reservoir feeds the engulfed DGs. In another object, He 2-10, the direct collision with an intergalactic gas cloud [@kob95] is obviously triggering a huge SB. Reasonably, for their measurable surface density the SFRs of most of these objects exceed those of ”normal” gas disks (Fig.\[fig:KS\]).

Yet it is not clear, what happens to dIrrs if they move into a region with increasing external pressure as e.g. by means of a denser IGM and of ram pressure when they fall into galaxy clusters. In sect.\[dEs\] we will discuss the effect of ram pressure on the structure of the ISM for which numerical models for spiral galaxies (e.g. [@roe05]) as well as for gas-rich DGs (e.g. [@mor00]) exist, but only hints from observations (as e.g. for the Magellanic stream). The effect on the SFR due to compression of the ISM is observed, but not yet fully understood and convincingly studied by models. [@cor06] e.g. observed a coherent enhancement of SF in group galaxies falling into a cluster.

![ The abundance ratio N/O as a function of oxygen abundance observed in spiral and irregular galaxies (shaded area, after [@zee98a]) overlayed with evolutionary loops due to infall of primordial intergalactic gas clouds. These have 