Hexactinellids are sponges with a skeleton made of silica skeletons, often referred to as glass sponges. Hexactinellids are relatively uncommon and are mostly found deep oceans. There are differences between glass sponges and other poriferans. Many are cup-shaped animals with fused spicules. Glass Sponges are commonly found as fossils because their spicules are sturdier than other sponges and fossilize better. They do not possess the ability to contract.
Photo Credit: (Venus' Flower Basket, ''Euplectella aspergillum'' PD-USGov From [http://oceanexplorer.noaa.gov/explorations/02sab/logs/aug27/media/heterotella_600.jpg])