The structures of protozoa are not referred to as organs as in higher animals but as organelles, organs being composed of cells and organelles being differentiated portions of a cell.
Protozoa contain one or more nuclei, which may be of several types. In the protozoa other than ciliates, the nucleus is vesicular, and all the nuclei in the same individual look alike. There are two types of vesicular nuclei. In one type, an endosome is present. The endosome is a more or less central body with a negative Feulgen reaction and therefore without deoxyribonucleic acid. The chromatin, which is Feulgen positive and which forms the chromosomes, lies between the nuclear membrane and the endosome. This type of nucleus is found in the trypanosomes, parasitic amoebae and phytoflagellates. In the other type of vesicular nucleus, there is no endosome, but there may be one or more Feulgen-positive nucleoli. In these, the chromatin is distributed thruout the nucleus. This type of nucleus is found in the Telosporasida, hypermastigorid flagellates, opalinids, dinoflagellates, and radiolaria.
In the ciliates there are two types of nucleus which look different, and each individual has at least one of each. The micronucleus is relatively small; it divides by mitosis at fission and apparently controls the reproductive functions of the organism. The macronucleus is relatively large; it divides amitotically at fission and apparently has to do with the vegetative functions of the organism. Both these nuclei appear quite homogeneous in composition in contrast to the vesicular nuclei of other protozoa.
Protozoa move by means of flagella, cilia or pseudopods. A flagellum is a whip-like organelle composed of a central axoneme and an outer sheath. The axoneme arises from a basal granule or blepharoplast in the cytoplasm. The axoneme has been shown by electron microscopy to be composed of 9 peripheral and 2 central fibrils. In some species a flagellum may pass backward along the body, being attached to it along its whole length or at several points to form an undulating membrane. Flagella are found in the Mastigasida and in the flagellate stages of the Sarcodasida and Telosporasida.
A cilium is an eyelash-like organelle resembling a small flagellum. It has a sheath, basal granule and axoneme. In Paramecium and other forms, the axoneme is composed of 9 peripheral and 2 central fibrils. Cilia are found in the Ciliasida. The less specialized ciliates have large numbers of cilia which are arranged in rows and beat synchronously. In the more specialized ciliates, special locomotory organelles have been developed by fusion of cilia. A cirrus is a tuft of fused cilia embedded in a matrix. A membranelle is a more or less triangular flap formed by the fusion of two or more transverse rows of cilia; membranelles are found especially around the mouth. An undulating membrane (not to be confused with the undulating membrane of flagellates) is formed by the fusion of one or more longitudinal rows of cilia; they occur in the oral groove of some ciliates.
A pseudopod is a temporary locomotory organelle which can be formed and retracted as needed. There are four types of pseudopod. A lobopod is a relatively broad pseudopod with a dense outer layer and a more fluid inner zone; lobopods are found in the amoebae and some flagellates. A filopod is a slender, hyaline pseudo pod which tapers from its base to its pointed tip; filopods tend to anastomose and may fuse locally to produce thin films of cytoplasm; they contain no cytoplasmic granules. A myxopod (rhizopod, reticulopod) is a filamentous pseudopod with a dense inner zone and a more fluid outer layer in which cytoplasmic granules circulate; myxopods branch and anastomose to form complex networks which are used for trapping food and also for locomotion; they are found in the Foraminiferorida. An axopod is a slender pseudopod which projects from the body without branching or anastomosing; it is composed of a thin outer layer of fluid cytoplasm and an axial filament composed of a fibrillar tube containing a homogeneous core; axopods are found in the Heliozoorida and Radiolariorida.
Locomotion can also be effected by bending, snapping or twisting of the whole body. A number of protozoa employ this method.
There is still another type of locomotion, gliding, exemplified by Toxoplasma, Sarcocystis, coccidian merozoites, gregarines and Labyrinthula, in which the body glides smoothly along without benefit of any apparent locomotor organelles, change in shape or other visible cause. Kummel (1958) found by means of electron micrographs that the outer surface of certain gregarines (Gregarina cuneata, G. polymorpha, Beloides sp.) is thrown into a series of deep, microscopic folds which he thought produce mucus. Beneath these folds in the pellicle are fibrils which he thought contract to move the organism along a mucous track. Jarosch (1959) thought that the gliding of Gregarina, Euglena and various single-celled plants is caused by superficial fibrils thrusting against extruded mucus. Beams et al. (1959) found numerous ultramicroscopic folds in the surface membrane and an ultramierqscopic network of fibrils about 50 to 200 A in diameter in the ectoplasm of the trophozoites of Gregarina rigida from the grasshopper. They believed that gliding is probably accomplished by movement of the body surface in contact with the substrate and that the mucus which is secreted may possibly provide a suitable surface for locomotion. Ludvik (1958) observed superficial, longitudinal fibrils in electron micrographs of Sarcocystis tenella. However, a definitive explanation of the mechanism of gliding still eludes us.
Organelles associated with nutrition
Nutrition among the protozoa may be of several types. Rather elaborate classifications have been proposed by some authors, but three types are sufficient for our purposes. In holophytic nutrition, which is characteristic of the phytoflagellates, carbohydrates are synthesized by means of chlorophyll which is carried in chromatophores, which vary considerably in size, shape and number.
In holozoic nutrition, particulate food material is ingested thru a temporary or permanent mouth. A temporary mouth is formed by amoebae when they engulf their food. A permanent mouth is a cytostome. It may be simple or it may lead into a cytopharynx. In many ciliates the area around the cytostome forms a peristome, and there may be a number of other specialized structures associated with it. Particulate food passes into a food vacuole in the cytoplasm, where it is digested. The indigestible material may be extruded from the body either thru a temporary opening or thru a permanent cytopyge.
In saprozoic nutrition, no specialized organelles are necessary, nutrients being absorbed thru the body wall. This type is found in many protozoa, and may be present along with holophytic or holozoic nutrition.
Excretion in the Protozoa is either thru the body wall or by means of a contractile vacuole which may be simple or may be associated with a system of feeder canals or vacuoles. Contractile vacuoles are probably more important as osmoregulatory organelles than for excretion. They maintain water balance by removing excess water from the cytoplasm and passing it out of the body. They are found in fresh-water protozoa but are absent in most marine and parasitic protozoa. However, some of the latter, including Balantidium and trypanosomes, contain them.
Protozoa have many other specialized organelles which are found in different groups. These will be described in the appropriate places below.