6 THE ANIMAL PARASITES OF MAN 



inhabited by the latter, almost always reach the open, and after a 

 shorter or longer period of free existence seek new hosts. During 

 their free period, moreover, a considerable growth may be attained, 

 or metamorphosis may take place, or even procreation. In the 

 exceptional cases in which the young remain within the same host, 

 they nevertheless always quit the organ inhabited by the parents. 

 They likewise NEVER attain maturity within the host inhabited by 

 the parents, but only, as in other cases, after having gained access 

 into fresh hosts. 



These transmigrations play a very important role in the natural 

 history of the internal parasites, but they frequently conceal the cycle 

 of development, for sometimes there are INTERMEDIARY GENERATIONS, 

 which themselves invade intermediary hosts. Even when there are no 

 intermediary generations, THE SYSTEM OF THE INTERMEDIARY HOSTS 

 is frequently maintained by the endoparasites. 



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

 recently become usual to separate the true parasites from those 

 creatures 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 amongst 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 epidermal 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- 



1 The Ricinidae are by no means advantageous to their hosts. These Hemipterous 

 parasites give rise to an intolerable itching which may cause loss of rest, emaciation, 

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

 in bad health. L. W S. 



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. 



