Today's parasite - Genarchella astyanactis - belongs to family of digenean trematodes called the Derogenidae, which have evolved a unique way of infecting their second intermediate hosts. They use copepods and other small crustaceans as second intermediate hosts, and their strangely-shaped cercariae are propelled from the snail first intermediate host by a forked appendage. The cercariae are also armed with a coiled structure call a "delivery tube" (shown extended in the drawing) that they use to gain access to the copepod host. For some reason, copepods identify the strange cercaria as a food item and avidly seize any that they encounter. However, as the crustacean manipulates the parasite with its mouth parts, the delivery tube of the cercaria rapidly extends and penetrates the copepod's gut wall. At the same time the cercaria body is propelled through the delivery tube into the body cavity. Imagine eating a muffin which stabs the back of your throat the moment that you bite it, and then injects a squirming parasite into your body!
Ditrich, O., Scholz, T., Aguirre-Macedo, L. and Vargas-Vázquez, J. 1997. Larval stages of trematodes from freshwater mollusc of the Yucatan Peninsula, Mexico. Folia Parasitologica 44:109-127.
Contributed by Tommy Leung.