Sponges, or poriferans, are animals of the phylum Porifera. They are sessile, mostly marine, waterdwelling, filter feeders, pumping water through their matrix and filtering out particulates of food matter. Sponges are among the simplest of animals, with partially differentiated tissues but without muscles, nerves, and internal organss. In some ways they are closer to being cell colonies than multicellular organisms. There are over 15000 modern species of sponges known, and more are being discovered every day. The fossil record of sponges dates back to the precambrian era. The structure of a sponge is simple: it is shaped like a tube, with the end stuck to a rock or other object. The open end is called the osculum, and the interior is the spongocoel. The walls are perforated with microscopic holes to allow water to flow through the spongocoel.

Sponges have only four types of cells:

  • Choanocytes, which line the spongocoel and function as the sponge's digestive system, are remarkably similar to the protistan choanoflagellates. See that article for a further description.
  • Porocytes are tubular cells that make up the pores.
  • Flat epidermal cells line the outside of the sponge and form its skin.
  • Amoebocytes live between the choanocytes and the epidermis. They carry out many of the sponge's functions, such as transport of nutrients, secretion of the spicules, and production of gametes.

Sponges have only three body types: asconoid, syconoid, and leuconoid

It is thought that the earliest animal life on Earth resembled sponges. The earliest known multicelled animal fossils are sponges from China that are roughly 600,000,000 years old. Sponges have not been as extensively studied as some other phyla and there may be some suprises still to be found. For example, it has recently been shown that some sponges are not sessile and can move to more favorable locations as rapidly as few cm a day. Another sponge, the Venus Flower Basket has some newly discovered uses involving fiber optics

spicules need to be explained.

Sponges are divided into Demosponges, Hexactinellid sponges, and Calcaerous sponges based on the type of spicules in their skeleton. Similar fossil animals known as Chancelloria are no longer regarded as sponges. 90% of modern sponges are demosponges. Demosponges are less common than other sponges, but that is probably because many demosponges have skelatons made only of relatively soft spongin that does not fossilize well. The fossil Archaeocyantha may also belong here, though their skeletons are solid rather than separated into spicules. It has been suggested that the sponges are paraphyletic to the other animals. Otherwise they are sometimes treated as their own subkingdom, the Parazoa.

In common usage, the term sponge is usually applied to the skeletons of these creatures alone, from which the animal matter has been removed by maceration and washing. The material of which these sponges are composed is spongin. Calcareous and siliceous sponges are too harsh for similar use. Commercial sponges are derived from various species and come in many grades, from fine soft "lamb's wool" sponges to the coarse grades used for washing cars. They come from the fisheries in the Mediterranean and West Indies. The manufacture of rubber, plastic and cellulose based synthetic sponges has significantly reduced the commercial sponge fishing industry over recent years.

Sponge is also the name of a Indie rock band; see: Sponge (band)