Body consists of single cell. The cells are large in size due to presence of numberr of cell organelles. Division of labour is at the organelle level. It gives a low level of operational efficiency. The cell size ranges from 11um to 1mm. Any injury or damage of cell may cause death of organism. Growth is carried out as a result of enlargement of cell.
A single cell carries out all the life processes. The cell body is exposed to the environment on all sides. Lifespan is short due to heavy load of work. Power of division is not lost. A well-marked capacity of regeneration is present.The cell has the same role for itself and the organism.