70 Papers from the Department of Marine Biology. 



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 n. 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 n; and 9 

 and 10, plate in. 



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. 



INTERNAL ORGANS. 



Of no less interest than the structures just described are the internal 

 organs. Of fish No. 10 (3 feet 10 inches in extreme length) careful 

 dissection and full notes were made. The abdominal cavity was 18.5 

 inches long from the diaphragm to the anus, with a 1-inch post-anal 

 extension to be described later. 



