Members of the Phylum Porifera are simple multicelluar aquatic animals that are collectively known as sponges. The majority of the 5,000 or more sponge species are marine, though some are found in freshwater environments.

Sponges lack tissues, organs, and digestive systems. Instead, sponges have several different kinds of specialized cells that carry out growth, repair, feeding, and reproduction. A sponge's body is supported by a skeleton of collagen fibers and mineral spicules of glass or calcium carbonate, which are secreted by specialized cells. The sponge's surface is equipped with pores, canals, and channels that allow water to pass through.

While the larval form is free-swimming, as adults sponges are sessile and attached to submerged objects such as rocks, shells, corals, or even other living animals. Some form broad mats, others are branched or lobed, and some species bore holes into their substrate.

Most sponges are colonial, and they range widely in size from a few millimeters to two meters or more. Many other animals live as commensals or parasites in or on sponges.