Why were fish, amphibians, reptiles called cold-blooded?

Cold-blooded animals are animals whose body temperature varies with changes in ambient temperature.
Amphibians, reptiles, fish are highly dependent on ambient temperature, their body temperature becomes the same as the external temperature, hence the name. Unlike warm-blooded animals, these species of animals become slower during the onset of cold weather and fall into hibernation. The warm-blooded animals are not affected by the ambient temperature, their body temperature always remains constant, therefore warm-blooded animals can continue their activity even when severe colds occur.

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