[46] _Arch. f. Protistenkunde_, Suppl. i, p. 169. In all these cases _L. intestinalis_ has been observed in the small intestine, or in the evacuations of patients with intestinal diseases. It has also been found in the intestine of healthy subjects. Just as _Trichomonas intestinalis_ may be found inhabiting the stomach in diseases of that organ, in which an alkaline reaction is present (carcinoma), so has _L. intestinalis_ been found to occur under similar circumstances (Cohnheim, Zabel). However, in Schmidt’s case, 1 per cent. hydrochloric acid was certainly stated to be present. Infection takes place by the ingestion of cysts (fig. 20, _e_), as was established by Grassi, experimentally on himself. Cereal food-stuffs, contaminated with Lamblia cysts from vermin of the locality, such as rats and mice, serve to convey the infection to man. Such cysts may probably be found in street-dust, etc. Stiles induced infection in guinea-pigs, and Perroncito in mice and rabbits, by means of cysts of Lamblia from human beings. Stiles suspected that flies could transport Lamblia cysts. Mathis (1914) found that _L. intestinalis_ was not amenable to emetine, at any rate in its cystic stage. Order. *Protomonadina*, Blochmann. The smallness of the Protomonadines and their less superficial situation than the Polymastigines, may be the cause that so far as the species occurring in man are concerned, they were formerly less well known. As regards parasitic species, this group may be divided as follows, according to the number of flagella and the presence or absence of an undulating membrane:-- (1) _Cercomonadidæ_, with one flagellum at the anterior extremity, without an undulating membrane. (2) _Bodonidæ_, with two flagella, without an undulating membrane, except in Trypanoplasma. (3) _Trypanosomidæ_, with one flagellum, and an undulating membrane along the length of the body in some genera. Family. *Cercomonadidæ*, Kent emend. Bütschli. Small uniflagellate forms, without cytostome. Genus. *Cercomonas*, Dujardin emend. Bütschli. Oval or rounded organisms, with the aflagellar end often drawn out into a tail-like process. *Cercomonas hominis*, Davaine, 1854. Davaine found flagellates in the dejecta of cholera patients. They had pear-shaped bodies, lengthening to a point posteriorly. Their length was from 10 µ to 12 µ, and a flagellum about twice as long as the body projected from one extremity (fig. 21). A nucleus was hardly recognizable. Occasionally a somewhat long structure (cytostome?) appeared at the anterior extremity. The animals moved with remarkable activity. They also attached themselves by means of their posterior extremities and swung about around the point of attachment. Davaine found a smaller variety, only about 8 µ long, in the dejecta of a typhoid patient (fig. 21, _b_). [Illustration: FIG. 21.--_Cercomonas hominis_, Dav. _a_, larger, _b_, smaller variety. Enlarged. (After Davaine.)]