Ammeter is used to measure electric current in a circuit, so it is connected in series as the internal resistance of ammeter is very low(zero) so if it is connected in parallel the it will draw maximum current and will get damaged. so it is connected in series.
Voltmeter is used to measure electrical potential difference between two points in an electric circuit, internal resistance of voltmeter is very high(infinite)therefore it is connected in parallel. If voltmeter is connected in series it would increase the equivalent resistance of circuit so no current will flow through the circuit. that's why it is connected in parallel so that it won't disturb the equivalent resistance and flow of current of the circuit.