Animal KingdomClass 11 Biology Notes

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Basis of Classification

To classify the vast diversity of animals, scientists look for fundamental common features despite differences in appearance and structure. These core features form the basis for grouping animals together. Key characteristics include the arrangement of cells, body symmetry, the nature of the body cavity (coelom), and patterns in the digestive, circulatory, and reproductive systems.

Levels of Organisation

All animals are multicellular, but their cells are organized in different ways, showing increasing complexity.

  • Cellular Level: This is the simplest level, where cells are arranged as loose clumps or aggregates. There's some division of labour, but the cells don't form organized tissues.
    • Example: Sponges (Phylum Porifera).
  • Tissue Level: In this level, cells that perform the same function are grouped together to form tissues.
    • Example: Coelenterates (Phylum Cnidaria).
  • Organ Level: Here, different tissues are grouped together to form organs, each specialized for a specific function.
    • Example: Platyhelminthes (Flatworms).
  • Organ System Level: This is the highest level of organization, where different organs work together in functional systems, like the digestive or circulatory system.
    • Examples: Annelids, Arthropods, Molluscs, Echinoderms, and Chordates.

Patterns in Organ Systems

Organ systems can also show varying levels of complexity.

  • Digestive System:
    • Incomplete: Has only a single opening that serves as both mouth and anus. Example: Platyhelminthes.
    • Complete: Has two separate openings: a mouth for ingestion and an anus for egestion.
  • Circulatory System:
    • Open Type: Blood is pumped by the heart into open spaces or cavities, directly bathing the cells and tissues.
    • Closed Type: Blood is always contained within a network of vessels (arteries, veins, and capillaries) and circulates throughout the body.

Symmetry

Symmetry refers to how the body parts of an animal are arranged.

  • Asymmetrical: The body cannot be divided into two equal halves by any plane passing through the center.
    • Example: Most sponges.
  • Radial Symmetry: The body can be divided into two identical halves by any plane passing through the central axis. This is common in animals that are sessile (fixed in one place).
    • Examples: Coelenterates, Ctenophores, and adult Echinoderms.
  • Bilateral Symmetry: The body can be divided into identical left and right halves by only one specific plane. This arrangement is associated with active movement.
    • Examples: Annelids, Arthropods, and Chordates.

Diploblastic and Triploblastic Organisation

This classification is based on the number of embryonic cell layers (germ layers) from which all body tissues develop.

  • Diploblastic: Animals in which the cells are arranged in two embryonic layers: an outer ectoderm and an inner endoderm. A non-cellular, jelly-like layer called mesoglea is present between them.
    • Example: Coelenterates.
  • Triploblastic: Animals in which the developing embryo has a third germinal layer, the mesoderm, located between the ectoderm and endoderm. This allows for the development of more complex organs.
    • Examples: All phyla from Platyhelminthes to Chordates.

Coelom

The coelom is a body cavity located between the body wall and the gut wall. Its presence or absence is a crucial feature for classification.

  • Coelomates: These animals have a true coelom, which is a body cavity lined on all sides by mesoderm.
    • Examples: Annelids, Molluscs, Arthropods, Echinoderms, Hemichordates, and Chordates.
  • Pseudocoelomates: In these animals, the body cavity is not fully lined by mesoderm. Instead, the mesoderm is present as scattered pouches between the ectoderm and endoderm. This type of cavity is called a pseudocoelom.
    • Example: Aschelminthes (Roundworms).
  • Acoelomates: These animals have no body cavity at all.
    • Example: Platyhelminthes (Flatworms).

Segmentation

In some animals, the body is divided both externally and internally into a series of repeating segments. This pattern is called metameric segmentation, and the phenomenon is known as metamerism. This allows for specialization of different body regions.

Example
An earthworm's body is a clear example of metameric segmentation, where you can see the repeating ring-like segments.

Notochord

The notochord is a flexible, rod-like structure derived from the mesoderm. It forms on the dorsal (back) side of the animal during embryonic development and provides support.

  • Chordates: Animals that possess a notochord at some stage of their life.
  • Non-chordates: Animals that do not have a notochord. This group includes all phyla from Porifera to Echinoderms.