CHAPTER II 



TREMATODA 



CHARACTERS OF TREMATODES HABITS AND STRUCTURE OF TREMA- 



TODA ECTOPARASITICA (MONOGENEA) LIFE-HISTORIES OF 



POLYSTOMUM INTEGERRIMUM, DIPLOZOON PARADOX UM, AND 



GYRODACTYLUS ELEGANS TREMATODA ENDOPARASITICA 



(DIGENEA) OCCURRENCE AND HABITS OF DIGENEA LIFE- 

 HISTORY OF DISTOMUM MACROSTOMUM DISTOMUM HEPATI- 



CUM AND ITS EFFECTS BILHARZIA HAEMATOBIA BISEXUAL 



TREMATODES TABLE OF HOSTS CLASSIFICATION. 



From the Turbellaria we now pass on to a consideration of the 

 second great subdivision of the Platyhelminthes, the Trematodes 

 or " flukes," of which the " liver-fluke " is the best known, since 

 it is one of the most dangerous parasites that infest domestic 

 animals. 



It has been pointed out that the Polyclads, Triclads, and Ehab- 

 docoels are carnivorous, and that in each of these groups sporadic 

 cases of parasitism occur. In other words, when the prey is 

 much larger than the Turbellarian, the latter tends to become 

 a parasite, and we can trace the development of the parasitic 

 habit from the gradual association of Turbellaria with Ascidians, 

 Crustacea, Molluscs, and Polyzoa merely for protective purposes, 

 through the adoption, not only of the body of the host for shelter, 

 but of its flesh for food ; though it is only in some Ehabdocoels 

 (Graffilla, etc.) that there exists a degeneration corresponding to 

 the easier mode of nutrition and simpler life. The Trematodes, 1 

 however, are wholly parasitic, either on the outer surface, the 

 gills, or internal organs of their host, which is almost always a 

 1 Compare the remarks on Trematodes, pp. 4-5. 



