Which of the following characteristic features holds true for the corresponding group of animals?
Solution:
Let's analyze each option:
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Air bladder is present – Chondrichthyes: Chondrichthyes (cartilaginous fishes like sharks and rays) lack an air bladder or swim bladder. They must swim constantly to avoid sinking. Air bladder is characteristic of Osteichthyes (bony fishes). So, this statement is false.
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External fertilisation – Osteichthyes: Osteichthyes (bony fishes) typically exhibit external fertilisation, where the female releases eggs and the male releases sperm into the water for fertilisation. So, this statement is true.
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True placenta is present – Reptiles: Reptiles are primarily oviparous (egg-laying) or ovoviviparous. A true placenta, which facilitates nutrient and gas exchange between mother and embryo, is a characteristic feature of most mammals (eutherians), not reptiles. So, this statement is false.
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External ear openings are present – Reptilia: Reptiles generally do not possess external ear openings (pinnae). They have a tympanum (eardrum) that is either exposed or slightly recessed. External ear openings are characteristic of mammals. So, this statement is false.
Therefore, the only true statement is that external fertilisation is characteristic of Osteichthyes.