ECOLOGY, C0MMENSAL1SM AND PARASITISM 371 



ments have shown indeed that L. donovani lives and multiplies in 

 the digestive tracts of various kinds of blood-sucking arthropods- 

 mosquitoes, sand flies, fleas and bed-bugs, but no experiments 

 involving transmission to man have been successful. With L. 

 tropica the evidence is more positive and numerous successful 

 experiments in producing skin ulcers from leptomonas forms in 

 sand flies of the genus Phlebotomus have led to the general belief 

 that this type of insect at least is capable of transmitting not only 

 L. tropica but L. braziliensis as well. 



The genus Herpetomonas, while not a parasite of vertebrates, is 

 interesting in having a stage in which it resembles a trypanosome. 

 In this stage the axial filament, as a rhizoplast, runs along the 

 margin of the cell, without however raising the periplast to form 

 an undulating membrane (Fig. 169, B). 



Trypanosoma, in its fully-developed phase, differs from related 

 forms of protomonads in having an undulating membrane, the 

 margin of which is formed by the axial filament of the flagellum 

 which ends in a free whip or terminates at the anterior end (Fig. 

 169, D, I). The axial filament arises posterior to the nucleus. Near 

 it is a conspicuous granule, homologized by Kofoid and his school 

 as a parabasal body. This, by use of the Feulgen nucleal reaction, 

 has been shown to contain thymonucleic acid (see p. US). The 

 combination of blepharoplast and parabasal is termed the kineto- 

 plast by Wenyon. The nucleus is usually spherical, with the 

 usual protomonad endosome lying in a clear space within a nuclear 

 membrane. The cytoplasm is usually clear and homogeneous but 

 contains volutin granules, as a rule, and a small vacuole frequently 

 lies near the kinetoplast. Reproduction is always by longitudinal 

 division which begins with the kinetoplast. The cell divides first at 

 the flagellar end, the posterior end with the kinetoplast dividing last. 



These few structural characters afford very little basis for divi- 

 sion of the genus into species, while the numerous changes which 

 the same species may undergo in the course of its life history make 

 it still more difficult. Size is some help, the largest forms occurring 

 in cold-blooded vertebrates. Other characters are relative length 

 of flagellum, distance from kinetoplast to posterior end, rounded 

 or pointed posterior end, position of the nucleus, etc. The ten- 

 dency is to name a trypanosome according to the host in which it 

 is found provided there are no specific structural characters by 

 which it can be identified — such methods may swell the synonyms 

 but they are relatively harmless until the full life history is worked 

 out in each case. 



The following list of species, 1 while not complete, gives some idea 

 of the distribution of trypanosomes and of the enormous literature 

 on the subject: 



1 Compiled from Wenyon, Protozoology; Biological Abstracts; Zoological Record, 

 and miscellaneous sources. 



