8 University of California Publications in Zoology [Vol. 16 



The dividing flagellate moves rapidly and vigorously throughout 

 the process of division. As .soon as the two flagella are present the 

 longitudinal division of the protoplasm is hastened by the antagonistic 

 movements on the part of the two daughter individuals. 



V. LIFE-HISTORY 



The life-history of Crithidia leptocoridis as outlined in plates 1-4 

 has been based largely upon a study of the living material supple- 

 mented by stained preparations. However, the interpretation of the 

 life-history of siich a parasitic flagellate is subject to much error since 

 it is largely a matter of conjecture owing to the fact that continuous 

 development can not be followed step by step. It must also be con- 

 sidered, in the first place, that some of the parasites may be degenerat- 

 ing forms, secondly that fixation and staining have distorted the 

 structure, and thirdly that the life-cycle of several parasites could be 

 readily confused. 



For these reasons it is with .some hesitation that a great number 

 of doubtful forms have been included in this paper, namely, two 

 different types of preflagellates and the so-called multiple fission forms 

 of the life-cycle. Fragmentary references and figures in the papers 

 of prior investigators to .such doixbtful forms have occurred, as will 

 be pointed out in a later paper. 



At the present time it seems best to follow the old system of nomen- 

 clature for the several stages of the crithidial life cycle, namely, pre- 

 flagellate, flagellate and postflagellate. The terms stomach and rectal 

 phase (Minchin and Thomson, 1915), together with the term vegeta- 

 tive phase would seem also to be appropriate in the light of the 

 present investigations of this flagellate. 



The life-cycle of Crithidia leptocoridis is divided up into three 

 stages: the preflagellate (pi. 1, figs. 1-28; pi. 2, figs. 21-33) which 

 includes the stomach phase of the life-cycle; the flagellate (pi. 2, figs. 

 34-56; pi. 3, figs. 57-73), which includes the flagellates in the vege- 

 tative phase as found in the mid-intestine, ilium and colon ; and the 

 postflagellate (pi. 4, figs. 74-108) which includes the rectal forms. 



2. Preflagellate (Stomach Phase) 

 The preflagellate forms (pi. 1, figs. 1-28; pi. 2, figs. 24-53) are 

 found in the stomach of young nymphs. Immediately after the in- 

 gestion of the spores, which are small, round, or oval forms from 1.5 



