🧬 Introduction
The Animal Kingdom includes multicellular, eukaryotic, heterotrophic organisms that ingest food. Despite the vast diversity in animal forms, fundamental features are common to various individuals. To study this immense diversity, animals are classified based on structural and functional organization.
Classification helps understand evolutionary relationships, ecological roles, and complexity levels.
🧫 Basis of Classification
Levels of Organisation: How cells are organized.
- Cellular Level: Cells arranged as loose aggregates (e.g., Sponges - Phylum Porifera).
- Tissue Level: Cells performing the same function are arranged into tissues (e.g., Coelenterates).
- Organ Level: Tissues are grouped together to form organs, each specialized for a particular function (e.g., Platyhelminthes).
- Organ System Level: Organs associate to form functional systems (e.g., Annelids, Arthropods, Molluscs, Echinoderms, Chordates). Organ systems can show varying complexity (e.g., digestive system: incomplete vs. complete; circulatory system: open vs. closed).
Symmetry: Arrangement of body parts.
- Asymmetrical: Body cannot be divided into equal halves through any plane (e.g., Sponges).
- Radial Symmetry: Body can be divided into identical halves by any plane passing through the central axis (e.g., Coelenterates, Ctenophores, Echinoderms - adults).
- Bilateral Symmetry: Body can be divided into identical left and right halves by only one plane (e.g., Annelids, Arthropods, Chordates).
Diploblastic and Triploblastic Organisation: Number of embryonic germ layers.
- Diploblastic: Cells arranged in two embryonic layers - external ectoderm and internal endoderm. An undifferentiated layer, mesoglea, may be present in between (e.g., Coelenterates, Ctenophores).
- Triploblastic: Cells arranged in three embryonic layers - ectoderm, endoderm, and a middle mesoderm (e.g., Platyhelminthes to Chordates).
Coelom: Presence or absence of a body cavity between the body wall and the gut wall.
- Acoelomates: Animals in which the body cavity is absent (e.g., Platyhelminthes).
- Pseudocoelomates: Animals in which the body cavity is not lined by mesoderm; instead, the mesoderm is present as scattered pouches. The cavity is called a pseudocoelom (e.g., Aschelminthes).
- Coelomates: Animals possessing a true coelom – a body cavity lined by mesoderm on all sides (e.g., Annelids, Molluscs, Arthropods, Echinoderms, Hemichordates, Chordates).
Segmentation: Serial repetition of at least some body parts.
- Metameric Segmentation: Body is externally and internally divided into segments with a serial repetition of at least some organs. (e.g., Earthworm - Annelida, Arthropods).
Notochord: Presence or absence of a supportive rod.
- Notochord: A mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals.
- Chordates: Animals with a notochord at some stage in their life.
- Non-chordates: Animals without a notochord.
🧠 Evolutionary Significance
The progression from cellular → tissue → organ → organ-system level and acoelomate → coelomate condition marks increasing complexity in body organization.
🧩 Key Differences Table
| Feature | Diploblastic | Triploblastic |
|---|---|---|
| Germ layers | 2 | 3 |
| Mesoderm | Absent | Present |
| Example | Hydra | Planaria |
| Feature | Acoelomate | Pseudocoelomate | Coelomate |
|---|---|---|---|
| Body cavity | Absent | False cavity | True cavity |
| Example | Planaria | Ascaris | Earthworm |
💡 Questions And Answers
Q1. Differentiate between diploblastic and triploblastic animals.
A1. Diploblastic animals have two germ layers (ectoderm and endoderm), while triploblastic have an additional mesoderm.
Q2. What is the significance of coelom?
A2. Coelom provides space for internal organs, aids growth, and enables complex organ development.
Q3. Define segmentation with one example.
A3. Division of body into repeating units called metameres — e.g., Earthworm.
Q4. Why are sponges considered cellular level animals?
A4. They lack tissues; their cells are loosely arranged and function independently.
Q5. Which symmetry is found in adult echinoderms?
A5. Radial symmetry (secondary radial symmetry in adults).
Q6. Differentiate between radial and bilateral symmetry.
A6:
- Radial Symmetry: The body can be divided into identical halves by any plane passing through the central axis. Animals are often sessile or slow-moving and circular in shape (e.g., Hydra, Jellyfish, adult Starfish).
- Bilateral Symmetry: The body can be divided into identical left and right halves by only one specific plane (the sagittal plane). This is associated with cephalization (head formation) and active movement (e.g., Humans, Insects, Earthworms).