[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.)]
Like Trichomonas, Lamblia can multiply under inflammatory conditions of the alimentary tract. Thus they are found in cases of diarrhœa, carcinoma of the stomach, etc. The parasites attach themselves by their sucking discs to the epithelial cells of the gut (fig. 20, _c_), and though their numbers may be very great, their direct pathological significance is not fully known. Their occurrence in cases of diarrhœa has been explained as being due to the increased peristalsis, which has detached the parasites from the epithelium. Free flagellate forms perish in stools if kept, more especially if the temperature falls below 0° C. or rises above 40° C. Lamblia has often been found in dysenteric diseases, especially in the East, and is said to be the causal agent of certain diarrhœas in India. Mathis (1914)[45] found Lamblia in cases of diarrhœa with dysenteriform stools in Tonkin. He also discovered healthy carriers of Lamblia cysts.