Why is an ammeter connected in series in a circuit?
An ammeter can only read the current that goes through it ! Thus, it has to be connected in series with the part of the circuit where the current has to be measured, because this will ensure that this current flows through it.
Of course, connecting the ammeter disturbs the circuit by changing its resistance. So, a good ammeter must have a very low resistance - thus ensuring that connecting it does not affect the value to be measured too much.