Defination: Removal of waste or harmful substances from the body is called excretion.
Many harmful and waste substances like urea, uric acid, ammonia, etc. are produced in living organisms.
If these substances accumulate in the body or are retained in the body for long, it can lead to serious harm or even death.
It is very necessary to remove harmful and waste substances from the body.
In unicellular organisms these waste materials are directly eliminated across the cell surface.