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236 LEEUWENHOEK AND HIs “ LITTLE ANIMALS” 
explained, and would necessitate the assumption that 
Leeuwenhoek found his ciliates not merely in two different 
frogs, but in frogs of two different species—of which there is 
no indication whatever in his own writings. But if, on the 
other hand, it be agreed that Fig. A shows O. dimidiata, then 
the frog was a specimen of R. esculenta, and the smaller ciliate 
was Clearly Nyctotherus—not an opalinid at all. The figure 
certainly suggests this strongly, and everything else supports 
this obvious, natural, and easy interpretation. 
When we come to consider Leeuwenhoek’s Fig. C, however, 
we are faced with very grave difficulties. This organisin is 
said to have been like a “‘ river-eel ”’, and to have been abundant 
in the frog’s faeces: and in size it appears, from the figure, to 
have been longer than a Nyctotherus—though the drawings 
were not made accurately to scale. Elsewhere in Leeuwen- 
hoek’s writings we find nematodes (Anguillula, etc.), spirilla, 
and spirochaetes, all likened to “eels”: and in the rectal 
contents of . esculenta we may find not only various species 
of nematodes, spirilla, and spirochaetes, but also long flexible 
and actively motile bacilli (Bacillus flexilis and similar forms). 
To determine which—if any—of these Leeuwenhoek may have 
seen on this occasion, is I think impossible. I must therefore 
leave Fig. C unidentified, though I am inclined to believe that 
it depicts a larval nematode. 
I conclude, consequently, that Leeuwenhoek discovered 
and described Opalina, Nyctotherus, and Trichomonas (or 
Trichomastix) in the faeces of frogs, in addition to various other 
protozoa and bacteria which are not now identifiable.’ 
And now we come to a letter which is, perhaps, as famous 
as any Leeuwenhoek ever wrote to the Royal Society—the 
one containing his account of the ‘‘ animalcules”’ in the human 
mouth. This letter is frequently quoted—or rather mis- 
" For example, there is a definite indication of the mouth, and the 
outline is—to me—quite convincing. The interpretation of Fig. B as 
Balantidium coli—recently put forward by Pritze (1928)—is so outrageous 
as to deserve no further notice. 
* I ought perhaps to point out that Prowazek (1913) has identified some 
of these organisms with Balantidiwm coli. Although he has been copied by 
others (e.g. Pritze, 1928), his interpretation is manifestly absurd. There is 
no evidence that L. ever saw any species of Balantidiwm in the frog, and 
B. colt certainly does not live in this host. 
