A number of modern and fossil sponges made their homes inside the shells of other organisms. Rather than taking an empty gastropod (snail) shell like a hermit crab, these sponges literally move into a shell. Clionid sponges use a combination of physical and chemical abrasion to create openings in shells. They then spread out throughout the internal structure of the shell to create an extended home for themselves. I often imagined these sponges waiting until an organism dies to create the network of galleries, but it seems this isn't always the case. A detailed study published in 2005 by Stefaniak et al. in the Journal of Experimental Marine Biology and Ecology examined the effects of clionid borings on living Littorina snails. They found that the snails reacted to the invading sponges by thickening the interior lining of their shell (thus reducing their living space). In addition, the shells were much weaker structurally after the sponges bored out their galleries.
To get an idea of what these sponges do, here's a pectinid bivalve that has been extensively bored by clionid sponges. Since these borings occur on both the interior and the exterior of the valve, this pectinid was likely dead prior to the borings (at least on the inside).
While this may not seem particularly damaging at a glance, a look at the cross-section of another bored shell shows what is really going on. Much of the structural integrity has been worn away by the sponges relentless chemical and physical attack. On the plus side, though, the sponges get a safe home in a nice hard substrate.