These transmigrations play a very important _rôle_ in the natural history of the internal parasites, but they frequently conceal the cycle of development, for sometimes there are INTERMEDIATE GENERATIONS, which themselves invade intermediate hosts. Even when there are no intermediate generations, THE SYSTEM OF INTERMEDIATE HOSTS is frequently maintained by the endoparasites.
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 Ricinidæ 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 advantageous association known as “symbiosis.”[2]
[1] According to Sambon, the Ricinidæ 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.
[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.
_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, _Tænia solium_ and _Tænia saginata_ in their adult condition have only been found in man; _Tænia crassicollis_ only in the cat; _Brandesia_ (_Distoma_) _turgida_ and _Halipegus_ (_Distoma_) _ovocaudatas_ 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 amphibia, _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.