Cellularity

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.