vi NEMATODA PARASITISM 163 



the parasites within the body of their host we have a double habitat 

 to consider. Many forms, such as Trichina spircdis, wander from 

 the intestine to the muscles ; others, such as Filaria medinensis, 

 from the alimentary canal to the lymphatics or blood vessels or sub- 

 cutaneous tissues. Others pass from the body-cavity to the intes- 

 tine, as the Mermithidae, which infest Insects, or from the stem and 

 leaves of a plant to its flower, as in the case of Tylenchus tritici. 



With regard to their occurrence in the different classes of the 

 animal kingdom, they have been most frequently observed in 

 Vertebrates and in Insects. They are comparatively rare in 

 the other large divisions. Many genera are confined to certain 

 hosts : thus Ascaris, Filaria, Trichosoma occur only in Vertebrates ; 

 Spiroptera (with one exception) in Mammals and Birds ; Cucul- 

 lanus in Fishes and Amphibia ; Strongylus and Physaloptera in 

 Mammals, Birds, and Eeptiles ; Dochmius, Pseudalius, Tricho- 

 cephalus in Mammals ; Dispharagus, Hystrichis, Syngamus in 

 Birds ; Nematoxys, Hedruris in Amphibia and Eeptiles ; Ichthyo- 

 nema in Fishes ; and Isacis and Mermis in Insects. 



Twenty-two species .have been described as parasitic in man, 

 of which perhaps the most dangerous are Filaria medinensis, 

 the three varieties of F. sanguinis hominis ; Dochmius (Ancylo- 

 stomum) duodenalis, and Trichina spiralis. The Ascaridae, as 

 Ascaris lumbricoides and Oxyuris vermicularis, though painful, 

 seldom cause death. 



The enormous number of parasites harboured by one host is 

 shown by the fact mentioned in Leuckart's Parasites of Man, that 

 Nathusius * took from a single black stork 24 specimens of 

 Filaria labiata from the lungs, 16 Syngamus trachealis from the 

 trachea, more than 100 Spiroptera alata from the coats of the 

 stomach, besides several hundred Trematodes belonging to several 

 different species (see p. 63). Even this has been surpassed in 

 the case of a young horse, in whose body Krause found 500 

 Ascaris megalocephala, 190 Oxyuris curvula, several millions of 

 Strongylus tetracanthus, 214 Sclerostomum armatum, 287 Filaria 

 papillosa, 69 Taenia perfoliata, and 6 Cysticercus forms. 



It is impossible here to enter into a full description of the de- 

 struction caused to domesticated animals and crops by the presence 

 of these parasites ; full details will be found in books dealing 



1 Arch. Naturg. Jahrg. Hi. Bd. i. 1837, p. 52 ; and van Beneden, Animal Para- 

 sites, p. 91. International Sci. Series. 



