- 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:
- Cellular level of organization: Here, the cells are arranged as loose cell aggregates. E.g. Porifera.
- Tissue level of organization: Here, the cells are arranged into tissues. E.g. Cnidarians and Ctenophores.
- Organ level of organization: Here, tissues are arranged into organs. E.g. Platyhelminthes and other higher phyla.
- 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).
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.
- 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.
- 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:
- Acoelomate: No coelom. The space between body wall and digestive cavity is filled with matrix (parenchyma). E.g. Porifera to Platyhelminthes.
- Pseudocoelomate: False coelom. Here, the body cavity is not lined by mesoderm. Mesoderm is scattered pouches. E.g. Aschelminthes.
- 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.
