LIFE-HISTOKY 343 



brate hosts. As regards the forms which occur in vertebrates, it will be 

 found that the organisms may assume any of the forms between the try- 

 panosome and the leishmania types. In the blood or other fluids of the 

 body they are usually provided with flagella, and have the trypanosome 

 structure, but occasionally, as in the case of Trypanosoma lewisi, free- 

 swimming forms of the crithidia or leptomonas type occur also. If the 

 flagellates are intracellular, they tend to be of the leishmania type, as in 

 the case of Leishmania donovani, L. tropica, and T. cruzi. In the case of 

 T. cruzi, however, after a number of intracellular leishmania forms have 

 been produced, they gradually become transformed through a crithidia 

 phase into flagellates having the trypanosome structure, while maintaining 

 their intracellular position. Any of these various forms, whether free- 

 swimming in the fluids of the body or in the cytoplasm of cells, can 

 reproduce by binary fission. 



In the invertebrate host, the members of the genera Trypanosoma and 

 Herpetomonas may occur in any form between the leishmania and the 

 trypanosome. The members of the genus Crithidia never pass beyond 

 the crithidia stage, while those of the genus Leptomonas never pass beyond 

 the leptomonas stage. Multiplication of all these forms by binary fission 

 takes place as in the vertebrate. During the development in the inverte- 

 brate, the flagellates may be provided with flagella, by means of which 

 they swim freely in the lumen of the gut, proboscis, salivary gland, or in 

 other situations. These free forms were termed nectomonads by Minchin 

 and Thomson (1915). On the other hand they may attach themselves 

 to the lining cells by their anterior extremities, in which case the 

 flagella are lost, but in the arrangement of the nucleus, kinetoplast, and 

 axoneme they may have the trypanosome, crithidia, leptomonas, or leish- 

 mania structure. Such attached forms have been called haptomonads by 

 Woodcock (1914). The attached forms may retain their elongate character 

 in the anterior portions of the intestine. In the hind-gut there is a general 

 tendency for the elongate flagellates to become much shortened, though 

 the nuclei and kinetoplasts may retain their relative positions. The 

 haptomonad forms occur very commonly in the case of those flagellates 

 which are limited entirely to invertebrates, but they are also found in the 

 invertebrate phase of development of trypanosomes. Thus, they occur 

 in the case of T. lewisi in the hind-gut of fleas, T. vivax in the proboscis of 

 Glossina morsitans, and T. gambiense in the salivary glands of G. palpalis. 



The flagellates belonging to the genera Herpetomonas, Crithidia, and 

 Leptomonas have but a single invertebrate host, infection being spread 

 by means of encysted leishmania forms passed in the dejecta. When 

 such an encysted form is eaten by a new host, the liberated leishmania 

 form gradually grows to the adult flagellate form. During this period of 



