52 SOME MINUTE ANIMAL PARASITES 



a nick or constriction appears at its centre, much 

 movement occurs, and finally the organism parts 

 in two, the daughter individuals swimming away in 

 different directions as soon as separation is effected. 

 Sometimes the new individuals are equal in width, 

 at other times marked dissimilarity occurs, and hence 

 there is a considerable variation among the parasites 

 encountered, due to differences both of division and 

 of rate of growth. 



Ultimately the flagellate commences to prepare for 



FIG. ii CRITHIDIA PULICIS : FOUR STAGES IN THE FORMATION 

 OF POST- FLAGELLATE FORMS 



life outside the body of its host, and to pass into 

 a form adapted for transference to a new host. 

 Gradually the Crithidia concentrates its body sub- 

 stance into a more or less oval mass. Its membrane 

 and flagellum are absorbed, its nuclei approximate, 

 and finally a thin, gelatinous cyst appears around the 

 organism. This cyst rapidly hardens, forming a 

 varnish-like coat around the organism (Fig. n). In 

 this form the post-flagellate stage of the Crithidia is 

 expelled from the body of its first host, only to be 

 swallowed by some small larval flea feeding on the 



