Invertebrate Zoology/Protozoans
One of the earliest forms forms of life, protozoans have simple body plans and are always single-celled, like the protozoan depicted to the right.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/a/a7/Dileptus.jpg/220px-Dileptus.jpg)
Protozoans exist as individuals, but some are colonial.
All the metabolic activities of an organism like respiration, digestion, excretion and reproduction are carried out by these single celled organisms.