MANNER OF BREATHING.
In the preceding section, reference has several times been made and attention has been called to the figures of the breathing valves. These structures are developed to such a degree in the barracuda, that the fish is enabled to breathe with its mouth open or at most only par tially closed. The head, including both the upper and lower jaws, is remarkably elongated and sharply pointed, as may be seen by reference to figures 3 and 4, plate xi. Across the narrow anterior end of the roof of the upper jaw is stretched a fold of flexible membrane having a cres cent-shaped posterior edge. In similar position in the floor of the mouth, just posterior to the symphysis of the mandibular bones, is a similar but larger breathing valve, whose hinder crescentic edge, how ever, swings free of the anterior end of the somewhat movable tongue. In this valve are marked depressions to receive the great premaxillary teeth. These valves may be plainly seen in figures 6, plate ir, and 9 and 10, plate m.
The action of these valves is very interesting. Ordinarily they lie horizontally, the one having above it and the other below it a space large enough for one to move about therein the handle of a scalpel, or, in a large specimen, one's finger. In gentle breathing, these valves swing slightly forward and downward for the one, forward and upward for the other. But in violent expiration, in consonance with the sharp
upward rise of the floor of the mouth, the hyoid region, and the strong pulling together and downward of the gill-covers, the water would be forced forward out of the mouth but for these valves. Being elastic and markedly distensible, they swing together in a horizontal median line, catch and hold the forwardly moving column of water, which is then forced out backwardly over the gills and out under and behind the gill covers. With the relaxation of the hyoid region and opercula, the elasticity of the breathing valves brings them back to their normal horizontal position.
The reason why this fish has such well-developed breathing valves results from the fact that it does not close its mouth in expiration. This in turn is probably to be correlated with the extraordinary devel opment of the great teeth, especially those on the anterior parts of the premaxillaries, since when the mouth is closed these must fit into the depressions in the anterior part of the mandibular valve. Because of these great teeth it is probably both more convenient and more com fortable for this fish to keep its mouth more or less open, even while breathing.