Respiratory Organs in Different Animals (Diffusion, Skin, Gills, Trachea, Lungs)
Biology · Breathing and Exchange of Gases · NEET
Animals breathe in five main ways: simple diffusion over the whole body (sponges, coelenterates, flatworms), moist cuticle (earthworms), tracheal tubes (insects), gills or branchial respiration (aquatic arthropods, molluscs and fishes), and lungs or pulmonary respiration (amphibians, reptiles, birds, mammals). Frogs are special: they also breathe through moist skin (cutaneous respiration). Memory hook: "Simple animals Diffuse, Worms use Skin, Insects use Tubes, Water animals use Gills, Land animals use Lungs" — habitat and body organisation decide the organ.
Five respiratory strategies from simple to complex: diffusion over the body surface (sponges), moist cuticle (earthworm), tracheal tubes (insects), gills or branchial respiration (aquatic arthropods, molluscs, fishes), and lungs or pulmonary respiration (amphibians, reptiles, birds, mammals). Frogs also use cutaneous (skin) respiration.
Your doubts, answered
How do sponges, coelenterates and flatworms exchange gases?
These lower invertebrates have no special breathing organ. Oxygen and carbon dioxide move directly across their whole body surface by simple diffusion. This works because their bodies are small and thin, so every cell is close to the outside water. NEET often frames this as the answer 'simple diffusion over their entire body surface'.
Do earthworms have gills or lungs?
Neither. Earthworms breathe through their moist cuticle (moist skin surface). Gases diffuse across the thin, wet body wall into the blood vessels below it. The skin must stay moist, which is why earthworms live in damp soil and die if they dry out.
What do insects use to breathe, and is it the same as gills?
No. Insects use a network of air tubes called tracheal tubes that carry atmospheric air directly inside the body to the tissues. This is different from gills. Gills are vascularised (blood-rich) structures used by aquatic animals; tracheal tubes carry air, not blood, to the cells.
Which animals use gills (branchial respiration)?
Gills are special vascularised structures. Most aquatic arthropods and molluscs use gills, and among vertebrates, fishes use gills. Gills are for gas exchange in water, taking dissolved oxygen from the water. Gill breathing is also called branchial respiration.
How does a frog breathe in water compared to on land?
A frog is amphibious. In water it respires mainly through its moist skin (cutaneous respiration), where dissolved oxygen diffuses across the wet skin. On land it uses its lungs (pulmonary respiration), plus skin and the lining of the buccal cavity. This dual ability is a favourite NEET point.
Why does the respiratory organ change from one animal to another?
NCERT states the breathing mechanism varies mainly with two things: habitat (water or land) and level of organisation (simple or complex body). Simple, small aquatic animals manage with diffusion; larger and terrestrial animals need lungs so gas exchange is fast enough and water loss is controlled.
⚠️ The NEET trap ✗ Earthworms breathe using gills because they live underground in a moist place. ✓ Earthworms breathe through their moist cuticle (moist skin), not gills. Gills belong to aquatic arthropods, molluscs and fishes. 🧠 Match the organ to the exact animal from the NCERT line: sponge = diffusion, earthworm = moist cuticle, insect = tracheal tubes, mollusc/fish = gills, frog = skin + lungs. NEET swaps these to trick you.
Real NEET questions
ReNEET 2026
Sponges exchange O2 with CO2 by __________.
A · Simple diffusion over their entire body surfaces ✓
B · Moist cuticle
C · Tracheal tubes
D · Gills
Solution: Lower invertebrates such as sponges, coelenterates and flatworms exchange gases by simple diffusion over their entire body surface. Earthworms use a moist cuticle, insects use tracheal tubes, and aquatic arthropods/molluscs use gills.
ReNEET 2026
In water, frogs respire using _____________.
A · Skin ✓
B · Buccal cavity
C · Lungs
D · Trachea
Solution: Frogs use different respiratory surfaces in water and on land. In water the moist skin acts as the respiratory organ, where dissolved oxygen diffuses across it (cutaneous respiration). On land the buccal cavity, skin and lungs are all used.
Solved Breathing and Exchange of Gases NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What are the five types of respiratory surfaces in animals?
Body surface (simple diffusion), moist cuticle, tracheal tubes, gills (branchial respiration) and lungs (pulmonary respiration). NCERT lists these in one paragraph, so memorise the animal for each.
Which invertebrates breathe by simple diffusion?
Lower invertebrates like sponges, coelenterates and flatworms exchange O2 and CO2 by simple diffusion over their entire body surface, because their bodies are small and thin.
Is branchial respiration the same as gill respiration?
Yes. Branchial respiration means breathing through gills. It is used by most aquatic arthropods, molluscs and by fishes.
Which animals use pulmonary respiration?
Terrestrial forms use lungs. Among vertebrates, amphibians, reptiles, birds and mammals respire through lungs. This lung breathing is called pulmonary respiration.
Why must earthworm and frog skin stay moist for breathing?
Gases can only diffuse across a wet surface into the blood. If the skin dries, oxygen cannot dissolve and cross it, so these animals need a moist skin and damp habitats to breathe.
