Strictly speaking the term filariasis should . mean the invasion of the animal host by any species of f ilaria, but in recent years at least it has gradually become more or less restricted to the description of these species which pour their embryonic or larval forms into the blood stream, that is to say the blood filariae proper. Such being the case the subject is considerably simplified, forms like the Filarie medinensis (the guinea worm), Filaria volvulus, Filaria conjunctivae, Filarie hominis oris, Filaria labialis, &c., being excluded.
In the present study it is intended therefore to use the term filariesis in this sense and to limit the discussion to species of filariae found only in the human subject, animal ones only being mentioned in so far as they throw any analogical light on problems occurring in the former. The life histories of some of the human blood filariae have now been cleared up very considerably but there are still many points of supreme interest remaining, amongst these being the question of doubtful species, the question of intermediate hosts, pathological points and so on.
Four good species certainly are to be found, viz.
2. Filaria bancrofti, Fileria loe., Filaria perstans and. Filaria demarquayi, and it is with these that the paper will be chiefly concerned, indifferent ones such as Filaria magalhaesi, doubtful ones such as Filaria ozzardi, and spurious ones such as Filaria gigas and possibly Fileria philippineneis being briefly mentioned at the end. The habitat of the adult forms to be described as will be seen later varies, but they have all this common characteristic that their embryos inhabit the circulating blood and so the diagnosis of their presence can be easily accomplished by the simple procedure of examining the blood.