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Difficulty: MediumVertebrates: Poikilothermic Classes (Pisces, Amphibia, Reptilia)

An adult toad (*Sclerophrys regularis*) and an adult rainbow lizard (*Agama agama*) both possess hearts with two atria. Which anatomical feature of the cardiac structure distinguishes the heart of the lizard from that of the toad?

  1. A partially divided single ventricle containing an incomplete septumAnswer
  2. B
    A completely undivided single ventricle with no internal partition
  3. C
    Four completely separate heart chambers with two independent ventricles
  4. D
    A two-chambered heart arrangement consisting of one atrium and one ventricle as found in *Agama Agama*

Answer

The heart of the lizard is distinguished from that of the toad by possessing a partially divided single ventricle containing an incomplete septum.
Class Reptilia represents an evolutionary advancement over Amphibia in circulatory efficiency. Adult amphibians have a single ventricle without internal partitioning. In contrast, non-crocodilian reptiles like *Agama agama* possess an incomplete septum within their single ventricle, which significantly reduces the mixing of oxygenated blood returning from the lungs with deoxygenated blood returning from the body tissues.

Step-by-Step Solution

1
Identify the circulatory structure of class Amphibia (adult toad).
Adult amphibians possess a 3-chambered heart consisting of two atria and one completely undivided ventricle.
Amphibian ventricles lack internal partitioning, allowing partial mixing of blood.
2
Identify the circulatory structure of class Reptilia (lizard).
Reptiles possess a 3-chambered heart with two atria and a single ventricle that is partially divided by an incomplete septum.
The partial septum in the reptilian ventricle provides partial separation of oxygenated and deoxygenated blood streams.
3
Compare the cardiac anatomy to determine the distinguishing feature.
The presence of an incomplete ventricular septum in the lizard differentiates its cardiac anatomy from that of the toad.
This evolutionary adaptation increases oxygen delivery efficiency for terrestrial life.

Key Concept

Comparative cardiac anatomy of poikilothermic vertebrates (Pisces, Amphibia, Reptilia)
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Topics

Vertebrates: Poikilothermic Classes (Pisces, Amphibia, Reptilia)

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