like an inverted crown at the base of their skeleton. These fibers are long and about the thickness of a human hair. Glass sponges are different from other sponges in a variety of other ways. For example, most of the cytoplasm is not divided into separate cells by walls but forms a syncytium or continuous mass of cytoplasm with many nuclei (e.g., Reiswig and Mackie, 1983). These creatures are long-lived, but the exact age is hard to measure; one study based on modelling gave an estimated age of a specimen of Scolymastra joubini as 23,000 years (with a range from 13,000 to 40,000 years), but due to changes in sea levels since the last glacial maximum, its maximum age is thought to be no more than 15,000 years, wherefore its listing of c. 15,000 years in the AnAge Database. The shallow-water occurrence of hexactinellids is rare worldwide. In the Antarctic two species occur as shallow as 33 meters under the ice. In the Mediterranean one species occurs as shallow as in a cave with deep water upwelling (Boury-Esnault & Vacelet (1994)) Reefs The sponges form reefs (called sponge reefs) off the coast of British Columbia, southeast Alaska and Washington state, which are studied in the Sponge Reef Project. Reefs discovered in Hecate Strait, BC have grown to up to 7 kilometres long and 20 metres high. Previous to these discoveries, sponge reefs were thought to have died out in the Jurassic period. Reports of glass sponges have also been recorded on the HCMS Saskatchewan and HCMS Cape Breton wrecks off the coast of Vancouver Island. Classification The earliest known hexactinellids are from the earliest Cambrian or late Neoproterozoic. They are fairly common relative to demosponges as fossils, but this is thought to be, at least in part, because their spicules are sturdier than spongin and fossilize better. Like almost all sponges, the hexactinellids draw water in through a series of small pores by the whip like beating of a series of hairs or flagella in chambers which in this group line the sponge wall. The class is divided into five orders, in two subclasses: Class Hexactinellida Subclass Amphidiscophora Order Amphidiscosida Subclass Hexasterophora Incertae sedis Dactylocalycidae Gray, 1867 Order Lychniscosida Order Lyssacinosida Order Sceptrulophora See also Sponge reef Cloud sponge Sponge Reef Project References External links Extant Cambrian first appearances