CHAPTERS ON EVOLUTION. 



rc\ 



of the air-bladder in the adult fish, all air-bladders are provided with 

 a duct in the young state. The duct becomes obliterated in such 

 fishes as the cod and perch, leaving the air-bladder a closed sac, 

 whilst it persists in the herring, carp, &c., as already noted. In all 

 ordinary fishes the air-bladder has one settled function it acts as a 

 hydrostatic apparatus. By compressing or expanding this structure 

 with its included gas, the fish is enabled to preserve a due relation 

 between its own specific gravity and that of the water, and is thus 

 enabled to rise or sink at will. It is interesting, moreover, to note, 

 that so far as the history of both lungs and air-bladder can be con- 

 structed by the light of development, both structures appear to be 

 modifications of one and the same primitive organ. The first use of 

 both structures was probably that seen in the ordinary air-bladder 

 to-day; namely, to form a receptacle for gas ; this gas becoming used 

 for breathing when the functions of the gills were interrupted. In 

 this way, the functions of lungs were foreshadowed, and as the 

 sequel will show, there is ample evidence at hand of such a modify 

 cation having actually taken place. 



But the story of the swim-bladder ends not thus. The mere 



knowledge of its functions and 

 use in no wise aids us towards 

 the understanding of what it is 

 or of its origin. Yet we may 

 trace this organ, from its form 

 and nature in our common 

 fishes, to the ancient ganoid 

 group of fishes now sparsely 

 represented in our seas by the 

 sturgeons, by the bony pikes of 

 North American lakes, by the 

 Polypteri of the Nile and other 

 African rivers, and by the still 

 more curious Lepidosirens or 

 mud-fishes (Fig. 54) of the 

 Gambia and Amazon, and the Ceratodus or "Barramunda" (Fig. 50) 

 of Australian fresh waters. 



In the ganoid fishes, the air-bladder presents us with varying 

 forms. In all, it communicates with the throat or stomach by a tube 

 or duct, as in the familiar carp, which, however, is not a ganoid fish. 

 It may be single or paired in the ganoid group, and we must note a 

 more special feature of the swim-bladder of these fishes in that it 

 frequently presents a cellular or divided structure internally. In 

 the Polypteri of African rivers, the swimming-bladder (Fig. 53, a) 

 is thus not only double, but divided internally into cells or small 

 compartments; whilst it also opens into the throat by a distinct 



FlG. 53. AlR-BLADDKRS OF FlSHES. 



