The Vorticella has a most wonderful mode of reproduction common to a few other Infusoria. A gelatinous substance is secreted by the bell, which hardens and envelopes it in a cyst; the encysted bell then separates from its stalk, and is transformed into an infusorial animal called an Acineta (fig. 108), closely resembling the Actinophrys sol with radiating filaments which it continually stretches out and draws in. A motile ciliated embryo, or Vorticella bud, is then formed within the Acineta, which, after a time, comes out at a slit in its side, swims about, gets a stem, fixes to some object, and is developed into a Vorticella. The slit closes again, and the Acineta keeps moving its filaments as usual, and another motile embryo is formed within it, which is emitted by a slit in the same manner, and is also developed into a Vorticella. As these young Vorticellæ, or bell animals, may undergo the same transformations, there may be an indefinite alternation of the two forms. The Vorticella-bud, when it issues from the slit in the Acineta, has an oval form, with a circlet of long cilia at its narrow end, a mouth at the more obtuse, a nucleus, and contractile vesicles, and, after swimming about till it finds a suitable place, it becomes fixed by one end of its oval body, a style or stem is formed, which rises rapidly, and the adult shape is developed. The Acinetæ are said to live upon Infusoria: they apply the dilated apex of their rays as sucking discs to the animal, and suck its contents till it dies. The Tricoda linceus undergoes metamorphoses analogous to those of the Vorticella, but more numerous and complicated.[21]