INCIDENTAL AND PSEUDO-PARASITES 7 



colis only in the cat ; Distomum turgidum and D. ovocaudatum only 

 in Rana esculenta, and so forth. In many other cases, however, 

 certain species of parasites are common to several, and sometimes 

 many, species of hosts ; Tcznia cucumerina s. elliptica is found in 

 the domestic cat as well as in the dog ; Distomum hepaticum is found 

 in a large number of herbivorous mammals (nineteen species), Am- 

 phistomum subclavatum in numerous urodela and anuric amphibia, 

 Holostomum variabile in about twenty-four species of birds, and so 

 on. In these cases the. hosts are almost invariably closely related, 

 appertaining, as a rule, to the same family or order, or at any rate 

 to the same class. Trichina spiralis, which is found in man, and 

 in the bear, pig, rat, mouse, cat, fox, polecat and marten, and is 

 capable of being artificially cultivated in the dog, rabbit, sheep, horse 

 and in other mammals, and even in birds, is one of the most 

 striking exceptions. 



Some parasites are so strictly confined to one species of host that, 

 even when artificially introduced into animals very closely related 

 to their normal host, they do not thrive, but sooner or later, often 

 very quickly, die off, and very rarely establish themselves. For 

 example, repeated attempts have been made to rear the adult 

 Tcenia solium in the dog, or to rear Cysticercus cellulose in the ox, 

 or Cysticercus Tcenia * saginata in the pig, but they have always 

 proved unsuccessful ; only exceptionally has it been possible to 

 transfer Ccenurus cerebralis, the larval stage of a tapeworm of the 

 dog (Tcznia ccenurus), from the brain of the sheep to that of the 

 domestic goat. On the other hand in the case of the Trichinae 

 transference into a different host is easily accomplished. 



Under natural conditions it is not uncommon for certain kinds of 

 specific parasites to occur occasionally in unusual hosts ; they there 

 stand to the latter as " INCIDENTAL PARASITES." Thus Echinorhynchus 

 gigas, a specific parasite of the pig, is only an incidental parasite of 

 man ; Distomum hepaticum and Distomum lanceolatum are specific to 

 numerous kinds of mammals, but may be found incidentally in man ; 

 on the other hand, Bothriocephalus lotus, a specific parasite of man, 

 may occasionally take up its abode in the dog, cat and fox. As a 

 rule all those parasites of man that are only rarely met with, not- 

 withstanding that human beings are constantly being observed and 

 examined by medical men, are termed INCIDENTAL PARASITES OF MAN. 

 In many cases we are acquainted with the normal or specific host 

 of these parasites. Thus we know the specific host of Balantidium 

 coli, Coccidium oviforme, Distomum hepaticum, T&nia cucumerina, &c. ; 

 in others it is as yet unknown to us. In the latter case the question 



