Biology · Breathing and Exchange of Gases · NEET
Sponges have no special breathing organ. Every cell is close to water, so oxygen and carbon dioxide move directly between the cell and the surrounding water by simple diffusion over the entire body surface. NEET groups sponges, coelenterates and flatworms together as animals that respire by diffusion across the general surface.
Both use the body surface, but NCERT names them differently. Sponges use simple diffusion over the entire body surface. Earthworms use their moist cuticle (skin). In an exam pick the exact wording: sponge = simple diffusion over body surface, earthworm = moist cuticle/skin.
In water a frog cannot use its lungs, so its moist skin becomes the respiratory organ — dissolved oxygen diffuses across the skin (cutaneous respiration). On land the frog uses its skin, the lining of its buccal cavity, and its lungs together. So the frog changes its respiratory surface with its habitat.
Gases can only diffuse across a surface when they are dissolved in a thin film of water. A dry surface stops diffusion. That is why earthworms and frogs keep their skin wet and thin, and why they cannot survive if the skin dries out.
Diffusion works only over very short distances. Small or thin animals have a high surface-area-to-volume ratio, so every cell is near the surface. Large, active animals have deep tissues that diffusion cannot reach, so they evolved gills, tracheae or lungs to carry oxygen deeper.
In water, frogs respire using _____________.
Sponges exchange O2 with CO2 by __________.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Cutaneous respiration is gas exchange across the skin. The skin must be moist and thin so oxygen and carbon dioxide can diffuse through it. Frogs (especially in water) and earthworms use this method.
Lower invertebrates such as sponges, coelenterates (like Hydra) and flatworms exchange gases by simple diffusion across their entire body surface, without any special respiratory organ.
No. Earthworms have no lungs or gills. They breathe through their moist cuticle (skin) by diffusion, so they must stay in damp soil to keep the skin wet.
No. The moist skin is the main organ in water, but on land the frog also uses its lungs and the lining of its buccal cavity. It switches between surfaces depending on its habitat.
NEET tests the exact respiratory surface for each animal group, and ReNEET 2026 asked two direct questions on frogs and sponges. One wrong word (skin vs cuticle vs diffusion) loses the mark, so memorise the exact NCERT terms.
