AIK-BLADDER AND GILLS OF FISHES. 259 



to scale the heavens or to descend again upon the earth. The 

 position of the air-bladder immediately under the spine and 

 above the centre of gravity, causes the fish to rise without the 

 danger of turning over on its back. Thus the air-bladder is a 

 highly important auxiliary organ of locomotion, and affords an 

 illustration of one of the many evidences of design in the primary 

 formation of aquatic animals. 



In the flying-fishes, whose peculiar habits rendered a greater 

 lightness of body extremely desirable, it is of enormous size, so 

 that when distended it fills almost the entire abdominal cavity ; 

 while in those fishes which are destined to live at the bottom of 

 the sea, or habitually to conceal themselves in the mud, it is 

 either very small or entirely wanting for economical Nature 

 constantly regulates her gifts according to the wants of her 

 creatures. 



The gills of the fishes are as beautifully constructed for aquatic 

 respiration as the lungs of the terrestrial vertebrate animals for 

 breathing in the air. In most fishes, comprising all the bony 

 fishes and the sturgeons, among those which have a cartilaginous 

 skeleton, we find, in the interior of the mouth at each side, five 

 apertures separated from each other by four crooked parallel 

 and unequal bones, and leading to a cavity which is closed on 

 the outside by an operculum or cover. In this cavity, and to 

 the external convex surface of each of the four bones or branchial 

 arches, is attached a double series of flat elongated cartilaginous 

 Iamina3, tapering gradually towards their extremities ; the whole 

 forming a crescent-shaped framework toothed like a comb, over 

 which is spread the delicately- fringed and highly vascular mem- 

 brane that constitutes the respiratory surface. Over this the 

 water taken in at the mouth is made to pass as it issues through 

 the opercular cavities, and in this way the branchias, being per- 

 petually bathed with aerated water, perform the same office as 

 the lungs of an air-breathing animal. 



This transmission of the water in one direction is in more 

 than one respect a most wise provision of Nature, for if the 

 fishes were obliged to receive and reject the water by the 

 same aperture (as we do the air), each respiration would evi- 

 dently drive them backwards, and consequently retard their 

 movements. It is also evident that the delicate fringes of the 

 gills must have been liable to perpetual derangement, if the 



