Sponges belong to the Subregnum Parazoa – Phylum Porifera. This is an intermediate group between one-celled and more-celled animals. The Phylum Porifera consists only of sponges. Sponges have existed for over 200 million years and have not changed in that time – they are perfectly adapted to what they do. Sponges are a group of cells working together, with no true tissues or organs. Each sponge consists of 3 types of cells. The cells are organized around a system of pores, channels and chambers. All cells consist of soft tissue and needle like structures (spirules). The spirules form the body of the sponge. The shape and size of the spirules are specific to the type of sponge. There are two main groups. First sponges that take the shape of a vase and secondly, sponges that spread flat on the bottom. All are attached to hard bottom, like a rock.
The outside of any sponge is covered with small pores that are lined with special cells called collar cells. These cells are flagellated. The only thing these cells do is moving the little flagella. This pulls water into the body of the sponge. The cells filter the food particles from that water and in vase shaped sponges all water exits through a big hole called the osculum. Flat sponges have a series of smaller oscula.
An average sized sponge filters hundreds of liters of water every day, which makes them the cleaning mechanism of the sea. Sponges are vulnerable to silt. When a diver stirs up silt and it falls on a sponge, some of the silt will fall off, but if too many passages are blocked by silt, the sponge will die of starvation.