doubt on mosfet symbols

[vamshi]

the arrow mark in the symbol of a transistor/jfet indicates the conventional

direction of hole current 

but why does mosfet have symbols opposite

i.e.,in  symbols for npn transistor and n channel fet why arrow diections are different

[Sukhbinder]

a N channel mosfet does not have any holes ( all FET and Mosfets are unipolar devices).