GENUS: GIARDIA 693 



The members of the genus, which vary little in minute structure, 

 are characterized by having a body which in shape resembles a longi- 

 tudinally split pear. The dorsal surface is convex, while the ventral one 

 is flat. The tapering posterior end or tail is a flexible structure which can 

 be turned up over the convex dorsal surface. The rest of the body is 

 rigid. On the ventral surface is the sucking disc, which is almost circular 

 in outline save for a posterior indentation or notch. It has a raised edge, 

 and by its means a flagellate is able to rest attached to the surface of epi- 

 thelial cells. The four pairs of flagella, which are symmetrically arranged, 

 originate in a series of blepharoplasts, the exact distribution of which has 

 been variously described by different observers. Two nuclei are present, 

 one lying on each side of the middle line of the body. There is no cyto- 

 stome, though some observers incorrectly refer to the sucking disc by 

 this name. Reproduction is by binary fission, which usually takes place 

 within an ovoid cyst. Occasionally, division occurs in the unencysted 

 condition. The body of a typical representative of the genus is distinctly 

 flattened dorso-ventrally, though the degree of convexity varies consider- 

 ably (Fig. 291). In some, which are probably the products of a recent 

 division, the body is not more arched than a watch-glass, while in others, 

 which are fully grown, it is almost hemispherical. When swimming in 

 fluid media, the flagellate sways from side to side as any flattened object 

 does when progressing through a liquid. The exact arrangement of the 

 flagella, blepharoplasts, and axonemes is difficult to elucidate; so much 

 so that the various observers who have undertaken the study of these 

 flagellates have given different accounts. The difficulty of interpretation 

 refers particularly to the region between the nuclei. The writer (1907) 

 described what he considered to be the arrangement in Giardia muris of 

 mice, and subsequent observations on G. intestinalis of man and other 

 forms convince him that his original description was substantially correct. 

 In the internuclear region the structures are so closely packed that the 

 separate blepharoplasts cannot be recognized except in specimens which 

 have been almost completely discoloured after staining with iron haema- 

 toxylin. Ordinary dried films stained by Giemsa stain not infrequently 

 show the granules and axonemes distinctly, especially in individuals which 

 have been flattened or even fortuitously dissected. Thin sections of the 

 intestine in which the flagellates have been cut often show the structures 

 more clearly than in flagellates mounted whole. Kofoid and Swezy 

 (1922), Simon (1922), and Hegner (1922) state that there is only a single 

 anterior blepharoplast on each side, but it appears from their figures that 

 the single elongate blepharoplast is really composed of at least two closely 

 applied blepharoplasts. They suppose that when two are present on each 

 side this is an indication of commencing division. The following, in the 



