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 coordinated 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 coordinated 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.