200 University of California Publications in Zoology [Vol. 20 



Species of tlie genus Giardia are found in the tadpoles (6r. agilis) 

 of Amphibia, in the blood (?) of the falcon (G. sanguinis), and in 

 the intestine {G. muris, G. microti, G. duodenalis, G. enterica) of 

 mammals, especially of rodents, eats, and man. There are morpho- 

 logical grounds to support the view (Bensen, 1908; Kofoid, 1920) that 

 there are several species in the genus, and that the species in rodents 

 differ from that found in man. This matter is important in the pre- 

 vention of human infection, for if any of the species in mice and rats 

 should be identical with that in man, the sources of infection are 

 greatly increased because of possibilities of contamination of food 

 supplies ,in the granary, warehouse, mill, bakerj', and stoi-e by the 

 infected faeces of these rodent pests. On the other hand, if the species 

 in man is distinct from those in rodents, the sources of human infection 

 are more circumscribed, and are limited mainly to the contamination 

 of food or water by water-borne and fly-borne cysts from human faeces, 

 by the dirty hand of the infected food handler, and by contaminated 

 washbowl or towel. 



It is the purpose of this paper to define accurately the morphological 

 features of the Giardia parasitic in man, and thus to assist in the 

 clearer definition of the morphological distinctions which may be used 

 to separate this parasite from related species of Giardia in other 

 mammals, notably in rodents. The accurate description will also afford 

 a basis for the accurate microscopical diagnosis of the infection in the 

 examination of h\iman stools. 



We liave elsewhere (Kofoid, 1920) set forth the grounds under the 

 Zoological Code of Nomenclature for using the name Giardia enterica 

 for the parasite widely designated in medical treatises as Lamilia 

 intestinalis. Earlier investigators (Grassi, 1881; Grassi and Schewia- 

 koff, 1888; Metzner, 1901, among others) regarded these flagellates in 

 man and in rabbits, rats, mice, and cats as but a single species desig- 

 nated as Megastoma entericuni, and later commonly known as Lamblia 

 intestinalis. In consequence of this fact figures of the Giardia ascribed 

 to man have often been drawn from specimens taken from rodents and 

 have been widely copied in textbooks as figures of the human parasites. 

 Thus, even Fantham, Stephens, and Theobald, in their recent work 

 on the Animal Parasites of Man (1916), reproduce the widely copied 

 fiigures of Grassi and Schewiakoff (1888) as Lamblia intestinalis, 

 although the latter authors explicitly state that their material came 

 from rats and mice. 



