This is a Cercaria (species?) - a larval form of the Trematode worms. The cercariae as parasites can cause sickness also for humans (at least a dermatitis). You can see they have two suckers (one is not enough !).
Live is not only beautiful but it can be also cruel!
Obj. 10x and Obj. 20x DIC, flash
Images made through a microscope. All subject types.