No. 3-] VALVES OF TELEOST FISHES. 123 



in twenty minutes had apparently recovered from the effects. 

 The rate was now fifty-nine per minute, and the manner of 

 breathing had entirely changed. The enlargement of the oral 

 cavity was fully a third greater than before (on a rough esti- 

 mate), and at or before the beginning of expiration the^'mouth 

 was tightly closed with an effort in order, apparently, to prevent 

 the regurgitation of water. In four days this fish was breathing 

 normally again and the valves were apparently repaired, the 

 scar being visible on the edge as a notch, with a white' line 

 running from this notch to the line of attachment of the valve. 



Summary. 



In the light of the above observations and experiments the 

 act of breathing in the teleost fishes may be described as 

 follows: 



The respiratory stream enters the oral cavity by the mouth 

 and leaves by the two gill openings, coming in contact with the 

 respiratory surfaces of the gills just before it passes out. It is 

 urged on its course by the pump-like construction and action 

 of the oral cavity and its two sets of valves, an anterior set, 

 which are those under consideration, and a posterior set, the 

 branch iostegal valves. 



In inspiration the stream enters at the mouth, in response to 

 a dilation of the oral cavity produced by the outward lateral 

 movement of the opercular frames. 



At the same time water is prevented from entering at the 

 gill openings by the branchiostegal valves which, although they 

 are attached to the opercular frames, move independently of 

 and contrary to them; so that, while this outward movement of 

 the frames extends the gill openings, the branchiostegal valves 

 close them automatically by the action of the water which tries 

 to enter. 



In expiration the water is forced out of the gill openings by 

 a corresponding contraction of the oral cavity. At the same 

 time the water is prevented from regurgitating through the 

 mouth, not by the contraction of the latter, but by the automatic 

 operation of the maxillary and mandibular breathing valves 



