266 PROTOZOOLOGY 



arises a single flagellum, around which occurs a ring of extremely fine 

 filopods; nucleus central; a contractile vacuole; holo zoic; fresh water. 



P. pulex P. (Fig. 114, d). 6-12m broad. 



Genus Histomonas Tyzzer. Actively amoeboid; mostly rounded, 

 sometimes elongate; a single nucleus; an extremely fine flagellum 

 arises from a blepharoplast, located close to nucleus; axostyle (?) 

 sometimes present; in domestic fowls. One species. 



H. meleagridis (Smith) {Amoeba meleagridis S.) (Figs. 114, e; 115). 

 Actively amoeboid organism; usually rounded; 8-21)u (average 10- 

 14 n) in the largest diameter; nucleus circular or pyriform with a large 



abed 



Fig. 115. Histomonas meleagridis as seen in smears from culture. 

 X1650 (Bishop) a, an amoeboid form; b, rounded form with axostyle 

 (?); c, nucleus preparing to divide; d, a stage in nuclear division. 



endosome; a fine flagellum; food vacuoles contain bacteria, starch 

 grains and erythrocytes; binary fission; during division flagellum 

 is discarded; cysts unobserved; in young turkeys, chicks, grouse, and 

 quail (Tyzzer). 



This organism is the cause of enterohepatitis known as "black- 

 head, "an infectious disease, in young turkeys in which it is often 

 fatal and also in other fowls. Smith (1895) discovered the organism 

 and considered it an amoeba. It invades and destroys the mucosa of 

 the intestine and caeca as well as the liver tissues. Trophozoites 

 voided in faeces by infected birds may become the source of new 

 infection when taken in by young birds with drink or food. Tyzzer 

 however demonstrated that the organism is transmissible from bird 

 to bird in the eggs of the nematode Heterakis gallinae. Bishop (1938) 

 made a cultural study of the organism. Dobell (1940) points out the 

 similarity between this flagellate and Dientamoeha fragilis (p. 368). 

 Wenrich (1943) made a comparative study of forms found in the 

 caecal smears of wild ring-neck pheasants and of chicks. The organ- 

 isms measured 5-SOfj. in diameter and possessed 1-4 flagella, though 

 often there were no flagella. 



Genus Rhizomastix Alexeieff. Body amoeboid; nucleus central: 

 blepharoplast located between nucleus and posterior end; a long 



