234 ENTO-PAKASITES AND HYGIENE. 



such a long torpor in order to proceed later in their regu- 

 lar development. 



Others have the remarkable faculty of drying up com- 

 pletely without losing their power of recovering and 

 continuing their existence (Ascaris). They can also live 

 in surroundings which would doubtless destroy any other 

 organism, putrescent, decaying material, in corrosive li- 

 quids, in alcohol, oil of turpentine, etc. If these species 

 had not such remarkable power of resistance they would 

 have perished long ago on account of their manner of life. 



However different the fates of the various kinds of 

 parasites may be, they all have one thing in common: 

 they must resume parasitism sooner or later, in order to 

 reach their full development. This return may be ef- 

 fected in more than one way, either by active immigra- 

 tion, the young animals entering their new host spon- 

 taneously — the rarer case — or by passive transportation 

 back into the definite host, the young parasite being 

 taken up by chance in the food or the drink of the fu- 

 ture host. The eggs or the larvae dried up cling to 

 leaves or little animals, and the possibility of an impor- 

 tation is at once on hand. This explains why children 

 are so frequently infected by round-worms (Ascaris, 

 Oxyuris) — they are not so strict concerning cleanliness 

 as it would be desirable, and the eggs of those parasites, 

 sticking to fruits, leaves, etc., which are handled by 

 children, gain easy access to their hands and mouths. 



The drinking water furnishes another opportunity for 

 carrying the germs of parasites into suitable hosts. This 

 way is used by Ankylostoma duodenale, the worm men- 

 tioned before. It is found among people of the lower 

 classes, in Italy for instance, who are not distinguished 

 by an abundance of delicacy concerning the use of water- 

 closets. These people often prefer to get rid of their 

 faeces in the immediate neighborhood of their working 

 places; here those excrements come into contact with the 



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