ABSTRACT
Holthuisana valentula Riek and H. agassizi (Rathbun) both ventilate their lungs by lateral oscillations of the thoracic walls within the branchial chambers. Air enters and leaves the lungs via the prebranchial apertures and the Milne-Edwards apertures. At rest, active ventilation in H. transversa was low (0·95 mlg−1 h−1) and a high diffusional component was evident. After disturbance, 
rose to 45·4mlg−1h−1 and % extraction was low (1·1%). Yentilation increased during moderate hypercapnia and marked hypoxia. 
was high after disturbance (2·95 μmolg−1 h−1) but declined to very low levels at rest (0·45 μmolg−1 h−1). The haemocyanin had a relatively low affinity for oxygen during air-breathing (P50 = 13 Torr) but it was saturated with oxygen at normal 
(56·1 Torr). CCO2 in the haemolymph (13 mmol l−1) was characteristic of air-breathing crabs. The respiratory performances of H. transversa in air and water are compared.
