90 ILLINOIS BIOLOGICAL MONOGRAPHS pO 



Tetracotyle pipientis (Fig. 47), where these grooves remain rudimentary, no 

 I ateral lappets develop. 



As von Linstow (1877:189) pointed out, species characters in Strigea 

 (Holostomum) have been treated very superficially, since the group members 

 are not readily distinguished by external markings and the internal anatomy 

 is difficult to interpret. Yet the writer has found that even in the early 

 larva the points of differentiation are well marked. The parthenogenetic 

 egg of the holostome developing into the cercaria is at the time of matura- 

 tion structurally different from the ova developing into a redia. Thus the 

 actual phylogenetic history of the group is hidden by its precocity and the 

 developmental stages of the holostome show only in telescopic fashion the 

 actual ancestral history. 



Extraordinary nerve modification in the holostomes (Fig. 53) is related 

 directly to the modification of the muscle complex. This modification con- 

 sists usually in the degeneration of the posterior dorsales and laterales, and 

 a relative increase in size and importance of the posterior ventrales. The 

 other systems of the holostomes, especially the genital and excretory organs, 

 are equally highly modified and equally well formed in the larva. Ssinitzin 

 (1910, 1911) has suggested that the change in the genital pore from the pre- 

 acetabular position to the posterior ventral extremity has come about thru 

 the formation of a new opening rather than thru a shifting of the old pre- 

 acetabular pore. The original pore is still present in the larvae, altho in most 

 cases there is no clue to its former connection or function. The extent of 

 these changes indicates a long period of gradual adjustment to a modifying 

 environment. 



The study of the cercariae of the various groups of the Digenea not only 

 serves to supplement relationship studies in adult hermaphroditic generations 

 of trematodes, but also brings out structural relationships very considerably 

 if not entirely hidden in the adult. The most constant of all the systems in 

 the group Digenea is the nervous system. Any marked modification from 

 the characteristic distomate type is indicative of a considerable period of 

 divergent growth. 



The fundamental systems of the hermaphroditic generation of the tre- 

 matode are deep seated; they are well formed in the cercaria, and little signi- 

 ficant differentiation takes place during metamorphosis. 



RELATION OF TREMATODES TO OTHER GROUPS 



Among the early systematists Trematoda were classified with the Hiru- 

 dinea because of the common superficial resemblance of the two groups. 

 Even as late as 1871 Schmarda separated the Trematoda from the Turbellaria 

 and Cestoda, and placed them with the Hirudinea in the Cotylidea. Balfour 

 (1881:316, 317) considered a direct relationship of all Metazoa above Coel- 

 enterata entirely unsatisfactory and conceived the idea of referring them 

 all back to the trochophore larva which possessed radial symmetry. In his 



