Molluscs

Snails, clams and octopuses look like strangers, yet they are one family. Underneath, the starting plan is much the same: a head at the front, a strong foot underneath for movement, and soft organs packed at the back under a fleshy fold. In most groups, that fold builds a hard shell, though some groups have lost or changed parts of the plan.

That one plan makes wildly different bodies. Snails and slugs hold the most species, about seven in ten, and most crawl on a broad foot. Most snails carry a single shell, while slugs have lost or shrunk theirs. Clams, mussels and oysters grow two hinged shells and no head, and most adults stay in one place, fixed to rock, buried in sand or gripping with tough threads. Octopuses, squid and cuttlefish have arms around the mouth and no outer shell, and squid and cuttlefish add two much longer tentacles. Octopuses can change colour to hide.

Molluscs live in the sea, in fresh water and on land, from city gardens and rocky shores to reefs and the deep cold sea. A garden snail is about as wide as a bottle cap, at about 3.5 centimetres, while a blue mussel can grow to about as long as a finger, at about 10 centimetres. At the large end, giant clams reach about 1.2 metres and giant squid can stretch longer than a bus, up to about 15 metres including their long tentacles.