Question

Why do animals that take in air from their skin have a low metabolic rate?

Answer 1

Most animals need to maintain their core body temperature within a relatively narrow range
Endotherms use internally generated heat to maintain body temperature. Their body temperature tends to stay steady regardless of environment.
Ectotherms depend mainly on external heat sources, and their body temperature changes with the temperature of the environment.
Animals exchange heat with their environment through radiation, conduction—sometimes aided by convection—and evaporation