434 ESSENTIALS OF ZOOLOGY 



The third means is used by only a few parasites, most of them 

 Protozoa. " The spirochete, Treponema, which causes syphilis, a flagel- 

 late protozoan called Trichomonas vaginalis, and a trypanosome para- 

 sitic in the reproductive organs of horses are examples. Some 

 nematode parasites of insects are also transmitted in this way. 



The fourth means includes a few cases, such as the acquiring of 

 dog tapeworm by people who kiss dogs and the transmission of pin- 

 worm from the hands of infected people who do not have cleanly 

 habits. 



Hookworms and the human blood flukes, called schistosomes, are 

 examples of parasites that invade new hosts under their own power. 

 Their larvae are able to penetrate the skin. 



Parasitism and Host Specificity. 



Since the beginning of the scientific study of parasitism, it has 

 been recognized that different animals have different parasites; for 

 instance, the parasites found in and on goats are nearly all different 

 from those of man. Some of the early parasitologists leaped to the 

 conclusion that each species had its own peculiar species of parasites 

 found nowhere else, and carried this idea of species specificity so 

 far that they considered presence in different hosts to be sufficient 

 evidence of specific difference of the parasites. 



Modern knowledge reveals that while some parasites are actually 

 species specific others have a wide range of hosts. Thus the beef 

 tapeworm, Taenia saginata, is found in the adult stage in man only, 

 but the fish tapeworm, Diphyllohothrium latum, seems to be able to 

 live in nearly all mammals which eat fish. 



Three main factors determine whether a parasite will infect any 

 given host: (1) opportunity for infection of host, determined by 

 habits or mode of life of parasite and host (malaria parasites may 

 be injected into any land animal by bite of mosquito, but strictly 

 aquatic animals, such as fish, would not be bitten) ; (2) the environ- 

 mental condition of the habitat furnished by the body of the host, 

 involving such factors as body temperature, nature of outside sur- 

 face, size, chemical content of internal organs, etc. (intestinal para- 

 sites of birds are seldom found in mnmmals, which have lower body 

 temperatures, and parasites adapted to the oxygen-rich interior of 

 a frog's lung can find no suitable habitat in a lungless fish) ; and 

 (3) ability of the parasite to adapt itself to the wide range of environ- 

 mental conditions found in different hosts ; thus Z>. latum, though it 



