Cell:- It is a basic structural and functions unit of life. Based on the no. of cell, organisms have been categorized as:

  • Unicellular: Organisms whose body is made up of a single cell. E.g. amoeba, paramecium etc.
  • Multicellular:  Body is made up of multi (more than one) cells.

Body Grade (based on number and co-ordination of cells):

a)      Protoplasmic Grade:

  • Animal having single cell
  • Also have cellular grade
  • E.g. protozoans

b)      Cellular Grade:

  • Body having cellular level of organization
  • Cells are not co-ordinated to form tissue i.e. each cell perform a special function though the body has multi cells. E.g. Porifera

Tissue Grade:

Many cells are co-ordinated to perform a designated/ specific function. E.g. nerve fibres of coelenterates

Organ-System Grade:

  Different tissues combine to form organs. Different organs combine to form a system to perform a specific function. All the animals from phylum Platyhelminthes to phylum chordate exhibit organ-system grade of organization. In some systems are incomplete/absent.

Body coelom/Body cavity:

Coelom is a cavity which lies between body wall (part of alimentary canal; intestine)

Coelomates: Organisms having coelom in their body.

Acoelomates: Organisms not having coelom in their body. E.g. protozoans, poriferans, coelenterates etc.

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