AQUEOUS CURRENT IN THE SPIRACLE OF THE DOGFISH. 87 



expansion can be seen both from above and from below ; and consists therefore 

 in the lowering o£ the wall of the pharynx as well as in the outward move- 

 ments of its sides. It has the appearance of being the result of a considerable 

 effort on the part of the fish. 



Just before the end of the inspiratory phase the mouth shuts : then the 

 branchial region suddenly collapses, water rushes out of the gill-slits, and the 

 expiratory phase has begun ; and scarcely has it begun before it is at an end, 

 for it takes about half the time occupied by the inspiratory phase. 



We must now consider the question of the direction of the water in the 

 mouth and spiracle. In order to determine the direction of the water in these 

 apertures, I placed the Dogfish in 3 to 4 inches of water in a flat-bottomed 

 glass dish. This dish was raised on four corks about 2 inches high ; so that, 

 by placing a mirror below the dish, one could examine the fish from the 

 ventral aspect. The method I adopted for determining the direction of the 

 current in the mouth and spiracle was to gently liberate from a pipette a little 

 finely powdered carmine suspended in sea-water in the immediate proximity 

 of these apertures. 



When the mouth opens at the beginning of the inspiratory phase, water is 

 vigorously drawn in : and this inhalation continues so long as the mouth 

 remains open. Water containing carmine which enters the left side of the mouth 

 emerges only from the gills of the left side ; and water entering at the right 

 side emerges only from the gills of the right side. The lumen of the pharynx 

 is, therefore, divided in front of the first gill-slit into two channels which do 

 not communicate with one another. Carmine liberated in front of the middle 

 of the mouth emerges from the gills of both sides, as a result, presumably, of 

 the equal distribution of the cloud of carmine to the two channels. 



The carmine sucked in through the mouth does not emerge equally from 

 all the gills ; but in considerably greater volume from the last three, especially 

 from the last one. My attention was first called to this by Dr. Allen, who 

 was watching one of my observations. 



Lastly, if carmine is liberated sufficiently near the nostril, the very slightest 

 current, entering it, can be detected. 



To turn now to the spiracle. At first sight this aperture seems to give rise 

 to two channels entering the head, a dorsal and a ventral one. The dorsal 

 one is not a channel, but is the spiracular csecum ; the ventral one is the 

 inspiratory aperture. During the inspiratory phase the inspiratory aperture 

 is open ; but during the expiratory phase it is closed by a valve which extends 

 backwards over it like a curtain, from the anterior wall of the spiracle. 



Carmine liherated near the sjyirade is vigorously drawn into the insjjiratori/ 

 aperture during the inspiratory phase. During normal breathing there is no 

 expulsion of water from the spiracle, though the faintest outward disturbance 

 in the cloud of carmine caused, possibly, by the shutting of the valve can just 

 be detected. Occasionally, however, the Dogfish clears his spiracle, so to speak. 



