360 
Crithidia gerridis 
Patton (1908) described an insect parasite found in Gerris fossarum, 
Microvelia and Perittopus sp., studied the life-histoiy most carefully and 
divided it into three phases, a pre-flagellate, a flagellate and a post- 
flagellate phase. He followed out the entire development of the parasite, 
and showed the similarity between the pre-flagellate stages of a Herpe- 
tomonas and Leger’s immature parasite, noting also the great likeness 
between the adult flagellate, Crithidia fasciculata -and the adult 
G. gerridis. 
Patton (1908, p. 142) sums up the situation thus:—“The genus 
Crithidia of Leger is therefore for the present best represented by all 
such flagellates in which the adult form is characterised by an attenu¬ 
ated anterior end to which the flagellum is attached by a rudimentary 
undulating membrane; the blepharoplast in all such flagellates is 
situated close up to the nucleus, never posterior to it.” 
Flu (1908) and Swingle (1908) working on a parasite recorded first 
by Pfeiffer (1905) from the gut of Melophagus ovinus have accepted 
the genus Crithidia as outlined above, and their flagellate is named 
Crithidia melophagia. I have worked myself on the latter parasite 
and consider it to belong to the genus Crithidia. 
Patton (1909) has described a Crithidia from two species of Tabanids, 
and has written (xn. 1908) a critical review of the relations of blood 
sucking invertebrates to the life-cycle of the Trypanosomes of verte¬ 
brates. Dealing there with the vexed question of the three genera 
( Trypanosoma, Crithidia, Herpetomonas ), he defines (p. 338) the genus 
Crithidia in the manner above stated, and in addition, the three 
characteristic stages in the life-cycle are mentioned—“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.” 
Having regard to the above definition, there is no doubt that the 
parasite with which I have been dealing is a true Crithidia. Further, 
Patton has described a similar organism from a very nearly allied host, 
another species of Gerris. Though the Crithidia here described from 
Gerris paludum, grows larger than that from Gerris fossarum, and has 
now been found in the ovaries as well as in the gut, the great simi¬ 
larities between the two Crithidia are such that it would be quite 
