A fluid-filled cavity that forms the main body cavity of vertebrate and most invertebrate animals. It is formed by the splitting of the mesoderm. Ciliated ducts (coelomoducts) connect the coelom to the exterior allowing the exit of waste products and gametes; in higher animals these are specialized as oviducts, etc. The coelom is large and often subdivided in annelid worms (in which it functions as a hydrostatic skeleton) and vertebrates. In arthropods it is restricted to the cavities of the gonads and excretory organs, the body cavity being a blood-filled haemocoel.