Sponges are very simple multicelled animals, comprised solely of groups of cells arranged with pores between the cells. A type of collagen protein called spongin makes up most of the structure of the sponges. The largest group of sponges, called the demosponges represents 90% of the 9,000 species of sponges. A small group of 120 more primitive sponges called homoscleromorphs also has that composition. One additional group of about 800 calcareous sponges fashions small spicules of calcium carbonate for additional structural support. Another group of about 700 deepwater sponges called glass sponges has small spicules of silica (glass) for additional support.

The motion of small flagella sticking out from the cell walls drives water from the outside surface and through the passages. Food particles and oxygen are delivered to the cells of the sponge by filtering both from the passing water. The filtered water from many pores discharges first into a large plenum and then out a large opening in the plenum and back out into the sea. Sponges take many forms around that discharge plenum, including cones, vases, tubes and large barrels.

The images here are all of demosponges, since that is the largest group and hence the most commonly observed. The final image shows a specimen with a very unusual shape, though. This ligament sponge grows with many branches several inches in diameter and more than several feet long. The outlet openings are arrayed along the tops of the branches. This species is known only from the area near Papua New Guinea and West Papua.