Why a Whale Must Surface to Breathe Though It Never Leaves the Sea
Use the three basic chordate characters to separate Urochordata, Cephalochordata and Vertebrata, distinguish jawless, cartilaginous and bony fishes, identify amphibians and reptiles and the importance of the amniote egg, and compare birds with mammals.
What unites a sea squirt, a shark, a frog, a peacock and you?
A sea squirt clinging to a rock looks nothing like a tiger. Yet both belong to Phylum Chordata, marked by features that appear at some stage of life in every member.
This part covers chordate characters and subphyla, cyclostomes and fishes, amphibians and reptiles, and birds and mammals.
This part covers chordate characters and subphyla, cyclostomes and fishes, amphibians and reptiles, and birds and mammals.
What are the three fundamental chordate characters, and how do they separate the three subphyla?
All chordates have, at least at some stage, a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits; in Urochordata the notochord is present only in the larval tail, in Cephalochordata it runs from head to tail throughout life, and in Vertebrata it is replaced by a backbone in adults.
Other common features: a post-anal tail and a heart on the ventral side.
The subphyla:
- Urochordata (tunicates) — notochord only in the larval tail: Ascidia, Salpa, Doliolum
- Cephalochordata — notochord from head to tail, throughout life: Branchiostoma (lancelet)
- Vertebrata — notochord in the embryo, replaced by a vertebral column; a muscular ventral heart with two, three or four chambers; kidneys; paired fins or limbs
Urochordata and Cephalochordata together are called protochordates.
An everyday example. Your own spine is the vertebrate replacement for the notochord that formed in your embryo.
The substance. All vertebrates are chordates, but not all chordates are vertebrates — protochordates have no backbone.
Other common features: a post-anal tail and a heart on the ventral side.
The subphyla:
- Urochordata (tunicates) — notochord only in the larval tail: Ascidia, Salpa, Doliolum
- Cephalochordata — notochord from head to tail, throughout life: Branchiostoma (lancelet)
- Vertebrata — notochord in the embryo, replaced by a vertebral column; a muscular ventral heart with two, three or four chambers; kidneys; paired fins or limbs
Urochordata and Cephalochordata together are called protochordates.
An everyday example. Your own spine is the vertebrate replacement for the notochord that formed in your embryo.
The substance. All vertebrates are chordates, but not all chordates are vertebrates — protochordates have no backbone.
How do Cyclostomata, Chondrichthyes and Osteichthyes differ?
Cyclostomes are jawless, scaleless animals with a round sucking mouth; cartilaginous fishes (Chondrichthyes) have jaws, placoid scales, gill slits without a cover and no air bladder; and bony fishes (Osteichthyes) have a bony skeleton, gills covered by an operculum, cycloid or ctenoid scales and an air bladder.
Cyclostomata:
- No jaws — a circular sucking mouth; many are ectoparasites on fishes
- No scales and no paired fins; cartilaginous skeleton
- Marine, but migrate to fresh water to breed
- Examples: Petromyzon (lamprey), Myxine (hagfish)
Chondrichthyes (cartilaginous fishes):
- Cartilaginous skeleton; powerful jaws with backward-pointing teeth
- Tough skin with tiny placoid scales; gill slits separate, no operculum
- No air bladder; two-chambered heart; cold-blooded
- Males have claspers on the pelvic fins; fertilisation internal
- Examples: Scoliodon (dogfish), Pristis (sawfish), Trygon (stingray), Torpedo (electric ray)
Osteichthyes (bony fishes):
- Bony skeleton; gills covered by an operculum
- Cycloid or ctenoid scales; an air bladder controls buoyancy
- Two-chambered heart; mostly external fertilisation
- Examples: Labeo (rohu), Catla (katla), Clarias (magur), Hippocampus (sea horse), Exocoetus (flying fish), Betta (fighting fish)
An everyday example. Rohu and katla sold in fish markets across India are bony fishes with an operculum you can lift to see the gills.
The substance. Sharks and rays have no air bladder, so they must keep swimming to avoid sinking.
Cyclostomata:
- No jaws — a circular sucking mouth; many are ectoparasites on fishes
- No scales and no paired fins; cartilaginous skeleton
- Marine, but migrate to fresh water to breed
- Examples: Petromyzon (lamprey), Myxine (hagfish)
Chondrichthyes (cartilaginous fishes):
- Cartilaginous skeleton; powerful jaws with backward-pointing teeth
- Tough skin with tiny placoid scales; gill slits separate, no operculum
- No air bladder; two-chambered heart; cold-blooded
- Males have claspers on the pelvic fins; fertilisation internal
- Examples: Scoliodon (dogfish), Pristis (sawfish), Trygon (stingray), Torpedo (electric ray)
Osteichthyes (bony fishes):
- Bony skeleton; gills covered by an operculum
- Cycloid or ctenoid scales; an air bladder controls buoyancy
- Two-chambered heart; mostly external fertilisation
- Examples: Labeo (rohu), Catla (katla), Clarias (magur), Hippocampus (sea horse), Exocoetus (flying fish), Betta (fighting fish)
An everyday example. Rohu and katla sold in fish markets across India are bony fishes with an operculum you can lift to see the gills.
The substance. Sharks and rays have no air bladder, so they must keep swimming to avoid sinking.
How do you identify amphibians and reptiles, and why did the amniote egg allow fully terrestrial life?
Amphibians have moist, scaleless skin, breathe through gills, lungs and skin, have a three-chambered heart and lay eggs in water; reptiles have dry skin with scales or scutes, breathe by lungs, usually have a three-chambered heart, and lay shelled amniote eggs on land that protect the embryo from drying out.
Amphibia:
- Live in water and on land; usually two pairs of limbs
- Moist skin without scales; eyelids; a tympanum for hearing
- Respiration by gills, lungs and skin; three-chambered heart; cold-blooded
- Gut, urinary and reproductive tracts open into a cloaca
- External fertilisation; eggs in water; larval stage
- Examples: Rana (frog), Bufo (toad), Hyla (tree frog), Salamandra, Ichthyophis (a limbless amphibian)
Reptilia:
- Mostly terrestrial; dry, cornified skin with scales or scutes
- Lungs only; heart usually three-chambered, but four-chambered in crocodiles
- Internal fertilisation; eggs with leathery shells; direct development
- Examples: Hemidactylus (wall lizard), Calotes (garden lizard), Chelone (turtle), Crocodilus, Naja (cobra), Bungarus (krait)
The amniote egg. Its shell and membranes keep the embryo moist, cushion it, store food and allow gas exchange — so reptiles no longer need water to breed.
An everyday example. The house lizard on your wall is Hemidactylus, a reptile that never needs a pond to lay its eggs.
The substance. Amphibians still depend on water for reproduction, which the shelled egg of reptiles overcame.
Amphibia:
- Live in water and on land; usually two pairs of limbs
- Moist skin without scales; eyelids; a tympanum for hearing
- Respiration by gills, lungs and skin; three-chambered heart; cold-blooded
- Gut, urinary and reproductive tracts open into a cloaca
- External fertilisation; eggs in water; larval stage
- Examples: Rana (frog), Bufo (toad), Hyla (tree frog), Salamandra, Ichthyophis (a limbless amphibian)
Reptilia:
- Mostly terrestrial; dry, cornified skin with scales or scutes
- Lungs only; heart usually three-chambered, but four-chambered in crocodiles
- Internal fertilisation; eggs with leathery shells; direct development
- Examples: Hemidactylus (wall lizard), Calotes (garden lizard), Chelone (turtle), Crocodilus, Naja (cobra), Bungarus (krait)
The amniote egg. Its shell and membranes keep the embryo moist, cushion it, store food and allow gas exchange — so reptiles no longer need water to breed.
An everyday example. The house lizard on your wall is Hemidactylus, a reptile that never needs a pond to lay its eggs.
The substance. Amphibians still depend on water for reproduction, which the shelled egg of reptiles overcame.
How do birds and mammals compare, and what features are unique to mammals?
Birds have feathers, wings, hollow bones and a four-chambered heart and lay eggs, while mammals have hair and mammary glands, a four-chambered heart and usually give birth to live young; both are warm-blooded.
Aves (birds):
- Feathers; forelimbs modified into wings; beak with no teeth
- Hollow, air-filled bones that keep the skeleton light; air sacs support the lungs
- Crop and gizzard in the gut
- Four-chambered heart; warm-blooded (homoiothermous)
- Internal fertilisation; eggs with hard shells
- Examples: Pavo (peacock), Columba (pigeon), Corvus (crow), Psittacula (parrot), Struthio (ostrich), Neophron (vulture)
Mammalia:
- Mammary glands that produce milk — the defining feature
- Hair on the skin; external ears (pinnae); different types of teeth
- Four-chambered heart; warm-blooded; breathe by lungs
- Mostly viviparous; a few lay eggs
- Examples: Panthera tigris (tiger), Elephas (elephant), Pteropus (flying fox), Delphinus (dolphin), Balaenoptera (blue whale), Macropus (kangaroo), Ornithorhynchus (platypus)
Worked example — tricky cases.
- Bat — flies but has hair and mammary glands: mammal
- Platypus — lays eggs but feeds young on milk: mammal
- Penguin — cannot fly but has feathers: bird
An everyday example. The peacock and the tiger, symbols seen across India, are a bird and a mammal — both warm-blooded with four-chambered hearts.
The substance. Mammary glands and hair, not live birth or life on land, are what make an animal a mammal.
Aves (birds):
- Feathers; forelimbs modified into wings; beak with no teeth
- Hollow, air-filled bones that keep the skeleton light; air sacs support the lungs
- Crop and gizzard in the gut
- Four-chambered heart; warm-blooded (homoiothermous)
- Internal fertilisation; eggs with hard shells
- Examples: Pavo (peacock), Columba (pigeon), Corvus (crow), Psittacula (parrot), Struthio (ostrich), Neophron (vulture)
Mammalia:
- Mammary glands that produce milk — the defining feature
- Hair on the skin; external ears (pinnae); different types of teeth
- Four-chambered heart; warm-blooded; breathe by lungs
- Mostly viviparous; a few lay eggs
- Examples: Panthera tigris (tiger), Elephas (elephant), Pteropus (flying fox), Delphinus (dolphin), Balaenoptera (blue whale), Macropus (kangaroo), Ornithorhynchus (platypus)
Worked example — tricky cases.
- Bat — flies but has hair and mammary glands: mammal
- Platypus — lays eggs but feeds young on milk: mammal
- Penguin — cannot fly but has feathers: bird
An everyday example. The peacock and the tiger, symbols seen across India, are a bird and a mammal — both warm-blooded with four-chambered hearts.
The substance. Mammary glands and hair, not live birth or life on land, are what make an animal a mammal.
Exam tip
What earns full marks on Phylum Chordata?
Compare classes in one table with columns for skeleton, scales or skin covering, respiration, heart chambers, temperature control and egg type.
- Chordate characters: notochord, dorsal hollow nerve cord, paired gill slits
- Cyclostomes: jawless, no scales — Petromyzon
- Cartilaginous fishes: placoid scales, no operculum, no air bladder — Scoliodon
- Bony fishes: operculum, air bladder — Labeo, Catla
- Amphibians: moist skin, three-chambered heart, cloaca — Rana
- Reptiles: dry scaly skin, amniote egg; crocodile heart four-chambered
- Birds and mammals: warm-blooded, four-chambered heart; mammals have hair and mammary glands
The trap. Giving an air bladder to sharks. Cartilaginous fishes have no air bladder.
- Chordate characters: notochord, dorsal hollow nerve cord, paired gill slits
- Cyclostomes: jawless, no scales — Petromyzon
- Cartilaginous fishes: placoid scales, no operculum, no air bladder — Scoliodon
- Bony fishes: operculum, air bladder — Labeo, Catla
- Amphibians: moist skin, three-chambered heart, cloaca — Rana
- Reptiles: dry scaly skin, amniote egg; crocodile heart four-chambered
- Birds and mammals: warm-blooded, four-chambered heart; mammals have hair and mammary glands
The trap. Giving an air bladder to sharks. Cartilaginous fishes have no air bladder.
Did you know
Why must a whale come up for air?
Whales and dolphins spend their entire lives in water, yet they are mammals, not fishes. They have no gills — they breathe air through lungs, using a blowhole on top of the head.
So every whale must rise to the surface regularly to breathe out stale air and take in fresh air. The spout seen at sea is the whale's warm breath meeting cooler air.
Like other mammals, whales are warm-blooded, have some hair at some stage of life, and feed their young on milk — clear signs of their place in Mammalia.
So every whale must rise to the surface regularly to breathe out stale air and take in fresh air. The spout seen at sea is the whale's warm breath meeting cooler air.
Like other mammals, whales are warm-blooded, have some hair at some stage of life, and feed their young on milk — clear signs of their place in Mammalia.
Exam relevance
How is Phylum Chordata tested in NEET?
Phylum Chordata completes Animal Kingdom in NEET Biology, and its class-by-class features and genus names appear constantly in match-type questions.
What gets asked. Fundamental chordate characters, where the notochord occurs in each subphylum, features that separate cartilaginous and bony fishes, heart chambers across classes including the crocodile exception, the amniote egg, and matching genera such as Scoliodon, Ichthyophis, Hemidactylus, Pteropus and Ornithorhynchus with their classes. Vertebrate organ systems are studied further in Structural Organisation in Animals and human physiology.
Question types. Match-the-column lists, statement-based questions and assertion-reason questions.
The trap that costs marks. **Placing Ichthyophis among fishes** — it is a limbless amphibian.
What gets asked. Fundamental chordate characters, where the notochord occurs in each subphylum, features that separate cartilaginous and bony fishes, heart chambers across classes including the crocodile exception, the amniote egg, and matching genera such as Scoliodon, Ichthyophis, Hemidactylus, Pteropus and Ornithorhynchus with their classes. Vertebrate organ systems are studied further in Structural Organisation in Animals and human physiology.
Question types. Match-the-column lists, statement-based questions and assertion-reason questions.
The trap that costs marks. **Placing Ichthyophis among fishes** — it is a limbless amphibian.
Key takeaways
What must you be able to do from this part?
- Chordate features and subphyla: notochord, dorsal hollow nerve cord, gill slits; notochord in larval tail (Urochordata), lifelong (Cephalochordata), replaced by backbone (Vertebrata)
- Fishes: cyclostomes jawless; Scoliodon with placoid scales and no air bladder; Labeo and Catla with operculum and air bladder
- Amphibians and reptiles: Rana with moist skin and three-chambered heart; Hemidactylus with dry scaly skin and amniote eggs; crocodiles have four chambers
- Birds and mammals: feathers and hollow bones versus hair and mammary glands; bat and platypus are mammals
Place these in their class — Myxine, Pristis, Hippocampus, Salamandra, Chelone, Neophron, Delphinus — and give the feature that decided each.
- Fishes: cyclostomes jawless; Scoliodon with placoid scales and no air bladder; Labeo and Catla with operculum and air bladder
- Amphibians and reptiles: Rana with moist skin and three-chambered heart; Hemidactylus with dry scaly skin and amniote eggs; crocodiles have four chambers
- Birds and mammals: feathers and hollow bones versus hair and mammary glands; bat and platypus are mammals
Place these in their class — Myxine, Pristis, Hippocampus, Salamandra, Chelone, Neophron, Delphinus — and give the feature that decided each.