Halipegus eccentricus is a trematode parasite of North American frogs such as the bullfrog, Rana catesbeiana. Like many of the other trematodes we have met, H. eccentricus has a complex life cycle involving many different hosts. The first intermediate host is a snail of the genera Physa or Planorbella. The cercariae are then ingested by the second intermediate host, a small crustacean, such as a copepod or an ostracod. The metacercariae were then thought to be ingested by tadpoles where they waited for the amphibian to develop into a mature frog, at which point, the parasite would migrate to the frog's eustachian tubes (yes, these worms live in frog ears.) A recent study, however, showed that odonate insects (damselflies, dragonflies) serve as paratenic hosts for the trematodes and that only adult frogs are becoming infected. We are always learning more about parasites!
The image comes from the paper above and shows a redia of H. eccentricus, with the minute cercaria developing inside.