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Coccidia parasitic in man 
The genera Eimeria and Isospora. 
Eimeria. This genus is one of the three original genera of coccidia founded 
by Aime Schneider (1875). After passing through various vicissitudes, it can 
now be regarded as firmly established and well characterized. For present 
purposes, the most distinctive characters of the genus are those supplied by 
the oocysts and spores of the parasites. Each oocyst contains, when fully 
developed, four spores, each of which encloses two sporozoites. Coccidium 
Leuckart (1879) is a synonym of Eimeria, and is the name still preferred by 
some authors 1 , though indefensible on grounds of priority. The type species 
of Eimeria is E. falciform,is Eimer (1870) 2 , the common parasite of the mouse. 
The researches of Schaudinn (1900) removed most of the difficulties connected 
with the genus, which was afterwards firmly re-established by Stiles (1902), 
and Liihe (1902). It will be unnecessary to enter into further details here. 
Isospora. This genus is not so easily dealt with; in fact, it presents several 
difficulties and cannot even now be regarded as definitively established. The 
difficulties are due to our present ignorance regarding the type species, and 
they cannot be decisively removed until this has been reinvestigated. 
The genus Isospora was founded by Aime Schneider (1881) for an organism 
which, unfortunately, he did not study thoroughly, and of which he has left 
a very imperfect description. He found the parasite in “a little black slug'’— 
he does not say in what organ—which he captured at La Fere (Aisne). He 
did not record its size. He proposed to call it Isospora rara n.g., n.sp. 3 , but 
later referred to it himself as Isospora incerta 4 . He characterized the genus 
(and species) as follows: “psorosperm spherical, as in the preceding genus 5 , 
with contents undergoing division into two sporoblasts, which develop into 
two regularly pear-shaped spores, containing pretty numerous sporozoites 6 .” 
Schneider’s figures are hardly more illuminating, though it seems probable 
from them that the spores each contained approximately four sporozoites— 
at all events, probably not more than four. It is thus possible to maintain 
that Isospora denotes, according to Schneider’s own definition, a coccidian 
whose oocyst contains two polyzoic spores: or alternatively, that the oocyst 
really contained two tetrazoic spores. Subsequent authors have sometimes 
adopted the first interpretation, sometimes the second. Nobody, however, 
appears to have found the parasite again; and it offers certain difficulties to 
1 Minchin (1912) even goes so far as to say that the proposal to replace the name Coccidium 
by Eimeria is “contrary to public policy, and should not be followed, anything in the law of 
priority notwithstanding” (p. 346). I do not agree with this, and I do not believe that public 
policy is violated by obeying the law in this case. At all events the substitution of names referred 
to has already been effected by most writers, so that to revert to the name Coccidium now would 
not only contravene the law, but would also lead to still greater confusion. 
2 Called “Gregarina” falciformis by Eimer. 
3 Aime Schneider (1881), p. 401. 
4 Op. cit., description of PI. XXII, figs. 65-72, p. 403. 
5 This refers to the genus Klossia Ai. Schn. 
c “A corpuscules falciformes assez nombreux.” 
