738 ON SOMIC TREMATODI;; PARASITES, 



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

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

 tomin.Te and Psilostominje. They should be looked upon, I think, as 

 members of a sub-family MehlisiinjB, with, for the present, the 

 characters of the genus, intermediate in position between those 

 groups. They differ from the Fasciolinge 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 Cotylotretus rugosus Odhn., (H), and 

 Mesaulus grundis R. (f)). 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 Psilostoraina? parasitic in birds and 

 reptiles. 



LITERATURE. 



1. Braun,M. — " Ueber Clinos/omum Leidy." Zool. Anz., xxii., 1S99. 

 2. " Zur Kenntnissder Tieniatoden der Saugetliiere." Zool 



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

 3. " Z\ir Verstandiguiii; iiher die (iiiltigkeib eiiiiger Nanieii 



von Fascioliden-Gattungeii."' Zool. Anz., xxiv., 1901. 

 4. "Die Arten der (iattuiig GLinostomnm Leid}'." Zool. 



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

 5. — " Fascioliden der Vogel." Zool. Jahrb. (Syst.), xvi., 1902. 



6. CoHN, L. — "Zwei neue Distonien," Central!). Bakt., xxxii., 1902 



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



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



8. Looss. — " Weitere Reitrage zur Kenntniss der 'rrernatoden-Famia 



^Egyptens, etc." Zool. Jahrb.(Syst.), xii., 899. 



