Favosites (“Brain Coral”) grows as giant domes or extensive thick crusts and can live for more than 100 years. It lived in warm, shallow water environments. Each Favosites is formed by genetically identical polyps which secrete a hard exoskeleton of calcium carbonate. Its hard outer shell, built by the tiny polyps that live within its walls, make it a type of stony coral. Hundreds of these polyps can live within a brain coral at one time. The polyps have two layers – an outer ectoderm and an inner endoderm, which are separated by a jelly-like mesogloea. They also have a single opening which acts as both mouth and anus, which is surrounded by a ring of tentacles containing stinging cells or ‘nematocysts’. The polyp mouths tentacles are only extended at night filtering small drifting organisms. This Favosite is from the Silurian.