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

Arrange the following poikilothermic vertebrates in increasing order of cardiac structural complexity, starting from the organism with the simplest heart layout to the one with the most completely divided cardiac chambers:

  1. 1Tilapia (*Oreochromis niloticus*)
  2. 2Toad (*Sclerophrys regularis*)
  3. 3Lizard (*Agama agama*)
  4. 4Crocodile (*Crocodylus niloticus*)

Answer

The correct sequence from simplest to most complex cardiac chamber layout is: Tilapia (*Oreochromis niloticus*), Toad (*Sclerophrys regularis*), Lizard (*Agama agama*), and Crocodile (*Crocodylus niloticus*).
The correct sequence reflects progressive cardiac chamber division in poikilotherms: Tilapia has 2 chambers (1 atrium, 1 ventricle); Toad has 3 chambers with an undivided ventricle; Lizard has 3 chambers with a partially divided ventricle; and Crocodile has 4 complete chambers with a fully formed septum.

Step-by-Step Solution

1
Identify the cardiac chamber configuration for the fish species (Tilapia).
Fish have a 2-chambered heart (1 atrium, 1 ventricle) with single circulation, representing the baseline poikilothermic vertebrate structure.
Pisces exhibit the simplest vertebrate heart design.
2
Identify the cardiac chamber configuration for the amphibian species (Toad).
Amphibians have a 3-chambered heart (2 atria, 1 completely undivided ventricle) enabling dual systemic and pulmocutaneous circuits.
Amphibia possess two receiving chambers but share a single pumping ventricle.
3
Identify the cardiac chamber configuration for the non-crocodilian reptile species (Lizard).
Lizards have a 3-chambered heart with an incomplete septum partially partitioning the single ventricle.
Squamate reptiles demonstrate an intermediate evolutionary step that reduces mixing of oxygenated and deoxygenated blood.
4
Identify the cardiac chamber configuration for the crocodilian reptile species (Crocodile).
Crocodilians possess a fully 4-chambered heart with two atria and two completely separated ventricles.
Crocodilia represent the highest degree of cardiac chamber division among poikilotherms.

Key Concept

Evolutionary progression of heart chamber separation in poikilothermic vertebrates
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Topics

Vertebrates: Poikilothermic Classes (Pisces, Amphibia, Reptilia)

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