Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Protozoa
Sarcomastigophora
Class Sarcodina

General characteristics

The phylum Sarcomastigophora comprises approximately 18,000 species of protozoa whose locomotor Organelles are either temporary cytoplasmic extensions known as pseudopodia or whip-like flagella. In some species, both types of organelles function simultaneously or sequentially during The life cycle. The phylum Sarcomastigophora is divided into two classes: Sarcodina and Mastigophora.

The Class Sarcodina encompasses over 11,000 species characterized by A wide variety of forms. The majority of sarcodines are marine inhabitants, though a considerable number are freshwater species; some dwell in soil and in the moss cushions of bogs and forests. Parasitic forms account for no more than 1.3% of free-living species. The length of representatives within this class ranges from 10-15 µm to 2-3 mm. Most possess a single Nucleus enclosed by a membrane. In the Cytoplasm, the denser ectoplasm can transition into a more fluid endoplasm and vice versa. This lack of a rigid, stable pellicle accounts for the indefinite and changeable shape of sarcodine bodies, enabling them to form pseudopodia (see Fig. 1). In many floating protozoa, these structures serve as trapping devices.

Sarcodines respire through their entire body surface. Their diet consists of Bacteria, Algae, or other protozoa.

Fig. 1 Amoeba (Amoeba proteus):

1 — pseudopodium; 2 — ectoplasm; 3 — endoplasm; 4 — food vacuole and its formation; 5 — contractile vacuole; 6 — nucleus.

Food capture is carried out by pseudopodia at any point on the body. This mode of feeding is known as phagocytosis. A food vacuole—a vesicle containing digestive Enzymes secreted by the cytoplasm around the food particles—forms around the food item once it enters the cytoplasm.

Indigested residues can be expelled through any part of The Cell. Freshwater sarcodines feature contractile vacuoles, which primarily function to excrete excess Water and Metabolic waste products from the cell; marine species lack such vacuoles. Sarcodines are capable of locomotion in response to environmental changes. Movement directed by various stimuli (such as Temperature fluctuations, light intensity, or Chemical properties of the medium) is called taxis. Movement toward a stimulus is termed positive taxis, whereas movement away from it is negative taxis. Upon environmental changes (such as drying or freezing), excess water is expelled from the cell, and a thick, durable proteinaceous envelope known as a cyst is secreted. In this dormant state, cysts are carried by the wind or persist until favorable conditions return.

The majority of sarcodines reproduce asexually: The Nucleus and subsequently the entire cell divide in two via a deepening constriction. Certain species produce Gametes (sex Cells), which may develop 1 flagellum, though 2 are more common. The fusion of gametes forms a zygote, The Development of which gives rise to a new generation of organisms.

The class Sarcodina includes several orders. Among these, Representatives of the order of naked amoebas are most common in freshwaters, with the common amoeba, or *Amoeba proteus* (about 0.5 mm in length), being particularly large. It can easily be found On the surface of silt in puddles and ponds. Under a Microscope, it appears as a colorless, watery mass that constantly changes shape. The amoeba either engulfs food from two sides with its pseudopodia or draws in filamentous algae without noticeable movement.

Many species of amoebas have adapted to living in the intestines of animals and humans. However, only a few of these can be considered true parasites.

Among parasitic forms, The most significant is the dysentery amoeba (*Entamoeba histolytica*), discovered by the Russian professor F. A. Lösch in 1875 in St. Petersburg, although it is more frequently encountered in southern regions. Cysts of the dysentery amoeba enter the Human digestive tract via contaminated food. In the Large Intestine, they release small, motile, uninucleate amoebas that begin to multiply rapidly and feed on bacteria, initially causing no harm to the host Organism. Subsequently, these amoebas penetrate the intestinal mucosa, where, acting as parasites, they feed and multiply. As a result, ulcers form in the intestinal mucosa, leading to bloody diarrhea. Passing through the intestinal wall, the amoebas enter Blood Vessels, and the bloodstream may carry them to the Liver, Brain, Lungs, and other Organs, causing dangerous abscesses. Many amoebas from the ulcers re-enter the intestine, forming cysts in its lower sections. Along with feces, the cysts of the dysentery amoeba are expelled into the external environment, where they can remain viable for extended periods.

A source of infection can also be an infected individual shedding 300–600 million cysts per day. Amoebic dysentery is a severe, debilitating, and dangerous disease. Treatment of patients is mandatory using specialized medications.

Species of the order of testate amoebas, such as the freshwater *Arcella* and *Difflugia*, are characterized by the unicellular organism residing inside a protective shell (test) from which pseudopodia protrude.

Marine waters are inhabited by foraminiferans, radiolarians, and heliozoans. Their cells are housed within intricately structured shells or unique skeletal frameworks. Floating suspended in the water, these animals are passively transported by currents.