how in multicellular organism, various body parts have specialised in the functions they perform?
The multicellular organisms are made up of different cells that perform specialised functions. All muticellular organisms posses type of cells called stem cells. For example, stem cells in embryonic stage in humans, meristematic cells in plants. These cells have the ability to divide and produce new types of cells with ability to perform specilaised functions.
Now when a group of cells perform a similar function, they form tissue like epithelial tissue, connective tisssue etc present in humans.
The tissues that waok togather are organised into organs. For example, heart, kidney etc.
Organs are further organised into organ systems which carry the whole functions. For example, respiratory, digestive system etc.
This way various body parts have specialised in multicellular organisms to perform different functions.