1. The respiratory movements of the dogfish, Scyliorhinus (Scyllium) canicula (L.), and the ‘skate’, Raia clavata L. (thornback ray), have been studied by the use of cinematographic and mechanotransducer recording methods. Simultaneous determinations of the time-course of pressure changes in the oro-branchial and parabranchial cavities were also made by means of Hansen condenser manometers.

2. In both species movements of the mouth precede those of the spiracular valve and of the branchial region. Adduction and abduction of the branchial region spreads serially from the first to last gill slit in the dogfish, but movements of the individual gill arches are more nearly synchronous in the skate. Opening of the flap valves formed by extensions of the inter-branchial septa are synchronous in both species.

3. Water entering one side of the mouth leaves by the three posterior gill slits of the same side. Water entering the spiracle leaves through the anterior slits of the same side. This separation of flow is less marked in the skate.

4. The pressure curves recorded in all parts of the system have both positive and negative phases with respect to the external medium. The positive phase, associated with closing of the mouth and spiracle, is larger in the oro-branchial than in the parabranchial cavities and vice versa. The time-course of the pressure changes indicates that the flow across the gills is maintained by the action of a pressure pump in front and a suction pump behind.

5. The suction pump plays a more important role than the pressure pump in the skate and its contribution to the flow across the gills is by no means negligible in the dogfish.

6. The differential pressure curves suggest that the flow across the gills is continuous except in the dogfish for a brief period when the gradient is reversed. The absence of this reversal in the skate suggests that the external gill slit openings are controlled by an active mechanism. This is probably an adaptation to bottom-living habit.

7. All these observations relate to animals which are stationary with respect to the water. During swimming at a reasonable speed leopard sharks (Triakis semifasciata) have been observed to make few or no respiratory movements, although they immediately ventilate actively on coming to rest at the bottom of the aquarium.

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