There is a division of labour …

ANSWER
Division of labour is differentiation of certain components or parts to perform different functions for increased efficiency and higher survival. Multicellular organisms often possess millions of cells. All the cells are not similar. The cell present on the surface is dead and impermeable to protect the internal cells from the harsh external environment. They also form structures for offence and defence of the organism.
Every cell of a multicellular organism cannot obtain food from outside. The organism requires a system for obtaining requires a system for obtaining food. Its digestion and distribution. Therefore, a digestive system and system of transport are also required.
Certain cells of the body take over the function of reproduction. Others take part in repair and replacement of worn out or injured portions.
For optimum functioning of cells, a multicellular organism also comes to have an internal favourable environment.