Abstract
Microscopic heterophyid trematode parasitic worms of the Genus Ascocotyle infect certain amnicolid and hydrobiid snails and certain cyprinodont and poeciliid estuarine fishes as first and second intermediate hosts. Adult trematode worms are found to mature in the intestines of particular definitive hosts, most often piscivorous birds, but also certain mammals. A survey of these parasites, harvested from fish hearts and gills collected in Mississippi, Texas and California, shows that they are are obligated to complex life cycles requiring at least three disparate and different hosts to achieve fecundity. Methods of infection, host infection site and host specificity are often unique to each different species of these parasites. Additionally, Ascocotyle worms demonstrate highly specialized structures such as HCl resistant cysts, HCl sensitive penetration glands and sensory organs which may serve to guide them to the specific infection site. These heteroecious life cycles and specialized structures are shown to be too complex to have developed by chance, therefore, evolutionary mechanisms appear insufficient to explain them. A creationist design argument for the presence of such parasites is promulgated.