Which skeletal materials occur in demosponges?

The skeletal materials found in Demospongiae are diverse, reflecting the class's varied structural adaptations. These materials include:

• Siliceous Spicules: Composed essentially of silicic acid, these spicules are categorized as either megascleres (for support) or microscleres (for protection and support). They may be 1-rayed (monaxon) or 4-rayed (tetract) in form.1234
• Spongin: This is a protein-based material that forms flexible, horny fibers. Spongin can exist as the primary skeletal component, or it may serve to cement siliceous spicules together into bundles or meshes, enhancing the skeleton's elasticity.5263
• Desmas: These are specialized, irregularly branched spicules formed by secondary deposits of silica around ordinary spicules. In some species, these interlock to create a compact, stony skeleton known as a lithistid skeleton.63
• Calcareous Elements: While less common, some demosponges—particularly those found on coral reefs—possess a compact skeleton that incorporates both siliceous spicules and organic fibers alongside calcareous material.6

A few primitive genera of Demospongiae lack a skeleton entirely.63