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Animal Kingdom - Notes | Class 11 | Part 1: Basis of Classification

Animal Kingdom

4. Animal Kingdom

  • Animals are multicellular and heterotrophic organisms without cell wall and chlorophyll.
  • Kingdom Animalia includes 11 major phyla:
  1. Porifera
  2. Cnidaria
  3. Ctenophora
  4. Platyhelminthes
  5. Aschelminthes
  6. Annelida
  7. Arthropoda
  8. Mollusca
  9. Echinodermata
  10. Hemichordata
  11. Chordata

Basis of Classification

1. Levels of Organization

  • Based on this, animals are grouped into four levels:
  1. Cellular level of organization: Here, the cells are arranged as loose cell aggregates. E.g. Porifera.
  2. Tissue level of organization: Here, the cells are arranged into tissues. E.g. Cnidarians and Ctenophores.
  3. Organ level of organization: Here, tissues are arranged into organs. E.g. Higher animals (Platyhelminthes to chordates).
  4. Organ system level of organization: Here, organs are associated to organ system. Each system performs a specific physiological function. E.g. Higher animals.
  • Organ systems of various animals show complexities. E.g.
  • 👉 Digestive system is 2 types:
    • Incomplete: It has only a single opening that acts as mouth & anus. Seen in Cnidaria and Platyhelminthes.
    • Complete: It has 2 openings (mouth & anus).
  • 👉 Circulatory system is 2 types: open & closed.

2. Body Symmetry

  • It is the arrangement of similar body parts on 2 sides of main axis of the body. So, body can be divided into 2 equal halves.
  • Based on symmetry, animals are 3 types:
  • Asymmetrical: Here, body cannot be divided into 2 equal halves. E.g. Most Poriferans, Snails etc.
  • Radial symmetry: Here, body can be divided into 2 equal halves in any vertical plane along central axis (oral-aboral axis) of the body. E.g. some Poriferans, Cnidarians, Ctenophores and Echinoderms (adult).
  • Bilateral symmetry: Here, body can be divided into equal right & left halves in only one plane. E.g. Platyhelminthes to Chordata (except adult Echinodermata).

Radial Symmetry

Bilateral Symmetry

3. Germinal Layers (Embryonic Layers)

  • These are layers of embryo from which all the body organs are formed.
  • Based on the number of germ layers, animals are 2 types - Diploblastic and Triploblastic.
  1. Diploblastic animals: 2 germ layers - outer ectoderm and inner endoderm. In between these layers, an undifferentiated jelly-like layer called mesoglea is present. E.g. Cnidaria & Ctenophora.
  2. Triploblastic animals: 3 germ layers - Outer ectoderm, middle mesoderm and inner endoderm. E.g. Platyhelminthes to Chordata.

4. Coelom (Body Cavity)

  • It is the cavity lined by mesoderm.
  • It is seen between body wall and gut wall.
  • Coelom separates the muscles of gut and body wall.
  • Based on the nature of coelom, animals are 3 types:
  1. Acoelomate: No coelom. The space between body wall and digestive cavity is filled with matrix (parenchyma). E.g. Porifera to Platyhelminthes.
  2. Pseudocoelomate: False coelom. Here, the body cavity is not lined by mesoderm. Mesoderm is scattered pouches. E.g. Aschelminthes.
  3. Coelomate: True coelom. Here, the coelom arises from the mesoderm. E.g. Annelida to Chordata.

5. Metamerism (Segmentation)

  • It is the phenomenon in which the body or organs is externally and internally divided into repeated segments (metameres).
  • E.g. Annelids (earthworm etc.), Arthropods.

6. Notochord

  • It is a mesodermally derived supporting rod formed on the dorsal side during embryonic development in some animals.
  • Animals with notochord are called chordates and those without notochord are called non-chordates.

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