46 LIFE-HISTORY OF PARASITES. 



single parasite ^ that undergoes all its life stages without a change 

 of locality. The number of eggs evacuated with the fseces varies 

 of course with the fertility and the number of the parasites, and 

 is sometimes so considerable, that a very superficial microscopic 

 examination is sufficient to show their presence. Moreover, the 

 intestinal parasites are not the only ones whose eggs are evacuated ; 

 the same thing takes place in animals living in other organs — the eggs 

 of Bistomum hepaticum reach the intestine through the bile duct, and 

 are thus shed from the body. In the same way the eggs of the 

 bronchial parasite of the sheep, Strongylusfilaria, are removed with the 

 tracheal mucus, and the eggs of Pentastomum tcenioides, which lives 

 in the nasal cavity of the dog, leave the body along with the 

 secretion of the Schneiderian membrane ; the eggs of Strongylus gigas 

 and the embryos of Filaria Bancrofti are passed out along with the 

 urine. ^ Nor is it necessary that the parasites should live in organs 

 that are in direct communication with the exterior ; there are instances 

 where such communications are made by some subsequent abnormal 

 process. The eggs and embryos of Distomum hcematobium break 

 through the wall of the urinary and rectal blood-vessels in which they 

 are originally laid, into the surrounding spaces, where they form 

 abscesses. The same thing is seen in Stephanurus (the " kidney-worm " 

 of the Americans), which lives near the kidneys in swine, and bores 

 its way into the pelvis of the kidney. The embryos of Bracunculus 

 {Filaria Medinensis), which, as is well known, live between the muscles, 

 reach the exterior through an abscess, which is formed as soon as the 

 head of the worm comes into contact with any part of the cutis. 



If we bear these and other similar cases in mind, and also 

 keep in view the fact that by far the greater number of sexual 

 Helminths live in the alimentary canal, it is evident that we are 

 right in considering the widespread occurrence of these migrations 

 to be important in the life-history of parasites. But those other 

 cases, where the eggs remain upon the spot where they were de- 

 posited until the escape of the young, become, on this account, all 

 the more striking and interesting. We have already mentioned 

 that, to attain this latter end, the eggs of the Epizoa are fastened in 

 various ways to the outer skin. There is no need of anything of this 

 kind in the internal organs, where the inaccessibility of the position 



' In the German text AnguiUula {Rhabditis) stercoralis was here mentioned as an 

 exception, but, as above mentioned (p. 21, footnote), this form has proved to be merely 

 a developmental stage of a parasite already known under the name of AnguiUula 

 mtestinalis. — R. L. 



^ A proper microscopical examination of the fseces and excreta under such circum- 

 stances generally furnishes a reliable diagnosis. 



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