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

Consider the following three representative poikilothermic vertebrate species: *Agama agama* (rainbow lizard), *Tilapia zillii* (redbelly tilapia), and *Bufo regularis* (African toad). What is the correct sequence of these organisms when arranged in order of increasing structural complexity of their heart and circulatory system?

  1. 1*Tilapia zillii* (Pisces)
  2. 2*Bufo regularis* (Amphibia)
  3. 3*Agama agama* (Reptilia)

Answer

The correct evolutionary order of cardiac structural complexity is *Tilapia zillii* (Pisces), followed by *Bufo regularis* (Amphibia), and finally *Agama agama* (Reptilia).
The correct sequence follows the evolutionary advancement of the circulatory system across poikilothermic classes: Class Pisces (*Tilapia zillii*) features a simple two-chambered heart; Class Amphibia (*Bufo regularis*) possesses a three-chambered heart consisting of two atria and one ventricle; and Class Reptilia (*Agama agama*) displays an advanced three-chambered heart with a partial septum dividing the ventricle, reducing cardiac mixing.

Step-by-Step Solution

1
Identify the vertebrate class and heart structure for each organism
*Tilapia zillii* belongs to Class Pisces (bony fish) and has a 2-chambered heart. *Bufo regularis* belongs to Class Amphibia and has a 3-chambered heart. *Agama agama* belongs to Class Reptilia and has a 3-chambered heart with a partially partitioned ventricle.
Evolutionary progression in poikilothermic vertebrates advances from 2-chambered single circulation in fish to 3-chambered double circulation in amphibians and partially divided 4-chambered systems in reptiles.
2
Order the organisms based on cardiac chamber separation and efficiency
Fish (*Tilapia zillii*) < Amphibian (*Bufo regularis*) < Reptile (*Agama agama*).
Pisces has no separation of pulmonary and systemic circuits in the heart, Amphibia has separated atria but an undivided ventricle, and Reptilia introduces partial ventricular septation for improved oxygen transport efficiency.

Key Concept

Evolutionary trends in cardiac anatomy among poikilothermic vertebrate classes (Pisces, Amphibia, Reptilia)
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Topics

Vertebrates: Poikilothermic Classes (Pisces, Amphibia, Reptilia)

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