_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. 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 _Tænia solium_ in the dog, or to rear _Cysticercus cellulosæ_ in the ox, or the _Cysticercus_ of _Tænia saginata_ in the pig, but they have always proved unsuccessful. Only exceptionally has it been possible to transfer _Cœnurus cerebralis_, the larval stage of a tapeworm (_Tænia cœnurus_) of the dog from the brain of the sheep to that of the domestic goat. On the other hand, in the case of the Trichinellæ transference to a different host is easily accomplished.