In the second category, we may place those which live at complete liberty, and only require for their sustenance the superfluities of their neighbours; they take great care of the skin of their host, 104 and use it sparingly. Some also are found which cannot live without assistance, but repay it with some service. Often, indeed, they associate with their host, and live on a footing of perfect equality with him; and besides these are found associations in which equality is by no means recognized, and where labourers or even slaves perform the work disdained by their masters. In the last category we shall arrange true parasites, which take both their lodging and their food. And here, again, we shall meet with three distinct subdivisions. The first includes those which travel from one hotel to another before they arrive at their destination; to-day they lodge in a prawn, to-morrow in a gudgeon, then in some fish which preys upon others, as the perch or the pike. These are nomadic parasites, which do not stop or think of family life until they have found the hotel for which they are destined. Sometimes the parasite gets into a wrong train, and not being able to retrace his steps, he remains at a station where no other train will take him up. He is condemned to die in a waiting-room. In the last subdivision, we have parasites that have arrived at their destination, occupying themselves in future only with the joys of a family. Thus we find some which are really at home, and others which are on their journey, sometimes on the right road, and at others, wandering and lost in an alien "host." The former are _autochthonic_ parasites, the others are foreigners. We may say that each animal species has its proper parasites, which can live only in animals which have at least more or less affinity with their peculiar host. Thus 105 the _Ascaris mystax_, the guest of the domestic cat, lives in different species of _Felis_, while the fox, so nearly resembling in appearance the wolf and the dog, never entertains the _Tænia serrata_, so common in the latter animal. The same host does not always harbour the same worms in the different regions of the globe which it inhabits. This relates both to the parasites of man, and to those of the domestic animals. Thus the large tapeworm of man, which naturalists call _Bothriocephalus_, is found only in Russia, Poland, and Switzerland. A small tape-worm, _Tænia nana_, is observed nowhere except in Abyssinia; the _Anchylostoma_ is known at present only in the south of Europe and the north of Africa; the _Filaria_ of Medina, in the west and the east of Africa; the _Bilharzia_, that terrible worm, has only been found in Egypt. There are also parasitic insects dreaded by man, as the _Chigoe_ (_Pulex penetrans_) which, happily, is only known in certain countries. Some, however, have become cosmopolitan, since man has introduced them wherever he has established himself.