178 The Mastigophora 



stages may arise either from leptomonad or trypanosomal forms in the 

 natural host, and the crithidial stage typically lacks an undulating mem- 

 brane. 



Trypanosoma Gruby (Fig. 4. 41, H-L). The life-cycle usually involves 

 both vertebrates and invertebrates (arthropods, leeches). The trypano- 

 somal stage occurs in the blood of vertebrates, while leptomonad and 

 crithidial forms are rare, if they are found at all. Intracellular leishmanial 

 stages may occur, as in T. cruzi. All four stages may occur in the inverte- 

 brate host. Haptomonads may be expected in insects infected with T. 

 lewisi (hind-gut of fleas), T. vivax (proboscis of Glossina morsitans), or 

 T. gamhiense ("salivary glands" of Glossina palpalis), for example. The 

 stage infective for vertebrates — the metacyclic trypanosome — is typically 

 an active trypanosomal form often derived from crithidial haptomonads. 



Methods of transfer from invertebrate to vertebrate vary with the 

 species of Trypanosoma. In one group, which includes T. cruzi of man, 

 T. lewisi of rats, and T. melophagiu7n of sheep, metacyclic forms are 

 voided from the hind-gut of the vector, and infection of the vertebrate 

 follows contamination of wounds or mucous membranes. The metacyclic 

 stages of T. gamhiense, T. rhodesiense, T. evansi, and similar species de- 

 velop anteriorly in the vector and are transferred to the vertebrate host 

 by inoculation. A third type, represented by T. equiperdum, is trans- 

 ferred in vertebrates by coital contact and the vector has dropped out of 

 the cycle. 



Vertebrate hosts of trypanosomes include fishes. Amphibia, aquatic and 

 terrestrial reptiles, birds, and various groups of mammals. Most species 

 of Trypanosoma, if not all, are probably non-pathogenic in their natural 

 hosts, or at least produce no serious damage. In man and domesticated 

 ungulates, however, several species cause diseases of considerable medical 

 and economic importance. This is particularly true in the tsetse fly areas 

 of Africa, where sleeping sickness of man (Chapter XII) and trypanosomi- 

 asis in cattle, sheep, horses, and goats have been important hindrances 

 to economic and social progress. 



Family 4. Cryptobiidae. These are biflagellate parasites with a kineto- 

 plast somewhat larger than that of the Trypanosomidae. One of the 

 flagella extends anteriorly. The other, which is usually adherent to the 

 body and may or may not form part of an undulating membrane, extends 

 posteriorly as a free trailing portion. 



The genera Cryptobia Leidy (Fig. 4. 42, A) and Trypanoplasma La- 

 veran and Mesnil are usually included, although some workers believe 

 that Trypanoplasma is a synonym of Cryptobia. However, this question 

 needs further study, since an undulating membrane has been described 

 in various species of Trypanoplasma but is absent in Cryptobia helicis 

 (166). Furthermore, the aciculum of C. helicis may be lacking in T^ypano- 



