Animals with backbones, called vertebrates, form only a tiny fraction of all known creatures. There are five classes of vertebrates: fishes, amphibians, reptiles, birds and mammals. The structure of fishes restricts them to water. Fishes are vertebrates with a backbone of either bone or cartilage. The adaptation of aquatic life is gills. The internal gills of fishes have large surface areas over which gaseous exchange between the water and the blood occur. The mouth and gill cavities form a mechanism that pumps water over the gills, a one way flow being ensured by flaps of skin in the mouth. The gills are so arranged that the blood in the gill filaments and the water carrying oxygen flow in opposite directions and this counter current system allows for the most efficient exchange of gases.
FrogIn evolutionary terms, one of the most profound modifications needed for animals to survive the emergence from an aquatic environment was the ability to absorb oxygen from air rather than from water. In water, amphibians can both walk and swim. Frogs and toads use their hindlimbs for swimming, whereas newts propel themselves with their tails, keeping their hindlegs pressed together. Since amphibians are carnivores, their main sources of food are insects and small invertebrates. The reproduction and life expectancy are that most amphibians lay eggs and fertilization usually takes place outside the body.
CrocodilesEgo's Crocodiles are small amphibious reptiles, growing no longer than three feet. Crocrodilians use their long vertically-flattened tail for swimming. They are carnivorous and adults prey on small animals caught either underwater or on land.
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