixed, and live in water.

Cnidaria, such as Hydra, jellyfish and corals, have true tissues, so groups of cells can specialise; tentacles catch prey instead of waiting for food to drift in. But one opening must take in food and push out waste.

Platyhelminthes (flatworms) have bilateral symmetry: one cut along one plane gives two matching halves, with a head end and a tail end. That gives movement a direction. Being flat, they need no lungs or gills: gases simply seep in and out. The gut, though, still has only one opening. Many are parasites that grip the host with hooks and suckers.

Nematoda (roundworms) are long tubes, a shape that slides through soil, water or a host's tissues and lets them live in many places. Food goes in one end and waste leaves the other: two openings, mouth and anus. Male and female are separate worms, each built at the organ system level.

Annelida (segmented worms) such as the earthworm have a body of segments and a body cavity, with organ systems, muscles for moving and a nerve cord for control. Segments make movement flexible and precise.

Arthropoda (insects, crabs, spiders) keep the segments but give them different jobs, and wear a hard external skeleton. Think of it as armour, a raincoat and a frame for muscles in one, and you can see why insects and spiders thrive even in deserts and dry grassland.

Mollusca (snails, squids, octopuses) are soft-bodied animals with organ systems; the body has a head, a hump and a strong muscular foot. Many protect the soft body with a shell: one basic body plan bent in many directions.

Echinodermata (echinos, spiny; derma, skin), starfish and sea urchins, have a hard internal skeleton of calcium carbonate that gives protection and controlled movement, yet no notochord.

Across the invertebrates, then, body organisation grows more complex, and each new feature solves one problem of feeding, moving or protection while raising new ones.

Protochordates, Amphioxus for example, carry a notochord at some point in life, if only for a while. The rod stiffens the body from inside yet still lets it bend and swim, and that was a big step. Because they are such simple chordates, they give a glimpse of the road from simple animals to backboned ones.

Vertebrates have a vertebral column, the backbone. It is a frame and a shield at once: it carries the body's weight and wraps the spinal cord, while the skull guards the brain. A strong inner frame is what lets an elephant be big and a cheetah be fast, and vertebrates pair it with sharp senses and well coordinated behaviour. Habitat, body covering and the way they reproduce sort them into five groups.