DISCOVERY OF COCCIDIA AND LEPTOMONADS 221 
Leeuwenhoek’s next observations on ‘ animalcules”’ 
inhabiting the bodies of other animals were written down 
in 1680 and printed some few years later. ‘They occur in the 
course of his description of the spermatozoa of insects—various 
species of which he dissected in his attempts to study their 
“seminal animalcules”. After recording some findings in 
flies, he casually adds the following: ' 
I have also seen, in the summer,’ in a big horse-fly 
(which was a female, out of which I pulled many eggs), a 
lot of small animalcules ; though these were not a sixth 
of the length of the animalcules aforesaid,* but a good 10 
times thicker.* They lay mingled with the thin matter ” 
that was in the fly’s guts, and moved forwards very 
quick. 
From the size and proportions of the “ animalcules”’ here 
mentioned, and from their situation, it can hardly be doubted 
that Leeuwenhoek actually saw on this occasion a protozoon 
(or possibly a bacterium) in the gut of a horse-fly. The 
dimensions of the organism are against the bacterial inter- 
pretation, but agree very well indeed with the supposition that 
he saw a Orithidia (or Leptomonas).° If this was so, then we 
1895; a practising London physician, and English minor poet) in 1839. 
He interpreted the lesions as “carcinoma”, and the oocysts as “a new 
form of the pus globule.” See Dobell (1922). 
* From Letter 38. 12 November 1680. To R. Hooke. MS.Roy.Soc. 
Published in Brieven, Vol. I (Dutch), and Opera Omnia, Vol. II (Latin). 
The full letter first appeared in print (in Dutch) in 1684. An incomplete 
English translation was published earlier, however, in Hooke’s Phil. Collect. 
(No. 3, pp. 51-58; 1681). The passage here translated is on p. 23 of the 
Dutch printed version. 
* Presumably 1680. 
° L. here refers—as the context shows—to the spermatozoa of “ the 
smallest sort of our common house-fly.”’ 
* On L.’s system this means that they had rather more than thrice the 
diameter of the fly’s spermatozoa. 
° 4.e., liquid or watery material. 
° These flagellates oceur—as is now well known—in many different 
species of Tabanidae. Several species of Crithidia have been described, in 
recent times, from the intestines of HKuropean species of Tabanus.—I may 
add that, forty years ago, Biitschli (1887-89; Vol. III, p. 1101) pointed out 
that the organisms here described by L. were probably flagellates; but he 
wrongly dated the observations 1695—the date of publication of the first 
Latin translation. 
