What defines an animal
The animals are multicellular eukaryotes that feed by ingesting other organisms (heterotrophic nutrition), have no cell walls, and are usually able to move. The kingdom splits into the invertebrates (no backbone) — the vast majority of animals — and the vertebrates (with a backbone).
Invertebrates
The invertebrate phyla include the simple cnidarians (jellyfish, corals), the worms (flatworms, roundworms and segmented worms like the earthworm), the soft-bodied molluscs (snails, octopuses), the spiny-skinned echinoderms (starfish), and — the largest group of all animals — the arthropods, with a hard exoskeleton and jointed legs: insects, arachnids (spiders), crustaceans (crabs) and myriapods (centipedes).
Vertebrates
The vertebrates all have a backbone and an internal skeleton, and fall into five classes: fish (gills, scales, lay eggs in water), amphibians (moist skin, larvae in water — frogs), reptiles (dry scaly skin, lay leathery eggs on land), birds (feathers, wings, hard-shelled eggs), and mammals (hair, feed young on milk, mostly give birth to live young).
| Class | Key features |
|---|---|
| Fish | gills, scales, eggs in water |
| Amphibians | moist skin, larval stage in water |
| Reptiles | dry scaly skin, leathery eggs on land |
| Birds | feathers, wings, hard-shelled eggs |
| Mammals | hair, milk, mostly live young |
A worked example
An animal has a dry, scaly skin, lays leathery-shelled eggs on land, and is cold-blooded. To which vertebrate class does it belong, and why not amphibian?
- Dry scaly skin and leathery eggs laid on land are reptile features.Reptiles are adapted to reproduce away from water.
- Amphibians have moist skin and lay jelly-covered eggs in water.Their larvae develop in water.
- The dry skin and land-laid eggs rule out amphibian.These features specifically distinguish reptiles.
- So the animal is a reptile.It matches all the reptile characteristics.
It is a reptile — dry scaly skin and leathery eggs laid on land distinguish it from amphibians, which have moist skin and lay eggs in water.
Warm-blooded and cold-blooded
Vertebrates handle body temperature in two ways. Ectotherms ('cold-blooded' — fish, amphibians, reptiles) rely on their surroundings for warmth, so they bask to warm up and are sluggish when cold, but need less food. Endotherms ('warm-blooded' — birds and mammals) generate their own heat and hold a steady body temperature, so they stay active in the cold and can live almost anywhere — but must eat much more to fuel it. This is one reason birds and mammals colonised such varied habitats.
Why arthropods dominate
Arthropods are the most numerous and varied animals on Earth, and their body plan explains why. Their tough exoskeleton supports the body, prevents water loss (vital on land) and protects against predators; their jointed legs allow fast, precise movement; their often small size lets huge numbers live in tiny niches; and many undergo metamorphosis, so larvae and adults do not compete for the same food. Together these adaptations made insects, in particular, extraordinarily successful.
Ectotherm vs endotherm trade-off
Endotherms stay active in any temperature but need lots of food; ectotherms need far less food but slow down when it is cold. Each strategy suits different environments.