Concentrations (mmol N l⁻¹) of urea in saliva samples collected from various regions of the buccal cavity (anterior upper jaw, posterior upper jaw, lower jaw inclusive of the tongue) of Pelodiscus sinensis at 3 or 6 h post-injection with saline (0.9% NaCl; 1 ml 100 g⁻¹ turtle), NH₄Cl (7.5 μmol N g⁻¹ turtle) or urea (20 μmol g⁻¹ turtle or 40 μmol N g⁻¹ turtle)