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Difficulty: Very hardVertebrates: Poikilothermic Classes (Pisces, Amphibia, Reptilia)

A comparative physiological study evaluates nitrogenous waste excretion, embryonic protection, and cardiac structures across three poikilothermic vertebrate species (X, Y, and Z). Organism X undergoes metamorphosis from an ammonia-excreting aquatic larva to a urea-excreting adult possessing a 3-chambered heart with a completely unpartitioned single ventricle. Organism Y maintains a single-circuit circulation powered by a 2-chambered heart and excretes ammonia throughout its lifecycle. Organism Z produces cleidoic eggs with extraembryonic membranes, possesses a 3-chambered heart with an incomplete ventricular septum, and excretes uric acid. Which of the following correctly identifies the taxonomic classes of Organisms X, Y, and Z?

  1. Organism X is an amphibian, Organism Y belongs to Class Pisces, and Organism Z is a reptile.Answer
  2. B
    Organism X is a reptile, Organism Y is an amphibian, and Organism Z belongs to Class Pisces.
  3. C
    Organism X belongs to Class Pisces, Organism Y is an amphibian, and Organism Z is a reptile.
  4. D
    Organism X is an amphibian, Organism Y is a reptile, and Organism Z belongs to Class Pisces.

Answer

Organism X is an amphibian, Organism Y belongs to Class Pisces, and Organism Z is a reptile.
Amphibians (Organism X) undergo metamorphosis from aquatic ammonotelic larvae to terrestrial ureotelic adults with a 3-chambered heart lacking a septum. Fishes (Organism Y, Class Pisces) retain a 2-chambered heart with single-circuit blood flow and excrete ammonia throughout life. Reptiles (Organism Z) lay amniotic cleidoic eggs, have a partially divided ventricle via an incomplete septum, and excrete uric acid to conserve water. Therefore, the option identifying X as an amphibian, Y as a fish, and Z as a reptile is correct.

Step-by-Step Solution

1
Analyze Organism X characteristics
Metamorphosis from an aquatic larva (ammonotelic) to an adult (ureotelic) alongside a 3-chambered heart (two atria, one undivided ventricle) uniquely characterizes Class Amphibia.
Amphibians transition from gill-breathing larvae to lung/skin-breathing adults while retaining a 3-chambered heart without a ventricular septum.
2
Analyze Organism Y characteristics
Single-circuit blood circulation driven by a 2-chambered heart (one atrium, one ventricle) and persistent excretion of ammonia defines Class Pisces.
Fishes pump deoxygenated blood directly to the gills and body in a single circuit through a 2-chambered cardiac structure.
3
Analyze Organism Z characteristics
Cleidoic (shelled) amniotic eggs, uricotelic waste excretion for water conservation, and a 3-chambered heart with a partial (incomplete) ventricular septum define Class Reptilia.
Reptiles are adapted for terrestrial life through amniotic eggs, water-conserving uric acid waste, and partial separation of ventricular blood.
4
Synthesize anatomical and physiological traits
Organism X = Amphibia, Organism Y = Pisces, Organism Z = Reptilia.
Matching all physiological markers yields the correct sequence of poikilothermic vertebrate classes.

Key Concept

Comparative Anatomy and Physiology of Poikilothermic Vertebrate Classes (Pisces, Amphibia, Reptilia)
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Topics

Vertebrates: Poikilothermic Classes (Pisces, Amphibia, Reptilia)

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