The Distribution of Po2 and Hydrostatic Pressure Changes Within the Branchial Chambers in Relation to Gill Ventilation of the Shore Crab Carcinus Maenas L

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Abstract
1. A technique is described for replacing part of the branchiostegite of Carcinus maenas by a transparent plastic ‘window’ for direct observation of the gills in situ with minimum disturbance. 2. Observation of dye streams shows that most water enters the hypobranchial space through the Milne-Edwards openings above the chelae, flowing anteriorly and/or posteriorly to ventilate most of gills 3-8. Water also enters above the pereiopods to ventilate the rest of the gills. Water passes from the hypobranchial to the epibranchial space, confirming that there is a counterflow with respect to the circulation of blood through the gill lamellae. 3. By sampling water at different points in the branchial system, patterns of oxygen removal were studied. The gradients confirmed the direction of water flow observed by the use of dyes. 4. Rhythmic changes in hydrostatic pressure in normal forward-pumping of 3-12 mm. H2O were recorded from the branchial cavities, superimposed on a maintained negative pressure relative to that outside the crab of 0-10 mm. H2O. Reversals produced a brief positive pressure change of 0-22 mm. H2O. 5. The possible relationships of the rhythmic pressure changes to scaphognathite movements are discussed. 6. The role of reversals is discussed and it is concluded that their primary function during ventilation is in helping to clean the ventrally facing gill surfaces. But they are also important in respiration under certain special conditions which arise during the normal life of the animal. 7. The utilization of O2 during its passage over the gills is low (7-23%) in spite of the counterflow. Possible explanations of this are discussed in relation to a model of the whole ventilation system.

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