This is a picture I took of the brain of a common East Coast killifish, the mummichog (Fundulus heteroclitus). The red arrows point to metacercariae, which are a stage of trematode found in intermediate hosts. I believe that this parasite is Diplostomum nassa. This trematode will only be transmitted to its definitive host (i.e., the host in which the parasite can sexually reproduce) if the mummichog gets eaten by this host. This is called “trophic transmission”.