Do you know what are the simplest animals? The protozoans are simple organisms that consist of only one cell. They make up the phylum protozoa.
What are the simplest animals?
It is difficult to refer to some of them as either animals or plants because although many are extremely active and catch and eat their food, others, like plants, use the energy of the sun to make their own food.
A protozoan is made up of a single microscopic unit of protoplasm (the complex mixture of substances of which all animals and plant are made) enclosed by a membrane and controlled by a central nucleus.
Protozoan’s live in remarkable variety of habitats. Although they are essentially aquatic they are found in all bodies of water from small muddy puddles of rainwater to all the oceans of the world. They can even exist in the thin film of moisture which surrounds soil particles and many are parasitic, living within the bodies of other plants and animals.
They are very resistant and, if the pond or marsh in which they live dries up. They are able to survive and it returns to life again when favorable conditions occur.
Although extremely small in size and basic in structure, the protozoans are very significant to other members of the animal kingdom. They form the heads of food chains and so provide proteins and vitamins for other more advanced animals. Parasitic forms cause diseases in man, for example, malaria.
How do these single-celled animals move?
Protozoans move in a variety of ways. The amoeba has no characteristic shape, in contrast to the other protozoans, and move along the substratum, or surface of objects, in a flowing movement, continually changing its form. The jelly-like portoplasm streams out into thin extensions called pseudopodia or false-feet and the rest of the amoeba flows into them. In this way the amoeba slowly creeps about.
The majorities of protozoans, however, are more active and lash the water with whip-like flagellae. Chlamydomonas and Euglena swim jerkily around.
A modification of the same method is seen in Paramecium in which the entire surface is covered with rows of short cilia. Their movements are co-ordinate and they beat rythmically in waves, propelling the animal smoothly through the water.
How does Volvox differ from other Protozoans?
Volvox is a colony of protozoans. It is a group of several thousand individual protozoans arranged to form the wall of a jelly-filled sphere. Some of these individual cells have different functions and in this way Volvox and other similar colonies of protozoans are among the first examples of the arrangement of many-celled animals.
Each protozoan of the colony has a pair of flagellae which lash the water in a co-ordained rhythmic sequence. Volvox can be seen in pond water as a tiny green ball moving smoothly along in a rolling motion.
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