Question:
The depressed central area on the planispiral coiled shell of Nautilus is called :
The depressed central area on the planispiral coiled shell of Nautilus is called :
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Think of the "Umbilicus" as the "belly button" of the planispirally coiled shell, representing the exact center of coiling.
Updated On: Jul 21, 2026
- Whorl
- Columella
- Varix
- Umbilicus
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The Correct Option is D
Solution and Explanation
Step 1: Understanding the Concept:
Shell morphology is a vital aspect of molluscan taxonomy, particularly for cephalopods like Nautilus which possess external, chambered shells.
Understanding the anatomical terms of shell spirals is essential for identification.
Step 2: Detailed Explanation:
The shell of a Nautilus is planispiral, meaning it is coiled in a single horizontal plane, resulting in a symmetrical left and right side.
At the center of this flat spiral, there is a depressed central depression or opening around which the outer whorls of the shell are wrapped. This central depressed area is called the Umbilicus.
Let us examine the other terms:
1. Whorl: A single complete \(360^{\circ}\) turn or coil of the shell.
2. Columella: The central structural pillar or column around which a helically coiled (conispiral) gastropod shell winds (absent in planispiral shells like Nautilus).
3. Varix: A prominent ridge or thickening on the outer surface of a gastropod shell, representing a growth pause.
Hence, the umbilicus is the correct term for the central depressed area of a planispirally coiled shell.
Step 4: Final Answer:
The depressed central area on the planispiral coiled shell of Nautilus is called the Umbilicus.
Shell morphology is a vital aspect of molluscan taxonomy, particularly for cephalopods like Nautilus which possess external, chambered shells.
Understanding the anatomical terms of shell spirals is essential for identification.
Step 2: Detailed Explanation:
The shell of a Nautilus is planispiral, meaning it is coiled in a single horizontal plane, resulting in a symmetrical left and right side.
At the center of this flat spiral, there is a depressed central depression or opening around which the outer whorls of the shell are wrapped. This central depressed area is called the Umbilicus.
Let us examine the other terms:
1. Whorl: A single complete \(360^{\circ}\) turn or coil of the shell.
2. Columella: The central structural pillar or column around which a helically coiled (conispiral) gastropod shell winds (absent in planispiral shells like Nautilus).
3. Varix: A prominent ridge or thickening on the outer surface of a gastropod shell, representing a growth pause.
Hence, the umbilicus is the correct term for the central depressed area of a planispirally coiled shell.
Step 4: Final Answer:
The depressed central area on the planispiral coiled shell of Nautilus is called the Umbilicus.
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