Asponge is supported by a skeleton made of hard crystals called spicules, which can be calcareous (made of calcium carbonates, the material found in limestone) or siliceous (made of silica—essentially, glass). Either type of sponge forms into a delicate network, giving the sponge its characteristic “holey-spongy” look.

A type of sponge called the demosponge has siliceous spicules and a network of fibrous proteins called sponging; they are the source of all natural household sponges. These utilitarian sponges are made by soaking the dead sponges in shallow water until all the once living cellular material has decayed, leaving the spongin network behind. Because this process is so labor intensive—and it also depletes the numbers of natural sponges—most sponges now sold for household use are plastic or fiber and have nothing to do with real sponges.

The two forms of cnidarians are called the polyp stage and the medusa (plural, medusae), or jellyfish, stage. Polyps generally live attached to a hard surface and bud to produce more polyps and, in some cnidarians, to produce the medusa stage of the life cycle. These medusae, or jellyfish, drift with the ocean currents or swim by pulsating their umbrella-shaped bodies. They also release sperm and eggs into the water; after external fertilization, the embryo develops into a larva that eventually settles to the ocean bottom, becoming another polyp and completing the life cycle. Not all cnidarians go through both polyp and medusa stages; some, such as corals and sea anemones, exist only as polyps.