Worm is a term that covers very different groups of animals that all happen to be long and wriggly. Flatworms, for example, are simple creatures that have no proper body cavity and a single opening that acts as both mouth and anus. Segmented worms are sophisticated animals on a par with crustaceans and molluscs. Some worms, such as the sea mouse (pictured), dont even look like worms.

The most obvious worms to divers are those that build tubes. Some make these from mucus alone, others glue sand grains together, while some extract calcium from the sea water and mix a type of cement which they mould to produce a hard, chalky tube.

The bootlace worm, a simple ribbon worm, is Britains largest animal - it can reach up to 30m long! More usually, it is around 5m long when fully extended, and contracts rapidly to a fraction of this size on being touched.

Peacock worms use sand grains to build tubes that can reach up to 30cm long. The worms sort carefully through grains of sand and mud captured by their tentacles. Large particles are discarded, small ones are eaten and medium-sized ones are mixed with mucus for tube-building.

Even within the segmented worm group, there are big variations in lifestyle. Some are active hunters and roam the seabed in search of prey. Others, such as the lugworm, eat mud within their burrows and dont emerge even to mate. Males shed sperm into the water, and females draw it in with their breathing current.

The feathery tentacles of fan worms serve several functions. They extract oxygen from the sea water, catch food and carry sense organs such as eyes. Underwater photographers know these worms have an unparalleled ability for detecting a camera shutter about to fire, and withdrawing into their tubes accordingly.