The body cavity is a well-developed coelom lined with somatic and splanchnic mesodermal epithelium. It contains a lymph-like coelomic fluid. It is enterocoelic in origin. Behind the pharynx the coelom is a spacious cavity around the intestine being suspended in the coelom by a dorsal mesentery.
But it is reduced on the right side of the hindgut by a posterior extension of the atrium. Coelom also surrounds the midgut diverticulum lying within the atrium on the right side of the pharynx.
But in the region of the pharynx the coelom is much reduced due to formation of gill-clefts, though in the larva it surrounds the pharynx all around except mid-dorsally. Thus, in the pharyngeal region of the adult it is reduced to three types of spaces, a mid-ventral sub-endostylar coelom running longitudinally below the endostyle, two dorsal longitudinal canals lying above the pharynx, one on either side and enclosing the brown funnels, and vertical coelomic canals in the primary gill-bars which connect the subendostylar coelom with a dorsal longitudinal coelomic canal on each side of the pharynx. Small coelomic spaces are also present within the gonads, these are the gonocoels. In higher chordates the coelom is completely lost in the region of the pharynx.
