The Mechanism of Urine Formation in Invertebrates

1. In Carcinus maenas: (a) The blood may be hypertonic, isotonic or hypotonic to the external medium. (b) The urine may be hypertonic, isotonic or hypotonic to the blood, and its concentration may differ in the two antennary glands. (c) The hydrostatic pressure of the body fluid is c. 13 cm. of water. (d) The colloid osmotic pressure of the blood is c. 11 cm. of water. (e) The urine probably contains protein and has a colloid osmotic pressure of c. 3 cm. of water. 2. In Potamobius fluviatilis: (a) The blood is hypertonic to the external medium. (b) The urine is hypotonic to the blood but hypertonic to the external medium and its concentration may differ in the two antennary glands. (c) The hydrostatic pressure of the body fluid is c. 20 cm. of water. (d) The colloid osmotic pressure of the blood is c. 15 cm. of water. (e) The urine may contain protein and has a colloid osmotic pressure (calculated) of c. 2 cm. of water. 3. In Peripatopsis spp.: (a) The blood is hypertonic to the urine. (b) The hydrostatic pressure of the body fluid is c. 10 cm. of water. (c) The colloid osmotic pressure (calculated) of the blood is c. 5 cm. of water. (d) The urine may contain protein and has a colloid osmotic pressure (calculated) of c. 2.5 cm. of water. 4. It is concluded that filtration is possible and that secretion and resorption almost certainly occur in the formation of the urine. 5. A microthermopile is described. 6. Methods are described for measuring the hydrostatic pressure and the colloid osmotic pressures of the body fluids in small animals.