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In fishes, again, the gills only are fully developed; while the lung structure either makes no advance at all, or only appears in the rudimentary form of an air-bladder. So, also, the baleen of the whale and the teeth of the land mammalia are different organs.
A point of Natural History being suggested, he gave an excellent account of the air-bladder of fishes. He was very full upon the subject of agriculture, but retired from the conversation when horticulture was introduced in the discussion. So he seemed well acquainted with the geology of anthracite, but did not pretend to know anything of other kinds of coal.
Some of the amphibious animals possess both lungs and gills, even when matured; but the higher vertebrates living in the water breathe through lungs which are evolved from the air-bladder of fishes, which in turn have been evolved from the primitive gullet of the lower forms. There are fishes known which are warm-blooded.
The most signal differences in kind are wont to arise when organs originally developed for a certain purpose come to be applied to a very different purpose, as that change of the fish's air-bladder into a lung which accompanied the first development of land vertebrates.
No part of a sturgeon is wasted: the roe is taken out, salted, and stowed away in casks; this is known by the name of 'caviare, and is esteemed a great luxury. From the sound or air-bladder isinglass is made, simply by being hung in the sun for a time; and the fish itself is dried, and exported to various parts of the world. Astracan is the chief seat of Caspian commerce."
How the noise is made is a question. Cuvier was of opinion that it was made by the air-bladder, though he could not explain how: but the truth, if truth it be, seems stranger still. These fish, it seems, have strong bony palates and throat-teeth for crushing shells and crabs, and make this wonderful noise simply by grinding their teeth together.
Like others of his kind Sammy had a very strong spine in which was an air-bladder. By pressing the air out of this he could swim easily at a great depth, and by inflating it to let the air in, like a balloon, he could rise and swim along the surface. Sammy's eyes were large and round, and he could see splendidly, especially when the water was clear.
There on the black bottom were a dozen young trout, all freshly caught, and all with the air-bladder punctured by the mother bird's sharp bill. She had provided their dinner, but she brought it to a good place and made them dive to get it. As I paddled back to camp, I thought of the way the Indians taught their boys to shoot.
Secondly, a Salpa, of the class which lives apart and has fine long fibres projecting from the hinder part of the body. Thirdly, a small animal, nearly allied to the Diphyes, the soft part of the body, which contains the tube for receiving nourishment, having no air-bladder. Fourthly, a small Beroe, having the power of drawing in its fins. Fifthly, a very small Porpita.
In every form man has passed through, he left behind some old member or power and took on some new. He left his air-bladder and his gills and his fins with the fishes; he got his lungs from the dipnoans, the precursors of the amphibians, and from these last he got his four limbs; he left some part of his anatomy with the reptile, and took something in exchange, probably his flexible neck.
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