What a Tiny Sea Squirt Larva Reveals About Your Own Backbone
Tell chordates from non-chordates by the notochord, meet the protochordates Urochordata and Cephalochordata, learn the seven vertebrate classes with examples, and compare the adaptations of aquatic, amphibious and terrestrial vertebrates.
What do fish, frogs, birds and humans have in common?
A shark, a frog, a peacock and a person look utterly different, but at some stage of life each has a stiff rod along its back, a nerve cord above it and slits or pouches in the throat region. Those shared features place them all in phylum Chordata.
This lesson covers chordates versus non-chordates, the protochordates, the seven classes of vertebrates, and how vertebrates are adapted to water and land.
This lesson covers chordates versus non-chordates, the protochordates, the seven classes of vertebrates, and how vertebrates are adapted to water and land.
How are chordates different from non-chordates, and what are Urochordata and Cephalochordata?
Chordates have a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits at some stage of life, which non-chordates lack; Urochordata and Cephalochordata are marine protochordates that never develop a backbone.
Chordates versus non-chordates:
- Notochord — present in chordates; absent in non-chordates
- Nerve cord — dorsal, hollow and single in chordates; ventral, solid and double in non-chordates
- Pharyngeal gill slits — present in chordates; absent in non-chordates
- Heart — ventral in chordates; dorsal, if present, in non-chordates
- Post-anal tail — present in chordates; absent in non-chordates
Urochordata (Tunicata):
- Notochord present only in the tail of the larva
- Adults are often attached to rocks and enclosed in a tunic
- Examples: Ascidia, Salpa, Doliolum
Cephalochordata:
- Notochord extends from head to tail and lasts throughout life
- Small, fish-shaped, burrowing marine animals
- Example: Branchiostoma (amphioxus or lancelet)
Vertebrata. The notochord is present in the embryo and is replaced by a cartilaginous or bony vertebral column in the adult — so all vertebrates are chordates, but not all chordates are vertebrates.
An everyday example. The backbone you can feel down the middle of your back took the place of a notochord that you had as an early embryo.
The substance. A sea squirt larva swims using a notochord in its tail but loses it on settling as an adult — larval features, not adult ones, reveal its chordate identity.
Chordates versus non-chordates:
- Notochord — present in chordates; absent in non-chordates
- Nerve cord — dorsal, hollow and single in chordates; ventral, solid and double in non-chordates
- Pharyngeal gill slits — present in chordates; absent in non-chordates
- Heart — ventral in chordates; dorsal, if present, in non-chordates
- Post-anal tail — present in chordates; absent in non-chordates
Urochordata (Tunicata):
- Notochord present only in the tail of the larva
- Adults are often attached to rocks and enclosed in a tunic
- Examples: Ascidia, Salpa, Doliolum
Cephalochordata:
- Notochord extends from head to tail and lasts throughout life
- Small, fish-shaped, burrowing marine animals
- Example: Branchiostoma (amphioxus or lancelet)
Vertebrata. The notochord is present in the embryo and is replaced by a cartilaginous or bony vertebral column in the adult — so all vertebrates are chordates, but not all chordates are vertebrates.
An everyday example. The backbone you can feel down the middle of your back took the place of a notochord that you had as an early embryo.
The substance. A sea squirt larva swims using a notochord in its tail but loses it on settling as an adult — larval features, not adult ones, reveal its chordate identity.
What are the seven classes of vertebrates and how do you recognise each?
Vertebrates are divided into Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia, each recognised by features of its skin, skeleton, breathing, heart and reproduction.
Cyclostomata:
- Jawless, with a sucking, circular mouth
- Many pairs of gill slits; no scales or paired fins
- Examples: Petromyzon (lamprey), Myxine (hagfish)
Chondrichthyes (cartilaginous fish):
- Cartilaginous skeleton and a mouth on the underside
- Gill slits without an operculum; skin with tiny placoid scales
- Examples: Scoliodon (dogfish), Pristis (sawfish), Trygon (stingray)
Osteichthyes (bony fish):
- Bony skeleton; gills covered by an operculum
- Cycloid or ctenoid scales; an air bladder controls buoyancy
- Examples: Labeo (rohu), Catla, Hippocampus (sea horse)
Amphibia:
- Moist, scaleless skin; breathing by gills, lungs and skin
- Three-chambered heart and a cloaca
- Examples: Rana (frog), Bufo (toad), Ichthyophis (limbless amphibian)
Reptilia:
- Dry, cornified skin with epidermal scales or scutes
- Usually a three-chambered heart, four-chambered in crocodiles; shelled eggs
- Examples: Chelone (turtle), Naja (cobra), Crocodilus (crocodile)
Aves:
- Feathers, forelimbs modified into wings, and a toothless beak
- Hollow, air-filled pneumatic bones; four-chambered heart; warm-blooded
- Examples: Pavo (peacock), Columba (pigeon), Struthio (ostrich)
Mammalia:
- Mammary glands that produce milk, and hair on the skin
- External ears or pinnae, different kinds of teeth, and a four-chambered heart
- Examples: Macropus (kangaroo), Pteropus (flying fox), Balaenoptera (blue whale)
An everyday example. Rohu and catla at a fish market are bony fish with an operculum over the gills, while a small shark from the coast shows open gill slits.
The substance. Flying is not what makes a bird — bats fly but are mammals, and ostriches cannot fly but have feathers.
Cyclostomata:
- Jawless, with a sucking, circular mouth
- Many pairs of gill slits; no scales or paired fins
- Examples: Petromyzon (lamprey), Myxine (hagfish)
Chondrichthyes (cartilaginous fish):
- Cartilaginous skeleton and a mouth on the underside
- Gill slits without an operculum; skin with tiny placoid scales
- Examples: Scoliodon (dogfish), Pristis (sawfish), Trygon (stingray)
Osteichthyes (bony fish):
- Bony skeleton; gills covered by an operculum
- Cycloid or ctenoid scales; an air bladder controls buoyancy
- Examples: Labeo (rohu), Catla, Hippocampus (sea horse)
Amphibia:
- Moist, scaleless skin; breathing by gills, lungs and skin
- Three-chambered heart and a cloaca
- Examples: Rana (frog), Bufo (toad), Ichthyophis (limbless amphibian)
Reptilia:
- Dry, cornified skin with epidermal scales or scutes
- Usually a three-chambered heart, four-chambered in crocodiles; shelled eggs
- Examples: Chelone (turtle), Naja (cobra), Crocodilus (crocodile)
Aves:
- Feathers, forelimbs modified into wings, and a toothless beak
- Hollow, air-filled pneumatic bones; four-chambered heart; warm-blooded
- Examples: Pavo (peacock), Columba (pigeon), Struthio (ostrich)
Mammalia:
- Mammary glands that produce milk, and hair on the skin
- External ears or pinnae, different kinds of teeth, and a four-chambered heart
- Examples: Macropus (kangaroo), Pteropus (flying fox), Balaenoptera (blue whale)
An everyday example. Rohu and catla at a fish market are bony fish with an operculum over the gills, while a small shark from the coast shows open gill slits.
The substance. Flying is not what makes a bird — bats fly but are mammals, and ostriches cannot fly but have feathers.
How are aquatic, amphibious and terrestrial vertebrates adapted to where they live?
Aquatic vertebrates have streamlined bodies, fins and gills; amphibians combine a moist, breathing skin with lungs and return to water to breed; and terrestrial reptiles, birds and mammals have waterproof body coverings, lungs alone and eggs or embryos protected from drying out.
Aquatic — fish:
- Streamlined bodies and fins for swimming
- Gills take up oxygen dissolved in water
- An air bladder in bony fish adjusts buoyancy; cartilaginous fish lack one and must keep swimming to avoid sinking
- Fertilisation is often external in bony fish
Amphibious — amphibians:
- Moist, glandular skin that exchanges gases in water and in damp air
- Lungs for breathing on land, while tadpole larvae breathe with gills
- Eggs without shells, laid in water; fertilisation usually external
Terrestrial — reptiles, birds and mammals:
- Scales in reptiles, feathers in birds and hair in mammals reduce water loss
- Breathing only through lungs
- Internal fertilisation; reptiles and birds lay shelled eggs, while most mammals give birth to live young
- Birds and mammals are warm-blooded, with four-chambered hearts that keep oxygen-rich and oxygen-poor blood fully separate
An everyday example. A frog that sits on a pond bank during the monsoon and dives when disturbed uses its lungs in air and its skin for breathing underwater.
The substance. Returning to the sea does not bring back gills — whales and dolphins live in water but still breathe air through lungs, because they are mammals.
Aquatic — fish:
- Streamlined bodies and fins for swimming
- Gills take up oxygen dissolved in water
- An air bladder in bony fish adjusts buoyancy; cartilaginous fish lack one and must keep swimming to avoid sinking
- Fertilisation is often external in bony fish
Amphibious — amphibians:
- Moist, glandular skin that exchanges gases in water and in damp air
- Lungs for breathing on land, while tadpole larvae breathe with gills
- Eggs without shells, laid in water; fertilisation usually external
Terrestrial — reptiles, birds and mammals:
- Scales in reptiles, feathers in birds and hair in mammals reduce water loss
- Breathing only through lungs
- Internal fertilisation; reptiles and birds lay shelled eggs, while most mammals give birth to live young
- Birds and mammals are warm-blooded, with four-chambered hearts that keep oxygen-rich and oxygen-poor blood fully separate
An everyday example. A frog that sits on a pond bank during the monsoon and dives when disturbed uses its lungs in air and its skin for breathing underwater.
The substance. Returning to the sea does not bring back gills — whales and dolphins live in water but still breathe air through lungs, because they are mammals.
Exam tip
What earns full marks on chordates and vertebrate classes?
Answer vertebrate questions with a matching set for each class: one skin feature, one breathing or heart feature, and two examples with their common names.
- Chordate features: notochord, dorsal hollow nerve cord and pharyngeal gill slits
- Urochordata: notochord only in the larval tail; Cephalochordata: notochord throughout life
- Heart: two chambers in fish, three in amphibians and most reptiles, four in crocodiles, birds and mammals
The trap. Listing whales as fish. Whales, dolphins and bats are mammals — they have hair, mammary glands and lungs.
- Chordate features: notochord, dorsal hollow nerve cord and pharyngeal gill slits
- Urochordata: notochord only in the larval tail; Cephalochordata: notochord throughout life
- Heart: two chambers in fish, three in amphibians and most reptiles, four in crocodiles, birds and mammals
The trap. Listing whales as fish. Whales, dolphins and bats are mammals — they have hair, mammary glands and lungs.
Did you know
How does a flying fish glide above the sea without wings?
The flying fish, Exocoetus, is a bony fish that escapes predators by leaping out of the water. It spreads its very large pectoral fins like wings and glides over the waves.
As it drops back towards the surface, it can beat the lower lobe of its tail in the water to push off again and stretch the glide further.
Despite its name, it never flaps its fins — it glides, much as a paper plane does, and remains a true fish with gills and scales.
As it drops back towards the surface, it can beat the lower lobe of its tail in the water to push off again and stretch the glide further.
Despite its name, it never flaps its fins — it glides, much as a paper plane does, and remains a true fish with gills and scales.
Exam relevance
Why does NEET keep asking about vertebrate classes and their examples?
Animal Kingdom is a recurring NEET chapter, and its chordate section supplies direct questions on features and examples.
What gets asked. Matching genus names to classes and common names, features such as placoid scales, the operculum, the air bladder, pneumatic bones and pinnae, heart chambers across classes, and differences between Urochordata and Cephalochordata.
Question types. Mostly match-the-column and statement-based questions, often built around a list of scientific names.
Why it matters later. Vertebrate organ systems return in Structural Organisation in Animals through the frog, and the sequence from fish to mammals returns in Evolution.
The trap that costs marks. Placing Petromyzon among the bony fish — the lamprey is a jawless cyclostome, while Hippocampus, the sea horse, is the bony fish.
What gets asked. Matching genus names to classes and common names, features such as placoid scales, the operculum, the air bladder, pneumatic bones and pinnae, heart chambers across classes, and differences between Urochordata and Cephalochordata.
Question types. Mostly match-the-column and statement-based questions, often built around a list of scientific names.
Why it matters later. Vertebrate organ systems return in Structural Organisation in Animals through the frog, and the sequence from fish to mammals returns in Evolution.
The trap that costs marks. Placing Petromyzon among the bony fish — the lamprey is a jawless cyclostome, while Hippocampus, the sea horse, is the bony fish.
Key takeaways
What must you be able to do from this lesson?
- Chordates: notochord, dorsal hollow nerve cord and pharyngeal gill slits, with Urochordata and Cephalochordata as protochordates
- Seven vertebrate classes: Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia
- Adaptations: fins and gills in water, moist skin and lungs in amphibians, and waterproof coverings with shelled eggs or live birth on land
Can you name a mammal that flies, a bird that cannot, and a fish without jaws?
- Seven vertebrate classes: Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia
- Adaptations: fins and gills in water, moist skin and lungs in amphibians, and waterproof coverings with shelled eggs or live birth on land
Can you name a mammal that flies, a bird that cannot, and a fish without jaws?