CHAPTER IV

THE ONE-CELLED ANIMALS OR PROTOZOA

Some idea of the general characteristics of the Protozoa has already been given by the description of _Amoeba_. We may now say something about special groups of the Protozoa, which have minor characteristics of their own.

Amoeba belongs to the class Rhizopoda, as has been already stated; but there are many of the Rhizopoda that greatly differ from Amoeba 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.

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.

_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.

It is interesting to note that the main types of the unicellular animals are repeated again in the cells of different parts of the bodies of multicellular animals. Amoeboid cells, so called because of their mobility and general resemblance to Amoeba, are found in various parts of the higher animals. The lymph corpuscles of vertebrata, and the white corpuscles of vertebrate blood, as well as the blood corpuscles of invertebrates, are among the instances of this. There are cells, on the contrary, such as those that line the mucous tracts, which are of a Vorticella type, so to speak; fixed to their bases, and presenting cilia on the free aspect.

Two things must be noticed before we leave the subject of the Protozoa. One is, that some forms present the beginning of a multicellular condition. Several units sometimes join together, and in this way a complex object may be formed, in which there are several nuclei; or the original unit may keep on growing till it consists of many successive portions, and in some of them a fresh nucleus may arise. This occurs in some of the Foraminifera.

The next thing to be noticed is, that there are a number of organisms which constitute a debateable ground, and are claimed now by the botanist, and now by the zoologist. While the latter insists on calling them Protozoa (Primitive Animals) the former would have them Protophyta (Primitive Plants). The fact is that in these organisms of the first grade, the distinction between "plant" and "animal" has not become a hard and fast line; and the disputed forms may be best described as links between the two. The chemistry of nutrition is probably more to be relied upon as a distinction, than the difference of structure. It is here that the two groups, plants and animals, start upon different roads, and many of the differences in structure must be regarded as the direct result of the fundamental difference in the mode of nutrition. The following very instructive remarks on the subject are taken from Professor Hertwig's valuable book "The Biological Problem of To-Day,"[B] pp. 111, 112.

[B] "The Biological Problem of To-Day, Preformation or Epigenesis," by Professor O. Hertwig. Translated by P. C. Mitchell. Heinemann, 1896.

"The different mode of nutrition of animals results in a totally different structural plan. Animal cells absorb material that is already organised, and that they may do so their cells are either quite naked, so affording an easy passage for solid particles, or they are clothed only by a thin membrane, through which solutions of slightly diffusible organic colloids may pass. Therefore, unlike plants, multicellular animals display a compact structure with internal organs adapted to the different conditions which result from the method of nutrition peculiar to animals. A unicellular animal takes organic particles bodily into its protoplasm, and forming around them temporary cavities known as food vacuoles, treats them chemically. The multicellular animal has become shaped so as to enclose a space within its body, into which solid organic food-particles are carried and digested thereafter in a state of solution, to be shared by the single cells lining the cavity. In this way the animal body does not require so close a relation with the medium surrounding it; its food, the first requirement of an organism, is distributed to it from inside outwards. In its further complication the animal organisation proceeds along the same lines. The system of internal hollows becomes more complicated by the specialisation of secreting surfaces, and by the formation of an alimentary canal, and of a body-cavity separate from the alimentary canal. In plants it is the external surface that is increased as much as possible. In animals, in obedience to their different requirements, increase takes place in the internal surface. The specialisation of plants displays itself in organs externally visible--in leaves, twigs, flowers, and tendrils. The specialisation of animals is concealed within the body, for the internal surface is the starting-point for the formation of the organs and tissues."

TABLE SHOWING THE CLASSIFICATION OF THE PROTOZOA

=Grade I.= { RHIZOPODA, OR { GYMNOMYXA. UNICELLULAR ANIMALS. =PROTOZOA= { { INFUSORIA, OR { CORTICATA.