The body of a sponge consists of jelly-like material (mesohyl) made mainly of collagen and reinforced by a dense network of fibres also made of collagen.sandwiched between two thin layers of cells. Many also have a skeleton made up of spicules of calcium carbonate or silica. Spicules vary in shape from simple rods to three-dimensional "stars" with up to six rays. Some sponges also secrete exoskeletons that lie completely outside their organic components whilst others, e.g. Spongia officinalis, the bath sponge, have no spicules at all.

Sponges do not have a nervous, digestive or circulatory system. They rely on keeping up a constant water flow through their bodies to obtain food and oxygen and to remove wastes.

Sponges have a unique feeding system among animals. Instead of a mouths they have tiny pores (ostia) in their outer walls through which water is drawn. Cells in the sponge walls filter food from the water as the water is pumped through the body and the osculum ("little mouth"). The flow of water through the sponge is in one direction only, driven by the beating of flagella which line the surface of chambers connected by a series of canals. Sponge cells perform a wide range of bodily functions and appear to be more independent of each other than are the cells of other animals.

Sponges are usually hermaphrodites, however they are either male, female or neuter at any time. Most sponges reproduce sexually by releasing sperm cells into the water. In viviparous species the cells that capture most of the adults' food capture the sperm cells and transport them to ova in the parent's mesohyl. The fertilized eggs begin development within the parent and the larvae are released to swim off in search of places to settle. In oviparous species both sperm and egg cells are released into the water and fertilisation and development take place outside the parent's bodies.