Summary

1. Blood pressure has been measured in several parts of the haemocoele of a representative terrestrial pulmonate, Helix pomatia.

2. A venous pressure of 8 cm of water is required to overcome the resistance presented by the branchial plexus and to effect the process of ultrafiltration in the renal plexus.

3. Visceral sinus and sub-renal sinus blood is at a pressure barely exceeding this value and need only perform the two named functions before returning to the auricle.

4. Blood in the cephalopedal sinus and general haemocoele of the foot is at a higher pressure, of up to 16 cm, and has obvious hydraulic functions. Partly by review of the literature and partly by direct observation these functions are discussed.

5. The distribution of blood during different states of activity and the importance of the heart in maintaining the high haemocoelic pressure of pulmonates is also briefly discussed.