BiologyISC Class 11 · Animal Kingdom

Phylum Chordata: Fundamental Characters, Subphyla and Vertebrate Classes

Updated 17 October 2026 · Fact-checked

Chordates have a notochord, a dorsal hollow nerve cord, paired pharyngeal gill slits, a post-anal tail and a ventral heart at some stage of life. Subphyla: Urochordata, Cephalochordata and Vertebrata. Vertebrates include Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia.

Understand Chordates and Classes of Vertebrates

Fundamental chordate characters. (1) Notochord, a mesodermal rod-like structure along the back. (2) Dorsal, hollow, single nerve cord (non-chordates have a ventral, solid, double nerve cord). (3) Paired pharyngeal gill slits. (4) A post-anal tail (in many). (5) A closed circulatory system with a ventral heart.

Subphyla. *Urochordata* (tunicates: Ascidia, Salpa, Doliolum): notochord only in the tail of the larva. *Cephalochordata* (Branchiostoma, amphioxus): notochord extends from head to tail throughout life. *Vertebrata*: notochord replaced by a cartilaginous or bony vertebral column in the adult; a ventral muscular heart with 2 to 4 chambers; kidney for excretion and osmoregulation; paired appendages (fins or limbs).

Classes of vertebrates.

Class	Main features	Heart	Examples
Cyclostomata	jawless, circular suctorial mouth, no scales or paired fins	2 chambers	Petromyzon (lamprey), Myxine (hagfish)
Chondrichthyes	cartilaginous endoskeleton, placoid scales, gills with no operculum, no air bladder	2	Scoliodon, Pristis, Trygon
Osteichthyes	bony endoskeleton, operculum covering the gills, air bladder, cycloid or ctenoid scales	2	Exocoetus, Hippocampus, Labeo, Catla
Amphibia	live in water and on land, moist skin without scales, cloaca, tympanum	3	Rana, Bufo, Hyla, Ichthyophis
Reptilia	dry cornified skin with scales, mostly 3-chambered heart (crocodile 4), lay shelled eggs	3 (4 in crocodiles)	Chelone, Testudo, Calotes, Naja, Crocodilus
Aves	feathers, forelimbs as wings, pneumatic bones, beak, no sweat glands, oviparous	4	Corvus, Columba, Psittacula, Struthio
Mammalia	mammary glands, hair, external ears (pinnae), viviparous (mostly)	4	Ornithorhynchus, Macropus, Pteropus, Homo

Body temperature. Fish, amphibians and reptiles are cold-blooded (poikilotherms), their body temperature changing with the surroundings; birds and mammals are warm-blooded (homeotherms) and maintain a constant body temperature. Aquatic mammals (Delphinus, Balaenoptera) are mammals though they live in water. Ornithorhynchus lays eggs.

Chordates and Classes of Vertebrates: key facts

Chordate characters: Notochord
Dorsal hollow nerve cord
Pharyngeal gill slits
Post-anal tail
Ventral heart; Present at some stage of life.
Subphyla: Urochordata Ascidia
Cephalochordata Branchiostoma
Vertebrata all vertebrates; Notochord in tail only versus throughout.
Heart chambers: Fishes 2
Amphibians 3
Reptiles 3 (4 in crocodiles)
Birds 4
Mammals 4; Increasing separation of blood.
Body temperature: $P o ik i l o t h er m s : f i s h es, am p hibian s, r e pt i l es ∣ H o m eo t h er m s : bi r d s, mamma l s$; Cold-blooded versus warm-blooded.

Urochordata	Ascidia
Cephalochordata	Branchiostoma
Vertebrata	all vertebrates

Fishes	2
Amphibians	3
Reptiles	3 (4 in crocodiles)
Birds	4
Mammals	4

Chordates and Classes of Vertebrates in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Why a bony fish floats at any depth

Suppose a student compares a shark with a rohu (Labeo) in an aquarium display and notices that the shark must keep swimming but the rohu can hover.

Explain using one structural difference.

Try it first, then show the solutionSolution

Rohu is a bony fish (Osteichthyes) with an air bladder, which regulates buoyancy.
A shark (Chondrichthyes) has no air bladder, so nothing offsets its body density (its light cartilaginous skeleton and oily liver help only partly).
So the shark must keep swimming to avoid sinking, while the bony fish adjusts the air bladder.

Answer: The air bladder of bony fishes gives buoyancy; sharks have none.

Memory hook: Air bladder: Osteichthyes only.

Tricks to remember Chordates and Classes of Vertebrates

Short memory aids. Say each one aloud once, then use it on a question the same day.

2-3-3-4-4
Heart chambers in fish, amphibians, reptiles, birds, mammals.
Use it when: Heart structure across classes.
Fish with bones have a flap
Osteichthyes have an operculum covering the gills; Chondrichthyes have exposed gill slits.
Use it when: Distinguishing fishes.

Chordates and Classes of Vertebrates: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Subphylum with notochord only in the larval tail · 1 mark · MCQBoard level

The subphylum in which the notochord is present only in the larval tail is:
(a) Cephalochordata
(b) Urochordata
(c) Vertebrata
(d) Hemichordata

Try it first, then show the solutionSolution

In Urochordata (tunicates) the notochord is limited to the tail of the larva.

Answer: (b) Urochordata

Example 2: Chordates and non-chordates · 3 marks · Short answerBoard level

State any three differences between chordates and non-chordates.

Try it first, then show the solutionSolution

Chordates have a notochord; non-chordates do not. (1 mark)
Chordates have a dorsal, hollow, single nerve cord; non-chordates have a ventral, solid, double nerve cord. (1 mark)
Chordates have pharyngeal gill slits and a post-anal tail at some stage; non-chordates do not. (1 mark)

Answer: Notochord, dorsal hollow nerve cord and gill slits.

Example 3: Cartilaginous and bony fishes · 4 marks · Short answerHarder board level

Compare Chondrichthyes and Osteichthyes on skeleton, scales, gill cover, air bladder and heart, giving two examples of each.

Try it first, then show the solutionSolution

Skeleton: cartilage in Chondrichthyes; bone in Osteichthyes. (1 mark)
Scales: placoid in Chondrichthyes; cycloid or ctenoid in Osteichthyes. (1 mark)
Gill cover and air bladder: absent in Chondrichthyes; present in Osteichthyes. Heart is two-chambered in both. (1 mark)
Examples: Scoliodon, Trygon; Labeo, Hippocampus. (1 mark)

Answer: Cartilage versus bone; placoid versus cycloid scales; operculum and air bladder only in bony fishes.

Example 4: Heart and temperature · 4 marks · Short answerHarder board level

Arrange the vertebrate classes by the number of heart chambers (2, 3, 4). Name the exception among reptiles. Explain homeothermy and name the two classes that show it.

Try it first, then show the solutionSolution

Two chambers: fishes (Cyclostomata, Chondrichthyes, Osteichthyes); three: Amphibia and most Reptilia; four: Aves and Mammalia. (1.5 marks)
The exception: crocodiles have a four-chambered heart. (0.75 mark)
Homeothermy is the maintenance of a constant body temperature regardless of the surroundings. (1 mark)
It is shown by birds and mammals. (0.75 mark)

Answer: 2: fishes; 3: amphibians and reptiles; 4: birds and mammals (and crocodiles).

Example 5: Identifying classes · 5 marks · Long answerHOTS / competency

A vertebrate has feathers, a beak, pneumatic bones and a four-chambered heart. Another is limbless, has a moist skin, a cloaca and lives in soil. A third lays eggs but also has mammary glands. A fourth is jawless with a sucker-like mouth. Identify each class and give an example, and explain why the second is not a reptile.

Try it first, then show the solutionSolution

First: Aves, e.g. Corvus (crow) or Columba (pigeon). (1 mark)
Second: Amphibia, e.g. Ichthyophis. (1 mark)
Third: Mammalia, e.g. Ornithorhynchus (an egg-laying mammal). (1 mark)
Fourth: Cyclostomata, e.g. Petromyzon or Myxine. (1 mark)
The second is not a reptile because it has a moist skin without scales, unlike the dry, cornified, scaly skin of reptiles. (1 mark)

Answer: Aves, Amphibia, Mammalia, Cyclostomata; reptiles have a dry scaly skin.

Where students lose marks in Chordates and Classes of Vertebrates

The slips examiners see most often, and what to write instead.

Mistake: Wrong heart chambers
Giving four chambers for amphibians.
Do this: Amphibians and most reptiles have three; birds and mammals have four.
Usually costs: 1 mark.
Mistake: Calling Ornithorhynchus a reptile
Because it lays eggs.
Do this: It has mammary glands and hair; it is a mammal.
Usually costs: 1 mark.
Mistake: Mixing the characters of fishes
Putting an operculum in sharks.
Do this: Operculum is a feature of bony fishes only.
Usually costs: 1 mark.

Exam tips

Make a table with class, skin, heart and example for revision.
State the five fundamental characters of chordates.
Use genus names for examples.
Mention both body temperature and heart chambers when comparing classes.

Practice questions on Chordates and Classes of Vertebrates

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which of the following is a jawless vertebrate?
(a) Scoliodon
(b) Petromyzon
(c) Labeo
(d) Rana
Hint
Lampreys have a circular suctorial mouth.
Answer
(b) Petromyzon
Q2 · 2 marks · Short answerBoard level
Name two features of birds that help them to fly.
Hint
Think about the skeleton and forelimbs.
Answer
Forelimbs modified into wings with feathers, and pneumatic (hollow, air-filled) bones that reduce weight; the body is streamlined.
Q3 · 3 marks · Short answerHarder board level
Write the differences between Branchiostoma and Vertebrata.
Hint
Compare the notochord and the heart.
Answer
In Branchiostoma (Cephalochordata) the notochord runs from head to tail throughout life and there is no vertebral column or true heart. In Vertebrata the notochord is replaced by a vertebral column in the adult and a ventral muscular heart is present.
Q4 · 3 marks · Short answerHarder board level
Why are whales and dolphins classified as mammals though they live in water?
Hint
Look at their characteristic features.
Answer
They have mammary glands, breathe by lungs, are warm-blooded, have a four-chambered heart and bear live young, which are mammalian characters.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): Reptiles are poikilothermic. Reason (R): Their body temperature changes with the environment. Choose: (a) both true and R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.
Hint
Define poikilothermy.
Answer
(a) Both true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a zoo visitor compares a frog (Rana), a lizard (Calotes) and a pigeon (Columba).
(i) Name the class of each. (1 mark)
(ii) State the number of heart chambers in each. (1 mark)
(iii) State which two are cold-blooded. (1 mark)
(iv) State which one is oviparous with a skin of feathers. (1 mark)
Hint
Apply the table of classes.
Answer
(i) Amphibia, Reptilia, Aves. (ii) 3, 3, 4. (iii) Frog and lizard. (iv) Pigeon.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Subphylum with a vertebral column?Show answer
Vertebrata.
2.Heart of fishes?Show answer
Two-chambered.
3.Egg-laying mammal?Show answer
Ornithorhynchus.
4.Scales of Chondrichthyes?Show answer
Placoid scales.

Chordates and Classes of Vertebrates: frequently asked questions

Do all chordates have a vertebral column?

No. Urochordates and cephalochordates have a notochord but no vertebral column; only vertebrates have it.

Why is the notochord replaced in vertebrates?

In the adult vertebrate the notochord is replaced by a vertebral column, which gives better support and protects the nerve cord.

Is a crocodile a typical reptile?

It differs in having a four-chambered heart, though it is cold-blooded with scaly skin like other reptiles.