In vertebrate zoology, which of the following classes of animals normally have a three-chambered heart consisting of two atria and one ventricle?

Introduction / Context:
The structure of the heart varies among different vertebrate classes and reflects their evolutionary adaptations and metabolic needs. A common exam topic is the number of chambers in the hearts of fishes, amphibians, reptiles, birds, and mammals. This question asks which classes typically have a three chambered heart, with two atria and one ventricle.

Given Data / Assumptions:

• The classes mentioned are Pisces (fishes), Amphibia, and Reptilia.
• We assume standard textbook descriptions of vertebrate circulatory systems.
• Crocodilians, which are reptiles, are known exceptions with a four chambered heart.

Concept / Approach:
In fishes (Pisces), the heart usually has two chambers: one atrium and one ventricle. Amphibians generally have a three chambered heart with two atria and one ventricle, allowing partial mixing of oxygenated and deoxygenated blood. Most reptiles also have a three chambered heart, though their ventricle may be partially divided. Crocodilians are an important exception; they possess a four chambered heart similar to birds and mammals. Therefore, the correct answer must include Amphibia and most Reptilia, while excluding Pisces and acknowledging crocodilian exceptions.

Step-by-Step Solution:
Step 1: Examine option A, Pisces and Amphibia. Pisces have two chambered hearts, not three, so this option is partly incorrect.Step 2: Examine option B, Amphibia and Reptilia (except crocodilians). Amphibians have three chambered hearts and most reptiles also have a functionally three chambered heart, making this option correct.Step 3: Examine option C, Reptilia only, including crocodiles. This is incorrect because it excludes amphibians and mistakenly includes crocodiles, which actually have four chambered hearts.Step 4: Examine option D, Amphibia only. While amphibians do have three chambered hearts, many reptiles share this feature as well, so limiting it to amphibians alone is incomplete.Step 5: Conclude that option B best describes the vertebrate classes with three chambered hearts.

Verification / Alternative check:
You can verify from standard zoology tables that fish have a single circulation with a two chambered heart, amphibians and many reptiles have double circulation with a three chambered heart, and birds and mammals (and crocodilians) have a fully separated four chambered heart. The presence of two atria and a single ventricle in amphibians and most reptiles is consistently noted in textbooks, confirming this classification.

Why Other Options Are Wrong:
Option A is wrong because Pisces do not have three heart chambers; they have only one atrium and one ventricle. Option C is wrong because it incorrectly includes crocodiles, which have four chambers, and ignores amphibians. Option D is wrong because it omits reptiles that also have a three chambered heart.

Common Pitfalls:
Students sometimes overgeneralize and assume all reptiles have three chambered hearts without recalling the crocodile exception. Another pitfall is confusing the two chambered heart of fishes with the three chambered hearts of amphibians and reptiles. A helpful way to remember is to think of a progression: fish (2 chambers), amphibians and most reptiles (3 chambers), and birds, mammals, and crocodilians (4 chambers). This pattern reflects increasing separation of oxygenated and deoxygenated blood across vertebrate evolution.

Final Answer:
Amphibia and Reptilia (except crocodilians).