18 



which groups them together. Though this distiuctive feature is suffi- 

 cient for the preseut purpose, it is a very variable character, for the 

 degree of parasitism manifested by each of the species varies through 

 all the scale possible from the transient momentary parasites to the 

 l)ermauent. 



The animal parasites of sheep are all embraced within three great 

 branches of the animal kingdom : The Protozoa, Vermes, and Arthro- 

 poda. None of the first branch, which includes the Coccidia and Bal- 

 liana gigantea, Raillet, are described in this volume. Examples of the 

 second, which includes all the worms, and of the last, which includes 

 the insects, mites, and liuguatula, are abundant. 



The worms live, as a rule, in the open cavities of the body — in air 

 spaces of the lungs, the ducts of the liver, and the lumen of the intes- 

 tine. The exceptions to this rule arise from those immature forms 

 which penetrate into the substance and closed cavities of the bodies, 

 e. g., the bladder stages of the tape-worms and the young embryos 

 of (Esophagostoma. The worms are called internal parasites. It is 

 easily understood, however, that being held in the cavities of the body 

 which have communication with the exterior, they are really external to 

 the body tissues, and only those embryonic forms which penetrate into 

 the tissues of the infested animal or host are true internal parasites. 



As a rule the insects live on the surface of the body. They are 

 called external parasites. The exceptions are the larva of (Estrus avis, 

 which lives in the nasal cavities, and Linguatula, whose young stage 

 infests various organs of sheep. CEJstrns is usually classed with th^ 

 external parasites, and Lingiiatula is in sheep truly internal. As has 

 been stated, those parasites which in their young stages penetrate the 

 tissues of sheep are alone true internal parasites. Even these spend 

 their adult stages in the open cavities of some other host and then be- 

 come true external parasites, so that no one of these parasites is, strictly 

 speaking, an internal one throughout its life. That every parasite 

 should be an external one in its adult stage is a necessary condition of 

 its existence and of the perpetuation of its species, for it is only in the 

 open cavities that they can obtain sufficient air and food, and can mate. 

 From these cavities, too, the eggs and young can escape for the infec- 

 tion of other sheep. The facility offered for mating and distribution 

 is the most important reason. In order to avoid confusion of terms 

 those parasites infesting the surface of the body will be called, in con- 

 formity with custom, external parasites; the others, which inhabit the 

 tissues of the body and its cavities, internal. 



Though the animal organisms that infest the living bodies of sheep 

 be small, they are endowed with all the vital functions of life. All can 

 move, feed, feel, and reproduce. None of the worms can see or hear. 

 The insects are more highly specialized than the worms. All of them 

 have in the past become so adapted to their surroundings that they can 

 live in no other, and while sheep thrive better if not infested by para- 



