A group of living things
Mouth-first animals
Protostomia
Insects, snails, worms and octopuses make up most kinds of animals
Its members' last common ancestor lived about 570 million years ago.
You and mouth-first animals last shared an ancestor about 580 million years ago (on 11 November, if life's history were one year).
Most kinds of animals are mouth-first animals. Their moulting side alone is the largest major animal group by number of kinds, with insects, spiders, crabs and roundworms. Add snails, octopuses and segmented worms, and this one branch holds most of them. We sit on the branch right beside it, with starfish and sea squirts.
The name means the mouth forms first in the growing embryo. Inside, the branch splits in two: animals that shed their outer coat to grow, like insects, and others such as snails, octopuses and segmented worms. Looks can be misleading. An earthworm is closer kin to a snail or a squid than to an insect, even though earthworms and insects both have bodies built from segments.
They live almost everywhere, from deep seas and rock pools to soils, forests, mountains and the air for flying insects. They also span wild sizes, from water bears about half a millimetre long to colossal squid about 4 metres long, with eyes as big as soccer balls and a hard beak like a parrot's.
Same babies, different adults
Many sea snails and segmented sea worms start life as almost the same kind of tiny swimming speck, with two rings of tiny hair-like threads for swimming and feeding. Grown up, a sea worm and a squid can look nothing alike.
The toughest nap
Water bears can dry into a tiny ball and wait up to about 30 years without food or water. In this state they can live through icy cold, heat above boiling and even the vacuum of space.
Two lives in one
In most insect groups the young change shape as they grow. The young insect can look nothing like the adult and can live in quite a different place.