Why a Frog Can Breathe Through Its Skin but a Lizard Cannot
Learn the three chordate characters and the five vertebrate classes, compare fish and amphibians, see how birds are built for flight, and tell cold-blooded animals from warm-blooded ones.
Why can a frog breathe through its skin when a lizard cannot?
Because a frog's skin is moist and scaleless, so gases can pass straight through it. A lizard's skin is dry and covered in scales, which blocks that exchange but prevents water loss.
One feature, two opposite trade-offs — and that is how vertebrate classes are told apart. This page covers everything in the ICSE Class 7 Biology chapter's second part: the characters of Chordata, and the five classes Pisces, Amphibia, Reptilia, Aves and Mammalia.
One feature, two opposite trade-offs — and that is how vertebrate classes are told apart. This page covers everything in the ICSE Class 7 Biology chapter's second part: the characters of Chordata, and the five classes Pisces, Amphibia, Reptilia, Aves and Mammalia.
What makes an animal a chordate?
Three characters, present at some stage of life:
- A notochord — a flexible rod running along the back, which supports the body. In vertebrates it is replaced during development by the vertebral column.
- A dorsal, tubular nerve cord — lying above the gut, hollow and single.
- Pharyngeal gill slits — openings in the throat region, used for breathing in fish and present only in the embryo of land vertebrates.
Chordates also have bilateral symmetry, an organ-system level of organisation and a true coelom.
The five classes of vertebrates within Chordata are Pisces, Amphibia, Reptilia, Aves and Mammalia.
The words dorsal and tubular carry marks of their own, and they exist to draw a contrast. An invertebrate such as an earthworm has a nerve cord too, but it is ventral — below the gut — solid, and double. Chordates reverse all three.
The phrase at some stage of life also matters. A human being has no gill slits and no notochord as an adult, yet both appear in the embryo — which is enough to place us firmly in Chordata.
- A notochord — a flexible rod running along the back, which supports the body. In vertebrates it is replaced during development by the vertebral column.
- A dorsal, tubular nerve cord — lying above the gut, hollow and single.
- Pharyngeal gill slits — openings in the throat region, used for breathing in fish and present only in the embryo of land vertebrates.
Chordates also have bilateral symmetry, an organ-system level of organisation and a true coelom.
The five classes of vertebrates within Chordata are Pisces, Amphibia, Reptilia, Aves and Mammalia.
The words dorsal and tubular carry marks of their own, and they exist to draw a contrast. An invertebrate such as an earthworm has a nerve cord too, but it is ventral — below the gut — solid, and double. Chordates reverse all three.
The phrase at some stage of life also matters. A human being has no gill slits and no notochord as an adult, yet both appear in the embryo — which is enough to place us firmly in Chordata.
How do fish and amphibians compare?
Both are aquatic to some degree, but they are built for different lives.
Pisces — the fishes:
- Skin covered with scales and slimy mucus.
- Respiration by gills.
- Heart with two chambers.
- Reproduction by laying eggs in water, with external fertilisation; they are oviparous.
- Body temperature — cold-blooded (poikilothermic).
- Move with fins and a streamlined body; most have an air bladder to stay afloat.
Examples: rohu, catfish, shark, seahorse.
Amphibia — the double-life animals:
- Skin moist, smooth and scaleless.
- Respiration by gills in the tadpole, and by lungs and skin in the adult.
- Heart with three chambers.
- Reproduction by laying eggs in water; the young are tadpoles that undergo metamorphosis.
- Body temperature — cold-blooded.
Examples: frog, toad, salamander.
The frogs that begin calling from ponds and ditches with the first monsoon rain are following exactly this pattern — they must return to water to breed.
The feature that defines the class is that double life. An amphibian is aquatic as a larva and terrestrial as an adult, and its skin must stay moist for cutaneous respiration — which is why frogs are found in damp places and never in a desert.
Pisces — the fishes:
- Skin covered with scales and slimy mucus.
- Respiration by gills.
- Heart with two chambers.
- Reproduction by laying eggs in water, with external fertilisation; they are oviparous.
- Body temperature — cold-blooded (poikilothermic).
- Move with fins and a streamlined body; most have an air bladder to stay afloat.
Examples: rohu, catfish, shark, seahorse.
Amphibia — the double-life animals:
- Skin moist, smooth and scaleless.
- Respiration by gills in the tadpole, and by lungs and skin in the adult.
- Heart with three chambers.
- Reproduction by laying eggs in water; the young are tadpoles that undergo metamorphosis.
- Body temperature — cold-blooded.
Examples: frog, toad, salamander.
The frogs that begin calling from ponds and ditches with the first monsoon rain are following exactly this pattern — they must return to water to breed.
The feature that defines the class is that double life. An amphibian is aquatic as a larva and terrestrial as an adult, and its skin must stay moist for cutaneous respiration — which is why frogs are found in damp places and never in a desert.
How are birds built for flight?
Reptilia — the creeping animals:
- Skin dry and covered with horny scales or plates, preventing water loss.
- Respiration by lungs only.
- Heart with three chambers, except the crocodile which has four.
- Reproduction by laying shelled eggs on land, with internal fertilisation.
- Body temperature — cold-blooded.
Examples: lizard, snake, turtle, crocodile.
Aves — the birds:
- Skin dry, covered with feathers; scales on the legs.
- Respiration by lungs, assisted by air sacs.
- Heart with four chambers.
- Reproduction by laying shelled eggs; they are oviparous.
- Body temperature — warm-blooded (homoiothermic).
Examples: crow, pigeon, sparrow, peacock.
Their adaptations for flight are the standard question:
- Streamlined body offering little air resistance.
- Forelimbs modified into wings, with strong flight muscles on a keeled breastbone.
- Feathers for lift and for insulation.
- Hollow, light bones filled with air, reducing weight.
- Air sacs connected to the lungs, supplying extra oxygen and lightening the body.
- No teeth and no urinary bladder, cutting weight further; a beak is used instead.
A crow lifting off from a rooftop shows several at once — light bones, keeled breastbone and wings.
The reptile-to-bird link is the interesting one. Birds keep scales on their legs and lay shelled eggs just as reptiles do, which is exactly why the two classes are studied together.
- Skin dry and covered with horny scales or plates, preventing water loss.
- Respiration by lungs only.
- Heart with three chambers, except the crocodile which has four.
- Reproduction by laying shelled eggs on land, with internal fertilisation.
- Body temperature — cold-blooded.
Examples: lizard, snake, turtle, crocodile.
Aves — the birds:
- Skin dry, covered with feathers; scales on the legs.
- Respiration by lungs, assisted by air sacs.
- Heart with four chambers.
- Reproduction by laying shelled eggs; they are oviparous.
- Body temperature — warm-blooded (homoiothermic).
Examples: crow, pigeon, sparrow, peacock.
Their adaptations for flight are the standard question:
- Streamlined body offering little air resistance.
- Forelimbs modified into wings, with strong flight muscles on a keeled breastbone.
- Feathers for lift and for insulation.
- Hollow, light bones filled with air, reducing weight.
- Air sacs connected to the lungs, supplying extra oxygen and lightening the body.
- No teeth and no urinary bladder, cutting weight further; a beak is used instead.
A crow lifting off from a rooftop shows several at once — light bones, keeled breastbone and wings.
The reptile-to-bird link is the interesting one. Birds keep scales on their legs and lay shelled eggs just as reptiles do, which is exactly why the two classes are studied together.
What makes mammals different, and what does warm-blooded mean?
Mammalia — the distinguishing features are:
- Mammary glands that produce milk to feed the young; this is the defining character.
- Hair or fur on the skin, with sweat and oil glands.
- External ears (pinnae).
- Respiration by lungs, with a muscular diaphragm.
- Heart with four chambers.
- Viviparous — they give birth to young ones rather than laying eggs.
- Warm-blooded, and they have teeth of different kinds set in sockets.
Examples: cow, dog, bat, whale, human being.
Cold-blooded (poikilothermic) animals cannot control their body temperature, so it changes with the surroundings. When it gets cold they become sluggish and may hibernate. Fishes, amphibians and reptiles are cold-blooded.
Warm-blooded (homoiothermic) animals keep a constant body temperature whatever the surroundings, which lets them stay active in heat and cold alike. Only birds and mammals are warm-blooded.
This is why lizards are seen sunning themselves on a wall on a cool morning, and why a buffalo is equally active in winter and summer.
Two exceptions are examined, and both must be stated carefully. A bat is a mammal, not a bird — it has fur, gives birth to young and feeds them milk, and its wings are skin, not feathers. A whale is a mammal too, despite living in water: it breathes through lungs, is warm-blooded and suckles its young.
- Mammary glands that produce milk to feed the young; this is the defining character.
- Hair or fur on the skin, with sweat and oil glands.
- External ears (pinnae).
- Respiration by lungs, with a muscular diaphragm.
- Heart with four chambers.
- Viviparous — they give birth to young ones rather than laying eggs.
- Warm-blooded, and they have teeth of different kinds set in sockets.
Examples: cow, dog, bat, whale, human being.
Cold-blooded (poikilothermic) animals cannot control their body temperature, so it changes with the surroundings. When it gets cold they become sluggish and may hibernate. Fishes, amphibians and reptiles are cold-blooded.
Warm-blooded (homoiothermic) animals keep a constant body temperature whatever the surroundings, which lets them stay active in heat and cold alike. Only birds and mammals are warm-blooded.
This is why lizards are seen sunning themselves on a wall on a cool morning, and why a buffalo is equally active in winter and summer.
Two exceptions are examined, and both must be stated carefully. A bat is a mammal, not a bird — it has fur, gives birth to young and feeds them milk, and its wings are skin, not feathers. A whale is a mammal too, despite living in water: it breathes through lungs, is warm-blooded and suckles its young.
Exam tip
Exam tip: answering with skin, heart and temperature
Vertebrate-class questions almost always ask for the same five points, so answer in that order every time: skin, respiratory organ, heart chambers, mode of reproduction, body temperature.
Five short phrases, five marks. Reptilia: dry scaly skin, lungs, three-chambered heart, shelled eggs on land, cold-blooded.
Count the heart chambers correctly — two in fish, three in amphibians and reptiles, four in birds and mammals — and note the crocodile exception.
For flight adaptations, give at least four features and include hollow bones and air sacs, which are the ones most often missed.
For mammals, name mammary glands first, since that is the defining character, and add hair and external ears.
And when a question offers bat or whale, say mammal and give the reason — fur or milk, not habitat.
Five short phrases, five marks. Reptilia: dry scaly skin, lungs, three-chambered heart, shelled eggs on land, cold-blooded.
Count the heart chambers correctly — two in fish, three in amphibians and reptiles, four in birds and mammals — and note the crocodile exception.
For flight adaptations, give at least four features and include hollow bones and air sacs, which are the ones most often missed.
For mammals, name mammary glands first, since that is the defining character, and add hair and external ears.
And when a question offers bat or whale, say mammal and give the reason — fur or milk, not habitat.
Did you know
Why do lizards bask in the sun in the morning?
Because they cannot generate their own warmth, so they borrow it from the Sun.
A cold-blooded animal's body temperature follows its surroundings. After a cool night a lizard's muscles work too slowly for it to hunt or escape, so it spreads itself on a warm wall until it has heated up enough to move quickly.
A warm-blooded animal has no such delay — it burns food to keep its own temperature steady, which is why a sparrow on the same wall is already active at dawn. The cost is that the sparrow must eat far more often than the lizard does.
A cold-blooded animal's body temperature follows its surroundings. After a cool night a lizard's muscles work too slowly for it to hunt or escape, so it spreads itself on a warm wall until it has heated up enough to move quickly.
A warm-blooded animal has no such delay — it burns food to keep its own temperature steady, which is why a sparrow on the same wall is already active at dawn. The cost is that the sparrow must eat far more often than the lizard does.
Key takeaways
Chordates and the five classes: quick revision
- Chordates have a notochord, a dorsal tubular nerve cord and pharyngeal gill slits at some stage; invertebrate nerve cords are ventral, solid and double.
- Pisces: scaly skin, gills, two-chambered heart, eggs in water, cold-blooded.
- Amphibia: moist scaleless skin, gills then lungs and skin, three-chambered heart, eggs in water, cold-blooded.
- Reptilia: dry scaly skin, lungs, three chambers (four in the crocodile), shelled eggs on land, cold-blooded.
- Aves: feathers, lungs with air sacs, four chambers, shelled eggs, warm-blooded — with streamlining, wings, hollow bones and no teeth for flight.
- Mammalia: mammary glands, hair, external ears, diaphragm, four chambers, viviparous, warm-blooded — so a bat and a whale are both mammals.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Pisces: scaly skin, gills, two-chambered heart, eggs in water, cold-blooded.
- Amphibia: moist scaleless skin, gills then lungs and skin, three-chambered heart, eggs in water, cold-blooded.
- Reptilia: dry scaly skin, lungs, three chambers (four in the crocodile), shelled eggs on land, cold-blooded.
- Aves: feathers, lungs with air sacs, four chambers, shelled eggs, warm-blooded — with streamlining, wings, hollow bones and no teeth for flight.
- Mammalia: mammary glands, hair, external ears, diaphragm, four chambers, viviparous, warm-blooded — so a bat and a whale are both mammals.
You will remember all of this far better after answering five questions on it than after reading it twice.