It was not noticed again until it was re-discovered by Lankester in 1871, who published the above figure of it in the _Quarterly Journal of Microscopical Science_ in that year.] As to the parasite itself--the trypanosome--a long and very interesting story has now to be told. The first blood-parasite ever made known to naturalists and medical men was that to which Gruby, in 1843, gave the name _Trypanosoma sanguinis_. He found it in the blood of the common frog. We have here reproduced a figure of this original trypanosome (fig. 49). Similar parasites had been seen, but not named, in the blood of fishes. These trypanosomes are all very minute and of a somewhat elongated form, a fair average length being one thousandth of an inch. They are simple protoplasmic animals, consisting of one single nucleated corpuscle. The protoplasm is drawn out at one end of the creature into a motile undulating thread, and from the point where this joins the body a membranous undulating crest extends along the greater part of the animal’s length. There is no mouth, nutrition being effected by the imbibition of soluble nutrient matter. After a long interval Gruby’s trypanosome was re-discovered in 1871; and then several kinds were described in the blood of tortoises, fishes and birds. In 1878, Dr. Timothy Lewis found a parasite in the blood of rats, at first in India, and subsequently in the common rats of London sewers. This parasite resembles a trypanosome in many respects (fig. 46A), but was very properly given a distinct name by Savile Kent, who called it “Herpetomonas.” This name has, however, been dropped; and the rat’s-blood parasite is spoken of as a trypanosome. It is the _Trypanosoma Lewisii_, and was the first of these trypanosomes to be found in the blood of a mammalian animal. The _Trypanosoma Lewisii_ of the rat’s blood seems to do no harm to the rat, in which it swarms, multiplying itself by longitudinal fission; nor is it at present known to produce any trouble in other animals when transferred to their blood. Similarly, the frog’s trypanosome seems to exist innocently in the frog’s blood. The next trypanosome discovered (1880) was, however found in the blood of camels, horses, and cattle suffering from a deadly disease known in India by the name “surra.” It is called _Trypanosoma Evansii_, after the observer who detected it. Trypanosomes now began to get a bad name, for the next was discovered in animals afflicted by a North African disease known to French veterinaries as “dourine.” This trypanosome was called _T. equiperdum_.