BY S. J. JOHNSTON. 



287 



itely separating D. i-latngeriini, D. medians and D. confusura. 

 This was followed several years later by those two magnificent 

 works of Looss, the "Versuch"(50), and the "Trematodes of 

 Turtles, etc. "(55), and with these a new era in the systematic 

 history of the Malacocotylea began. 



Up to this time, the modern views as to what constituted 

 specific and generic, and even subfamily and family characters of 

 these worms, had scarcely begun to make their appearance, and 

 the detailed classification of the group was in a very unsatisfac- 

 tory state. A very comprehensive account of the Trematodes, in 

 regard to anatomy and life-history, covering practically all that 

 was known of them up to that time, had been given, indeed, by 

 Braun in his admirable and monumental volume on this Class in 

 Bronn's "Classen u. Ordnungen, etc. "(4), but the dawning of 

 our modern conceptions regarding the definition of the species 

 and genera was not foreshadowed in this work. The genus Dis- 

 tomum Retz., comprised at this time what are now looked upon 

 as a number of separate families, including a very large number 

 of genera. Many of the old species {e.g., D. variegatutn and D. 

 cygnoides) have since been shown to have really comprised a 

 number of species and in some cases even several genera. Owing 

 to the real specific difterences and the limitations of the natural 

 genera of these worms not having been recognised, a natural 

 classification of the distomes had not been able to be propounded. 

 Looss points out(55, p. 809) as a result of this want of recognition 

 of the real specific characters " . . . . dass verschiedene Species, 

 die moistens derselben natiirlichen Gattung angehoren, friiher in 

 eine einzige zusammengeworfen, und dass die betreff'enden natiir- 

 lichen Gattungen damit auf das Niveau bloser Species herab- 

 oedriickt worden sind." 



When, trematodes from frogs, for instance, were first recorded 

 from America, they were referred to the European species to 

 which they appeared to be most closely related, e.g. Wright(102). 

 Later, some became distinguished as special varieties, e.^.Bensley(3) 

 and later still, when the meaning of the small but constant differ- 

 ences in structure which they exhibited, came to be realised, they 

 were described as independent species. 



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