VOL. XXVIII.] PHILOSOPHICAL TRANSACTIONS. 43 



motion, moving any part of their body, we presently conclude, that these parts 

 are not formed in vain ; and consequently we may conclude, that the wheel- 

 like motion of those animalcula is useful to their bodies, though we cannot tell 

 exactly how. 



The latter end of July, and beginning of August, I caused some of those 

 green weeds, commonly called duck-weed, to be taken out of the water, that 

 runs with a gentle stream through the town (Delft), for the pleasure of observ- 

 ing these animalcula, with others of several sorts, that were fastened to the 

 duck-weed, or ran about upon it. Among others, I have found some animal- 

 cula, whose sheaths, at the extreme part, were a little thicker than a head 

 hair, and composed of small globules, were very easy to be distinguished. 



I viewed one of these animalcula a good while together, and observed several 

 times, one after another, that when the animalculum thrusts its body out of the 

 sheath, or case, and that the wheel-like or indented particles moved in a circle, 

 at the same time, out of a clear and transparent place, a little round particle 

 appeared, which, without nicely viewing, could hardly be perceived ; which par- 

 ticle growing larger, moved with great swiftness, as it were, about its own axis, 

 and continued without any alteration in its place, till the animalculum had drawn 

 part of its body back into its sheath; in doing which, it placed the said round 

 particle on the edge of its sheath, which thus became augmented with a round 

 globule : and whereas the animalculum had placed the said globule on the east 

 part of its sheath, another time it fixed it on the south or north side; by which 

 means the sheath was regularly increased on all sides. 



Having further, and with great exactness, viewed the circulating indented 

 wheel-work, I observed that it caused an exceedingly great motion in the water 

 about it; by which means many very small particles, which were only visible 

 through the microscope, were wafted to the said animalculum, and others were 

 driven away. The animalculum made use of some of these particles, that were 

 thus drawn to it by its circulating instrument, for food and nourishment; 

 and other particles that were thus drawn to it, were with great nimbleness 

 driven away, and, as if rejected by the animalculum: from whence I inferred, 

 that those particles which were thus thrust away, were not proper for its food. 

 From this discovery we may conclude, that since this kind of animalculum can- 

 not move from place to place in the water, nor consequently pursue its food, 

 as other creatures do, that are endued with motion, being fastened by the tail 

 or other parts of the body, it must necessarily be provided with such instru- 

 ments as are fit to move the water, and by that means come at the particles 

 floating in it, which serve for the nourishment, increase^ and defence of 

 its body, 



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