A sponge is one of the simplest animals on Earth. In fact, its description hardly sounds like a fully formed animal at all. It has no muscles, no internal organs, and no nervous system. It simply anchors itself to the bottom of the ocean and waits for food to come along.
Perhaps because of this simplicity, sponges are a hearty lot. They’ve been around for at least 500 million years, and today, there are more than 5,000 species, scattered across all the oceans of the world. And they come in a lot of shapes and sizes -- from small, spiny lumps to long, narrow tubes.
One of the sponge’s most interesting traits is the way it eats.
A sponge’s body is filled with tiny pores. Water flows in through the pores, and is channeled through the body by the beating of tiny hair-like structures. The water contains oxygen, as well as microorganisms like bacteria and plankton. The sponge’s individual cells absorb both the oxygen and these food particles, and expel waste products. The waste is expelled into the ocean. So the sponge basically operates on a cell-by-cell basis.
Most sponges actually have somewhat rigid skeletons. But some have more pliable skeletons -- a structure made of proteins, known as spongin. When the living cells are stripped away, you can use these pliable skeletons to wash your back or car or kitchen sink. So while most commercial sponges today are made of synthetics, some are still the remains of living organisms.