The approximately 10,000 species of animals commonly known as sponges are largely ocean dwellers, with all but 150 or so species inhabiting freshwater. Sponges are among the simplest of the invertebrates, being comprised of aggregations of cells and lacking true organs. These sessile creatures are important components of their environments, and many species are hosts to communities of other animals such as shrimp and fish. Identification of individual species is complicated by the fact that many take the shape of the objects upon which they have attached, or assume different forms depending upon the rapidity of the water currents in which they live. Although the bright colors of many advertise their toxicity, a variety of marine animals such as turtles, fish, and sea slugs do manage to consume sponges regularly. Sponges have amazing powers of regeneration.

Living sponges are given their texture and shape by a material known as spongin, which supports the rest of the body's cells. Movement of water into the sponge is accomplished by the action of hairlike flagellum. Water enters and waste products exit the animal through a single opening, with microscopic organic material being removed to nourish the sponge in the process. Sponges reproduce by releasing eggs and sperm into the surrounding water. The newly hatched young become part of the plankton and float around for a period until attaching to the substrate, a rock, or a similar surface, where they spend the rest of their lives as sessile organisms.