Annette Miller, age 13, of Eugene, Oregon, for her question:
How and where does the ameba live?
This miniature miracle of life requires moisture, preferably sunny fresh water, teeming with scraps of organic material, bacteria and tiny algae. There are sure to be thriving ameba populations in fresh water ponds and lakes, streams and rivers. A few also exist in the surface waters of the ocean, though there they are far outnumbered by more agile relatives. Many amebas thrive in swamps and moist soils. And a few species live as parasites inside the bodies of animals and human beings.
The average sized ameba measure 1/100th part of an inch and a giant specimen is just about big enough to be seen as a milky spot. They are classified with thousands of other single celled animals of the Phylum Protozoa. Within this vast group of assorted midgets, the amebas share Class Rhizopoda with a variety of close relatives. The genuine amebas, and there are many of them, belong in the Order Amoeba. Each species carries on all the functions of life within the soft cytoplasm of a single cell. What's more, the miniature miracle is so successful that he may achieve immortality.
When time comes to reproduce, he multiplies by dividing himself into a pair of newborn twins, leaving no aging parents. In a suitable environment and barring accidents, he can perpetuate his youth indefinitely. The bitsy blob may be destroyed by strong chemicals or eaten. If an accident severs him in half, one piece can restore itself to normal. He can survive a period of drought by encasing his soft body in a waterproof cyst and reducing his life activities to a minimum.
Moisture is the key to his success. It provides oxygen and other vital ingredients. The delicate membrane around his gelatinous blob of cytoplasm can select and absorb the useful chemicals and also keep out certain harmful substances. This thin, elastic membrane is always changing its shape. It pokes forth fingery pseudopods and the soft cytoplasm flows into them, moving the ameba's whole body in this or that direction.
The tiny hunter can detect suitable morsels and even sense the difference between a meat and vegetable course. The pseudopods surround a scrap of food, close in and absorb it to be digested inside. If the item is a passive alga cell, they move close and make a quick grab. If it is an active little animal cell, they spread wide and sneak up in slow motion to prevent escape. These miniature safaris go on in ponds and streams, where amebas teem in every drop of water, in moist soil and to some extent in the sea.
Naturally we consume numbers of these teeming amebas unawares. Most species do us little or no harm at all. However, a dangerous parasitical species thrives in soils and streams polluted with untreated sewage contaminated with human wastes. It enters the body in the dormant cyst stage. In the warm moist intestines it sheds its shell and attacks its host. This unsanitary little parasite causes a serious and sometimes fatal disease called amoebic dysentery.