The Vorticella nebulifera and some others of the Infusoria are remarkable for the diversity of their reproductive powers; for, besides division and gemmation, they are reproduced by a kind of alternate generation, accompanied by singular metamorphoses. The Vorticella, one of the most beautiful animals of its class, lives in pools of fresh water: groups of them are found on almost every mass of duckweed like little blue bells upon slender stalks, creating active currents in the water by the vibrations of long and powerful cilia with which the margin of the bell is fringed. The lip or edge of the bell is bent outwards into a permanent rim, and a deep groove cleaves the rim on one side, in which a wide cavity forming the mouth is placed. The mouth, the short throat or gullet, and the whole bell, are bristled with vibratile cilia.
The Vorticellæ feed on vegetable organisms, chiefly diatoms, and are exceedingly voracious. The cilia round the rim of the bell entangle the food, draw it into the mouth, and those in the gullet force particle after particle mixed with water into vacuoles which they make in the interior of the soft sarcode which fills the bell, and there the particles undergo rotation till digested and absorbed, and, if refuse remain, it is ejected through a softer part in the outer layer of the bell.
The stem that fixes the animal to a solid object is a tubular continuation of its outer membrane, containing a highly contractile filament; and, as the creature is extremely sensitive to external impressions, it folds up the ciliated rim of its bell, and its stalk shrinks down in a spiral on the slightest alarm, but the bell opens and the stalk stretches out again as soon as the alarm is over. When a Vorticella is reproduced by division, the bell separates longitudinally into two parts; one is often smaller than the other, and separates from its parent, swims about till it gets a stem, and fixes itself to an object. When the two parts are of equal size, the division extends to a greater or less distance down the stalk, and as each of these become perfect bells, and do not fall off but subdivide in the same manner, it follows that, by successive divisions, a whole group of these beautiful animals may spring from the same stem, as in fig. 107.