22 



The destructiveness of each species is dependent on the numbers of 

 the invading ]>arasites, the organ invaded, the method by which they 

 produce disease, and the age of the host. As a rule, most parasites 

 produce disease by their numbers, each causing its inlinitesimal amount 

 of annoyance. The sheep-grub, the broad tape-worms, Boclimius cer- 

 nuus, and Twnia cwnnrus, are notable exceptions to the rule. But 

 few individuals of each of these species are found invading the organs 

 of sheep. Their destructiveness depends on the character of the 

 annoyance produced and on the organ invaded. A few of nearly all 

 species may infest sheep, and seemingly cause no loss, but when any 

 of the factors favoring the development of either of the species appears 

 they increase innumerably and destroy the lives of their hosts. 



Parasites effect injury to the health of sheep in many ways, some of 

 which are very evident to all, while others are indefinable and illusive. 

 The injuries effected by sheep-scab, by the hair lung-worms, and by 

 (Esophagostoma, are easily discovered; but the injuries produced by the 

 tape-worms and various species of round worms can not always be de- 

 tected in the intestinal walls. In these the method of determining dis- 

 ease is not by testing the various organs by the microscope or chemical 

 analysis, but by comparing the patient with a healthy animal of the 

 same age in action, appearance, weight, etc., and by comparing the 

 organs of the patient with an actual or assumed standard. In these 

 ways the effects of the subtle diseases produced by the parasites are 

 learned and just allowance made for their importance. 



Most parasites mechanically injure tissues. They may either force 

 their way through soft tissues as the cysts of Twnia marginata, break 

 through the structure of lung tissue as Strongylus ovis-puUnonalis, en- 

 ter into the intestinal walls as (Esophagostoma, or lacerate the tender 

 mucous membrane as Dochmius cernuus. The adult tape-worms seem 

 to irritate the intestines, derange their functions, and cause nervous dis- 

 turbances. Dochmius cernuus and perhaps Strongyhis contortus seem to 

 abstract blood. Other round worms may live on the intestinal contents. 



The destruction caused by the different diseases produced by para- 

 sites is varied. Scab, lung- worms, flukes, and tape-worms often destroy 

 entire flocks. The ravages of other species are 'less patent. There is 

 no doubt, however, that each causes more trouble than has yet been 

 assigned to it. The least destructive of the species common to sheep in 

 this country seems to be Trichodectes sphwrocephalus, Strongylus JiUcollis, 

 IS. ventricosus, and Trichocephalus affinis. 



No rigid system has been adhered to in the succeeding pages. Nev- 

 ertheless, a certain plan of arrangement and treatment will be noticed. 

 The highest insects in the zoologic classification precede. These are 

 followed by the Acarinw or scab mites. The Helminths, embracing 

 the Platoda or flat worms, and the Nematoda or round worms, follow in 

 turn. A systematic classification of the worms among each other is not 

 closely adhered to. It may be noticed further that such an arrange- 



