438 THE POPULAR SCIENCE MONTHLY. 



into its intestine. And, as I have already suggested, it may be that 

 the narrow posterior canal which leads from the air-bladder to the 

 exterior is a sort of safety-valve allowing the air to escape, when the 

 fish, rapidly ascending or descending, alters the pressure of the water 

 upon the contained air. 



This hypothesis may be put forward with some show of probability, 

 but I really find it difiicult to suggest anything with respect to the 

 physiological meaning of the connection between the air-bladder and 

 the ear. Nevertheless such an elaborate apparatus must have some 

 physiological importance ; and this conclusion is strengthened by the 

 well-known fact that there are a great many fishes in which the air- 

 bladder and the ear become connected in one way or another. In the 

 carp tribe, for example, the front end of the air-bladder is connected 

 by a series of little bones with the organ of hearing, which is, as it 

 were, prolonged backward to meet these bones in the hinder end of 

 the skull. But here the air-bladder, which is very large, may act as a 

 resonator ; while in the herring the extreme narrowness of the passages 

 which connect the air-bladder with the ear renders it difficult to sup- 

 pose that the organ can have any such function. 



In addition to the singular connection of the ear with the exterior 

 by the roundabout way of the air-bladder, there are membranous spaces 

 in the walls of the skull by which vibrations can more directly reach 

 the herring's" ear. And there is no doubt that the fish is very sensitive 

 to such vibrations. In a dark night, when the water is phosphorescent 

 or, as the fishermen say, there is plenty of " merefire," it is a curious 

 spectacle to watch the effect of sharply tapping the side of the boat 

 as it passes over a shoal. The herrings scatter in all directions, leav- 

 ing streaks of light behind them, like shooting-stars. 



The herring, like other fishes, breathes by means of its gills the 

 essential part of which consists of the delicate, highly vascular fila- 

 ments which are set in a double row on the outer faces of each of 

 the gill-arches. The venous blood, which returns from all parts of 

 the body to be collected in the heart, is pumped thence into the gills, 

 and there exchanges its excess of carbonic-acid gas for the gaseous 

 oxygen which is dissolved in sea-water. The freedom of passage 

 of the water and the great size and delicacy of the gills facilitate 

 respiration when the fish is in its native element ; but the same pecu- 

 liarities permitting of the rapid drying and coherence of the gills, and 

 thus bringing on speedy suffocation, render its tenure of life, after 

 removal from the water, as short as that of any fish. It may be 

 observed, in passing, that the wide clefts behind the gill-covers of the 

 herring have some practical importance, as the fish, thrusting its head 

 through the meshes of the drift-net, is caught behind them, and can 

 not extricate itself. In the herring, the upper end of the last gill-cleft 

 is not developed into a sac or pouch, such as we shall find in some of 

 its near neighbors. 



