My last letter to you was that of July 28th with enclosure to Mr. Boyle. . . . I hope that these numerous observations will not seem tedious to you, and that you will communicate them to the Curious.

In the year 1675 about mid-September being busy with studying air when I had much compressed it by means of water I discover'd living creatures in Rain water, which had stood but few days in a new [cask, painted] blew within. This invited me to view this water with great attention, especially those little animals appearing to me ten thousand times less than those represented by Mons. Swamerdam, and by him called Water-fleas or Water-lice, which may be perceived [to live and to move] with the naked eye.

The first sort by me discover'd in the said water, I after divers times observed to consist of 5, 6, 7, or 8 [very] clear globuls, without being able to discern any film [or skin] that held them together, or contained them. When these animalcula did move, they put forth two little horns, [like a horse's ears] continually moving themselves: The place between these two horns was flat, though the rest of the body was roundish, sharpning a little towards the end, where they had a tayl, near four times the length of the whole body, of the thickness (by my Microscope) of a Spiders-web; at the end of which appear'd a globul, of the bigness of one of those which made up the body; which tayl I could not perceive, even in very clear water, to be mov'd by them. These little creatures [were the most wretched ones I ever saw;] if they chanced to light [with the globule] upon the least filament or string, or other such particle, of which there are many in water, especially after it hath stood some days, they stood intangled therein, extending their body in a long round, and striving to dis-intangle their tayl [by strong extension]; whereby it came to pass, that their whole body lept back towards the globul of the tayl, which then rolled together Serpent-like, and after the manner of Copper- or Iron-wire that having been wound about a stick, and unwound again, retains those windings and turnings. This motion of extension and contraction continued a while; and I have seen several hundreds of these poor creatures, within the space of a grain of gross sand, lye fast cluster'd together in a few filaments.

I also discover'd a second sort, the figure of which was oval; and I imagined their head to stand on the sharp end. These were a little bigger than the former. The inferior part of their body is flat, furnished with divers incredibly thin feet, which moved very nimbly, and which I was not able to discern till after several Observations. The upper part of the body was round, and had within 8, 10, or 12 globuls, [otherwise the said animals were very clear]. These little Animals did sometimes change their figure into a perfect round, especially when they came to lye on any dry place. Their body was also very flexible; for as soon as they hit against any the smallest fibre or string, their body was bent in, which bending presently also yerked out again, [just as if you press a bladder full of water with your finger; on removing your finger the bladder loses the impressed dent. Yet the greatest marvel was] when I put any of them on a dry place, I observ'd, that changing themselves into a round, their body was raised pyramidal-wise with an extant point in the middle, and having lain thus a little while with a motion of their feet, they burst asunder, and the globuls [and the watery liquid] were presently diffus'd and dissipated, [without my being able to] discern the least thing of any film, in which the globuls had doubtless been inclosed: And at this time of their bursting asunder I was able to discover more globuls than when they were alive [, which I figure to happen like this: for instance imagine that you have a sheep's bladder filled with shot, peas and water; then, if you were to dash the bladder in pieces on the ground, the shot, peas and water would be scattered in all directions].

But then I observ'd a third sort of little Animals, that were [about] twice as long as broad, and to my eye yet eight times smaller then the first. Yet for all this, I thought I discern'd little feet, whereby they moved very briskly, both in a round and streight line.