338 
A Critical Review, etc. 
yet they undoubtedly belong to the same parasite. Again, we would 
refer to the striking difference in size between the cysts of Herpetomonas 
lygaei(S'o p to 4 p,)and the adult flagellate which may measure as much 
as 25 p. 
In order, therefore, to make our conception of the genus Crithidia 
quite clear we will define it as follows :— 
Crithidia Leger, 1902 (emended by Patton, 1907). 
Flagellates which in their adult stages have a fusiform body with a 
blepharoplast, usually a large rod-shaped structure, situated close to the 
nucleus either anterior or a little distance posterior; their anterior ends 
are attenuated and drawn out along the flagella to which they are 
attached by a narrow undulating membrane, which never has the 
characteristic folded appearance seen in adult flagellates of the genus 
Trypanosoma. Their posterior ends may be blunt or pointed. They have 
three characteristic stages in their life-cycles: pre-flagellate, round or oval 
bodies with a nucleus and blepharoplast which multiply by simple fission ; 
flagellate stage when they multiply by longitudinal division which may 
be either equal or unequal; in this stage they often exhibit marked 
polymorphism; post-flagellate stage when the flagellates shorten, divide 
and then encyst, some species (in ticks, leeches and Melophagus ovinus) 
pass this stage in the eggs of their hosts. 
Fig. 9 represents two adult flagellates of Crithidia haemaphysalidis, 
in one the blepharoplast is well behind the nucleus; note the pointed 
posterior ends. Fig. 10 is an adult flagellate of Crithidia gerridis. 
These flagellates differ markedly from the genus Herpetomonas 
Figs. 9—12, 
