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The Robinson Library >> Phylum Protoza

[uh mE' buh] a one-celled animal that usually can only be seen under a microscope

an amoeba

Some amoebas live in water and moist soil, while others live in the bodies of humans and other animals. One type of amoeba causes a serious ailment called amoebic dysentery when it gets into the large intestine of humans.

The single cell of an amoeba is a shapeless mass of protoplasm, the jellylike material found in the cells of all living things. A thin, elastic membrance surrounds the protoplasm and holds it together. Water and gases pass in and out of the amoeba through the membrane.

Since the amoeba has no feet it must move about by changing its body shape. The protoplasm pushes out the elastic membrane to form a fingerlike pseudopod (false foot). All of the protoplasm then flows into the pseudopod, thus moving the amoeba forward (or backward, or sideways). For every "step" another pseudopod must be formed.

structure of an amoeba
structure of an amoeba

Amoebas eat other one-celled animals and tiny plant life such as bacteria. They engulf their food by slowly wrapping pseudopods around a food particle. In this way, the food gets inside the cell. The section of the cell that contains the food is called a food vascule. It floats in the protoplasm until the food is digested. All undigested food is forced out of the cell.

an amoeba taking in food
an amoeba taking in food

Amoebas in fresh water must control water taken into their bodies or they will burst. They have a contractile vacuole to collect the extra water that builds up in the cell. When the vacuole is full, it empties through the cell membrane.

Amoebas reproduce by fission when they reach a certain size. The nucleus, a mass of special protoplasm, divides first. Then the remainder of the body divides. The division results in two daughter cells, each able to grow, feed, and divide.


The World Book Encyclopedia Chicago: World Book-Childcraft International, 1979

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The Robinson Library >> Phylum Protoza

This page was last updated on 09/03/2018.