A. Porter 
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unjustifiable to create a new species for the parasite of G. paludum. 
It is probable that Crithidia gerridis is far more widely distributed 
among water-bugs than is at present known, and the present tendency 
to create new species of parasites, owing to slight differences of host, is 
to be deplored. Infection experiments are necessary to determine 
whether any parasite is absolutely restricted to one host or not, before 
exalting it to specific rank. 
The exact systematic position of the genus Crithidia is somewhat 
difficult to determine. The possession of nucleus, blepharoplast and 
definite undulating membrane and flagella places it very near the 
Trypanosomes, but there is a difference in habitat, the majority of the 
Trypanosomes being blood inhabiting. Perhaps, however, this difference 
in habitat has been somewhat exaggerated, for the medium in which most 
of the Crithidia live is of much the same density as blood—in fact, 
Crithidia melophagia is commonly found mingled with either freshly 
drawn or partially digested blood of the sheep contained in the gut of 
Melophagus ovinus. Again, in the case of Herpetomonas jaculum, found 
in the gut of Nepa cinerea, the flagellate sometimes occurs in a medium 
containing fish’s blood, when specimens of Nepa are examined just after 
attacking fish (sticklebacks). The importance of the density and com¬ 
position of the medium need not be so great, then, as is generally 
supposed. 
There is, then, little to debar Crithidia from a position in the family 
Trypanosomatidae, and when Leger created the genus Crithidia , he 
recognised the affinities of the two genera. The structure of Crithidia 
would place it somewhat higher in the scale of development than Herpe¬ 
tomonas, while its closer relation to the blood inhabiting Trypanosomes 
cannot be definitely stated until the early, probably pre-flagellate, 
stages of these latter parasites are known. The work of Moore and 
Breinl (1907) on the “latent” forms of Trypanosomes is very suggestive, 
as the latent forms are not unlike the post-flagellate stages of Crithidia k 
Summary. 
(1) Crithidia gerridis occurs in the alimentary tract of Gerris 
fossarnm, Microvelia and Perittopus sp. found in Madras. It is now 
1 In this connection see the recent work of Bruel and his colleagues in Uganda on the 
development of Trypanosoma gambiense in the stomach of the tse-tse fly ( Proc. Roy. Soc_ 
Ser. B, Vol. 81, No. B. 550, published Oct. 27th, 1909). PI. X, Figs. 15,16 show obviously 
rounded up forms of Trypanosomes from the midgut of the fly, such forms apparently 
being capable of resistance for a long time. 
