Prior to departure for a nearby aerodrome the pilot decided to carry out some practice night circuits. He subsequently advised that all necessary checks were completed for the first landing, however the aircraft landed with the gear retracted. When the aircraft came to rest the pilot noted that the gear hydraulic pump motor was still operating. It was determined that the aircraft had touched down on the gear doors, which were open at the time. Although the main gear doors had completed their extension cycle, a defect in the gear control valve spool prevented the gear itself from extending. Contamination of the hydraulic system was also evident, probably resulting from ineffective filtering of the fluid. The pilot had evidently not checked the gear position indicator lights prior to landing.