What is this symbol?
A1 and A2 are potentiometers.  (This diagram uses the European notation.)

This diagram could have been drawn in a general-purpose (not specialized for electrical engineering) diagramming tool such as MS Visio. It was not drawn in Altium, or Eagle, or OrCAD.

update:


  why would there be potentiometers in such a circuit ??? :(


Potentiometers are there to make adjustments.  A1 acts as a adjustable voltage divider (together with R16); it scales the output of OpAmp III.  A2 acts as an adjustable voltage divider; it scales the output of the OpAmp IV.  

[As an aside, I wouldn't design this circuit the way they did.  I would use instrumentation amplifiers (InAmp). I wold guess that the original designers had a goal that the circuit could be made with components that are cheap and available the world over.]