Written By Muhammad Yusuf on Friday, July 1, 2011 | 11:34 PM
: Entamoeba histolytica
: Entamoeba histolytica
The contractile vacuole is also respiratory, because carbon dioxide probably makes its way to the exterior by way of this organ. Oxygen dissolved in water is taken in through the surface of the body. This gas is necessary for the life of the animal; if replaced by hydrogen, movements cease after 24 hours; if air is then introduced, movements begin again; if not, death ensues.
Amoeba is common in fresh-water habitats, less so in marine habitats, and occur as parasites of many kinds of invertebrates and vertebrates.
Food vacuoles serve as temporary digestive organelles. After a food vacuole has become embedded in the endoplasm, its walls pour into it a secretion of some acid, probably HCl. Various enzymes are also part of the digestive fluid, which breaks down protein substance, and perhaps also fats and sugars, but not starch.
Both sexual and asexual reproduction occur among Protozoa. Some reproduce asexually only. As this is true of groups which are apparently primitive for other reasons, it seems probable that sexual reproduction appeared while organisms were at the protozoan grade of organization. Asexual reproduction involves the division of the parent body, either equally or unequally, to producee one more young individuals which develop into mature organism. It always involves a single parent and neither meiosis nor fertilization occurs.
Parasitic amoebae occur in coelenterates, man, and most kinds of animals between. The most important human amoeba is Entamoeba histolytica, which causes amoebic dysentery. A serious medical problem in tropical and subtropical regions, ir sometimes causes severe outbreaks in temperate climates as well. It is transmitted ehen cysts are eaten with contaminated food or water. The cysts germinate in the intestine; a quadrinucleated amoeba emerges which undergoes special divisions to reduce the nuclear number to the normal single nucleus. The amoebae grow, attacking the intestinal mucosa by secreting a histolytic enzyme. Active division occurs and large areas of the intestinal lining may be destroyed, causing severe diarrhoea and dysentery. The amoebae migrate by way of the blood stream from the intestinal wall to other organs where they produce amoebic abscesses. These are especially common in the liver, but have also been reported from lungs, brain, skin, and other body parts. Under some conditions, not yet completely characterized, the active amoebae become sluggish, lose their food vacuoles, and eventually begin the secretion of a cyst membrane. The precystic amoeba become rounded as the cyst wall develops, and the nucleus divides twice, so four are present in the mature cyst. This life cycle, in its general nature, is characteristic of most amoboid intestinal parasites.