Mode of classification:

Here,individuals are classified on the basis of:

1) Arrangement of cells (level of organization):  It is of three types,

• Cellular level→ In which cells are arranged as loose aggregates and cells perform all functions.
• Tissue level→ In which cells performing the same function are arranged as tissues, more advanced than cellular level.
•  Organ system→ Tissues are grouped together to form organs, to perform a specific function.

2) Symmetry: It is of three types,

• Asymmetrical→ Which cannot be divided into equal halves through any plane.
• Radial symmetry→ Which can be divided into equal halves, when cut through any plane.
•  Bilateral symmetry→ Which can be divided into two equal halves, only through one plane.

3) Arrangement of cells in two embryonic layers:  It is divided into two types, 

• Diploblastic→ They have two layers, outer ectoderm, inner endoderm and an undifferentiated middle layer called mesoglia is also present between them. They have total two layers.
• Triploblastic→ Here, middle layer is fully differentiated and now called mesoderm. They have total three layers.

4) Coelom: Coelom means a body cavity which is lined by mesoderm layer.  It is divided into three types,

• Coelomates→ That have body cavity lining the mesoderm.
•  Acoelomates→ They don’t have any body cavity called coelom.
• Psuedo-coelomates→ Here, body cavity is present in the form of scattered pouches between endoderm and ectoderm.

5) Segmentation: Here, the body has segments in at least some organs. The phenomenon of segmentation is called metamerism.

 

6) Notochord: It is a rod like structure, developed at back side(dorsal). It is of two types,

• Chordates→ They have notochord.
•  Non-chordates→ They don’t have such structure.