162 University of California Publications in Zoology [Vol. 20 



The recent investigation by Alexeieff (1914) of the finer structure 

 of Giardia and our own more extended investigation (Kofoid and 

 Christiansen, 1915 a and ft) bring convincing proof of morphological 

 similarity of the species referred to Giardia to those heretofore included 

 in Lamblia. They have the same organelles in similar relationships. 

 Their differences consist solely in contour, shape, proportion, size, 

 hosts, and shape of the parabasal bodies, and are valid only to separate 

 species. The species of Giardia and Lamblia should therefore all be 

 placed in one genus, as in the case of the various species of TricJio- 

 monas and Trypanosoma. By the accepted laws of nomenclature 

 Giardia has priority. 



The several species which have been described, especially by Bensen 

 (1908), are based on slight differences in form and proportions, and 

 Noc (1908) raises the question as to the validity of these distinctions. 

 After our experience with the material which we have examined, 

 which includes five species, Giardia alata, G. muris, G. microti, G. 

 duodenalis, and G. enterica, and a wide experience with the genus 

 Trichomonas, we are inclined to regard Noc's skepticism as not well 

 grounded. One learns to detect those differences in size and propor- 

 tions, due to movement and growth, and to recognize others, usually 

 a group of characters, which constitute the specific distinctions, such 

 for example as those between G. muris and G. microti. We believe 

 therefore that Bensen (1908) was correct in making distinctions 

 though it may be that not all his species will stand the test of fuller 

 examination, and that not all his distinctive characters will prove valid. 



This matter is one of more than restricted interest for it impinges, 

 on the one hand, on the general biological question as to how far 

 specificity of parasites to particular hosts obtains, and on the other 

 hand on the very practical question as to whether or not rodents, 

 e-specially mice, rats, and Belgian hares, are sources of infection to 

 man, for these flagellate parasites, in mice and men at least, are often 

 associated with chronic intestinal disturbances (Noc, 1909 ; Roden- 

 waldt, 1912; Jollos, 1913; Kofoid and Christiansen, 1915). If the 

 species are distinct, it is less probable that rodents serve as sources 

 for human infection. If these so-called specific differences are illusory, 

 on the other hand, the dangers from rodents as carriers and sources 

 are greatly enhanced. 



There remains the further problem which experiment only can 

 solve a.s to whether or not the same species of flagellate may pass 

 through a series of host species and assume in each, or in certain 



