Demosponge

We know that at least some living calcified demosponges grow extremely slowly (0.2 mm/year Willems and Hartman 1985) and, since individuals can grow up to 1 m in diameter, are also very old (up to 5000 years).

Demosponges have siliceous spicules and a network of fibrous protein, spongir, that is similar to collagen. The demosponges are the source of natural household sponges, which are made by soaking dead sponges in shallow water until all the cellular material has decayed, leaving the spongin network behind.

Demosponges, the class to which the barrel sponge belongs, are the oldest still existing multicellular animals for which there are unambiguous traces in the Cryogenian, or ‘snowball Earth’, period of the late Proterozic.