But perhaps the most singular and beautiful of all the infusorial animalcules are the _Vorticellæ_, which resemble minute cups or flower-bells, mounted upon slender retractile threadlike stalks, by which they are moored to the surface of the weeds and stones. They are called Vorticellæ on account of the little vortices or whirlpools which they continually create in the water, by means of a fringe of very minute cilia placed round the brim of their cups. These cilia are so minute as to require a very high microscopic power to make them visible, and even then they are not easily detected, on account of their extremely rapid vibration, which never relaxes while the animal is in full vigour. On the other hand, when near death, their velocity diminishes, and ample opportunity is afforded for observing that the movements consist of a rapid bending inwards and outwards, over the edge of the cup. This is best seen in a side view. The action is repeated by each cilium in succession, with such rapidity and regularity that, when viewed from above, the fringe looks like the rim of a wheel in rapid revolution. A similar appearance, produced by the same cause, in another class of animalcula, of much more complex structure than the Vorticellæ, has procured for it the name of _Rotifera_, or wheel-bearers. The result of this combined movement of the cilia is, that a constant stream of water is drawn in towards the centre of the cup, and thrown off over the sides, when, having reached a short distance beyond the edge, it circles rapidly in a small vortex, curling downwards over the lips. These currents are rendered evident by floating particles in the water. The possession of these vibratile cilia is not peculiar to this class of animals; indeed, there is good reason to believe that there is scarcely a living creature, from the lowest animalcule, or plant germ, up to man himself, that is not provided with them in some part or other. In many of these Infusoria the cilia constitute the organs of locomotion; while in the higher forms they serve various other purposes, but chiefly that of directing the flow of the various internal fluids through their proper channels. But the peculiar and perhaps most wonderful organ of the Vorticella, is its stalk or mooring thread. This though generally of such extreme tenuity as to be almost invisible with ordinary microscopes, yet exhibits a remarkable degree of strength and muscular activity in its movements, which apparently are more voluntary than those of the cilia. Its action consists of a sudden contraction from a straight to a spiral form with the coils closely packed together, by which the head or bell is jerked down almost into contact with the foot of the stalk; after a few seconds the tension seems gradually relaxed, the coils are slowly unwound, and the stalk straightens itself out. This action takes place at irregular intervals, but it is seldom that more than a minute elapses between each contraction.