Bill, it did look like it could be blood worms, but at least the more mature blood worms seem to have a head with some 'feelers' and segments in their body. These would have been very 'miniature or immature' red worms, very smooth, featureless body and no defined head or tail section.
how they got inside the snail shell with the snail is beyond me...
This is a neat page showing a ghost shrimp eating a blood worm. I imagine these larvae don't stand much of a chance with shrimp, scuds, and fish of all types around waiting to snap them up.
see this:
Blood worm bye bye...
