Hitch-hiker

The rain had stopped by the time I reached the allotment, but everything was very, very wet. Too wet to cut any grass, so I sowed some seeds and did some weeding. The spuds are coming along nicely. When I was tying on a few more strings for the peas to straggle up I disturbed a snail who was roosting on one of the sticks - when it started to move I noticed that it had another, smaller snail attached to its shell. Friend, relative, or opportunistic hitch-hiker?