  We derive the atomic hydrogen content for 1900 spirals in the fields of
eighteen nearby clusters. By comparing the HI deficiency distributions of the
galaxies inside and outside one Abell radius ($\ra$) of the center of each
region, we find that two thirds of the clusters in our sample show a dearth of
neutral gas in their interiors. Possible connections between the gaseous
deficiency and the characteristics of both the underlying galaxies and their
environment are investigated in order to gain insight into the cause of HI
depletion. In the clusters in which neutral gas deficiency is pronounced, we
see clear indications that the amount of depletion is related to the morphology
of the galaxies: early-type and, probably, dwarf spirals are more easily
emptied of gas than the intermediate Sbc-Sc types. Gas contents below one
tenth, and even one hundredth, of the expected value have been measured,
implying that gas removal must be very efficient. Our 21-cm data also show that
in the HI-deficient clusters the proportion of gas-poor spirals decreases
continuously towards the outskirts of these systems, the zone of significant
deficiency reaching as far out as $2\ra$. In an independent analysis of the
Virgo cluster, we find suggestive indications that gas losses are driven by the
interaction of the disks with the inner hot intracluster gas around M87. We
also report evidence that gas-poor spirals follow more radial orbits than those
of the gas-rich objects. We conclude that ISM-IGM hydrodynamic effects appear
as the most plausible cause of HI removal.
