INCIDENTAL AND PSEUDO-PARASITES 7 



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 

 soliitiii in the dog, or to rear Cysticercus cellulose? in the ox, or the 

 Cysticercus of Tcenia saginata in the pig, but they have always proved 

 unsuccessful. Only exceptionally has it been possible to transfer 

 Ccenunis cerebralis, the larval stage of a tapeworm (Tcenia ccenums) of 

 the dog from the brain of the sheep to that of the domestic goat. 

 On the other hand, in the case of the Trichinellae transference to 

 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. Their 

 relationship to the latter is that of INCIDENTAL PARASITES. Thus 

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

 incidental parasite of man ; Fasciola hepatica and Dicroccelium 

 lanceatum are specific to numerous kinds of mammals, but may 

 be found incidentally in man. On the other hand, Dibothriocephalus 

 latus, a specific parasite of man, may occasionally take up its abode 

 in the clog, cat and fox. As a rule, all those parasites of man that 

 are only rarely met with, notwithstanding 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, Eimeria stiedce, 

 Fasciola hepatica, Dipylidiiun cauinuin, etc.; in others the host is as 

 yet unknown. In the latter case the question partly relates to such 

 forms as have been so deficiently described that their recognition 

 is impossible, partly to parasites of man in various regions of the 

 earth, the Helminthes and parasites of which are totally unknown 

 or only slightly known, or finally to early developmental stages that 

 are difficult to identify. Animals that usually live free, and exception- 

 ally become parasitic, may likewise be called incidental parasites. 

 In this category are included a few Anguillulidce that have been 

 observed in man ; also Leptodera appendiculata, which usually lives 

 free, but may occasionally become parasitic in black slugs (Arion 

 empiricorum) : when parasitic it attains a larger size, and produces 

 far more eggs than when living a free life. In order to avoid errors, 

 the term " incidental parasites " should be confined to true parasites 

 which, besides living in their normal host, may also live in other 

 hosts. Leuckart speaks of FACULTATIVE PARASITISM in such forms 

 as Leptodera. L. Oerley 1 succeeded in artificially causing Leptodera 

 (Rhabditis) pellio to assume facultative parasitism by introducing these 



1 Oerley, L., " Der Rhabditiden und ihre medizinische Bedeutung," Berlin, 1886, p. 65. 



