We shall have occasion again to notice the _Protophytes_, and now pass to the _Protozoa_, of which we have a good illustration in the _Vorticella_ already spoken of. In the group before us a number of elegant bells or vases stand at the end of long stalks, as shown at the top of the frontispiece, while round the tops of the bells, the vibrations of a wreath or cilia produce little vortices or whirlpools, and hence comes the family name. This current brings particles of all sorts to the mouth near the rim of the bells, and the creature seems not entirely destitute of power to choose or reject the morsels according to its taste. Every now and then the stalk of some specimen is suddenly twisted into a spiral, and contracted, so as to bring the bell almost to the ground. Then the stem gracefully elongates again, and the cilia repeat their lively game. The general effect can be seen very well by a power of about sixty linear, but one of them from one to two hundred is necessary to bring out the details, and a practised observer will use still more magnification with good effect. They should be examined by a moderately oblique light, or most of the cilia are apt to be rendered invisible, and also by _dark ground_ illumination. This may be accomplished in a well-made microscope by turning the mirror quite out of the plane of the axis of the instrument, that is to say, on one side of the space the body would occupy if it were prolonged. By this means, and by placing the lamp at an angle with the mirror, that must be learnt by experiment, all the light that reaches the eye has first passed through the object, and is refracted by it out of the line it was taking, which would have carried it entirely away. Or the object may be illuminated by an apparatus called a _spotted lens_, which is a small bull's-eye placed under the stage, and having all the centre of its face covered with a plaster of black silk. In this method the central or direct rays from the mirror are obstructed, but those which strike the edge of the bull's-eye are bent towards the object, which they penetrate and illuminate if it is sufficiently transparent and refractive. Another mode of dark ground illumination is by employing an elegant instrument called a _parabolic illuminator_, which need not be described. [Illustration: Vorticella, with posterior circlet of cilia in process of separation, 300 linear.--_Stein._] [Illustration: Vorticella in process of self-division. A new frontal wreath in formation in each of the semi-lunar spaces.] Different specimens and species of _Vorticellæ_ vary in the length of their bells from one three or four thousandth to one hundred and twentieth of an inch, and when they are tolerably large, the dark ground illumination produces a beautiful effect. The bells shine with a pearly iridescent lustre, and their cilia flash with brilliant prismatic colours.