that these were all members of either the Calcarea or the Demospongiae. So far scientific publications have identified about 9,000 poriferan species, of which: about 400 are glass sponges; about 500 are calcareous species; and the rest are demosponges. However, some types of habitat, vertical rock and cave walls and galleries in rock and coral boulders, have been investigated very little, even in shallow seas. Classes Sponges were traditionally distributed in three classes: calcareous sponges (Calcarea), glass sponges (Hexactinellida) and demosponges (Demospongiae). However, studies have shown that the Homoscleromorpha, a group thought to belong to the Demospongiae, is actually phylogenetically well separated. Therefore, they have recently been recognized as the fourth class of sponges. Sponges are divided into classes mainly according to the composition of their skeletons: These are arranged in evolutionary order as shown below in ascending order of their evolution from top to bottom: {| class="wikitable" ! Class !! Type of cells !! Spicules !! Spongin fibers !! Massive exoskeleton !! Body form |- ! Hexactinellida | Mostly syncytia in all species|| SilicaMay be individual or fused || Never || Never || Leuconoid |- ! Demospongiae | Single nucleus, single external membrane || Silica || In many species || In some species.Made of aragonite if present.|| Leuconoid |- ! Calcarea | Single nucleus, single external membrane|| CalciteMay be individual or large masses || Never || Common.Made of calcite if present.|| Asconoid, syconoid, leuconoid or solenoid |- ! Homoscleromorpha | Single nucleus, single external membrane|| Silica || In many species || Never || Sylleibid or leuconoid |} Fossil record Although molecular clocks and biomarkers suggest sponges existed well before the Cambrian explosion of life, silica spicules like those of demosponges are absent from the fossil record until the Cambrian. An unsubstantiated 2002 report exists of spicules in rocks dated around . Well-preserved fossil sponges from about in the Ediacaran period have been found in the Doushantuo Formation. These fossils, which include: spicules; pinacocytes; porocytes; archeocytes; sclerocytes; and the internal cavity, have been classified as demosponges. Fossils of glass sponges have been found from around in rocks in Australia, China, and Mongolia. Early Cambrian sponges from Mexico belonging to the genus Kiwetinokia show evidence of fusion of several smaller spicules to form a single large spicule. Calcium carbonate spicules of calcareous sponges have been found in Early Cambrian rocks from about in Australia. Other probable demosponges have been found in