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


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


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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