The protozoa are one-celled animals and the smallest of all animals. Most of them can only be seen under a microscope. They do breathe, move, and reproduce like multicelled animals. They live in water or at least where it is damp. Animals in this group include the paramecium, the euglena and the ameba.
Some protozoans are harmful to man as they can cause serious diseases. Others are helpful because they eat harmful bacteria and are food for fish and other animals.
Below is a link to a description of three types of protozoa.
| System Type | Protozoas System |
| Muscular-Skeletal | A protozoa has no inner or outer skeleton. They move a variety of ways. The ameba has a false foot that extends as it moves. The paramecium is covered with hairs and the euglena has a whip-like tail to move. |
| Digestion | A protozoa takes in food via the water and stores the food in sacs called vacuoles. They eat tiny algae and bacteria. |
| Nervous | A protozoa has a very low level reaction to the world around it and does not have a brain per se. They can react to light and temperature changes. |
| Circulation | A protozoa has water flow in through the pores. The water contains the food and oxygen the protozoa needs. |
| Respiration | A protozoa takes in oxygen through the cell membrane and gives off carbon dioxide through the cell membrane. |
| Reproduction | A protozoa reproduces by splitting in half. This is called fission. |
| Excretion | A protozoa has sacs called vacuoles that take in and get rid of water. |
| Symmetry | A protozoa is usually asymmetrical. |
| Coloration | A protozoa is very microscopic and is pale in color generally. |
Life cycle
Some protozoa have life stages alternating between proliferative stages (e.g. trophozoites) and dormant cysts. As cysts, protozoa can survive harsh conditions, such as exposure to extreme temperatures or harmful chemicals, or long periods without access to nutrients, water, or oxygen for a period of time. Being a cyst enables parasitic species to survive outside of a host, and allows their transmission from one host to another. When protozoa are in the form of trophozoites (Greek, tropho = to nourish), they actively feed. The conversion of a trophozoite to cyst form is known as encystation, while the process of transforming back into a trophozoite is known as excystation.Protozoa can reproduce by binary fission or multiple fission. Some protozoa reproduce sexually, some asexually, while some use a combination, (e.g. Coccidia). An individual protozoon is hermaphroditic.
Sub-groups
Protozoa have been divided traditionally on the basis of their means of locomotion, although this character is no longer believed to represent genuine relationships:- Flagellates (e.g. Giardia lamblia)
- Amoeboids (e.g. Entamoeba histolytica)
- Sporozoans (e.g. Plasmodium knowlesi)
- Ciliates (e.g. Balantidium coli)
