On the coast of Chile, a few leagues north of Concepcion, the "Beagle" one day passed through great bands of muddy water, exactly like that of a swollen river; and again, a degree south of Valparaiso, when fifty miles from the land, the same appearance was still more extensive.
Some of the water placed in a glass was of a pale reddish tint; and, examined under a microscope, was seen to swarm with minute animalcula darting about, and often exploding.

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.