522 Vniversitu of California Pithlicafioiis in Zoology [Vol.20 



dominance of gram-positive rods, Bacillus acidophilus. Cannon and 

 McXease have further shown that the pH of the caeca of rats, fed on 

 meat only, averages 7.1, while the caeeal contents assume an acidity 

 of pH 4.5 when the animals are fed on milk only. 



It would seem that one or both of two main factors are responsible 

 for the disappearance of the amoebae from the digestive tract of the 

 rats. Fir.st, it has previously been noted in this paper that amoebae 

 ingest gram-negative bacilli rather than gram-positive bacilli. The 

 change in the flora from gram-negative to gram-positive rods may 

 result in the starvation of the amoebae. Second, the great increase 

 in the acidity of the caeeal contents as the result of the lactic acid 

 produced from the milk may have an antagonistic effect on the 

 amoebae. 



DISCUSSION 



Classification and systematic relationships. — Differentiation into 

 species of animals so minute as amoebae and with such a limited num- 

 ber of characteristics has always been a difficult problem. In non- 

 parasitic amoebae, species characters are generally based on the type 

 of pseudopodia and the nuclear .structure (Schaeffer, 1920). Schaef- 

 fer said that these characteristics are hereditary and are due to 

 "fundamental chemical structure of the protopla-sm which is specific 

 for the species." In speaking of parasitic amoebae, Dobell (1921) 

 states that the important structural characters distinguishing the 

 various genera and species are supplied by the nuclear apparatus and 

 by the cysts. Differences in the types of pseudopodia formed and 

 structural differences in the protoplasm are also generally regarded 

 as bases for differentiation of species. It is recognized by the writer 

 that much unwarranted differentiation into species has occurred in 

 the pa.st, involution forms of the same species having been regarded 

 as showing specific differences. The mechanism of mito.sis is similar 

 in all species of parasitic amoeba and in certain pha.ses of mitosis it 

 is often impossible to distinguish different species from each other. 

 The tendency to confuse these different pha.ses should be carefully 

 guarded against and only those characteristics which are permanent 

 at some definite phase of the life cycle should be considered in differ- 

 entiation of .species; e.g., the resting stage of the nucleus. 



The further facts that it is possible for the same species of para- 

 sitic amoeba to be transferred from host to host without any recogniz- 

 able morphological change (Kes.sel, 19236) and that transfer of 



