4 University of Calif ornia Puhlications in Zoology [Vol. 18 



individual each (pi. 1, fig. 12) were found at the posterior end of the 

 duodenum and close to the caecum ; in series 29 and 33 the cysts were 

 found not only in the small intestine but also in the colon and rectum, 

 the smears having been made from the faeces. These cysts showed 

 single individuals with two nuclei (pi. 1, fig. 12) and the so-called 

 "copulation" types (Schaudinn, 1903; Rodenwaldt, 1911) with two 

 individuals in the back-to-back, end-to-end position, and a total of four 

 nuclei. The latter type is shown in plate 1, figures 13 and 16. In series 

 33 and 36, cysts with many nuclei (pi. 1, fig. 14) were very abundant 

 in the small intestine, but only a very few were present in the colon. 



With regard to the pathogenicity of these parasites, Kofoid and 

 Christiansen (1915&) found that Giardia niuris caused a chronic enter- 

 itis ; that the walls of the intestine become orange or j^ellow in color, 

 flaccid and often inflated with gas. In the case of the meadow mouse, 

 a chronic enteritis accompanied by the erosion of the mucous lining, 

 and the disintegration and falling away of epithelial cells and even of 

 whole villi into the lumen, are the main visible pathogenic effects. 



The gas inflation did not occur in mice examined by me ; this con- 

 dition as found in Peromyscus infected with G. muris is probably due 

 to the presence of certain gas-producing bacteria and not to the pres- 

 ence of the flagellates. There is no evidence at hand that Giardia 

 microti causes a dysentery in the meadow mouse as Giardia intestinalis 

 causes in man. 



That these parasites in turn undergo parasitism, which leads to 

 deleterious results, is seen in the fact that they are at times covered 

 with rod-like bacteria, many adhering to the flagella and some occur- 

 ring as inclusions within the body itself (pi. 1, figs. 3, 15) . The identity 

 of these bacteria could not be determined. They occurred in only two 

 series of preparations. They appear to be surrounded by a clear 

 space, filled with a liquid and walled off from the rest of the body by 

 a membrane, or they may be situated in a vacuole. 



Morphology 

 The parasites were found to vary from 6 to 1 6/x in length and from 

 4.5 to S/x in width. The body as seen from the dorsal or ventral sur- 

 face is pyriform in shape, and, from a partial side view, the dorsal 

 surface or back is arched (pi. 1, fig. 8) . The anterior end of the parasite 

 is semi-circular. Posteriorly the body tapers gently or abruptly to form 

 the caudal area. This caudal area is well developed in this species. 



