2 WRIGHT AND MACALLUM. [Vol. I. 



that the two latter forms are genealogically connected, which 

 is further strengthened by Zeller's ^ account of a two-suckered 

 (or Sphyranura) stage of the developing Polystomum. 



It was further urged that, when the hosts of the forms men- 

 tioned are taken into account, a certain parallelism between the 

 phylogenv of both is apparent, for Necturus occupies a half- 

 way position between the fishes and the frogs, and the thought 

 lies upon the surface that both the frog and Necturus have 

 inherited their parasites from fish-like ancestors, the gills of 

 which were infested by Gyrodactylus-like forms. Such a direct 

 transmission of parasites from one generation of the host to 

 another is less likely to have been disturbed in the case of mono- 

 genetic than of digenetic Trematodes, for the development of 

 the latter provides for a wider dispersion of the species than 

 does that of the former. 



When the description of Sphyranura was originally published. 

 Van Beneden's ^ arrangement of the families of digenetic Trema- 

 todes was accepted as the best expression of their relationship. 

 He recognized five families, — Tristomida, Polystomida, Octo- 

 cotylida, Udonellida, and Gyrodactylida. The second of these 

 was formed for the reception of the genera Polystomum, Oncho- 

 cotyle, and Erpocotyle ; while in the fifth were included Gyro- 

 dactylus and Dactylogyrus, Calccostoma, Tetraonchus, and 

 Diplectanum. The existence of a form, however, intermediate 

 between Gyrodactylus and Polystomum, as well as the develop- 

 ment of the latter, seemed to point to a closer relationship 

 between these two families, and it was suggested that both 

 might be included in a family Gyrodactylida, Gyrodactylus 

 being obviously nearest the ancestral form. 



In the same year (1879) Taschenberg-' suggested a somewhat 

 different grouping of the monogenetic Trematodes. He recog- 

 nizes two families, Tristomcae (incl. Tristomidae, Monocotylidae, 

 Udonellidse) and Polystomeae (incl. Octobothriidae, Polysto- 

 midae, Microcotylidae, Gyrodactylidae), the sub-families, Poly- 

 stomidae and Gyrodactylidae being coextensive with Van Bene- 

 den's similarly named Families. If this grouping be accepted 

 (which may be done provisionally until a more complete knowl- 



'Zeit. wiss. Zodl., XXII., Tab. I, Fig. 7. 



' Recherches sur les Trematodes marins, p. 64. Van Beneden & Hesse. 



'Zeitsch. fiir die gesammt. Naturwiss., LIL, p. 235. 



