Basis of Classification: Key Criteria

Animals are classified based on fundamental characteristics such as symmetry, segmentation, body cavity (coelom), and germ layers. These criteria help in understanding the evolutionary and structural differences among organisms.

1. Symmetry

  1. Symmetry refers to how an organism’s body parts are arranged around a central axis.
  2. There are three main types of symmetry: Asymmetry, Radial Symmetry, and Bilateral Symmetry.
  3. Asymmetry: No definite shape or body pattern. Example: Sponges (Porifera).
  4. Radial Symmetry: Body can be divided into identical halves along multiple planes. Example: Jellyfish, Sea anemones, Starfish (Echinoderms).
  5. Bilateral Symmetry: Body can be divided into two equal halves only along one plane. Example: Humans, Insects, Fish.
  6. Bilateral symmetry allows for cephalization (development of a head region).

2. Segmentation

  1. Segmentation refers to the division of the body into repetitive segments.
  2. Provides flexibility, movement efficiency, and specialized functions.
  3. Three types of segmentation: Pseudometamerism, Metamerism, and External Segmentation.
  4. Pseudometamerism: Superficial segmentation, not internally divided. Example: Tape Worms.
  5. Metamerism: True segmentation where internal structures repeat. Example: Annelids (Earthworms), Arthropods, Chordates.
  6. External Segmentation: Seen in the body structure but not internally divided. Example: Leeches.

3. Body Cavity (Coelom)

  1. Body cavity or coelom is the fluid-filled space between the body wall and internal organs.
  2. Helps in circulation, movement, and organ development.
  3. Three types based on coelom presence: Acoelomates, Pseudocoelomates, and Coelomates.
  4. Acoelomates: No body cavity; organs embedded in solid mesoderm. Example: Flatworms (Platyhelminthes).
  5. Pseudocoelomates: False body cavity (not lined by mesoderm). Example: Roundworms (Nematoda).
  6. Coelomates: True body cavity lined by mesoderm. Example: Annelids, Mollusks, Arthropods, Echinoderms, Chordates.
  7. Coelomates allow for better organ specialization and complex body functions.

4. Germ Layers

  1. Germ layers are layers of embryonic cells that develop into different tissues.
  2. Based on germ layers, animals are classified into Diploblastic and Triploblastic.
  3. Diploblastic: Have two germ layers - Ectoderm (outer layer) and Endoderm (inner layer). Example: Cnidarians (Jellyfish, Hydra).
  4. Triploblastic: Have three germ layers - Ectoderm, Mesoderm (middle layer), and Endoderm. Example: Platyhelminthes to Chordates.
  5. Triploblastic organisms have complex organ systems due to the presence of mesoderm.

Additional Key Points

  1. Symmetry influences movement and organ placement in animals.
  2. Radial symmetry is found in sessile or drifting animals, while bilateral symmetry is common in motile organisms.
  3. Segmentation provides evolutionary advantages like better movement and organ specialization.
  4. Animals with true segmentation (metamerism) show higher functional efficiency.
  5. Coelom is crucial for organ development and physiological processes.
  6. Acoelomates lack a coelom and have limited body complexity.
  7. Pseudocoelomates have a false coelom that still provides some mobility to organs.
  8. Triploblastic animals are more advanced than diploblastic ones due to an additional mesoderm layer.
  9. Chordates (vertebrates) are coelomates with advanced organ systems.
  10. Arthropods are the largest group of coelomates with segmented bodies.
  11. Echinoderms exhibit radial symmetry but are coelomates.
  12. Symmetry, segmentation, and germ layers play a role in evolutionary adaptation.
  13. Higher animals exhibit cephalization (concentration of sense organs in the head).
  14. Organisms with a well-developed coelom have specialized circulatory, digestive, and nervous systems.
  15. Understanding these criteria helps in taxonomy, evolution, and biodiversity conservation.

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