Q.There has been an increase in the number of chambers in heart during evolution of vertebrates. Give the names of the class of vertebrates having two, three or four-chambered heart.
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🔒 Start your 14-day free trial to unlock the full solution →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. …
Two-chambered heart — Chondrichthyes and Osteichthyes (the fish classes under Pisces).
Three-chambered heart — Amphibia, and Reptilia in general (except crocodiles). …
Heart complexity rises through the vertebrate classes: two chambers in fishes, three in amphibians and most reptiles, and four in crocodiles, birds and mammals.
The chapter traces an increase in heart complexity as vertebrate classes are described in sequence.
- Two-chambered heart (one auricle, one ventricle): found in the fish classes of super class Pisces — Chondrichthyes (cartilaginous fishes) and Osteichthyes (bony fishes) both have a two-chambered heart.
- Three-chambered heart (two auricles, one ventricle): found in Amphibia, and generally in Reptilia as well — reptiles are described as usually having a three-chambered heart.
- Four-chambered heart: found as the exception among reptiles in crocodiles, and as the standard condition in Aves (birds, completely four-chambered) and Mammalia (also four-chambered). …
Method: Exception-first, then build outward
Rather than listing each class's chamber count independently from the start, anchor the answer on the ONE well-known exception (crocodiles) first, and build the rest of the classification outward from that anchor by comparing every other class to it.
Anchor fact: crocodiles are reptiles, yet they are explicitly the exception within Reptilia that has a full four-chambered heart, unlike ordinary reptiles.
Build outward — compare every class to the crocodile anchor:
- If ordinary reptiles (lizards, turtles, snakes) do NOT get the crocodile's exceptional four-chambered upgrade, they must sit at the class's normal reptilian level, which is three-chambered (two atria, one ventricle) — the same level as amphibians.
- Amphibians, having no comparable "exception" of their own noted anywhere, stay at the baseline three-chambered level alongside ordinary reptiles.
- Since crocodiles reach FOUR chambers despite being reptiles, and birds/mammals are consistently described as the most advanced/highest-energy classes, it follows that birds and mammals sit at or above the crocodile's four-chambered level, i.e., also four-chambered (matching their universally warm-blooded physiology, which demands strict separation of oxygenated and deoxygenated blood). …
- 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. …
- 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). …
- 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). …
- 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'. …
- 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". …
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