Amœba belongs to the class Rhizopoda, as has been already stated; but there are many of the Rhizopoda that greatly differ from Amœba in appearance. The possession of a shell or skeleton gives a special importance to several groups. For, as the reader has no doubt already learnt from an earlier volume in this series, such skeletons or shells have played an important part in the history of the earth's surface, building up geological strata of vast extent, by the accumulation of the shells left after the decay of the owners' tiny bodies, during long periods of time. The chalk rocks that form the "white cliffs of Albion," and that are so widely distributed in other parts of the globe, are formed in this manner; while the ooze of the Atlantic and other oceans, similarly composed of Protozoan débris, is now at the present time building up what will be the chalk rocks of future ages. Some of these Protozoans attain a remarkable size, instead of being microscopic, as is the case typically with the one-celled animals. Some forms of the Foraminifera found on the coast of North America measure as much as one-fifth of an inch across, while in warmer seas there are kinds which attain, as did the extinct Nummulite of Egypt, the size of a bean. Two inches across is mentioned as the maximum diameter, however, of either extinct or living forms. The Foraminifera are sometimes named Reticularia, because their pseudopodia interlace.
Fig. 8.—Fossil Skeletons of Polycystina, from the so-called "Infusorial Earth" of Barbadoes, highly magnified.
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 Amœba, 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 Amœba, 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.
Vorticella, the Bell Animalcule, is a stalked form living in ditches, which is usually selected as a typical form of the Infusoria. It receives its name, the Whirlpool Animal, from the current which its cilia create in the water. The purpose of this current is to wash food particles into the mouth. Associated with the Infusoria under the name of Corticata are the Gregarina and some other parasitic forms.