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.

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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.

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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.

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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.

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    $\begingroup$ Hi welcome to Bio.SE! This is a very well thought out and well written question that shows prior research effort on your part and provides useful pictures and specimen characteristics (including size and your location). These are all excellent qualities we hope to see in all of our questions! Thanks. Unfortunately, I find it unlikely that anyone here will know this answer. My hope is that my bounty (though little, given how unlikely I think a good response will be) will attract someone to dive in and do a little more research for a good answer. We'll see. Good luck! $\endgroup$ – theforestecologist Nov 30 '18 at 21:11
  • $\begingroup$ Thanks a lot! If no one here can provide an answer, I'm not sure where else to turn. Anything to narrow it down a bit would helpful. I've been trying to find any parasitic worms that feature these "legs" (or even what the proper term for them is if not that), and so far my search has been futile. $\endgroup$ – 560812508 Dec 1 '18 at 3:39
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    $\begingroup$ I would guess the pseudopod/protoleg looking structures and the mouth parts should be the most helpful in narrowing this down... $\endgroup$ – theforestecologist Dec 5 '18 at 22:04
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    $\begingroup$ The oral sucker and association with snails almost certainly makes this a trematode (aka fluke). The cercaria life stage usually has one or more tails, which I don't see on your specimen. However, I don't know of any adult flukes with the leg-like structures seen in your images (though this old drawing identified as Phyllodistomum americanum appears somewhat similar) $\endgroup$ – theforestecologist Mar 12 at 18:50

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