1. The hearts of Squalus lebruni and Mustelus antarcticus beat at intervals that approximate to some simple multiple (1-4) of respiratory cycle length.

2. The beat tends to be initiated during a particular phase of the respiratory cycle, most commonly just before or just after the opening of the mouth.

3. Possible mechanisms of this co-ordination are considered and evidence is presented to show that it is reflexly mediated. Fibres from receptors fired by pharyngeal dilatation are believed to constitute the afferent limb, and the cardiac vagi the efferent limb of the reflex arc.

4. Simultaneous records of blood pressure have been made from the 1st afferent branchial artery and the coeliac artery. By subtraction, the differential pressure across the gill vessels has been calculated and found to have a peak early in the cardiac cycle.

5. Simultaneous photo-electric records of gill opacity and branchial blood pressure show the gill vessels to be compliant. It is concluded that there is a brief period of rapid blood flow early in each cardiac cycle.

6. Calculations from simultaneous records of the electrocardiogram and branchial blood pressure showed that heart beats originating near the time of mouth opening caused a rapid flow of blood through the gills at the time that water was expelled across them.

7. It is suggested that this synchronization of the periods of rapid flow of blood and water serves to maintain the diffusion gradient of oxygen across the gill epithelium.

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