Sponges are frequently found on the lower shore in gullies and under rocky overhangs. Here they avoid desiccation and display a range of colours and growth forms giving them a superficial resemblance to plants. Indeed, it was not until the eighteenth century that they were classified as animals and as late as 1825 before the issue was put beyond doubt by the research of Dr R. E. Grant of Edinburgh.

Sponges are sessile, some forming encrusting, irregular growths covering large areas of rock, while others are vase-like and attached to the substratum at the base. They exhibit the cellular grade of organization, in which the cells do not form tissues and organs so characteristic of higher animals, and as a result they are often referred to as primitive animals. Despite this simplicity, they exhibit a range of complexity in body structure and have colonized both freshwater and marine habitats, extending from the shore to the greatest depths.

The basic design of a simple sponge is that in which the body wall is perforated by many tiny pores, ostia, through which water enters the sponge, and a larger opening, the osculum, through which water leaves. The cavity of the sponge is lined by flagellated cells known as choanocytes which maintain a flow of water through the body.