6 THE ANIMAL PARASITES OK MAN 



According to the kind of food ingested by parasites, it has 

 recently become usual to separate the true parasites from those 

 animals that feed on the superfluity of the food of the host, or on 

 products which are no longer necessary to him, and to call the latter 

 MESSMATES or COMMENSALS. As examples, the Ricinidae are thus 

 designated, because, like actual lice, they dwell among the fur of 

 mammals or the plumage of birds. They do not, however, suck 

 blood, for which their mouth apparatus is unsuited, but subsist 

 on useless epidermic scales. These epizoa, according to J. P. van 

 Beneden, are, to a certain extent, useful to their hosts by removing 

 deciduous materials which under certain circumstances might become 

 harmful to them. 1 This investigator, who has contributed so greatly 

 to our knowledge of parasites, assigns the Ricines to the MUTUALISTS, 

 under which term he comprises animals of various species which 

 live in common, and confer certain benefits on one another. The 

 mutualists are usually intimately connected in a mutually advan- 

 tageous association known as " symbiosis." 2 



Incidental and Pseudo Parasites. In many cases the parasites are 

 confined to certain hosts, and may therefore be designated as specific 

 to such hosts. Thus, hitherto, Tcenia solium and Tcenia saginata 

 in their adult condition have only been found in man ; Tcenia crassi- 

 collis only in the cat ; Brandesia (Distoma) turgida and Halipegus 

 (Distoma) ovocaudatns 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 ; Dipylidium caninum is found 

 in the domestic cat as well as in the dog; Fasciola hepatica is found in 

 a large number of herbivorous mammals (nineteen species), Diplodiscus 

 (Amphistomum) subclavatus in numerous urodele and ecaudate am- 

 phibia, Holostomum variabile in about twenty-four species of birds, 

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

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

 same class. Trichinella spiralis, which is found in man, and in the pig, 

 bear, rat, mouse, cat, fox, badger, polecat and marten, and is capable 

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



1 According to Sambon, the Riciniclse are by no means advantageous to their hosts. These 

 Hemipterous parasites give rise to an intolerable itching which may cause loss of rest, emacia- 

 tion, and sometimes even death. Birds suffering from phthiriasis of the Ricines are usually 

 in bad health. 



2 For further information on these conditions, see ''Die Schmarotzer des Thierreichs," by 

 P. J. van Beneden, Leipzig, 1876 ; and " Die Symbiose," by O. Hertwig. 



