Class 11 Biology | Animal Kingdom - III

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In this video (Page 17 of Chapter 4 – The Animal Kingdom), we continue the Basis of Classification with three key structural features — Coelom, Segmentation, and Notochord.
These are crucial for understanding the evolution of body organization in animals and are frequently asked in NEET 2025 and CBSE Class 11 Biology exams.

📚 Topics Covered (Page 17):

🧫 1️⃣ Coelom (Body Cavity):

Definition: Fluid-filled cavity between body wall and gut wall, lined by mesoderm.

Importance: Provides space for organ development and movement.

⚫ Acoelomate:

No body cavity; mesoderm forms a solid mass.

Example: Platyhelminthes.

🟢 Pseudocoelomate:

Cavity present but not lined by mesoderm; false coelom.

Example: Aschelminthes (Nematodes).

🔵 Coelomate:

True body cavity completely lined by mesoderm.

Example: Annelids, Arthropods, Molluscs, Echinoderms, Chordates.

🧩 2️⃣ Segmentation (Metamerism):

Division of the body into repeated segments (metameres).

External segmentation: Visible externally (e.g. earthworm).

Internal segmentation: Seen internally (e.g. chordates).

Provides flexibility and specialization of body parts.

🧬 3️⃣ Notochord:

Definition: A rod-like, flexible structure formed from mesoderm.

Supports the body and forms the basis of the vertebral column in higher animals.

Present in all Chordates at some stage of life.

Non-chordates lack notochord (e.g. arthropods, annelids, molluscs).

💡 Highlights:

Clear classification of animals by coelom and segmentation

Key difference between chordates and non-chordates

Based on Nectar’s Class 11 Biology Book & NCERT Biology

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📺 Previous Video: Animal Kingdom – Symmetry & Germ Layers (Page 16)

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