_Herpetomonas_ is a generic name for certain flagellates possessing a vermiform or snake-like body, a nucleus placed approximately centrally, and a blepharoplast (kinetic nucleus) near the flagellar end. There is no undulating membrane (fig. 49, _a_). The organisms included in this genus certainly possess one flagellum, while according to Prowazek (1904) _Herpetomonas muscæ-domesticæ_, the type species, possesses two flagella united by a membrane. Patton,[115] Porter[116] and others affirm, however, that the biflagellate character of _H. muscæ-domesticæ_ (from the gut of the house-fly) is merely due to precocious division. The matter is further complicated by the generic name _Leptomonas_, given by Kent in 1881, to an uniflagellate organism found by Bütschli in the intestine of the Nematode worm, _Trilobus gracilis_. This parasite, _Leptomonas bütschlii_, has not yet been completely studied. Until these controversial points relating to the identity or separation of _Herpetomonas_ and _Leptomonas_ have been satisfactorily settled, we may retain the better known name _Herpetomonas_ for such uniflagellate, vermiform organisms. However, the name _Leptomonas_, having been used by Kent two pages earlier in his book (“Manual of the Infusoria”) than _Herpetomonas_, would have priority if the two generic names were ultimately shown to be synonymous. [115] _Arch. f. Protist._, xiii, p. 1. [116] _Parasitology_, ii, p. 367. A full discussion of these interesting and important flagellates hardly comes within the purview of the present work; brief mention can only be given here to certain species. The Herpetomonads occur principally in the digestive tracts of insects, such as Diptera and Hemiptera. They are also known in the guts of fleas and lice, but are not confined to blood-sucking insects. One example, _H. ctenocephali_ (Fantham, 1912)[117] occurs in the digestive tracts of dog fleas, _Ctenocephalus canis_, in England, France, Germany, Italy, India, Tunis, etc. It is a natural flagellate of the flea, and might easily be confused with stages of blood parasites in the gut of the dog flea. Dog fleas are stated by Basile to transmit canine kala-azar, which is believed to be the same as human infantile kala-azar. Confusion is further likely to arise since herpetomonads pass through pre-flagellate, flagellate and post-flagellate or encysted stages; pre- and post-flagellate stages being oval or rounded and _Leishmania_-like. The post-flagellate stages are shed in the fæces, and are the cross-infective stages by means of which new hosts are infected by the mouth. The possible presence of such natural flagellates must always be considered when experimenting with fleas, lice, mosquitoes, etc., as possible vectors of pathogenic flagellates like _Leishmania_ and _Trypanosoma_. _H. pediculi_ (Fantham, 1912) occurs in human body lice.[118] See further remarks on pp. 107, 112. [117] _Bull. Path. Exot._, vi, p. 254. [118] _Proc. Roy. Soc._, B, lxxxiv, p. 505.