TREMATODA. 775 



reference to the well-known disease in sheep termed rot. This 

 disorder is unquestionably due to the presence of the common 

 liver fluke {Distoma hepatimm), a parasite that is seldom more 

 than an inch in length. Other animals than sheep are liable to 

 be infested by it, but, except in the case of cattle, hares, and 

 rabbits, it only very rarely occasions severe disease. In about a 

 score of instances this entozoon has been detected in the human 

 body. It sometimes also infests the horse. 



For many years past investigations have been conducted with 

 tlie special purpose of ascertaining the manner in which cattle and 

 sheep infest themselves with this parasite ; and although we 

 have not succeeded in tracing out all the stages of growth of the 

 common fluke itself, researches amongst the trematodes generally 

 have enabled observers to arrive at conclusions of the highest 

 practical importance. Intelligent cattle-breeders and agricul- 

 turists have all along observed that the rot was particularly 

 virulent after long-continued wet weather, and more especially 

 so when there had been a succession of such seasons; and 

 further, that the flocks grazing in low pastures and marshy 

 districts were much more liable to invasion than those which 

 pastured on higher and drier grounds ; but, what is most 

 interesting, they also observed that an exception occurred 

 in the case of those sheep feeding in the extensive salt 

 water marshes bordering our eastern shores. It was probably 

 this latter circumstance which suggested the common and 

 useful practice of mixing salt with the food of sheep and cattle, 

 both as a preventive and curative agent. At all events, as will 

 appear in the sequel, the intelligible explanation of the good 

 effected by this simple practice, is intimately associated with a 

 correct understanding of the mode of development of the parasite 

 in question. 



The symptoms, treatment, and pathological appearances con- 

 nected with rot are not here discussed, the present chapter 

 being limited to the natural history of the entozoa. Even 

 within this limitation . the subject is too large to be treated 

 of exhaustively and in detail; but for all practical purposes 

 it is probably sufficient to follow the plan we have adopted 

 in the case of trichina, namely, to offer a series of conclusions, 

 such as appear to be well established by the independent re- 

 searches of various helminthologists. In this connection it is 



