I collected some bladder snails several months ago. While checking them out one day, I noticed that they were now infested with external, parasitic worms.
Here's a picture of a snail with a bunch of the worms hanging off of it's head. I've also circled one of the worms crawling along the glass.
I sampled some of the worms and took a look at them under a microscope. Here's a video I recorded of one of them crawling around. They have some notable characteristics.
First, they appear to have some sort of sucker mouth.
Second, they have some interesting "legs" tipped with hairs (?) at the ends of their tails. Here's a short video of a worm writhing around that shows them off a little better than pictures do. They appear to be in pairs of two.
I have witnessed the snails shedding tons of cercariae into the water, which eventually disappear after a couple of hours (reinfect the snails?). I'm not sure if those are related to this worm, or if the snails are also infected with some other type of fluke.
After some research, my best guess is that they're trematodes from the class Digenea-- They appear have sucker mouths, can parasitize snails exclusively, can have a free-swimming form, and can shed cercariae. That said, I'm not terribly confident, and would ideally like to be able to identify them more specifically.
These snails were gathered in Canada, in a still-water area connected to a river. The river is accessible by cows and deer. The length of the worms ranges from 1-5 mm.
In the end, I would like to eliminate these worms if possible (I can't gather more snails any time soon, and they eventually kill off smaller snails). My first attempt was dosing with praziquantel, which is supposedly effective against flukes while being snail-friendly. Unfortunately, three days later, one of the snails died, and the worms appear to be unharmed.