Most of the Vorticellæ, and probably the majority of Infusoria, remain unchanged for a time within their cysts, being then in a state analogous to the hybernal sleep of some of the reptiles. The cyst shelters them from cold and draught, and, when heat and moisture are restored, they resume their active vitality. The motions of the Infusoria are probably automatic, and in some instances consensual; they have neither true eyespecks, though their whole body seems to be conscious of light and darkness; nor have they ears; and, with the exception of touch, which the Vorticellæ have in a marvellous degree, it may be doubted whether the Infusoria have any organs of sense whatever, though they avoid obstacles and never jostle one another. The vibrations of their cilia are involuntary as in plants, an instance of the many analogies which perpetually occur between the lowest tribes of the two great kingdoms of nature. In both there are examples of propagation by bisection, conjugation, budding, and the alternation of generation, which occurs more frequently among Protozoa than among any other class of animals. There is a perfect resemblance between Zoospores and Protozoa; they both cease to move, the Zoospore when it secretes its cellulose coat and becomes a winter or resting spore, the Protozoon previous to encysting, a process presumed to be universal among that class of animals, before subdivision or reproduction begins. It is the dried cysts or germs of the Infusoria that float in the atmosphere as winter spores do, and it is believed that, like the fungi, the same germs may develope themselves into several different forms according to the nature of the liquid into which they may chance to be deposited; consequently, it is not necessary that the variety of germs should be very great, although the Infusoria themselves are of numerous forms.[22]