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:
| Type of cells | Spicules | Spongin fibers | Massive exoskeleton | Body form | |
|---|---|---|---|---|---|
| Calcarea | Single nucleus, single external membrane | Calcite May be individual or large masses |
Never | Common. Made of calcite if present. |
Asconoid, syconoid, leuconoid or solenoid |
| Glass sponges | Mostly syncytia in all species | Silica May be individual or fused |
Never | Never | Leuconoid |
| Demosponges | Single nucleus, single external membrane | Silica | In many species | In some species. Made of aragonite if present. |
Leuconoid |
| Homoscleromorpha | Single nucleus, single external membrane | Silica | In many species | Never | Sylleibid or leuconoid |
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