288 BIOLOGY OF THE PROTOZOA 



substance of the cell as a rhizoplast thus forming an axial fibril 

 (Cercomastix). The method of insertion is quite variable (see 

 Chapter II). Fertilization processes, involving macrogametes and 

 microgametes have been described in detail by Goldschmidt (1905), 

 but there is considerable scepticism over his results. 



Order II. PROTOMASTIGroA. 



The flagellates included in this Order are little known for the most 

 part, and their affinities are obscure. They are amongst the 

 smallest of the Protozoa and are abundant in waters ever\^'here, 

 particularly in stagnant pools and infusions. Here also are included 

 some of the most pernicious of the protozoan parasites of man. 

 They well illustrate the power of continued adaptation to new con- 

 ditions in the host, leading to progressive parasitism w^hereby an 

 original commensal may become a lethal parasite. Such a transi- 

 tion is shown b^' the genera Leptomonas, Ilerpetomonas, Crithidia, 

 Leishmania and Trypanosoma. The first of these is apparently a 

 harmless commensal of the intestine (Leptomonas of nematode 

 worms, Herpetowonas of the common house-fly) w^ith quiescent, 

 non-flagellated stages free in the lumen of the gut. Crithidia 

 species live in the intestinal tracts of various larval forms of insects 

 and are remarkable in having a free-swimming or nectomonad stage 

 and a quiescent usually attached, resting, haptomonad stage. The 

 nectomonads become transformed into haptomonads by attach- 

 ment to the surface of epithelial cells, where they lose their flagella, 

 live and multiply as extracellular parasites (P'ig. 138). With 

 Leishmania, parasitism goes a step further; the nectomonads are 

 in the blood and are carried to various organs of the body where as 

 intracellular parasites they live and multiply, ^'arious forms of 

 leishmaniasis are caused by different species of this genus. Kala- 

 azar of India is due to L. donovani, Mesnil; tropical ulcer of the 

 near East to L. tropica; infantile ulcer of the Mediterranean regions 

 to L. infantum (Nicolle) ; and the Brazilian disease known as Espun- 

 dia to L. tropica var. Americana or Braziliensis (Vianna). In all 

 of these diseases the quiescent phases are passed in cells of different 

 organs. Trypanosoma leicisi and T. cruzi, the former a rat para- 

 site, the latter a human parasite and cause of American trypano- 

 somiasis (Chagas' disease) have similar intracellular stages, the 

 latter in the mammalian host, the former in the intestinal cells of- 

 the transmitting host a rat-flea CeratojihyUns fasciatus (Fig. 116, 

 p. 238). 



The trypanosomes of African sleeping sickness (T. gainhiense and 

 T. rhodesiense) affect the human host in (juite a different \va\. This 

 is a disease of the lymphatics and the characteristic symptoms are 



