Which of the following organs are used for gaseous exchange in a mature toad?

Analyze the question: It asks which organs a mature (adult) toad uses for gaseous exchange. The word "mature" matters because tadpoles and adults breathe differently.

Concept: Amphibians use three main surfaces for gaseous exchange:

1. Skin (cutaneous respiration): The skin is thin, moist and richly supplied with blood capillaries. Oxygen dissolves in the moisture and diffuses into the blood, while carbon dioxide diffuses out.
2. Lungs: These are simple, sac-like lungs. The toad fills them by buccal pumping, lowering the floor of the mouth to draw air in and then raising it to force the air into the lungs.
3. Buccal cavity (mouth lining): The lining of the mouth is also thin, moist and well supplied with blood vessels. Gaseous exchange takes place across it, especially when the toad is at rest.

Eliminating the wrong options:

• Gills: Only tadpoles (larvae) have gills. They are lost during metamorphosis, so adults do not use them. This rules out A and C.
• Eardrum (tympanic membrane): It is for hearing, not gaseous exchange. This rules out A and B.

Answer: Option D, Skin, lungs and mouth, is the only one that lists three organs an adult toad really uses for breathing.

Exam tip: Remember the changes through life:

• Tadpole: gills (external, then internal) and skin
• Adult: lungs, skin and mouth lining

If a question says "mature" or "adult" amphibian, cross out any option with gills straight away. Also, any option containing the eardrum can be eliminated, since it is a hearing structure.