First appearing in the fossil record in the Pre-Cambrian Eon, over 600,000,000 years ago, sponges have shown little change over time. Currently, about 5000 different species of sponges have been identified ranging from 1 cm to over 200 cm in length. All sponges have a similar body plan which basically resembles a sac filled with holes. Water enters the body through these holes where it is kept moving by thousands of microscopic cells which have beating flagella, tiny whip like hairs which move back an forth. Once in the sponge body, the water is filtered of tiny plankton which it uses as a food source.
Most sponges are hermaphroditic, meaning that a single organism has both eggs and sperm.
One unique feature of sponges is there ability to repair themselves. When a sponge gets damaged, or when cells have been separated from the main body, they re-aggregate to form a healthy individual. Today, sponges are used for a variety of jobs from medical research to fertilizers. Have students research some of the uses of natural sponges and if possible, bring in samples of the material. Calcispongiae and Hexactinellida actually secrete a hard skeleton made of calcium carbonate and silica glass respectively.