Many slug species tend to aggregate when they are resting. That behavior is called huddling. Here is a note I had in issue No. 15 of Tentacle that was about a specimen of the native U.S. slug Philomycus carolinianus that was huddling with an introduced Arion subfuscus.
I have many rocks in my backyard and almost every large rock has one or more resident slugs underneath it. Even more numerous in the same places are isopods, which appear to be more sensitive to dehydration than are slugs. On many occasions I have seen isopods clustered around resting slugs. This morning I was able to photograph 2 such clusters under one rock.
The smaller slug was probably a juvenile Limax maximus, while the larger one in the picture below is probably an Arion subfuscus.
If we define huddling loosely as the clustering of animals for mutual protection against environmental extremes, then what we have here would be an example of interphylar huddling. However, the isopods are probably benefiting much more from huddling against a slug than is the slug from having isopods lined up against its body.