Parasitology has now become a vast subject, and it is impossible for any parasitologist to have a complete knowledge of the classification of all the groups of parasites with which he may be expected, from time to time, to deal. Almost all helminthologists, for example, are obliged to confine their special studies to one of the main groups—one dealing chiefly with Nematodes, another with Trematodes, another with Cestodes, and so on. Even within these large groups many findit impossible to keep in touch with the literature concerning more than one or two special families. But whether the field they attempt to cover be large or small, it is probable that most parasitologists find the name of the host a very valuable clue to the identity of the parasite which they are endeavouring to determine. For some groups more or less comprehensive host-lists have been published from time to time, and authors frequently include such lists even in short memoirs, presumably for the assistance of their fellow-workers. In all probability many workers find it useful to attempt to compile for their own use some kind of host-list.