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THE fourth and last class of the Vertebrated sub-king- dom, comprises the animals known as FISHES. These are destined to live altogether in the water ; and this circumstance has im- pressed a peculiar character on their entire organization : but the most important differences which they present, when we compare them with the other Vertebrata, consist in the confor- mation of their apparatus for respiration and circulation. They have no lungs at any period of their lives ; and they breathe by gills only. Their heart contains but two cavities ; and receives only venous blood. This liquid, after having been brought into contact with oxygen, passes into a dorsal vessel, where no new force accelerates its course through the different parts of the body. next |