298 



THE PROTOZOA 



undergo the principal phase of their cycle. In the development of 

 the trypanosomes of fresh-water fish in the leech Hemidepsis, the 

 parasites do not pass farther back than the crop (Robertson, 503). 

 Finally, in the many species of pathogenic trypanosomes which are 

 transmitted by tsetse-flies of various species, two types of develop- 

 mental cycle can be distinguished : in the one, the parasite invades 



Fid. 129. Trypanosoma granulosum of the common eel : four different sizes, 

 probably stages of growth. After Minchin (478), x 2,000. 



the whole alimentary canal of the fly ; in the other it undergoes the 

 greater part of its development irr the proboscis and pharynx alone. 

 The details of the developmental cycle in the invertebrate host 

 are very inadequately known, and have only been studied in a 

 very few instances. As a rule the characteristic form of this 

 part of the life-history is a crithidial or trypanomonad type, repre- 



