_Lamblia intestinalis_ possesses eight flagella (fig. 19). The first pair of flagella, which cross one another, arise in a groove formed by the anterior edge of the sucking disc. Two pairs of flagella (lateral and median) are inserted on the posterior edge of the disc, while the posterior flagella occur at the tapering posterior extremity of the body. Basal granules are found at the bases of the flagella. The median flagella are most active in movement, the anterior and lateral flagella being less motile, as they are partially united to the body for part of their length. The nuclear apparatus is situated in the thin, anterior, hollowed part of the body. It is at first dumb-bell shaped, the “handle” of the dumb-bell being formed by a very slight connecting strand, which eventually separates, so that the flagellate becomes binucleate, and thus completes the general bisymmetry of the organism. There is a karyosome in each nucleus. Other bodies of unknown function, and possibly composed of chromatin, occur on or near the axostyles. [Illustration: FIG. 19.--_Lamblia intestinalis_. A, ventral view; B, side view; N, one of the two nuclei; _ax._, axostyles; _fl_^1, _fl_^2, _fl_^3, _fl_^4, the four pairs of flagella; _s_, sucker-like depressed area on the ventral surface; _x_, bodies of unknown function. (After Wenyon.)] Division has not been observed in the flagellate stages of the Lamblia, but it occurs within the cysts. The resistant cysts (fig. 20, _e_) are oval and are surrounded by a fairly thick, hyaline cyst wall. They measure 10 µ to 15 µ by 7 µ to 9 µ, and may be tetranucleate. According to Schaudinn, the cysts arise from the conjugation of two individuals, and nuclear rearrangement occurs. _L. intestinalis_ occurs in its flagellate stage in the duodenum and jejunum, and rarely as such in the other parts of the intestine. Normally it is found in the large intestine as cysts, which are voided with the fæces. The hosts of Lamblia include _Mus musculus_, _M. rattus_, _M. decumanus_, _M. silvestris_, _Arvicola arvensis_ and _A. amphibius_, the dog and cat, rabbit, sheep and man. Cysts voided with the fæces of infected animals reach plants or drinking water, and thence are transferred to man. The flagellate in these different hosts exhibits some variation in size and in the problematic chromatic bodies. Bensen has suggested the species _L. intestinalis_ from man, _L. muris_ from the mouse and _L. cuniculi_ from the rabbit. It is not certain whether these different species are necessary, as the variation may be due to differences of environment. [Illustration: FIG. 20.--_Lamblia intestinalis._ _a_, from the surface; _b_, from the side; _c_, on intestinal epithelium cells; _d_, dead and _e_, encysted. (After Grassi and Schewiakoff.)]