Recently I encountered these tiny white snails clustered on a California Mussel (Mytilus californianus). They're called odostomes and appear to be Evalea tenuisculpta. Each one is less than 5 mm long.
I saw one snail approaching the egg mass above, and was surprised to see it turn and start to evert a very long proboscis (an elongated tubular appendage). It extended the proboscis between the two valves (shells) of the gaping mussel!
I could see fluid or particles moving inside the proboscis towards the snail, and thought perhaps it was stealing food from the mussel.
In turning the mussel slightly for a better view, I was puzzled to see the proboscis tip flat against the mantle (outer tissue) of the mussel.
Here's an even closer perspective. The orange is the mussel's tissue.
I was then further shocked to see something inside the proboscis stabbing the mussel, followed by distinct pumping action drawing fluids up into the proboscis.
After doing some research, I found this somewhat frightening description of a related species feeding on a worm:
"Then suddenly the sucker grips the epithelium of the filament, and the stylet may be seen to drive outwards so that it is clearly being used to perforate the body of the worm. Vigorous pumping movements of the buccal apparatus may then be seen and it is obvious that fluid, blood, and perhaps, cells loosened from the worm are being sucked into the gut of the pyramidellid." (Fretter & Graham 1949)
These small snails are ectoparasites (parasites living on the outside of another animal) that practice suctorial feeding!
Here's a nice illustration of yet another species feeding on blue mussels:
(From Advances in Marine Biology 1967, redrawn after Fretter & Graham 1962)
Another unusual feature of these snails is the shape of their tentacles. They are broad and flap-like, with a concave surface, and can be adjusted to face in different directions. In combination with the very long trunk-like proboscis, they give this diminutive snail a very elephant-like appearance.
Who knew there were tiny elephants in our local mussel beds?