Animal Kingdom

Animal Kingdom

AMPHIBIANS
The best-known amphibians are frogs, toads, and salamanders. Compared to the classes of other higher animals -- animals with backbones -- amphibians are very primitive creatures.
Life originated in the seas. The first animals were simple ones without backbones -- "invertebrate" animals such as segmented worms, sponges, and corals. The first fish were so simple that they didn't even have jawbones for opening and closing their mouths. The first land animals with backbones were the amphibians. The oldest amphibian fossils look like fish with thick, muscular fins. That's because amphibians arose from early fish. The first amphibians lived more in water than on dry land.
They can't mate on dry land. Like their fish ancestors, amphibians must return to water to reproduce. Female amphibians lay gelatinous eggs in water, and then the male deposits clouds of sperm over them. This is one reason why amphibians are seldom found far from water, and one reason they may be rare or nonexistent in your backyard.
They can't retain water well in their bodies. Scales, which cut down drastically on water loss from skin, didn't appear until reptiles arose millions of years after the first amphibians. This is another reason why amphibians need to stay near water.
They can't keep their bodies warm during cold weather. Amphibians are "cold blooded." When the air cools, so does their body temperature. This causes the animal to become sluggish, which can be bad if a predator is near. We think of reptiles as being cold-blooded, but some evidence suggests that certain dinosaurs were "warm blooded," like the reptiles' descendents, the birds and mammals.
Amphibians lead double lives—one in water and one on land. Many begin life with gills, then develop lungs as they age. They are vertebrate animals that include frogs, toads, salamanders, and newts as well as odd, wormlike caecilians. They are cold-blooded, using the environment to regulate their body temperature. Early...