128 RESEARCH IN PROTOZOOLOGY 



parasite relationships. Species should, however, be recognizable 

 morphologically before their status as distinct species is granted. 

 The chief problems are (i) to determine the characters which 

 differentiate any one species from others; (2) to determine the 

 limits of variation for these characters; and (3) to determine the 

 physiological characteristics as regards to host-parasite specificity 

 and their relation to cultural and other conditions outside the host. 

 In this section morphological features will be considered primarily. 

 Host-parasite specificity is considered in another section. 



2. Key to genera and species 



The following key is based primarily on the characters that are 

 revealed in adequately fixed and stained slides. 



I ( 12) MONOZOA, without bilateral symmetry 2 



2(3) Body elongately pear-shaped, rounded anteriorly, tapering rather 

 abruptly to a posterior spike or tail, no axostyle, cytostome 

 large, containing an undulating flagellum, nucleus far forward, 

 three anterior fiagella, 10-18 11 long by 5-8 /i wide (Fig. 13, a) 



Chilomastix bettencourti 

 3(2) Body pear-shaped to spindle-shaped, with axostyle, trtcho- 



MONADID^ 4 



4(11) Posterior flagellum attached to body by a spirally wound un- 

 dulating membrane 5 



5(10) With three anterior flagella, genus Tritrichomonas 6 



6(9) Anterior flagella about half as long as body; undulating mem- 

 brane and chromatic basal rod well developed ; axostyle hyaline, 

 of uniform diameter through the body, tapering rapidly to a 

 sharp point at the posterior end, with chromatic ring just above 

 the point of emergence from the body 7 



7(8) Body 8-20 /i long, inner and outer rows of chromatic granules 

 well developed ; late prophases show six double chromosomes 

 besides the waning karyosome (Fig. 12, a) . . . .Tritrichomonas muris 



8(7) Body 3-10 fi long, inner and outer rows of chromatic granules 

 not well developed, late prophases show 3 (or 4?) chromosomes 

 (Fig. 12, c) Tritrichomonas minuta 



9(6) Body 5-11 fi long; three anterior flagella as long as or longer 

 than the body; undulating membrane not well developed, 

 usually sharply spiral in position not extending beyond the 

 middle of the body; no chromatic basal rod; anterior third or 

 half of body usually packed with chromatic granules, obscur- 

 ing the nucleus; nucleus, when visible, has prominent, usually 

 central round karyosome ; axostyle slender, tapering gradually 

 to the posterior end which may protrude for % io Ys its length 

 beyond the body; no chromatic ring at point of emergence 

 (Fig. 12, d) Tritrichomonas parva 



