Animal Kingdom

Classification Of Animals

Animals are classified on the basis of the following fundamental features :

1. Level of Organization

2. Body symmetry

3. Circulatory system

4. Number of germ layer

5. Presence of absence of notochord

6. Segmentation

7. Nature of coelom

8. Reproductive system

9. Body Plan

10. Digestion

Level of organization

There are five patterns of levels of organization of cells in the body : 

1. Acellular level of organization

2. Cellular level of organization

3. Tissue level of organization

4. Organ level of organization

5. Organ system level of organization

1. Acellular level of organization

• It is the mass of protoplasm.
• All the activities is done by cell organelles.
• Cells are surrounded by plasma membrane.

Example : Protozoans

2. Cellular level of organization

• Multiple cells co-ordinate to work together.
• They are not able to form tissue.
• This is the simplest organization.
• Cells are loosely  aggregated.

Example : Porifera

3. Tissue level of organization

• Cell aggregates and form tissue of similar type.
• When tissue is formed, the division of labour is seen.

Example : Coelenterata ( Cnidaria)

4. Organ level of organization

• Tissue aggregates and form organ.
• Tissue join and they perform as a unit.

Example : Phylum Platyhelminthes

5. Organ system level of organization.

• All organ get jointed and they perform or work co - ordinately.

Example : Nemetehelminthes Onwards

Body Symmetry

It is the distribution of the body parts along with its axis.

The external form of animals can be described as asymmetrical or symmetrical.

Asymmetrical Animals

When body is divided by tiny plane through its central axis, does not divide them into two equal halves.

Example : Sponges

Symmetrical Animals

These are the animals whose body can be cut into two identical halves along one or more vertical planes, passing through the center.

They are of two types :

1.Radial symmetry

2.Bilateral symmetry

Radial Symmetry

Body is divided into two equal halves by any plane passing though central axis. 

Radial Symmetry of Starfish (Echinodermata)

Example : Coelenterata, Echinodermata, Ctenophora.

Larva of Echinodermata has Bilateral Symmetry

Bilateral Symmetry

The body can be divided into two identical left and right halves only in one plane. 

Bilateral Symmetry

In Mollusca, Snail lost its symmetry in Torsion ( Twisting During Growth )

What type of symmetry do Humans have?

Humans display the property of bilateral symmetry, meaning both sides of the body are virtually identical mirror images that allows duplication of key parts. This symmetry is characteristic of most vertebrate animals as well. It provides functional and structural advantages compared to radial or other types of symmetries.

What is Cephalization?

Cephalization refers to the evolutionary process where the nervous system and sensory organs of an organism become concentrated at the anterior or head end of the body.

In short, the formation of Brain is known as Cephalization.

Circulatory system

It is of two types:

1. Open Type
2. Closed Type

1. Open Type Circulatory system

In this type, blood is pumped by heart and cells and tissues are directly bathed in it.
Example: From Phylum Arthropoda to Hemichordata.

2. Closed Type Circulatory system

In this type, blood is circulated in to the closed or series of closed blood vessels varying in diameters ( Arteries → Veins → Capillaries ), with a pumping organ heart.
Example: Annelida, Class of Mollusca (Cephalopoda) and in Vertebrates.

Number of germ layer

Depending on the number of germ layers of cells in the gastrula, animals are grouped into two categories:

1. Diploblastic organisms
2. Triploblastic organisms

1. Diploblastic Organisms

These are the animals which have two germ layers - Ectoderm and Endoderm.

Example: Porifera, Cnidaria, and Ctenophora.

Mesoglea is a Undifferentiated space filling layer.

Diploblastic Organisms

2. Triploblastic Organisms

These are the animals which have three germ layer in their gastrula - Ectoderm, Endoderm, Mesoderm.

Example: Platyhelminthes onwards

Presence of absence of notochord

• Notochord - Chorda dorsalis
• It is a cartilaginous, tubular (Rod like) structure present between the dorsal nerve cord and the alimentary canal.
• Notochord is found in lower chordates.
• Vertebral column is found in higher chordates.
• Notochord is the part of endoskeleton.

Chordates are those who possess a notochord at some stages in their life time.

Non-chordates are those which do not possess the notochord e.g., All phyla from Porifera up to Hemichordata.

Segmentation 

It is the serial repetition of the body parts along the length of the animal.

In Earthworm, the body shows this pattern called Metameric Segmentation, and the phenomenon is called Metamerism.