CLASSIFICATION OF DIGENEANS

The life cycles of the great majority of digeneans display a remarkable and highly characteristic alternation of asexual and sexual reproductive phases.... —P. J. Whitfield" he digenetic trematodes, or flukes, are among the most common and abundant of parasitic worms, second only to nematodes in their distribution. They are parasites of all classes of vertebrates, especially marine fishes, and nearly every organ of the vertebrate body can be parasitized by some kind of trematode, as adult or juvenile. Digenean de-velopment occurs in at least two hosts. The first is a mollusc or, very rarely, an annelid. Many species include a second and even a third intermediate host in their life cycles. Several species cause economic losses to society through infections of domestic animals, and others are medically important par-asites of humans. Because of their importance, the Digenea have stimulated vast amounts of research, and the literature on the group is immense. This chapter will summarize the morphology and biology of the group, illustrating them with some of the more important species. Trematode development will be considered in detail later (p. 217), but a "typical " life cycle is as follows: A ciliated, free-swimming larva, the miracidium, hatches from its shell and penetrates the first intermediate host, usually a snail. At the time of penetration or soon after, the larva discards its ciliated epithelium and metamorphoses into a rather simple, saclike form, the sporocyst. Within the sporocyst a number of embryos develop asexually to become rediae. The redia is somewhat more differentiated than the sporocyst, possessing, for example, a pharynx and a gut, neither of which are present in a miracidium or sporocyst. Additional embryos develop within the redia, and these become cer-cariae. The cercaria emerges from the snail. It usually has a tail to aid in swimming. Although many species require further development as metacercariae before they are infective to the definitive host, the cercariae are properly considered juveniles; they have organs that will develop into the adult digestive tract and suckers, and genital primordia are often present. The fully developed, encysted metacercaria is infective to the definitive host and develops there into the adult trematode