738 ON SOMIC TREMATODE PARASITES, 



Relationship. — These two species seem to me to be related, on 

 the one hand to the Fasciolinae, and, on the other, to the Echinos- 

 tominae and Psilostominae. They should be looked upon, I think, as 

 members of a sub-family Mehlisiina?, with, for the present, the 

 characters of the genus, intermediate in position between those 

 groups. They differ from the Fasciolinae in their form and size, in 

 the simpler form of the gonads and intestinal limbs, for the most 

 part, and especially in the character of their excretory apparatus ; 

 while they differ from the Psilostominae in the absence of a reeep- 

 taculum seminis, the very large size of the gland of Mehlis, the 

 larger and more complex form of the testes, in the form of the 

 excretory system in some respects, and in the more extended dis- 

 position of the yolk-glands. They find their nearest relatives, per- 

 haps, in such forms as CoUjlotretus rugosus Odhn., (14), and 

 Mesaulus grandis R. (5). In determining the phylogenetic relation- 

 ships of Trematodes, it should be borne in mind that they were 

 parasitic in the early ancestors of the vertebrates, and, owing to 

 the conditions under which their lives are passed, have been much 

 less subjected to evolutionary changes in form than their hosts, 

 so that we should rather expect to find the nearest relatives of 

 trematode parasites of primitive mammals amongst such groups as 

 the Echinostominae and Psilostominae parasitic in birds and 

 reptiles. 



LITERATURE. 



1. Braun, M. — " Ueber Clinostomum Leirly." Zool. Aiiz., xxii., 1899. 

 2. " Zur Keniitnissder Tiematoden der Siiiigetliiere." Zool. 



Jahil). (Syst.), xiv., 1901. 

 3 "Zur Verstiiiuligung iiher die (liiltigkeit einiger Nanieii 



von Fasciolideii-Gatlungen.'' Zool. Atiz., xxiv., 1901. 

 4, "Die Arten der Gattung Clinostomum Leid^'." Zool. 



Jahrb.(Syst.), xiv., 1901. 

 5. " Fascioliden der Vcigel." Zool. Jalirb.(Sy.st.), xvi., 190'2. 



6. CoHN, L. — " Zwei iieue Distomeii." Central!). Bakt., xxxii., 19U2 



7. Johnston, S. J. — "On a new Species of DisLomum from the Platypus." 



Proc. Linn. Soc. N. S. Wales, 1901. 



8. Looss. — " Weitere Beitr.ige 7,nr Kenntniss der Treinatodeii-Faiina 



/Egyptens, etc." Zool. Jahrb.(.SysL.), xii., S99. 



