Q.200 Zoology ⭐ 360° Option Elimination Active

NEET 2021 Zoology Question 200 Solution

Topic: Structural Organisation in Animals · Official NTA Booklet Code M1

Question

Following are the statements about prostomium of earthworm. (a) It serves as a covering for mouth. (b) It helps to open cracks in the soil into which it can crawl. (c) It is one of the sensory structures. (d) It is the first body segment. Choose the correct answer from the options given below.

(1)(a), (b) and (c) are correct
(2)(a), (b) and (d) are correct
(3)(a), (b), (c) and (d) are correct
(4)(b) and (c) are correct - o 0 o -

✓

Verified Against NTA Final Answer Key

Option (1) : (a), (b) and (c) are correct

📚 Structural Organisation in Animals Medium

📋 Given Parameters & Problem Setup

Anatomical Structure Prostomium of Earthworm (Pheretima posthuma)

Statement (a) It serves as a covering for mouth

Statement (b) It helps to open cracks in the soil into which it can crawl

Statement (c) It is one of the sensory structures

Statement (d) It is the first body segment

⚡

The 30-Second Exam Hall Shortcut: ⚡ "PERISTOMIUM is the First Segment; PROSTOMIUM is the Sensory Upper Lip Wedge!"

⏱️ Save ~25 seconds

The first body segment of the earthworm is the PERISTOMIUM (buccal segment). The prostomium is NOT a body segment; it is a pre-segmental fleshy lobe acting as a mouth covering, a soil-wedge, and a sensory organ. Statement (d) is FALSE! Discard any option with (d) -> Options 2 and 3 vanish. Option (1) is the correct answer!

1 Critical Anatomical Distinction: - Prostomium = Pre-segmental sensory lobe overhang serving as mouth covering and wedge. - Peristomium = True 1st body segment containing the ventral mouth.

2 Evaluate Statement (d): "It is the first body segment" -> Factually FALSE (peristomium is the 1st segment).

3 Eliminate options containing (d): Disqualifies Option (2) and Option (3).

4 Evaluate remaining statements: (a) mouth covering (True), (b) wedges cracks in soil (True), (c) sensory in function (True).

5 Option (1) is 100% correct!

🛡️

Why You Can 100% Trust This Shortcut: NCERT Biology · Structural Organisation in Animals (Earthworm Morphology: The anterior end consists of the mouth and the prostomium, a lobe which serves as a covering for the mouth and as a wedge to open cracks in the soil into which the earthworm may crawl. The prostomium is sensory in function. The first body segment is called the peristomium (buccal segment) which contains the mouth).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Anatomy of the Anterior Region of Pheretima posthuma

The anterior cephalic end of the earthworm features specialized adaptations for subterranean burrowing in soil: - Prostomium: A fleshy, muscular dorsal lobe projecting above and overhanging the ventral mouth opening. It is pre-segmental (not a true metamere / body segment): - (a) Mouth Covering: It acts as a protective dorsal hood over the mouth. - (b) Soil Wedging: It functions as a powerful mechanical wedge to force open crevices and fissures in compact soil, allowing the worm to crawl forward. - (c) Sensory Function: It is heavily innervated with photoreceptors and chemoreceptive tactile cells to sample light, chemical gradients, and soil moisture.

Step 2

Dissection of Statement (d): Prostomium vs Peristomium

- Peristomium (Buccal Segment): The first true body segment of the earthworm is called the peristomium, which completely encircles the crescentic mouth opening. - Because the prostomium is merely a pre-segmental fleshy lobe overhanging segment 1, stating that the prostomium is the first body segment ((d)) is a direct anatomical error. - Therefore, statements (a), (b), and (c) are correct, while statement (d) is incorrect.

\begin{cases} \textbf{Prostomium:} & \text{Pre-segmental Sensory Wedge \& Mouth Covering } [\text{(a), (b), (c) True}] \\[4pt] \textbf{Peristomium:} & \mathbf{First\,True\,Body\,Segment\,(Buccal\,Segment)} \implies [\text{(d) is False}] \end{cases}

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 1

✓ 100% Correct

Verdict & Scientific Reason:

Correct. Confirms all three genuine functions of the prostomium while correctly excluding the false statement (d).

💡

Why this is airtight: None. Candidate knows the distinction between prostomium and peristomium.

2 2

✗ Incorrect

Verdict & Scientific Reason:

Incorrect because (d) is false; peristomium is the first body segment.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing prostomium with peristomium.

3 3

✗ Incorrect

Verdict & Scientific Reason:

Incorrect due to the false inclusion of (d).

⚠️

Student Fallacy (Why candidates pick this trap): Assuming all four statements are true without checking the definition of peristomium.

4 4

✗ Incorrect

Verdict & Scientific Reason:

Incorrect because statement (a) is also a genuine verified function (serves as a covering for the mouth).

⚠️

Student Fallacy (Why candidates pick this trap): Failing to realize that prostomium covers the mouth.

⚠️

NTA Trap Alert: Prostomium vs Peristomium Terminology Trap

NTA intentionally uses "Prostomium" and includes "first body segment" as statement (d). Candidates who confuse the prefixes "Pro-" (before) and "Peri-" (around) fail to realize that *peristomium* is the first segment, not the prostomium.

🛡️ How to avoid the -1 mark: Remember: "Peri- = Perimeter (around the mouth, segment 1); Pro- = Projection in front of mouth".

📖

Core NEET Syllabus Standard

NEET / NCERT Biology · Structural Organisation in Animals Standard Benchmark

Core Scientific Principle Tested:

"Prostomium covers mouth, opens soil cracks, and is sensory; peristomium is the first body segment."

Syllabus Focus: Morphology of Earthworm

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NEET 2021 Zoology Question 200 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "PERISTOMIUM is the First Segment; PROSTOMIUM is the Sensory Upper Lip Wedge!"

Step-by-Step Rigorous Workflow & Mathematical Proof

Anatomy of the Anterior Region of Pheretima posthuma

Dissection of Statement (d): Prostomium vs Peristomium

360° Option Elimination Engine Key Feature

NTA Trap Alert: Prostomium vs Peristomium Terminology Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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