Hexactinellid sponges are sponges with a skeleton made of four or six pointed spikes, often referred to as glass sponges. Glass sponges are rather uncommon and are mostly found at depths from 450 to 900 meters under the water. They are found in all oceans of the world, although they are normally found in Antarctic waters. They are basically cup-shaped animals, ranging from 10 to 30 centimeters in height, with sturdy internal skeletons made up of fused spicules of silica. The body is symmetrical, with a large central cavity that, in many species, opens to the outside through a sieve formed from the skeleton. Unlike other sponges, they tend to be alone, rather than forming large populated colonies.