les and cell walls. An example is the transportation of soluble products of digestion in the blood plasma of animals.
             Fish are thought to have originated in the sea, but it is believed that these animals adapted to fresh water early in their evolutionary history. The external surface of the fish is more of less impervious to water and to dissolved substances. However, this does not mean that the external solution is unimportant. The respiratory current of water through the mouth and the pharynx of the fish brings the aquatic environment into close contact with the thin-walled gills and the blood they contain. Water flows through the gill surfaces by osmosis. The kidne
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