THE EFFECT OF DILUTION OF SEA WATER OX THE ACTIVITY AXD LONGEVITY OF CERTAIN .MARINE CERCARLE, WITH DESCRIPTIONS OF TUX) NEW SPECIES II. \V. STUNKARD AND C. RUTH SHAW (I-row the Biological Laboratory, New York University, and the Marine Biological Laboratory, Woods Hole, Mass.) INTRODUCTION The present investigation was undertaken to secure data bearing on the question of the origin, distribution, and evolution of present groups of digenetic trematodes. The specific problem under consideration de-velops from the observation that several families of the digenetic trema-todes have representatives in both marine and fresh-water hosts. A brief statement of the problem was given by Stunkard (1930). If the trematodes found in both marine and fresh-water hosts and assigned to common families, and even to common genera, have true phylogenetic relationships; i.e., if they have descended from common ancestors, rather than consisting of groups that through convergence show mor-phological and developmental similarities, their distribution raises an exceedingly difficult biological problem. There appears to be little doubt but that the parasites in question are actually closely related. Among the gasterostomes, Bucephalus poly-tnorphns von Baer, 1826 was described from fresh-water fishes and Bucephalus haimeanus Lacaze-Duthiers, 1854 was described from marine fishes. Tennent (1906) traced the life cycle of the latter species, and the recent studies of Woodhead (1929, 1930) have demonstrated the development of two species that occur in fresh-water hosts. The similarity in structure and development between the marine and fresh-water species is so striking that it strongly indicates close relationship. In the Prosostomata there are a number of families whose members occur in both marine and fresh-water hosts. The family Aspido-gastricke, e.g. (see account by Stunkard, 1917), contains species that infest mollusks, fishes and turtles of frrsh water, and others that occur abundantly in marine fishes. In this aberrant family, also, the mor-phological and developmental agreement is too close to be satisfactorily explained on the basis of convergence. Several other families of the Digenea manifest the same type of 242
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