In which of the following vertebrates does the skin function as a respiratory surface

Analyze the question: The question asks which vertebrate uses its skin as a respiratory (gas exchange) surface. This is called cutaneous respiration.

Explain the concept: For skin to work as a respiratory surface, it must be:

• Thin
• Moist (gases dissolve in water before diffusing)
• Well supplied with blood capillaries

Go through the options:

• A. Rat: A mammal. It breathes with lungs, and its skin is covered with fur and is dry, so it is not a respiratory surface.
• B. Lizard: A reptile. It has dry, scaly skin that prevents water loss, so it breathes only with lungs.
• C. Fish: It breathes mainly with gills. Some fish can take in a little oxygen through the skin, but the gills are the main respiratory organ.
• D. Frog: An amphibian. Its skin is thin, moist, and richly supplied with blood vessels. A frog uses its skin for gas exchange, especially when under water or during hibernation. It also uses its lungs and the lining of its mouth (buccal cavity).

Answer: The frog is the classic example of a vertebrate that uses its skin for respiration, so the correct option is D.

Tip: Remember that amphibians (frog, toad, newt, salamander) breathe through three surfaces: skin, buccal cavity, and lungs. Whenever JAMB asks about "skin as a respiratory surface" among vertebrates, think amphibian. Rats and lizards have dry, protected skin, and fish are the "gills" answer.