A group of living things
Two-sided animals
Bilateria
Left, right, front and back: the body plan you share with worms and octopuses.
Its members' last common ancestor lived about 580 million years ago.
You are part of this group. See how.
You have a left and a right, a front and a back. So do worms, insects and octopuses. You all share the same basic body plan: mirror-image halves with a head end, a tail end, a back and a belly. In most, the head leads the way, packed with sense organs like eyes and feelers to meet the world first.
This big family splits in two. One side holds insects, snails, clams and earthworms. The other holds starfish and people like you. Starfish look like rule-breakers, but their young start two-sided and grow into adults with usually five arms. The mirror match is mostly skin-deep: insides are often lopsided, like your heart sitting off-centre.
Between them, they live almost everywhere, from mountain tops to deep-sea hot vents. Some snails are as tiny as a grain of sand, about a millimetre long. Others, like the giant squid, grow to about 13 metres including their long tentacles, a little longer than a bus.
Three layers, not two
Early in growth, your body forms from three layers of cells. Jellyfish and corals, next door on the tree, build theirs from only two.
Food goes one way
Most have a mouth and a separate anus, joined by a food tube.
Same genes, different animals
The same family of Hox genes draws the head-to-tail plan in a worm, a fly, a mouse and you.