Their shape is oval,
and contracted in the middle by a ring of vibrating curved ciliae.
It was, however, very difficult to examine them with care, for
almost the instant motion ceased, even while crossing the field of
vision, their bodies burst. Sometimes both ends burst at once,
sometimes only one, and a quantity of coarse, brownish, granular
matter was ejected. The animal an instant before bursting expanded
to half again its natural size; and the explosion took place about
fifteen seconds after the rapid progressive motion had ceased: in a
few cases it was preceded for a short interval by a rotatory
movement on the longer axis. About two minutes after any number
were isolated in a drop of water, they thus perished. The animals
move with the narrow apex forwards, by the aid of their vibratory
ciliae, and generally by rapid starts. They are exceedingly minute,
and quite invisible to the naked eye, only covering a space equal
to the square of the thousandth of an inch. Their numbers were
infinite; for the smallest drop of water which I could remove
contained very many. In one day we passed through two spaces of
water thus stained, one of which alone must have extended over
several square miles.
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