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Zoology Question 48 – AP-EAMCET 2025

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It is the fore runner of thyroid gland of vertebrates

Consider the concept of homologous structures, which are similar in different species due to common ancestry, even if their functions have diverged.

Step 1: Understand the Question's Core Concept✦ Active

The question asks to identify the evolutionary precursor or 'forerunner' of the thyroid gland in vertebrates. This requires knowledge of comparative anatomy and the evolutionary history of endocrine glands, particularly within the chordate lineage.

💡 Teacher's Secret Hint

Focus on structures in primitive chordates that perform functions analogous to the thyroid gland.

Step 2: Recall the Function and Origin of the Thyroid Gland○ Expand

The thyroid gland in vertebrates is an endocrine gland that produces thyroid hormones (e.g., thyroxine) which are crucial for regulating metabolism, growth, and development. These hormones are unique because they contain iodine.

💡 Teacher's Secret Hint

Remember that iodine uptake and hormone production are key characteristics of the thyroid gland.

Step 3: Evaluate the Given Options for Homology○ Expand

Let's examine the options: 1. **Atrium:** A chamber of the heart, involved in circulation, not endocrine function. 2. **Endostyle:** A ciliated groove in the pharynx of lower chordates (e.g., amphioxus, larval lampreys). It secretes mucus for filter feeding and, importantly, has the ability to trap iodine and synthesize iodine-containing compounds. This function is homologous to that of the vertebrate thyroid gland. 3. **Pharynx:** The part of the throat behind the mouth and nasal cavity. While the endostyle is located in the pharynx, the pharynx itself is a broader anatomical region, not the specific homologous structure. 4. **Test:** Typically refers to the outer covering of some invertebrates (like tunicates) or a reproductive organ (testis). Neither is related to the thyroid gland's evolutionary origin.

💡 Teacher's Secret Hint

The endostyle's ability to metabolize iodine is a strong indicator of its evolutionary link to the thyroid gland.

Step 4: Conclude the Correct Forerunner○ Expand

Based on comparative studies and the shared function of iodine metabolism, the endostyle of lower chordates is widely recognized as the evolutionary forerunner of the thyroid gland in vertebrates.

💡 Teacher's Secret Hint

This is a classic example of evolutionary homology in chordates.

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