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.

Two-chambered heart — Chondrichthyes and Osteichthyes (the fish classes under Pisces).

Three-chambered heart — Amphibia, and Reptilia in general (except crocodiles). …

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). …