Primitive multicellular organisms belonging to the phylum Porifera, sponges can be occasionally found in freshwater, but thrive more often in the sea. Sponges do not have definite organs to carry out their functions and generally do not move from place to place, which resulted in the early mistaken belief that they were plants rather than animals.

Possessing a simple cellular anatomy, sponges are composed of an outer layer of protective surface cells and an inner layer of flagellated cells that help move food and water through its inner cavities. A gelatinous substance, through which amoeboid cells move and carry out a variety of functions, is located between the two layers. The cells aid digestion, transport of nutrients, reproduction, and produce the structures that compose the skeleton of the sponge. The structures, known as spicules, give the sponge its shape and remain intact even after the animal has died.

Sponges are taxonomically classified according to the composition of their skeletons and spicules. The vast majority of sponges have a skeleton made of spongin, which makes them soft and elastic. These sponges belong to the class Demospongiae and are sometimes harvested for household and commercial use. The remaining sponges either have spicules made of calcium carbonate and belong to the class Calcispongiae or are members of Hyalospongiae, which have skeletons composed of silicic acid. The latter are sometimes referred to as glass sponges because their skeletons have a beautifully delicate, glass-like texture.