A sponge is member of the phylum Porifera—one of the most primitive animals in the world. Overall, approximately 5,000 species of marine (saltwater) sponges and 150 species of freshwater sponges exist. These living invertebrates may be brightly colored— green, blue, yellow, orange, red, or purple—or they may be white or drab. The bright colors are due to the various bacteria or algae that live on or within the sponge.

A sponge’s body contains holes that lead to an inner water chamber. The organisms pump water through those pores and expel it through a large opening at the top of the chamber. As water passes through the body, the sponge gathers nutrients, oxygen is absorbed, and waste is eliminated. Sponges are distinctive in possessing choanocytes (special flagellated cells whose beating drives water through the body cavity) that characterizes them as suspension feeders (also known as filter feeders). A marine sponge that is 4 inches (10 centimeters) tall and 0.4 inch (1 centimeter) in diameter pumps about 23 quarts (22.5 liters) of water through its body in one day. To obtain enough food to grow by 3 ounces (100 grams), a sponge must filter about 275 gallons (1,000 kilograms) of seawater!