is represented only by the thin envelope or periplast, so that the whole body is practically endoplasm. When the two layers are well differentiated the endoplasm is more fluid and coarsely granular, and contains various organs, chief amongst them in importance being the nucleus, which must be considered specially and may be put aside for the present.
In considering the functions of ingestion and assimilation of food a distinction must be drawn between those Protozoa which absorb solid food-particles, that is to say, which are holozoic in habit, and those which, being holophytic, saprophytic or parasitic in habit, absorb their nourishment in a state of solution. Only in holozoic forms is a special apparatus found for ingestion or digestion of food; in all other forms nutriment is absorbed by osmosis through the body-wall, presumably at any point of the surface. In holozoic forms we must distinguish further those in which the protoplasm is naked at the surface from those in which the body is clothed by a firm cuticle or cell-membrane. In naked forms food-particles are taken in at any point of the body-surface, either by means of the pseudopodia, or by the action of flagella causing them to impinge upon the surface of the body. In either case the food is absorbed by the protoplasm simply flowing round it and engulfing it, and the food passes into the interior of the body in a tiny droplet of water forming what is termed a food-vacuole. Into the food-vacuole the surrounding protoplasm secretes digestive enzymes, so that each such vacuole represents a minute digestive cavity, in which the food is slowly digested, rendered soluble, and absorbed by the surrounding protoplasm. The insoluble residue of the food is finally rejected by expelling the food-vacuole and its contents from the surface of the body at any convenient point.
The simple process of food-absorption described above for the more primitive naked forms is necessarily modified in detail, though not in principle, in corticate Protozoa, that is to say, in forms provided with a cuticle. In the first place, it becomes necessary to have a special aperture for the ingestion of food, a cell-mouth or cytostome. Primitively the cytostome is a simple pore or interruption of the cuticle, but in forms more highly evolved the aperture is prolonged inwards in the form of a tube lined by ectosarc and cuticle, forming a gullet or oesophagus which ends in the endoplasm. Food-particles are forced by the action of cilia or flagella down the oesophagus and collect at the bottom of it in a droplet of water which, after reaching a certain size, passes into the endoplasm as a food-vacuole in which the food is digested. For rejection of the insoluble residue of the food-vacuoles, a special pore or cell-anus (cytopyge) may be present. In the Ciliata there is often a distinct anal tube visible at all times, but as a rule the anus is only visible at the moment that faecal matter is being ejected from it, though fine sections show that the pore is a constant one. In the higher Flagellata, on the other hand, the oesophageal ingrowth forms commonly a sort of cloacal cavity, into which the contractile vacuole or vacuoles discharge themselves, and into which also the food-vacuoles evacuate their residues.
Besides the food-vacuoles already described, and the nuclear apparatus presently to be dealt with, the endoplasm may contain various metaplastic products, that is to say, bodies to be regarded as stages in the upward or downward metabolism of the protoplasmic substance. Such substances may take the form of coarse granules of various kinds, crystals, vacuoles or droplets of fatty or oily nature, pigment-grains, and other bodies. In the holophytic Flagellata the endoplasm contains also various organs proper to the vegetable cell, such as chlorophyll-bodies (chromatophores), pyrenoids, grains of a starchy nature (paramylum), and so forth, which need not be described here in detail.
The nucleus in Protozoa is usually a compact, fairly conspicuous structure, composed of chromatin combined in various ways with an achromatic substance or substances. Sometimes the chromatin is distributed in smaller masses through the nucleus, producing a granular type of nucleus; more often the chromatin is more or less concentrated in a central mass forming a so-called karyosome, consisting of an achromatic plastinoid substance impregnated with chromatin. If the karyosome is large and there is very little chromatin between it and the nuclear membrane, the nucleus is of the type termed vesicular. A nuclear membrane is not, however, always present, and true nucleoli, of the type found in the nuclei of metazoan cells, are not found in Protozoa.
A given individual may have more than one nucleus, and the number present may amount to many thousands, as in the plasmodia of Mycetozoa. In such cases the nuclei may be all of one kind, that is to say, not markedly different in size, structure or function, so far as can be seen; or there may be a pronounced morphological differentiation of the nuclei correlated with a difference of function. Thus in the class Infusoria two nuclei are found in each individual; a macronucleus which is somatic in function, that is to say, which regulates the metabolism and vital processes of the body generally, and the micronucleus, which is generative in function, that is to say, which remains in reserve during the ordinary, “vegetative” life of the organism and becomes active during the act of syngamy, after which the effete macronucleus is absorbed or cast out and a new somatic nucleus is formed from portions of the micronuclei which have undergone fusion in the sexual act. Thus the micronucleus of the Infusoria can be compared in a general way with the germ-plasm of the Metazoa, like which it remains inactive until the sexual union. On the other hand, in some Flagellata a differentiation of the nucleus of quite a different type is seen, a smaller, kinetic nucleus being separated off from the larger, trophic or principal nucleus. The kinetic nucleus has the function, apparently, of controlling the locomotor apparatus, so that the specialization of these two nuclei is of a kind quite different from that seen in the Infusoria.
Besides the nuclear substance which is concentrated to form the principal nucleus or nuclei, there may be present also extranuclear granules of chromatin, so-called chromidia, scattered throughout the whole or some part of the protoplasmic body. Chromidia may be normally present in addition to the principal nucleus, or may be formed from the principal nucleus during certain phases of the life-cycle. In some cases the entire nucleus may become resolved temporarily into chromidia, from which a new nucleus may be formed again later by condensation and concentration of the scattered granules. When the chromidia are numerous and closely packed they may form a so-called chromidial network (Chromidial-Netz). Recent observations on the reproduction of some Sarcodina have shown that the chromidia may possess great importance in the life-cycle as representing generative chromatin which, like the micronucleus of the Infusoria mentioned above, remains in reserve until, by the process of syngamy, the nuclear apparatus is renewed; while the principal nuclei represent, like the macronuclei, somatic or vegetative chromatin which becomes effete and is cast off or absorbed when syngamy takes place. These questions will be discussed further below.
It was formerly supposed that the lowest Protozoa were entirely without a nucleus, and on this supposition E. Haeckel attempted to establish a class named by him Monera, defined as Protozoa consisting of protoplasm alone, in which a nucleus was not differentiated. To this class were referred various organisms whose alleged archaic nature was expressed by such names as Protogenes primordialis, organisms which, like so many other of the primitive forms of animal life described by Haeckel, have been seen by that naturalist alone up to the present. In all Protozoa that have been examined by modern methods a nucleus in some form has been demonstrated to exist, and it must be supposed, until proof to the contrary be forthcoming, that in the case of the so-called Monera either the nucleus was overlooked owing to defective technique, or it had been temporarily resolved into chromidia.
The nuclear apparatus may be supplemented by other bodies of which the nature is not always clear. Such is the so-called “Nebenkern” of Paramoeba eilhardi, apparently of the nature of a centrosome. Sometimes the karyosome acts like a