Although sponges are found in a wide variety of body forms, they have three basic types of body construction. Cross sections of the body walls of these three forms are shown above. The simplest is the asconoid type, where there is typically a central spongocoel lined by choanocytes , and a single osculum where water exits from each spongocoel . These are usually smaller, simple sponges of the Class calcarea , and tend to be radially symmetrical. In the Syconoid condition, the body wall folds to form secondary choanocyte chambers, which then empty into the spongocoel through a system of canals.
Many calcareous sponges have a syconoid plan of construction.
The leuconoid condition arises from even more complex folding of the body wall, and the central spongocoel is replaced by a complex system of excurrent canals. Most of the Calcarea , and all species of demospongiae , have a leuconoid plan of construction. There are about 9,000 species of sponges alive today. Sponges are a very old group, having evolved in the pre-cambrian era, so their earliest affinities are unknown, although sponges have left a good fossil record since the Cambrian . Most sponges are marine, however there are about 200 species that inhabit freshwaters.