A group of living things
Molluscs, worms and kin
Spiralia
Clever octopuses and garden worms, named for a spiral start
Its members' last common ancestor lived about 550 million years ago.
You and molluscs, worms and kin last shared an ancestor about 580 million years ago (on 11 November, if life's history were one year).
A clever octopus and an earthworm are cousins. Both belong to this group of mostly soft-bodied animals. The group is named for how early cells divide in a spiral in many members. The cells split at an angle, so the cells at the top sit twisted out of line with those below. This group forms one side of the mouth-first animals, where the mouth appears before the rear opening. Its sister group is the moulting animals.
Many molluscs and segmented sea worms start life as the same kind of tiny swimming young, with two rings of beating hairs and a tuft on top. Earthworms were once grouped with arthropods because both look segmented. But those jointed-leg animals lack this swimming young, and DNA links earthworms to molluscs instead.
Molluscs show the range. Many snails glide on a broad foot under a single coiled shell. Clams and mussels shelter between two hinged shells. Octopuses have no shell at all and grip with eight arms lined with suckers around a hard beak. With squid and cuttlefish, they share the most advanced nervous system of any animal without a backbone, and see shapes and textures well.
As long as a bus
Female giant squid reach about 13 metres from fin to tentacle tip. That is as long as a bus. Males reach about 10 metres.
A shell that sees
Chitons cling to wave-washed rocks with a strong foot and carry eight overlapping plates. They scrape algae to feed. Many have simple eyes in the shell that tell light from dark.
Living camouflage
Squid can match the seabed by squeezing tiny colour sacs in the skin. Muscles under nerve control open and shut each sac.