In molluscs, the coelom is represented by the pericardial cavity and the cavities of the kidneys and gonads. The pericardial coelom opens into the renal cavity via the renopericardial duct. Thus the molluscan kidney is a modified coelomoduct of the metanephridial type opening into the pericardial cavity at one end and the ' mantle cavity at the other.
Figure: Kidney and its relationship to the pericardium in the snail
In the molluscs, an initial fluid is formed in the kidney by ultrafiltration of the blood through the wall of the heart. Reabsorption of valuable materials hke glucose and secretion of unwanted compounds occur subsequently resulting in the formation of the final urine. There is some evidence that in the abalone, Haliotis, glucose re-absorption takes place primarily in the left kidney, while secretion of unwanted compounds occurs in the right kidney. Hydrostatic pressure of the blood provides the filtration pressure and application of back pressure stops urine production.