Ventral masts- what are they?
Normally, those masts are located on the upper fuselage and sometimes on the wings. They are used to string antennas between them and other airplane parts, or are antennas themselves.
In case of the Boulton-Paul Defiant, those antenna masts could not be located in the upper hemisphere in order to keep the field of fire of its gun turret free. Together with the Blackburn Roc and the Hawker Hotspur, those airplanes had a unique layout for a fighter with a single pilot and a gun turret placed behind the cockpit. How well this idea turned out in practice can be read in this answer.
In case of the Defiant, those antenna masts were made retractable to allow take-off and landing. The antennas were needed for communication and navigation.

The different antennas on a Grumman TBF Avenger (picture source).