Syn.: _Macrostoma mesnili_, _Chilomastix mesnili_, _Fanapapea intestinalis_. The genus _Tetramitus_ differs from _Trichomonas_ in possessing an undulating membrane inserted in a deep groove or cytostome. There are three anterior flagella. The pear-shaped organism measures 14 µ by 7 µ, but smaller examples occur. _T. mesnili_ occurs in the human intestine, having been described by Wenyon[43] (1910) from a man from the Bahamas in the Seamen’s Hospital, London. Its occurrence is widespread. Alexeieff considers that _Macrostoma_ and _Tetramitus_ are synonymous. The parasite is the same as _Fanapapea intestinalis_, Prowazek, 1911, from Samoa. Brumpt (1912) found _T. mesnili_ to be the causal agent of colitis in a Frenchwoman. Nattan-Larrier (1912) considers it of little pathological importance. [43] _Parasitology_, iii, p. 210. Gäbel[44] (1914) described an interesting case of seasonal diarrhœa acquired in Tunis, in which a new Tetramitid was the causal agent. The organism was pear-shaped, without an undulating membrane, and measured 6·5 µ to 8 µ by 5 µ to 6 µ. The cytostome was large, and there was no skeletal support. Encystment occurred. Gäbel named the organism _Difämus tunensis_ and considered that it was pathogenic. [44] _Arch. f. Protistenk._, xxxiv, p. 1. Genus. *Lamblia*, R. Blanchard, 1888. Syn., _Dimorphus_, Grassi, 1879, _nec_ Haller, 1878; _Megastoma_, Grassi, 1881, _nec_ de Blainville. The body is pear-shaped, with a hollow on the under surface anteriorly. It has four pairs of flagella directed backwards, of which three pairs lie on the borders of the hollow disc, and the fourth arises from the pointed posterior extremity. *Lamblia intestinalis*, Lambl, 1859. Syn.: _Cercomonas intestinalis_, Lambl, 1859 (_nec_ 1875); _Hexamitus duodenalis_, Davaine, 1875; _Dimorphus muris_, Grassi, 1879; _Megastoma entericum_, Grassi, 1881; _Megastoma intestinale_, R. Blanch., 1886; _Lamblia duodenalis_, Stiles, 1902. The organism is pear-shaped and bilaterally symmetrical. It is from 10 µ to 21 µ long and 5 µ to 12 µ broad and possesses a thin cuticle. Anteriorly an oblique depression is present, which functions as a sucking disc (fig. 19, _s_). Its edges are raised above the general surface and are contractile. It corresponds to a peristome and acts as an adhesive organ (fig. 20, _b_, _c_). No true cytostome is present. A double longitudinal ridge, representing axostyles, extends from the sucking disc to the tapering posterior extremity, which is prolonged as two flagella from 9 µ to 14 µ long.