20 



The ability to select tlieir final lodgment belongs to each species, and 

 is the one character on which their own life and that of the species de- 

 pends. This is self-evident in the case of external parasites. After 

 hosts are once infected by the internal parasites and the young embryos 

 are endowed with activity, the^^ either select their i^roper place while 

 being carried along b^^ intestinal fluids, or force their way to it through 

 all opposing tissues and against all counter currents of fluids. Those 

 embryos which fail to reach these places finally die for want of the nec- 

 essary conditions of life. The very ability that is so absolutely neces- 

 sary to enable certain of the parasites to reach their chosen organ often 

 proves the means of their premature death. Twnia marginaia cysts 

 invading the liver become lost in the mass of this organ and perish. 

 Multitudes of these parasites injure the capsule of the liver and cause 

 the sheep invaded to die long before they have matured sufficiently to 

 pass into dogs. The embryos of (Esophagostoma often wander into the 

 mesenteries, the retro-peritoneal glands and liver, and perish. 



Parasites escape from their ovine hosts either actively, e. g., the 

 young and adults of the louse-flies, lice, mites, and the larva; of Oestrus, 

 or passively as eggs or young embryos, the young embryos of the 

 Strongyliis filaria and Twnia expansa, the completely segmented eggs of 

 the Strongyhts contortus, and as eggs incompletely segmented. In 

 the latter case they are rejected with the excreta of the lungs or intes- 

 tines. A very few (the cystic tape- worms) escape only after the death of 

 their host by the intervention of some carnivorous animal which swal- 

 lows them with its food and liberates them from their imprisonment by 

 the processes of digestion. The death of the host is usually caused by 

 the carnivora in search of their food. The continuance of the parasites' 

 life into the adult stages depends, therefore, on the destruction of their 

 host. This fact is contrary to the usual rule of parasitism, which de- 

 mands that the host continues to live in order that the parasite may 

 live and reproduce its species. 



The length of time and the stage of development at which parasites 

 infest their host varies considerably. Lambs have no parasites at birth. 

 Within a month or two after, the}' become infested by a few individ- 

 uals of certain species of round worms, and by external parasites. 

 From this time on they may harbor any of the species to which they 

 become exposed. It will be noticed that the commencement of infection 

 begins when the lambs first nibble grass. The louse-flies, lice, and scab 

 insects infest the fleeces and skin from generation to generation. Unless 

 it should subsequently be proven that the hair-lungworm {iStrong- 

 ylus ovis-j)ulmonalis)j and the stomach round worm {Strongyhts contortus)^ 

 may also perpetually infest sheep, they harbor no other species through- 

 out their entire life cycles. (Estrus ovis is parasitical only in its larval 

 stage, and consumes months in developing. Because it can not take nour- 

 ishment when adult, it is believed to pass a very ephemeral adult stage. 

 The broad tape- worm develops rapidly and disappears, its six-hooked 



