The Foraminifera have shells composed of carbonate of lime, but there are other forms that build up geological deposits, in which the shell is flinty. The diagram (Fig. 8) shows some fossil shells of Protozoa from the marl of Barbadoes. These constitute a deposit which was named "Infusorial earth," in the earlier days of microscopic observation, when all Protozoans were spoken of as Infusoria. The name, Infusoria, it must be recollected, is now restricted to a special class, to which the forms in question do not belong. These fossil forms were named Polycystina, and are still often spoken of under that name, although the animals that present the peculiar feature of possessing "more than one cyst" now are called Radiolarians. The "cyst" consists of a basket-work supporting skeleton of flint; there may be several, one inside the other, and connected by radial bars. A living species named _Actinomma_ has three such layers of basket-work, one in the outer layer of protoplasm, one in the inner layer, and a central one. It will perhaps be remembered by the reader that the animals of this group, Radiolaria, are forms described in a previous volume of the series, as so curiously associated in Symbiosis with the algæ known as Yellow Cells. The famous polishing slate of Bilin in Bohemia consists of flinty Protozoan shells; it is 14 feet thick, and a cubic inch has been estimated to contain 41,000,000,000 of the shells. While the Radiolarians are marine, the Heliozoa, a group in which the skeleton is also present, but not usually so greatly developed, are predominantly fresh-water forms. Both classes take their name (Ray-animals, Sun-animals) from the stiff radiating rods of the skeleton. Strongly to be contrasted with the above groups belonging to the Rhizopoda are the Infusoria proper, which are characterized by the usual possession of cilia. Cilia (literally "eyelashes") are fine hair-like processes of the protoplasm of the cell, which fringe its exterior; by their constant movement they enable the animal to swim, and at the same time they create a current in the water, which washes up to the region of the mouth particles which may serve for food; for these creatures have this very great advantage over Amoeba, and the other forms above referred to, that they possess something which may be called a mouth. That is to say, there is one particular spot of the surface where particles are taken in. This may seem to be a restriction, when we compare the Infusorian with Amoeba, which is apparently able to take in food at any part of the surface. But it is a restriction which is associated with an advantage; the Infusorian cell, namely, has a firm exterior with a definite outline, instead of being soft and mobile all over. The firmer exterior layer of protoplasm, which is in turn covered by a thin cuticle or limiting membrane, is called the cortex or rind. For this reason the name Corticata is sometimes given to the group, _i.e._, Protozoa with a rind.