ANIMAL KINGDOM

Basis of classification

ANIMALS ARE CLASSIFIED BASED ON THE FOLLOWING COMMON FEATURES

• Arrangement of cells
• Body symetry
• Nature of coelom
• Digestive system
• Circulatory system
• Reproductive system

Most animalia are multicellular and hence different levels of cellular organisation can be used as the basis of classification

ON BASIS OF ORGANISATION OF CELLS ANIMALIA ARE CLASSIFIED INTO FOUR LEVELS

• Cellular level
• Tissue level
• Organ level
• Organ system level

CELLULAR LEVEL OF ORGANISATION

• Cells are loosely arranged
• Different activities are done by each cells
• Example: sponges

TISSUE LEVEL OF ORGANISATION

• Arrangement of cells is complex
• Cells performing same functions are arranged into tissues
• Example: Coelenterates

ORGAN LEVEL OF ORGANISATION

• Tissues are grouped to form organs
• These tissues perform specialised functions
• Example: Platyhelminthes

ORGAN SYSTEM LEVEL OF ORGANISATION

• Organs are associated to form functional systems.
• Each system is concerned with specific function.
• Examples:Annelids, Arthropods, Molluscs, Echinoderms, Chordates.

Digestive and circulatory systems are organised at organ system level.
But, digestive systems can also be of two types.

TYPES OF DIGESTIVE SYSTEMS

• Incomplete digestive system: single opening acts as mouth and anus, such as in Platyhelminthes.
• Complete digestive system: has two distinct openings for mouth and anus.

Similarly, circulatory system is also of two types.

TYPES OF CIRCULATORY SYSTEMS

• Open type: blood is pumped out of the heart and the cells and tissues are bathed in it.
• Closed type: blood is circulated through a series of vessels of varying diameters.

Symmetry

RADIAL SYMMETRY

RADIAL SYMMETRY

• Any plane passing through the central body axis divides it into two equal halves
• Example:Coelenterates, Ctenophores, Echinoderms

BILATERAL SYMMETRY

BILATERAL SYMMETRY

• The body can be divided into identical left and right halves in only one plane.
• Example: Annelids, Arthropods.

ASYMMETRICAL

• Any plane which passes through the Center does not divide it into two equal halves.
• Example: sponges

Germinal layers

DIPLOBLASTIC ORGANISATION

DIPLOBLASTIC

• Cells are arranged into two embryonic layers
• The two layers are: ectoderm, endoderm
• Endoderm is internal where as ectoderm is external
• An undifferentiated layer called mesoglea is present in between endoderm and the extoderm

TRIPLOBLASTIC ORGANISATION

TRIPLOBLASTIC

• In triploblastic organisms, cells are arranged into three layers in the developing embryo
• The three layers are: ectoderm, endoderm and mesoderm
• Example: Platyhelminthes

COELOM

• Coelom is the body cavity which is lined by mesoderm
• Presence or absence of coelom also plays a major role in animal classification
• Animals in which body cavity is absent are called acoelomates

In some animals, the body cavity is not lined by mesoderm,instead,the mesoderm is present as scattered pouches in between endoderm and ectoderm.
This type of coelom is known as pseuo coelom

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Classification of animals

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