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The protozoa are a heterogeneous assemblage of some 50,000 single-cell organisms possessing typical (eukaryote) membrane-bound cellular organelles. Because most are motile and many are heterotrophic, this assemblage was treated in the past as a single phylum within the Animal Kingdom — the phylum Protozoa. They are now known to consist of a number of different unicellular phyla, which together with most algal phyla are placed in the Kingdom Protista. Some of these protozoan groups are related to each other, some probably evolved independently from remote eukaryote ancestors, and some are members of various algal groups.

The unicellular level of organization is the only characteristic by which the protozoa as a whole can be described; in all other respects they display extreme diversity. Protozoa exhibit all types of symmetry, a great range of structural complexity, and adaptations for all types of environmental conditions. As organisms, the protozoa have remained at the unicellular level but have evolved along numerous lines through the specialization of parts of the protoplasm (organelles) or of the skeletal structure. Thus, simplicity and complexity in protozoa are reflected in the number and nature of their organelles and skeletons in the same way that simplicity and complexity in multicellular animals can be reflected in the development of tissues and organ system s. A protozoan cell may be far more complex than a metazoan cell, but a protozoan cell is an entire organism , not part of an organism, as is a metazoan cell.

Protozoa occur wherever moisture is present—in the sea, in all types of fresh water, and in the soil. There are commensal, mutualistic, and many parasitic species.

Although most protozoa occur as solitary individuals, there are numerous colonial forms. Some colonial forms, such as species of Volvox, attain such a degree of cellular interdependence that they approach a true multicellular level of structure. Both solitary and colonial species may be either free moving or sessile.

Protozoan Organelles and General Physiology

The protozoan body is usually bounded only by the cell membrane, which possesses the typical bilayered lipid ultrastructure of cells in general. The rigidity or flexibility of the protozoan body is largely dependent on the nature of the underlying cortical cytoplasm, called ectoplasm , which is rather gelatinous, in contrast to the more fluid, internal cytoplasm called endoplasm. Nonliving external coverings or shells occur in many different groups. Such coverings may be sim ple gelatinous or cellulose envelopes, or they may be distinct shells, com posed of various inorganic and organic materials, or sometimes foreign particles cemented together.

Depending on the species, there are one to many nuclei. The locomotors organelles may be flagella, cilia, or flowing extensions of the body called pseudopodia. Since the type of locomotors organelle is important in the classification of the phylum, discussion of the structure of these organelles is deferred until later.

All types of nutrition occur in protozoa. Some are autotrophic or saprozoic; many ingest food particles or prey and digest this food intracellular within food vacuoles. Food reaches the vacuole by engulfment, or phagocytosis, often through a mouth, or cytostome. Soluble food may enter by pinocytosis. Intracellular digestion has been most studied in amoebas and ciliates. The food vacuoles undergo definite changes in hydrogen ion concentration (pH) and in size during the course of digestion. Following ingestion, the vacuole contents become increasingly acid and smaller, as excess water is removed. Lysosomes deliver hydrolytic enzymes (Fig. 2-1), and the vacuole increases in size and becomes alkaline. The enzymes digest the vacuole contents, and products of digestion then pass into the cytoplasm by pinocytosis. The indigestible remnants are egested.

Protozoa that live in water where there is active decomposition of organic matter or in the digestive tract of other animals can exist with little or no oxygen present. Some protozoa are facultative anaerobes, utilizing oxygen when present but also capable of anaerobic respiration. Changing availability of food supply and of oxygen associated with decay typically results in a distinct succession of populations and protozoan species.

Metabolic wastes diffuse to the outside of the organism. Ammonia is the principal nitrogenous waste, and the amount eliminated varies directly with the amount of protein consumed.

Characteristic of many protozoa is an organelle system called the contractile vacuole complex. The complex is com posed of a spherical vesicle the contractile vacuole proper and a surrounding system of small vesicles or tubules termed the spongiome. The complex functions primarily in water balance (osmoregulation), pumping excess water out of the organism. The spongiome provides for the collection of water, which is delivered to the contractile vacuole. The latter expels the fluid to the outside of the organism through a temporary or perm anent pore. In some protozoa (some amoebas and flagellates) the vacuole completely disappears following contraction and is reformed by fusion of small vesicles. In others (many ciliates) the vacuole collapses at discharge and is refilled by fluid from the surrounding tubules of the spongiome.

Reproduction and Life Cycles

The protozoan reproductive processes and life cycles are varied. Only a few of the more common terms are described here.

Asexual reproduction occurs in most protozoa and is the only known m ode of reproduction in some species. Division of the animal into two or more daughter cells is called fission. When this process results in two similar daughter cells, it is termed binary fission; when one daughter cell is much smaller than the other, the process is called budding. In some protozoa, multiple fission, or schizogony, is the rule. In schizogony, after a varying number of nuclear divisions, the cell divides into a number of daughter cells. With few exceptions, asexual reproduction involves som e replication of missing organelles following fission.

Sexual reproduction may involve fusion (syngamy) of identical gametes (called isogametes) or gametes that differ in size and structure. The latter, called anisogametes, range from types that differ only slightly in size to well-differentiated sperm and eggs. Meiosis commonly occurs in the formation of gametes, but in many flagellate protozoa and sporozoans meiosis is postzygotic, that is, it occurs following the formation of the zygote as in most algae. In ciliate protozoa there is no formation of distinct gametes; instead, two animals adhere together in a process called conjugation, and they exchange nuclei. Each migrating nucleus fuses with a stationary nucleus in the opposite conjugant to form a zygote nucleus (synkaryon). Less com m on is a process called autogamy, in which two nuclei, each representing a gamete, fuse to form a zygote, all within a single individual.

Encystment is characteristic of the life cycle of many protozoa, including the majority of freshwater species. In forming a cyst, the protozoon secretes a thickened envelope about itself and becomes inactive. Depending on the species, the protective cyst is resistant to desiccation or low temperatures, and encystment enables the animal to pass through unfavorable environmental conditions. The simplest life cycle includes only two phases: an active phase and a protective, encysted phase. However, the more complex life cycles are often characterized by encysted zygotes or by for motion of special reproductive cysts, in which fission, gametogenesis, or other reproductive processes take place.

Protozoa may be dispersed long distances in either the motile or encysted stages. Water currents, wind, and mud and debris on the bodies of water birds and other animals are com m on agents of dispersion.