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12. Life
| 12.29 |
Advanced mono-cellular creators (protozoa, sacodina, ciliata)
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A microscopic picture of a paramecium. You can see the tiny
hairs around its single cell body- flagelettes that it uses to wim around.
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| Protozoa, collective name for
animal-like, single-celled organisms, some of which may form colonies. In the
classification followed in this encyclopedia the protozoa are placed in the
kingdom Protoctista with other single-celled organisms that have
membrane-enclosed nuclei. Protozoa have little or no differentiation into
tissue systems. Several phyla are commonly recognized. They include flagellated
Zoomastigina, many species of which live as parasites in plants and animals;
the amoeboid Sarcodina, which includes the Foraminifera and Radiolaria, both
important components of the plankton; ciliated Ciliophora, many with
specialized structures suggesting the mouth and anus of higher organisms;
Cnidosporidia, parasites of invertebrates, fish, and a few reptiles and
amphibians; and Sporozoa, many species of which are parasites of animals
(including humans). More than 20,000 species are known, including such familiar
forms as paramecium and amoeba.
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| Most species are found in such aquatic habitats as oceans, lakes, rivers, and ponds. They vary in length from 2 to 70 micrometres (one micrometre equals one-millionth of a metre). Protozoa obtain their food by ingesting bacteria, waste products of other organisms, algae, or other protozoa. Most species are motile (can move), either by whip-like structures called flagella, hair-like structures called cilia, or amoeboid motion, a streaming type of movement involving the formation of pseudopods (foot-like extensions). | ||
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