708 ORDER: DIPLOMONADIDA 



axonemes of the tail flagella. As pointed out by Simon (1922), none of 

 these characters is of sufficient constancy to be of any value for separating 

 species. According to him, it is only by the average dimensions that 

 species can be recognized. His measurements for G. niuris are: length, 

 7-25 to 12-75 (average 9-75) microns; breadth, 5-25 to 9-75 (average 7-26) 

 microns. The dimensions of the cysts are very similar to those of G. intes- 

 tinalis of man. Simon believes that in one white rat examined by him 

 there occurred two distinct species. One of these was evidently G. muris. 

 The other is referred to as G. sp. It was larger than G. muris, and varied 

 in length from 10-25 to 16-75 microns (average 13-25) and in breadth from 

 6-25 to 9-25 microns (average 7-49). Both white rats and wild rats known 

 to be free from natural infections were readily infected with G. 7nuris from 

 mice by Simon. He was unable to infect these animals with G. intestinalis 

 or G. microti. Fantham (1925) records G. muris from Rnttus concha and 

 Tatera lohengula. 



White mice and rats are commonly infected with G. muris, which quickly 

 spreads when introduced to a batch of these animals. There seems little 

 reason to regard it as in any way pathogenic, though Kofoid and Christi- 

 ansen (1915) maintain that the intestines of infected animals are altered 

 to a yellow colour, which is most evident at the site of heaviest infection. 



Hegner (1923a) has found G. muris in wild rats and mice in America. 

 From a study of Rattus norvegicus in Paris, Lavier (1924) concludes that 

 these rodents harbour two species of Giardia. One of these is G. muris, 

 while the other appears to be morphologically identical with G. intesti- 

 nalis of man. As all attempts to infect rats with the human Giardia have 

 failed, Lavier believes that the form in the rat is a distinct species, for 

 which he proposes the name Giardia simoni. It is apparently the form 

 referred to by Simon as G. sp. 



Giardia microti Kofoid and Christiansen, 1915. — This is a form which was 

 discovered by Kofoid and Christiansen (1915) in meadow mice {Microtus 

 californicus californicus) in California. They supposed that it could be 

 distinguished from G. muris on morphological grounds, but Simon (1922) 

 has shown that this is not the case, and that the species can only be distin- 

 guished by its measurements (Fig. 296). He gives these as: length, 8-25 to 

 13-75 (average 11-11) microns; breadth, 5-25 to 10-25 (average 7-58) microns. 



Simon was unable to infect rats or mice with G. microti obtained from 

 M. pennsylvanicus acadicus. It is not improbable that the form seen by 

 Grassi in Arvicola {Microtus) arvalis in Italy, and studied by Lavier (19216) 

 in France, and that described by Splendore (1920) as G. jpitymisi of 

 Pitymys savii of Italy are identical with G. microti. 



G. viscacise Lavier, 1923. — This species was discovered by Lavier 

 (1923) in the viscacha {Viscacia viscacia), a rodent of South America. 



