174 Dynamic Theory. 



competition between different breathing apparatus has awarded the pre- 

 ference to another locality as the best place for. a lung. That locality 

 is the interior cavity of the body. It is essential that every cell that 

 goes to help make up the various tissues of the body should respire, 

 but it is impossible in a large body that every cell should have direct 

 communication with the air. The air must be conveyed at second hand 

 to a great majority of them. The blood, in every animal that has 

 blood, acts as the medium of this conveyance. Any specially differen- 

 tiated aerating apparatus must and does, therefore, have reference 

 chiefly to getting air into contact with the blood. * As the blood vessels 

 lie in. the body cavity between the intestinal tube and the outside skin 

 la}'er, the air must reach them by passing through either the outside 

 skin or the membranes of the stomach. The gills are a section of the 

 former, the internal lungs a section of the latter. There can be no 

 doubt that the air bladder of all fishes that possess it, contributes more 

 or less toward the aeration of the blood, whatever other service it may 

 perform in regulating the specific gravity of the body. It may be ad- 

 mitted that in those fishes in which there is no air duct connecting the 

 bladder with the esophagus, the bladder receiving its air by osmosis, 

 the service of aeration by the bladder is comparatively inconsiderable. 

 But wherever the bladder is connected with the gullet by an air duct, 

 its respiratory function is increased. Almost all the more active fishes 

 get part of their respiration through this air bladder, and they are 

 therefore obliged to have recourse to the surface of the water in order 

 to renew the air. If they are prevented from doing this by ice or if a 

 net be stretched across an aquarium just under the surface of the water 

 so as to prevent the fish from reaching the air, they will die of suffoca- 

 tion. According to Semper, if the air duct to the bladder of certain 

 Brazilian fishes viz. , the Sudis-gigas, ErytJirinus taeniatus and Ery- 

 thrinus Braziliensis be ligatured so as to prevent the inhalation of air, 

 suffocation is the result. (Semper 190.) The gills are insufficient for 

 the oxidation of all the blood and are reinforced by the bladder, which 

 is, therefore, to these fish a true lung and an essential organ of respira- 

 tion. The Ganoid fishes, including the Amia, Sturgeon, Gar-pike and 

 Polypterus of Africa, all possess the bladder connected with the throat 

 by a tube. 



In some of them the bladder is double, and in some, especially the 

 Polypterus, it has a cellular or divided structure internally composed of 

 folds of the mucous membrane, and giving a greater surface of exposure 

 to the air. 



There is no air bladder in the Blennies, Flatfishes, Sand-eels, Lorica- 

 rini, Symbranchii, and some members of other families. But all that 

 have the bladder have also in embryo life a duct leading to it from the intes- 



