A group of living things
Moulting animals
Ecdysozoa
The animals that change clothes to grow.
Its members' last common ancestor lived about 550 million years ago.
You and moulting animals last shared an ancestor about 580 million years ago (on 11 November, if life's history were one year).
Imagine growing by stepping out of your skin, like changing clothes. All moulting animals grow like this. They include crabs, insects and spiders, plus tiny roundworms and water bears. That family link surprises many people. Roundworms look like earthworms, but they are cousins of crabs, not of earthworms.
Each animal wears an outer coat that works like a skeleton. It is often smooth and shiny. To get bigger, it must shed this coat. For example, an insect or crab grows a soft new coat underneath, takes in air or water to crack the old shell, then wriggles out. The insect or crab is pale and soft at first, then its new coat hardens and darkens.
Moulting animals have more known kinds than any other big animal group. You can find them almost everywhere, from deep sea mud to mountain tops and houses. Many are too small to see clearly without a microscope, like water bears about half a millimetre long. The biggest crabs have leg spans about as long as a small car.
A crowd in one apple
Roundworms can be amazingly common. In one study, about 90,000 were found in one rotting apple.
Sleeping dry
When their damp homes dry out, water bears shut down. They lose more than 90 out of every 100 drops of water in their bodies. They shrivel up and pause until water comes back, then wake up.
Slime trappers
Velvet worms are soft and look like caterpillars. They squirt slime to trap small prey for food.