trunghieu1985
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Fish and Amphibians
Publisher: Encyclop.dia Britannica, Inc. | 2008 | 56 pages | ISBN: 159339389X | PDF | 11,6 mb
Fish were the first vertebrates with bony skeletons to appear on the Earth. They doubtless form the most numerous group of vertebrates. Unlike today's fish, the earliest fish had no scales, fins, or jawbone, but they did have a type of dorsal fin. Over time they have been changing in form and size to adapt to different environments, in both fresh water and salt water. Their bodies are generally streamlined, being covered with smooth scales and having fins that enable them to move with energy, direction, and stability. In place of lungs, these complex creatures normally breathe through gills that capture oxygen dissolved in the water, and they are cold-blooded.
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