The procercoid stage develops within the hemocoele of the first intermediate host, in this case a copepod. This photo is of a female copepod about one month following exposure to coracidia. The copepod was fixed, cleared and stained. The ovary of the copepod has stained red and can be seen on the midline of the copepod's body, at the far right of the body.
The large, red, oval structures anterior to the ovary is a procercoid (arrow). It is large relative to the size of the hemocoele of the copepod, and can contribute to host mortality. The anterior end (top of photo) of this infective procercoid has an invagination. It is not the start of the future scolex, but is a site of glandular activity. When the copepod is eaten by a second intermediate host fish these glands will help the procercoid penetrate through its intestinal wall and begin a migration through the musculature.