Protozoa are a difficult to define group of single celled organisms, They fall within the general classification of protists. The protists are believed to be the early ancestors of the multicellar plant and animal kingdoms. The protists are single celled organisms that contain a nucleus. As such they can be distinguished from bacteria; bacteria fall into the category of single celled organisms that do not contain a nucleus.
Biologists have traditionally divided the protists subkingdoms describing organisms as either having animal or plant like behaviour. The animal like protists were identified as those capable of movement and classified as protozoa. The plant like protists were identified as those capable of photosynthesis and were defined as protophyta. The most famous protozoa are probably the amoebas. Most photophyta fall into the general category of algae.
Although used for classification purposes modern science regards the distinction between protozoa and protophyta is inadequate. There are many examples of algal cells that are both motile and capable of photosynthesis. Genetic studies also indicate that closely related organisms do not necessarily lie entirely within the protozoa or protophyta classes. The reclassification of protists on the basis of DNA similarities is far from complete.
Protozoa, meaning single celled orgasms with animal like characteristics, can be found widely in aqueous environments throughout the world. Most are between 10 and 50 micrometers long, However, the largest known protozoas, the xenophyophores can grow to a staggering 20 cm in diameter.
Despite genetic evidence suggesting that the classification is manmade concept, protozoa are classified into seven groups depending upon their means of locomotion. The slowest moving are the tulodens. They have whiplike tails called flagella.
Protozoa play an important part in the micro food chain. They prey on bacteria, micro fungi and single celled algae. They feed by a variety of mechanisms. Some absorb food through their cell membranes. Others such as the amoebas engulf their prey. Others sweep food towards specialist mouth parts. Some protozoa live as parasites on their hosts.
One unwelcome feature of the protozoas is that many cause disease as human parasites. Dysentery, malaria and African Sleeping Sickness are all caused by a protozoa parasites.
Protozoa have very clever mechanisms to ensure their survival in difficult times. Many protozoas can alternate between proliferative and dormant phases. The dormant cysts can survive long period without nutrients,. It is an ideal lifestyle for a parasite that has to endure long periods outside the host.
The protozoa are a large and diverse group of organisms that are currently being reclassified according to their genetic composition or in preference to their behavioural traits. In time this process will give insight into the evolution of multi-cellular organisms.