The boring sponge is not named after its humdrum lifestyle. It bores into soft limestone, using an acid digestion technique, and forms systems of interconnecting chambers within the rock. If these are outgrown, obvious yellow bulges are formed which can reach 1m across. The sponge also bores into mollusc shells, and can be a serious pest on oyster beds.

Long thought to be plants because of their static nature, sponges were not positively identified as animals until the 19th century. Some of the numerous species in UK waters, such as the sea orange and shredded carrot sponge, have plant-based names. Others rejoice in the more mysterious titles of elephants hide, golfball, black tar or breadcrumb.

Many sponges contain tiny needle-like spikes (spicules) made from hard calcium or silicon compounds. A supportive skeleton is formed by these spicules, by protein fibres or by a mixture of both. In some tropical species, the fibrous skeleton produces a traditional bath sponge when the other constituents are stripped away.

Sponges are essentially living filter pumps. They take in water through the multitude of tiny pores with which they are covered, and expel it through the fewer but more obvious vents (which can resemble small chimneys), having extracted any food material. A sponge only a few centimetres across can filter more than 20 litres of sea water in a day.

With no mobility to escape predators, and no hard shell for protection, sponges need some other form of defence. This is usually provided by secreting unpleasant-tasting substances which, along with their sharp spicules, make them unpalatable. Some animals, such as sea slugs and starfish, remain undeterred and can be found happily grazing on sponges.

Sponges are simple creatures which lack many of the features characteristic of higher animals, including digestive and nervous systems. Their low level of sophistication has been highlighted in experiments in which sponges are broken up by being pushed through muslin. The resulting soup of cells can simply regroup into new sponges!