 In amniotes and some invertebrates the coelom is the large cavity lined by mesothelium, an epithelium derived from mesoderm.Organs formed inside the coelom can freely move, grow, and develop independently of the body wall while fluid in the peritoneum cushions and protects them from shocks.Arthropods and most molluscs have a reduced (but still true) coelom, usually the pericardial cavity and the gonocoel.