Phylum Protozoa

The Protozoa – “first animals” – are one-celled heterotrophs possessing typical cellular structures. Classically, the Protozoans have been treated as a single phylum within the Animal Kingdom. But the unicellular level of organization is the only characteristic by which the phylum can be described; in all other respects the phylum displays extreme diversity. Protozoans exhibit all types of symmetry, a great range of structural complexity and adaptations for all types of environmental conditions. As organisms, the protozoans have remained at the unicellular level, but have evolved along numerous lines through the specialization of parts of the protoplasm. That is, specialization has occurred through the evolution of organelles. Protozoan evolution parallels that of multicellular animals in which specialization has occurred through the differentiation of cells within a multicellular body.

Protozoans occur wherever moisture is present—in the sea, in all types of fresh water and in soil. There are commensal, symbiotic and many parasitic species. In fact, the sporozoans are entirely parasitic. Although most protozoans occur as solitary individuals, there are numerous colonial forms. Both solitary and colonial species may be either free-moving or sessile.

The great majority of protozoans are microscopic. Plasmodium, the malarial parasite, is so small that it occupies only 1/4 or 1/5 of a human red-blood cell. At the other extreme, the freshwater ciliate, Spiroslomum, may reach a length of 3 mm. and be seen with the naked eye. The protozoan body is usually bounded only by the cell membrane. The rigidity or flexibility of the protozoan body is largely dependent upon the nature of the underlying cytoplasm. This cortical cytoplasm is usually gelatinous and called ectoplasm, in contrast to the more fluid, internal cytoplasm called endoplasm. Nonliving external coverings or shells occur in many different groups. Such coverings may be simple gelatinous or cellulose envelopes; or they may be distinct shells, composed of various inorganic materials or sometimes foreign particles cemented together.The nucleus is most commonly vesicular containing considerable nucleoplasm and one or more nucleoli.

Characteristic of many protozoans is an organelle called the contractile vacuole. Contractile vacuoles are water-balancing structures acting as pumps to remove excess water from the cytoplasm. These usually spherical vacuoles periodically collapse, releasing their fluid contents to the outside. Contractile vacuoles are most commonly encountered in freshwater protozoans with cytoplasm hypertonic to the aqueous environment. However, contractile vacuoles are also present in some marine groups.

All types of nutrition occur in protozoans. Some are autotrophic, and others are saprozoic; many are holozoic, and digestion occurs intracellularly within food vacuoles. Intracellular digestion has been most studied in amoebas and ciliates.

Gas exchange occurs by the diffusion of oxygen across the cell membrane. Protozoans that live in water where there is active decomposition of organic matter, or live in the digestive tract of other animals, can exist with little or no oxygen present. Some protozoans are facultative anaerobes, utilizing oxygen when present but also capable of anaerobic respiration. Metabolic wastes are diffused to the outside of the organism. Ammonia is the principal nitrogenous waste, and the amount of protein consumed.

The protozoan reproductive processes and life cycles are varied.

Asexual reproduction occurs in all protozoans and is the only known mode 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.

Sexual reproduction may involve fusion (syngamy) of identical gametes (called isogametes), or gametes that differ in size and structure. In ciliate protozoans there is no formation of distinct gametes; instead two animals adhere together in a process called conjugation, and they exchange nuclei.

Encystment is characteristic of many protozoans, including the majority of freshwater species. In forming a cyst, the protozoan secrets 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.

Classification of the Protozoa into three of their major groups is based upon their characteristically different methods of locomotion:

1) by flagellar movement (the zooflagellates — “animal flagellates” — or mastigophores),

2) by pseudopodia (the sarcodines),

3) by ciliary movements (the ciliates).

The fourth major group, the sporozoans, are nonmotile during the major phases of their lives and all are parasites.

Date: 2014-12-22; view: 968