BiologyCBSE Class 11 · Animal Kingdom

Phylum Chordata: Vertebrate Classes from Fishes to Mammals

Updated 17 October 2026 · Fact-checked

Chordates have a notochord, a dorsal hollow nerve cord, paired pharyngeal gill slits and a post-anal tail at some stage. Vertebrates (subphylum Vertebrata) include Cyclostomata, Pisces (Chondrichthyes and Osteichthyes), Amphibia, Reptilia, Aves and Mammalia, separated by jaws, skin covering, heart chambers and mode of development.

Notice the first split is jawless versus jawed, and only the four-limbed Tetrapoda branch gives Amphibia, Reptilia, Aves and Mammalia.

Illustration drawn for QuestionBuddy.

Understand Chordate Classes: Fishes to Mammals

Chordates are bilaterally symmetrical, triploblastic, coelomate and metamerically segmented, with organ-system level of organisation. The fundamental features: notochord, dorsal hollow nerve cord (central nervous system), paired pharyngeal gill slits and a post-anal tail. A ventral heart and a closed circulatory system are present. Phylum Chordata has three subphyla: Urochordata and Cephalochordata (together protochordates, exclusively marine) and Vertebrata. In vertebrates the notochord is replaced by a vertebral column in the adult.

Vertebrata. Divided into Agnatha (jawless: Cyclostomata) and Gnathostomata (jawed vertebrates; Pisces and Tetrapoda).

Class Cyclostomata. Ectoparasites on some fishes; elongated body with 6 to 15 pairs of gill slits for respiration; circular mouth without jaws; sucking and circular mouth; cranium and vertebral column are cartilaginous; no scales or paired fins; marine but migrate to fresh water for spawning; larvae return to the ocean after metamorphosis. *Petromyzon* (lamprey), *Myxine* (hagfish).

Class Chondrichthyes (cartilaginous fishes). Marine, streamlined, cartilaginous endoskeleton; mouth ventral; notochord persists throughout life; gill slits separate, without operculum; skin tough with placoid scales; teeth are modified placoid scales, projecting backwards; no air bladder, so they must swim constantly to avoid sinking; cold-blooded (poikilothermous); heterocercal tail; heart two-chambered (one atrium, one ventricle); some have electric organs (*Torpedo*) and some poison sting (*Trygon*); internal fertilisation, mostly viviparous. *Scoliodon*, *Pristis*, *Carcharodon*.

Class Osteichthyes (bony fishes). Marine and fresh water; bony endoskeleton; terminal mouth; four pairs of gills covered by an operculum; air bladder to regulate buoyancy; skin with cycloid/ctenoid scales; cold-blooded; two-chambered heart; external fertilisation, mostly oviparous, direct development. Marine: *Exocoetus* (flying fish), *Hippocampus* (sea horse); freshwater: *Labeo* (rohu), *Catla*, *Clarias* (magur); aquarium: *Betta* (fighting fish), *Pterophyllum* (angel fish).

Class Amphibia. Live in both water and land. Body: head and trunk, tail may be present. Skin moist, without scales; eyes with eyelids; a tympanum represents the ear; alimentary canal, urinary and reproductive tracts open into a cloaca; respiration by gills, lungs and skin; three-chambered heart (two atria, one ventricle); cold-blooded; oviparous, external fertilisation, indirect development. *Bufo*, *Rana*, *Hyla*, *Salamandra*, *Ichthyophis* (limbless).

Class Reptilia. Creeping or crawling; dry, cornified skin with epidermal scales or scutes; no external ear opening (tympanum represents ear); limbs, when present, two pairs; heart usually three-chambered but four in crocodiles; cold-blooded; snakes and lizards shed their scales as skin cast; oviparous with internal fertilisation, eggs with tough shells. *Chelone* (turtle), *Testudo* (tortoise), *Chameleon*, *Calotes*, *Crocodilus*, *Alligator*, *Hemidactylus* (wall lizard), *Naja* (cobra), *Bangarus* (krait), *Vipera*.

Class Aves. Feathers and forelimbs modified into wings; beak; hindlimbs with scales adapted for walking, swimming, clasping; pneumatic (air-filled) long bones; digestive tract with additional crop and gizzard; four-chambered heart; warm-blooded (homoiothermous); no skin glands except the oil gland at the base of the tail; oviparous, with internal fertilisation and direct development. *Corvus* (crow), *Columba* (pigeon), *Psittacula* (parrot), *Struthio* (ostrich), *Pavo* (peacock), *Aptenodytes* (penguin), *Neophron* (vulture).

Class Mammalia. Mammary glands for milk; hair on the skin; external ears (pinnae); two pairs of limbs; four-chambered heart; warm-blooded; different types of teeth (heterodont dentition); mostly viviparous. *Ornithorhynchus* (platypus, oviparous), *Macropus* (kangaroo, pouched), *Pteropus* (flying fox), *Camelus*, *Macaca*, *Rattus*, *Canis*, *Felis*, *Elephas*, *Equus*, *Delphinus*, *Balaenoptera* (blue whale), *Panthera tigris*, *Panthera leo*.

Table: heart chambers: Pisces 2, Amphibia 3, Reptilia 3 (crocodile 4), Aves 4, Mammalia 4.

Chordate Classes: Fishes to Mammals: key facts

Chordate characters: Notochord
dorsal hollow nerve cord
paired pharyngeal gill slits
post-anal tail; At some stage
Heart chambers: $F i s h es 2∣ A m p hibia 3∣ R e pt i l ia 3 (cr oco d i l e 4) ∣ A v es 4∣ M amma l ia 4$; Counts
Scales: $P l a co i d (C h o n d r i c h t h y es) ∣ cy c l o i d, c t e n o i d (O s t e i c h t h y es) ∣ e p i d er ma l (R e pt i l ia)$; Skin covering
Body temperature: $C o l d - b l oo d e d : f i s h es, am p hibia, r e pt i l ia ∣ w a r m - b l oo d e d : a v es, mamma l ia$; Thermoregulation

How to answer Chordate Classes: Fishes to Mammals questions

Questions give a feature and ask for the class, or ask for a table of differences.

1Look for the discriminating feature: jaws, skin, gills (operculum), cloaca, feathers, hair.
2Link it to heart chambers and body temperature.
3Add fertilisation (internal or external) and development (direct or indirect).
4Give two examples with correct genus names.
5Check for exceptions: crocodile heart, platypus egg-laying.

Chordate Classes: Fishes to Mammals 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.

Aquarium and fish market

Suppose a shop sells a small fighting fish for tanks and a large rohu for food, while a visitor asks if a shark is the same type of fish.

Classify the three and state one feature separating the shark.

Try it first, then show the solutionSolution

Fighting fish (Betta) and rohu (Labeo) are bony fishes (Osteichthyes).
The shark is a cartilaginous fish (Chondrichthyes).
Shark has a cartilaginous skeleton, placoid scales, no operculum and no air bladder.

Answer: Bony fishes: Betta, Labeo; shark: Chondrichthyes with cartilage skeleton.

Memory hook: Bone has gill cover; cartilage has slits.

Tricks to remember Chordate Classes: Fishes to Mammals

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

2-3-3-4-4
Heart chambers: Pisces 2, Amphibia 3, Reptiles 3, Birds 4, Mammals 4.
Use it when: Heart
Placoid = sharks
Placoid scales in Chondrichthyes.
Use it when: Scales
FH
Feathers - Hair: Aves has feathers, Mammals have hair.
Use it when: Skin cover

Chordate Classes: Fishes to Mammals: solved examples

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

Example 1: Chordate characters · 2 marks · Very short answerBoard level

List the four defining features of chordates.

Try it first, then show the solutionSolution

Notochord and dorsal hollow nerve cord (1 mark).
Paired pharyngeal gill slits and post-anal tail at some stage (1 mark).

Answer: Notochord, dorsal nerve cord, gill slits, tail.

Example 2: Differences in fishes · 4 marks · Short answerBoard level

Distinguish Chondrichthyes from Osteichthyes on skeleton, gill cover, air bladder, scales.

Try it first, then show the solutionSolution

Skeleton: cartilage versus bone (1 mark).
Gill cover: absent versus operculum (1 mark).
Air bladder: absent versus present (1 mark).
Scales: placoid versus cycloid or ctenoid (1 mark).

Answer: Cartilage, no operculum, no bladder, placoid versus bone, operculum, bladder, cycloid/ctenoid.

Example 3: Heart comparison · 3 marks · Short answerHarder board level

Give the number of heart chambers in a fish, a frog, a lizard, a crocodile and a pigeon, and relate it to the body temperature of each.

Try it first, then show the solutionSolution

Fish 2, frog 3, lizard 3, crocodile 4, pigeon 4 (1 mark).
Fish, frog, lizard, crocodile are cold-blooded (1 mark).
Pigeon is warm-blooded, with the four-chambered heart separating oxygenated and deoxygenated blood (1 mark).

Answer: 2, 3, 3, 4, 4; only birds and mammals are warm-blooded.

Example 4: Oviparous mammal · 3 marks · Short answerHarder board level

Ornithorhynchus lays eggs but is a mammal. Which features place it in Mammalia?

Try it first, then show the solutionSolution

It has mammary glands to nourish the young (1 mark).
Hair on the body (1 mark).
Lays eggs, an exception among viviparous mammals (1 mark).

Answer: Mammary glands and hair define it as a mammal.

Example 5: Identify the class · 5 marks · Long answerHOTS / competency

A vertebrate has a moist skin without scales, a cloaca, a tympanum, a three-chambered heart, lays eggs in water and has larvae. Identify the class, explain each clue, and say why it must return to water.

Try it first, then show the solutionSolution

Amphibia (1 mark).
Moist scaleless skin used in respiration, cloaca and tympanum are class features (1 mark).
Three chambers: two atria, one ventricle (1 mark).
Eggs without shells need water to avoid drying, and the larvae (tadpoles) breathe with gills (1 mark).
Indirect development, external fertilisation in water (1 mark).

Answer: Amphibia; needs water for eggs and larvae.

Where students lose marks in Chordate Classes: Fishes to Mammals

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

Mistake: Operculum in sharks
Fish types are mixed.
Wrong working
Sharks have an operculum.
Right working
Sharks have separate gill slits; operculum is in bony fishes.
Do this: Cartilage = no operculum.
Usually costs: 1 mark.
Mistake: Three chambers in all reptiles
Crocodile forgotten.
Wrong working
All reptiles three-chambered.
Right working
Crocodiles have four chambers.
Do this: Exception.
Usually costs: 1 mark.
Mistake: Mammals all viviparous
Platypus forgotten.
Wrong working
No mammal lays eggs.
Right working
Ornithorhynchus lays eggs.
Do this: Exception.
Usually costs: 1 mark.

Exam tips

Learn the heart chamber table.
Always mention exceptions.
Give genus names correctly.
Use feature plus example in every answer.

Practice questions on Chordate Classes: Fishes to Mammals

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
The presence of operculum and air bladder is a feature of
(a) Chondrichthyes (b) Osteichthyes (c) Cyclostomata (d) Amphibia
Hint
Bony fishes.
Answer
(b).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Birds have pneumatic bones.
Reason (R): They need to reduce body weight for flight.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Air-filled bones.
Answer
(a).
Q3 · 2 marks · Very short answerBoard level
Name two jawless vertebrates and one feature of their mouth.
Hint
Cyclostomata.
Answer
Petromyzon and Myxine; circular, sucking mouth without jaws.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a zoo labels P: heterocercal tail, no air bladder; Q: wings, crop, gizzard; R: pinnae, mammary glands, four-chambered heart.
(a) Name classes. (1)
(b) Which two have the same number of heart chambers? (1)
(c) Which is cold-blooded? (1)
(d) Name one example of Q. (1)
Hint
Chondrichthyes, Aves, Mammalia.
Answer
(a) P Chondrichthyes, Q Aves, R Mammalia. (b) Q and R (four chambers each; P has two). (c) P. (d) Columba or Corvus.
Q5 · 4 marks · Short answerHOTS / competency
Suppose an animal is cold-blooded, breathes through lungs, has dry scaly skin, lays shelled eggs and has a four-chambered heart. Identify it, and explain how it differs from a bird and from a typical lizard.
Hint
Crocodile.
Answer
A crocodile (class Reptilia). Unlike a bird it has scales and is cold-blooded with no feathers; unlike a typical lizard (three-chambered heart) it has a four-chambered heart.

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.Which class has placoid scales?Show answer
Chondrichthyes.
2.Which class has a cloaca and tympanum?Show answer
Amphibia.
3.Which class has pneumatic bones?Show answer
Aves.
4.Which has mammary glands?Show answer
Mammalia.
5.Name one jawless vertebrate.Show answer
Petromyzon.
6.What replaces notochord in vertebrates?Show answer
Vertebral column.

Chordate Classes: Fishes to Mammals: frequently asked questions

What is Agnatha?

Jawless vertebrates, i.e. Cyclostomata.

What are warm-blooded animals?

Animals that maintain constant body temperature: birds and mammals.

What is the oviparous mammal?

Ornithorhynchus.

What is Pisces?

Fishes: Chondrichthyes and Osteichthyes.

What is the function of the air bladder?

Regulating buoyancy in bony fishes.