Q.Which one of the following sets of animals share a four chambered heart?
Concept understanding — Vertebrate Heart Evolution
Think about how different animals get around — a fish glides through water, a frog hops between pond and land, a bird flies, a mammal runs. Their bodies demand different amounts of energy, and energy needs oxygen. The heart is the pump that delivers oxygen-carrying blood, and across the vertebrates it becomes steadily better at keeping fresh, oxygen-rich blood separate from stale, used blood. That gradual improvement in heart design across groups is what we mean by vertebrate heart evolution.
Every vertebrate has a muscular, chambered heart, but the number of chambers is not the same in every group. The pattern runs like this:
- Fishes have a two-chambered heart — one atrium and one ventricle.
- Amphibians and reptiles (crocodiles aside) have a three-chambered heart — two atria but only a single ventricle.
- Crocodiles, birds and mammals have a four-chambered heart — two atria and two ventricles.
The number of chambers matters because it decides how well oxygenated and deoxygenated blood are kept apart. In a fish, the heart handles only deoxygenated blood, which is sent to the gills to pick up oxygen. In amphibians and reptiles, oxygenated and deoxygenated blood both empty into the single ventricle and mix there, so the blood pumped to the body is a blend of the two. Only in birds and mammals do two separate ventricles keep the two streams completely apart, so pure oxygenated blood reaches the tissues.
More chambers is not just "more parts" — it means cleaner separation of fresh and used blood, which lets an animal supply its tissues with better-oxygenated blood and sustain a more active life.
Understanding this progression makes the human heart easier to appreciate: our four-chambered heart sits at the fully separated end of this series, which is exactly why humans have a complete double circulation.
This explanation lines up with searches such as "heart chambers in fish, amphibians, reptiles, birds and mammals" and "vertebrate heart evolution class 11 biology," a comparative sequence drawn directly from the NCERT/CBSE Class 11 Biology syllabus. The two-, three- and four-chambered heart progression above is one of the most frequently asked comparison questions in NEET biology.
The correct option is (B) Crocodiles, Birds, Mammals.
- Among reptiles, the heart is usually three-chambered, but crocodiles are the exception with a fully four-chambered heart.
- Birds (class Aves) have a completely four-chambered heart.
- Mammals also have a four-chambered heart.
- Amphibians and most reptiles (other than crocodiles) have only a three-chambered heart, so options built on "Amphibian" or "Lizards/Turtles" cannot be correct.
Crocodiles, Birds and Mammals (option B) share a four-chambered heart.
A four-chambered heart is found in crocodiles (uniquely among reptiles), all birds, and all mammals — not in amphibians or non-crocodilian reptiles.
Heart structure becomes progressively more complex across the vertebrate classes: fish have a two-chambered heart, amphibians a three-chambered heart, most reptiles a three-chambered heart, and birds and mammals a fully four-chambered heart.
- Amphibians always have a three-chambered heart (two auricles, one ventricle), so any set including "Amphibian" as a four-chambered example is wrong.
- Reptiles as a class usually have a three-chambered heart, with crocodiles being the specific exception that has evolved a complete four-chambered heart. Lizards and turtles, both reptiles other than crocodiles, remain three-chambered, which rules out option (C).
- Birds always have a completely four-chambered heart, supporting their high metabolic demands as warm-blooded, flying animals.
- Mammals also always have a four-chambered heart, matching their warm-blooded physiology.
So the only combination where every member genuinely has a four-chambered heart is crocodiles (the reptilian exception), birds and mammals.
In short, crocodiles, birds and mammals (option B) are the set that shares a four-chambered heart.
Method: Exception-first table building
Rather than trying to recall each option's heart-chamber count directly, build a small mental table of "default heart type per class," flag the ONE known exception, and use that exception to test the options.
Step 1 — lay out the class defaults: Fishes = two chambers (default, no exceptions taught). Amphibians = three chambers (default, no exceptions taught). Reptiles = three chambers (the DEFAULT for the class). Birds = four chambers (default, no exceptions). Mammals = four chambers (default, no exceptions).
Step 2 — flag the one exception explicitly: among reptiles, crocodiles break the class default and have evolved a full four-chambered heart, unlike lizards, turtles and other reptiles which stay three-chambered.
Step 3 — test each option against the table plus the exception:
- (a) Amphibian, Reptiles, Birds — amphibian is three-chambered (default), so this set already fails; discard.
- (c) Crocodiles, Lizards, Turtles — lizards and turtles are ordinary reptiles, so they stay three-chambered even though crocodiles are the exception; this set fails.
- (d) Lizards, Mammals, Birds — lizards are three-chambered (ordinary reptile default); this set fails.
- (b) Crocodiles, Birds, Mammals — crocodiles use the EXCEPTION (four-chambered), while birds and mammals use their own four-chambered DEFAULT. Every member of this set is genuinely four-chambered.
Building the table of defaults first, and treating the crocodile as a deliberately flagged exception rather than trying to remember it alongside every other reptile, makes it obvious which set is internally consistent.
Conclusion: Laying out each class's default heart-chamber count and specifically flagging crocodile as the one reptile exception reaches the same answer as direct recall — option (b), crocodiles, birds and mammals — while explaining precisely why lizards and turtles cannot join that set.
- AP EAPCET 2026Set ap-2026-05-20-FN1 markMCQQ.Which of the following animal has three chambered heart? (A) Pteropus (B) Scoliodon (C) Hippocampus (D) Chelone
›Reveal solutionSolution
Fish have 2-chambered hearts, mammals and birds have 4-chambered hearts, and most reptiles (except crocodilians) have a 3-chambered heart. Among the choices, only the turtle Chelone fits "three-chambered."
Concept and Intuition
Heart chambering tracks with the demand for separating oxygenated and deoxygenated blood. Fish need only a single circuit (2 chambers); reptiles are transitional with partial separation (3 chambers, incomplete ventricular septum); birds and mammals need full separation for endothermy (4 chambers). Crocodilians are the reptilian exception with a fully divided, functionally 4-chambered heart.
Step-by-Step Solution
- Pteropus is a fruit bat — Mammalia — 4 chambers (2 atria + 2 ventricles), ruled out.
- Scoliodon is a cartilaginous fish (dogfish shark) — 2 chambers (1 atrium + 1 ventricle, plus sinus venosus/conus arteriosus as accessory chambers, but functionally 2), ruled out.
- Hippocampus is a bony fish (seahorse) — 2 chambers, ruled out.
- Chelone is a marine turtle — Reptilia — has two atria and one ventricle that is incompletely divided by a septum, giving a 3-chambered heart. This matches the question.
Common Mistakes
- Assuming all reptiles have a fully 3-chambered heart with NO exception — remember crocodilians are 4-chambered.
- Confusing "3 chambers" with fish having 3 chambers by counting the sinus venosus and conus arteriosus as separate chambers — by syllabus convention fish are 2-chambered.
✓Final answerThe correct option is (D) — Chelone.
ANSWER: D
- AP EAPCET 2023Set ap-2023-05-22-FN1 markMCQQ.The heart receives only deoxygenated blood in (A) Aves (B) Mammals (C) Fishes (D) Reptiles
›Reveal solutionSolution
Fish have single circulation with a two-chambered heart handling only venous (deoxygenated) blood, unlike birds, mammals, or reptiles where the heart also receives oxygenated blood.
Concept and Intuition
Circulatory pattern determines what kind of blood the heart handles. Fish use single circulation: blood goes heart → gills (oxygenated) → body → back to heart (deoxygenated) → gills again. The heart itself, therefore, never receives freshly oxygenated blood; it only ever receives venous (deoxygenated) blood returning from the body. In double-circulation animals (birds, mammals), oxygenated blood from the lungs returns to the LEFT side of the heart before going to the body — so the heart does receive oxygenated blood (in a separate chamber from the deoxygenated side). Reptiles have incomplete septation of the ventricle, causing some mixing of oxygenated and deoxygenated blood within the heart itself.
Step-by-Step Solution
- Fishes: 2-chambered heart, single circuit — heart gets ONLY deoxygenated blood from the body, sends it to gills; oxygenated blood bypasses the heart on its way to the body. Matches the question.
- Aves/Mammals: 4-chambered heart, double circuit — right heart handles deoxygenated blood, left heart handles oxygenated blood; the heart as a whole DOES receive oxygenated blood (in its left chambers).
- Reptiles: 3-chambered (or partially septated) heart — mixing of oxygenated and deoxygenated blood occurs within the ventricle.
- Only fishes satisfy "receives ONLY deoxygenated blood."
Common Mistakes
- Forgetting that in double-circulation animals the heart is divided into two functionally separate sides, so it does handle oxygenated blood too (just not mixed with deoxygenated blood).
- Assuming reptiles, like fish, only handle deoxygenated blood — reptiles actually have partial mixing, not exclusive deoxygenated flow.
✓Final answerThe correct option is (C) — Fishes.
ANSWER: C
- AP EAPCET 2023Set ap-2023-05-22-FN1 markMCQQ.Identify the group in which sinus venosus is present but conus arteriosus is absent. (A) Amphibia (B) Reptilia (C) Aves (D) Mammals
›Reveal solutionSolution
Reptile hearts still retain a distinct sinus venosus chamber but have lost the conus arteriosus as a separate chamber — a transitional stage between amphibian and bird/mammal hearts.
Concept and Intuition
The vertebrate heart evolved by progressively reducing/losing accessory chambers found in fish. Fish hearts: sinus venosus → atrium → ventricle → conus arteriosus (all four in series). Amphibian hearts: sinus venosus + two atria + one ventricle + conus arteriosus (both extra chambers retained). Reptilian hearts: sinus venosus is still present as a distinct chamber, but the conus arteriosus has disappeared as a separate structure (its role is taken over by separate arterial trunks arising directly from the ventricle). In birds and mammals, sinus venosus itself regresses into the sino-atrial node embedded in the right atrial wall, and conus arteriosus remains absent as a chamber.
Step-by-Step Solution
- Fish: sinus venosus present AND conus arteriosus present — doesn't match (conus present).
- Amphibia: sinus venosus present AND conus arteriosus present — doesn't match (conus present).
- Reptilia: sinus venosus present, conus arteriosus ABSENT (lost as distinct chamber) — matches exactly.
- Aves/Mammals: sinus venosus itself is reduced/absent as a distinct chamber (become SA node) — doesn't match "sinus venosus present".
- Only Reptilia satisfies both conditions simultaneously.
Common Mistakes
- Assuming amphibians already lack conus arteriosus — they still have it; the loss occurs at the reptilian stage.
- Assuming birds/mammals retain sinus venosus as a chamber — it is reduced to the SA node, not a true chamber.
✓Final answerThe correct option is (B) — Reptilia.
ANSWER: B
- AP EAPCET 2022Set ap-2022-07-12-FN1 markMCQQ.Heart in fishes is described as (A) Neurogenic and bronchial heart (B) Myogenic, bronchial and venous heart (C) Neurogenic and venous heart (D) Neurogenic and alternate heart
›Reveal solutionSolution
Fish hearts are myogenic (the heartbeat originates from within the cardiac muscle itself), pump only deoxygenated blood toward the gills (making it a 'branchial'/venous heart).
Concept and Intuition
The heart of fishes is myogenic — the impulse that initiates each heartbeat arises intrinsically from specialised cardiac muscle (the sinus venosus, functioning like a pacemaker) rather than from an external nerve centre, unlike the neurogenic hearts of some invertebrates (e.g., crustaceans). Since a fish heart is two-chambered and receives and pumps only deoxygenated (venous) blood — which it sends forward to the gills (branchiae) for oxygenation before it reaches the rest of the body — it is described as a myogenic, branchial, and venous heart.
Step-by-Step Solution
- Determine origin of heartbeat: fish hearts are myogenic (originates in the heart muscle itself), not neurogenic.
- Determine blood type handled: fish hearts receive/pump only deoxygenated (venous) blood, making it a 'venous heart'.
- Determine destination: this venous blood is pumped toward the gills (branchiae) for oxygenation, making it also a 'branchial heart'.
- Combining all three correctly describes it as myogenic, branchial and venous heart.
Common Mistakes
- Incorrectly calling the fish heart neurogenic (that's true of some invertebrate hearts, not vertebrates).
✓Final answerThe correct option is (B) — Myogenic, branchial (bronchial) and venous heart.
ANSWER: B
- AP EAPCET 2021Set ap-2021-09-07-FN1 markMCQQ.Match the following? List I | List IIi) Fishes | a) Incompletely divided ventricleii) Reptilia | b) Branchial heartiii) Amphibia | c) Incomplete double circulationiv) Mammalia | d) Four chambered heart (A) (i-b),(ii-a),(iii-c)&(iv-d) (B) (i-c),(ii-b),(iii-a)&(iv-d) (C) (i-b),(ii-d),(iii-c)&(iv-a) (D) (i-b),(ii-c),(iii-a)&(iv-d)
›Reveal solutionSolution
This tests the evolutionary progression of vertebrate heart structure and circulation type from fish to mammals.
Concept and Intuition
Vertebrate heart complexity increases along the evolutionary line from fish to mammals, tracking the shift from single to complete double circulation:
- Fish: a two-chambered heart (one atrium, one ventricle) that receives and pumps only deoxygenated venous blood towards the gills for oxygenation — often described in this context as the "branchial heart" since its entire output goes to the gill (branchial) circulation before reaching the body.
- Reptiles (except crocodilians): a three-chambered heart with two atria and a single, incompletely divided ventricle — a partial septum reduces (but doesn't eliminate) mixing of oxygenated and deoxygenated blood.
- Amphibians: a three-chambered heart (two atria, one undivided ventricle), so oxygenated blood from the lungs/skin and deoxygenated systemic blood mix in the single ventricle — this partial mixing is called incomplete double circulation.
- Mammals (and birds): a fully four-chambered heart with complete separation of oxygenated and deoxygenated blood, giving true complete double circulation.
Step-by-Step Solution
- Fish → two-chambered heart pumping only venous blood to the gills → matches (b) "Branchial heart".
- Reptilia → three-chambered heart, ventricle only partially septated → matches (a) "Incompletely divided ventricle".
- Amphibia → single, undivided ventricle mixing arterial and venous blood → matches (c) "Incomplete double circulation".
- Mammalia → complete four-chambered heart → matches (d) "Four chambered heart".
- Assemble: (i-b),(ii-a),(iii-c),(iv-d) → option (A).
Common Mistakes
- Swapping reptile and amphibian descriptors, since both involve a degree of ventricular mixing — the key distinguishing detail is that reptiles have a partially divided ventricle while amphibians have a fully undivided single ventricle.
- Forgetting that fish circulation is single (not double at all), so labeling it "incomplete double circulation" (which applies to amphibians) would be wrong.
✓Final answerThe correct option is (A) — (i-b),(ii-a),(iii-c)&(iv-d).
ANSWER: A
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