56 



The Rot in Sheep. 



no sexual organs can be detected, although in other respects, 

 when deprived of the tail, their resemblance to distomata is so 

 exact. It is evident from this that cercarice have to undergo 

 further development, which they can only attain by becoming 

 parasitic to higher forms of creatures. Some varieties of them 

 have been observed to bore their way into water-snails, to cast 

 off their tails, and develop into flukes with perfect sexual organs — 

 thus completing the series of changes. After entering the body 

 of the snail, and before being transformed into the fluke, the 

 cercarice roll themselves into little balls and pass into the pupa 

 state, by emitting from the surface of their bodies a mucous 

 secretion which hardens and encloses them. This change was 

 first observed by Nitzsch, and afterwards by Siebold and others. 



Dr. Cobbold, in describing the process, in the work pre- 

 viously referred to, says, " in the cases of some species of fluke 

 there is reason to believe that before the cercariw gain access to 

 their final or definite host, they re-enter the bodies of the mol- 

 lusks. This they accomplish by means of a boring apparatus ; 

 and, having previously cast off their tails, they encyst themselves 

 beneath the surface of the skin. In this new situation they 

 develop into the so-called pupa, which is at length passively 

 transferred with the fodder or drink to the digestive organs of 

 the host." This conveyance is the more readily effected by the 

 pupa adhering in large numbers, not only to mollusks, but to 

 the larvae of aquatic insects, &c, and being also found free in 

 water. How long the pupa state may continue is not known, 

 but, according to the experience of Steenstrup, in some varieties 

 of the cercaria it persists " for many months." Thus it will be 

 seen that the pupa state of the cercaria is the penultimate form 

 of the fluke, and that in this condition the entozoon enters the 

 bodies of cattle, sheep, and other animals. 



The opinion here expressed of the means by which flukes 

 enter the digestive organs of animals, and thus become the 

 cause of rot, receives considerable support from the statements 

 of Kiichenmeister, who says that " when De la Valette set about 

 administering the tailed, free living forms — that is to say, the 

 ccrcariai themselves unencysted — the result of a metamorphosis 

 of these forms into mature distomata did not occur. He then 

 directed his attention to the forms originating from the cercaria 

 which are enclosed in cysts, and, although still asexual, are 

 already in other respects somewhat further developed. . . . When 

 administered in this state the young distomata are quickly pro- 

 vided with germ stock, testes, and ovaries. 



The annexed engraving {fig. 13) represents the pupa state of 

 the Cercaria ephemera. The letters indicate — a, the oral sucker, 

 and c, r/, the urinary organ. 



