THE MORPHOLOGY AND LIFE-HISTORY OF THE DIGENETIC TREMATODE, ASYMPHYLODORA AMNICOLAE N. SP.; THE POSSIBLE SIGNIFICANCE OF PROGENESIS FOR THE PHYLOGENY OF THE DIGENEA

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Abstract
The genus Asym-phylodora is reviewed, noting its morphological characters, constituent species, and the larval types that have been referred to its members. Progenetic metacercariae from Amnicola limosa and sexually mature worms from fresh-water fishes, taken in the region of Woods Hole, Massachusetts, are described as a new species, Asymphylodora amnicolae. Laboratory-reared snails have been infected by feeding eggs taken from progenetic metacercariae and successive stages in the life cycle are described and figured. The significance and meaning of progenesis in larvae of digenetic trematodes is discussed. It occurs in many genera, scattered in more than a dozen different families. Such distribution among widely separated groups suggests that progenesis may be a relict, the survival of an earlier developmental method. The attainment of maturity in longer-lived wider-ranging vertebrate hosts would have advantages for survival and distribution of the parasite and would gradually replace a cycle in which the mollusk was the original and perhaps the only host.