ostia lead to numerous incurrent canals, but there is no large central cavity.

The skeleton of spicules and spongin provides the support to keep the pores open. The spicules can be either siliceous or calcareous of a variety of shapes (see right), and can be used in identifying species. The spongin is a form of collagen.

Regeneration. Sponges have the amazing ability to regenerate an entire individual from just a few cells.

There are four Classes of sponges.

Class Hexactinellida

These are commonly known as the glass sponges and include the Venus' flower basket (Euplectella spp.) which can be seen on the right. And Regardrella okinoseana, a lacy basket sponge on the left. They are often radially symmetrical and vase- or funnel-shaped and can be up to 1 m across and 1 m high, and are very beautiful. Their distinguishing feature is the network formed by the six-rayed siliceous spicules. There are about 500 species. They occur mainly in deep, cold waters between 200 - 2000 m, although some can be found below 6000. Euplectella sp. is rather atypical as it is found in tropical waters at around 150 m, and is rarely taller than 30 cm. Some species of Euplectella have a commensal relationship with a shrimp species. A young male and female shrimp enter the central cavity and live there. As they grow they become too large to escape through the sieve-like covering of the osulum, and so spend their entire life inside the sponge. Synconoid and leuconoid canal systems occur in this class.

Class Calcarea

As the name suggests sponges in this class have spicules of calcium carbonate. The spicules are either free or fused. They tend to be relatively small, mostly less than 10 cm, and tubular or vase-shaped. All three types of canal system occur in this class. There are about 100 species, mainly marine in water no deeper than 1000m.

Class Demospongiae

This is the largest Class containing over 90% of living sponges, and nearly all the larger species. They can be found at all depths in both fresh and salt water. The skeleton can be siliceous, spongin, or both. The spicules are either simple or four-rayed. All have the leuconoid canal system. Demospongiae are often brightly coloured. The bath sponges are in this Class, see Spongia sp. right. Bath sponges are simply the spongin skeleton which has been left behind after all the other cells have been removed in various treatments. Spongin is a substance similar to the keratin of hair and fur. Bath sponges do not have siliceous spicules. The barrel sponge, Xestospongia testudinaria, below right is also in the demospongiae. It can grow so large that a man can fit inside. They are found growing on solid surfaces, and can reach over 1 m tall and wide. Below left is Spongilla lacustris, the pond sponge. It is found in fresh still water. It forms a green/yellow growth over stones and plants. The colour varies according to the light level, with the greener sponges growing in good light.

Class Sclerospongiae

This is a small group of sponges that resemble corals. They are usually found in dark tunnels in coral reefs. The skeleton consists of siliceous spicules and spongin on a thick basal layer of calcium carbonate. All in this class have the leuconoid canal system.