234 LEEUWENHOEK AND HIS “LITTLE ANIMALS”’ 
last spoken of was so great, that I judged there were 
several thousand of ’em in every bit of matter as big as a 
sand-grain. 
On seeing this, I took it for certain that the animalcules 
which I said before were in the blood (which I had taken 
out of the dish), came to be there only in this way: 
namely, because the frog, when it was being cut up, 
voided its dirt into the dish, or because I had unwittingly 
wounded the guts of the frog, in whose dirt there must 
have been many animalcules, which got mixed with the 
blood. 
It has been generally allowed that the larger animalcules 
which Leeuwenhoek here described and figured (his Figs. A 
and B) were ciliates—which occur so commonly in the guts 
of frogs. But it is possible, I believe, to identify them more 
exactly. To anybody familiar with the intestinal fauna of 
the two common frogs of Northern Europe (Rana temporaria 
and R. esculenta), it must be obvious that the abundant long 
organisms (Hig. A) were Opalina (Cépédea) dimidiata, while 
the smaller, scantier, and more rounded creature (Fig. B) was 
Nyctotherus cordiformis. Both of these occur very commonly 
in Lana esculenta, and I therefore infer that the frogs which 
Leeuwenhoek was studying on this occasion were of this 
species.” 
It would have been impossible to determine the exact 
species of these ciliates before the intestinal protozoa of frogs 
had been adequately studied. Consequently, most of the early 
identifications are not worth serious consideration. The first 
correct determination we owe, I think, to Biitschli, who says 
of frogs’ faeces under the microscope. The “animalcules’”’ responsible for 
the phenomenon are flagellates (Trichomonas, Trichomastix, Chilomastiz, 
and Hexamuta), and various motile bacteria. 
" O. dimidiata is well known as the characteristic species of opalinid 
from this host: Nyctotherws, however, is usually described as occurring in 
i. temporaria only. That the above statement is correct I know from my 
own observations. 
* The frog in which L. found the worms previously was—as already 
noted—f. temporaria: but there is nothing in his words to prove that the 
frogs of the second batch were not of the other species. If they were, then 
the above identification amounts almost to a certainty. 
