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sites the latter can not live without sheep. The only exceptions are 

 those species which are also parasitic on other animals, as goats and 

 cattle. The modifications of organs which have arisen out of the needs 

 of parasitism are too many to give in detail. The great central fact of 

 their lives is that all the parasites have arisen from their kind, and under 

 favorable circumstances will reproduce their species, and that they are 

 to be treated as the originators of disease and not as the products of 

 disease. 



The methods by which sheep hecome infested differ with the species. The 

 external parasites are usually transmitted by actual contact of sheep 

 against sheep. The parasites may, however, be dislodged from their 

 former host and afterwards make their way to another sheep. The first 

 is known as mediate, and the second as immediate contact. The dis- 

 eases produced by the external parasites are true contagious diseases, 

 and should be regarded as such fully as much as any of the more act- 

 ively virulent maladies. The transmission with this class of parasites 

 is usually an active one; they may, however, be borne from one sheep 

 to another by people, cattle, goats, or by locks of wool, when the trans- 

 mission would be passive. 



(Estrus ovls, which seems to bridge the gap between the external and 

 adult internal parasites, differs from these groups in being able to act- 

 ively infest its host with its young, without an actual contact or inter- 

 mediate bearer. Lice, louse-flies, and scab insects may do this in a 

 less degree, butnot to that possessed by the (Estrus. The (Estrus larvae 

 are never transmitted by contact; they must mature, fall to the ground, 

 metamorphose, and emerge as adults before the females can infect sheep. 

 The internal parasites are passively conveyed into sheep along with 

 the food and drink consumed, ^nd never actively enter into the trans- 

 mission. They may be conveyed either as eggs or very young embryos. 

 Oestrus forms the single exception. 



The terms ^^ contagious ^^ and '■'•infectious'''' can be applied to these 

 parasites. The former is apj)licable to those parasites which usually 

 transmit themselves to other hosts, the latter to those which are trans- 

 mitted to their hosts along with food and drink. The young of Oestrus 

 have no agency in their transmission, and hence infect sheep. 



Parasites are frequently said to invade the hosts which harbor them. 

 This is only true of those species which actively undertake migration, 

 as scab insects and sheep ticks. A few species invade the organs of 

 their hosts after the latter have been infected, thus : The larvse of (Estrus 

 crawl from the margins of the nostrils to the sinuses of the head ; the 

 lung worms migrate into the lungs; the young embryos of Tcenia mar- 

 (/wafa tunnel the liver; T. c<xnur\is innnois, the brain; (Esophagostoma 

 penetrates into the intestinal walls. Those internal parasites which 

 undertake active migration in the bodies of their hosts seem to form a 

 minor class in the parasitic world, those which lodge in the intestine 

 and ducts emptying into it forming the majority. 



